Beam axle shimmy 1

[formula94lt1]

If all joints are rod ends(tight) and wheels are not damaged mis-made or out of balance, where, if anywhere, left is there potential for shimmy? I keep hearing shimmy is caused by gyroscopic forces which act through loosness in the suspension, but what are these gyroscopic forces caused by generally?

 

[patprimmer]

Side to side out of balance of the tyres or out of round of the tyre or hard and soft spots on the sidewall, or the wheel being mounted eccentric or not exactly perpendicular to the spindle, insufficient preload on the wheel bearings.

Regards

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[evelrod]

Lot of experience with "Ford" shimmy!

As Pat says, it is generated by an out of balance/round of the tires. An out of balance, side to side, wheel/tire could cause gyroscopic precession and contribute, but in some of my earlier experiments, I found this not to be the main cause. The shimmy is almost always caused by wear pattern/out of round in the tires combined with insufficient toe in and a general 'looseness' in the steering box/tie rod, etc.

Rod

 

[carnage1]

kingpins? made a huge difference on our 33 dodge, the other was the font axle not being shimmed properly so had too little caster.
also all the stuff already said

 

[Tmoose]

Hartog's vibration book has a section on shimmy and tramp.
The book's at home, but I kind of recall a one wheel bump providing a true gyroscopic input that could trigger misbehaviour.

 

[NormPeterson]

Wouldn't that be a precession effect of some sort? Unlike a beam rear axle, the only "stiffness" against wheel/tire rotation about the Z-ish (kingpin) axis is due to operator arm/upper body strength, so it's really easy to get Z-ish rotation of the wheels.

I remember being in a car that exhibited that sort of behavior, though I don't remember the year/make/model involved. Too many decades ago, I guess. Anyway, whoever was driving had to slow the car down every so often to make the shaking stop.


Norm

 

[evelrod]

Old solid axle Fords, Norm. I have owned a slew of them over the years. Three ways to stop the shimmy...Slow down (really ticks off the other traffic)...Speed up (works better if you have the hp, but difficult in traffic)...and...Turn left or right if you can or zig zag right and left really hard. Any one thing in combination with another also works...Could get you a ticket, too!

Rod

 

[GregLocock]

Yes there is an issue with camber gain causing shimmy due to the gyro forces, it is mentioned in a couple of the books. A beam axle has a camber gain of about 30 deg/m which is not especially high, so I'd be a bit surprised if the gyro force was the sole contributor.

Cheers

Greg Locock

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[evelrod]

Agree. We tried to force the shimmy but gyro forces are quite low at the slow speeds the shimmy usually occurs. In about 90% of the cases I've seen it's a combination of worn or worn out steering components along with old tires.

Rod

 

[Tmoose]

Hartog's "Mechaniucal Vibrations" p 331 - technological reference frame is beam axles (probably leaf springs) and those newfangled balloon tires. "In case the proper phase relations for instability do exist, the vibration wil be all the more violent the smaller the flexibilities and the stronger the coupling."