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Radial rim imperfections

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Radial rim imperfections

Old 06-15-13, 03:49 PM
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Radial rim imperfections

I replaced the hub in my rear MTB wheel so I could use 9 speed cassettes. The rim is Rigida.

I re-laced the wheel keeping the original spokes, also keeping them in the same order. For truing I used one of those McDonald's coffee stirrers, paper clip and some superglue.

For lateral truing I can get it to within 0.5mm or so.

For radial truing I can get it to 1mm roughly. However when looking at the rim when it spins, even slowly, I get the feeling that the rim radial rim thickness (distance between rim inner and outer diameter) is not even. Is this a possibility? Or do I just need to true a bit more?

If there is a discrepancy in rim radial rim thickness should I be using the internal rim edge for radial truing?
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Old 06-15-13, 04:49 PM
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When the rim left the factory, the height was perfectly uniform, except possibly at the joint. Whether to use the inner or outer surface is a judgement call. If it all looks about the same, but the outer edge is locally a bit off near the joint, either from filing on a welded rim, or distortion on a pinned rim, you need to make a judgement where the true circle is.

I never try to use the inner section because rims have curves there and its hard to get true reading on the true position. In theory, you could use the edge of the inner wall right at the bead seat, since this is what counts, but that's too much work.

My method is to use the outer edge for everything, but if I suspect it's low because of fuiling at the joint, I watch the inner wall as it spins to see if the whole rim is moving in, or it's just a filing issue.

BTW- the sevret to getting good radial true, is to do so very early in the process as the wheel first gets tight. Get radial spot on and lateral decent, then add tension fine tuning the radial if it moves at intervals. By the near end, as the wheel is coming together and you're fine tuning diah and lateral true, radial should be spot on and stable there. Fine tuning radial on a tight wheel is difficult so you want to avoid that as much as possible.
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Old 06-16-13, 05:56 AM
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My bet is the imperfection you are speaking of is exactly opposite the valve hole at the rim seam. That's common.
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Old 06-16-13, 06:12 AM
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Thanks for the very valuable tip. I only wish I knew this before I jumped into rebuilding the wheel as the spokes are already kinda tight by now. I will definitely try out that method the next time I (re)build a wheel.
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Old 06-16-13, 06:21 AM
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I true to the outer edge of the rim. That's where the tire seats, and it's easier for me to see it. I ignore the distortion caused by the joint, and I bring the tension up enough to cause a little dip at every drive side spoke hole.
There can definitely be enough tension to distort the rim a little, especially from the drive side spokes. You can see that from the uneven wear pattern that develops on the brake track after a couple of hundred miles.
OTOH, if you can see a distortion in the rim cross section with your naked eye, maybe you have some cracks or other imperfections somewhere. The extrusions are very consistent when they leave the die, so any change in shape indicates some kind of damage.

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Old 06-16-13, 09:30 AM
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Originally Posted by rhlee

I re-laced the wheel keeping the original spokes, also keeping them in the same order. For truing I used one of those McDonald's coffee stirrers, paper clip and some superglue.
There's your problem- you need to be using a Starbucks stirrer.

Seriously however- If you're certain the rim is totally round and you induced the run-out initially, go back and start over. My guess however, since the rim is used, it's either been banged up or there is a low spot at the joint. You'll need to make that call since you have it in front of you. Short of an attempt to straighten an out of round rim, you could live with it. 1mm run-out on the radial (or a dip), though it could be better, ain't all that bad- 'specially on a used rim. If it bugs you find the high spot on the tire and mount it to the low spot on the rim. Then again it's a mountain bike. You'll not feel that little run-out, or a dip.
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Old 06-17-13, 08:47 AM
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Even though people mentioned the distortion at the join. There was one there, but I easily straightened that out.

Originally Posted by eddy m
There can definitely be enough tension to distort the rim a little, especially from the drive side spokes.
I think what eddy m says here is what I'm having. When I inspect the the distortions closer, they seem to be inline with the spokes. I didn't think this would happen. I generally thought the deflection caused by spoke tension would be smoothed out by the rigidity of the rim.

Another wierd thing I noticed was that these distortion caused the rim to be pulled in in-between the spokes rather than at the pokes.
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