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NOTE - please read update below and all comments.

I have a Mavic Cosmic carbon wheel which is perfectly true, but it has some vertical hop (3-4 mm). I thought at first that it was out of round, but deep carbon rims are very strong and rigid, and I don't see how it's possible for it to bend.

I did a test to check the roundness of the wheel - I measured the radius at different points around the wheel (from the hub to the outside of the rim). It was 306 mm all around. So the wheel is round, right?

I took the quick-release axle out and rolled it along a flat surface - I noticed that it is slightly bent, maybe 1 mm max deviation. I guess this is at least partially contributing to the hop, but I don't think it fully explains it. I will replace the axle - hopefully that corrects the issue, but if not, what else could be contributing to it?

UPDATE

I've replaced the quick-release skewer. The wheel still hops, so at least it's eliminated that possibility.

Here's a video showing the hop. https://imgur.com/a/icMfN7q

I've also noticed there is a slight raised area at the edge of the wheel at one point. This is on both sides, like the wheel edge is slightly "flared" at that point. This is very slight but enough to alter the braking force at that point - I can feel pulsing through the brakes as the friction increases at this point. Looking at the wheel rotating again, it seems to pull in sharply at this point. So it's like the opposite of hop - what's that, dip? This causes the excessive rubbing and you can see the brake marks below:

enter image description here

  • Get a new axle. Should be easy to buy and nothing exotic. Mavic isn't exactly the smallest back-yard company. – Carel May 7 at 19:19
  • @Carel It's a Novatec axle, but yeah I will get it replaced. I should clarify, when I say axle I mean the Quick-Release rod. – Kev May 7 at 19:23
  • quick release rods are generally called skewers if this helps to disambiguate the question – Swifty May 7 at 20:48
  • Is the bead seated evenly all the way around? This seems like a long shot, but you appear to have dealt with the likely causes. – DavidW May 8 at 20:58
  • I would note that when I notice a "hop" while riding the bike the first thing I check is that the tire is evenly seated all around. Especially check the area near the valve, as the tube tends to get caught under the tire beads there and cause this problem. – Daniel R Hicks May 24 at 16:37
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By 'hop' I assume you mean radial run-out (as opposed to lateral run-out).

A bent axle will have no effect on the rim running radially true or not, because the hub rotates around the static axle. A slightly bent quick release shaft will just conform to the bore through the axle and have no effect.

A rim can be perfectly circular but still have radial run-out - if the hub axis is not at the center of the circle. That may be what is going on here. In any case the solution is to have the wheel re-trued.

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  • Note that Kev said "I measured the radius at different points around the wheel (from the hub to the outside of the rim). It was 306 mm all around," which means it's much less likely the case that the hub is not centred. – DavidW May 8 at 20:57
  • Yes, radial is what I meant. – Kev May 24 at 16:32

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