4 Link Suspension

The 4 Link suspension has gained popularity in the racing industry due to its ability to turn freely in the center of the turn and have no effect on tire bite as the car rounds the turn. The concept behind this very sensitive suspension is this, 2 bars mounted on each side of the chassis, one higher then the axle and one lower mounted horizontally to the chassis in front of the axle, two different mounting styles are utilized, the first is rigid mounted to the axle, the second is a free floating bird cage that allows the axle to pivot as the up and down motion is put through its range,  the rigid mounted style relys on the link arms to maintain pinion angle of the U-joint to drive shaft angle. The free floating mounting style requires a pull bar mounted above the axle to maintain the same pinion angle.

First off, the complexity of this suspension can make even the best crew chief scratch his head to dial it in making the chassis adjust to track conditions.  We are going to explore what the upper and lower bars actually does, and how to adjust them making the car handle better.

The top bar of this suspension works the following way: The upper link bar controls forward bite, when the front of the mount is higher then the axle mounting position gives side bite and forward bite out of the turn, as the bar is leveled out bite is decreased using time, if the bar is more than 15-18 degrees uphill the side bite through the turn will make the car stick like glue, the problem you will run in to is that as soon as the car exits the corner, forward bite will fade fast, usually by mid straight on a short track, aprox 20 yards out of the turn, as the bar is laid horizontal the bite stays longer glueing the tires to the track as you go down the straights, forward bite is maintained longer.  the length of the upper bar will also dictate how fast bite is obtained as the suspension works through the turns.  A shorter bar will allow the changes to happen faster allowing faster bite.

Normally the left side upper bar should be between 15-18 degrees uphill angle. The right side upper bar should be between 10-15 degrees to compensate for exessive suspension motion as the car is in the turn.

 

The lower bar controls rear steering: as the chassis rounds the corner, the lower bar moves in a vertical motion much like the upper bar, but its roll in this movement actually changes the axles positon and the rear geometry of the axle.  the left side unloads as the right side loads up as you enter the corner, as the bars move up and down the geometry length changes making the axle postion from left side and right side shorten and lenthen making the axle want to steer the rear end to the outside of the track, one way to correct or improve on the way the axle steers is to shorten the left side bar to compensate for over steer.  also bar angle helps correct this situation,  the lower bar normally should be horizontal with 0 degrees of uphill angle, normal adjustment would give a negative setting to help control roll steer.

 

The pull bar should be mounted with a down hill angle to allow the weight of the chassis to push down on the rear end when the transition of forward bite takes place, the use of a dampener spring, and shock is useful to help load up the rear end and hold the power to the wheels.

 

Now the best for last:   Do not make major changes when adjusting this suspension, the slighted adjustment can make major noticable changes, mounting holes are great but I recommend the use of slot driven adjustment mounts, this gains valuable adjustment space as one hole dimension to the next can make a larger change then what is needed.