Old 01-22-22, 07:00 AM
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Frkl
Must be symmetrical
 
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Installing an Italian ITA cartridge bottom bracket "backwards"

Hello,I know there is an older thread about this topic started by Landgolier , but it would be great to hear any long term or new experiences with doing this.

The situation:
I purchased an Italian cartridge bottom bracket that turned out to be asymmetrical because, wait for it, it was actually a BSA cartridge pressed into Italian cups. The cups, however, were not machined in a way that held the cartridge symmetrically in the frame (ie to account for the different width of the BB shell). The result was a shift to the fixed, drive side, with the removable cup threading deep into the shell before making contact.

It was hard to find the right spindle length for a reasonable price in the first place, and everything is sparse in the spare parts dept right now, so I was hesitant to just return it. So I tried to thread it in "backwards" with the fixed side on the non-drive side. This was possible, of course, because the threads on both sides of an ITA bottom bracket run in the normal direction. I wasn't worried having the removable cup on the drive side from a strength perspective--Shimano (maybe others too?) makes (or at least made) bottom brackets with the fixed side on different sides of the shell. I had one such unit on a bike for years with no problem.

The result was actually perfect, and it meant that I could get rid of a 1.5mm spacer I usually needed on this crank set to get the 1x chain line the way I like it (biased inboard because I do a lot of climbing).

The older thread also mentions a possible side benefit, too, in that this might reduce the tendency of ITA brackets to unscrew due to precession. It makes theoretical sense to me, and I am wondering if anyone has real world experience. To wit, precession forces are going to try to turn the cartridge unit counter clockwise from the perspective of the drive side. This usually unscrews Italian bottom brackets with the fixed cup on the drive side. However, if the cartridge is installed with the fixed cup (which is part of the cartridge unit) on the non drive side, the same precession forces will turn the fixed cup clockwise from the perspective of the non-drive side, thus tightening the fixed cup's flange onto the frame. This should dramatically slow the precessional rotation. The precession forces acting on the removable cup on the drive side come from the cartridge body itself (ie, clockwise axle rotation causes the cartridge body to rotate counter clockwise, which exerts a counter clockwise force on the removable cup). However, since precession of the cartridge body is slowed/stopped by the fixed cup's flange, the precession of the removable cup might also be slowed/stopped.

I don't think the same would apply to a classic non-cartridge ITA bracket, since the above relies on the fact that the cartridge is a unit that connects drive and non drive sides, forcing them to rotate as a single object. Traditional cups can rotate independently.

Last edited by Frkl; 01-22-22 at 01:18 PM.
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