Procar Tuning
Henry Gallagher
For more than 20 years we have handled detailed finishing and tuning of Toyota vehicles. In our company, we exclusively install in-house developments as well as products of renowned international manufacturers. Quality control at the highest level is an absolute must for us and is step-by-step further developed.
We offer parts and accessories for BMW M1 Series and Procar in the areas engine / electrical, fuel preparation, transmission, front and rear axle, steering, brakes, body, vehicle equipment, exhaust system, clutch, lighting system and much more.
A lot has been written about four-link tuning. Instant centers, long, short, high and low - various approaches have given different results and with so much attention being given to the intersection point, the location of the bars is rarely mentioned. This piece won't give you the actual dimensions your car will need to run its best; rather we'll cover some of the basics and theories that seem to work and give you something to tune around.
While prior experience with terms like instant center, anti-squat and wheel speed will help when reading this article; I'll try to give a quick primer for those unfamiliar. Instant center is the theoretical point that a pair of four link bars would intersect at when viewed from the side. There is a ton of info on this in various chassis books, or on the Internet. Anti- squat (or hit) is the very first move made by the suspension when the trans brake of clutch is released. It is a downward move into the sidewalls made by the housing, and every car has some degree of anti squat. It is caused by the effect of a rotating pinion coming in contact with a stationary ring gear. It happens very quickly, often too quickly to see, and even cars that squat excessively experience it. Wheel speed is pretty much productive spin. Some cars need it to cushion a high horsepower launch.
Before we get too involved, a little primer in four-link dynamics. When the trans brake or clutch is released, the effect of the pinion trying to turn the ring gear cocks the housing, rotating the pinion up. This causes anti-squat. As power continues to be applied the upper four link bars pull, while the lower bars push. This causes pitch rotation, or rise/squat. The angles that these bars are at relative to the housing make a big difference on the way they will push and tug on the body, regardless of the instant center they make.
Five measurements need to be taken into account when setting up a four-link suspension.
On the housing, the distance of the top and bottom bar respectively from axle centerline must be taken into account. The spread of the bars on the housing is critical; it has a lot to do with the amount of hit applied. The amount of rake in the bars from the housing toward the chassis brackets can make a big difference in the way the car launches, and the overall height of the bottom bar must be within spec for chassis center of gravity and power available.
Beginning with the housing side, the positioning of the upper bar from axle centerline sets up both the hit on the tire and the amount of bite the tire will see as the car drives out. In our experience, the higher the power, the closer to the housing centerline you want to set the bar. As its position moves away from axle centerline, the strike, or hit on the tire at launch becomes greater, but the overall load applied to the tire diminishes as the car moves away from the starting line. In a low powered car, a more violent anti-squat will get the tire dead hooked, but its relative lack of torque will not be able to spin the tire after the initial hit. In a car with some serious power, the tire typically wants wheel speed, so a lower bar setting will allow the tire to turn at the hit, but the greater leverage controls the rate of slip as the car moves out.
The spread on the rear of the housing is the next dimension we set up. Maximum and minimum spreads are adhered to at ProCar with the lower powered car having the wider reach. (Since the upper bar typically rides lower on the high power car, you can see how this stands to reason.) Wide spreads move the housing very quickly at launch, and can effectively eliminate wheel speed in moderate powered cars. Since keeping the tire hooked is important in most bracket cars going slower than 7.50, a lower bottom bar will set up an IC that regains leverage lost by the high top bar, pinning the tire as it moves off the starting line. The faster cars need to move forward with less housing movement and less anti-squat. They have torque to apply the sidewall, and excessive mechanical leverage may wad the tire and cause shake.
I hope this offers a different spin on the way we look at the rear suspension of our cars- it intertwines with conventional instant center theory, but offers another aspect to consider. Above else, be safe when testing!
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