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Slight hop on wheel

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Slight hop on wheel

Old 03-20-11, 04:46 PM
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Slight hop on wheel

Hi All, I'm trying to true the wheel, but I can't get rid of a small hop. Honestly it doesn't effect ride, but it's now a quest. It seems easier, or I've been lucky, to repair/true a well out of true wheel than this one so I think it's something subtle I'm missing.

The wheel is a 700C and the hop is about 1/4-1/3" at midpoint and runs along the rim for about 8".

TIA, Brad
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Old 03-20-11, 05:14 PM
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Getting rid of hop, especially a low spot, is the hardest thing to do when aligning a tight wheel. Unlike wobble, hop requires the rim to move along the circumference because higher radius implies longer circumference. That means that when you bring a hop in the extra material needs to be spread along the entire circumference, against the resistance of the other spokes.

1/4" is quite a lot, and I wonder how it got there in the first place. Pulling it down will require staggering tension, and will also cause tension unbalance as the material tries to flow out. I suggest you carefully lower the overall tension by degrees, working about 1/4 turn at a time, until you're at about half tension. Then you can work on the hop, getting the wheel round, and tension fairly even before reloading back to normal tension by degrees.
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Old 03-20-11, 05:48 PM
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That's a tacoed rim. Unlace the rim. Repair the rim so that it is true to within +/- 1/16". Re-lace the rim. You're not going to be able to fix a 1/4" hop by messing around with a few spokes and expect the wheel to stay true.
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Old 03-20-11, 06:07 PM
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FB, This is a rim that came with a used bike, I don't know how the hop worked it's way in either, but it is a rim that looks to have considerable life left in it. The excessive tension was the primary reason I posted as I thought it could be avoided with a better plan. I'll start backing off all of the spoke's tension and see where that goes.

furball, Once the tension is released we'll find out the rim's condition.

Thanks for the help.

Brad
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Old 03-20-11, 06:25 PM
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On most rims the strength of the rim is far less than the forces generated by the spokes, so the condition of the unspoked rim can be compensated in the build. It's the spoking that determines the final roundness.

Obviously that needs to be qualified. Rims come in a variety of weights and shapes, and for example a deep rim will be far less amenable to the spoke forces a in radial direction than a shallow one. Likewise wider rims resist the spokes more in terms of wobble. Also, long gentle distortions are less of a problem than local ones.

Back in the dark ages of light squirrely rims, most had distortion at the joint and you'd get a quick low, high,low zone (or the other way around) We considered a wheel aligned when the whole wheel was better than the joint zone.

On your wheel you have no way of knowing if hop is in the rim itself or, as I suspect, is imposed on it by the spokes. Easing the tension will help give you an idea, plus give the rim some freedom to flow. There's no need, and little to gain by taking it apart entirely.
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Old 03-20-11, 08:52 PM
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FB, The hop is 90 d. from the joint or valve stem hole. Looks like someone has had a go at it in the past. I have to put it aside for a couple of days and I'll update the thread.

Brad
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Old 03-29-11, 06:56 AM
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Update

Hi All, Well, I found the problem. The rim had been damaged, looks like a head-on curb strike and was repaired, actually very well with only the slightest dip in the braking surface and a slight .2 mm decrease in circumference for about 20 mm. There are no cracks or other stress related damage.

When checking runout using the inside of the rim there was actually a very slight dip, which gave the appearance of a hop as the rim returned to normal. Very little work was required to true the wheel.

Brad
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