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PostPosted: Sun Aug 18, 2019 6:24 pm 
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Location: Essex, UK
Hi,

I have a 94 Marin Pine Mountain with a supplied BB which is way to short and the chainrings foul before the crankset is even fully screwed on.
I’m guessing it’s a 68 she’ll but I can measure this. I plan to just buy a shimano UN55 but don’t know what length.
Also the “too short” one has no markings on it to know the length or chain line.
Can I just measure it? I never know if the “length” is physically the overall size or the mystical chain line :)

Worst case I have one or two I can try but I’d rather not just do trial and error if someone knows the correct length

Thanks :)


Last edited by Tricky1977 on Mon Aug 19, 2019 9:13 am, edited 1 time in total.

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PostPosted: Sun Aug 18, 2019 11:20 pm 
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There's the shell length - 68mm in this case, by the sounds of it - and the overall length, which you just measure and is, er, just the overall length (spindle length). The shell length depends on the frame; the overall length depends on the chainset. Sheldon Brown's bottom bracket database is useful for finding the right BB for a chainset: https://www.sheldonbrown.com/bbsize.html. The chainline is something else and this article will tell you more than you probably ever wanted to know: https://www.parktool.com/blog/repair-he ... e-concepts.


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PostPosted: Mon Aug 19, 2019 8:18 am 
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So the length isn’t dictated by the frame at all? Rather by the chainset?

Edit: Sheldon has it, thanks for the explanation Cassidy - Deore DX MT60 Triple 122.5 Square


Last edited by Tricky1977 on Mon Aug 19, 2019 9:12 am, edited 1 time in total.

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PostPosted: Mon Aug 19, 2019 9:08 am 
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Joined: Tue Oct 09, 2007 1:55 pm
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Location: New Forest, UK
94 Pine Mtn has a 68mm shell.


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PostPosted: Mon Aug 19, 2019 9:12 am 
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Thanks Hamster, a 68 x 112.5mm BB-UN55 on its way via Amazon delivered this very evening for some further tinkering!


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 Post subject: Re:
PostPosted: Mon Aug 19, 2019 2:11 pm 
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I hope that's a typo: wasn't it 122.5?

The correct overall BB length is not entirely dictated by the chainset. It appears that way when the rear dropout OLD follows a given standard, e.g. 135mm; then, because the rear spacing is a constant across bikes, the only significant variable is the chainset. However, if you put your DX chainset on a fat bike, it would be a whole different story: the rear spacing would have varied as well and a 122.5mm BB wouldn't produce nice effective chain lines. (I don't know much about fat bikes but I think they need BBs of about 140-odd mm for the chain to run parallel to the centre line of the frames.)


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 Post subject: Re: Re:
PostPosted: Mon Aug 19, 2019 2:33 pm 
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Lol yes, sorry 122.5 :)
CassidyAce wrote:
I hope that's a typo: wasn't it 122.5?

The correct overall BB length is not entirely dictated by the chainset. It appears that way when the rear dropout OLD follows a given standard, e.g. 135mm; then, because the rear spacing is a constant across bikes, the only significant variable is the chainset. However, if you put your DX chainset on a fat bike, it would be a whole different story: the rear spacing would have varied as well and a 122.5mm BB wouldn't produce nice effective chain lines. (I don't know much about fat bikes but I think they need BBs of about 140-odd mm for the chain to run parallel to the centre line of the frames.)


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