Steering judder

flying dutchman said:

Is this not an effect of rear wheel drive? The front tires must push the front of the car side ways but the rear tires are pushing the car in a straight forward line. So the front tires are just skidding on the surface. It is worse on slippery surfaces such as garages.
On a front wheel drive car the front wheels actually pull the front of the car sideways.

Click to expand...

Well, kind of. Mostly it's just the size (width) of the tires themselves, really.

Try an experiment. You'll look silly but should feel enlightened ...

Stand comfortably with both feet in line with each other, as if you're standing at a starting line. Then, slide your left foot forward one foot-length while turning it in an arc, as though you're a car turning left in a tight circle. Next, slide your right foot forward one foot-length (only one foot-length!) while making the same turn, which, since your feet are apart, is in a wider arc. Note that your right foot is now behind your left foot.

Slide your left foot one more foot-length forward in the arc, continuing the turn. Slide your right foot forward similarly. Now your right foot is even more behind your left one.

When you drive in a circle, your tires have to do the same thing: the side of the tire "towards" the turn (the left edge if turning left, right edge if turning right) moves in a tighter arc than the other side of the same tire. The wheel on the outside of the turn has to go much slower than the wheel on the inside of the turn, but the back wheels can do this due to the differential, and the front wheels can do it because they're not driven in the first place, so each one turns freely. But it's true not only of the wheel, but also of the tire itself: the "inside edge" wants to turn more RPMs than the "outside edge"—but this is impossible.

A skinny tire has less differential RPM rates on inside-vs-outside, and the less sharply you turn (in a bigger arc), the less "turn scrub" you apply to the tires.

Incidentally, this is also why you don't drive a locked, full-time 4WD drive system on dry pavement, only on slippery surfaces (mud, snow, sand, etc): the inside and outside wheels need to turn at different speeds whenever you go around any turns.