Four Link Tuning- IC's Versus Geometry
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.
This article is about expanding way many of us look at four link tuning. Again, the theoretical intersection point, or ‘instant center' is often all we pay attention to- even though testing has shown that the holes we select is almost as important as the IC itself. If you spend a little time noodling with 4 link wizard or several other programs, you can see that you can hit nearly identical IC's with several bar arrangements. Anti-squat values may change considerably, but the IC is still the same.
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
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.
Different cars with different power levels want varying amounts of rake in the bottom four-link bar, too. If the bar is too ‘nose down' for the application, it will bury the quarters, driving the tubs toward the tires and wad the sidewall. If you run too flat a bottom bar, or one that travels uphill to the chassis, the anti-squat dialed in by the top bar may cause the bottom bar to separate the car from the axle centerline, compromising hook. Again, we split racecars into two groups, those with enough power to use torque to keep the tire applied without a ton of leverage, and those that do not. For those cars that do not have a bunch of power, some rake in the bottom bar is appropriate. In fact, many high eight to mid-ten second cars will tolerate a significant amount of pitch, resulting in an instant center that is at or beneath ground level. The raked bar sets up a lower IC, and since low=long, it makes up for the high anti-squat/low ‘bite' setting of their top bar. Higher-powered cars won't tolerate all that bite- so dialing a bunch of hook will result in a tire that wads up and either spins or shakes. They demand a flatter bar, yet one that won't go over center during anti-squat. This pushes the car forward, not appreciably up or down, and once again the available torque will work the sidewall sufficiently to keep the car hooked.
The angle of the top bar is the final ingredient to determining how much bite the tire will have, and for how long. The flatter the top bar, the easier it is to use its pull on the chassis to plant the tire. It makes sense, since this is the bar that also dictates the IC once the rest of the parameters are set- flatter is longer, and longer is associated with greater hook. The more angular the bar, the greater the ‘snapping' force (anti squat) it will apply to the sidewall at the hit, and since it is at a mechanical disadvantage to pull on the chassis, it will generate less load- as IC theory dictates.
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!