Question about steering wheel shaking while driving

[Luce]

Wide tires on a light car with a manual rack and pinion steering is very susceptible to polar imbalance.

Imagine there is a heavy spot on the outside edge of a tire, and they try to fix the imbalance with a weight on the inside edge of the wheel, but opposite side. A static balance will look good, but as the wheel spins, the 2 heavy spots will try to move the wheel so that the 2 heavy spots are in plane and make the wheel wobble. At the perfect resonant frequency of the steering system, it will become a very obvious left right shake.

ALSO, if the other wheel has a similar polar imbalance, when the wheels are synced, it'll be more pronounced, then you make a slight turn and they become antisynced and the 2 shakes will cancel.

Most heavier cars with narrower tires and power steering are far less susceptible and most shops are not that picky about the polar balance. The only way to see the polar balance is with spin balancing. The old bubble thing won't show it.

[SBSerpent]

Originally Posted by Luce

Wide tires on a light car with a manual rack and pinion steering is very susceptible to polar imbalance.

Imagine there is a heavy spot on the outside edge of a tire, and they try to fix the imbalance with a weight on the inside edge of the wheel, but opposite side. A static balance will look good, but as the wheel spins, the 2 heavy spots will try to move the wheel so that the 2 heavy spots are in plane and make the wheel wobble. At the perfect resonant frequency of the steering system, it will become a very obvious left right shake.

ALSO, if the other wheel has a similar polar imbalance, when the wheels are synced, it'll be more pronounced, then you make a slight turn and they become antisynced and the 2 shakes will cancel.

Most heavier cars with narrower tires and power steering are far less susceptible and most shops are not that picky about the polar balance. The only way to see the polar balance is with spin balancing. The old bubble thing won't show it.

Very interesting concept indeed! I have had some similar thoughts along this line in trying to understand my wheel shake that comes and goes at speed.

When I drive along a smooth, straight road keeping a constant throttle and maintaining a constant speed (lets say 65 MPH), I have a wheel shake that comes and goes as I drive. Roughly speaking, its cycling every several seconds.

I have 255/40s on the front and 315/35s on the rear (17" wheels). The diameters of these tires are 25.0" and 25.7" respectively, which comes out to a difference of 2.2" between the tire circumferences. What this means is that both front and rear will not remain constantly in synch when you travel in a straight line. Based on some rough calculations, after about 18 revolutions, the tires will be 180 degrees off from each other. Another 18 revolutions and they will be back to where they were with respect to each other 36 revolutions ago (again, approximate).

So I'm thinking that there is some sort of harmonic going on where, depending on where the tires are with respect to each other, i am getting a pronounced shake or nothing at all since whatever imbalance there is is cancelling itself out. Hmmmmmm.

[Gaz64]

Originally Posted by SBSerpent

Very interesting concept indeed! I have had some similar thoughts along this line in trying to understand my wheel shake that comes and goes at speed.

When I drive along a smooth, straight road keeping a constant throttle and maintaining a constant speed (lets say 65 MPH), I have a wheel shake that comes and goes as I drive. Roughly speaking, its cycling every several seconds.

I have 255/40s on the front and 315/35s on the rear (17" wheels). The diameters of these tires are 25.0" and 25.7" respectively, which comes out to a difference of 2.2" between the tire circumferences. What this means is that both front and rear will not remain constantly in synch when you travel in a straight line. Based on some rough calculations, after about 18 revolutions, the tires will be 180 degrees off from each other. Another 18 revolutions and they will be back to where they were with respect to each other 36 revolutions ago (again, approximate).

So I'm thinking that there is some sort of harmonic going on where, depending on where the tires are with respect to each other, i am getting a pronounced shake or nothing at all since whatever imbalance there is is cancelling itself out. Hmmmmmm.

A front imbalance cannot be cancelled out by a rear imbalance.

Two front wheels can cancel each other out, and so the steering wheel can shake at around 50-70 mph typical, and then cycle on/off.