A short-course, off-road racing truck is a bit of a dichotomy. It must be able to handle through the turns and then also absorb some pretty big bumps on the whoops and landing after the tabletops. That's a lot to ask of a suspension and why, when it comes to the shock absorbers, they are basically two-in-one.
Shock absorbers, in every case, must be able to handle two basic types of compression – the roll of the body as weight transfers under cornering (low speed) and the thump as a tire rolls over uneven surfaces (high speed). Most street shocks do one or the other well, but not both. High-end racing shocks have different adjustments for both high- and low-speed compression and rebound, but in that case the bumps are usually pavement irregularities and the occasional FIA curb. In the case of off-road racing, those bumps are huge….
To make the same shock work controlling roll in a 4000lb truck and still absorb the landing off a big tabletop requires some pretty extreme and specialized engineering.
Because the two ends of this spectrum are so far apart, an off-road racing shock is really two shocks in one. There's the coilover part that controls the low-speed movement and the bypass portion that allows a lot of oil to move through the shock quickly so that the suspension can go from full extension to full compression – in the case of most of the trucks in the Lucas Oil Off Road Racing Series, that's about 18 inches of travel – in about a nanosecond.
“There are two shocks on there and each shock controls a different zone,” says Sean Keppler, Sales and Tech representative for King Off-Road Racing Shocks.“ One controls the high speed and one controls the low speed. The bypass is for the high-speed stuff; and the corners, where we get the body roll action, is where the coilover comes into play. The coilover is what we tune for the low speed tuning damping and the bypass would be used for the high speed.”
The inside of the coilover portion is typical shock absorber – a piston moving up and down through fluid that keeps the vehicle from bouncing on the springs. How fast the piston can move through that fluid determines how stiff the shock is. Rapid absorption of shock, as in landing a jump, is where the bypass comes into play. The bypass allows the oil in the shock absorber to go around the piston so the suspension can compress quickly.
“It's an external tube that allows the oil to bypass the valving piston, so the piston can move more freely by not having to go through as much oil. Each tube gets smaller and smaller until you get to the area where there are no tubes and it progressively gets stiffer. You can tune the amount of flow you get around the piston,” explains Keppler.
The typical shock used has two compression bypass tubes and two rebound bypass tubes, which Keppler says actually gives three different zones. That, plus the internal valving of the shocks themselves, gives the racers a lot of tuning options to make their truck, buggy or UTV handle the way they want.
Because the shocks in off-road racing – and the construction for the short-course racing is the same as for desert racing – take so much abuse, they are built a little differently from a standard street application. The first difference is the size – not only are they much longer to accommodate the foot-and-a-half of travel, but the shaft is also much bigger, an inch or an inch and a quarter. The shaft is also induction hardened to handle the rocks thrown up in the roost. Ideally, though, if the suspension is working properly, there should be little, if any, lateral load on the shaft. The body and shaft should be moving in perfect linear fashion, even when the vehicle lands sideways.
The off-road racing shock is a special breed, unlike most street and motorsports applications. Sheer size is a factor, of course, but the special engineering to make it handle the extremes of off road racing make it a unique and interesting machine.