Oversteering wildly, backing into the turn before launching off a tabletop jump and landing hard, then sliding into the next corner, the motions of a short course off road truck don't scream “refined chassis tuning.” Yet to get the truck to work right under all those circumstances, not to mention as the track surface moves from wet and slick to perfectly tacky to dry and slippery during the course of a race, takes some serious thought and engineering.
A Pro 4 or Pro 2 Unlimited truck in the Lucas Oil Off Road Racing Series has about a foot-and-a-half of suspension travel. So, no, the adjustments aren't as fine as they are in dealing with the fractions of an inch that a modern Formula 1 or IndyCar suspension moves during the course of a lap. But while everything is bigger and beefier on the truck, the principles are much the same once you move beyond the idea of sticky asphalt vs. the dirt, where drivers are constantly searching for the elusive grip.
“Everything we do on dirt is traction-limited,” says Dave Clark (RIGHT), crew chief at Stronghold Motorsports. Clark joined the team at the beginning of 2011 with immediate results, as the team immediately became a contender. Now, with one race left, Stronghold has clinched the 2012 Pro Lite championship with RJ Anderson. “But traction is the most important thing. It comes down to 4x8-inch pieces of rubber, the most important thing on the car,” he adds, referring to the contact patch of the tires.
“We make the driver comfortable by adjusting air pressures and tuning the swaybar. You have to have good throttle response, because they drive more with the throttle than the steering wheel. The motor has to pick up in an instant,” he adds.
There are several options available for tuning the traction and handling of an off-road truck: spring rates, swaybars, external and internal shock adjustments, tire pressures and tire grooving, to name a few. These are all things that can be adjusted fairly easily at the track, and each has a particular effect.
“Most of the shocks here are external bypass shocks,” says Todd Tenbroek, race support and special projects manager for Fox Racing Shocks. “You can do a lot with the external adjustability, but there are times you've got change internal valving.”
Tenbroek explains that there is a check valve inside the shock that lets more or less oil past the piston. The external bypasses activate at different points along the shock's travel. The first is the ride height zone, then comes bump, and then the smallest tube, where the shock is compressed the most, allows for the fine tuning.
The tires in Pro 4 and Pro 2 are grooved to each driver's specifications, explains Bryan Shackleford, who does all the grooving at the track for Toyo Tires. Circumferential grooves add side bite; grooves across the tire aid forward and braking traction. The size of the grooves matter as well – a hard-pack track with blue grooves demands as much rubber on the surface as possible. If there's more fluff, bigger grooves will help the tire shed the mud.
The swaybar is a central point for tuning the handling of the truck, says Clark. “With the swaybar, we can make a quick change. If the driver is having trouble getting the back end to come around, then we can put a heavier bar in the back and that will make that end of the car break traction and come around. If it's coming around too much, we can put a smaller bar on the rear, or add more bar to the front to make it push. Whatever end you make the spring heavier, that end will break away first. That's how we balance it,” he says.
Off road teams have to rely on driver feedback and video to determine what the truck is doing. Data acquisition is almost useless – things like throttle position, steering angle and wheelspeed don't mean much when the wheels are spinning faster than the truck is moving, and often in a different direction. So it takes a good crew chief to interpret the limited data, use it to make the truck handle well and, if at all possible, keep it in the middle of the adjustment range.
“It's all compromise,” explains Clark. “There are a hundred things you can adjust, and if you have any of them not in concert with one another, you're not going to have predictability. If you know what it's doing, even if it's evil, you at least know what it's going to do if you have predictability.”
To the external viewer, it may all look like chaos. But getting a short course off road truck to handle correctly and predictably is the difference between a happy driver and a miserable one – and that is as important as talent and luck when it comes to winning.