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Adding spacers onto a crankset with a square taper

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Adding spacers onto a crankset with a square taper

Old 07-28-20, 06:55 AM
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Egglesworth1
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Adding spacers onto a crankset with a square taper

I bought a new crankset as my old one was busted, but when I put it on, I realised that it was too close to the frame to be able to get into first gear. Is there a way to add spacers onto this even though it has a square taper?
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Old 07-28-20, 07:01 AM
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Originally Posted by Egglesworth1 View Post
I bought a new crankset as my old one was busted, but when I put it on, I realised that it was too close to the frame to be able to get into first gear. Is there a way to add spacers onto this even though it has a square taper?
you can’t add spacers to the crank itself - it has to engage the BB taper. However, you might be able to shift the BB itself to the right by a few mm, depending on the BB design. However, cranks generally require a specific BB spindle length, so unless you installed an identical replacement crank, you’ll likely have to revisit BB length
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Old 07-28-20, 07:02 AM
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No, you will need a bottom bracket with a longer spindle.
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Old 07-28-20, 08:50 AM
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I'm a little confused as to how the crank being too close to the frame keeps it from going into 1st gear. I understand that if the crank is hitting the frame that the crank can't make a complete rotation...and thereby be unable to switch gears. Is THAT what's keeping it from going into 1st? I mean just the fact that the crank can't completely rotate doesn't otherwise impact the chain being able to go onto the various gears. Otherwise...as indicated above...you need to either get a BB with a wider spindle, or cranks with an angle that will prevent it from hitting the frame.

Dan
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Old 07-28-20, 08:58 AM
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Originally Posted by _ForceD_ View Post
I'm a little confused as to how the crank being too close to the frame keeps it from going into 1st gear. I understand that if the crank is hitting the frame that the crank can't make a complete rotation...and thereby be unable to switch gears. Is THAT what's keeping it from going into 1st? I mean just the fact that the crank can't completely rotate doesn't otherwise impact the chain being able to go onto the various gears. Otherwise...as indicated above...you need to either get a BB with a wider spindle, or cranks with an angle that will prevent it from hitting the frame.

Dan
I think he's saying that his FD can't shift to the small chainring because it (the chainring) is now too far inboard.
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Old 07-28-20, 09:32 AM
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Originally Posted by Litespud View Post
I think he's saying that his FD can't shift to the small chainring because it (the chainring) is now too far inboard.
Yep, that's exactly right
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Old 07-28-20, 09:35 AM
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Thanks to everyone for the replies, I think I'll try and get a replacement bottom bracket (ideally with the UK governments bike voucher!)
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Old 07-28-20, 10:13 AM
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It is possible to shim the crank flats. Use thin aluminum sheet. I've used Coke cans and .007" sheet. Cut two "bow ties" with the bows being thaper of the spindle, then coming across the top of the spindle (ie the knot part of the tie, then a similar taper to match the opposing flat., Bend each shim into hard cornered "U"s so that the shim fits tight ti the spindle and both flats and there is no excess at the spindle end. Cross the two shims so all four tapers are covered. Now slide the crank on. Perhaps tap the crank a few times with a padded mallet. Punch a hole in the shims at the crank bolt hole and very gently dill the shim out of the bolt. Insert bolt and tighten as usual.

Properly done, the shims last until you take off the cranks. I always make new ones for the re-install.

I just did the math. Crank shim distance = shim thickness / sin (2 degrees) = shim thickness / 0.0349 = 29 X shim thickness. Yes, thin shims do quite a lot, hence the Coke cans. THis jibes with my experience, The .007" sheet made a huge difference; a whole new spindle length. Coke cans make a good adjustment.

Edit: No one mentioned the two tapers. Bottom bracket spindle come in ISO and JIS tapers. Both use 2 degree taper but he difference is how thick they are. The difference makes a 4.5 mm difference on where the cranks sits. Lifted from Sheldon Brown's excellent website "If you install an ISO crank on a J.I.S. spindle, it will sit about 4.5 mm farther out than it would on an ISO spindle of the same length." (https://sheldonbrown.com/bbtaper.html)

Ben

Last edited by 79pmooney; 07-28-20 at 10:23 AM.
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Old 07-29-20, 08:19 AM
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Originally Posted by Litespud View Post
I think he's saying that his FD can't shift to the small chainring because it (the chainring) is now too far inboard.
Wondering if the "L" limit screw is, or can be adjusted.

Dan
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Old 07-30-20, 04:05 AM
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It's fairly normal to shim the BB cups to adjust the left and right crank arms to get them symmetric. One thing you could do is to measure your front chain line and crank symmetry first before choosing a new BB spindle length. Also note that Shimano BB of different lengths have different symmetry.
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Old 10-03-22, 08:55 PM
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Beer can BB shims

I used a modified beverage can shim method, simply making four tiny shims that cover the flats and bend 90 degrees over the tip of the spindle by about 1 mm, enough to hold them in place while the crank is placed into position, but not enough to block the threads. I used a drop of bearing grease to adhere them to the spindle long enough to get the crank pushed on, and tightened it up. There's still plenty of thread engagement on the fixing bolt. This gained me 3mm in "spindle length" and will suffice for this occasionally ridden vintage bike until I can acquire a 68 x 127.5mm spindle. In truth, my DIY 125.5mm spindle pushed the inner chain ring to a safe distance away from the chain stay and, as a bonus, set the chain line pretty spot on. I doubt that 127.5mm spindle will ever get ordered.
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Old 10-03-22, 10:38 PM
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First of all, I'm confused by the premise.

Generally we want cranks inboard as much as possible, with typical chainstay clearance of only a few millimeters.

You reference problems shifting to 1st gear. Are we talking FD shift to inner chainring, or something in back?

As mentioned there are workarounds to move the crank 1-3mm, but usually no more (certain BBs are an exception). Or if you have a classic cup/spindle BB, there are plenty of alternate spindles to choose from.

But before looking at ways to move the crank, think about exactly WHY you need to move it, and if there are easier ways to solve that problem.
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Old 10-04-22, 06:50 AM
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The point was that the inner chain ring was so far inboard that the derailleur "bottomed out" on the seat tube before it carried the chain to the ring. Having the chain ring(s) as far inboard as possible while clearing the chain stay is not the goal -- clearing the chain stay, getting the Q-factor right, and getting the chain line right are the goals. Sometimes Q-factor and chain line cannot both be optimized.

Last edited by dy60; 10-04-22 at 06:51 AM. Reason: Added thought
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Old 10-04-22, 10:30 AM
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Originally Posted by 79pmooney View Post
Edit: No one mentioned the two tapers. Bottom bracket spindle come in ISO and JIS tapers. Both use 2 degree taper but he difference is how thick they are. The difference makes a 4.5 mm difference on where the cranks sits. Lifted from Sheldon Brown's excellent website "If you install an ISO crank on a J.I.S. spindle, it will sit about 4.5 mm farther out than it would on an ISO spindle of the same length." (https://sheldonbrown.com/bbtaper.html)

Ben
For clarity: the Rene Herse site says it moves each crank arm by between 2mm and 2.5mm.

Both are credible sources of info. Rene Herse and the late Sheldon Brown are consistent if Sheldon Brown was quoting the total difference in spindle length required for interchange, which would be divided between the two crank arms.

I believe that to be the case, but I haven't tried mounting the same crank on both to verify.

Last edited by Hondo6; 10-05-22 at 05:05 AM.
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