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A new "Setting Toe" question....

luderchris

Bronco Guru
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Oct 26, 2008
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I know how to set the toe, but I was thinking.......if you do the "mark a point on each front tire, measure, roll back 180 degrees, measure" etc, wouldn't the size of the tires change the difference you would want to see in the 2 measurements?

With 30 inch tires, 1/8th inch difference would be more than when doing the same procedure with 38" tires. Right?

I am thinking with 38.5" tires I should be looking for more than a 1/8" to 1/4" toe in difference. Maybe more like 1/4" to 3/8" ???

Thoughts?
 
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Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,835
I remember back in alignment class...
Toe setting is based on a standard 29" (I think) tire. Using an alignment machine it won't know the diameter of the tire, all it sees is angles. Actually I did an alignment today and the machine was defaulted to degrees.
Now doing an alignment at home it is easier to measure toe in inches then it is in fractions of a degree. For some reason that has become the standard in refering to toe, not the actual angle that matters or is measured by a machine.

Now when I do it at home I also consider the age of the front end parts. If everything is brand new I tend to set the toe a lot closer to zero. IF the stuff is older and is more free to move around, I go to the spec further away from zero. Now lets say the spec is 1/8" Doing it at home with normal sized tires (say 31") the difference is (lets do the math 31/29=) 1.07 error factor. A little more math and the error is only a hair under .009" You cannot set toe that accurately.
Lets step it up to your 38" tire. 38/29=1.31 That is a little more signifient. Factor that into the 1/8" and we get .039" difference. That is the difference of a spark plug gap. Well withen acceptable tolerance of a home driveway alignment. You are likely to have just as much if not more error because you are not really measureing 90° to the piviot plane of the ball joints front and rear of the axle.
On top of all that who says that the spec is any good with tires that are not even close to any sort of factory spec?
When doing custom alignments (stuff that has no specs) my general guideline is put about ½° negative camber, 5 degrees caster and zero toe in. While not optimal most of the time it generally will get most everything down the road very nicely.
 
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luderchris

luderchris

Bronco Guru
Joined
Oct 26, 2008
Messages
1,140
Loc.
SouthEast PA
Thanks for the help.

I guess I will start at zero and play with it a bit to see how it effects the steering, etc. I won't go more than 5/16" toe in.
 

broncnaz

Bronco Guru
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May 22, 2003
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24,341
If your marks are on the center of the tire it wont matter as then 1/8" is 1/8" at the center of the tire. No matter what size it is.
 
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luderchris

luderchris

Bronco Guru
Joined
Oct 26, 2008
Messages
1,140
Loc.
SouthEast PA
If your marks are on the center of the tire it wont matter as then 1/8" is 1/8" at the center of the tire. No matter what size it is.

1/8" inch is an 1/8", but it does matter because with a bigger tire you are further from the center axis of the wheel. 1/8" is a smaller angle when looking from the front of the truck the bigger the tires are (not that you could tell with the naked eye).

And it doesn't matter if the marks are in the center of the tire tread either. They don't need to be to get an accurate toe reading, they just need to be at the same spot on each tire with both readings.
 

broncnaz

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May 22, 2003
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1/8" at the center of the tire is not 1/8" at the outside edge of the tire. Doesnt matter how far from the center of the wheel you are. Yes bigger tires may appear to be toed in more but they are not really. As was mentioned earlier you may need more toe but probably only because the tierod ends have wear in them. Not really saying the marks you use have to be exactly centered but as close as possible. as it keeps the measurement more constant and true even if the tire size is changed. I've done mine both ways centerred and from the inner edges. always seems like the center measurements produce better results.
 
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luderchris

luderchris

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1)1/8" at the center of the tire is not 1/8" at the outside edge of the tire. 2)Doesnt matter how far from the center of the wheel you are. 3)Not really saying the marks you use have to be exactly centered but as close as possible. as it keeps the measurement more constant and true even if the tire size is changed.

1) Yes it is -- as long as you use the same point for the in front of axle and the behind axle measurement.
2)Yes it does -- the angle would be the same if you were measuring in degrees, but not the inch measurement.
3) See #1 -- doesn't matter where as long as you use the same point for the in front of axle and the behind axle measurement.

Broncnaz -- you have been a big help to me and a wealth of knowledge in the past, but I think you are off on this one. ;)
 

broncnaz

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No problem I've been wrong before. My point is mostly that a measurement from the center will always be correct vs a measurement taken from other points. If you set toe from the center line to 1/8 toe in measurement will not be 1/8 in at the edge of the tire ift will be more than 1/8 so yes your setting at the edge will require more than the 1/8" probably as you stated before probably will take 1/4-3/8 toe in at the edge to get the centerline at 1/8 toe. Diameter of the tire only matters when your not using the tire center line to set toe.
 

W650Mike

Sr. Member
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May 5, 2009
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Eastland, Texas
I think what he’s trying to convey is that 1/8” on a 38” tire is a shallower toe angle than 1/8” on a smaller tire. The linear measurement is the same on any diameter. But the angular measurement varies by the diameter given the same linear measurement.

Inversely, the larger tire would need a larger measured linear distance for the same angular toe setting.

I hope I’m not butting in too much here.
 

broncnaz

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May 22, 2003
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Also I think the angles are a better way to adjust toe as with many of todays steerng setups you have more than one adjustment point so the angles tell you that both tires are set to the specified angle otherwise you could just adjust one side and get the toe in specified but actually not have both tires toed in.
 
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