O's_Car

I’m probably over-thinking this, but here goes. Like so many others, I’m considering a change in wheel size on my Cutlass. The stock setup is 14x7 SSI wheels with G78-14 tires (27” diameter, 8.5” width). At the front I’d like 17x7 wheels, staying close to the stock tire diameter and width (225/55-17). The question has to do with the offset of the wheel. The stock SSIs have 4.375” backspace – that works out to a +10mm offset. The 17x7s have 4” backspace and 0mm offset. That means the center of each wheel/tire will be shifted 10mm outboard of the stock location. That’s ok in terms of wheel well fitment, but I’m not sure what that does to the steering. I’ve done some searching on the internet and found that decreasing wheel offset can create (or enlarge) a positive scrub radius, which in turn affects the handling of the car. Apparently, a little positive scrub radius improves road feel in the steering, but too much can have bad consequences. Not knowing what the stock scrub radius is, my question is whether a 10mm (0.375”) increase in it would be significant or not. If anyone out there has knowledge about suspension and steering alignment, maybe you could help me understand this.

If you’re interested, here are a couple of articles I found that describe scrub radius:

http://www.hrsprings.com/technical/scrub_radius
http://www.miata.net/garage/offset.htm

Like I said, I’m probably over-thinking this. I could just order the new wheels with custom offsets to match the originals, but I’d like to know what other people have done in this situation.

Dapapadon

Interesting topic! I've wondered how the guys with super large dia. wheel/tire combos dealt with this. Anyway, generally you hear street rods guys talking about this. They tend to modify their front suspensions more than other car enthusiasts.

I did find the first article interesting. I thought you needed to shoot for zero offset. But per the article, zero offset would induce "squirm". Odd though that the author didn't state what was the ideal offset to shoot for.

I have read that too much offset positive or negative will cause excessive tire wear and premature wheel bearing failures. But I don't recall what quantifies "too much".

Hopefully Joe, Norm or some of the other knowledgeable folks will know more about this. Sorry I could not help with your question. I would like to know the answer though!

Don

83hurstguy

Like you saw, it is determined by spindle axis (or kingpin) inclination and the incidence of the tire centerline. Trying to pinpoint an optimal amount of scrub radius depends on a lot of factors... it is a lot more sensitive if you are going to run 8" wide R-compound tires vs some tall sidewall radials or bias ply street tires. I'm not sure what stock SAI value is for an earlier A-body, or what the stock scrub radius is (probably fairly large). Modern cars run less than 2". A racecar may run anywhere from 0.5-1.5". It's just part of a complex suspension package, in which you need to optimize a lot of factors to work together.

Without any scrub radius, you develop no turning moment around the top view axis of the tire, which results in little to no feedback at all in the steering wheel. If you ran with 0* SAI, you'd also lose most of your self centering effect from "jacking" the car while turning (if you don't have any positive caster, you lose it all)

Where do you have the rest of the alignment settings at? Caster has just as much to do with steering feeling and straight line stability as the SAI does.

I guess what I am getting at... is that if you have power steering, and aren't going racing, you probably won't notice less than a 1/2" change.