Rear toe question C2

[John D.]

Had the wheel alignment checked on my 67 a few weeks back and as expected the front toe in was way off which was easy to correct. The rear toe was also off :

-.63 degrees (toe out) on the LH side
-.23 degrees (toe out) on the RH side

Had no shims to correct it at the time. Is there a correlation or rule of thumb for shim thickness to rear toe reading. I know I'll have to remove from the inner pack and add to the outer but if I had an idea how much it would help.

thanks in advance

[Duke W.]

Re: Rear toe question C2

The first thing you do is convert degrees to inches.

Toe (in.) = Tire OD (toe in deg.)(2pi/360)

OE and OE replacement size tires are typically 27-27.5" OD, but measure them to be sure.

Assuming the shim pack is snug with the nut loose you will need to swap some shims from the inside to he outside. First compute the ratio of trailing arm length to tire radius. I think the trailing arms are about double tire radius, but you should be able to measure reasonably accurately with a tape measure. Or if someone has a loose trailing arm laying around maybe they can measure the pivot centerline to the axle centerline.

So lets say the toe is 1/8" out and you want 1/16" in. That's 3/16" total movement at the tire OD, so if the tire radius to trailing arm length ratio is 1/2 you would need to move 3/32" (3/16 times 1/2) worth of shims from the inside to the outside. It's a linear relationship.

Sketch it out on paper and start working on the numbers.

Duke

[Richard G.]

Re: Rear toe question C2

To expound on Duke's comment on the best method to graph this out;

Use graph paper. The X axis use inches and fractions or mills on the other axis.
One line is the intended goal or alignment line.
The other is where the wheel is actually located.
Counting the grid gives the solution.


The same method can be used for equipment alignment.
Much more information is located online if you want to follow up with more information.
Search "equipment alignment using graph paper."

Once the lines are laid out it is easy to calculate the corrections for each side multiple times using the same piece of paper.
It is an much easier than completing the math each time. And as a bonus it supplies a visual of where your wheel is located.
Rick

[Duke W.]

Re: Rear toe question C2

The first thing you do is convert degrees to inches.

Toe (in.) = Tire OD (toe in deg.)(2pi/360)

OE and OE replacement size tires are typically 27-27.5" OD, but measure them to be sure.

Assuming the shim pack is snug with the nut loose you will need to swap some shims from the inside to he outside. First compute the ratio of trailing arm length to tire radius. I think the trailing arms are about double tire radius, but you should be able to measure reasonably accurately with a tape measure. Or if someone has a loose trailing arm laying around maybe they can measure the pivot centerline to the axle centerline.

So lets say the toe is 1/8" out and you want 1/16" in. That's 3/16" total movement at the tire OD, so if the tire radius to trailing arm length ratio is 1/2 you would need to move 3/32" (3/16 times 1/2) worth of shims from the inside to the outside. It's a linear relationship.

Sketch it out on paper and start working on the numbers.

Duke

Sorry, I screwed up my original post. When swapping shims from inside to outside the toe will move about one-half of the shim swap. I copied and pasted my original post with the corrections in bold face type.



The first thing you do is convert degrees to inches.

Toe (in.) = Tire OD (toe in deg.)(2pi/360)

OE and OE replacement size tires are typically 27-27.5" OD, but measure them to be sure.

Assuming the shim pack is snug with the nut loose you will need to swap some shims from the inside to he outside. First compute the ratio of trailing arm length to tire radius. I think the trailing arms are about double tire radius, but you should be able to measure reasonably accurately with a tape measure. Or if someone has a loose trailing arm laying around maybe they can measure the pivot centerline to the axle centerline.

So lets say the toe is 1/8" out and you want 1/16" in. That's 3/16" total movement at the tire OD, so if the trailing arm radius to tire radius is 2 you would need to move 6/16" (3/16 times 2) worth of shims from the inside to the outside. It's a linear relationship.

Sketch it out on paper and start working on the numbers.

I'll also add that you should have at least one 1/32 and 1/16 spare shims on hand. The shims should be a slight interference fit, and require light tapping with a hammer to drive the last one home before you tighten the nut, and the one you tap in should be the thickest shim regardless of which side it's on. That's why you should have the extra thin shims available... in case you need a bit more shim stack to have that slight interference fit. Then with the bolt properly torqued it's very likely that the slotted type shims will ever become dislodged.

Duke