Brake caliper position?

[GuitarTech]

I've been driving cars since 1971, but today something occured to me that I've never thought about before: On cars with disc brakes, sometimes the caliper is at the front of the disc (looking from the side), and sometimes at the back. And on cars with discs at the front and rear some of them have the calipers both at the front or both at the rear, and some have front calipers at the front, and rear calipers at the rear, or indeed the other way round. The question is: Why? What are the pros and cons of all the different configurations? I'm sure some suspension expert here will be able to tell me

[GuitarTech]

It took 10 minutes for my post to slip onto page 2, what's up here? Doesn't anybody know? No trackday aces here today?
If I'd started yet another "what car" thread, people would be tripping over themselves to tell me what I should buy and why, but an interesting tech question? No takers....

[HustleRussell]

Basically it doesn't matter. The caliper is designed to work in a certain orientation and that orientation is determined by the front suspension arrangement i.e. where it'll 'fit' after the steering arms, lower arms, upper arms, track rods, anti roll bar droplinks etc, are fitted.

ETA: That's my understanding / best guess anyway. Certainly braking performance for a given clamping force, radius and friction material will be the same regardless of what position the caliper is fitted.

Oh and hanbrake mechanism. That too.

[trickywoo]

Brakes are most efficient mounted at 12, 3, 6 or 9 O'clock.

Which one is used is most often due to 'packaging' i.e. where its easiest to mount the calipers and run the brake hoses.

Nissan 350Z are a notable exception and they have calipers at 10:45

[HustleRussell]

trickywoo said:

Brakes are most efficient mounted at 12, 3, 6 or 9 O'clock.

For heat dissipation reasons?

[8bit]

I was told at the "front" was better as it was "more aggressive" and that position means that better airflow to the caliper can be provided for cooling. Not sure how that position can be more aggressive, I'm assuming it means most braking force, unless that's a by-product of better cooling, but I'd have thought that would reduce fade rather than increase bite.

[GuitarTech]

thanks all for the answers so far. So it has no effect on suspension dive under braking? I'm thinking about the effect of newtons second law here? Or is it irrelevant?

[CraigyMc]

GuitarTech said:

thanks all for the answers so far. So it has no effect on suspension dive under braking? I'm thinking about the effect of newtons second law here? Or is it irrelevant?

I've wondered this in the past too. I don't know the answer, just thought you'd like to know you're not alone.

C

[8bit]

GuitarTech said:

thanks all for the answers so far. So it has no effect on suspension dive under braking? I'm thinking about the effect of newtons second law here? Or is it irrelevant?

Just checked with my more mechanically-learned friend, he's confirmed that - if the front calipers are at the rear of the disc then there will be less dive under braking than if they were at the front.

He's also said that normally FWD cars have the front calipers at the front, to do with packaging (clearing drive shafts etc. I guess) and cooling. RWD cars tend to place front calipers at the rear of the disc. I've got one FWD car and one RWD car and they follow this convention.

[john2443]

Whether they are at 9 or 3 o clock depends which side of the car you look at .

I can't see that they are more efficient in one position or another, it's the friction and distance from the axle that matter, and that's the same wherever they are round a circle.

There might be better airflow to the disc if the caliper is at the back, but if it made a lot of difference you'd think everyone would mount them at the back and they don't so it probably isn't.

[GuitarTech]

Calling all brake and suspension gurus! Where are they all? I'd like to know though: My VW has them both at the front, and in front of my workshop an Audi and a BMW are parked, and they both have front and rear at the rear. And I saw the Mclaren has featured in one thread has rear-mounted at the front, and front-mounted at the rear. Till someone corrects me, I'm going with the squat/dive characteristics theory.

[Superhoop]

To really bake your noodle - Some supercars have rear brakes with two calipers per disc, one mounted at 3 o'clock and the other at 9 o'clock!!

And yes I do understand why!!

[LaurasOtherHalf]

Squat & dive does come into it but it's more of a concern for race cars. For road applications, ease of packaging & bleeding is of more importance with the emphasis on packaging.

Basically, as long as they can be bled correctly, whichever position doesn't necessitate a new calliper/hub design from one already tooled up for will get preference. All things equal you start looking at which will use less ancillaries such as brake lines, sensor cables etc.

[RenesisEvo]

8bit said:

Just checked with my more mechanically-learned friend, he's confirmed that - if the front calipers are at the rear of the disc then there will be less dive under braking than if they were at the front.

AIUI, anti-dive (and squat) is determined by the suspension geometry itself (for double wishbones, it's the amount of convergence of the wishbone inboard rotation axes), I'd be very surprised if the caliper position makes any difference, although the braking effect itself probably does. I'd be interested to hear your friend's reasoning.

There are five factors: 1) packing around the desired suspension geometry 2) packaging for enough cooling flow 3) being able to easily bleed the brakes 4) centre of gravity (6 o clock is best) 5) inertia (towards the rear at the front, front on the rear). For a racing car, the position is largely dictated by a compromise of 1) and 2), with some consideration for 3), 4) resolved by not placing it at the top, and 5) largely ignored. For a road car, 2) is much less of an issue, so it is a matter of arriving at the cheapest upright/hub design whilst working around the suspension.

[Mike600F]

This is something I thought about a lot when designing a suspension system for a formula student car.

The position has no effect on braking force. None. Friction is friction. It also doesn't affect suspension dive etc. That's all based on geometry.

What it does affect is the centre of gravity (the lower the better) and the polar moment of inertia (the lower the better).

For low Center of gravity it's best to mount them at the bottom (I'm sure we're all aware of the benefit a low Center of gravity gives?).
For low moment of inertia, in the "middle" (back on the front wheels and front on the rear wheels) is best.
For packaging reasons, the optimal solution is normally to reduce the moment of inertia rather the Center of gravity. Why is this important?

The polar Moment of enertia of your car is important for cornering. More specifically when initiating and terminating a corner.
When turning in you must set the car "rotating". The polar Moment of enertia is the rotational "mass" of the car. The closer the Center of the car all the mass is, the lower the polar moment of inertia is. (Think about weights on a carousel).

Producing this rotation uses tyre grip - grip which then can't be used for trail braking/lateral loading/acceleration.
Once the corner is complete, you must then stop the rotation, again by using more grip.

So why put the calipers in the middle?
It makes the car turn in quicker and put down more power on turn exit as it has a lower polar moment of inertia.

I think this is the reason the inside wheel initiates the turn-in: because the grip-force has more "leverage" around the polar moment of inertia (until weight transfer to the outside wheel produces more grip there and offers the increased lever-arm). But we weren't able to confirm this with testing.

M

[DoubleD]

Have you spent the past 4 years thinking about this?

[Arnold Cunningham]

Even if he has, it's a good response.

[8bit]

Mike600F said:

Lots of interesting stuff.

Thanks for the post! Nice to have some genuinely informed input.

[anonymous-user]

Mike600F said:

This is something I thought about a lot when designing a suspension system for a formula student car.

The position has no effect on braking force. None. Friction is friction. It also doesn't affect suspension dive etc. That's all based on geometry.

What it does affect is the centre of gravity (the lower the better) and the polar moment of inertia (the lower the better).

For low Center of gravity it's best to mount them at the bottom (I'm sure we're all aware of the benefit a low Center of gravity gives?).
For low moment of inertia, in the "middle" (back on the front wheels and front on the rear wheels) is best.
For packaging reasons, the optimal solution is normally to reduce the moment of inertia rather the Center of gravity. Why is this important?

The polar Moment of enertia of your car is important for cornering. More specifically when initiating and terminating a corner.
When turning in you must set the car "rotating". The polar Moment of enertia is the rotational "mass" of the car. The closer the Center of the car all the mass is, the lower the polar moment of inertia is. (Think about weights on a carousel).

Producing this rotation uses tyre grip - grip which then can't be used for trail braking/lateral loading/acceleration.
Once the corner is complete, you must then stop the rotation, again by using more grip.

So why put the calipers in the middle?
It makes the car turn in quicker and put down more power on turn exit as it has a lower polar moment of inertia.

I think this is the reason the inside wheel initiates the turn-in: because the grip-force has more "leverage" around the polar moment of inertia (until weight transfer to the outside wheel produces more grip there and offers the increased lever-arm). But we weren't able to confirm this with testing.

M

it also affects wheel bearing loads as brake torque reaction can either add too, or subtract from the normal force carried by those bearings.....

[SoCalDave]

I also don't know the answer but it would be interesting to know of course. Following this for sure!