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how do I determine BB spindle length

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how do I determine BB spindle length

Old 08-29-10, 09:34 AM
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Slow Riding
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how do I determine BB spindle length

I have a specialized Tricross frame, and a set of unknown square taper cranks.
How do I determine the spindle length for a set of unknown cranks. I only plan on running a 46 tooth front ring.
I am currently running an old bottom bracket from my mountain bike, and the cranks are so far from my frame it is killing my knees, and robbing me of serious power.
I plan on purchasing a nice titanium spindle BB, so I need to get the spindle right the first time.
My big concern is that I really don't know how far these old square taper cranks will "seat" onto the spindle.
Any help will be appreciated.
Thanks
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Old 08-29-10, 09:51 AM
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Good luck, there is a different suituation for each crank brand /model

Id suggest get your unidentified crank Identified at the local Bike shop

since I dont have a clue , none offered.. couldnt even post a picture.. & My Crystal ball is out of order.

buy a whole new crankset and its BB as a set,
they are all 2 piece tubular BB part of right arm
and
External bearing now , get a Mountain bike one for the triple crank, low gears..
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Old 08-29-10, 10:25 AM
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on a more heuristic note, you could buy a cheap Shimano BB (say, a 108mm UN54) for $25 and figure it out from there. Shouldn't be more than 1-2mm either way from there. Or maybe someone at your LBS can tell you, or search the brand of crank you have (or post it here). OR you can measure the chain-line distance you need to clear the rear stays.
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Old 08-29-10, 10:42 AM
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Tha tapers are all more or less standard as long as it's a regular ISO taper and not a JIS. JIS being rather rare over here. So the taper SHOULD seat in the cranks the same to within a mm or so.

So if you measure your present spindle length and work with measurements of both your crank arm to stay gap and chainline measurements some simple math will get your selection to within a couple of mm's. Keep in mind that the length is overall and you'll be measuring the ONE side so allow for double the difference when selcting the length. If you're still not trusting yourself then as mentioned buy a cheapie cartridge BB or get a discarded one of that length from one of the LBS's in your area and fit it to your bike just for sizing. Note that unless you're pretty careful that measuring the length of the current BB spindle would most likely require removing the crank arms to be sure you get the distance correct. Or do up a little diagram and then use calipers to measure from the outside faces of the arms, then after removing the crank bolts measure how deep from that same spot the end of the spindle is and do some more math. But if hte area around the crank bolts are sloping or have other fancy shaps I'd suggest to be sure that you just remove the arms and measure the actual spindle.
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Old 08-29-10, 11:14 AM
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Abundant unspoken of context complications..
FWIW One one bike, my Shimano M730 used a 118 BB.
I've got it on a different bike
with a wide chainline (rohloff hub)
and wide wheel clearance chainstays , now the same crank needs a 127.5..

Let me add : take the BB you have, fit it into the crankarms and/or

Take, perhaps the arms to the bike shop and compare with their inventory..

have them facing on one side, measure the distance between the inside faces of the crank arms. .

Compare that with how wide they have to be to not hit your frame..

Last edited by fietsbob; 08-29-10 at 11:21 AM.
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Old 08-29-10, 11:18 AM
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if you have a bike shop that will allow you to trade or return BBs you've purchased from them, that would be the best solution.
Buy a few different lengths and see what works.

It's even better if you can guesstimate the length before hand, as that'll narrow down the choices.
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