I do my own alignments and I've been looking at equipment for measuring toe-in. I've been struggling to understand what a toe-in spec in inches (as opposed to degrees) really means, since the CSM specifies toe-in in inches. Obviously where this is measured will affect the angle of the tire and the real toe setting. I'm ASSUMING that the factory spec is supposed to be measured at the center of the tread face at spindle height, front and back of the tire. The problem with this is that changes in tire diameter (ie, larger or smaller than stock) will change the distance between the front and back surfaces of the tire and thus will change the angle if the toe setting dimension is kept the same. Similarly, I've been looking at buying a set of "toe plates", which are simply aluminum plates that sit against the outside faces of the tires and allow you to use a tape measure front and back to measure toe. These plates all seem to be about 23.5" wide, which means that if you use the factory toe spec, the actual toe measured at the faces of the tire will be different.



I ran some numbers (hey, trig is your friend). Assume the factory toe spec is 1/4". With 26.5" diameter tires (typical factory size for a 1960s A-body), that's 0.27 degrees of toe, total. If I use a pair of these toe plates, that same 1/4" total toe would give me 0.30 degrees, or equivalent to 0.282" measured at the tire faces. That's more than a 10% difference. Sounds like the right answer is to take the factory spec and tire diameter, convert that to degrees, then convert it back to inches for the width of the toe plates. You'd have to do the same thing if you were measuring to the tire faces on a tire of non-stock diameter. Alternately, since there's a pretty wide tolerance band on the toe spec, you could go a hair under spec if the distance between measurement points was smaller than stock tire diameter and a hair over if larger.

 

I also do my own alignments, with the Longacre tool for caster and camber. For toe-in, I use two straight boards sitting on blocks and held against the tire sidewalls. Board height is the max allowing a tape measure to go between the wheels. I go for 1/8" toe in with that method and it has given stable steering at up to 119 mph. My guess is that for stability you don't want 0 or toe-out, and you want a small amount of toe-in (vs. larger amount) to minimize tire wear/dragging.

 

Agreed. I suspect this just isn't that critical a measurement.

 

I prefer a simple toe stick (beam style gauge) and can of silver or white spray paint. Paint a ring around the center of the tire and scribe a line near the center of the tread. Measure front and rear using the scribed lines adjust, move the car forward and back, double check and your done. The paint wears off pretty quick. I don't think tire height compensation matters as much in the figures, but the measurements need to be taken at the same height front and rear on the tire.

Depending on the tire type, tire width, and driving style, you can use 0, 1/16, 1/8, 3/16, or 1/4. Some like to to run settings toe'd in and some like them toe'd out. Its not a matter of what the factory recommends because those settings were for the most part geared towards bias ply tires.

It's not an exact science, more like trial and error.

 

But how often do you change tire size? I would run into this problem often with race quads. People would use small 18 inch MX tires for racing and 20-22 inch for play. I tell them I set them up for racing and just live with it for play. That or readjust every time but that was never going to happen.