Shock Geometry - Simple Coilover Facts
Absorbing the Impact
Suspension can be everything in the off-road world. Yeah, a strong engine gets you moving, but suspension keeps you in control so you can use that engine power. The use of coilover shocks, their spring rates, and their mounting geometry can have a big impact on how the overall suspension performs.
In general, for maximum stability, shocks should be mounted as far outboard as reasonably possible. Times when this is not possible are often due to long suspension travel goals or limited mounting space. Angling shocks inward can also provide clearance, however, as you deviate from vertical, the shock effectiveness decreases. (To determine the effective damping value, we would calculate the cosine of the angle from vertical. If the shocks are leaned 20 degrees from vertical, they would have a vertical damping effectiveness of about 94 percent of what a vertical shock would have.) Coil springs themselves are a simple item, but their characteristics are often misunderstood. Each is just a bar of spring steel rod wound in a circle for compact packaging. The spring has a rating based on how much weight is needed to compress it 1 inch. The thicker the rod that is used, the higher the spring rate, or the stiffer the spring. On the other hand, increasing the overall diameter of a spring coil or adding more turns to a spring makes the spring rate lighter.
When choosing mounting locations for coilovers, physical constraints often dictate their final location and may force compromises. One can play some tricks with angles and leverage ratios to maximize overall wheel travel. Just beware how those choices affect your spring rate, damping, and shock efficiency. It's sometimes best to cycle the suspension and check dimensions before choosing shocks. This will help you maximize travel, and also account for any unforeseen mount changes you may have to make.
Coilover shocks can have one, two, or three coil springs in a stack. Two springs with different spring rates is very common and is often set up as a dual-rate shock. In the initial portion of the travel stroke, both springs are being compressed, but in different amounts based on their stiffness. This initial spring rate will be relatively low, below the rate of either individual spring. Once the lighter rate top spring is completely compressed (coil bind), the spring rate will increase to that of the higher rate bottom spring for the remainder of the shock stroke.
On an IFS setup, the coilover often attaches somewhere at a mid-point on the lower A-arm. The resulting leverage effect is why spring rates used for A-arm front suspensions may exceed those used on comparable live, straight-axle suspensions. Coincidentally, shock lengths needed for IFS are less than those needed on comparable straight axle rigs, but spring and damping rates of the shock will be higher. Preload may be added to the shock by tightening the collar pressure on the captured coil spring to raise vehicle ride height, but this does not appreciably affect the spring rate of the shock.
On this Ultra 4 class buggy, a single 18-inch-travel coilover is used near the rear axle. Resulting travel will be near the length of shock travel, and damping will be very effective at this mount location in the suspension. This is a dual-rate shock with the shorter, lighter rate coil on top. Occasionally, a very light weight top spring will be used and is fully compressed at ride height. In this case, the shock is being used as a single rate coilover and the light spring is a tender spring used to keep the main spring from rattling around when the shock is fully extended.
On this desert buggy, the coilover is mounted near the center point between the trailing arm pivot and the rear axle. In this way a 16-inch-travel coilover may yield about double, or 32 inches, of rear wheel travel. The bypass shock behind it sits a little further down the trailing arm than the coilover, so a bit longer travel is required for that shock. Unlike the Ultra 4 shock configuration, the spring rate and valving will have to be relatively higher since the rear axle is pushing on the shocks through the mechanical advantage of the trailing arm as a lever.