Terms reference:

Diff: is short for the rear torque differential mechanism.

Trailing arms: are control arms that mount from the diff housing to a forward position on a car. There is a pair either side and are always called the upper and lower tailing arms. These arms control the forward/rear movement of the diff housing.

Pan-hard Rod: is a single rod that joins from the far side of a vehicle body and joins onto the diff housing to control its lateral (side to side) movement. On some 3 / 4 link setups a “Watts link” is used in its place.

Pinion angle: is the angle between the road when the car is on a flat surface and the imaginary line from the diff center to the diff pinion, example: when the diff has a zero degree angle the pinion will point directly forward




Linkage principles:

Under the normal use the diff moves in proportion to the body of the car as it goes through its suspension travel over irregularities of the surface in which the car is traveling. To allow the car to lean around a corner the diff is only mounted by the single pan-hard rod bar which lets the diff rotate side to side. However because the angle of the diff in the forward/aft direction needs to be controlled to allow power to be transmitted twin bars are used in the form of Trailing arms. Because the diff is solidly linked to the ends of the arms and the arms rotate to allow for the suspension travel the diff inadvertently moves through an arc (see diagram below), this is known as the “arc of travel”.

This diagram shows the arc of travel and how anything located on the end of a pivoting arm will move in an arc shape.

Because of packaging reasons the top arms are usually shorter than the bottom which has the effect of the diff being subjected to two different arcs of travel between the top and bottom mounts. The real world effect of this is that as the diff moves through its suspension travel the diff rotates and the pinion angle changes. To minimize this the top shorter bar on cars is fitted completely flat (zero degrees), this is because if the arm was lets say angled upwards then there would be a greater change in the pinion angle due to the greater length change from the angle of rotation.




Tuning the 4/5 link setup:

Top trailing arm: As stated above the trailing arms are usually unequal lengths which have the effect of causing the diff to have an unwanted rotation through its suspension travel. One way used some race tuners is to chop into where the rear seat usually is to facilitate the movement of the top pivot point inward (see picture below). This lets a spare set of lower control arms be used in the tops position and therefore allows for two equal arcs of rotation. This stops all diff rotation bar those that are a by-product of the anti-squat settings (more about this later).

	A view of how the floor has been modified in this Japanese AE86 to allow for equal length upper control arms.
	Aftermarket weld in floor pannel sold in Japan for those who wish to perform the modification.

As mentioned previously cars usually have the top trailing arm in a zero degree position to minimize the forward/aft movement that’s a bi-product of the arc of rotation. When cars like this are modified they are usually lowered in ride height and because of that the diff is closer to the body than originally. This effectively means that the top bar no longer is at zero degrees and instead is leaning upwards from the front to the rear of the car. This can in theory be corrected by redrilling the diff housing at a lower position, however in practice the bar quite often will hit against the housing at full compression. To get around this some custom arms could be made that have an offset end or even angled. As part of the rear end suspension tuning the top bars angle may be changed but as a rough rule a good starting point to have is zero degrees.




Bushes/joints:

Just about all cars originally have pressed in rubber bushes in the suspension joints, this allows for deflections in the suspension and also more importantly for the original manufacturer it provides good vibration from the NVH (noise/vibration/harshness) coming from the rear suspension. Unfortunately these original bushes allow for a considerable amount of undesirable deflection in the joints which has the effect of allowing unpredictable suspension geometry changes. The rubber bushes also don’t allow easy rotation through their axis so therefore provide a restriction is response.

Most tuners “fix” this problem by replacing the original rubber bushes with those made from a harder Polyurethane material. Quite often this is seen as a very cost effective upgrade as it usually does not have any increased road noise but allows the rear suspension to be much more predictable and feeling “more tight” as phrased by drivers. Another more drastic option is to do away with the deflective bushes all-together and use a metal bush [see pic of bush]. This type of bush has a few names such as a rose joint/ spherical joint and a rod end. This lets there be no deflection at all and therefore allows for the maximum possible predictability and “feel”. However because this effectively removes all possible shock dampening there is a significant increase in road noise inside the cabin.

The multi-link rear end has a fundamental flaw in its design that is not common knowledge. Because the diff naturally wants to rotate from the different arcs of movement from the unequal length control arms there is a problem when only one side wants to move, for example if a car hits a shoulder while going straight ahead. In this instance the side of the diff that is being lifted wants to rotate and face the pinion on more of a upward angle but the other side wants to not twist. So therefore the other side either moves down, the bushes take up the deflection or lastly the housing itself has to bend. To make up for this a setup nicknamed the “3.5 link rear end” is used. What this requires is if all the bushes are changed to a harder type that the top upper left trailing arm is excluded and instead has multiple holes drilled into the original rubber bush for maximum deflection. This would have the effect of allowing the diff to move freely throughout its movement by effectively only using one of the two trailing arms.




Anti-squat:

The term anti-squat refers to setting up a rear suspension system to convert some of the reactive twisting torque into downward pressure away from the car body.

How this works is that the twisting force against the diff housing causes a force which effectively puts a rotational force on the housing. Depending on the configuration of the rear geometry that twisting force can be converted into a force that tries to push the car body away from the diff housing. This causes a reactive force that reduces a cars tendency to squat downwards and because of the extra force required to keep the body from falling ends up putting a higher pressure onto the tires. This why the term “anti-squat” and is basically used to gain traction. The effect is however limited as its greatest effect is in first gear where it has the greatest torque multiplication from the gearing and a reduction in any gear after that. Regardless it is very useful in initial traction off the line and a necessity in drag racing to “get out of the hole”. The amount of anti-squat can be worked out as shown by the diagram below:

 

This diagram shows how the anti-squat amount is worked out. Pictures courtesy of Desert rides

As mentioned earlier a good starting point is to have the top arms level and then adjust the pivot point of the lower arms to achieve the right level of anti-squat. The bottom pivot point can be changed by the means of bolt on brackets (see picture below) which are called “traction brackets” or by modification of the housing itself (see second picture below).

	Bolt on "Traction Brackets" Picture courtesy of Vision R
	Modified diff housing for a lower pivot point on the lower control arm.

The method the drag racing teams use is to have diff housing with multiple holes drilled in them for easy adjustments. The holes are also drilled in a row that forms an arc shape to make it so when the arms are moved in their pivot points there is minimal pinion angle change. Too much anti-squat reportedly causes the rear wheels to hop on heavy braking so an in-between medium is needed. For an AE86 with standard top arms the bottom pivots can be lowered all the way to the minimum 100mm ground clearance without a problem.




Pinion Angle:

The initial angle of the pinion can be adjusted at times for some beneficial gains. It is believed amongst some suspension tuners that having the pinion angle facing upwards when the car is in a neutral position aids in traction and vice versa. If this is so then its likely to only have an effect on cars that haven’t been converted to equal length arms. This can be achieved by using adjustable length control arms which can either be bought as an aftermarket option or by modification of the original arms.

Pictured are some Madhouse branded adjustable arms that use spherical joints, are adjustable in length and are made from aluminum.   Picture courtesy of Madhouse

Dialing in the right settings:

With the basic principles in place its possible to have a starting point for tuning the rear suspension for maximum traction or even the opposite if wanted. To find any of the ideal settings the best way can’t be calculated instead it needs to be from experimentation. The best way to do this is to the best possible effort to change a single part of the suspension at a time and then do testing. For example if a lower pivot point is used for greater anti-squat then the length of the lower arms be adjusted to keep the pinion angle the same as before the adjustment. This is so during test the difference each suspension adjustment can be individually identified in how the car has responded in testing. If multiple adjustments are made at once then it’s hard to identify which change made the difference in performance. After an adjustment is made and the differences noted more aggressive settings can be tried till the gains go backwards again, going off this an ideal setting can be found.