Trigonometry and Toe Plates
Thread Starter
Instructor
Joined: Apr 2024
Posts: 152
Likes: 36
From: Katy, Texas
Trigonometry and Toe Plates
Toe plates are an inexpensive and accurate tool for adjusting front and rear wheel toe on our cars, but many folks use them incorrectly, resulting in a fairly significant error.
Prescribed toe specifications are based on the difference between the front and back measurements across the diameter of the rims. In contrast, toe plates have measuring slots that are typically spaced considerably farther away than the rim diameter. For instance, the spacing between my toe plate slots is 22” and measuring the toe using 22” slot spacing will not be the same as measuring toe based on, say, a 15” rim diameter (my '76 C3). For example, 1/16” (0.063”) toe-in based on 15” diameter rims becomes 0.094” when measured using 22” toe-plate slot spacing. The result is an error of 0.031”.
To achieve accurate results using toe plates you must take into account the difference between slot spacing on the plates and the rim diameter. First, trigonometry is used to determine the toe angle based on the rim diameter and the prescribed toe in fractions of an inch. This angle is then used to determine what the prescribed toe will become when measuring with toe plates.
I hope this helps folks to use toe plates correctly.
Jason
Prescribed toe specifications are based on the difference between the front and back measurements across the diameter of the rims. In contrast, toe plates have measuring slots that are typically spaced considerably farther away than the rim diameter. For instance, the spacing between my toe plate slots is 22” and measuring the toe using 22” slot spacing will not be the same as measuring toe based on, say, a 15” rim diameter (my '76 C3). For example, 1/16” (0.063”) toe-in based on 15” diameter rims becomes 0.094” when measured using 22” toe-plate slot spacing. The result is an error of 0.031”.
To achieve accurate results using toe plates you must take into account the difference between slot spacing on the plates and the rim diameter. First, trigonometry is used to determine the toe angle based on the rim diameter and the prescribed toe in fractions of an inch. This angle is then used to determine what the prescribed toe will become when measuring with toe plates.
I hope this helps folks to use toe plates correctly.
Jason
Safety Car
Joined: Nov 2008
Posts: 3,717
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From: Seattle Washington
I then do the opposite for the Front, I shoot backwards to my 28 inches on the Frame and then measure the distance off my rear Axels. This gets more complicated due to my suspension changes because my cars are not square, in other words my Axel Widths are not the same front to rear.
Thread Starter
Instructor
Joined: Apr 2024
Posts: 152
Likes: 36
From: Katy, Texas
Thread Starter
Instructor
Joined: Apr 2024
Posts: 152
Likes: 36
From: Katy, Texas
Drifting
Joined: Nov 2021
Posts: 1,736
Likes: 386
From: People's Republic of Nueva York
Considering that they don't make calipers big enough to measure the track width of a car and these things are getting done with tape measures I suspect you guys are cutting it a little too fine. But if it makes you feel better, have at it.
Race Director
Joined: Nov 2006
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From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
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Cruise-In VIII Veteran
I think it's just terrific that Jason assumes that we all align our cars incorrectly. And have no idea how to correctly use tools.
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Old Pro Solo Guy
Joined: Dec 2017
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From: Marlton NJ
Ummm... toe-in is measured in inches at the tire tread, not the wheel rim.
According to BFG Racing Tire guys who trained me, and other racing groups.
The rim is just a convenient locating point.
But the confusion over that very issue is probably why they switched to measuring toe-in in degrees at some point.
And if you are measuring it in degrees, it does not matter where your locating / measuring points are.
According to BFG Racing Tire guys who trained me, and other racing groups.
The rim is just a convenient locating point.
But the confusion over that very issue is probably why they switched to measuring toe-in in degrees at some point.
And if you are measuring it in degrees, it does not matter where your locating / measuring points are.
Thread Starter
Instructor
Joined: Apr 2024
Posts: 152
Likes: 36
From: Katy, Texas
Race Director
Joined: Jun 2000
Posts: 10,046
Likes: 675
From: FL
Ummm... toe-in is measured in inches at the tire tread, not the wheel rim.
According to BFG Racing Tire guys who trained me, and other racing groups.
The rim is just a convenient locating point.
But the confusion over that very issue is probably why they switched to measuring toe-in in degrees at some point.
And if you are measuring it in degrees, it does not matter where your locating / measuring points are.
According to BFG Racing Tire guys who trained me, and other racing groups.
The rim is just a convenient locating point.
But the confusion over that very issue is probably why they switched to measuring toe-in in degrees at some point.
And if you are measuring it in degrees, it does not matter where your locating / measuring points are.
https://www.vansteel.com/PDFs/alignm...82-degrees.pdf
https://www.vansteel.com/PDFs/alignmentspecs6382.pdf
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
Not really an issue either as we are using "custom" specs that were not measured or provided "at the rim".
Angle specs are more precise and can be calculated no matter how long the plates are. Longer the plate, the better the precision will be.
Thread Starter
Instructor
Joined: Apr 2024
Posts: 152
Likes: 36
From: Katy, Texas
Yes, the difference is quite small ~1/32" but the error increases when using the same toe plates with smaller wheels, such as the 14" wheels on my '66 MGB.
Jason
Old Pro Solo Guy
Joined: Dec 2017
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From: Marlton NJ
But toe-in (using the unit of inches) is measured (or more correctly reported) at the tire tread, say ~27".
Anything reported at 22" or 15" or 14" is not correct.
You are correct the math does not work out the same at the various distances.
This "toe-in at the rim" myth has been around for ages.
It happens because that is where most of the equipment attaches.
But examine their charts, or the calculations, and the reported measurement works out to the tire tread distance.
And some people just calculate it wrong.
If you do not want to listen, to either reasoning, or experience, I'm done.
Anything reported at 22" or 15" or 14" is not correct.
You are correct the math does not work out the same at the various distances.
This "toe-in at the rim" myth has been around for ages.
It happens because that is where most of the equipment attaches.
But examine their charts, or the calculations, and the reported measurement works out to the tire tread distance.
And some people just calculate it wrong.
If you do not want to listen, to either reasoning, or experience, I'm done.
Last edited by leigh1322; Dec 11, 2024 at 04:40 PM.
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
way so .031" per tire....
The difference is not measurable.
Thread Starter
Instructor
Joined: Apr 2024
Posts: 152
Likes: 36
From: Katy, Texas
Measuring toe at the rim is not a total myth; see the attached.
Jason
Thread Starter
Instructor
Joined: Apr 2024
Posts: 152
Likes: 36
From: Katy, Texas
Calculate the toe angle on your left-hand sketch and then calculate the toe angle in the right-hand sketch and you'll find quite a difference between the two angles. I calculated the following: left toe angle = 0.081^0 and right toe angle = 0.12^0.
Jason
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
Jason,
If we want to adjust for the 22" toe boards.....What is the correct toe measurement (in inches) with the longer 22" toe boards if you are shooting for your "true" 1/16" toe-in at the 15" rim?
If we want to adjust for the 22" toe boards.....What is the correct toe measurement (in inches) with the longer 22" toe boards if you are shooting for your "true" 1/16" toe-in at the 15" rim?
