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Old 08-20-12, 04:31 PM   #1
beer goggles
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One piece crank to cartridge BB conversion

The bottom bracket on my Roadmaster MTB recently seized up and I'm in the process of converting it to a cartridge type BB. I've purchased a Truvativ American to Euro BB adapter and a Shimano M171 crankset but I'm unsure about the specific BB and spindle length I need to get.

The existing chain line measurement is 43.5mm (as best as I can determine). Do I need to maintain this? The paperwork included with the M171 calls for a UN26 BB with a 122.5 spindle length to achieve a 47.5 mm chain line. Is this 4 mm difference going to matter that much? Also the UN26 has a plastic left cup, can I switch to a BB with a metal cup such as a UN55 as long as I match the spindle length?

BTW - Your two most common recommendations: (1) getting a REAL bike, is financially out of the question and (2) my local bike shop is full of elitist a-holes so this is why I'm asking a bunch of total strangers for advice...
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Old 08-20-12, 06:15 PM   #2
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I would go with the crank recommendation; The existing chain line measurement is 43.5mm (as best as I can determine), could mean you are off estimating the chainline. As long as the dropouts are 135mm than 47.5 is the industry standard for MTB. As long as the spindle is the same, then you can use any SQ JIS BB. BTW Shimano plastic left cups are pretty strong and usually install without any problems. Unless you are really hard on components, the cheaper un26 should work just fine (sense you said money was an issue).
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Old 08-20-12, 10:07 PM   #3
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The chain line measurement I made is accurate to within .5 mm, it's certainly not 47.5. The reason I said "as best as I can determine" is because with my failing eyesight & the color and shape of my downtube, it's a little difficult to guesstimate the *exact* center. Last summer I replaced the original rear wheel with a Specialized BX23 that I found at Goodwill. I just measured the dropouts with the rear wheel installed and they ARE 135 mm, however the wheel I replaced measured 128 mm so it makes sense to now go with a wider chain line.

Since I'm using a BB adapter, I think I'll get a BB that uses a metal left cup. The UN55 is only 5 bucks more expensive than the UN26.

thanks for the help...
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Old 08-21-12, 12:12 AM   #4
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Originally Posted by beer goggles View Post
The chain line measurement I made is accurate to within .5 mm, it's certainly not 47.5. The reason I said "as best as I can determine" is because with my failing eyesight & the color and shape of my downtube, it's a little difficult to guesstimate the *exact* center. Last summer I replaced the original rear wheel with a Specialized BX23 that I found at Goodwill. I just measured the dropouts with the rear wheel installed and they ARE 135 mm, however the wheel I replaced measured 128 mm so it makes sense to now go with a wider chain line.

Since I'm using a BB adapter, I think I'll get a BB that uses a metal left cup. The UN55 is only 5 bucks more expensive than the UN26.

thanks for the help...
To be honest I never gave a second thought to the chaineline of an American one piece triple crank, so I really don't know and there seems to be very little info on it. At this point, because you have apparently pinched the rear dropouts to 128mm, you need to measure the chainline of the rear (which is always the case anyway), however since the axle is shorter it will have a more narrow chainline not wider. Also which ever Shimano BB you choose should work great!

Edit: well I finally found some info and it is as it seems; the chain lines seem to differ greatly on bikes with American BB. The best was is to measure the rear chain line (to the middle cog of the cassette) and find a spindle length that will fit. I.E. if 43.5 is the chainline (probably not with a narrower rear axle/dropout width) you will probably need a 118mm for the Deore 171 depending on the asymmetrical measurements of the spindle if be even narrower. Good Luck!

Last edited by onespeedbiker; 08-21-12 at 12:38 AM.
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Old 08-21-12, 12:37 PM   #5
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Maybe I was a little fuzzy in my previous post... My rear dropout width was ORIGINALLY 128 mm. I determined this by measuring the old wheel that I removed. The rear dropout width with the new back wheel installed is NOW 135 mm. (I distinctly remember having to spread the dropouts apart when I installed it.)

I ordered the UN55 (122.5) from Amazon last night and once it arrives, my plan is to take everything down to the Sopo Bike Co-op and have them walk me through the installation. I'll post a followup after I get it completed...

thanks again
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Old 08-21-12, 01:43 PM   #6
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I think your parts should work fine.
When I removed the original crank & BB from my hybrid, the were the SAME crank & BB length that you are installing. Bike has 135MM drops.
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Old 08-22-12, 11:11 AM   #7
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Maybe I was a little fuzzy in my previous post... My rear dropout width was ORIGINALLY 128 mm. I determined this by measuring the old wheel that I removed. The rear dropout width with the new back wheel installed is NOW 135 mm. (I distinctly remember having to spread the dropouts apart when I installed it.)
OOps, bewteen your fuzzy writing and my fuzzy reading I did get it wrong; indeed a wider wheel axle will need a wider BB spindle. So I'll return to my original idea
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I would go with the crank recommendation
It does make sense that a one piece BB would work with a narrower axle similar to a road bike as the original chain line was in the road bike range..
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Old 08-22-12, 11:51 AM   #8
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Chain line on a triple is measured from the middle CR.. lining up with the middle of the rear cogs
3 of 5, 4 of 7. or the space between the 2 middle cogs of even numbered 'speeds', 6, 8 ..


The crank profile design determines how the crank sits on the end of the axle..
so the 2 are best considered together..

Just Text on a computer, I cannot see the job from here .. good luck.

Last edited by fietsbob; 08-22-12 at 11:55 AM.
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Old 08-23-12, 04:26 PM   #9
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I removed the old BB a few minutes ago, can you guys help me determine why it seized up?



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