I think I know the answer but worth a sanity check from the collective.
I'm thinking of changing the rear disk on my CAADX from 140mm to 160mm. I have a pair of Avid 160mm disks spare and find they improve braking when compared to the current Tektro discs.
Cannondale fit and recommend 140mm for their rear disc size. Am I going to destroy the frame with the forces generated via an extra 20mm?
I know the front brake does most work/braking is limited by contact patch stuff. This question is about the forces on the mounts/chainstay/seatstay from the extra 20mm. I'm inclined to think there's not going to be a problem as I ride this bike in the flatlands unloaded but it's worth asking the question.
Any opinions from engineery types please?
Absolutely categorically no issue whatsoever. The rear wheel will loose traction way before force on Mount / frame starts to become an issue.
Do you struggle to lock the back wheel with a 140 disc on skinny tyres?
If you want more braking power you should be upgrading the front brake.
Upgrading the back will just have you locking the back wheel easier.
I think I just want to optimise what I have. 160mm Avid discs front and rear and I have the discs and adapter so no £'s need be involved. I just don't want to cock up what I have.
The back wheel has something like half the grip of the front - which is why nearly every bike runs a smaller disc behind. All you will do is lock the rear wheel quicker and add weight to your bike.
It's personal preference really, depends on brake and massively on what tyres you use. I put a post on here previously about hydro sti performance.... Mine were not brilliant. But after a bit of experimenting, I am sure this is to do with the awesome 42mm wide marathon supremes I am using. Plus my weight 13 stone fully loaded with beer and pizza.
The 160 rotors struggled with the supremes. I stuck the old tyres 32mm conti speed cross back on and these locked up easily with one finger.
I now run 180s front and 160 rear which is great. I like power from the hoods. Just need to be careful not to grab a lever full from the drops but would rather have the power if I need it...