XE007

Hello everyone.

Some time ago I began looking into building my own bicycle. Throughout this time, I have put considerable effort into learning the inner workings of bicycle components so that I may not have to depend much on a mechanic.

Having said that, I have a Schwinn Loop folding bicycle stripped with just the frame alone and fork. Its a nice bicycle but at 33 pounds (15kg), its quite heavy when folded.

Anyhow, I have run into an issue. The bike originally had a square taper bottom bracket measuring 68mm and 118mm at the spindles. I am seeing the length as problematic as I am looking for a good set of crank arms using a new Shimano UN55 68mm, 118mm BB.

It seems that finding crank arms to fit these specs is quite hard. I just found a set of Andel crank arms 144BCD on Ebay but to my dismay, they require a BB of 110mm at the spindles with a chainline of 42mm. I think my chainline with the older components is 45-48mm. The loop has a rear frame spacing of 136mm which I think will not be much of an issue however the chain runs straight to my 7th gear which is what this bike has.

I want to cut this short and ask; Is there anyway to make this fit? I am afraid that if I get a Shimano UN with 110mm, I will have an issue with the chainring touching the frame. Is 118mm too long for most cranks?

Thank you for any help you can provide.

dsbrantjr

Find a crankset which suits you and purchase a bottom bracket to fit if necessary. Trying to do otherwise is letting the tail wag the dog. In the event you find one which uses a 118mm BB you are fortunate, but it is putting an unnecessary constraint on your search. The current BB may end up not being any good, anyway.

XE007

Thanks for your reply. As I wrote above, my concern is that the spacing of the chainline will get smaller as I have almost no room because of the chainstay and my rear sprocket (lowest); If I get the UN 110mm will chainring spacers help with this offset?

Thx

Andrew R Stewart 

It's a simple math problem to, on paper, design a crank/BB length combo and the resulting ring/frame clearance if you have the same crank mounted on a known BB length. Just measure the amount od current ring/stay clearance and subtract twice that (for symmetrical BB spindles) and add about 4 or 5mm for the 2-3mm of working clearance on one side. Of course in real life and with manufacturing tolerances being just goals and not constant facts a real fit up is the final say.

But the OP mentioned replacing the crankset with another. So my above isn't going to help unless that new crankset is mounted on the current BB spindle to test the fit/clearance/chainline. I will say that chainline is not an absolute but both ring/stay clearance is and the ability for the ft der to move through a range that will shift across the rings final position. Andy

XE007

Thanks for your reply Andrew. Yes, I will be replacing the ring and I only have a enough space for a 50T ring even by manually pushing the crank in on the drive side as a test. I cannot post any images unfortunately but if I could send images (not enough post count) through PM that would be amazing and you can post your findings.

Cheers,
Xavier