Chris F

Hey all, I was about to ask this question, but google found it for me. I figured it was useful to a lot of people so I'd post it.

http://www.sccaforums.com/cgi-...2;p=0

Summary:

Toe is measured (in inches) from the front of the tire to the rear of the same tire. So, if I have 195/55 R15 tires, the total diameter is 15 + 2x 7.7*.55 = 23.5 inches in diameter. This will be the "hypotenuse" site of our triangle. The opposite side (from where the toe out andle is measured) is our toe out measurement in inches.

Here's an example for .25 inches of toe out (for 1 tires), and equivalent degrees. (Soh Cah Toa to the rescue!)





This measurement is for one tire. If you have two identically toed-out tires like this, then you'd have .25+.25=.5 inches of total toe out, or about 1.5 degrees.

So, I'm running about .5 degrees of total toe out, which is close to .21 inch of toe out.

p.s. there's nothing wrong with using arctan or tan here instead of sin. It depends on whether you measure toe out from a right angle to the car or a right angle to the tire. Measuring from the car makes the most sense to me. If you had 90 degrees of toe out, it would be equivalent to 23.5 inches of toe out, rather than infinity. For small angles, they are almost exactly the same though.

Later,
Chris



Modified by ccfries at 12:30 PM 11/20/2004

nsxtasy

Here is a conversion table...

(Just don't ask me how to use it.)

Chris F

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nsxtasy &raquo;</TD></TR><TR><TD CLASS="quote">Here is a conversion table...
</TD></TR></TABLE>

Ugh, that's hard to read (but thorough!)

So, .2488 inches toe out on a 19" wheel works out to be 45 minutes of a degree. Perfect, 45/60 == .75 degrees.

Thanks for posting that, although they use toe measured from the wheel, which is not what a lot of other people are measuring. It will differ by maybe 20% or something.

-Chris




Modified by ccfries at 12:31 PM 11/20/2004

kkim

So, on the chart it's listed as wheel diameter. Is that really wheel diameter or total diameter of wheel and tire combined?

Thanks for the chart... I've been looking for an easy way to convert from degrees to decimal.

nsxtasy

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kkim &raquo;</TD></TR><TR><TD CLASS="quote">So, on the chart it's listed as wheel diameter. Is that really wheel diameter or total diameter of wheel and tire combined?</TD></TR></TABLE>I'm pretty sure it's really wheel diameter.

kkim

If that's so, and I'm not saying it's not, that says sidewall height (tire aspect ratio) does not affect toe value?

Seems contrary to what I understood my alignment guy to say.

nsxtasy

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kkim &raquo;</TD></TR><TR><TD CLASS="quote">If that's so, and I'm not saying it's not, that says sidewall height (tire aspect ratio) does not affect toe value?

Seems contrary to what I understood my alignment guy to say.</TD></TR></TABLE>

Wheel diameters are typically in the range of 13-20 inches, and for most cars, 15-18 inches. Tire diameters are typically in the range of 23-24 inches. The table, which comes from a racer site, (a) claims it is showing wheel diameters, and (b) goes from 12 to 22 inches. Which do YOU think it is showing?

Chris F

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nsxtasy &raquo;</TD></TR><TR><TD CLASS="quote">

Wheel diameters are typically in the range of 13-20 inches, and for most cars, 15-18 inches. Tire diameters are typically in the range of 23-24 inches. The table, which comes from a racer site, (a) claims it is showing wheel diameters, and (b) goes from 12 to 22 inches. Which do YOU think it is showing?
</TD></TR></TABLE>

Good point Ken.

The SCCA guys (in forum link in first post) were taking sidewall height into account. It makes sense, too.

Why would switching from a 16" low profile tire to a 14" high profile tire change your toe? The DEGREES don't change, but the inches would if you use wheel instead of tire as your diameter measurement.

So, it seems, measuring inches of toe isn't really standardized. YMMV, IANAL, etc.

EDIT: Also, on that PDF file, the whole 12" column is way off! Actually the 12" column is decimal degrees, not toe for a 12" wheel.

EDIT #2: CLIFF NOTES:
toe out in inches equals .41 * toe out in degrees. If you're 1 degree toed out, you're .41 inches toed out on a 23.5 inch tire. Or, if you're .25 inches toed out, you can divide by .41 to get .61 degrees.



Modified by ccfries at 12:34 PM 11/20/2004

kkim

Ken,
Yes, I can see what the chart is saying... thought maybe it was only a partial list that Chris had somehow edited, maybe??? In any event, it does look/sound like alignment, while a very exact and precise measurement type of excercise, is still very much of a black art that only the very best can master. All I was saying was that if the chart was for wheel diameters, it was contrary to what the alignment tech was telling me about toe being affected by overall tire diameter.

Chris,
Thanks for the chart and the link. Gives me something to show my alignment tech so we can try to work the conversion from degrees to decimal when we align my car and at least confirm if we are close to what the values are supposed to be and work from there.

Thanks guys,
Kelly

Chris F

NOOO DON'T PRINT IT OUT QUITE YET!

I suck. Tire diameter is wheel + 2x sidewall

Editing in progress...
Edit done.

p.s. you can use GOOGLE to do this math for you, if you don't have a calculator handy.

Google sin(.610 degree) * 23.5
.610 degrees toe out in inches:
http://www.google.com/search?h...earch

Google arcsin(.25/23.5) in degrees
.25 inch toe out in degrees:
http://www.google.com/search?s...grees

Modified by ccfries at 12:34 PM 11/20/2004


Modified by ccfries at 12:38 PM 11/20/2004


Modified by ccfries at 12:38 PM 11/20/2004

kkim

Thanks, Chris.

To clear up what Ken was speaking of, I looked at the other chart on Smartracing.com...

http://www.smartracingproducts...s.pdf

In it it shows the relationship to tire diameter and toe measurement. The last paragraph explains why Ken's chart is correct, which shows toe angle and why the toe measurement changes with tire diameter which is toe measurement.

Thanks to the both of you for helping me to better understand exactly what I need to ask for when I go for an alignment in a few weeks.

Mahalo!