BB axle length question

Korszun · Jul 25, 2013

Just finished my first retro build (will do a thread soon), a '90 marin palisades 1x8 rat. Built it up with M730 to M739 XT parts plus existing good bits and some new parts.

I used a 122.5mm BB as recommended by Shelton brown for the M730 crank arms I used. However as I only have one chainring and it's in the outer position, I'm thinking the front chain line distance is too large. I measured it today and it's about 55mm from centre of seat tube to centre of chain. I've had the chain come off once when shifting quickly through the cassette (from 11t towards 32t) and my thought is that this could be the long chain line distance coupled with the small diameter 34t outer chainring?

Should I get a shorter bb?

Woz · Jul 25, 2013

You don't perhaps really need it, but if you strive to get your chain-line to be somewhere in the middle of the cassette it will help;
probably should be around the 48mm mark.

If you put the ring in the middle position it would be better, but I always think it doesn't look right. If you go for say a 113mm length
there is a risk that the crank arms / back of your foot may touch the frame as it will bring DS and NDS inwards by about 5mm. I doubt
a 34T chain ring would start to hit the chain-stay, but it would be good to measure up what distance you can play with.

Korszun · Jul 25, 2013

Thanks Woz,

You highlighted some things I wasn't aware of. Middle position would be better I'm sure but you're right, I'd have to get a bash ring to make it look anywhere near right and I just don't dig the look (I am a fan of function over form though as this will be my work bike so maybe this is the best solution).

The inside of the crank arms have >10mm between them and the stays so I reckon a shorter BB is a possibility.... Maybe the bash ring is just a better solution though.

Thoughts anyone? Recommendations for a bash ring? Crank is 110 BCD

Paul

Woz · Jul 26, 2013

There's always compromises and optimums. Since I'm a bit of a nerd in this area, and based on your measurements to play with I would not hesitate to get the Q-factor down. If it was me I would definitely spend the money on a useful moving part ie. a new bottom bracket to help keep your bike running in good order. UN55 are not that expensive and a 113mm will get the weight of your rat down a bit compared to a 122.5mm. They are probably cheaper than a bash guard which would function just like a glorified chain guard and adding more dead weight. If you go for a double later, then you've got the old BB (with less wear) in the spare part box ready.

hamster · Jul 26, 2013

I'd try a shorter BB, there is no harm in trying. It would also allow you to fit a chain guide if the problem of dropping the chain persists.

Korszun · Jul 26, 2013

Decided to move the chainring to the middle at least to see if that solves the problem. If it looks crap then I'll think again but I figure I should try this first as it's free!

BB axle length question

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