Converting Toe Inches to Toe Degrees (and vice versa)

[iDiaz]

Okay, geometry problem for you guys. How do you convert toe inches to toe degrees, and vice versa?

I'm thinkering with this trapezoid calculator, and I think I'm headed in the right direction:

http://www.1728.com/quadtrap.htm

For overall tire diameter, I'm using 205/45-17 (stock 17"), so for sides b and d, I'm using 20.6". Sides c and a represent measurements taken between the front of a toe plate (a flat plate that sits against the outside of the tire) and the rear of the toe plate, respectively. For c and a, so long as the angles are not extreme, the actual length doesn't matter, moreso just the difference between the two measurements.

[Adam]

Ooo, I can figure this out, gimme a min.

[iDiaz]

Okay, so given a 205/45-17 tire, it looks like it works out to this:

1/8" = 0.348 degrees (total, so 0.174 degrees per side)
7/64" = 0.304 degrees (0.152 per side)
3/32 = 0.26 degrees (0.13 per side)
5/64 = 0.218 degrees (0.109 per side)
1/16" = 0.174 degrees (0.87 per side)
3/64" = 0.13 degrees (0.065 per side)
1/32" = 0.087 degrees (0.0435 per side)
1/64" = 0.0435 degrees (0.02175 per side)

I'll continue adding more conversions as I calculate them.

[glnr13]

i was thinking triangles and came across this in a quick search:
http://www.mentorsoftwareinc.com/cc/...s/TIPS0699.HTM

[iDiaz]

Originally Posted by glnr13

i was thinking triangles and came across this in a quick search:
http://www.mentorsoftwareinc.com/cc/...s/TIPS0699.HTM

Yeah, I thought of that, too. You could use that on individual wheels, but the American standard of measuring toe as inches measures the difference between the length of the front toe tapes (distance between the front of the tires) and the length of the rear toe tapes (distance between the rear of the tires).

[glnr13]

this theorem youre building... wouldnt it be better to use the rim instead of the tire as a measuring point? i'd think that inconsistencies of a tire would hinder the exactness of the measurements.

also, question in regards to measurements of toe... when toe is set to be 1/8" out is that a reference to 1/8" away from the original position? i.e.- 0.348 degrees from 0?

[gandini]

Originally Posted by glnr13

this theorem youre building... wouldnt it be better to use the rim instead of the tire as a measuring point? i'd think that inconsistencies of a tire would hinder the exactness of the measurements.

also, question in regards to measurements of toe... when toe is set to be 1/8" out is that a reference to 1/8" away from the original position? i.e.- 0.348 degrees from 0?

Toe out is measured as a larger distance at the front of the tires than at the back. So a "string" encompassing the tires would be 1/8" longer at the front than the back. And you are right, toe out is indicated by a negative degree measurement (-0.17 is 1/16" toe out).

[iDiaz]

Originally Posted by glnr13

this theorem youre building... wouldnt it be better to use the rim instead of the tire as a measuring point? i'd think that inconsistencies of a tire would hinder the exactness of the measurements.

In theory, yes it would be inaccurate due to inconsistencies in tire diameter, but as far as I know, this is the way it's been done, using toe plates that are the size of the tire. I'll ask around and make sure, though.

Originally Posted by glnr13

also, question in regards to measurements of toe... when toe is set to be 1/8" out is that a reference to 1/8" away from the original position? i.e.- 0.348 degrees from 0?

If you were to place a flat plate as wide as the tire--on the face of each tire, then measure the distance between the front edges of both plates, then measure the distance between the rear edges of both plates, subtract the rear measurement from the front, and you'll have your total toe in inches. So, 1/8" toe out means the fronts of the tires are 1/8" further apart than the rears of the tires.

[Adam]

OK, someone correct me if I'm wrong but:

The angle should just be tan (diameter of wheel / amount of toe)


And the amount of toe should be arctan (angle) = radius of wheel / amount of toe

so:

amount of toe = diameter of wheel / arctan (angle)

[glnr13]

interesting...
so then, if one were to go and get an alignment with one set of tires and toed it out at 1/8" then flipped out to larger tires (diameter-wise) without an adjusting alignment, potentially the car could be toed out even further?

[iDiaz]

Originally Posted by glnr13

interesting...
so then, if one were to go and get an alignment with one set of tires and toed it out at 1/8" then flipped out to larger tires (diameter-wise) without an adjusting alignment, potentially the car could be toed out even further?

Well, as far as the contact patch is concerned the toe wouldn't change, but the measurement might appear to be different when represented as inches. A measurement of degrees would not change, unless it was derived from the measurement in inches. Most alignment machines nowadays measure in degrees, then convert to inches (not sure what width they use as the toe plate width), so they'll be consistent regardless of tire/wheel size.

Hm, this makes me think there has to be some standard toe plate width everyone uses when they code their alignment machine computers. I wonder what that length is. Also, based on what I'm finding on searches for "toe plates", most companies carry a single toe plate width.

[iDiaz]

BTW, this is what I mean when I say "toe plate":

[iDiaz]

Originally Posted by amg6975

OK, someone correct me if I'm wrong but:

The angle should just be tan (diameter of wheel / amount of toe)


And the amount of toe should be arctan (angle) = radius of wheel / amount of toe

so:

amount of toe = diameter of wheel / arctan (angle)

I donno, that's what trapezoid calculators are for.

[MrBattle]

so would the conversion of toe be different for everycar?

[iDiaz]

Originally Posted by MrBattle

so would the conversion of toe be different for everycar?

I can't imagine it would be, so I need to find out what the standard is for toe plate size. Problem is, what if you've wheels that are too big to use a toe plate on?