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Old February 22nd, 2005, 02:35 PM   #1
KenJ
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Alignment conversion factors

I tried looking at the archives and searching, but didn't find the right 'key word' to catch anything that would convert degrees to inches and vice versa.

I thought Kevin C. might have mentioned some sourcing for a conversion table, but I can't remember exactly.

Over a year ago, I went to Sonnen to get an alignment and the rear toe read: left +0.09' and right +0.08'. Total toe +0.16'. I've got 17" wheels. Does anybody know how +0.16minutes relates to inches? i.e. 1/8"... 1/4" total toe (in).

Thanks,

kj
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Old February 22nd, 2005, 02:59 PM   #2
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That's a good question. I guess one could to the math and figure it out if bored enough to do so

I ran across a site that had a generic answer - perhaps this is enough.
Q. How important is wheel alignment?

A. Research indicates that the average car is driven about 12,000 miles per year. A car with toe alignment just 0.34 degrees (Just 0.17 inches) out of specification has dragged its tires sideways for more than 68 miles by the end of the year!
If you need more it may be time to pull out the trigonometry books.
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Old February 22nd, 2005, 03:17 PM   #3
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I think I've dragged my tires

more than 68 miles. After maybe 170 units, I don't think I've saved one textbook. At least I can get a relationship out of this (if 17" wheels were the assumption)

thanks,

kj
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Old February 22nd, 2005, 03:46 PM   #4
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Yeah, but alignment angles aren't static. They change constantly. They change as your gas level changes. They change when you have cargo or a 2nd passenger. They have to change even some small amount when you eat a big meal or seat off some water weight. And that doesn't even talk about way they change as you drive going over bumps, dips, and varying cornering loads. I don't think any of here has a time:money ratio significant enough to allow us to obsess about tire wear. In terms of performance (with which I think Ken is at least somewhat concerned - hehe) I wouldn't put too much sleep on the line figuring out a perfect angle for perfect all around performance. Maybe just get a pyrometer and for any given course, take measurements in order to determine that on that day, on that surface, with that load of fluids and passengers, you either have the tires heating up evenly or you need to tweak the alignment a little hear or there. I'd say the only way Ken will be happy with his alignment is to get the portable equipment and make trackside adjustments as needed. But that's just what I gather from the alignment questions I've read so far. BTW, if you figure out how to do that, teach me. I would really love to do that sort of work myself as well.
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Old February 22nd, 2005, 04:10 PM   #5
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Alignment

What units is Sonnen using? You asked about inches, degrees and minutes.

If you want to convert degree to inches, multiply 0.01746 by the length in inches. Here is an example for 17 inch wheels and 1 degree of angle.

8.5 x 0.01746 = 0.148 displacement at the edge of the wheels

The distance from axel the edge of the wheel is 17/2=8.5 inches

Now if you want to convert minutes to inches use this:

http://quake.geo.berkeley.edu/convert/degree.html

this translates 0.16 minutes into 0.2667 degrees

so

.2667 x .01746 x 8.5 = 0.03957

which is damn close to a millimeter
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Old February 23rd, 2005, 09:57 AM   #6
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Thank you Victor!

This is good. I knew it was out there.

The actual reason behind my concerns are based on conversations picked up here and there about rear toe. I like knowing the 'approximate' number in order to translate my experiences and feel of the car. I now hope to learn "degreespeak".

I think Doug Mac might have been with me one time at Alameda when I tried negotiating a 'kink' in the road that was a real key to cutting time if you could keep the speed coming out of it. This car can get pretty squirrelly trying to power out. I've always felt the car was too loose in the rear end coming out of turns (as opposed to going in) - I just chalked it up to the Boxster weight balance vs. the Carerra and just not being as effective with planting the rear end coming out of turns.

Anyway -this tends to confirm my suspicion that I have too little toe (in) for the rear. I think most people's numbers are from 1/8" to 3/16" total toe. This tells me I'm ~1/25" total toe (1mm) - about 3 times below the minimim of my surveyed group. Woof. 1/8" total toe is a little less than 1/2 degree per side.

What's odd about the Porsche specs is that the Porsche targeted toe is +0.05' (minutes) per rear wheel. It seems they don't allow enough camber in front for safety reasons and yet have the rear end goosey at speed?

Thanks guys,

Okay Scott - I'll give Custom Alignment a call.

kj
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Old February 23rd, 2005, 11:16 AM   #7
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Glad it helped. I recall past threads discussing Porsche's alignment issues. I think the alignment is set up for everyday driving to maximize tire wear and not AX or DE performance. I believe the complaint is the Boxster has understeering issues that can be correctede with "proper" alignment, but at the cost of tire wear.
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