Discussion and questions related to the course Motorsport Wheel Alignment Fundamentals
Tim, you mentioned measuring to the rim. I believe toe in mm generally refers to measurements at the tyre tread.
Would yo agree that commonly specified toe figures in mm need to be reduced by the ratio of rim diameter/tyre diameter when measurements are taken at the rim?
In my experience, the normal convention is to always measure at some convenient place at the edge of the rim. Obviously, this means that different rim size and even design will affect the measurement you get, for a given toe setting.
To be more "correct", the toe should be defined as an angle. In the OEM world, this is how we normally define and measure it.
In motorsport, due to the measurement hardware, we tend to use, it's much more convenient to use a linear distance instead - most commonly mm. This obviously introduces the potential for confusion if we are comparing toe settings and we're not measuring the linear measurement at the same radius. However, with all that said, as long as you are consistent with the wheels you use to do your setup (which isn't usually a problem) then it all works out fine.
Tim, in line with your response, I found an example (UK Ford Sierra workshop manual) which specifies toe in mm, with no angle given. In the setting instructions, that manual says to measure to the rim - as you might have expected.
However, many general setup texts lead the reader to work to the tyre tread.
So it's important for people to be wary of toe specifications in mm: we must be sure to know where on the wheel they apply.
As you say, for comparison purposes in the development of a car, either place is OK - so long as we are consistent.
Absolutely, the rim size is important 😃
I would say overall, the tyre is a poor choice to measure toe from for 2 reasons at least-
1. It doesn't solve the problem is specifying the size - you still need the tyre size to be able to compare, you'd also need to specify the position on the tyre, and the shape of the sidewall can vary significantly by manufacturer.
2. A tyre is not a stable place to measure from as there is a lot more variability in its final dimensions compared to a wheel rim
Other than that, sounds like you're all over it 💪
We used to use trammel bars and scribe a line on the tread surface of tire (slicks, btw). Just spin the wheel and lay on a mark with a scribe. I've seen guys either put a bit of something colored (paint) on before scribing to make it stand out.
That said, I've been measuring with string and rims for the last 30 years.