Bizarre Bottom Bracket Baffles Bad Bicyclist
Blliteration aside,
Are bottom brackets self-centering? I've got a 68mm BB (110mm spindle) that seems to be off-center on my '02 MTB (with 68mm BB shell). I think I have installed it correctly...the fixed cup (DS) is screwed in as far as it can go, such that the flange is pressing against the side of the BB shell. The free cup is inserted as far as it can go (with a couple of threads showing). The BB is square taper. The gap between crankarm and chainstay is larger on the NDS by about 2-5mm. I swapped the crankarms and placed them in various positions on the spindle to rule them out as a source of the problem...in all cases, the gap was larger on the non-drive side. Assuming the frame is symmetrical (it appears to be, and is in good condition), is this kind of discrepancy acceptable/expected? Have I installed the BB wrong? Am I doing something else wrong? Thoughts are appreciated, Dave |
Bizarre Bottom Bracket Baffles Bad Bicyclist
Borrall Wonnell wrote:
Blliteration aside, Are bottom brackets self-centering? I've got a 68mm BB (110mm spindle) that seems to be off-center on my '02 MTB (with 68mm BB shell). I think I have installed it correctly...the fixed cup (DS) is screwed in as far as it can go, such that the flange is pressing against the side of the BB shell. The free cup is inserted as far as it can go (with a couple of threads showing). The BB is square taper. The gap between crankarm and chainstay is larger on the NDS by about 2-5mm. I swapped the crankarms and placed them in various positions on the spindle to rule them out as a source of the problem...in all cases, the gap was larger on the non-drive side. Assuming the frame is symmetrical (it appears to be, and is in good condition), is this kind of discrepancy acceptable/expected? Have I installed the BB wrong? Am I doing something else wrong? Thoughts are appreciated, Dave Sounds like you need to reverse the spindle to me. Most spindles are asymmetric, with one side (normally the drive side) longer. If this is one of those cartridge BBs for which the fixed cup cannot be removed or moved to the other side of the cartridge, then it sounds like your spindle is inappropriate for your crankset. Mark |
Bizarre Bottom Bracket Baffles Bad Bicyclist
On 2 Jul 2006 19:28:17 -0700, "Borrall Wonnell"
wrote: Blliteration aside, Are bottom brackets self-centering? Yes and no. Most do not provide for lateral position adjustment, which is what I think you're looking for. I've got a 68mm BB (110mm spindle) that seems to be off-center on my '02 MTB (with 68mm BB shell). I think I have installed it correctly...the fixed cup (DS) is screwed in as far as it can go, such that the flange is pressing against the side of the BB shell. The free cup is inserted as far as it can go (with a couple of threads showing). The BB is square taper. By any chance, was it a Shimano unit that had an unthreaded shoulder about 2mm wide next to the right flange? There's one design of UN-series BB that I've encountered which is intended specifically for use with BB-mounted front ders; using it with a tube-mounted fder will cause the cranks to be 2mm farther to the left than normal for the spindle length. The gap between crankarm and chainstay is larger on the NDS by about 2-5mm. I swapped the crankarms and placed them in various positions on the spindle to rule them out as a source of the problem...in all cases, the gap was larger on the non-drive side. Assuming the frame is symmetrical (it appears to be, and is in good condition), is this kind of discrepancy acceptable/expected? Have I installed the BB wrong? Am I doing something else wrong? I hink the BB that caused this problem for me was a UN52 or UN53; I've swapped over to the cheaper UN25 (and competitive units) as a result. I will note, however, that as long as the sprockets don't hit the chainstay, there's no real problem with having the cranks off center by an amount this small. -- Typoes are a feature, not a bug. Some gardening required to reply via email. Words processed in a facility that contains nuts. |
Bizarre Bottom Bracket Baffles Bad Bicyclist
I hink the BB that caused this problem for me was a UN52 or UN53; I've swapped over to the cheaper UN25 (and competitive units) as a result. The BB I am using is a generic Chinhaur (or something like that). After my post last night, I swapped it out for a new Shimano UN52 of the same specs (68/110)...and found the same problem. Hmmm...maybe I'll ask my LBS for a sample of the UN25. I will note, however, that as long as the sprockets don't hit the chainstay, there's no real problem with having the cranks off center by an amount this small. Part of the reason I'm concerned about the difference is that I plan on 'upgrading' my crankset to a 48-36-26 (still a Shimano crank w/square taper)....and I am a bit concerned about clearance between the frame and large ring. I'll probaby change the BB to a 113mm (still within chainline recommendations). Cheers, Dave |
Bizarre Bottom Bracket Baffles Bad Bicyclist
Borrall Wonnell wrote:
Blliteration aside, Are bottom brackets self-centering? I've got a 68mm BB (110mm spindle) that seems to be off-center on my '02 MTB (with 68mm BB shell). I think I have installed it correctly...the fixed cup (DS) is screwed in as far as it can go, such that the flange is pressing against the side of the BB shell. The free cup is inserted as far as it can go (with a couple of threads showing). The BB is square taper. The gap between crankarm and chainstay is larger on the NDS by about 2-5mm. I swapped the crankarms and placed them in various positions on the spindle to rule them out as a source of the problem...in all cases, the gap was larger on the non-drive side. Assuming the frame is symmetrical (it appears to be, and is in good condition), is this kind of discrepancy acceptable/expected? Have I installed the BB wrong? Am I doing something else wrong? Thoughts are appreciated, Dave I had the exact same thing after I changed the chainset on my bike. Different chainsets may need different spindle lengths. My spindle length was originally 110mm and the NDS (left) appeared to be approximately 4mm further out from the frame than the other (right) side. I then got the next length down, 107mm, and everything is just about OK. If you want some proof, then take your BB down to your LBS and compare it to a number of different sizes. You will probably find that the amount that the drive side spindle protrudes from the BB shell is the same for different lengths. That was my experience with Shimano BB's anyway. Hope that helps. |
Bizarre Bottom Bracket Baffles Bad Bicyclist
Borrall Wonnell says...
Blliteration aside, Are bottom brackets self-centering? I've got a 68mm BB (110mm spindle) that seems to be off-center on my '02 MTB (with 68mm BB shell). I think I have installed it correctly...the fixed cup (DS) is screwed in as far as it can go, such that the flange is pressing against the side of the BB shell. The free cup is inserted as far as it can go (with a couple of threads showing). The BB is square taper. The gap between crankarm and chainstay is larger on the NDS by about 2-5mm. I swapped the crankarms and placed them in various positions on the spindle to rule them out as a source of the problem...in all cases, the gap was larger on the non-drive side. Assuming the frame is symmetrical (it appears to be, and is in good condition), is this kind of discrepancy acceptable/expected? Have I installed the BB wrong? Am I doing something else wrong? Thoughts are appreciated, Dave An Isis BB I installed recently had a spacer and an o-ring that were supposed to be used or left off depending on your BB shell width and desired chainline. The o-ring is only there to cover up the gap on the non-drive side cup because it can't be fully inserted on a 68mm shell. The spacer was for the drive side 68mm shell. With a 73mm shell, both were left off. The new outboard bearing cranksets all have spacers on the drive side for MTB models with a 68mm shell and I believe MTB's have the drive side crankarm further from the BB than the non-drive side by about 5mm to accommodate triple chainrings and wide, thick chainstays to clear wide tires. For Isis and Octalink, MTB BB's have spindle widths of about 113mm and narrower 108mm spindles for road doubles. There are other typical spindle widths for road triples and downhill MTB's. The only way to know for sure the correct spindle width is to know the manufacturer specs, because some cranksets don't follow the typical patterns. Are you sure you have the correct spindle width? Were there spacers included that maybe you should have used? |
Bizarre Bottom Bracket Baffles Bad Bicyclist
The
only way to know for sure the correct spindle width is to know the manufacturer specs, because some cranksets don't follow the typical patterns. Are you sure you have the correct spindle width? Were there spacers included that maybe you should have used? I wasn't aware of any of this before I took the crankset apart. According to Sheldon, the recommended BB spindle length for my crankset is 110-113mm. I measured the BB and was able to physically confirm the 110mm length. The spindle is the same length on either side of the BB shell (about 21mm). No spacers were included...this is the original crankset, unfortunately I never examined the spacing before stripping the frame. I just measured the gap between the crankarm and chainstay on both sides of the frame, from the center of the pedal spindle. The crankarms are roughly parallel with the chainstay at this point. Here's what I came up with: DS: 6 mm gap NDS: 13 mm gap!! Wow, a 7mm difference....I never thought it was that large. Even worse...I swapped the crankarms around so that the chainrings were on the NDS and re-measured. This time, the gaps were 8mm and 11mm (DS, NDS respectively). Arggh! Ok. Obviously my crankarms aren't perfectly identical, otherwise I would have had the same gap measurement after switching them around. Something else must be fishy. My next guess...the square taper is slightly larger on the NDS, and as a result the crankarm cannot be installed as close to the frame as on the DS. I don't think I can bend the crankarms (and even if I could, it wouldn't be accurate). I *could* try filing down the inside of the NDS crankarm to make it slide down the tapered spindle a little further. Alternately, I could try filing down the spindle itself. I'm not a fan of either approach. The 7mm gap isn't a dealbreaker for me...I'm just wondering why the gap exists, and if it is a common 'problem' for square taper designs. Cheers, Dave |
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