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Need help selecting compatible crankset

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Need help selecting compatible crankset

Old 10-07-15, 11:41 AM
  #1  
ChrisJS
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Need help selecting compatible crankset

I am rebuilding the bottom bracket and crankset on an old Trek 830, 7x3 speed.
Original parts were Shimano Hyperdrive-C crank (FC-M290) 42/34/24T /175mm and Shimano LP-25 bottom bracket (110mm spindle length).
So far, I have ordered a Shimano UN-55 (110mm) bottom bracket that seems to match the existing set-up.
The problem I am having is finding a compatible (Shimano) chainwheel/crank set that fits the 110mm bottom bracket shaft of the new UN-55 (same length as previous LP-25).
Spoke with Shimano tech. support to no avail. They suggested the M371 or M361, neither of which appear to fit.
This is a basic bike and budget is limited.
Some have suggested tossing the new BB and getting a longer one to fit more cranksets.
Problem with this approachj ( I think??) is that this will most likely require me to replace the whole gear/chain assembly because it will alter the chain line.
Any suggestions for cranks that fit the present 100mm spindle would be appreciated.
Thanks, Chris
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Old 10-07-15, 11:46 AM
  #2  
fietsbob 
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I have used a 118 BB spindle on my M730 cranks .. I have no experience using the same crank you mention..

I suggest asking a Bike shop they can see stuff you cannot describe .

Others also writing from their job may have the same parts you do.
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Old 10-07-15, 12:01 PM
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You put yourself in a bind by ordering the BB first. It's best to buy BB and crankset at the same time to insure compatibility. If you are lucky enough to locate a crankset compatible with your new BB that's great, but I'd suggest checking on the chainline you need to match the rear chainline and then order a crankset/BB to that spec.
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Old 10-07-15, 12:04 PM
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Get a Shimano FC-M131 and a 123mm BB. Everything will line up, and work properly, after a MINOR (very minor) tweak of your FD's limit screws.

Metropolis offered a 48/36/26 9 speed at one time to fit a 110mm BB. They cost quite a bit more than a FC-M131 and UN55 BB.
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Old 10-07-15, 05:26 PM
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I have a similar problem and came up with the following procedure. I'd like to know what others think of it.

If the problem is matching a new BB to a new crank, I suggest acquiring the crank first. Then try taking the following measurements and putting them in a simple top-view diagram of your drive train:

1. Measure the distance from the center of your rear cogset to the middle of your rear axle between the dropouts (middle of "over-locknut distance"). That measurement establishes your chain line.

2. Measure the same distance from the center of the BB shell to the middle of your chain set. On a double, that will be the space between the rings; on a triple, it will be the center ring.

3. To the distance found in step 2, add the distance to the crank hole in the crank set. Measure only to the point where the crank bolt will actually seat, because that's where the BB spindle will end.

4. You now know half the length of the BB spindle. Double this length to get total spindle length, assuming the spindle is symmetrical, as most are.

5. The width of your BB shell (usually 68mm) and the thread type are the final determinants in selecting a BB. Otherwise, buy a threadless BB for your shell width, and with the spindle length you calculated.

I think I got it right, but if others disagree, I'd appreciate input. I'm sure others have had this chicken-and-egg problem.

Last edited by habilis; 10-07-15 at 05:37 PM.
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Old 10-08-15, 10:59 AM
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Originally Posted by habilis View Post
I have a similar problem and came up with the following procedure. I'd like to know what others think of it.

If the problem is matching a new BB to a new crank, I suggest acquiring the crank first. Then try taking the following measurements and putting them in a simple top-view diagram of your drive train:

1. Measure the distance from the center of your rear cogset to the middle of your rear axle between the dropouts (middle of "over-locknut distance"). That measurement establishes your chain line.

2. Measure the same distance from the center of the BB shell to the middle of your chain set. On a double, that will be the space between the rings; on a triple, it will be the center ring.

3. To the distance found in step 2, add the distance to the crank hole in the crank set. Measure only to the point where the crank bolt will actually seat, because that's where the BB spindle will end.

4. You now know half the length of the BB spindle. Double this length to get total spindle length, assuming the spindle is symmetrical, as most are.

5. The width of your BB shell (usually 68mm) and the thread type are the final determinants in selecting a BB. Otherwise, buy a threadless BB for your shell width, and with the spindle length you calculated.

I think I got it right, but if others disagree, I'd appreciate input. I'm sure others have had this chicken-and-egg problem.
close enough. On a 2 x X or 3 x X derailleur equipped bike, chain line is not supercritical; 5mm is good enough for the vast majority of applications.
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Old 10-10-15, 04:10 AM
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you might try the vuelta website, several of their cranksets use 110mm bb.
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