Wheel Alignment Question (Tow-In)

[Jaydebe]

First time Jeeper here and I'm curious about a few videos and posts I've seen talking about setting the tow-in.

I believe that I've read a good tow-in is set at around 1/16"-1/8" in at the front tires.
I've also seen that someone makes a bolt on tool (that attaches to the rotors) for setting a more precise measurement.

My question is, does tire size effect where (the distance in front of the hub center) the tow-in should be set from?

So, it seems to me that if you have 31s and set the tow-in at 1/16" at the treads or 37s set at 1/16" at the treads; that is really two different tow-ins, especially if you consider maybe using a more "fixed" point like the fronts of a stock rotors.

Am I overthinking this? Am I onto something?

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[Wet Willys]

Tire size won't change toe settings. 1/16"-1/8" means then the wheels are perfectly straight, you go 1/16-1/8" in from there (shorten the tie rod). This helps a lot with tracking, and steering wander.

 

[QuickOne]

Tire size won't change toe settings. 1/16"-1/8" means then the wheels are perfectly straight, you go 1/16-1/8" in from there (shorten the tie rod). This helps a lot with tracking, and steering wander.

This is potentially false, depending on how you measure it.

If you're using degree/angles, then tire size won't matter. If you're using distance (i.e., 1/16" or 1/8") and measuring at the tread, then tire size absolutely matters and will change it. Basic geometry.

I personally setup a perfect square (using SmartStrings) around the car and measure distance to the wheel edge to keep it consistent and not worry about tire size. I compute the degrees of toe-in based on the distance with a radius of the 17" wheel.

 

[Wet Willys]

This is potentially false, depending on how you measure it.

If you're using degree/angles, then tire size won't matter. If you're using distance (i.e., 1/16" or 1/8") and measuring at the tread, then tire size absolutely matters and will change it. Basic geometry.

I personally setup a perfect square (using SmartStrings) around the car and measure distance to the wheel edge to keep it consistent and not worry about tire size. I compute the degrees of toe-in based on the distance with a radius of the 17" wheel.

If you're measuring from center of tread left to right, front and back of the tire, and using that as your reference point, then adjusting toe from there, tire size makes no difference.

 

[QuickOne]

If you're measuring from center of tread left to right, front and back of the tire, and using that as your reference point, then adjusting toe from there, tire size makes no difference.

Sorry, but you are incorrect.

If your goal is to set 1/8" of toe-in on a 33" tire (Rubicon stock), then sure, you've achieved your goal. But 1/8" in on a 31" tire is different from a 33" tire, which is different from a 37" tire. However, the actual alignment values (toe values) are different and it impacts how the tire wears and drives on the contact surface.

To be specific
1/8" in on 31 is 0.116 degrees
1/8" on a 33 is 0.109 degrees
1/8" on a 37 is 0.097 degrees

Double those numbers for total toe in. As you can see, it is different and significant.

Details matter here.

 

[Wet Willys]

Sorry, but you are incorrect.

If your goal is to set 1/8" of toe-in on a 33" tire (Rubicon stock), then sure, you've achieved your goal. But 1/8" in on a 31" tire is different from a 33" tire, which is different from a 37" tire. However, the actual alignment values (toe values) are different and it impacts how the tire wears and drives on the contact surface.

To be specific
1/8" in on 31 is 0.116 degrees
1/8" on a 33 is 0.109 degrees
1/8" on a 37 is 0.097 degrees

Double those numbers for total toe in. As you can see, it is different and significant.

Details matter here.

You think a variance of .019 degrees is significant?? Look up the factory tolerance on toe.

 

[QuickOne]

You think a variance of .019 degrees is significant?? Look up the factory tolerance on toe.

That's a variance at a single wheel in degrees - double that for both. And yes, it absolutely matters regarding drivability and handling road variance.

Tolerance is not the same thing as specification. Tolerance also does not mean there isn't a perceivable difference in function. It simply means within specified range. Plus, I wouldn't call FCA's recommendations for alignment specs good - just look at their caster numbers. You get much better feel at 6, which is well outside specification.

But, hey, some of us are more in-tune (read: sensitive) to what's happening with the cars than others are. I'm quite tuned into it.

I build race cars and the details and feedback matters.

 

[Wet Willys]

That's a variance at a single wheel in degrees - double that for both. And yes, it absolutely matters regarding drivability and handling road variance.

Tolerance is not the same thing as specification. Plus, I wouldn't call FCA's recommendations for alignment specs good - just look at their caster numbers. You get much better feel at 6, which is well outside specification.

But, hey, some of us are more in-tune (read: sensitive) to what's happening with the cars than others are. I'm quite tuned into it.

I build race cars and the details and feedback matters.

This is not a race car, where that level of precision might be necessary, it's a Jeep. You're going to have this guy worrying about measuring his toe to the thousand of an inch.

I did alignments for years, and I can assure you that a variance .038 degrees of toe on a Jeep can not be felt.

 

[QuickOne]

This is not a race car, where that level of precision might be necessary, it's a Jeep. You're going to have this guy worrying about measuring his toe to the thousand of an inch.

I did alignments for years, and I can assure you that a variance .038 degrees of toe on a Jeep can not be felt.

Again, I disagree as I felt it on my own JL from the dealer.

My point was ultimately to not spread mis-information about technical details.

And I'm glad I never took my car to your alignment shop. I'm an engineer, and the details absolutely matter.

 

[Wet Willys]

Again, I disagree as I felt it on my own JL from the dealer.

My point was ultimately to not spread mis-information about technical details.

And I'm glad I never took my car to your alignment shop. I'm an engineer, and the details absolutely matter.

Take your Jeep to an alignment shop and tell them you want your total toe set within 38 thousandths of a degree from the spec on your Jeep and see how much they roll your eyes at you. That's probably within the measurement uncertainty of the alignment machine.

And congrats on being an engineer, so am I. I know when to sweat these kinds of details, and when not to.

Always one of these guys on these forums.....

 

[QuickOne]

Take your Jeep to an alignment shop and tell them you want your total toe set within 38 thousandths of a degree from the spec on your Jeep and see how much they roll your eyes at you. That's probably within the measurement uncertainty of the alignment machine.

And congrats on being an engineer, so am I. I know when to sweat these kinds of details, and when not to.

Always one of these guys on these forums.....

Dude, calm down. I'm just making sure the facts are correct without opinions or interpretations. I don't take my cars to alignment shops for a reason - TO ME, the details matter. Perhaps they don't to you.

One of these guys.. nice. Great comeback when the reality is you're giving out false information.

 

[Wet Willys]

Dude, calm down. I'm just making sure the facts are correct without opinions or interpretations. I don't take my cars to alignment shops for a reason - TO ME, the details matter. Perhaps they don't to you.

One of these guys.. nice. Great comeback when the reality is you're giving out false information.

Not false information, you're being super anal and ridiculous. 1/16"-1/8" total toe-in is a good setting for a 32" tire, or a 37" tire.

OP: the point of the tool that you're talking about is to give you a better way to take the measurement. It's hard to be precise when you're laying on your back measuring from the center of the tire tread, not to mention you're using a tape measure, not a high precision measuring tool. I always err on the more positive side of toe-in range for a more stable steering feel.

 

[Jaydebe]

Thank you guys; I get both points you are trying to make.
For the record; I used to do build VWs, build motorcycles (was relentless on specs & torque) and renovations (specialized in fine trim out) and I can be anal myself about measurements.

I asked the question because it became apparent to me that when adding different tire sizes and measuring tow from the treads that there is some weirdness.

For the record, I installed 37s on a factory alignment at 9k miles with ZERO issues. The wear pattern is amazing on Grabber ATx. Almost ZERO feathering at 17k miles of wear as long as I rotate every 5k +/- based on wear pattern.

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