Stringing a race car may be old-fashioned, but it is one of the most dependable and simple
The process of stringing a car really hasn't changed since guys like Richard Petty and Cale Yarborough were racking up Winston Cup championships back in the day. That's because it works. Yes, there are fancy systems available to do the job, complete with lasers and CNC-etched calipers. And yes, they do work, but unless you are setting up a half-dozen cars a week the cost often isn't worth the benefit.
That's why many racers still rely on a roll of string, four jackstands, and measuring tape when setting up their race cars. The process of stringing a car is straightforward, relatively efficient, and-probably best of all-you know that when the string is pulled tight it's always straight.
The greatest difficulty with stringing a car is establishing what "straight" is. For example, many racers string their cars off the framerails, but if a rail bends in a wreck or the entire frame is knocked out of line, you can string the car all day long and not get the wheels straight. You could string your car based off the driver's seat, but that's hardly realistic.
Instead, consider using the trailing arm crossmember as your starting point. On most cars, the trailing arm crossmember is the steel tube that runs between the framerails, and the rear suspension connects to it. Almost every chassis builder in the country begins a new chassis by placing the trailing arm crossmember in the chassis jig. Everything else on the chassis, framerails included, is based on the location of this piece of steel.
On virtually every tube-frame stock car, the trailing arm crossmember is the first piece o
Begin by using the trailing arm crossmember to square the rear end. You may prefer to race with a little bit of angle in the rear end to make the car more stable, but the car must be squared up in order to string it properly. After you are finished, you can put the preferred pitch back in the rear end. Also, the car should be off the ground because you need to get underneath it. Make sure the wheels are sitting on the stands (which, in turn, are sitting on level floor in your race shop) to properly simulate ride height; you cannot simply put jackstands underneath the framerails.
To square the rear end you will need four plumb bobs, a long straightedge, a tape measure (two is preferable), and tape. Suspend two plumb bobs on string using tape. Be sure to suspend them from the same side of the crossmember (front or back) and to the right and left sides of the crossmember as far outboard as you can comfortably get. Make sure they are extended until they are nearly touching the floor. Once the plumb bobs are perfectly still, line one edge of your straightedge underneath the plumb bobs. This replicates the line of the crossmember on the floor.
Next, secure the other two plumb bobs to the rear end. Like the crossmember, the string can be attached to either the front or the rear of the housing as long as it is consistent. And again, place them as far outboard as possible, making sure nothing gets in the way. The string should be attached to the housing itself.
Once the rear end is square, it can be used to square up the rest of the car. Run strings
Using measuring tape, measure from the straightedge to the tips of the plumb bobs hanging from the rear axle housing. Avoid nudging the straightedge out of position. Also, make sure your tapes aren't running at angles, as they need to be perpendicular to the straightedge. If both measurements are the same, you can be confident that the rear end is squared to the trailing arm crossmember. If they aren't the same, adjust the suspension until they are. The adjustments should be built into a tube-frame car, but if you are racing an OEM suspension you may need creativity to align the rear end. One of the primary problems with stock suspensions is that worn bushings and bent trailing arms throw off alignment. Check for those issues first.
Now that the rear axles are established as the baseline, the direction and location of the front wheels can be based off the rear wheels. The next task is to center the steering system and lock it down so that it cannot move.
Lining up the front wheels is futile if the steering wheel isn't centered. If you race a car with a rack-and-pinion system, all you have to do is count the turns of the steering wheel from lock to lock and then find the approximate center. But if you are racing a car with a more traditional drag-link system, finding the center is more involved.
To do this, simply take a set of cross measurements and adjust the system until those two measurements are equal. First, measure from the center of the pitman arm pivot to the center of the drag-link pivot on the opposite side. Then, measure from the center of the idler arm pivot to the center of the opposite drag-link pivot. If these measurements are equal, the steering rack is centered. If they aren't, turn the wheel in small increments until they are. Most steering boxes have a flat spot built into them at the center point, which you may be able to feel in the steering wheel. Once you have found the center, lock down the wheel so that it cannot be bumped accidentally and knocked out of line. The most common way to do this is with two locking pliers. Lock them onto the steering shaft with the handle against a framerail. They should be set so that one keeps the wheel from turning left while the other keeps the wheel from turning right.
This chassis is still under construction, but it gives you an excellent view of the traili
Racer Shaun Mangum makes measurements from the rear of the left-rear tire to the string at
Once the car is square based off the trailing arm crossmember and the framerails are paral
By drilling two holes in the rocker panels, Mangum can easily access the framerails. He in
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 a similar tool when stringing cars. However, a socket is welde
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 system at the rear of the car. The only problem with this method
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.