Comparing Alignment Specs to Measured Toe In

[ksagis]

Has anybody gotten an alignment and compared it to a measurement at home? I'm pretty decent at math, and just can't make any sense of things. I measured the difference in back to front at two difference places very carefully, getting back measurement of 65 7/16" and front measurement of 65 10/16", for a difference of 3/16" toe-out (yes, odd to have have toe-out, fixing that but wanted to get a baseline first)

The alignment sheet from a recent check shows total toe of -0.07 degrees and the first way I calculated degrees used my actual tire diameter (Krawlers are 34.5"):

arcsin (3/16 / 34.5") = 0.31 degree toe out

I'll bet their Hunter system doesn't account for after-market tire size and uses what is probably in their system for a 1972 Bronco for default tire diameter. That actually makes it worse since the factory tire diameter is smaller:

arcsin (3/16 / 29") = 0.37 degrees toe out

This website looks like they do the math the same as me: link

I know I'm overthinking it, mainly interested if somebody has gotten an alignment and also has an at-home measurement. If you have, can you post up the details please?

Is it possible that bigger tires and wider tires are totally throwing off their system. Seems like toe should be an easy thing to measure on alignment machine that wouldn't be affected by tire size.

P.S. I've checked for slop in front end that could be contributing to weird measurements and all seems tight.

 

[Oldtimer]

The alignment shops I have been in clamp their instruments to the rim, read angles (not inches), and don't analize at the tire diameter.

 

[Yeller]

The alignment shops I have been in clamp their instruments to the rim, read angles (not inches), and don't analize at the tire diameter.

Hence why they use degrees, they are consistent to infinity.

 

[jamesroney]

The alignment shops I have been in clamp their instruments to the rim, read angles (not inches), and don't analize at the tire diameter.

You must be going to one of those fancy computer alignment shops.

@ksagis Don't you find it funny that the manufacturer makes a specification in inches, based on a tire diameter, then that specification gets converted mathematically to degrees, by a front end alignment machine maker (so as to satisfy the @Yeller infinity requirement...) and then the angle is used from then on without regard to tire size? It is a incorrect assumption to believe that toe in is an angle that does not contemplate tire diameter.

Freightliner's NHTSA alignment spec bulletin sets the toe at the same 1/16 inch as a honda Civic. (but the angle is specified as .09)

...and don't ever measure your toe in from a feature on the tread. Take a piece of tape, and attach it to your tires. Mark it with a line. Roll the tires 1/2 rotation so the marks are in the rear, and measure the same marks. This will account for tire and wheel irregularities. Don't jack up the wheel to make the rotation.

And your measured caster and camber are perfect. Do NOT use the thing that is called "specs" from your PDF. 7 degrees of caster and 0 degrees camber is perfect for a Bronco with radial tires and power steering on 31 inch tires. You might go to 6 degrees of caster with the 34's, but I wouldn't. Give yourself 1/16 of toe IN, and tell me that it doesn't handle great.

 

[ksagis]

Yup, thats what I did, draw a crisp line on two pieces of tape, put the tape on the tire, and measure between the two crisp lines. I didn't worry about getting the tape in the middle of the tire and instead placed it on a tread lug to get good adhesion that I could use to hold measuring tape. I also offset and started reading at 2" mark to help me get the end of the tape placed well.

And yes, pretty happy with caster and camber based on everything I read here, and only messing with toe.

Some quick math shows that a 1/16" of measurement error translates to about 0.1 degrees of error (actually 0.09 deg for my size tires), I'm sure I was +/- 1/16" when measuring and probably closer to about half of that during my usual "that's a fat 1/16 or a that's a thin 1/16" approach.

I may need to go back and check how accurately my helper was holding the other end of the tape to make more sense of it. I'm struggling to make sense of my measurement of 3/16" toe out when based on alignment machine specs that I should have measured slightly under 1/16" toe out (if I believe the -0.07 degrees which is toe out)

(I only rolled it forward and kept tires on ground for all measurements)

Appreciate the feedback.

 

[lars]

You must be going to one of those fancy computer alignment shops.

@ksagis Don't you find it funny that the manufacturer makes a specification in inches, based on a tire diameter, then that specification gets converted mathematically to degrees, by a front end alignment machine maker (so as to satisfy the @Yeller infinity requirement...) and then the angle is used from then on without regard to tire size? It is a incorrect assumption to believe that toe in is an angle that does not contemplate tire diameter.

Freightliner's NHTSA alignment spec bulletin sets the toe at the same 1/16 inch as a honda Civic. (but the angle is specified as .09)

...and don't ever measure your toe in from a feature on the tread. Take a piece of tape, and attach it to your tires. Mark it with a line. Roll the tires 1/2 rotation so the marks are in the rear, and measure the same marks. This will account for tire and wheel irregularities. Don't jack up the wheel to make the rotation.

And your measured caster and camber are perfect. Do NOT use the thing that is called "specs" from your PDF. 7 degrees of caster and 0 degrees camber is perfect for a Bronco with radial tires and power steering on 31 inch tires. You might go to 6 degrees of caster with the 34's, but I wouldn't. Give yourself 1/16 of toe IN, and tell me that it doesn't handle great.

James, so what you are saying is that toe, measured in inches, should be independent of tire diameter. If running larger than stock tires, the method you describe would result in less toe than stock, measured in degrees. Just clarifying.

And full disclosure, when I installed new knuckles on my Dana 44 last summer, I clamped (very straight, yes I checked) steel bars to my brake rotors, measured fore/aft, converted linear distance to degrees, etc. Though I don't recall what angle I targeted offhand. I messed with it a couple of times out of curiosity (more angle, less angle) though always toe-in. I couldn't tell any difference. Tires are "metric 35's." I'm at zero camber and 6 degrees of caster, the latter accomplished by rotating the knuckles oh-so-long-ago, as we've both advocated exhaustively.

 

[jamesroney]

James, so what you are saying is that toe, measured in inches, should be independent of tire diameter. If running larger than stock tires, the method you describe would result in less toe than stock, measured in degrees. Just clarifying.

And full disclosure, when I installed new knuckles on my Dana 44 last summer, I clamped (very straight, yes I checked) steel bars to my brake rotors, measured fore/aft, converted linear distance to degrees, etc. Though I don't recall what angle I targeted offhand. I messed with it a couple of times out of curiosity (more angle, less angle) though always toe-in. I couldn't tell any difference. Tires are "metric 35's." I'm at zero camber and 6 degrees of caster, the latter accomplished by rotating the knuckles oh-so-long-ago, as we've both advocated exhaustively.

I think I'm saying that. Empirically, it appears that the OEM's specify 1/16 inch of toe regardless of diameter within the 22 to 40 inch diameter range. In the limiting case, if you use a constant angle, a tire of huge diameter would have huge toe in, as the tires would eventually touch each other.

I always thought that the purpose of toe was to bias the tread against the carcass, so that the tread belt is always pushing in a constant direction against the wheel. If the toe is exactly zero, then the tread can squirm relative to the wheel, and result in vague steering. Which is why some people run 1/16 of toe OUT successfully.

But I am truly speaking from a position of observation, and not fundamental knowledge. I cannot derive from first principles any reason why 1/16 inch of toe in would have become the industry standard. It is entirely possible that .08 degrees is the correct target specification for optimal performance in the practical tire diameter regime.

I would be interested to know the toe in specification for modern aircraft landing gear. (Like the nose wheels of a 737.)

 

[Yeller]

Here's another toe in/toe out analogy.

Inherently there is a miniscule amount of movement, and for practical purposes we'll call it slop or undesired motion in tie rod ends, ball joints, even in the wheel bearings. At zero toe those miniscule amounts of motion create a small amount of motion that creates vague steering and allows the wheels to wabble, which is the start of the dreaded death wabble. That is why sometimes you can add a touch more toe in or toe out and the problem will go away. As James pointed out it also consistently loads the tire carcass, so it doesn't squirm.

I'll also throw in a bit on caster, the magic target of 5-7 degrees of caster is also important. Besides the weight of the vehicle forcing the tires to run straight and true, it adds to that force on the components to eliminate the unwanted motion, thus removing the wobble.

Caster also adds to tracking by basic physics. I'll elaborate for those that may be following trying to learn something, I know from previous discussions that James, Lars, and myself are on the same page, we just describe it differently. I compare it to a hammock; we've all used one. Is a hammock that is perfectly vertical with an attachment near the ground and directly vertical above useable and stable? No, not at all, it flops and spins in wind like a flag and is completely unstable, you cannot sit or lay in it. Begin to lay that hammock down toward horizontal. There comes a point that it begins to stabilize and become usable, and it is well before being horizontal. Just like a hammock becomes stable and is able hold weight, adding caster does the same. The lower the caster, the more it wants to flop and be unstable, the more that is added the more stable it will become. There does become a point where it is not helpful and you can no longer steer, but that is another discussion.

I guess my point is toe and caster work together to create proper dynamics to have an enjoyable, stable, wobble free driving experience.

 

[ksagis]

Thought I'd share an update on my various posts asking about tips and tricks to get my rig to drive better. I finally got disgusted and changed tires and wheels, and long story short, the rig drives 1000 percent better. Here's a short summary of changes:

Changed from 15 x 10 wheels with 3.625 BS wheels to 17 x 9 with 4.5 BS wheels (Racelines)
Changed from 35x13.5 BFG Krawlers to 315R70x17 (basically a 35x12.5) Cooper STT Pro tires

Not entirely sure what exactly made it drive so much better as there were many things in direction of goodness

-Reduced the sidewall height going from 15 to 17 inch wheels (tires measure within 1/4" of same diameter so sidewall height reduced 1 inch)
-Reduced scrub radius about 1.5 inches (7/8 inch of BS change, and another 1/2 inch due to 1 inch less wide wheel)
-Somewhat less aggressive tread going from Krawlers to Cooper STT Pro
-The Krawlers were nearly 20 years old, but visually in perfect shape, no idea of the age of tire was contributing to poor driving habits

I'm still going to mess around with toe a bit more (currently about 1/8"), but am more in a mode of tweaking a good driving rig (which feels a lot better than where I started).

Got a question for folks that might be a minor contributor to freeway driving habits but want to ask in case folks have info. The original 1972 Bronco had 57.4" track on both the front and back axles based on Ford sales brochure. I've got disks front and back and measured WMS of 59.75" front and 58.06" rear. Based on some poking around the internet, it looks like wider front WMS promotes oversteer and I tend to think short wheel base Broncos would benefit from some degree of understeer. Any opinions out there a difference in WMS front to back matters much at all?

Thanks for all the good advice and folks sharing lots of good info.

 

[toddz69]

Thought I'd share an update on my various posts asking about tips and tricks to get my rig to drive better. I finally got disgusted and changed tires and wheels, and long story short, the rig drives 1000 percent better. Here's a short summary of changes:

Changed from 15 x 10 wheels with 3.625 BS wheels to 17 x 9 with 4.5 BS wheels (Racelines)
Changed from 35x13.5 BFG Krawlers to 315R70x17 (basically a 35x12.5) Cooper STT Pro tires

Not entirely sure what exactly made it drive so much better as there were many things in direction of goodness

-Reduced the sidewall height going from 15 to 17 inch wheels (tires measure within 1/4" of same diameter so sidewall height reduced 1 inch)
-Reduced scrub radius about 1.5 inches (7/8 inch of BS change, and another 1/2 inch due to 1 inch less wide wheel)
-Somewhat less aggressive tread going from Krawlers to Cooper STT Pro
-The Krawlers were nearly 20 years old, but visually in perfect shape, no idea of the age of tire was contributing to poor driving habits

I'm still going to mess around with toe a bit more (currently about 1/8"), but am more in a mode of tweaking a good driving rig (which feels a lot better than where I started).

Got a question for folks that might be a minor contributor to freeway driving habits but want to ask in case folks have info. The original 1972 Bronco had 57.4" track on both the front and back axles based on Ford sales brochure. I've got disks front and back and measured WMS of 59.75" front and 58.06" rear. Based on some poking around the internet, it looks like wider front WMS promotes oversteer and I tend to think short wheel base Broncos would benefit from some degree of understeer. Any opinions out there a difference in WMS front to back matters much at all?

Thanks for all the good advice and folks sharing lots of good info.

Congratulations on all the improvements! I'll go out on a limb and speculate that replacing those aggressive, 20 year old tires was the biggest factor in your driving experience improvement. I also like to see the decrease in scrub radius.

No meaningful input on the WMS differences but I can tell you that Ford set up the Bronco with understeer characteristics due to the track bar being below the drag link in the front suspension (except the '76-'77 rigs).

Todd Z.

 

[stupidboy]

What was this under steering ford built in??

 

[toddz69]

What was this under steering ford built in??

It's a characteristic built into the front suspension/steering that's dependent on the trac bar/drag link relationship to each other. If the drag link is above the trac bar, it promotes oversteer, if the trac bar is above the drag link, you get understeer, and if they're in the same plane (like '76-'77s), things are supposed to be pretty neutral.

Todd Z.

 

[stupidboy]

So then this is oversteer?

 

[toddz69]

So then this is oversteer?

No, that is an understeer configuration (drag link is "under" the track bar).

Todd Z.

 

[DirtDonk]

Thought I'd share an update on my various posts asking about tips and tricks to get my rig to drive better. I finally got disgusted and changed tires and wheels, and long story short, the rig drives 1000 percent better. Here's a short summary of changes:

I'm with Todd. A twenty year old tire is a dead-as-a-doornail tire and should never be expected to work well for the highway.

Changed from 15 x 10 wheels with 3.625 BS wheels to 17 x 9 with 4.5 BS wheels (Racelines)

Huge change in leverage against the steering.

Changed from 35x13.5 BFG Krawlers to 315R70x17 (basically a 35x12.5) Cooper STT Pro tires

Huge change in the rubber meeting the road.
Your tire is basically a "tall" 35 in the old days. Most were between 33.5 and 34 inches tall. Yours are probably in the 34.5 range if they follow the ones on a friend's new Bronco.
Maybe just take a measurement to see where yours fall in the range.

Not entirely sure what exactly made it drive so much better as there were many things in direction of goodness

It was without a doubt all of the above.
But the tires would have made a difference even if you had not changed the other aspects.

-The Krawlers were nearly 20 years old, but visually in perfect shape, no idea of the age of tire was contributing to poor driving habits

Visual means absolutely nothing with regard to tires, other than resale value for someone that does not know to look for the date codes. At 20 years even the best tire is going to be trash most likely.
There might be some situations where a tire is good enough to drive. But even then you would probably feel the difference between them and a new set of tires.
We talk about this here all the time. It can't be stressed enough that there is a very good reason why most larger tire shops won't even rotate, patch, plug or balance tires over 7-10 years old.
They don't want the liability for one, but for another they know that nothing is going to help them and the customer will be upset about it.

I'm still going to mess around with toe a bit more (currently about 1/8"), but am more in a mode of tweaking a good driving rig (which feels a lot better than where I started).

It's all great news. But definitely tweak the toe-in a bit here and there. Drive it, tweak it, drive it for a week, tweak it again until you find the sweet spot.
Toe is easy to adjust, repeatable and easy to put back to where you found it to be happiest.

Back to the age and mileage thing, whether on my cars or trucks I can usually feel when the tires are aging out. Tires wear out inside too. Sometimes long before they show their age on the outside.
Even Death-Wobble is usually "caused" by an internal fault with no visible scarring on the outside.

In more recent years I could typically get about 100k miles on tread wear, even with mud tires on a truck. But with the truck tires they are dead to the world at 65k to 70k miles. Or less.
It's been worse for me with the car tires.
I experimented with four sets of Goodyear tires on my Regal and they all had different characteristics when new and when old. My favorite set were great in every way (including fuel economy) but by the time they had 10k miles on them and showed no visible wear they were starting to make a notable noise. By 15k they sounded like a set of snow tires on your uncles old truck at the mountain cabin!
I put up with the noise until they started acting funny at about 50k miles. Got another set of a different model and while things felt new again, they did not match the cornering or road feel of the more performance oriented set I'd just gotten rid of. However they went for the next 40k miles before I started feeling the funny jitters again. Ran them another 15 or 20 thousand and got rid of them too.
Same for the next set, and finally the fourth set which had never felt quite right from the beginning. But I kept them until I got rid of the car.
Kind of the same thing for my Maxima, when my original set aged out at about 25k miles. Put on a new set of cheap Kumhos and got almost 75k miles on 'em. They started feeling crappy at about 45k but I knew I'd be selling the car and didn't want to pay money for a new set only to get rid of the car.

All by way of saying that even perfectly good looking tires can start acting up in as little as 10k miles, and in my experience were all played out near 50k on them. That's all mileage related though, as none of those sets above got to be more than two years old.
I've never had a decent set of tires that were low mileage but were over five years old. Some even earlier...

You did the right thing.

Paul

 

[ksagis]

Congratulations on all the improvements! I'll go out on a limb and speculate that replacing those aggressive, 20 year old tires was the biggest factor in your driving experience improvement. I also like to see the decrease in scrub radius.

No meaningful input on the WMS differences but I can tell you that Ford set up the Bronco with understeer characteristics due to the track bar being below the drag link in the front suspension (except the '76-'77 rigs).

Todd Z.

Interesting detail, thanks for sharing it. I can't visualize how the relationship of the height of the track bar to drag link affects under or oversteer. Could you elaborate?

Do you mean the height of the bottom end, the top end, the whole thing?

 

[67sport]

Yes, that configuration would promote more oversteer as compared to stock.

Todd Z.

In the interest of educating myself, that front end looks like a perfect example of one that would benefit from a TRO set up and then modifying the track bar brackets to suit?
I'm not looking to criticize the specific truck, I like it very much. Just looking to learn how the TRO option plays into the discussion.

 

[Yeller]

The closer to the same plane the track bar and drag link relationship are, the more neutral the handling. The beautiful truck above probably drives great, start pushing and oversteer will crop up and may even make it feel “loose” or twitchy.

There are other benefits of TRO too, it improves handling by flattening the angle of the links and gets the axle pivot closer to the center of gravity of the truck. Again why stock trucks drive so well.

 

[stupidboy]

The beautiful truck above
Thank you.

 

[ksagis]

The closer to the same plane the track bar and drag link relationship are, the more neutral the handling. The beautiful truck above probably drives great, start pushing and oversteer will crop up and may even make it feel “loose” or twitchy.

There are other benefits of TRO too, it improves handling by flattening the angle of the links and gets the axle pivot closer to the center of gravity of the truck. Again why stock trucks drive so well.

Yeller, when you say "same plane" I think you are referring to having track bar and drag link at same angle, is that right? If that's what you're referring to, I totally get that aspect, it's important to have the same angle for the track bar and drag link and also have the same length. If they aren't matched, for the same change in axle height, they will move laterally differently and disturb the orientation of the front tires due to that change.

I took what Todd was saying to be something about the height, not the angle or length. Maybe I misunderstood what he was saying. (to me oversteer and understeer is totally independent of "messing" up geometry as axle moves up and down).