Mirror Distance Using Wheel vs Tire Radius

In this video the distance between the laser and mirror is 5x the wheel radius. I am curious why the wheel radius is used and not the tire's radius?

Hi Patrick,

I would presume that it is because a 17" rim is going to be pretty consistent and thus repeatable, in its radius, where as a 225/45/17 tyre can have a quite varied radius depending on pressure, temperature wear and manufacturer.

Hi Patrick, just want to check which video you're referring to? Is this the bump steer checking procedure from the wheel alignment course?

Hi Tim. Yes, it is the bump steer checking procedure from the wheel alignment course.

Hi Patrick, we use the wheel radius as this is the same position you measure the toe. The convention for motorsport is to define toe in mm. Because we are measuring a distance, the radius at which you make the measurement will affect your measurement. Meaning you would get a different toe measurement if you measured at the tyre vs wheel.

It's normal to measure your toe at the rim rather than the tyre for the same reasons Gord mentioned - the tyre is not a very reliable place to measure from.

By measuring the bump steer relative to the same way we define the toe, this allows us to understand how much bump steer we have in terms of our toe measurements. Hope that helps.

It wasn't me, but I'll take the credit for it ;-)

The wheel diameter does bring something up, though, IMO for the most part it's better to use degrees/minutes/ seconds (?) for toe, bump steer, etc because it isn't unusual to use different diameter rims at different times - eg, daily driver and track tyres, or larger diameter aftermarket replacements for smaller OEM fitments - and if the manufacturer's spec', or one's base toe settings, were for one diameter, a wheel diameter change will affect that if it were using a comparative distance measurement. If using an angle it will be the same regardless.

My bad Gord, guess I had you on the brain from another post!

Strictly speaking about measuring the toe angle rather than distance, you're absolutely right. It is the more "proper" way to define it. It really just comes down to what's most practical. Obviously in racing, almost all of us are measuring the toe with a ruler giving us a length, and this obviously becomes problematic if you are changing wheel sizes a lot.

For me, it really just comes down to having a fixed, repeatable reference for every setup you do. Even if you are changing wheel sizes for some reason - let's say for wet tyres you run a different size rim. I don't think it's unreasonable to use just one rim size as part of your setup procedure. So maybe you do all your setups with your 18" dry wheels for example.