vince vette 2

Finally getting things in the rear suspension back together. Looking to get approximately close on the alignment while the car is still off the floor. I'm looking for a reference line to use to judge toe. It appears from frame schematics that the frame section under the doors are parallel, so a straight edge running along that section of the frame and back past the rotor should provide a reference good enough to adjust toe in close enough to drive to get a final alignment.

Assuming that will work, my plan is to set up as shown below. I'll put the straight edge on the frame. Measure the distances M1 and M2 from the straight edge to either side of the rotor. To get the tire toe in, I'll take the difference between M1 and M2, divide by M3, the distance between the measurement points on the rotor, and multiply by the diameter of the tire. In my case where I'll be measuring on the rotor the M3 is about 9 inches. My target toe in for one side is 0.125" and the tire to go on is the stock 255R15-60 with a 27 in diameter. So, to get a toe in of 0.125 M1 minus M2 should be about 0.040". Does all that sound right.

Note I'm supporting the rotor so the half shaft is basically level which seems close, maybe a little high, to it's normal position with the car sitting on the ground.


rough alignment set up

cagotzmann

To much work to set rear toe, and it won't be getting the trust angle and total toe setup within specs.

To setup the rear toe you need to setup the camber angle equal on both wheels. Set something 0 - - 0.5 degrees. Then set the thrust angles where the rear wheels have the same angle relative to the front axle.

Here is a picture for measuring the rear wheel relative to the front axle.



Make both side as close as possible. You won't be able to get 100% since the shim thickness won't allow a perfect setup. Once the thrust is set relative to the "front axle" not the frame then you can set total toe. Remember the alignment is the front axle & rear axle where the tires are installed, using the frame sounds ok, but there is a lot of connections (parts) that connect to the frame to set the actual wheel position.

Next to measure total toe lets look at the math to calculate toe angle. As you have indicated using a 9" distance to measure M1-M2 the actual accuracy is based on the difference of M1-M2. Using a ruler you could read 1/32" inch repeatable. If you use a digital caliper and you took a average of 5 measurements you will also get ~ 1/32" difference. So over a 9" distance this equals ~ 0.1989 degrees per wheel. This is not very accurate. Use this calculator to convert degrees to toe measurements.
https://robrobinette.com/ConvertToeDegreesToInches.htm
So total toe is ~ 5-6/32".

To make this easier to get more accurate measurements ( allows for less perfect setup to increase the margin of error ) we need to increase the distance for M1 & M2 difference.

Here is my setup.



I use a laser pointer to the front and rear bars for measurement ~ 108" distance. Simple to get the difference.
1. left wheel point to the front bar record the reading. lets say 908mm mark.
2. left wheel point to the rear bar and set the bar to read 908mm mark.
3. right wheel point to the front bar record the reading lets say 2400mm mark
4. right wheel point to the rear bar record the reading lets say 2405mm mark.




So total toe = inverse tan of (2405mm-2400mm)/2743mm (108") calculates to 0.1044 degrees total toe. ~ 3/32" total toe.

But the important point is its easy to setup and its easy to read a difference of 1mm over 2743mm giving a accuracy of ~ 0.02088 degree's total toe

Even if you misread a point by 1 or 2 mm the error is ~ 1/32" total toe.

Total time to take rear toe readings is about 5 minutes.

Place the rulers at the wheels. Set the front and rear bars. Setup the laser on 1 wheel. Set the rulers to a set point. Move to the other side and take 2 readings.
Subtract the readings and then if you make a chart from 0 - 10mm difference with the calculations pre done it simple to measure total toe on any car without a complicated setup.

Now for making the changes on the car is a different story. But once to setup the thrust angle and have measured your total toe, then all you need to do is move equal shims on both sides until you get the total toe you desire.

vince vette 2

Thanks for all the detail. And the issue of the absolute measurement error being constant but the error as a percentage of the length measured being reduced by taking a longer measurement is an excellent point. Basically where I'm at, the rear of the car frame on blocks with the whole car on a quick lift, I can't set up as you have. But hopefully what I've done will get me close and at least having right total shims in. I guess as part of that I should try to have at least a couple 1/32 shims on both sides of the TA bushing to make small adjustments easier later.. Once I get it on the floor I can set up more as you've shown to get it aligned relative to the front axle. Much appreciated.

gkull

Cagotzmann, have you ever driven your vette with zero rear toe? There are some real advantages to that. I have driven solid rear end cars and open wheel lapping race cars with all kinds of toe in settings

Toe in creates self correcting, but also causes less predictable rear drifting say if you lift the throttle. An extreme example is a drifter race car. Solid locked rear end and zero toe to create clouds of smoke going around the track with very predictable slides. My vette has had zero rear toe.


Very brilliant alignment that you do

ignatz

Zero toe? I take the simple route and use the string method to get zero toe. Works good enough for me and I would think good enough for the OP to get his car to an alignment shop.

Using the strings, I set my rear tires to zero toe .... but isn't the rear track slightly wider than the front track? That gives me a little toe in and I figure that rolling friction and compliance does the rest to take it back a bit to zero. Thoughts?

69autoXr

Yes, 58.7" front vs. 59.4" rear for a '69, for example.

vince vette 2

Same on '80.

But maybe the first question I should have asked, what is the stock rear camber and toe spec? I've seen two different listings (and my '80 shop manual conveniently has none that I can find).

One source states camber -0.2 to -0.9 camber and toe total -1/32" to +1/32"

The other stated -0.5 to -0.8 camber and toe total +1/8" to +1/4"

gkull

Because of my 13 inch rear wheels added about 9 inches to my rear track width compared to a stock 79. My fronts added 4 inches.

335 width tires

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cagotzmann

I run about 3-5/32" this I found works great with high speed braking. Running 0 may make the rear drift more in high speed braking. I find this reasonable and my track which only has 2 corners and mostly 1 where off throttle helps turn the car in the direction I want.