Some things to keep in mind for those who are setting or just checking their toe settings at home.
A typical alignment settings for total toe is 1/8 inch. But the question you should be asking is: "over what distance is this taken?" This distance is known as toe span. It is very important that the included angle of the tires is correct. Obviously the further apart the toe measurement is taken for a given toe angle the larger the toe measurement becomes. So, is the toe measured at the rim? tire sidewall? tread face, or some other spot? I have seen all of the above methods used and surprisingly this number is a hard to find. Actually I have never found it. On Hunter alignment machines, and on the specs tables they give for toe, total toe is based on:
2 degrees = 1 inch.
But since most of us do not have alignment machines at home, what is the toe span used to give toe settings?
So, based on 2 degrees is equal to 1 inch.
If we draw an isosceles triangle with two equal sides, and the angle formed by the two equal sides is 2 degrees, and the base of the triangle is 1 inch, we can calculate the height of the triangle or the toe span. If we bisect the 2 degree angle we get a right triangle with one side being ½ inch forming an angle of 1 degree with the other side.
So then,
tan 1 degree = ½ in / toe span
Toe span = ½ in/ (tan 1 degree)
= 28.64 inches
Now the typical front tire is a 245/60-15
so to get the tire diameter the following formula is used:
tire diameter =(tire width mm)(1 in/25.4 mm)(aspect ratio)(2)+(rim diameter)
=(245 mm)(1 in/25.4 mm)(60/100)(2)+15in
=27.05 inches
So if your toe setting is 1/8 inch, and you are only measuring to the tread face of the tire, you are putting in too much toe. To convert to what your toe should be based on tire diameter:
tan angle = toe/toe span = new toe/ new toe span
note that 1/8 = .125
so we get
new toe setting = (spec toe)/(spec span)*(new span)
=.125 in/28.6 in*27.05 in
=.118 inches
Now if we measure to the rim
new toe setting=(.125 in/28.6 in)*15 in
=.066 inches
As you can see it is very important where you are taking the measurements. Something else to keep in mind is: the smaller the toe span the more difficult it becomes to set the toe accurately.
To further complicate measuring toe, it is normally very difficult to measure straight across the tires because things like engines and frames are in the way...So some guys just measure up to where the tape measure just touches the frame. The problem is that they have made the toe span smaller yet...
That's why toe plates were invented.
:
http://www.longacreracing.com/instru....asp?instid=23
Hope this helps,
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