Scrub Radius

[BenG]

Does anyone here have an idea of how to explain scrub radius? I Know that it has to do with the relation between the pivot point of the steering tire and the centerline of the contact patch, but I do now know how it is affected by changing the offset with wider aftermarket tires. I've got a '96 Camry that I'm doing a Plus2 wheel upgrade on going from a 195/70R14 to probably a 205/50R16 (although I havn't worked out the exact size to keep the OD the same yet). Anyone here have a formula or rules of thumb to ensure that the steering capabilities and stability don't get changed in a negative manner by the upgrade?

Everyone talks about how a bigger wheel and tire combo will give you more grip and make your car look better, but I'd rather have well balanced handling characteristics for street driving over trying to make my car do 1.0g on the skidpad. I don't want to sacrifice stability and control, and if Plus sizing my wheels is going to do that, I'd rather stick with the stock wheel size.

[wedge]

By 'scrub radius' are you talking about slip angle?
I've never heard the term scrub radius before. But the way you describe it, that sounds like slip angle to me.

In any case, whatever size tire you do put on. Just try to get the overall diameter to be as close to OEM as possible. That way, the ONLY negative side effect will be a slightly rougher ride quality. Putting on a larger, lower profile tire will improve control and stability (rather significantly in some cases).
With stock suspension and tires my Paseo used to be (scarily) unstable at highway speed, moreso with lots of wind. My first ever upgrade was going from 185/70/14 tires to 195/50/15. I noticed a Huge improvement in stability at high speeds, in cornering grip, and in steering response.
A rule of thumb that I would draw is: Keep the OD as close as possible to OEM, and there is very little that can go wrong.

[Phi]

Originally posted by BenG
Anyone here have a formula or rules of thumb to ensure that the steering capabilities and stability don't get changed in a negative manner by the upgrade?

Here's a tire size calculator;
http://www.miata.net/garage/tirecalc.html

In terms of negatives...a by-product of larger rims is weight. So it's basically a trade off of wheel weight and ride comfort for handling.

[BenG]

I believe that 'slip angle' is pretty much the same as scrub radius. I kind of remember reading about that in an article in Sport Compact Car, and I got the term "scrub radius" from the H&R Spring website where they had an article on the same thing.

I asked my question because in the H&R article, they mentioned that offset can change the scrub radius/slip angle because you are physically changing the size and maybe location of the contact patch in relation to the central pivot point and since you have to change the offset when you upsize your wheels, I didn't know if there were any special things that I needed to know about the new offset and such.

I already know that tire overall diameter is very important to match within a +/- 2-3% so that will be taken care of for sure

[Deep]

The offset should be to oem specs or withing a few mm to keep the suspension geometry as close to its design as possible. Extending the wheel outward or inward would affect camber, which would affect the slip angle.

[Artsi]

BenG - You're on a right track to begin with.

Scrub radius has nothing to do with slip angle. There are literally several dozens of very good websites explaining suspension geometry. Just punch in those keywords in google search engine and you'll find more info.

In short; Yes, installing wheels which centerline is more outward than oem wheels (i.e. less offset) are, makes more positive scrub radius. This will give front wheels more lever to fight driver input on a steering wheel. Do this on a several hundred horsepower fwd car, and you'll be scared to let a hand off steering wheel for changing a gear.
Vast positive scrub radius makes steering wheel to jolt side to side under heavy braking. (Well, not necessarily - but it would be able to do so if road surface traction variations encourace such phenomena.)

In recent years megadollar super sportscars (Maserati, Bugatti, Ferrari, Lamborghini...) have started to use negative scrub radius to lessen said ill behaviour.

Heh, nice comment there on assumed Camber variation with altered wheel ET's. I don't wish to start an argument, but it isn't so. Camber will remain unchanged, no matter what ET wheel is installed on a car. Period.

In brief - to maintain oem scrub radius, keep to original ET value, but feel free to bolt on a wider wheel if such is felt to be a necessity.
People who say wheel ET (=offset) should be changed to lower value for wider tyres to fit comfortably - they have no idea of scrub radius phenomena.

Best racing teams are vary of ill effects of their suspension alteration, but tradeoff can well be worth it - car will lap faster times around a track. These fine tuned changes do however change from one racing track to another.