Making It Stick Part 4 - Advanced Suspension Geometry

Part 4: More lessons in geometry

This series began with handling basics, and moved to more difficult subjects involving suspension geometry. This month, there's more of the same. However, unlike previous installments, the suspension fundamentals we discuss this time are less tunable but just as important. Who knew paying attention in geometry class could make your car faster?

Toe steer

Like bump steer, which we discussed in the previous installment, toe steer can adversely affect your car's handling. Toe steer is a product of suspension components of different lengths moving through different arcs at the same time. The resulting changes in toe in the rear suspension can cause unpredictable handling.

Early sport compact suspension designs like the semi-trailing-arm type found in mid-'80s BMWs, Mk I and II Toyota Supras and the Datsun 510 have severe toe steer. Because of this geometry, some semi-trailing arm suspensions toe out under roll, which can cause severe trailing-throttle oversteer. Others, like those found on the Porsche 928 and FC RX-7, use links or bushings of different hardness to force the suspension to passively toe in under roll, which creates understeer at the limit.

These designs work well if they're operating the way the engineers intended them to. It's when speed junkies like us try to out-engineer the engineers that problems develop. This usually happens when lowering the car.

One easy solution to reduce toe change in lowered, semi-trailing-arm suspensions is to increase the spring rate to reduce wheel travel (effectively increasing the overall wheel rate). The less the wheel moves, the less toe can change. Obviously, overstiffening the rear suspension has its own drawbacks, but it worked well on the Datsun 510 and BMW 3 Series, both of which had a long and successful racing heritage.

Many current sport compacts use multilink rear suspensions designed to sweep through complex camber and toe angles to maximize street performance, comfort and stability. However, this combination of camber and toe can conspire against an enthusiast who's willing to accept the compromises of a more aggressive suspension setup.

Aftermarket tuners like Whiteline offer offset bushings for cars like the EVO that relocate the pivot points to reduce toe steer. If kits are unavailable for your application, a racecar fabricator can reposition the trailing arms and other links to a corrected position on severely lowered cars. It's also common to slot control arm mounting points to alter their path of travel and correct toe on lowered cars.

Excessive lowering can make toe steer worse by placing the suspension links in a static position and range of travel they were never designed for. A beam axle suspension with trailing arms, as found in the rear of many small front-wheel-drive cars is a good example of where this might occur.

At stock ride height, the trailing arms are usually parallel to the ground. When the car rolls, the outboard and inboard arms swing in different directions. The resulting arc-shaped axle path shortens the car's wheelbase during compression and droop. Since each arm swings equally in different directions, the axle's toe doesn't change because each end of the beam axle is pulled the same distance forward.

If the car is lowered too much, both trailing arms point downward toward the front of the car. At this angle, the arms don't move equally in opposing directions. Under roll, the inside arm will push its side of the axle rearward while the outer arm will pull its side of the axle forward, causing understeer.

Anti-dive and anti-lift

Anti-dive and anti-lift are tricks that can be applied to a car's front suspension geometry to control brake dive and acceleration lift.

Lift and dive can be mitigated by carefully locating suspension pivot points to take advantage of the "force reaction" on the chassis created by acceleration or deceleration. This avoids the need to increase spring rates to reduce pitching-an important issue for ride quality in all street cars.

Anti-dive helps prevent the nose of the car from diving during hard braking. Most cars have some degree of anti-dive designed into the stock suspension geometry. Anti-dive utilizes deceleration forces to increase the front wheel compression rate and reduce brake dive.

By changing the angle of the suspension links, the amount of anti-dive can be manipulated. However, excessive anti-dive can hinder performance by causing the front suspension to stiffen while braking for a corner, which can cause understeer. In extreme amounts, anti-dive geometry will cause wheel hop and caster changes under braking.