Really cheap and easy toe-in measurement
Old time solid axle vehicles have all the handling charm of ... well ... trucks. But they do have simple suspensions that can easily be checked and adjusted by any shade tree mechanic. In this demonstration I'll be measuring the toe-in on my 1996 G300DT. I've used this method and two hydraulic jacks to restore alignment to perfection after bending the track rod into a U (thanks for the help Karl).
This method will, of course, also give you the toe-in of more complex suspension geometries. However to do any meaningful alignment on these you will need more sophisticated equipment.
- A length of thin but strong string sufficient to circle the vehicle.
- Four small standoffs (Lego 2x4 full height bricks work really well)
- A fine pitch ruler, 0.5mm graduations work best
- A Flashlight
- Pencil & Paper
- Patience, this will take some fiddling the first time
1) Tie the string in a circle around all four tires
Center the steering wheel as much as you can. Then, starting at one tire, pull the string all the way around the four tires. Try to get the string near the centerpoint of each tire as you go. Tie the ends together making sure the string is good and taut. At the four corners of the vehicle insert the blocks. The blocks help to ensure that the string only touches at these four spots. The blocks should sit near the fattest part of the tire bulge.
2) Ensure the line really crosses the center point of each wheel
The accuracy of this method is all in the setup. Make sure the line crosses exactly at the center point. Mercedes has graciously provided a point in the exact center of the wheel, use it. While you are at it, make sure the blocks are in roughly similar positions on the four wheels.
Using the ruler, measure the distance from the string to the front and back edges of each rim. The flashlight comes in handy here. You can create a sharp shadow of the string on the ruler, making it easier to get an accurate reading. Be consistent about reading the outside or inside edge of the string.
4) Calculating the Results
Write your findings down as you go. A cartoon drawing of the car helps keep the values in their correct places.
Subtract the back measurement from the front measurement on each side. Then subtract the left side from the right. The end result is twice the toe-in at the front edge of the rim. In the example above one can see that the wheels are turned slightly to he left and that I've got 0.5mm of toe-in. That is within spec, however 0.0mm is ideal for the G. You will find that the specs for your vehicle are written in degrees. Generally 0 degrees is ideal for two solid axles, which translates to 0mm. If some angle is desired, trigonometry is your friend.
Also: If the suspension is worn, you may want to apply some outward pressure to the leading side of the front wheels to simulate the spreading that occurs with forward motion.
6) Fixing the alignment
Actually changing the toe-in is relatively simple. The catch is that the adjustment bolts are usually fiercely corroded on. Big wrenches are required. In this situation, I've found it easiest to remove the track rod and hold it in a really large and well anchored bench vise. This also gives you some more fighting room and lets you use a torch with less risk of expensive damage. If you like to drive in the rough, now might be a good time to get things loosened up and anti-seized.