Scrub Radius; Positive or Negative

[CTX-SLPR]

Howdy,

As I continue to plow through the parameters I can adjust on my Riviera, Designing the control arms pretty much centers around the offset of the wheels since I can't change the track width much. I had planned on cloning the C5/6 front suspension with narrower wheels and taller tires but as I read more and more on scrub radius it seems that the Corvette has too little scrub at only ~0.39in possitive. If I change the wheels to something with closer to 20mm offset, I can get a decent scrub radius in the 1.0in range but it's now negative. I didn't really find a lot of opinion on which to choose, just "0.5-2in" scrub radius.

Can someone steer me towards which side of 0 is preffered and why?

Thanks,

[Norm Peterson]

Take a look through this link.

http://www.cdxetextbook.com/steersusp/wheelsTires/alignFund/scrubradius.html

You probably want it to be positive, particularly if you expect to be running a significant amount of negative camber.


Norm

[CTX-SLPR]

I'm not sure I'm following your statement in relation to the link you posted. I get the idea of the negative scrub causing the car to pull to the side of less braking, the preverbial "into the puddle" and that not being the greatest thing in the world. However, how great is the effect if it's only -1.125in?

The wheel availablity is much higher for 15-40mm offsets than it is for 50+. I can't imagine that BMW would design thier suspensions with only 20mm of offset in an 18x8 (which I conjecture as generating a negative scrub) M-Parallel and have it be a bad thing.

Thanks,

[Norm Peterson]

I'm not sure I'm following your statement in relation to the link you posted. I get the idea of the negative scrub causing the car to pull to the side of less braking, the preverbial "into the puddle" and that not being the greatest thing in the world.
With positive scrub when you set positive static toe, that will tend toward zero toe while driving and help keep the two steer tires from slightly fighting each other while cornering and giving away a little front grip. That's arguably better than setting zero and having the drag cause more toe-in. Static toe-out is best reserved for FWD.


However, how great is the effect if it's only -1.125in?
Probably big enough to feel, particularly if it goes from slightly + to slightly - as that would reverse the direction of any steering corrections that might have to be made should there be vastly different drag forces at the two steered tires.



The wheel availablity is much higher for 15-40mm offsets than it is for 50+. I can't imagine that BMW would design thier suspensions with only 20mm of offset in an 18x8 (which I conjecture as generating a negative scrub) M-Parallel and have it be a bad thing.
BMW also uses double pivot geometry in the lower arms in some of its models, which puts the effective lower steering axis pivot somewhere out beyond the ball joints. You wouldn't need nearly as much offset in the wheel to end up with a small scrub radius.


Norm

[Hondo78]

Note to self: BMW 5 and 7 Series susp. is cheap, and wide too. Big brakes, and designed for a heavy car... What does that Riv weigh?

[CTX-SLPR]

Note to self: BMW 5 and 7 Series susp. is cheap, and wide too. Big brakes, and designed for a heavy car... What does that Riv weigh?4000lbs

However I'm not changing the front suspension, I don't have the time, skill, or budget to undertake that. I've already only driven it 3 times in 4yrs.