I saw a pic of this vehicle and it looked like they had the same size brake discs on the front and rear, a setup that's desirable in racing for keeping spares costs down. Turns out it actually has bigger brakes on the rear, by 3mm. Equal-sized brakes already make hydraulic brake bias selection (which OEMs often simply skip in favor of letting the ABS do the work these days, I hear) and heat management difficult, so this setup is downright bizarre. They didn't save any money with this setup since the front and rear brakes are still completely different. And if you're going to have different sized brakes, you might as well make the rear smaller to save unsprung weight and keep brake heat similar front to rear. So I'm stumped.
Ian F
MegaDork
6/26/17 1:50 p.m.
Maybe they found the smaller brakes to wear excessively due to traction control programming? (just a WAG)
Ian F wrote:
Maybe they found the smaller brakes to wear excessively due to traction control programming? (just a WAG)
If you're making a pretend limited slip diff (rear or center) by braking only the slipping wheels, you might have lots of need for large rears.
Or towing. If you are towing the rear brakes on the truck are like front brakes on the trailer.
It's a visual / aesthetic thing. No doubt the rear rotors are thinner than the fronts since they don't need as much thermal mass Back there, and also no doubt the rear piston area is smaller than the front because they don't need as much clamp force back there.
AngryCorvair wrote:
It's a visual / aesthetic thing. No doubt the rear rotors are thinner than the fronts since they don't need as much thermal mass Back there, and also no doubt the rear piston area is smaller than the front because they don't need as much clamp force back there.
Going off of that, might be a cost thing too? Bigger rotor but thinner and smaller piston area might equate to a lower cost, but they had to make the diameter larger to keep the same braking power?
-Rob
Most Miatas are the same way. 2001 Sport is 10.6 front, 10.9 rear. But the rear is non-vented with a much smaller piston.
It's easier to come up with a custom rotor size for a specific application than a custom piston size in the caliper. One's a dumb piece of cast iron, the other is a fairly heavily tested and standardized critical safety item. I'll bet you'll find that same caliper on other BMWs. So you get in the ballpark with your piston size, then fine-tune the brake torque with rotor diameter.
There's also the possibility they needed some clearance in the rear, so it worked better to mount the rotor further outboard.
Ian F
MegaDork
6/26/17 5:33 p.m.
Does the parking brake use shoes inside the rotor? Those are often larger as well.
Most vehicles with drum type parking brakes have larger OD rear discs.
Came here to say exactly that.
