On cars with triangulated upper control arms, you can’t set preload with adjustable arms. If you lengthen the same bar, more load is placed on the left rear tire. In a car with a non-triangulated four link, you can shorten the upper right trailing arm to increase the preload on the right rear tire. You can counteract this force by preloading the suspension. In turn, that bite tends to move the car toward the left.
In a high horsepower car, engine torque tends to rotate the car so the right rear tire has more bite. Preload is another tuning tool to counteract body roll rotation. If the car has OEM style rubber suspension bushings, a pinion angle of minus three to minus four degrees is more appropriate. For cars with spherical bearings ( rod ends) or solid rear suspension bushings, most racers use a pinion angle between minus one to minus two degrees (minus is negative angle, or pointing down).
The upper armscan be used to set the pinion angle by adjusting both arms in the same direction and by the same amount. Pinion angle is measured between the pinion gear flange and the driveshaft. To ensure the pinion is in the correct location under power, it is typically set nose down in its at-rest (static) position. As the suspension in a car “wraps up”, the pinion is driven upward. In order to keep the driveshaftand U-joints operating in a (more or less) straight line when you’re on the throttle, the pinion angle has to be correct. Setting Pinion Angle with Adjustable BarsĪdjustable trailing arms will allow you to set the static pinion angle. Avoid these types of trailing armsfor drag racing. Just as bad, some of the top-mount arms (ones that physically raise the upper trailing arm location on the rear end housing) have a tendency to hammer the trunk floor when you get on the throttle. That can work for a mild street or bracket car, but will make a high-horsepower car react violently at launch. The problem with some trailing arms made for Mustangs and GM’s A- and G-bodies is they tend to create a very short (further from the engine) instant center by changing the angle of the bars. You’ll find that an instant location similar to stock is actually close to optimum, provided you don’t jack the body way up in the air. Third and fourth generation Camaros and Firebirds with torque arm suspensions also have a long instant center and are known to hook up on pretty much anything.
Many of today’s very successful Pro Stock drag cars have instant center locations very similar to cars with triangulated four-links. The instant center is also rather high the angles of the bars can raise it somewhere above the front wheel spindle. Trailing Arms and Instant CenterĪ triangulated four-link puts the instant center (I/C) location-the imaginary point of where the rear suspension components intersect, and the point they rotate around in a given (instant) position–way out front, often close to the engine.
So how do you make it work? There are three items to address: shocks, anti-roll bar, and adjustable trailing arms. The system works well enough on the street, but not so well when it comes to hooking up at the drag strip, especially with big horsepower. Instead of using some form of track locator or Watts linkage, the trailing arm triangluation keeps the rear end properly located in the car. The upper arms are angled to form the triangle that gives the suspension its name. This type of suspension has coil springs and upper and lower trailing arms (also called control arms). If you have a Fox-body or SN95 Mustang, a 1980s GM G-body (Grand National, Malibu, El Camino, etc.) or something like an early Chevelle or a GTO, it has a triangulated four-link rear suspension.
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