Bottom Bracket Length question
12-05-12 | 07:51 PM
#1
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From: Florida
Bikes: Diamondback Parkway, Specialized Sirrus Sport, 1980 Concord Freedom Deluxe, Early 80s Schwinn LeTour, Trek 4600, Trek 360
Bottom Bracket Length question
I'm a newbie to the forum and want to get an opinion about a slight bottom bracket length mismatch.
My learning project bike is an '01 Diamondback MTB (curbside find treasure). The bottom bracket was dry and noisy so I pulled it. Its a square taper measuring 16.4 mm. The choices for replacement were 15.5 and 17.5 mm. I opted for the 17.5 which actually measures 17.8 mm. The entire 1.4 mm difference is on the drive side.
My question is whether that 1.4 mm is likely to give problems adjusting the FD and with shifting?
Thanks in advance and for the other great information presented here in the mechanics forum.
My learning project bike is an '01 Diamondback MTB (curbside find treasure). The bottom bracket was dry and noisy so I pulled it. Its a square taper measuring 16.4 mm. The choices for replacement were 15.5 and 17.5 mm. I opted for the 17.5 which actually measures 17.8 mm. The entire 1.4 mm difference is on the drive side.
My question is whether that 1.4 mm is likely to give problems adjusting the FD and with shifting?
Thanks in advance and for the other great information presented here in the mechanics forum.
12-05-12 | 08:44 PM
#2
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Bikes: 1991 Bianchi Eros, 1964 Armstrong, 1988 Diamondback Ascent, 1988 Bianchi Premio, 1987 Bianchi Sport SX, 1980s Raleigh mixte (hers), All-City Space Horse (hers)
Well, the good news is that everything will clear. My preference is to run as short of a bottom bracket as I can get away with -- this minimizes the "Q factor" and allows the big ring to work with more of the cassette/freewheel. Most of the bikes that came my way had room to come in a little.
12-05-12 | 09:28 PM
#3
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Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
if you were only speaking of 1.4mm change on the drive side that would be fine.
But reading your post I suspect you have the wrong units and your measurements are actually in cm. If that's right, 1.4cm or 14mm is way too much difference, and you need to find a better match.
But reading your post I suspect you have the wrong units and your measurements are actually in cm. If that's right, 1.4cm or 14mm is way too much difference, and you need to find a better match.
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An ounce of diagnosis is worth a pound of cure.
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FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
12-06-12 | 08:05 AM
#4
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From: Florida
Bikes: Diamondback Parkway, Specialized Sirrus Sport, 1980 Concord Freedom Deluxe, Early 80s Schwinn LeTour, Trek 4600, Trek 360
Oops. Guess I haven't successfully migrated to the metric system. A 17.5 mm BB would be a short one indeed.
Probably better go back and get the 15.5 CM BB. Thanks FBinNY and ThermionicScott for the helpful responses.
EDIT: Man, I need to get my head right. Dimensions were actually 116.4 and 117.8 mm, so 1.4 mm difference looks OK.
Probably better go back and get the 15.5 CM BB. Thanks FBinNY and ThermionicScott for the helpful responses.
EDIT: Man, I need to get my head right. Dimensions were actually 116.4 and 117.8 mm, so 1.4 mm difference looks OK.
Last edited by alabamatx; 12-06-12 at 08:17 AM. Reason: error.
12-06-12 | 10:12 AM
#5
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Bikes: 1991 Bianchi Eros, 1964 Armstrong, 1988 Diamondback Ascent, 1988 Bianchi Premio, 1987 Bianchi Sport SX, 1980s Raleigh mixte (hers), All-City Space Horse (hers)
12-06-12 | 10:14 AM
#6
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From: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Man, it's a good thing you're not cutting down a fork.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
12-06-12 | 11:40 AM
#7
Senior Member
Joined: Aug 2011
Posts: 22,676
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From: CID
Bikes: 1991 Bianchi Eros, 1964 Armstrong, 1988 Diamondback Ascent, 1988 Bianchi Premio, 1987 Bianchi Sport SX, 1980s Raleigh mixte (hers), All-City Space Horse (hers)
Yeah, that operation is a little more critical. 
The measurements in the OP confused me, but I figured he was measuring the length of the axle outside of the cups or something like that, where 1.4mm would indeed be a small change.
The measurements in the OP confused me, but I figured he was measuring the length of the axle outside of the cups or something like that, where 1.4mm would indeed be a small change.
12-06-12 | 11:44 AM
#8
Senior Member
Joined: Apr 2009
Posts: 39,897
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From: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
I love to pick on folks who make these UOM errors, but in all fairness the good folks at NASA crashed a lander into Mars the same way.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
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