# How to determine the correct bottom bracket length?



## Cinq (Jan 2, 2003)

Hi there, as you might know, I am upgrading my sons Hot Rock 20 inch from six to nine speeds.

I am still waiting for a bunch of parts but I figured that I could already mount the cranks, chain ring and chain guide. It looks great:










I did notice that the original cranks weren't 130 mm but more like 125, a measurement error on my part. I told my son to take more care when pedaling in the corners. The new chain ring is 34 teeth, the old one was 36. These two changes will hopefully help a bit when climbing, when the new setup is complete, he will have a 34-11 cassette compared to the current 28-14.

The only problem I see now is that the bottom bracket (127 mm) is too long. With the six speed cassette, the chain is perfectly straight when it's on the smallest cog! The result is that the chain drops off the chain ring when pedaling backwards in the first speed (biggest cog).










I know that the 9 speed cassette will be a bit wider (40 mm compared to the current 34 mm) but even then it would probably be straight on the 2nd or max 3rd cog. On my own bike, it's straight when it's on the middle cog.

There is almost an inch of clearance between the crank arms and the frame so I have no problem whatsoever there. Tire clearance is OK too, even taking into account that the 9 speed cassette is wider.

I took a picture to show how much space there is between the cranks and the frame:










I guess that I can easily mount it 5 to 7 mm closer to the frame, meaning a 117 or 113 mm bottom bracket. Looking at the available sizes, there are 113, 115 and 118 mm bottom brackets. Is there a way to calculate the exact size or isn't this an exact science and is it based on experience or even trial and error?

Kind regards,

Clemens


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## Demo9 (Nov 20, 2006)

I used a 113mm on my sons Scott with a 34t front and a 11-34 cassette. It seems to work pretty good. It will sometimes drop chain on the 34-34. I am going to put a chain guide on it soon, it should then solve the problem.


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## jlc30 (Apr 1, 2009)

Go with the 113mm bottom bracket. Cranks are designed to use a particular length bottom bracket. You can get close or play with shims to make incorrect ones work with varying results. With my kids bikes I've typically been able to spend a few minutes on the net and figure out which bottom bracket I'm supposed to be using. On my daughters mountain bike I picked up a set of closeout cranks and was able to find a couple bikes that used that particular crank. Looked up the specs on those bikes and it gave me the bottom bracket length.


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## Cinq (Jan 2, 2003)

Thank you for the replies. I have done some math myself based on the measurements and I agree that I should go for the shortest possible bottom bracket. Another reason is that the bash guard on the outside will prevent the chain from dropping off on that side.

I'm going to order a 113 mm bottom bracket and will see how things work out!

By the way, great looking Scott bike you have there! What type of Maxxis tires are that? They look huge!

Kind regards,

Clemens


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