One piece crank to cartridge BB conversion
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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...
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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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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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...
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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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...
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...
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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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
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
#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.
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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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 would go with the crank recommendation
#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.
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.