Cause and effect...

[Jon M.]

I'd bet it would be something called resonance frequency when things have the potential to vibrate in sync thereby amplifying the vibrations, rather than things vibrating out of sync and either being too small to feel by themselves, or by being out of sync actually cancelling certain vibrations out. Think noise-cancelling headphones (but the reverse at 50-60).

I would bet that you would feel the same vibration come back between 100-110. I know I did when I had wheel balancing problems and a slight shimmy between 50-60.

[sbcncsu]

I'd bet it would be something called resonance frequency when things have the potential to vibrate in sync thereby amplifying the vibrations, rather than things vibrating out of sync and either being too small to feel by themselves, or by being out of sync actually cancelling certain vibrations out. Think noise-cancelling headphones (but the reverse at 50-60).

I would bet that you would feel the same vibration come back between 100-110. I know I did when I had wheel balancing problems and a slight shimmy between 50-60.

Okay, the resonant frequency on these cars is set from the factory with the:

unsprung mass: mass of the control arms, the mass of the wheels and tires, mass of the knuckle and the mass of the strut assy.

spring rates: Coil spring rate, modulus of elasticity of the arms, the swaybar spring rate

Damping: Strut damping, damping effect of the bushings due to elasticity

If you turn this into a pure vibration problem from engineering, changing any one or a combination of these things will change the resonant frequency and/or amplitude of the vibration. That would explain why changing the thrust arm bushings would improve the "shimmy" as the frequency would have changed and its effect (amplitude) would be dampened by the bushings.

For those that have changed to light weight forged wheels, have you seen any change in the speed at which shimmy is observed?

What about those of you that have gone to larger wheels that are of the cast variety and are heavier? Have you seen a change in the speed where shimmy occurs?

[NoSpeedLimits]

Yah, I too, am curious to hear from folks who have altered their wheel type.

I have noticed that the shimmy comes and goes. IOW, some days it is more noticeable than others which has me confused. It appears that a few things play a role: the ambient temp., the duration the vehicle has been parked (indoor and outdoor), tires temp. Overall, I would say that the shimmy seems to be less noticeable when the tires are warm which leads me to believe that the stiffness of the side walls (ie. tire suspension) are effecting the cars resonant frequency. Perhaps the slight uneven ware on the tires is not playing a role. I wonder if anyone else has observed their shimmy being effected a similar manner?

[AllanS]

When I first get in and drive to work in the mornings, there's very little noticeable shimmy. But when I go out later in the day, or on extended trips, it gets more noticeable. But it's like you say- comes and goes.

My wheels are 17" Breyton Magics- I'm not sure about their weight, but the shimmy occurs as it did on my 525i, with stock 15" wheels.

[NoSpeedLimits]

Negative on the reoccurrence speed. I gave her two runs up to buck twenty this morning and there was no noticeable shimmy. Actually, she drove nice and stable at triple digit speeds. So, what does this mean or suggest? Dunno? Typically, for me, the shimmy tends to be at it's worst in the morning. The shimmy was unusually noticeable at about 53 MPH this morning at 4:00 AM -- it's great time of day for testing So, I figured the shimmy would reoccur around 106 MPH. No such proof was evident.

Anyway, I am not sure what I am going to gain from this discussion, but I enjoy attempting to make sense of a confusing situation. Plus, it's always nice to know others are having similar experiences. One of these days I hope to totally R&D my suspension, but for now, I will half to live with the occasional shimmy experience. If anyone else has any (and I do mean any) thoughts on why the shimmy occurs between 50 and 60 MPH, I would love to hear them.