bpcyclist

Few days ago doing hill repeats, began hearing a rather loud scraping sound only when out of the saddle only with left crank from the 730 or so position to the maybe 615 or so position. Loudest on 1st revolution, quieter on 2nd, quieter still on 3rd, then, after 4 or 5, it is gone, even if putting out maximum power--all out of the saddle. Sounds quite similar to the sound of a rim brake with metal shards hitting a brake track--very scrapy sounding. Quite loud on that 1st rev. Happens in all gears, though I am running 1x10 right now as FD cable snapped. Chain not hitting FD cage, plenty of room on both sides in all gears. Chainrings not rubbing against anything. Now, staying in-saddle and starting from standstill and applying max possible power canno0t reproduce sound, no matter how hard I push in any gear. Only happens when really stepping on that left crank at teh 730 to 615 positions. Did the little BB stability test with the cranks in the 3 and 9 positions--totally no movement, no motion, nothing.

Bike is a 2018 BMC SLR02, Ultegra/105, well-maintained. New chain/new cassette recently. Lubed up and clean. Had to order a Shimano crank removal tool and it has not arrived from Japan yet, so have not had a good look inside. Pedals recently removed, cleaned up, greased, reinstalled. No issues apparent doing that.

Thoughts? Obviously, it is concerning, given that it really does sound like something is scraping against something else.All LBS's closed here, pretty much. Just wondering if there is anything I can do in the mean time. Bike and BB probably have about, maybe, oh, only 6000 or so miles. Thanks a million!!!!

dsbrantjr

Did you check that? Perhaps one of your wheels is flexing under power/load and contacting a brake pad.

bpcyclist

Well, I do run my brakes tight-- I do. But not sure how I can really check that, since I have to be cranking hard to produce the sound. Oh, wait--duh. Just release the cable to zero tension. Okay--will go try that. Thanks!!

Miele Man

Look at the brake shoes to see if 1. any metal bits are embedded in them and 2. make sure the brake pads aren't worn to the point wherethe metal holding them is hitting the rim.

Cheers

bpcyclist

Welp, thanks a ton, guys. I learned something today. I have been riding CF bikes for tenish years or so, but I had not idea they could flex so much that they could induce brake rub (mine is in the rear and on the opposite side, the right, of the side to which the big force is inducing it is) in a bike that otherwise has well set-up brakes. No clue that could happen. Thanks a million for helping me out!!!

Trakhak

The flexing is more likely occurring in the wheel rather than in the frame, although both are possible.

smashndash

I wonder if you have a deep-ish carbon rim? It took people a while to reach a consensus but it seems that deep carbon rims are more likely to overwhelm the spokes and flex near the brake track than spindly alloy rims. Those just deflect near the ground.

I’d recommend going with stiffer spokes or a shallower ERD next time to mitigate brake rub, if that is a concern.

bpcyclist

Ah--got it. Rims are just fairly lightweight aluminum, but thank you.