Rumbles Subaru WRX

Finding your Scrub (Radius)

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Ever since I learned about scrub radius I have wondered what my general scrub radius was (positive or negative) without calculating my kingpin inclination and other assorted measurements.

I don’t remember who suggested this method for getting a general idea of your scrub but I finally got around to doing it. It’s pretty simple. Start your car and turn the wheel back and forth a few times. Then roll your car back and check out the pattern on your tire.

Positive Scrub Radius

I have a slightly positive scrub as the center of the tire is outboard of the rotating point of the tire. I believe stock is either neutral or negative scrub. Information on on OEM scrub radius is hard to come by.

However, too much positive scrub can lead to torque steer in FWD or high HP AWD cars. That is one of the reasons we are seeing Revoknuckles or similar style front suspensions on higher horsepower FWD cars now.

There are a couple of ways to change your scrub radius. Wheel spacers are the easiest way; add more spacers and gain more positive scrub. This obviously applies to wheels with a higher offsets as well. Adjusting camber via camber plates or camber bolts (MacPherson strut) can change scrub as well. Negative camber will generally add negative scrub although the effect can be mitigated with camber bolts at the knuckle.

And since I bent two of my SSR GT2 after hitting a pothole, I am in need of a new set of wheels. I could get the SSRs fixed but I don’t really want to put any more money into them.

I am looking at 18×9.5 wheels in the +30-40mm range. I am leaning towards getting something cheap since I do plan on going 5×114.3 one day. And since everything is on backorder my car probably won’t be back on the ground before the spring semester starts.

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