shimmy comes and goes - why

[BobM3]

While recently riding on the interstate at about 70 mph I noticed I had some steering wheel shimmy. I figure the tires/wheels must need balancing. But the shimmy would be there for a bit then go away for a bit - maybe even periodic. I'm trying to figure out why. My guess is that at 70 mph I might be near the wheel-hop natural frequency. The out-of-balance forcing function might be running at nearly the same frequency. The slight difference between the forcing function frequency and the wheel-hop natural frequency is causing a modulation to appear at low frequencies. Does this make sense to you guys? Other possibilities?

 

[GregLocock]

It is very common. You were running at wheelhop. Your tires have slightly different diameters, and the road has curves. Consequently the wheels do not rotate at exactly the same speed so the wheels speed force they generate is going in and out of phase.

OOB is not the only cause of wheelhop excitation, tire force variation does the same, and cannot be cured by balancing. OTOH TFV remains pretty constant over the life of the tire, so if it was OK originally it probably wasn't that.

Oh and then there's flatspotting...

Cheers

Greg Locock

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