Suspension & Steering
Steering Geometry Tutorial
>whilst scanning through the technical data pamphlet about the 928, it says
>under suspension geometry that it is a "negative steering scrub (roll)
>radius" can someone tell me what that means? i haven't a clue as
>suspension geometry is definately not my strong point.
If you were to draw a line through the upper and lower ball joint and extend it down to the ground. The distance that this point is from the centerline of the tire is the scrub radius. Basically means that instead of the tire pivoting on the center of the tire when you turn the steering wheel it pivots toward the inside or outside edge. The net effect is a change in tow when driving in a straight line, as the torque works on the tire and even more pronounced changes under braking load. Changing the width of the rims particularily on the front where there is little room on the outside, fender clearance, means pushing the rim in. So the scrub radius is changed which changes the static toe which should be used as with most things it is a compromise and street car /race car needs are very different.
Jim's explaination is correct, but perhaps a little more info might help.
If the imaginary line thru the balljoints intersects the ground at the center of the tire contact patch, the car will be very easy to steer, and will have little or no kickback when you hit a bump. Unfortunately, there will also be virtually no "road feel" or force feedback to tell you what the tires are doing, and the car will feel very "nervous" and "twitchy".
If the imaginary line thru the balljoints intersects the ground toward the inside edge of the tire contact patch, you have positive scrub radius. The rolling resistance will push the wheel back and turn it out, compressing the steering linkage (for rear steer cars), so we need some toe-in to compensate so that the wheels will end up rolling straight down the road. The car will be harder to steer, and there will be more lick-back on bumps. A blowout on the front will yank the wheel very hard, as the added drag will cause the tire to turn out, increasing the tendency to turn into the blown tire. There will also be much more road feel and force feedback, so you can feel the front tires start to lose grip in a corner.
If the imaginary line thru the balljoints intersects the ground toward the outside edge of the tire contact patch, you have negative scrub radius. The rolling resistance will push the wheel back and turn it in, stretching the steering linkage (for rear-steer cars), so we need very little toe-in, and in fact may need some toe-out, to keep the wheels rolling straight down the road. The effort required to steer the car is a little less than with positive scrub radius, and there will be very little kickback on bumps. A blowout will yank the steering wheel very much less, as the added drag tries to turn the wheel in, rather than out, reducing the total amount of effect from the blown tire. There will also be less road feel and force feedback, so it will be harder to feel the front tires start to lose grip in a corner.
One effect of having the wheel hubs pushed toward the outside of the wheel, such as on the 928, is that it lowers the loads on the wheel bearings, and spreads the loads more evenly between the inner and outer bearings. If you look at some of the "rice rockets" with extreme dish on the wheels, you can readily see that the outer (and smaller) wheel bearing is very heavily loaded. This also has the effect of softening the springs and shocks, due to the longer lever arm.
Sorry for the verbose answer - you really shouldn't ask either Jim or me
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