Brakes are all well and good, but you need some method of applying them in order for them to work. The method by which the force from your hand or foot reaches the brake itself is all to do with the brake actuator system.
This is about as basic as you get. A cable is connected to a lever at each end. You press on one lever with your foot or squeeze it with your hand, and it pulls the lever at the other end. On the back of the brake-end lever there’s an elliptical cam which rotates inside a circular cup in the brake shoe. As the long axis of the ellipse rotates, it forces the brake shoes to move apart. In the case of a bicycle brake, the brake-end of the cable just pulls the two calipers together.
One step up, and found on the rear brake of most Indian motorbikes, the solid bar connection. This allows the use of mechanical advantage (see below) to amplify your force on the pedal or lever before it gets to the brakes themselves. Typically these systems are used on drum brakes with the elliptical actuator described above. The disadvantage of this system is that it needs hinge and pivot points that match the position of the suspension components. If they’re not present, going over a bump could put the brakes on as the suspension moves relative to the lever.
Another step up and we get to the type of brake system used on most cars and motorbikes today. Gone are the cables and bars, replaced instead with a system of plungers, reservoirs and hydraulic fluid. Single-circuit hydraulic systems have three basic components – the master cylinder, the slave cylinder and the reservoir. They’re joined together with hydraulic hose and filled with a non-compressible hydraulic fluid (see brake fluid below). When you press your foot on the brake, or squeeze the brake lever, you compress a small piston assembly in the master cylinder. Because the brake fluid does not compress, that pressure is instantaneously transferred through the hydraulic brake line to the slave cylinder where it acts on another piston assembly, pushing it out. That slave assembly is either connected to a lever to activate the brakes, or more commonly, is the brake caliper itself, with the slave cylinder being the piston that acts directly on the brake pads. Because of the arrangement of the slave cylinder, heat from the brakes can be transferred back into the brake fluid.
Dual-circuit hydraulic systems are available on high-end luxury vehicles and newer motorbikes, in particular BMW bikes. These have two separate circuits. One is the command circuit – that’s the one you act on with your hand or foot. The second is a separate circuit controlled by an onboard computer, and that’s the one which is actually connected to the brakes. As you apply the brakes, you’re sending a pressure signal via the command circuit to the brake computer. It measures the amount of force you’re applying, and using a servo / pump system, applies the same force to the secondary circuit to activate the brakes. If you do something stupid like trying to slam on the brakes at 100mph, the computer will realise that this would result in a skid or spin, and will not send the full pressure down the secondary circuit, instead deciding to use it’s speed and ABS sensors to determine the optimal brake pressure to maintain control of the vehicle. The advantage of a dual-circuit system is that the command circuit never gets heat transferred into it because it is totally separated from the brakes themselves. The disadvantage of course is that you now have two hydraulic circuits to maintain.
The most advanced system of brakes to date are brake-by-wire. These are a direct copy of some styles of racing brakes and are very similar to the dual-circuit hydraulic system described above, but instead of the command circuit being hydraulic, its replaced with electronics. The brake pedal or lever is connected to a hypersensitive rheostat (measures electrical resistance). The more you push it, the greater the electrical signal sent to the brake computer. From there on, it performs just like the secondary circuit described above. The advantage to this system is that the brake pedal or lever can be placed just about anywhere you like as it no longer is encumbered by the plumbing that goes with a hydraulic circuit. To combat driver complaints of “lack of feel” in the brakes, most brake-by-wire systems have a reverse feedback loop built in. This measures the pressure being applied to the brakes on the secondary circuit, and actuates an electrical resistor in the pedal or lever assembly to provide resistance. This is needed because there is no physical connection to any part of the brake system at all.
Power brakes (also known as power assisted brakes) are designed to use the power of the engine and/or battery to enhance your braking power. Whilst you can generate a fair amount of force using your foot, using systems from elsewhere in the car to help you apply even more force means that you get more powerful brakes as a result. The four most common types of power brakes are: vacuum suspended; air suspended; hydraulic booster, and electrohydraulic booster. Most cars use vacuum suspended units (vacuum boosters). In this type of system, when you press the brake pedal, the push rod to the master cylinder opens a vacuum control valve. This allows vacuum pressure (normally from the intake manifold) to “suck” on a diaphragm inside the vacuum assist unit. This extra vacuum suction helps you to produce more force at the pedal end of the brake system.
Hydraulic booster systems usually utilise pressure from the power steering system to augment pressure on the master brake cylinder.
Electrohydraulic booster systems use an electric motor to pressurize the hydraulic system downwind of the brake pedal which has the effect of amplifying the internal pressure in the whole system.The advantage to this system is that as long as you have battery power, you have power brakes even if the engine fails. With vacuum-assist brakes, no engine means no assistance.
If you’re curious about how power brakes work, go out to your car and with the engine off, step on the brakes. They’ll have a slightly solid, almost wooden feel to them. Turn the engine on and do it again and you’ll notice a lot less back-pressure on the pedal. This is the power assist which is making it easier for you to depress the pedal.
Brake master cylinders are complicated affairs involving finely manufactured parts, minute tolerances, springs, o-rings and rubber seals. The diagram below is a simplified representation of a dual-circuit master brake cylinder. When you step on the brake, its connected to the main plunger (on the right side of this image). As this is pushed into the master cylinder it acts on the components inside. The rear plunger (in blue) is the first one to start moving. As it moves forward, brake fluid from the reservoir is sucked in through the fluid intake and return port. At the same time, fluid is sucked in through the equalisation port. As the second circuit rear seal passes the intake and return port (about 1.5mm after the plunger starts moving), it creates a fixed volume of fluid between the rear and front plungers. The more you step on the brake pedal, the more this fluid is now forced out into the second brake circuit to apply those brakes. At the same time, the pressure building up in this area overcomes the strength of the first circuit return spring and the front plunger (red) begins to move too. As with the rear plunger, it too sucks fluid from the reservoir until the first circuit rear seal passes the fluid intake and return port (again about 1.5mm), trapping fluid between it and the front of the master cylinder. This fluid is then forced out into the first brake circuit, applying those brakes. When you take your foot off the brakes, the return springs push the plungers back into their neutral position. Fluid returns to the brake fluid reservoir and the system goes back to an unpressurised state.
One last thing about brake master cylinders : they cost an absolute bomb to replace. If you find yours is leaking, patching it up is not an option. Brand new master cylinders can go for around $1500 without labour costs. Remanufactured ones come in slightly cheaper at around $900. Bear that in mind when your 20 year old beater develops a leak – it’s probably cheaper to buy another used car than to replace the master cylinder.
In the rendering of the master brake cylinder above, you’ll see there are two plungers and two brake circuits. This is the most common design for cars today. It’s a form of redundancy in the brake system. The idea is that only two brakes, one front and one rear, are on either of the brake circuits. For four brakes, you therefore need two circuits. But why? Well imagine one of your brake lines springs a leak – for the sake of argument, the front-left brake. If all four brakes were on a single circuit, when the master cylinder began to pressurise the brake system, fluid would spurt out of the broken line and pressure would never build up. In turn, that means none of the brakes would ever come on and you’ll sail merrily into the back of the vehicle in front of you. Imagine the same scenario with two circuits. As the first circuit pressurises the front-left and rear-right brakes, fluid spurts out of the broken line and those brakes are never applied. However because the master cylinder is also pressurising a separate second circuit connected to the front-right and rear-left wheels, those brakes do apply and you’ve still got braking force. Sure, it’s reduced, but it’s a hell of a lot better than no brakes at all. Because of the front-left to rear-right and front-right to rear-left linking of the brake circuits, this type of system is known as cross-linked brakes. The rendering below shows an example arrangement of cross-linked brakes.
It’s worth spending a moment here to talk about handbrakes. Or parking brakes, e-brakes or emergency brakes depending on where you come from. Whilst they’re good for doing handbrake turns, they’re not especially effective at actually slowing you down. They will – don’t get me wrong – but you won’t be seeing any stellar performance out of them so the term ‘emergency brake’ is a bit of a misnomer. So why is this? Well, handbrakes are cable-actuated for a start so the amount of power they have is wholly dependent on the amount of tug you have in your arm. There’s no hydraulic system to help you out. Apart from that, they only work on the rear wheels, so you’re not getting four-wheel braking. On drum-brakes, the handbrake is connected to a small lever that pivots against the end of one of the brake actuating pistons. When you pull the handbrake, the lever gets pulled and the brake shoes are pressed out against the inside of the drum. On disc brakes, the handbrake normally works a second set of brake pads in the rear caliper. They’re little spots, about the size of a grown man’s thumbprint and they’re clamped mechanically against the brake rotor. These pads never need changing because they’re normally only used at standstill so generally don’t wear much. Their small size is the other reason you shouldn’t expect stellar stopping performance if you yank on the handbrake. That being said, there are derivatives of disc-based handbrakes that use a mechanical arm to press the main brake pads against the rotor although these are less common as far as I know.
Typically you ought to use your handbrake whenever you’re stopped somewhere, be it parked, on a hill or waiting at traffic lights. The reason is simple : if you’re parked or stopped, you generally don’t want the car to run off without you. At traffic lights, it’s an accident minimisation function as much as anything. If you’re sitting there with your foot on the brake and someone drives into the back of you, the impact will cause you to take your foot off the brake and you’ll go sailing into the car in front, causing more accidents. If you have the handbrake on in the same scenario, your car will largely stay put (apart from the initial shove across the ground as the energy from the impact is dissapated through your tyres). Of course there are personal habits and mechanical complications to contend with here. For example in a car with an automatic gearbox, it’s force of habit to just use the footbrake. Even so, you should still use the handbrake when you’re parked, especially on an incline. The ‘park’ setting on automatic gearboxes isn’t sufficient to hold a car on a hill, and apart from that, it puts incredible strain on the transmission and clutch system if you let the whole weight of the car transfer into the transmission to try to keep it from moving. In some American cars, the handbrake isn’t a handbrake at all, it’s a second footbrake on the far left side of the footwell, which is basically totally useless because it’s a pain to put on and even more of a pain to get off because it’s a one-way ratchet system (you have to force the pedal all the way down to get it to release). Then there’s the ignorance factor. When I went to my new owners orientation evening after buying a Subaru in America, one lady asked what the parking brake was for. (Apparently the name wasn’t obvious enough). The dealer representative told her it was a relic of days gone by, not to be used, and he didn’t understand why manufacturers even put them in cars any more!
The first and most obvious answer to this is : when you’re going at any speed. If you yank on the handbrake at any speed much over 30km/h, the back end of your car will start to slide. Great for stunts and tricks, not so great if you’re trying to stop in 5 lanes of crowded highway traffic.
The other time you should not use your handbrake is in post-snow, freezing conditions. With the salt and grit that gets put down on the roads, you’ll be driving through a salty, snowy slush and it will be spraying all over the underside of your car. If you park and put the handbrake on, you risk it binding on by freezing. Why? Well handbrake cables are almost always exposed to the elements at some point under your car. If you put the handbrake on and the cable is covered in slush, as it freezes again it will lock the handbrake on. There’s no solution to this other than waiting for the weather to warm up. Well, not unless you fancy a crack at the Darwin Awards, because some people have tried using blowtorches to thaw the ice, not understanding that they were working right underneath the petrol tank. So here’s a tip : don’t. If you need to park in those types of conditions, try to find level ground and leave your automatic gearbox in “p” or your manual gearbox either in first or reverse gears.
By 2006, brake-assist and accident warning systems were starting to find their way into consumer cars. Volvo’s collision warning system (CWS), for example, constantly monitors your speed and uses a radar with a 15° forward field of view to determine the distance to any object in front of you. If the distance begins to shrink but you don’t slow down, the system sounds a buzzer and flashes a bright red light in a heads-up display to alert you. The brake pads are automatically placed against the discs and when the driver finally does use the brakes, the system monitors the pedal pressure. If the pressure is determined to be too light, the braking power is amplified by the system. Brake-assist and auto-brakes go one step further. In some high end vehicle now (top end BMWs and Mercedes’ for example), the collision-detection system is linked into the brakes like it is with the Volvo system, but it’s also been given the flexibility to do all the braking for you. Adaptive cruise control, for example, will control the throttle just like a normal cruise control system, but will also apply the brakes if it determines that you’re getting too close to the vehicle in front. Full auto-brakes will actually stop the car for you if you fail to respond. All these systems work in essentially the same way – they monitor the brake use and distance to the vehicle in front. If the computer thinks you’re not braking hard enough, it will assist you. These systems are all very clever but they tread the thin ethical line. Just because engineers can make their vehicles do this doesn’t mean they should. Consider this: with in-vehicle monitoring and tracking systems like OnStar, and the impending satellite-tracking systems for road tolling, it’s not too hard to imagine all those systems chained together in such a way that the vehicle will literally prevent you from speeding by limiting the throttle availability and controlling the brakes. If you really want to be driven like that in a vehicle over which you have no control at all, take the bus.
There are other brake technologies that are becoming available in vehicles now, and a lot of them are gathered together in the 2006 / 2007 BMW models. They’re the rolling embodiment of clever brake engineers just showing off. Three of the more notable features are:
All the above devices fall into that ethical grey area again, but unlike the brake-assist and collision-detection systems outlined earlier, these three brake technologies don’t actually attempt to compensate for any wrongdoing on the driver’s behalf. They simply help prepare the car for when the driver chooses to use the brakes. From that point of view, I would regard these as better technologies than those which go the whole hog and interfere with your driving.
Reference:
www.carbibles.com – It is a truly wonderful site for getting all of the information on basic automotive domains. Do give it a try.
The simple answer : they slow you down.
The complex answer : brakes are designed to slow down your vehicle but probably not by the means that you think. The common misconception is that brakes squeeze against a drum or disc, and the pressure of the squeezing action is what slows you down. This in fact is only part of the reason you slow down. Brakes are essentially a mechanism to change energy types. When you’re travelling at speed, your vehicle has kinetic energy. When you apply the brakes, the pads or shoes that press against the brake drum or rotor convert that energy into thermal energy via friction. The cooling of the brakes dissipates the heat and the vehicle slows down. This is all to do with The First Law of Thermodynamics, sometimes known as the law of conservation of energy. This states that energy cannot be created nor destroyed, it can only be converted from one form to another. In the case of brakes, it is converted from kinetic energy to thermal energy. Angular force. Because of the configuration of the brake pads and rotor in a disc brake, the location of the point of contact where the friction is generated also provides a mechanical moment to resist the turning motion of the rotor.
If you remember any sort of physics classes from school, you might recall something called mechanical advantage. In its most basic form, mechanical advantage is the ratio of force-in to force-out in a mechanical system. Mechanical Advantage = Effort Torque/Load Torque. For example a 20kg weight 1 metre from a pivot can lift a 40kg weight 0.5m from the pivot on the other side. The effort torque and load torque calculations are to do with force in Newtons and distance from pivot point. Hence torque is measured in Newton-metres, or Nm. A Newton is the amount of force required to accelerate a mass of one kilogram by one metre per second². On Earth, where acceleration due to gravity is 9.8m/s², the force exerted upon a mass of 1kg is 9.8N (usually rounded up to 10N). Another popular notation is lbf.ft – pound-force-feet, commonly referred to as foot-pounds. 1 Newton-metre is equivalent to 0.737 foot-pounds. The diagram below shows a simple lever system on a pivot. The load torque is 200Nm, and the effort torque is also 200Nm. Mechanical advantage = effort / load, which in this case is 200 / 200, which is 1. ie. the system is balanced.
Now imagine increasing the weight on the effort side to 30kg instead of 20kg, but leaving everything else the same. The load torque is still 200Nm, but the effort torque is now 300Nm. Mechanical advantage = effort / load, which is 300 / 200, which is 1.5. Any mechanical advantage value larger than 1.0 means that the effort has the advantage. In this case, a 30kg weight which is lighter than the 40kg load, is able to lift it off the ground.
If you now take your new-found / remembered knowledge about physics and look at the simple lever brake system, you’ll realise how it’s possible to generate enough force using your foot to stop a car or motorbike. Look at this diagram of the lever-operated cam brake.
This system has 4 levers in it. The middle two have no mechanical advantage as the levers are connected the same distance from the pivot in each case. However, look at the pedal. The values I’ve put in are arbitrary but they serve the purpose. On the pedal we have some amount of force 20cm from the pivot, but the other end of the lever is only 5cm from the pivot. This gives us a mechanical advantage of 4 on the brake lever (20cm / 5cm). At the other end, the lever attached to the cam is still a lever system – it’s just bent. The input lever is 10cm long but the cam is only 4cm across – or 2cm to the tip from the pivot. So at the brake cam we have a mechanical advantage of 5. (10cm / 2cm). So across this entire system, we have a total mechanical advantage of 20 – 4 from the brake pedal and 5 from the lever and cam. Apply force to this little system and be amazed. The units of force used are irrelevant – they’re multiplied just the same. To use easier-to-comprehend values, let’s imagine that when you’re braking, your foot is pushing on the brake pedal with about 60pounds of force – 27Kg. Through the brake pedal, that is amplified 4 times to 240pounds, and through the lever and cam its amplified a further 5 times from 240pounds to 1200pounds. You pushed the pedal with 60pounds of force, but the cam inside the drum brake is being forced out against the brake drum with 1200pounds of force – about 544Kg. Sweet.
Most braking systems now use hydraulics. This is a slight change in the equation but the concept of mechanical advantage still exists, this time by the use of pressure equations. Pressure = force / area. If you apply 20 Newtons of pressure to 1m², it’s the same as applying 200 Newtons to 10m². Why? Because 20 Newtons of force divided by 1m² of area generates 20 Pascals of pressure. Similarly, 200N / 10m² is also 20Pa.
If you now think of that in terms of a hydraulic braking system, it becomes clear how mechanical advantage works for you. Brake fluid is incompressible – it has to be. This is good because it makes calculation for hydraulic brake systems quite easy – you can eliminate the internal pressure from the equation. Split the system into two parts – input and output – the brake pedal and the brake caliper piston. For each part, Pressure = Force / Area. The Pressure is the same at all points in the system, so some basic algebra gives a simple formula:
Using our previous example, we apply 60pounds (27Kg) of input force to the brake pedal. This is attached to a master piston which (for example) is 1.25cm across – ie. it has a surface area of 0.000491m² (remember your maths? area = PI x r²). At the other end of the system is the caliper piston, which for example is 2cm across – ie. it has a surface area of 0.001257m². Using our sparkly new formula, the output force from the caliper piston is 60 x (0.001257m² / 0.000491m²) Get your calculator out and that comes out to 154pounds (69.8Kg) – more than double the force at the brake pedal. The ratio of output area to input area is sometimes referred to as the area differential.
So that, my friend, is why you can stop a speeding vehicle with a single foot.
If you ride a motorbike or drive a race car, you’re probably familiar with the term brake fade which is used to describe what happens to brakes when they get too hot. A good example is coming down a mountain pass using your brakes rather than your engine to slow you down. By the First Law of Thermodynamics, as you start to come down the pass, the brakes on your vehicle heat up, slowing you down. But if you keep using the brakes, the drums or discs and brake pads will stay hot and get no chance to cool off. The next time you try to brake, because the brake components are already so hot, they cannot absorb much more heat. Once they get to this stage, you have to look at the brake pads themselves. In every brake pad there is the friction material which is held together with some sort of resin. Once this lot starts to get too hot, the resin holding the pad material together starts to vapourise, forming a gas. That gas has to have somewhere to go, because it can’t stay between the pad and the rotor, so if forms a thin layer between the two trying to escape. The result is very similar to hydroplaning while going too fast in the rain; the pads lose contact with the rotor, thus reducing the amount of friction. Voila. Brake fade. The typical symptom of this would be to get the vehicle to a stop and wait for a few minutes. As the brake components cool down, their ability to absorb heat returns, the pads cool off which means they have more chance to heat up again before the resin vapourises, hence the next time you use the brakes, they seem to work just fine. This type of brake fade was more common in older vehicles. Newer vehicles tend to have less outgassing from the brake pad compounds but they still suffer brake fade. So why? Well it is again to do with the pads getting too hot. With newer brake pad compounds where outgassing isn’t so much of a problem, the pads transfer heat into the calipers because the rotors are already too hot and the brake fluid starts to boil as a result. As this happens, bubbles form in the brake fluid. Air is compressible, brake fluid isn’t, so you can put your foot on the brake pedal and get full travel but have no braking effect at the other end. This is because you’re now compressing the gas bubbles and not actually forcing the pads against the rotors. Voila. Brake fade again. So how do the engineers design brakes to reduce or eliminate brake fade? For older vehicles, you give that vapourised gas somewhere to go. For newer vehicles, you find some way to cool the rotors off more effectively. Either way you end up with cross-drilled or grooved brake rotors. While grooving the surface may reduce the specific heat capacity of the rotor, its effect is negligible in the grand scheme of things. The rotors will heat up to cool down no faster or slower. However, under heavy braking once everything is hot and the resin is vapourising, the grooves give the gas somewhere to go, so the pad can continue to contact the rotor, allowing you to stop.
The whole understanding of the conversion of energy is critical in understanding how and why brakes do what they do, and why they are designed like they are. If you’ve ever watched Formula-1 racing, you’ll see the front wheels have huge scoops inside the wheel pointing to the front (see the picture on the right). This is to duct air to the brake rotors to help them cool off because in Formula-1 racing, the brakes are used viciously every few seconds and spend a lot of their time trying to stay hot. Without some form of cooling assistance, the brakes would be fine for the first few corners but then would fade and become near useless by half way around the track.
Rotor technology. If a brake rotor was a single cast chunk of steel, it would have terrible heat dissipation properties and leave nowhere for the vapourised gas to go. Because of this, brake rotors are typically modified with all manner of extra design features to help them cool down as quickly as possible as well as dissapate any gas from between the pads and rotors. The following diagram shows some examples of rotor types with the various modification that can be done to them to help them create more friction, disperse more heat more quickly, and ventilate gas. From left to right. 1. Basic brake rotor. 2. Grooved rotor. The grooves give more bite and thus more friction as they pass between the brake pads They also allow gas to vent from between the pads and the rotor. 3. Grooved, drilled rotor. The drilled holes again give more bite, but also allow air currents (eddies) to blow through the brake disc to assist cooling and ventilating gas. 4. Dual ventilated rotors. Same as before but now with two rotors instead of one, and with vanes in between them to generate a vortex which will cool the rotors even further whilst trying to actually ’suck’ any gas away from the pads. An important note about drilled rotors: Drilled rotors are typically only found (and to be used on) race cars. The drilling weakens the rotors and typically results in microfractures to the rotor. On race cars this isn’t a problem – the brakes are changed after each race or weekend. But on a road car, this can eventually lead to brake rotor failure – not what you want.
Big rotors You know I’ve been drumming into you the whole mechanism that causes you to stop? How does it apply to bigger brake rotors; a common sports car upgrade? Well sports cars and race bikes typically have much bigger discs or rotors than your average family saloon car. The reason again is to do with heat and friction. A bigger rotor has more material in it so it can absorb more heat. More material also means a larger surface area, which as well as meaning more area for the pads to generate friction with, also translates to better heat dissipation. On top of that, the larger rotors mean that the brake pads make contact further away from the axle of rotation. This provides a larger mechanical advantage to resist the turning of the rotor itself. To best illustrate how this works, imagine a spinning steel disc on a pivot in front of you. If you clamped your thumbs either side of the disc close to the middle, your thumbs would heat up very quickly and you’d need to push pretty hard to generate the friction required to slow the disc down. Now imagine doing the same thing but clamping your thumbs together close to the outer rim of the disc. The disc will stop spinning much more quickly and your thumbs won’t get as hot. That, in a nutshell explains the whole principle behind why bigger rotors = better stopping power. Taking it one step further, composite brake rotors, as found on high-end Ferraris, the McLaren F1, and most Formula-1 race cars, are even better again at heat transfer.
All brakes work by friction. Friction causes heat which is part of the kinetic energy conversion process. How they create friction is down to the various designs.
I thought I’d cover these because they’re about the most basic type of functioning brake that you can see, watch working, and understand. The construction is very simple and out-in-the-open. A pair of rubber blocks are attached to a pair of calipers which are pivoted on the frame. When you pull the brake cable, the pads are pressed against the side or inner edge of the bicycle wheel rim. The rubber creates friction, which creates heat, which is the transfer of kinetic energy that slows you down. There’s only really two types of bicycle brake – those on which each brake shoe shares the same pivot point, and those with two pivot points.
The next, more complicated type of brake is a drum brake. The concept here is simple. Two semicircular brake shoes sit inside a spinning drum which is attached to the wheel. When you apply the brakes, the shoes are expanded outwards to press against the inside of the drum. This creates friction, which creates heat, which transfers kinetic energy, which slows you down. The example below shows a simple model. The actuator in this case is the blue elliptical object. As that is twisted, it forces against the brake shoes and in turn forces them to expand outwards. The return spring is what pulls the shoes back away from the surface of the brake drum when the brakes are released. See the later section for more information on actuator types.
The “single leading edge” refers to the number of parts of the brake shoe which actually contact the spinning drum. Because the brake shoe pivots at one end, simple geometry means that the entire brake pad cannot contact the brake drum. The leading edge is the term given to the part of the brake pad which does contact the drum, and in the case of a single leading edge system, it’s the part of the pad closest to the actuator. The diagram below shows what happens as the brakes are applied. The shoes are pressed outwards and the part of the brake pad which first contacts the drum is the leading edge. The action of the drum spinning actually helps to draw the brake pad outwards because of friction, which causes the brakes to “bite”. The trailing edge of the brake shoe makes virtually no contact with the drum at all. This simple geometry explains why it’s really difficult to stop a vehicle rolling backwards if it’s equipped only with single leading edge drum brakes. As the drum spins backwards, the leading edge of the shoe becomes the trailing edge and thus doesn’t bite.
The drawbacks of the single leading edge style of drum brake can be eliminated by adding a second return spring and turning the pivot point into a second actuator. Now when the brakes are applied, the shoes are pressed outwards at two points. So each brake pad now has one leading and one trailing edge. Because there are two brake shoes, there are two brake pads, which means there are two leading edges. Hence the name double leading edge.
Some background: Disc brakes were invented in 1902 and patented by Birmingham car maker Frederick William Lanchester. His original design had two discs which pressed against each other to generate friction and slow his car down. It wasn’t until 1949 that disc brakes appeared on a production car though. The obscure American car builder Crosley made a vehicle called the Hotshot which used the more familiar brake rotor and calipers that we all know and love today. His original design was a bit crap though – the brakes lasted less than a year each. Finally in 1954 Citroën launched the way-ahead-of-its-time DS which had the first modern incarnation of disc brakes along with other nifty stuff like self-levelling suspension, semi-automatic gearbox, active headlights and composite body panels. (all things which were re-introduced as “new” by car makers in the 90’s).
Disc brakes are an order of magnitude better at stopping vehicles than drum brakes, which is why you’ll find disc brakes on the front of almost every car and motorbike built today. Sportier vehicles with higher speeds need better brakes to slow them down, so you’ll likely see disc brakes on the rear of those too. Disc brakes are again a two-part system. Instead of the drum, you have a disc or rotor, and instead of the brake shoes, you now have brake caliper assemblies. The caliper assemblies contain one or more hydraulic pistons which push against the back of the brake pads, clamping them together around the spinning rotor. The harder they clamp together, the more friction is generated, which means more heat, which means more kinetic energy transfer, which slows you down. You get the idea by now.
Standard disc brakes have one or two cylinders in them – also know as one or two-pot calipers. Where more force is required, three, or more cylinders can be used. Sports bikes have 4- or 6-pot calipers arranged in pairs. The disadvantage of disc brakes is that they are extremely intolerant of faulty workmanship or bad machining. If you have a regular car disc rotor which is off by so much as 0.07mm (3/1000 inch) it will be Hell when you step on the brakes. That ever-so-slight warp or misalignment is going to spin through the clamped calipers at some ungodly speed and the resulting vibration will make you wonder if you’re driving down stairs. So you can imagine the kind of tolerances required for these components.
There is a quiet but major revolution happening in the world of brakes, and its being brought about by a Canadian company called NewTech. Rather than the piecemeal improvements we’ve seen over the last few years, with slight design changes, and materials improvements, the new system is a radical redesign from the ground up. NewTech have designed a disc brake system called “full contact disc brakes”. They looked at traditional pad and rotor design and figured that the pads only contact about 15% of the rotor surface at any one time. With a change of design, NewTech have been able to add 5 more pads to the system so that 75% of the brake rotor is in contact with the pads at any one time. With traditional pads and rotors, the brake rotor is clamped between the pad. With the NewTech design, the brake rotor itself becomes a floating rotor, similar to those found on motorbikes. It is covered with a ’spider’ (the red structure in my renderings below) and the spider has 6 brake pads on the inside of it. The hydraulic system acts on fully circular elastomer composite diaphragm behind the brake disc, mounted in the black structure in the renderings. This had 6 pads on it which push the entire disc out against the 6 pads inside the spider. This provides and even force across the entire disc to push it out, and the disc gets an even contact with all 12 pads. To ensure the brakes remain cool, the system is covered in cooling fins connected to the outer pads to dissipate heat. The inner pads are fitted with a moulded thermal barrier made of a composite material. Special inserts made of a variety of frictional materials are distributed evenly on the entire surface of the pad. The range of materials is used to ensure performance under diverse conditions. NewTech believe that the system has considerable advantages over conventional brakes with better cooling, higher strength and reduced noise and vibration. NewTech have sold truck and bus versions of these brakes into the haulage and public transport industry, but now Renault is considering introducing this system on its cars in conjunction with a new brake-by-wire system. Newtech’s first OEM customer was to be Saleen who were going to put the system on their S7 supercar, but in the end went with conventional six-piston monoblock calipers instead. NewTech’s website can be found here.
It’s worth nothing that this isn’t actually the first time this has been tried in cars. Bugattiexperimented with a system like this in the late 80’s for inclusion on their 1991 EB110 supercar; it was going to be available as an option for the car. People who had experienced the brakes said they were just otherworldy, that the braking power was way beyond capabilities of the average driver. They came from Aerospatiale, the French aerospace company, who also designed the chassis for the EB110 (this type of brake was being used in aircraft at the time). Bugatti dropped the idea because the brakes would have cost more than the rest of the EB110, which at $350,000 was by no means a cheap car.
Siemens VDO in Germany are trying to bring a prototype electric wedge brake (EWB) to the market. The EWB is an innovative idea based on technology developed by a company called eStop. Siemens acquired eStop early in 2005 and have been continuing their work on the wedge system ever since. The principle is both simple and clever. The brake pad is pressed against the brake rotor by means of a wedge-shaped thrust plate. The more the brake rotor turns, the harder the slope of the wedge forces the pads against it. Because of the shape of the wedge bearings and thrust plate and the rotation of the brake rotor, the pad is actually forced against the rotor harder the faster the rotor is spinning. In effect, a lot of braking force for very little input. The system runs off a normal 12v vehicle electrical system which means no more hydraulics. It also allows the system to eliminate all the plumbing associated with ABS as the EWB is entirely electronically controlled. The final advantage, if you could call it that, is that it allows the first true all-electronic brake-by-wire system. Current brake-by-wire systems use electronics behind the brake pedal to send signals to actuators in the hydraulic system. With the EWB there is no hydraulic system so the only link from the brake pedal to the brake caliper is a 12v electrical feed and signal actuation wire. The operation of the wedge system is based on several roller bearings and a wedge-shaped thrust plate connected to a pair of 12v electric motors. As the brake pedal is depressed, the signal is sent to the motors to start moving the thrust plate. Because of its shape and the design of the roller bearings, as the thrust plate moves, it forces the brake pad to press against the brake rotor. The reaction time of the electric motors can be measured in milliseconds – far quicker than any hydraulic system could react, so in theory, when connected to a full computer-monitored brake-by-wire system, the EWB ought to be able to shave milliseconds off brake reaction time. Doesn’t sound like much but if it means a few less metres in stopping distance, that can only be a good thing. The brake caliper unit itself has an intelligent wheel-braking module built into it. As well as the motors, bearings and wedges, the module also has a sensor system for monitoring movement and force – basically this is what replaces the traditional ABS items so each brake caliper becomes a self-governing ABS unit. Because there’s no physical link back to the brake pedal any more, the ABS doesn’t force the brake pedal to judder when it activates which will make it far more acceptable for a lot more drivers. Finally, because the system is totally electronic, the traditional cable-pulled handbrake can also be eliminated and replaced with a parking switch that simply activates all four EWB modules. Of course there are pros and cons to any new system like this. Obviously reducing the weight and complexity of the braking system is a good thing, and because of the design of the EWB, there’s a lot less space taken up in the engine bay, freeing up more room for the car designers to work with. But by removing the hydraulic lines, ABS actuators and sensors, and master and slave brake cylinders, the EWB concept becomes entirely reliant on the 12v electrical system and the vagaries of a computer. Knowing how often a single dodgy earth connections in a car can totally screw up the electrics, I’ve got to wonder what would happen if a grounding strap came loose and the electronic brake system started playing up. Will these brakes have a fail-safe or backup system like the double hydraulic circuits we use now, or will you sail off into some solid object because you’ve got no brakes left? Siemens aren’t clear on this matter. If you want to see a video demonstrating the EWB, Siemens VDO have one available here(27.8Mb mpeg).
Just a quick word on brake pad compounds. Most pads used to use asbestos but we all know what that stuff is like. Today they use all manner of combinations of materials. The pads themselves are made up of a friction material bonded to the backing plate. The brake caliper piston pushes against the backing plate and the friction material is pushed against the brake rotor. The material combinations typically fall into the following broad categories now.
Organic
These pads are well-suited for street driving because they wear well, are easy on the ears, don’t chew up the rotors and don’t spew dust everywhere. They’re favoured for your average family saloon because they work well when they’re cold. Of course the drawback is that they don’t work so well when they get hot.
Semi-metallic / sintered
This is a good compromise between street and track. These seem to be the pad of choice for sportier vehicles such as the Subaru Impreza WRX. They won’t work as well as organic pads when they are cold, so you need to be a bit wary of the first couple of stops. Conversely they do work well when hot. Occasionally the weak link in semi-metallic pads is the bonding material that holds the friction pad to the backing plate. There have been occasions where the friction material has come away completely. That’s infrequent though.
Metallic
These pads are typically reserved for racing or the extremely rich. They squeal and dust like crazy, are hard on rotors and don’t work well when cold.
Ceramic
Ceramic pads still have metal fibers (about 15% vs. about 40% for semi-metallic) but they are copper instead of steel and therefore cause less wear and transfer heat better. They don’t fade as easily as other pads, cool faster, last longer, and are effectively silent, as the sound they genereate is outside of the human range of hearing. Dogs will go crazy thought. The dust created by ceramic pads is also very light in color so your wheels look cleaner.
Here is my opinion on the current scenario of Car Crash Safety. Do post your views on this topic in the comments section. All kinds of feedback are welcome..:)
If carmakers are to be believed nowadays, you can take your modern car to 60km/hr, have a booze at the wheel, keep chatting with your passenger without bothering about the road, hit a big old tree, AND walk out like nothing happens. But it turns out that they are, erm.. true. (On the safety part obviously. You can obviously forget crashing your next car as your license will be stripped away)
I recently heard an interesting story. A guy just bought a new Fiat Linea, and drives out of the showroom with his family. And hits a lorry straight away. His bad luck worsens when that lorry happens to carry granite slabs. Not too happy with the crash, the lorry dumps some of its cargo on top of the car. But guess what, inspite of those massive granite slabs, none of the passengers have even a scratch to report. The car was totalled, but the people saved. He was so happy with the car that he bought another one right away!
A happy ending, then. The world is now a safer place to live, cars were never this good, end of the story. Not quite. You see, all of this safety bishbosh has come at a pretty big cost.
You may know the Hyundai Getz. It was introduced as a modern ‘European’ car, with all the modern safety features, a light but strong body, a highly efficient engine, blah blah blah. You will also know of the HM Ambassador. The longest selling car in history (over 50 years), its called the grandpa’ car, a massive behemoth of steel that’s widely considered to be the most outdated car sold anywhere.
Now comes the funny part. The petrol Ambassador gets 12 km per litre overall. Do you know how much the Getz gets? A humongous 11 km/l. That’s right. A small modern ‘hatchback’ gets lower fuel economy than an antiquated Grandpa car. Want to know why? Its this thing called weight. The two cars in question weigh exactly the same! 1050 kg. The difference in fuel consumption is because of the engine/gearbox matching in the Getz, which is not tuned properly to run in our ’stop-and-go’ traffic conditions.
You would have guessed where this is going. But hold on, before you lash out for not caring about human lives, let me tell you this. I’m not against those electronic safety curtains like ABS, traction control and those things, they add just about 50 kilos. Acceptable. I’m not even against those front & side crash tests. Their weight addition is perfectly ok since they actually save lives on a consistent basis.
Its those safety authorities. The ones called EuroNCAP, especially. They started out rating cars in terms of their safety in 1997, and since they were EURO-backed, customers started taking their safety ratings seriously. A great thing, since it spurred manufacturers to come up with all the electronic gizmodos and improve crash safety spectacularly. The NCAP test gives up to 5 stars, and within 4 years, almost every car sold in Europe, and the equivalent ones in US, got full marks. So almost all cars sold in the western nations were perfectly safe.
What would a teacher do if all her students got good marks? Pat them in the back and congratulate them, right? Not in this case. Since manufacturers were fighting for visibility and the NCAP guys wanted to be in business for a longer period, the tests were made a whole lot tougher. Cars which got 5 stars were now getting only 2. New tests took care of cases where cars were crashed from their side, hit from the rear (common in Europe), when they rolled over(!), and when they hit pedestrians in the front. They may even think of cases where they hit pedestrians in the side and back, of the car, effectively turning it into a jelly-shaped blob. But enough of that now.
The main problem is that adding all those safety features also added tons of weight. If a new car, say, a Honda Jazz were to be entirely made of steel, it would weigh nearly 50% more. Extra layers of metal were added to the front, the doors, the roof and boot to meet these safety norms, and they had to compensated with other costly materials. For example, the Hyundai i10 has some Titanium coated parts for its engine. As you’ll know, Titanium is more expensive than gold! This leaves us with cars that are heavier, more expensive, and normally less fuel-efficient than before. New engines can improve mileage, but they are simply a lot more expensive.
There are a few more issues. Each country (and state sometimes) in the world has different safety requirements, and they are all almost mandatory now. Which is fine if people specifically made cars for them. But our carmakers are too lazy to do that, focussing on global cars instead. So, the Suzuki SX4 sold here also takes care of Icelandic slalom tests, while handling 100 km/hr crash tests in Europe. And it is sold here where we hardly take it over 60. Great going. And EuroNCAP is going for another set of revisions, since all cars are getting 5 stars again.
Here are a few facts:
Am I saying cars should be made like Ambassadors again? Or that crash testing is useless? No way! Its just that making every car meet every kind of scenario (with a probability of 1 in a million) is just too much. Our cars are safe enough now. We’ll have them the way they are, thank you very much. No need to add any more weight in the name of crash safety. We can have safer roads instead. Easier on our pockets too..
For years, the trusty seat belt provided the sole form of passive restraint in our cars. There were debates about their safety, especially relating to children, but over time, much of the country adopted mandatory seat-belt laws. Statistics have shown that the use of seat belts has saved thousands of lives that might have been lost in collisions.
Air bags have been under development for many years. The attraction of a soft pillow to land against in a crash must be very strong — the first patent on an inflatable crash-landing device for airplanes was filed during World War II! In the 1980s, the first commercial air bags appeared in automobiles.
Since model year 1998, all new cars have been required to have air bags on both driver and passenger sides. (Light trucks came under the rule in 1999.) To date, statistics show that air bags reduce the risk of dying in a direct frontal crash by about 30 percent. Newer than steering-wheel-mounted or dashboard-mounted bags, but not so widely used, are seat-mounted and door-mounted side air bags. Some experts say that within the next few years, our cars will go from having dual air bags to having six or even eight air bags. Having evoked some of the same controversy that surrounded seat-belt use in its early years, air bags are the subject of serious government and industry research and tests.
In this article, you’ll learn about the science behind the air bag, how the device works, what its problems are and where the technology goes from here.
Before looking at specifics, let’s review our knowledge of the laws of motion. First, we know that moving objects have momentum (the product of the mass and the velocity of an object). Unless an outside force acts on an object, the object will continue to move at its present speed and direction. Cars consist of several objects, including the vehicle itself, loose objects in the car and, of course, passengers. If these objects are not restrained, they will continue moving at whatever speed the car is traveling at, even if the car is stopped by a collision.
Stopping an object’s momentum requires force acting over a period of time. When a car crashes, the force required to stop an object is very great because the car’s momentum has changed instantly while the passengers’ has not — there is not much time to work with. The goal of any supplemental restraint system is to help stop the passenger while doing as little damage to him or her as possible.
What an air bag wants to do is to slow the passenger’s speed to zero with little or no damage. The constraints that it has to work within are huge. The air bag has the space between the passenger and the steering wheel or dash board and a fraction of a second to work with. Even that tiny amount of space and time is valuable, however, if the system can slow the passenger evenly rather than forcing an abrupt halt to his or her motion.
There are three parts to an air bag that help to accomplish this feat:
The inflation system is not unlike a solid rocket booster.The air bag system ignites a solid propellant, which burns extremely rapidly to create a large volume of gas to inflate the bag. The bag then literally bursts from its storage site at up to 200 mph (322 kph) — faster than the blink of an eye! A second later, the gas quickly dissipates through tiny holes in the bag, thus deflating the bag so you can move.
Even though the whole process happens in only one-twenty-fifth of a second, the additional time is enough to help prevent serious injury. The powdery substance released from the air bag, by the way, is regular cornstarch or talcum powder, which is used by the air bag manufacturers to keep the bags pliable and lubricated while they’re in storage
The idea of using a rapidly inflating cushion to prevent crash injuries had a long history before the U.S. Department of Transportation called for the equipment to be adapted for automobiles in the 1980s. The first patent on an inflatable crash-landing device for airplanes was filed during World War II.
Early efforts to adapt the air bag for use in cars bumped up against prohibitive prices and technical hurdles involving the storage and release of compressed gas. Researchers wondered:
They needed a way to set off a chemical reaction that would produce the nitrogen that would inflate the bag. Small solid-propellant inflators came to the rescue in the 1970s.
In the early days of auto air bags, experts cautioned that the new device was to be used in tandem with the seat belt. Seat belts were still completely necessary because air bags worked only in front-end collisions occurring at more than 10 mph (6 kph). Only seat belts could help in side swipes and crashes (although side-mounted air bags are becoming more common now), rear-end collisions and secondary impacts. Even as the technology advances, air bags still are only effective when used with a lap/shoulder seat belt!
It didn’t take long to learn that the force of an air bag can hurt those who are too close to it. Researchers have determined that the risk zone for driver air bags is the first 2 to 3 inches (5 to 8 cm) of inflation. So, placing yourself 10 inches (25 cm) from your driver air bag gives you a clear margin of safety. Measure this distance from the center of the steering wheel to your breastbone. If you currently sit less than 10 inches away, you can adjust your driving position in the following ways:
The rules are different for children. An air bag can seriously injure or even kill an unbuckled child who is sitting too close it or is thrown toward the dash during emergency braking.Experts agree that the following safety points are important:
