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V10 wobble at speed caused by replacement tube


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I've got a V10 that I bought used and it broke.  I thought it was a bad motherboard so I replaced that (oops, try again) to no avail.  I asked the guy I bought it from to fix it and he was successful.  Turned out to be a failed cotter key on the axle shaft, which allowed the axle to spin and damage the wires coming out of the motor.   Anyway, while we were at it, I suggested he replace the tire and tube with a new one. So after replacing the motor, tire, tube and (for no good reason) motherboard, we got it back.  This was winter and months after it died originally so we went slow at first.  But as we got comfortable again and went faster, my son and I noticed a wobble at speed. My background includes many years fixing bicycles so I recognized this as an issue with a tire or tube, both of which were new. 

It turns out that the replacement tube he got had a slightly different bend to the valve stem  than what we had originally. He mentioned this to me when I got it back from him but it seemed to work well enough. But when we took it apart, we saw the difference.  The original tube has a brass valve stem and it is bent slightly more and turned to the side about 10 degrees, compared to the replacement. To get the replacement to fit in the rim and lay flat, so the housing doesn't hit it, it needed to be slightly twisted in the tire. This worked initially, but that slight twist causes a different deflection characteristic at the valve area and it would cause a wobble when that part of the tire was against the ground. At speed, it got worse.  A similar thing could happen when a tire is not seated properly but in this case, I believe because it caused a different deflection resistance on one side of the tire compared to the other, it got it to wobble sideways.  I doubt it could have caused a crash on it's own, but it sure did make us slow down and question it's safety. It had to be fixed. Oh, and the tire was seated just fine. 

In the end, we replaced the tube with the old tube and the wobble was gone.  

I just share this in case others have an issue with a mystery wobble. 



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