Cause and effect...

[NoSpeedLimits]

I was hoping someone could explain or give me their theory on why the frequently discussed 50-60 MPH shimmy occurs only between 50-60 MPH.

I have read many archives and have not been able to dig up any info that would clarify the effects of the cause. I, too, have recently replaced my thrust arms, got the tires rebalance, and a four-wheel alignment only to still be plagued with the infamous shimmy. Although, I must admit the medicine I gave my bimmer has definitely helped. And, you should know that as a result of being cheap, I elected to pass on the alignment when I purchased my last set of tires, I have worn the tires unevenly. Therefor, I realize my hopes of eliminating the shimmy are not going to be resolved until I buck up for some new tires or the tires manage to wear evenly from daily trips to work – not sure if that’s possible. Now, if I only had one of those Quaife Differentials, then I could help the wear along with a few burnouts. :) Again, I am being cheap (more like stupid) because letting go of a new set of tire with less than 10K miles ails me to death. Truth be told, (don’t want you to think I am stupid) I would have new rims and tires on her tomorrow (and the differential) if the wife was more financially secure. But, that's another story....

I am not looking for suggestions on how to fix my shimmy problem cause there is plenty of threads archived that have covered those possibilities. I am turning to you for understanding of cause and effect. I know about the many things that can cause the shimmy, but why does the effect only surface between 50 and 60 miles per hour?

[Tiger]

My theory is this... 50-60MPH is probably the midpoints of the vibration scale where it would be the most pronounced and any speed lower or higher will balanced themselves out. The cause is probably the tire itself because the tire are not perfectly round itself... it is close but not perfect and daily driving will also make it heavier here and there... etc.

It is rubber so that is why it can be round enough to drive and not feel that imperfections.

On ther other hand, you can have your tire shaved for nominal cost to retrue your 'roundness'... look for speed shop who are equipped to do this.

[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.

[632 Regal]

ditto dead nuts on!

[NoSpeedLimits]

Thanks for the responses...resonance frequency makes sense.

It's too bad that the things vibrate in sync between 50 and 60...a speed that I find most other travels love to endure on their daily commuters. Slow-down or pass... decisions, decisions... unfortunately, the later has not been the correct choice.

When conditions permit, I will try to test the reoccurrence speed. I am currently tiptoeing around due to back to back speeding tickets (nothing to write home about, they were bad ones...lets just say I hate 25 MPH speed zones and local cops picking on America’s working class). Anyway, I need to stay clean for at least a year or risk accumulating too many points and loosing my license. So, it may take a few days before I find an opportunity to hold it to the floor and report back with my findings.

In addition, thanks tiger, I will look around for local shaving...it can’t hurt. :)

[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.

[mike wong]

Jk ;)

[NoSpeedLimits]

Easier said then done. As I have come to learn, without make the appropriate mods, the engineers at BMW have made it very difficult to brake 'em loose. Although, I try when ever the road conditions permit... ie. I find a puddle of standing water or some loose gravel to help get things started. :)

[jplacson]

Ok... like you guys... I too have that problem... not as bad as before... I have changed my thrust arm bushings... front shocks... tie rods... stab links... alignment... balancing...

And like some of you... mine becomes more pronouned when the car has been driven for a while.

I'm not sure yet... but I think my shimmy is caused by my REAR subframe bushings. This is something that may have been overlooked as a slight contributor to the problem. Think about it, they generally do the same task as the thrust arm bushings, except they are located in the rear. This may not matter on a FWD car, but since Bimmers are RWD, it may cause the rear to be misaligned every now and then. This causes the more dramatic "rear trying to steer itself" ... but unless the roads are bad, or the rear bushings are totally gone, it may just present itself as the common shimmy!

I'll get back to you guys on whether or not replaceing my rear bushings (mounts) helps. I'll have to order the parts first, then schedule my car for the repairs. Will keep you posted.