Running a 140mm rotor on the rear - any issues or considerations?

daniel_b

Morning all,

I dropped a bike and newly acquired groupset off at an lbs on Saturday.

As we went through all of the parts, it became apparent that I'd been supplied with a 160mm and a 140mm.

The very helpful chap in the LBS said he thought it would be fine with the 140 on the rear, but that he had heard some stories of people taking bikes such as this to the mountains, and then having problems with warped rotors after the long descents.

I'm generally between 65-69kg, so not the heaviest or lightest, but I definitely will be taking this bike to the mountains in the coming years.

I happen to have a suitable rotor in my bag today, so when I go to collect the bike (Assuming it is all ready today) I can ask them to quickly swap to the 160, but was interested to hear what people's personal experiences were.

My gut instinct says to stay with the 140, but equally I don't want to compromise my riding abroad - though I could of course change rotors, but that seems a little bit faffy.
Although - having said that, I would need the wheel off as I would be swapping to an 11-34 cassette with a paired chain, so maybe it could fall into that overall job.

If it's relevant, the rotors are the Shimano XTR ones - and it's for a road bike.

First.Aspect

God almighty the crepe that you hear from some people about brakes is unbelievable.

140 is fine. I have that on the front as well.

First.Aspect

Think logically. 160 is fine for the front, yes and does about 80% of the braking. Why would 140 not be okay at the back?

MattFalle

how the flip do you warp a disc coming down a mountain road?

are people honestly this bad at riding bicycles?

First.Aspect

Most centrelock rotors are designed to accommodate expansion, via the rivets at the spider. Most 'warped' discs are most likely discs that heat up a bit and start to rub annoyingly like all disc brakes do from time to time. People think they have become warped, whereas in reality they were always warped but just not rubbing.

super_davo

I run two of my 3 disc brake bikes on 160/140 and absolutely fine going down mountains, at a similar weight to you.

Most of your braking is at the front, all that happens is you pull your back brake marginally more than you would if it were a 160 but not as hard as the front. Overheating isn't an issue unless you drag your brakes rather than letting the bike run until you get to sharp corners.

I will probably change the other over (160/160) as there are no real drawbacks and it just makes swapping wheels easier.

daniel_b

Yep no totally understand the vast majority of the braking will be down to the front brake, and we did discuss that, he was just retelling stories from some customers, but he did suggest they did not perhaps know how to ride their bike in the optimum manner ;-)

That's decided it then, I will stick with the 140, which will imo look a bit nicer, shed a bit of weight, and if it turns out the mounting adaptor needs to be removed to get the calliper in the right place, then a few more bonus grams :-)

No plans to be changing wheels on this bike at all, but yes I can see how that could prove an issue if you wanted easy swappability which is not a word.

Cheers all.