flats

I just installed a 108mm bottom bracket and a 50/34 compact crankset. Now the chain wobbles-- not sure how else to describe it-- so that each revolution the chain is alternately where it should be and rubbing against the front derailleur. "Purrrr Scrape Purrrrrr Scrape" etc.

Any ideas? Do I have the wrong spindle length BB? Crankset improperly installed?

Thanks in advance.

TampaRaleigh

make sure the crank arm fixing bolt is tight.

ThermionicScott 

Is this a square-taper BB? Using a tiny bit of grease to "moisten" the taper, then pressing on the crank at the interface to get it started helps it to go on straight.

fietsbob

42

zukahn1

Ounce you make sure the crank is on strait and tight and the BB is solid. The second step before you do anything major like like changing the BB is to adjust the stops and tension on both of the DR's. Anytime you change a crank you will typically need to readjust both DR's. Also when putting a new crankset or cassette on bike it is recomended practice that you also but a new chain on the bike.

pierce

did you properly torque down the BB lock ring? after doing a few to 'oomph' tight, I finally used a torque wrench on the last un55 I installed and was surprised that the correct torque was quite a bit more than I expected. also was annoyed to find that I couldn't use my clicker torque wrench on the reverse thread side, as it never clicked, but at least knowing how much ooomph the other side required to click, I got it a lot closer.

assuming this is a shimano cartridge BB you replaced, you grease metal-metal threads but don't grease the plastic side, if yours have one (UN26 etc).

flats

Thanks for the replies. I just tightened down the crank arm bolts-- they were a touch loose, and now everything is in good order

TampaRaleigh

cny-bikeman

It would usually take more than "a touch loose" for crank arms to be crooked, so I hope you tightened them more than a touch. If you have access to a torque wrench it would be a good idea to check for proper tightness. Park Tool is generally one of the most reliable sources for tech info: https://www.parktool.com/blog/repair-...s-and-concepts

dsbrantjr

"the correct torque was quite a bit more than I expected."

This is often the case with crank bolts as well. If they are insufficiently torqued and come loose the cranks can be quickly damaged and rendered useless. Even an inexpensive torque wrench is often better than guessing.

pierce

yeah, I've always torqued the crank bolts on darn hard, using a 3/8" drive ratchet, and pretty much all the torque I can muster without going postal on it. I'm of the 'lightly grease the taper' school, I know many say clean and dry. I always grease the threads, too. for the square taper cranks where the dust cap and center bolt are integrated, I use a 3/8" drive hex bit, as I have a full set of these for my BMW motorcycle, a regular L shaped allen key just can't supply sufficient torque without twisting.

cny-bikeman

I have always just used whatever residual grease/oil that was on my fingers on the flats. Pre-torgue wrench days (and now too, as I don't have a torque wrench) the rule was always to tighten until feeling a large, sudden increase in pressure, and a touch more for larger bolts. Rough but it was necessity when shops had neither torque wrenches nor specs for reference.

zukahn1

Yes for crank arm bolts on older stuff you crank it on about as hard you can your looking for 80-90lbs maybe more so as tight as you can get with most tools is about right.

Sixty Fiver

I am of the school that you use grease on threaded assemblies but avoid any grease on square taper interfaces as it will affect the torque values and one can then over tighten the fixing bolts.

In this circumstance, too tight is unlikely to cause much damage (unless you are a gorilla with a snipe) whereas too loose will cause serious damage to the cranks.

pierce

thats too high.

On the square post low end mountain bike stuff like Alivio, Tourney, Shimano says, 35 - 50 N·m {305 - 435 in. lbs.** which is about 25-36 ft-lbs.

90 ft-lbs is up there in hardened class 5 head bolts on a cast iron engine block. or the lug nuts on my BMW motorcycle rear wheel

zukahn1

Ok I'm a hack on crank bolts I just go for as tight as possible with the wrench on hand. Feel free to correct me on the tension.

davidad

This is the bst school. https://draco.nac.uci.edu/rbfaq/FAQ/8f.11.html

ultraman6970

+1.. the bolts wont strip or anything, and i have to agree the cranks might have been more than a tad lose to wobble. OP you need to eat more cereals man...