Tech FAQ: Solving a Sloppy Spindle

Dear Lennard,
I have an issue and keep getting what I think are Band-Aid fixes to a simple issue even after a couple of weeks of trying to play nice.

I purchased a new Gary Fisher Super Fly-100 a few weeks ago and the non-drive side bearing/spindle interface is loose. Please see the measurements below. The bike shop did get me a new bearing kit, which was labeled for a Trek Madone. Same issue.

• Drive side spindle OD = 23.96mm
• Drive side bearing ID = 23.98mm
• Slip fit =  .04mm   [ .0015″]
• Non-drive spindle OD = 21.95mm
• Non-drive side bearing ID (old one) = 22.18mm
• Slip fit =  .23mm [ .0095″] “very loose”
• Non-drive side bearing ID (new one) = 22.17mm
• Slip fit =  .22mm  [ .0086″]  “very loose”
•The measurements for the new SRAM crank that came on the Superfly-100 are exactly the same on my SRAM crank from last year.

Can you help me? I think the non-drive bearing ID is too big and the bike shop and the on-line Trek support just tell me to tighten down the crank bolt more and/or add spacers to it until tight. It was tight when I got it, as I turned red in the face taking the crank arm off. If I add spacers this will only load the inner race and take more energy to pedal and wear out the bearing prematurely — maybe even on both bearings.
-Nick

Dear Nick,
I understand that the bearing is loose on your spindle, and I agree with you that it should fit tighter than yours does. To compare with your issue, I just measured a random sampling of GXP spindles and bearings we have here in my shop. We have hundreds of each, since we use that system on the custom cranks we make. Strange thing is that my measurements (though they may be suspect, as I took them with a digital caliper, not a micrometer), are not that different from yours. I seem to consistently come out with around 21.96mm on the left-side spindle OD and 22.16 on the left bearing ID. However, while I can rock any left bearing on the left-end bearing seat on any spindle, it will not wallow around in there like yours does on your video, once it is installed through both bearings.

Setting that aside, and even given that dramatic play your’s is exhibiting, I still don’t understand why it would be loose when riding, since the SRAM GXP system should pinch that inner race between the left arm and the shoulder on the left end of the spindle tightly enough that it should not move around. Something else must be wrong in the system.

Are you aware that some GXP bottom brackets have the same bearing left and right and have an aluminum sleeve with a shoulder on it that goes through the bearing ID instead of a smaller ID with a wider flange on the left bearing? You can see the sleeve sticking out of the bearing on these replacement bearings for GXP.

If your crank had been tightened to torque, the only explanations for looseness while riding is that the extra flange on the inboard side of the bearing’s inner race is too short, the splined hole in the left crankarm is undersized, or the splined end of the spindle is oversized; any one of these would cause the crank to tighten onto the spindle before coming in contact with the bearing’s inner race. That is why Trek suggested you add a spacer if tightening it tighter (just tighten it to suggested torque, by the way) did not remove the play.

And regarding adding the spacer that was suggested to you, the dire things you predict should not happen due to the GXP spindle design, as it has a shoulder that stops the flange extending inboard from the back of the left bearing’s inner race. Adding a spacer should not axially load the right bearing, since it would effectively make the spindle longer, no matter which side of the left bearing you put it on, thus moving the drive crankarm further from the bearing, not closer to it. Furthermore, the added spacer will not axially load the left bearing any more, since the spindle’s shoulder stops the bearing’s inner race, rather than causing it to be pushed inward relative to the outer race as the crank is tightened on, as other non-GXP systems would with such a spacer.
-Lennard

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

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

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