Rear Caliper Position - Any Advantages?

[DeltaT]

I am building up a 9" rear, and since I'm welding on the brake brackets, I have some latitude on how I position the rear calipers - right at 90 degrees, slightly higher, etc.

For example, this guy has his clocked slightly higher than the centerline:

http://thegmr.com/wp-content/uploads/2012/01/moser-12-52.jpg

And this guy's is slightly lower:

http://thegmr.com/wp-content/uploads/2012/01/024.jpg

And most pictures I see, where the caliper is behind the axle line, the caliper is clocked right at 90 degrees.

Are there any advantages to clocking the calipers other than the standard 90 degrees, other than modest packaging considerations? Clearly I would get into bleeding issues if I went too far either direction.

Thanks,

Jim

[realcoray]

I think any benefit of position would be small. For example, putting them lower, moves the mass slightly lower, and probably gives them more airflow over the caliper but you aren't going to notice anything as a result.

[dontlifttoshift]

The second picture is my rear end. The calipers arent actually mounted that low, they are just hanging there at that point, they actually ended up near vertical, in other words the bleeders are pointed straight up.

I have looked into this a little, and found no evidence supporting one position to be better than the other, in front of or behind the axle, or clocking the caliper either. Even with the caliper mounted horizontal, they can be bled by removing the caliper, installing a shim to simulate the rotor and bleeding normally holding the caliper in your hand......that is a pain in the arse, though.

Mount the calipers in a way that gives you the best clearance and easiest maintanence. After that I try to match the clocking of the front caliper for aesthetic purposes.

[DeltaT]

Thanks, guys.

Jim

[Bryce]

I always wonder about the reaction load on the car.

[dontlifttoshift]

Bryce, can you elaborate?

In side view, the caliper will load the axle housing through the caliper mounts trying to rotate the pinion down, that won't change. Would there be a different reaction between the caliper mounted front or rear?

[Bryce]

If the caliper was mounted on the back side of the axle the load on the caliper would be in the vertical direction. the could reduce the load on the tire. The opposite would be true as well.

[dontlifttoshift]

So you think that mounting the caliper in the front would load the tire under braking? The advantage would be minute at best, yes?

[Vegas69]

If the caliper was mounted on the back side of the axle the load on the caliper would be in the vertical direction. the could reduce the load on the tire. The opposite would be true as well.

I visualize it the opposite but I doubt it plays out as they are both attached to the same component.

[James OLC]

That part of the equation notwithstanding... Keep in mind that calipers can (and are) built as leading or trailing and are designed for that orientation - piston size / order. And FWIW I've run both leading and trailing and noticed no appreciable difference.

[dontlifttoshift]

Good point James. When you say leading/trailing are talking about the caliper design itself or just differential bore pistons and their location.

I think suspension link location and arrangement would have to be determined before you could determine how the caliper would load/unload a tire.

[Bryce]

I will draw a free body diagram. The frictional force went somewhere, grab a spinning bike tire where does your hand go...

I thnk its going to endup being a force on the tire, either up or down. I dont think it would increase/decrease antidive.

[dontlifttoshift]

I will draw a free body diagram. The frictional force went somewhere, grab a spinning bike tire where does your hand go...


The emergency room........

Rear mounted caliper will load the axle then, in the direction that the tire is rotating. So will a front mounted caliper. In the case of a torque arm type setup with the outboard links mounted vertically at axle centerline, for example, the rear mounted caliper would load the tire and front mounted would unload it......in theory. I'm curious about this.

[realcoray]

I'd suggest looking at where OEMs or competition racing cars put their brakes. The assumption is that those people spend millions of dollars looking at these sorts of questions.

For the C6R and Stingray for example, GM has the brakes in front. On F1 cars though it appears like they are very low in the back.

Some of these things may be packaging issues, the F1 car for example has a lot stuff going on around the caliper, but you have to assume that their goal is to maximize everything to reach peak performance with their very light weight, high horsepower cars. Shifting of grams lower means more to them than it does on a production corvette or even for the C6R.

Again, it's pretty much not going to make any difference, but this is the approach that I will often take when considering something like this.

[Sieg]

Koenigsegg Agera R rear calipers are at 3 & 9 o'clock behind the axle. Their engineering is probably some of the more current and advanced.

Porche's Carrara GT had the rear calipers slightly lower than 3 & 9 o'clock in front of the axle.

Then you'll see some F1 cars with bottom mounted calipers.

http://www.formula1.com/photos/597x478/manual/rrbrakes02.jpg

:headscratch: What is best and how much influence does it have relative to the desired performance objective?

[intocarss]

Here's some discussion about it

http://fsae.com/eve/forums/a/tpc/f/125607348/m/80310023931

[dontlifttoshift]

Good Link!!

[sik68]

I would venture to guess that caliper position on high end cars like F1 is mostly dictated by efficient airflow in that area for brake cooling.

I agree that caliper position affects the wheel bearing loads during braking, but not the resulting load on the tire.

If caliper position did change the load on the tire, then under hard braking the tire could go into an "open feedback loop" and your car would a) become infinitely heavy and crush through the earth (3oclock), b) speed up (6oclock), c) levitate off the ground (9oclock), or d) instantly stop (12oclock) just by pushing harder and harder on the brake pedal. :p Just exaggerating for visualization purposes.

[ironworks]

I hope there is no issues because I just put a floater in my dirt racer and had to stagger the calipers to keep my track width the same in the rear and keep my stock staggered shock mount positions. So one caliper is forward one one is backward on the other side. The opposite of the shocks.

But its not like the brakes are really only used to park the car in the pits. HAHA

[intocarss]

I hope there is no issues because I just put a floater in my dirt racer and had to stagger the calipers to keep my track width the same in the rear and keep my stock staggered shock mount positions. So one caliper is forward one one is backward on the other side. The opposite of the shocks.

But its not like the brakes are really only used to park the car in the pits. HAHA

Are you testing at Bako on Sat?

[JasonElvisHeard]

realistically I would challenge any driver who can state that they can "feel" the difference behind the wheel as to where the caliper is mounted.

Packaging and looks are the #1 /#2 factors in caliper location. Performance will be good regardless of mouting in front or rear.

FYI, the C5/6 platforms only mount calipers like so because GM is cheap... yeah I said it.... they are flat-out cheap... they did this so that they don't have to make different uprights for front and rear. Simple as that. At the end of the day, they have to make money. Performance comes about 5th... behind legal issues and safety... haha.


Jason