By drilling two holes in the...
By drilling two holes in the rocker panels, Mangum can easily access the framerails. He inserts the tool until it touches the framerail, uses the bubble level to make sure it is level, and then places his string so that it barely touches the upright at both the front and the rear of the framerail. After those things are accomplished, he can begin squaring up the wheels.
Now you can run two strings along both sides of the car. Tie the ends of the strings to jackstands so that they will run a foot or less off the ground when the string is pulled tight and approximately a foot away from the car. Start at the right side of the car and make measurements from the edge of the rim to the string. (Nextel Cup teams keep a set of wheels and tires in the shop that are only used for stringing the car, but most of us cannot afford that luxury. Instead, just make sure you have a relatively new set of wheels on the car that aren't bent.) Make measurements at both sides of the wheel level with the string and move the jackstands until those measurements are equal. You must be accurate to within 1/32 inch. The exact distance between the wheel and the string isn't important, but the key is that the two measurements are the same. Also, it's better to have a round number for the distance-10 or 11 inches instead of a fractional amount-because it will make your calculations much easier later. Once you have that complete, move to the other side of the car and try to set the string at the same distance.
Now you can move to the front wheels and make the same measurements. The difference is that any adjustments you make are going to be at the wheel and not the string. For example, if your measurements at the right rear are 8 inches and you are getting 7.75 at the rear of the right-front wheel and 8.25 at the front of the right front, you have half an inch of toe-out. However, if you get 8 inches on both sides of the right rear and 8.25 on both sides of the right front, then your right-front wheel is outboard of the right rear by 0.25 inch. The same thing holds true for the left side of the car.
This Busch Series team uses...
This Busch Series team uses a similar tool when stringing cars. However, a socket is welded into the front and rear clips at precise locations. A piece of tubing is inserted into the socket. The length and direction is precisely determined so that a string can be tied to grooves in the tubing and be used to square the car.
Once you begin making measurements at the front wheels, you can make toe adjustments by adjusting the tie rods and track width by adding or removing wheel shims. Also, by keeping the string low on the wheel, you can measure how camber changes affect track width. If you string across the middle of the wheel, camber changes are invisible to you. The best practice is to calibrate the entire car, or make everything square. Then, once that is done and you know everything is right, begin putting in your setup package, including toe-out, tread width on the front and rear, and even rear-end alignment as necessary.
Most racers find that the best handling solution is to put all toe-out in the right-front wheel and leave the left front straight. This makes the car more stable on turn entry. Also, when it comes to track width on the front wheels versus the rear, you are almost always better off with the rear wheels slightly narrower-usually by half an inch. Chassis builder Charlie Barham recommends always starting out on the right side of the car and setting it so that the right-front and right-rear wheels are in line with each other. Then you can run the left rear inboard of the left front 1/4 to 1/2 inch. This tightens the car on turn entry and frees it on exit.
Here's a shot of the same...
Here's a shot of the same system at the rear of the car. The only problem with this method for stringing a car is that whenever the car is bumped, it must be thoroughly checked to make sure the sockets welded to the clips haven't been misaligned.
We mentioned earlier that many racers prefer to square their cars based off the framerails. This is mainly because it is faster than the method outlined above and doesn't require crawling underneath the car. The only drawback is making sure the framerails are parallel before reliably squaring the car off of them. Once you have gone through the complete process using your trailing arm crossmember as a baseline, make sure the framerails are parallel.
Without moving your strings, use your tape measure to check the distance from the framerail to the string at both the front and rear of the rail. Those measurements should be the same. You may have to cut holes in the sheetmetal to get to the framerail. Also, make sure you measure against a smooth, flat spot on the tubing. Then, after verifying that the framerails are parallel, you can set your strings off the framerails in the future and simply skip crawling underneath the car to square the rear end before running your strings. Remember, if you ever endure a big hit on the racetrack, you will need to make sure your framerails are square.