Size of brake rotors front vs rear

[TW14 9LF]

Noticed yesterday that the diameter of the rear brake rotors is significantly larger than the front. Every other car I've owned has been exactly the opposite. Thoughts on why?

'18 LR RWD w/ 18" wheels

Front


Rear


Keith

 

[tilt]

Look at the size of the calipers and hence the pads on the front. Much larger contact area. I'm sure they did the math and testing to end up with more braking power on the front despite the rotors being a bit smaller.

 

[TW14 9LF]

Agreed the contact patch with the pads is greater, even with the smaller rotor. Just curious why the rotor is bigger on the rear. Never seen that on any car, there must be a reason for it.

 

[tilt]

I'd guess it was a trade off between the larger calipers and larger pads. With a larger rotor, it just won't have the clearance with the wheel if you want larger calipers and pads. There's probably a sweet spot where a smaller rotor with larger calipers and pads out performs a larger rotor with smaller pads and calipers.

 

[03DSG]

Noticed yesterday that the diameter of the rear brake rotors is significantly larger than the front. Every other car I've owned has been exactly the opposite. Thoughts on why

Couple of reasons and just the basics here. Keep in mind the front brakes do the majority of the work but rotor diameter is not the defining factor. It’s about the front to rear balance and that can be accomplished in a number of different ways.

The front calipers are fixed 4 piston and the pads are longer and thinner (L x W) to allow the pad to contact the 2 pistons on each side. The rear calipers are floating single piston and the pads are shorter and wider (L x W) as the only contact is with the center of the 1 piston on the back side.
The longer thinner pad on the front can have a larger contact patch on a smaller diameter rotor whereas the more ‘square’ rear pad requires a larger diameter rotor to accommodate the width of the pad. It doesn’t mean the contact area of the pad is larger than the front.
The front rotors are also 5mm thicker than the rear to allow for more heat sink.

 

[Uncle Paul]

It may also be that the fronts, with their steering/suspension, don't have room for the larger diameter rotor. (Rear does not steer)

 

[jerry33]

And the parking brake works on the rear disks as well.

 

[Msjulie]

It may also be that the fronts, with their steering/suspension, don't have room for the larger diameter rotor. (Rear does not steer)

MPP as well as some others supply larger rotors up front with brackets to mount the caliper back out at the edge. Fits fine

I'm guessing the brakes are sized based on expected need; with regen these brakes get less work out than similar-weight ICE cars