1

I have an old Trek 8000 ZX that I am fixing up. I had to replace the bottom bracket and I believe I successfully did that. The bike did not have a crank set to work with so I salvaged one from a pile of other old bike scraps.

My problem is when I attached the crank arm and tried to spin the pedal, the smallest gear just rubs against the frame. It appears that the crank is too close the frame. It is just making contact 5mm would address the issue.I want to know what my options are. My initial guess is to find a crank set with smaller gearing and hoping to find one that fits. The other idea is try and get a spacer but I am not sure how safe that would be.

There are number of bikes available that I could steal parts from but I would like some guidance on what I should be looking for in terms of size. I am not sure of the age of the bike, but I am pretty sure the frame size is 21". The color is yellow if that helps.

I am sorry if some of my terminology is wrong/crude, I am new to bike repair/salvage (Having a blast though).

Thanks in advance.

  • Do you know if the bike was a double or triple front chainring originally ? Did you fit a double or a triple ? Front derailleur mech could have 2 or 3 positions on the left-hand shifter. Triples tend to be wider, so have a longer BB axle exposed. If you've put a triple on what was a double-sized BB axle then rubbing is possible. Note, spacers and washers will not help you. Frame size and colour are pretty irrelevant. – Criggie May 28 '16 at 6:41
  • 1
    It is a 3x7 gear system with 3 positions on the front derailer. So I know I need to find a certain size BB, but do I need to look for a triple BB? Thanks – TheCodeNovice May 28 '16 at 11:04
  • The part of the BB axle that pokes to the right will be longer than if it was a double. So as per other comments, 113mm (being more than the 106mm long one you have now. 7mm difference sounds what you want. – Criggie May 28 '16 at 21:29
4

Welcome to BB standards hell...... you are about to learn far more than you wanted to about bicycle gearing cranks and chain rings.....The only spec I could find on these was the 1999 model, which used a Shimano BB-UN52, 113mm spindle BB Shell Width 73mm - its pretty common and typical of that era MTB.

Your idea of another crank set is a possible solution, that might be easier if you have a supply of donor bikes, is a larger small chain ring.

Its likely though you need a longer bottom bracket spindle, as a short spindle will not only cause this to be a problem, but will affect the chain line. With incorrect chain-line you may have problems with getting gear shifts to work smoothly and reliably, and have problems with chain drops. Read up about chain line Here (as always, Sheldon Brown)

Presuming its square taper BB, they come in a range of lengths. You should be able to measure your current length, and might find a longer one on your donor bikes. Hopefully the one you have now is around 106mm, meaning all you need to do is source the correct 113mm sized one.

|improve this answer|||||
  • Yes it is a square taper. I tried to measure the spindle with calipers I will have to double check. I was going to try and order one, Is there a risk of buying one that is too large? – TheCodeNovice May 28 '16 at 11:06
  • Do I need a longer spindle if it is already 113? I am on amazon and I see one, Shimano BB-UN26, I was going to try and order a 73x122. – TheCodeNovice May 28 '16 at 11:12
  • OK just realized what the 52 and 26 mean. Still have questions about buying a longer spindle. – TheCodeNovice May 28 '16 at 11:44
  • A longer BB could cause chain line problems, but might be fine particularly in lower gears when on the large chain rings. You won't know till you assemble it if it works. BB selection when assembling from non original parts is a bit of a back art and trial by error. If you buy from your LBS, you may be able to do so on a return basis - even if you have to pay a restocking fee its not a bad idea. – mattnz May 29 '16 at 5:33
  • A longer BB will also affect Q Factor - not something I would worry about to start with - I did warn you there was a learning curve :) – mattnz May 29 '16 at 5:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.