Front – 322mm or 345mm rotors
Rear – 302mm solid rotor or 320mm vented rotors
All the online stores I looked at naturally lined up the smaller front rotors with the smaller rear rotors and the larger front with the larger rears. As I looked at my S90 it is easy to spot the rear rotors are vented, so it has the larger 320mm brakes equipped on the rear, so naturally I assume the front brakes are the larger 345mm brakes and ordered them. Much to my surprise when my rotors arrive, my car is equipped with the 322mm fronts and 320mm rears. This was absolutely astonishing to me and makes zero sense from an engineering standpoint. Considering the front brakes do 80%+ of the braking, I can’t imagine how volvo allowed a car to leave the factory with essentially the same size brakes on the front of the car as the rear. After I realized this, it makes sense that braking performance is so poor and the front rotors already warped on my car (not to mention the factory pads only lasted 35k miles on all four corners).
So, rather than taking the easy path and returning the larger brakes, I decided to give myself a “big brake” upgrade. I couldn’t find any online store selling new or rebuilt calipers for any of the 2017+ late model volvo’s so I took to searching local junk yards. I was able to find a junk yard that had the 345mm calipers with brackets off a totaled XC90 for $60 each with less than 10k miles on them (and the used pads that came in them are nearly new as well
). None of the parts websites show these calipers are interchangeable between the S90 and XC90, but if you look at the part numbers for the brake pads and rotors, you will notice that they are the same for both cars. So I took the risk and bought the XC90 calipers and it paid off big time. The calipers fit perfectly and greatly improved braking performance. It was no more complex than doing a normal brake job replacing the rotors and pads, the only extra step was swapping the brake line onto the new larger calipers and bleeding the brakes.
For those of you into the mathematical figures behind this change and how significant of a size upgrade it is.
surface area of a 322mm diameter circle is 81433mm/squared or 126.22in/squared
surface area of a 345mm diameter circle is 93482mm/squared or 144.89in/squared
This equates to 12049mm/squared or 18.67in/squared ADDITIONAL braking surface area of the rotor. Which is 15% increase in surface area.
I hope this is helpful for anyone else that is less than impressed with the braking performance of their S90 and notice they are in the same situation as I was with the smaller 322mm front brakes.
BONUS: For those of you that want REALLY big brakes on your S90, I also noticed the XC90 R-Line has an optional 366mm front rotor, now these were never equipped on the S90 from the factory, but my bet is they would be a direct bolt on upgrade as well, No Guarantees on this one though.