A question about bigger brakes

[djones]

Forgetting about brake fade for a second, how do bigger, more powerful brakes stop a car faster?

The way I'm looking at it is if you have the braking power to lock the wheels, there is no way you can brake any better?

Surely the point just before they lock is the best braking force you can ever get?

I don't understand it when people go on about upgrading the brakes on their car and how it's made a big difference in stopping distances.

[zac510]

Yes you are quite right, the maximum braking effort is dictated by the tyre grip too, but bigger brakes offer other very worthy advantages too like more resistance to fade (caused by overheating), longer pad/disc life, stronger initial bite at high speeds.

Any driver would appreciate the consistency and reliability offered by the biggest brakes he or she can get!

[kilcoo316]

Yeah, zac is right - but its also about how you apply the brakes that will change your maximum braking potential.


You stand on them, and the rear brakes will lock up as the weight is transferred forward very quickly.

You ease onto them a little (control the longitudinal weight transfer) and you can get much more overall braking force transmitted to the road. With a good technique, you will get much more from the brakes with little chance of locking up in the middle 3rd of the braking 'manouvre' - its here that bigger brakes come in handy.


Bigger brakes are always a better thing

[flynfrog]

Yeah, zac is right - but its also about how you apply the brakes that will change your maximum braking potential.


You stand on them, and the rear brakes will lock up as the weight is transferred forward very quickly.

You ease onto them a little (control the longitudinal weight transfer) and you can get much more overall braking force transmitted to the road. With a good technique, you will get much more from the brakes with little chance of locking up in the middle 3rd of the braking 'manouvre' - its here that bigger brakes come in handy.


Bigger brakes are always a better thing

not always you can reach a point where the added mass of the disk will acutly hurt performance

with a bigger break you can hold the tire on the edge of traction (its most effective point) longer thus increasing you stopping distance

in my experice this is all a moot point without the right tire

[NickT]

Bigger disks always help because they increase the mechanical leaverage being applied. Just like using a longer lever to help lift a mass. Its one a lot of aftermarket tuners use and a cheap and effective upgrade on mountain bikes to.

[RH1300S]

Bigger brakes give a mechanical advantage as already stated, so they will be able to work more effectively from higher speeds (in your example of locking the wheel, we will assume that the smaller brakes don't lock the wheel instantly from any speed ). I.e. you will get more stopping power sooner and for the same pedal effort.

Bigger brakes will tend to have more rotating inertia, so actually take more effort to stop the brakes turning before they stop the wheel - you don't get this for free

BUT, what you can do with multi piece discs (like a motorbike - where the inner is forged alloy and the disc itself is a ring). Keep the inner light and make the outer part as narrow (radially) as possible.

The effective lever is through the centre of the pads, so multi caliper brakes (e.g. 6 pot calipers) can have a narrower pad (radially) but keep the same surface area, which can move the lever point away from the centre without making the outside diameter of the disc bigger. So you get the mechanical advantage without adding to the rotating mass. Obviously the decision to use 2/4/6/8pots is one of getting the right pad/disc surface area for the vehicle being stopped.

Motorbikes seem to use this far more than cars, quite possibly because they have much more to gain in keeping everything light at the front (sprung/unsprung mass ratios are poor compared to a car - also gyroscopic forces make it hard to turn a bike, so ANY reduction in rotating mass is immediately felt by the rider).

I am pretty sure that in this there are heat advantages too in that the discs/pads will run cooler.

[flynfrog]

Bigger disks always help because they increase the mechanical leaverage being applied. Just like using a longer lever to help lift a mass. Its one a lot of aftermarket tuners use and a cheap and effective upgrade on mountain bikes to.

so if i put a 4 foot brake on my biek it will stop faster because it is bigger

as RH1300S said you dont get it for free

personaly id like to see a multi disk (like a clutch) brake you could have a 4diameter brake that is way more effective at stoping the car might be a challenge to get a good feel out of it though

[RH1300S]

Bigger disks always help because they increase the mechanical leaverage being applied. Just like using a longer lever to help lift a mass. Its one a lot of aftermarket tuners use and a cheap and effective upgrade on mountain bikes to.

so if i put a 4 foot brake on my biek it will stop faster because it is bigger

as RH1300S said you dont get it for free

personaly id like to see a multi disk (like a clutch) brake you could have a 4diameter brake that is way more effective at stoping the car might be a challenge to get a good feel out of it though

At some point in the mid 1980's some F1 cars (I think Williams with the FW11??) had twin disc setups - these were discarded because of issues with stiffness and effective disc cooling. Ideally a disc is cooled evenly on each side to avoid distortion. Also, caliper stiffness is massively important to good brake feel.

As far as disc cooling goes, a solid disc will have air directed to each face; ventilated discs should have air sent to the middle, from there it is "pumped" by wheel rotation through the vanes/webs running between the disc faces. Which is why F1 ducts aim the air at the centre of the brake

The clutch idea is interesting, I'm still trying to get my head around it - but back OT - My luddite, gut instinct is that it won't be as good; still an interesting line of thinking .

In most normal car/bike situations a larger diameter disc will benefit stopping power. There is a point of diminishing returns, but this is to some extent physically limited by wheel rim diameter anyway. So, for most practical arguments the biggest diameter disc that you can cram into your wheel the better as you can always deal with the rotating mass issues with multi piece brakes and multi pot calipers.

The "pumped" description leads me a little off topic (sorry ) - I wonder how much of the cooling air in an F1 brake duct is "pushed" in by air-speed or whether it is "pulled" in. I know that in cast iron car discs the vanes are shaped and can act as a pump, but in the F1 carbon discs I'm not sure this is the case.

[flynfrog]

Bigger disks always help because they increase the mechanical leaverage being applied. Just like using a longer lever to help lift a mass. Its one a lot of aftermarket tuners use and a cheap and effective upgrade on mountain bikes to.

so if i put a 4 foot brake on my biek it will stop faster because it is bigger

as RH1300S said you dont get it for free

personaly id like to see a multi disk (like a clutch) brake you could have a 4diameter brake that is way more effective at stoping the car might be a challenge to get a good feel out of it though

At some point in the mid 1980's some F1 cars (I think Williams with the FW11??) had twin disc setups - these were discarded because of issues with stiffness and effective disc cooling. Ideally a disc is cooled evenly on each side to avoid distortion. Also, caliper stiffness is massively important to good brake feel.

As far as disc cooling goes, a solid disc will have air directed to each face; ventilated discs should have air sent to the middle, from there it is "pumped" by wheel rotation through the vanes/webs running between the disc faces. Which is why F1 ducts aim the air at the centre of the brake

The clutch idea is interesting, I'm still trying to get my head around it - but back OT - My luddite, gut instinct is that it won't be as good; still an interesting line of thinking .

In most normal car/bike situations a larger diameter disc will benefit stopping power. There is a point of diminishing returns, but this is to some extent physically limited by wheel rim diameter anyway. So, for most practical arguments the biggest diameter disc that you can cram into your wheel the better as you can always deal with the rotating mass issues with multi piece brakes and multi pot calipers.

The "pumped" description leads me a little off topic (sorry ) - I wonder how much of the cooling air in an F1 brake duct is "pushed" in by air-speed or whether it is "pulled" in. I know that in cast iron car discs the vanes are shaped and can act as a pump, but in the F1 carbon discs I'm not sure this is the case.

i belive this is what some air planes are using not completly sure on that fact in a motor cycle engine you have 4-8 clutch disks that are clamped by a spring when you grab the clutch to pulls the disks away from the fibers

im at work so i dont have time to find a pick

try howstuffworks.com

[RH1300S]

Most motorbike use a multi-plate clutch. For reasons I don't understand it doesn't seem popular with cars. The clutch plates are held in a basket, some are attached the the engine output, others are attached to the gearbox input (alternating). Activating the clutch separates the plates and releasing it lets the springs hold the plates together. Don't worry, I know exactly what you mean - I just haven't quite worked out the pros/cons relating to brakes.

An obvious "con" is one of mechanical complexity. At the moment, the disc brake is a pretty simple system and it can be made to work very well indeed within the confines of the packaging space available, and it's fairly easy to get rid of the heat. Using multi discs would I presume make the packaging denser (otherwise it just gets bigger for no benefit), which might make heat a much bigger issue.

[kilcoo316]

multi-plate brake?


Its unpopular as it would use a smaller radius disk. Obviously that imparts a smaller braking torque to the wheel than a pad operating on a larger radius disk.

It'd give less braking power for more unsprung weight and more brake wear.



For motorbikes however, a smaller radius disk may more important for gyroscopic effects, which could explain why its more popular on 2 wheels rather than 4, where the wheels usually remain in the same plane.



Aircraft probably need it as they have a huge weight on the wheels and could use massive amounts of braking power without locking up wheels. One disk simply might not provide enough torque, so they use multiple. [I have to admit, I've never heard of them being used, but the brakes are not something I pay too much attention to on an aircraft - thrust reversers are more up my street]

[DaveKillens]

AS you pointed out, RH1300S, heat can be an issue. repeated use of the brakes can make them very hot, and lead to fade, if not outright failure. The transfer of heat to the brake fluid can make it boil, and parts warp and distort. In Formula One, the brakes are so esential, used so frequently an dhard, they are of very special construction. In fact, they suck when cold, and aren't truly effective until up to operating temperature.
A multi plate brake is possible, but being able to carry away the heat after repeated use is a problem. In aircraft, they have multi plate brakes, but they are used only once, on landing. Then they have a lot of time to cool down before the next need to use them again.
Having higher capacity and larger brakes not only gives headroom in heating and fading, but allows better controllability, it's easier to find that fine line where you get maximum grip before tires start skidding on the road surface. Instead of having the brake pedal mashed to it's maximum, you can have it depresed a lot less in distance and force, and thus be able to control it better.

[zac510]

Big planes do have brakes but they try not to use them upon landing due to massive heat, wear, etc etc.

[flynfrog]

AS you pointed out, RH1300S, heat can be an issue. repeated use of the brakes can make them very hot, and lead to fade, if not outright failure. The transfer of heat to the brake fluid can make it boil, and parts warp and distort. In Formula One, the brakes are so esential, used so frequently an dhard, they are of very special construction. In fact, they suck when cold, and aren't truly effective until up to operating temperature.
A multi plate brake is possible, but being able to carry away the heat after repeated use is a problem. In aircraft, they have multi plate brakes, but they are used only once, on landing. Then they have a lot of time to cool down before the next need to use them again.
Having higher capacity and larger brakes not only gives headroom in heating and fading, but allows better controllability, it's easier to find that fine line where you get maximum grip before tires start skidding on the road surface. Instead of having the brake pedal mashed to it's maximum, you can have it depresed a lot less in distance and force, and thus be able to control it better.

the cooling problem makes sense as most clutches are in oil and not used as much as the brakes



bike clutch so you can get an idea of what im talking about

if you were to reverse the springs to they pull them apart and have a push in action when the brakes are applied it oculd possible provide much more stopping power than a larger brake because it has alot more surface area and most of th weight it around the center of the wheel not on the out side of it

[RH1300S]

Lets flip this on it's head.......

A brake system has to get rid of far more energy than a clutch and more often..

Why not try a disc brake type setup as a clutch.............

Ok..obvious problem is that both parts of a clutch need to be able spin...........but hey