Bottom Bracket Advise, Please.
 

Thanks for taking the time to read through this!

I had a pedal strip out of the crank arm while under load on my 2011 Trek 3900 Disc MTB. That was a mystery of it's own on a bike that does have fairly heavy mileage, but I digress.

I am new to this whole bicycle mechanics thing, but have some experience with a total rebuild of another, much older bike.

After pulling the crank arms, I cleaned up the bottom bracket and took a look at it's condition. It's tight as far as any slop either end-to-end or up & down, even when using the crank arms to lever it around. What I feel when I rotate the spindle is considerably more "gravelly" than the silky smooth feel of the cup/cone/loose ball BB that's on my older bicycle. When I put the crank arms on the BB turns very freely and the cranks swing easily with not too much noise....but again, more noise than the cup-n-cone one. Is that "normal" for a cartridge type BB? Or..."as long as it's torn apart..."

One more quick question on this repair, please. I went with the same model/brand of crankset, but went 4 teeth more on the two smaller chainrings and 6 more on the largest. Is there a formula for lengthening the chain....or will the old number links + 6 work?

Thanks again!

A rough or gravelly feel isn't normal, and usually indicates some dirt or rust contamination and/or wear. If this is a typical cartridge style BB, your options for service are limited. You might want o take a look and see if it's at all serviceable, otherwise since it shows no play, go ahead and use it until it dies.

Thanks, Francis. I'm still holding $.50 in escrow for you after I used your wheel dishing trick on two wheels. Ya' never know...I may have to re-dish again!

let me know when it's enough to buy a few beers then I'll come out to collect.

so much of the bike production has gone over to fit and forget BB assemblies ..
If you got dirt in the bearings , take out the BB cartridge, and put a new one in ..

UN type BB are disposable Priced and designed to be so.

Regarding the chain length part of your question, if it was sized properly before, it will be short now. But, we don't know if it was sized properly before so either replace or add the number of links necessary to get it to the correct length.

Thanks, CACycling. Yes, it was the original and seemed to be fine for all gear combos and easy to remove the rear wheel. I'm replacing it, as well as the crankset and cassette, so will just add the 6 to the original chain link count. Probably replace the BB, too...it just doesn't feel right.

Just as an addendum: looking around on the web I can find several 122.5mm spindle length BB's but only a couple at the 123mm length specified by the crankset manufacturer....close enough?

That's a bit less than 1/100" shorter on each side.

Pretty sure how worn the taper is, or even how tight the cranks are, or even the tolerances of the BB shell make a bigger difference than that. Insignificant.