Steering wheel shimmy Options

[rbutler914]

I’ve been doing a bit of freeway driving lately and I have a weird steering wheel shimmy, noticeable between 60-80 mph. The strange thing is it dissipates after about 5 seconds, the returns about 5 seconds later and continues like that. There is no accompanying wheel vibration or shaking, just the steering wheel. Any thoughts would be appreciated.

73, 1.7, all stock, slightly lowered by previous owner.

[rbutler914]

Wow, thx for all the great suggestions. I’ve already tried balancing the fronts. Swapping with the rears will happen next. Btw Fuchs 2 liter wheels with 205/50 tires. I have a suspicion that front end geometry and/or worn components might be the issue, that’ll be next. The fact that it comes and goes and constant speed and road surface in puzzling though. I will definitely report back.

[Superhawk996]

The fact that it comes and goes and constant speed and road surface in puzzling though.

The reason it comes and goes is because of phasing between the right and left wheels.

The R1H component for right and left tire are not usually occurring at exactly the same place (in degrees of wheel rotation).

The right and left tires will have slightly different diameters due to tire pressure and tread wear. This means they rotate close the same amount between the sides but not not exactly the same.

Because of the slight difference in rotational speed between them, they will periodically align where both R1H forces occur at the same time - doubling the forces acting on the steering system. This is when the shimmy will be at its worst.

Then due to the rotational speed difference, R1H forces will eventually go out of phase between right and left. The shimmy will dissipate for a bit. This pattern repeats over and over.

Sometimes you can immediately break the shimmy if you are able to turn slightly. As you turn the right and left tires have to rotate at different speeds through the turn. Forcing that speed differential can move the R1H forces out of phase more quickly than it would naturally occur when travelling in a straight line.

[76-914]

The fact that it comes and goes and constant speed and road surface in puzzling though.

The reason it comes and goes is because of phasing between the right and left wheels.

The R1H component for right and left tire are not usually occurring at exactly the same place (in degrees of wheel rotation).

The right and left tires will have slightly different diameters due to tire pressure and tread wear. This means they rotate close the same amount between the sides but not not exactly the same.

Because of the slight difference in rotational speed between them, they will periodically align where both R1H forces occur at the same time - doubling the forces acting on the steering system. This is when the shimmy will be at its worst.

Then due to the rotational speed difference, R1H forces will eventually go out of phase between right and left. The shimmy will dissipate for a bit. This pattern repeats over and over.

Sometimes you can immediately break the shimmy if you are able to turn slightly. As you turn the right and left tires have to rotate at different speeds through the turn. Forcing that speed differential can move the R1H forces out of phase more quickly than it would naturally occur when travelling in a straight line.

Interesting info. So why doesn't this happen to all cars or new cars for instance. BTW, metal propellers have to deal with harmonics that overlap at a certain rpm. (IMG:style_emoticons/default/beerchug.gif)

[Superhawk996]

Interesting info. So why doesn't this happen to all cars or new cars for instance.

It does occur on new cars. I’ve spent many working hours looking at warranty data from unhappy customers.

The chances of it occurring on new cars is greatly minimised by the OEM controlling tire uniformity to tolerances that are less than what you can typically buy retail.

In fact, what often happens is that the OEM is paying the tire supplier to sort tires to a tire uniformity specification. Tires that don’t make the OEM spec are sold into the aftermarket by the tire manufacturer.

There is also another technique called match mounting.

Match mounting requires that the spot on the tire that has the high R1H force is mounted on the rim where the lowest radial runout spot is. This helps minimise the force disturbance that R1H causes.

The problem is match mounting is much more effective with steel wheels that are never perfectly round. Aluminum wheels on the other hand are machined round and have much less radial runout - therefore less effective at obtaining a benefit from match mounting.

Everything is a compromise between the OEMs willingness to pay for tire sorting and the ability of the tire supplier to produce enough tires without generating excess scrap or more high spec tires than the aftermarket can absorb.