Technical FAQ: Side-loading bottom bracket bearings and riding 650b on a Lefty

Lennard Zinn talks riding 650b on a Lefty and tells a reader that he irreparably messed up his bottom bracket

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Editor’s Note: Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

Dear Lennard,
I removed my XT crank set yesterday (it is of the Hollowtech II type) while doing some bike maintenance. I got a bit carried away with the tightening of the plastic part that screws into the axle over the non drive-side arm. I know it should only be finger tight…what can I say… Now the bottom bracket bearings on the non drive side spin very tightly. I uninstalled and (now correctly) reinstalled the crank set, which alleviated the problem, but not fully. I guess I compressed something in the bearing assembly. Is there something I can do to fix this, or did I earn myself the honor of having to purchase a new bottom bracket?
— Santiago

Dear Santiago
The reason that screw is plastic and the tool to tighten it is just a plastic disc about the size of a poker chip is to prevent people from doing what you did. It is only there so that you can just snug the left crankarm up against the bearing and take the play out. Standard cartridge bearings like the ones in a bottom bracket cannot take being side loaded. Once you have done that to them, they will never be the same again. If you want the original performance back, you’ll need new bearings, which for most people means buying a new bottom bracket, although somebody with the right tools can pull the bearings out of the cups and press new ones in.
― Lennard

Dear Lennard,
Regarding your response to Dan’s question about purchasing a 2008 Specialized Stumpjumper comp 29er: I actually own that particular frame and can confirm that it’s a standard English threaded bottom bracket, not BB30.

Also, you might make Dan aware that this particular frame has had some brake vibration issues.
— Greg

Dear Lennard,
I was wondering if it would be possible to set up a Cannondale Lefty fork with a 650b wheel. I know that all Lefty forks can be converted between 26″ and 29″, so it wouldn’t it seem that a middle ground could be found between the two settings?
— Carson

Dear Carson,
There is absolutely no reason why that would not work. Obviously, it lowers the bottom bracket height, but less than switching to 26-inch would.
― Lennard

Dear Lennard,
You wrote:

Rather than having a quill going down into the steerer, Ritchey silver-soldered a 7/8″ stub of a tube into the top end of the steering tube so that it stuck out above the headset. The triangulated tubes of the Bullmoose bar met at a cylindrical clamp with a pinch bolt that clamped onto that stub.

It was very elegant, as well as being lighter and stiffer than a quill with an expander wedge. Tom Ritchey was far ahead of his time on that, as clamp-on stems did not come to the mainstream until threadless headsets arrived.

Although Tom Ritchey certainly has a lot of original ideas, this was pretty common on French randonneurs even before WWII.

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