Alignment Toe Question

[bdtbdtbdt]

Should I measure toe from tire tread or wheel rim?

[Racecomp Engineering]

From the wheel. How are you doing the measuring?

- Andrew

[ZDan]

I'm pretty sure the reported/standard measurement in units of length is at the tire tread, not the wheel.

25" tire OD, 1/8" (0.125") toe-in conversion to degrees = (0.125"/25")*180°/pi = 0.29°

If you measured at the wheel, 1/8" (0.125") toe would equate to (0.125"/17")*180°/pi = 0.42°, that's a big difference!

[jamal]

What are you using? The rim provides a better place to measure with strings because the tire sidewall is not perfectly straight or parallel to the wheel surface so it is hard to measure precisely on 2 places on 4 tires. However, yes, that distance is shorter so it's a little harder to measure and adjust accurately. You could lean a rectangle of plywood against the tire like a toe plate and measure to the edges of that instead.

[CoolHandMoss]

Typically the wheel. The tire is too dynamic.

One good technique I've seen involves using a nail to just slightly score a center line on the tread of the tire as you spin it. Just enough mark to be visible. I suppose a good sharpie would work too. Then you measure to that line that you marked from a fixed point while the wheel turned. Still, the wheel is better.

ZDan's point about the difference of measurement is a good one but the spec is measured to the wheel as a standard. If you've ever watched a professional alignment, all the machines I've ever seen used are measuring to the wheel. Otherwise different tires, pressures, or even states of wear would be impacting alignment settings.

[Racecomp Engineering]

Originally Posted by ZDan

I'm pretty sure the reported/standard measurement in units of length is at the tire tread, not the wheel.

25" tire OD, 1/8" (0.125") toe-in conversion to degrees = (0.125"/25")*180°/pi = 0.29°

If you measured at the wheel, 1/8" (0.125") toe would equate to (0.125"/17")*180°/pi = 0.42°, that's a big difference!

It depends on the method. There are easy-ish methods to accommodate for either but it depends on how your doing your measuring. Many people that do at home alignments with toe plates measure at the tire but I would prefer not to do that.

There's at least 1 set of toe plates that are specifically designed to measure from the wheel, and if you're using strings it's not a big deal to use the wheel and IMO is preferable.

- Andrew

[ZDan]

The problem with using the wheel is that direct inches or millimeters of measured toe are not the same as the industry-standard which alignment specs are given in. You have to scale based on tire OD/wheel OD to get the conventional toe dimension.

0.125 inches of toe measured at the wheel on a 17" wheel with 25" tire = (25/17)*0.125" = 0.22 inches of toe in the standard measurement.

Again, it's a big difference and if you don't account for it you'll end up giving yourself way more toe than desired or intended...

Probably best to just convert directly to degrees which I prefer for units anyway, to remove ambiguity.

0.125" at 17" wheel = (.125/17)*180/pi = 0.42° (calculation for reference only, way too much for any usage IMO)

[bdtbdtbdt]

I measured with string at the rim edges. Autox specific alignment. Not a daily driver. I set 1/8 " toe out front. 1/8 toe in rear. I've got tarmac2's. Is that an OK starting point Andrew? Thanks everyone for your help. GREAT forum!

[ZDan]

Originally Posted by bdtbdtbdt

I measured with string at the rim edges. Autox specific alignment. Not a daily driver. I set 1/8 " toe out front. 1/8 toe in rear. I've got tarmac2's. Is that an OK starting point Andrew? Thanks everyone for your help. GREAT forum!

Like I said above, 1/8" (0.125") *at the wheel* is 0.42°, IMO way too much for any usage. I prefer close to zero toe at both ends, but anyway I don't think you'd want more than 1/8" of front toe-out *at the tire*, which is 0.085" or a little less than 3/32" *at the wheel*, or 0.29°.

All that toe-in at the back of the car is, IMO, good for nothing and bad for everything, including turn-in responsiveness which I would think you'd want for autoX. I would run zero to no more than 1/16" rear toe-in *at the tire*, or 0.0425" *at the wheel*, about 1.5/32", or 0.14°

As the above numbers illustrate, you have to be able to measure distances more precisely at the wheel to make up for the fact that the measurements are only ~17" apart instead of ~25" at the tire tread...

[Racecomp Engineering]

IMO yeah that's too much toe. 0 up front or close to it at least as a starting point. 1/16 in at the rear as a starting point.

Converting to degrees is a good way to be sure you're doing it right.

- Andrew

[gpvecchi]

So, when you're talking of 1/16, you're talking in inches, right?

[FirstWinter]

Originally Posted by gpvecchi

So, when you're talking of 1/16, you're talking in inches, right?

Yes, freedom units.

[new2subaru]

Originally Posted by FirstWinter

Yes, freedom units.

[gpvecchi]

I think here in Italy we measure toe at the rim edge, can you please tell me what 1/16 of inch is with 17" rims? I have total rear toe in of 1,6 mm... Thanks!