[Ron Sutton]

Hey Rob,

Originally Posted by FETorino

Ron

What would you consider Large-ish scrub radius in the case of these 275-315 range of tires?

I have heard that an inch or under can be thought of as zero.

Maybe a little more detail on scrub would be good.
I never created a descriptive verbal scale ... until now.

In my experience & by my personal scale ...
0-1/4" = Zero
1/4"-1" = Very Low
1-2" = Low
2-3" = Moderate
3-5" = High
5-7" = Very High
7" + = Extreme

* But this relative to the situation. When we designed a new Modified chassis that dropped the Scrub Radius from over 7" to under 5" ... we thought of that as "low scrub radius ... for the NASCAR Modified class we raced in on the West Coast.

So, with this new "descriptive verbal scale" ... I need to restate my advice with a slight wording change.

If your PT car has a scrub radius "Moderate or higher" … you need to be careful with how much caster you can put in the car … because caster combined with a high scrub radius creates a “jacking effect” when you turn the wheels. Dynamically, this jacking effect “de-wedges” the car … loading the inside front & outside rear tires more … while also unloading the inside rear tire & increasing the degree the car diagonally rocks & loads the outside front tire.

All of this helps the car to turn better. But go too far … and the car will get loose on entry. This is where track tuning comes into play. If you have a large-ish scrub radius … sneek up on the caster you put in the car … until you get the car “free” on entry … then reduce the caster a tick … or tune something else to allow you to keep that amount of caster, so the car turns well in the middle. But don’t keep a set-up that makes the rear step out on corner entry.


I was having a conversation stating there is a nominal effect on scrub due to height differences in tires. That difference in scrub between a 30 and 35 series tire of the same section width on the same wheel would be pretty minimal and in real world probably insignificant. But with all things it is best not to assume and really dig into what amount of scrub becomes noticeable or detrimental.

I agree with this statement: That difference in scrub between a 30 and 35 series tire of the same section width on the same wheel would be pretty minimal and in real world probably insignificant.

I have had talented race drivers we developed to the nth degree ... with amazing feel & feedback ... that can tell you if you changed the tire 1/4 pound of air pressure, .030" of bump stop shim, which suspension corner stopped traveling 1/16" earlier than the other or if there was extra clearance in your ring & pinion gap ... and that's not even a tiny bit exaggerated.

A Crew Chief/Tuner can have a lot of confidence in those situations. These particular drivers & I won a lot of races together.

And on the other hand ... I've seen drivers that couldn't tell you if they hit a pothole or not. So I think "noticeable" is also relative.

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