Technical > Four Wheel Finesse
The Physics of Rockcrawling
by Jim Allen
Extreme rock crawling involves having a feel for everything at once and adjusting the driving technique constantly... in this case from rock to rock. Weight transfer is a factor here, with the driver’s rear having the most weight and the passenger front having the least. The way things are right now, this driver is in a no-go situation, but with a little left rudder, he can make the climb.
Extreme rock crawling involves having a feel for everything at once and adjusting the driving technique constantly... in this case from rock to rock. Weight transfer is a factor here, with the driver’s rear having the most weight and the passenger front having the least. The way things are right now, this driver is in a no-go situation, but with a little left rudder, he can make the climb.
Tire Grip
Everything you do on the trail stems from the grip your tires have on terra firma. Weight on a patch of rubber supplies grip. Put grip and torque together and you get tractive effort. Simple. The problem is that grip is extremely variable. The variables include the obvious things like the ground surface, tire size and type, and the less obvious things like weight transfer and torque multiplication.
It’s best to avoid the situation when the front and rear suspensions are articulated opposite each other because the two lower tires can’t provide much tractive effort. The two remaining tires may be enough to get you through... or not. An articulated situation is the best argument for lockers because an open diff will wimp out here, as will the less aggressive limited slips.
The ground surface is out of our control. What’s there is there and we must adjust technique or equipment to match. In rockcrawling circles, the worst surfaces are loose rocks, loose, crumbly or sandy rocks, or rocks that are “lubricated” by water, mud or sand. The best rocks are those that are dry, clean and have lots of sharp irregularities for the tire to “bite” into.
As to tires, soft and flexible is the overall key. We want the tire to envelop and grab all those irregularities on the rock. The ideal tire has a soft compound with lots of edges in its tread design. The carcass of the tire should be flexible and that comes from both the way it’s designed and the air pressure. With air pressure, low is the only generality to keep in mind, but that has to be balanced against vehicle weight. Plus, a tire that is too soft may deform too much and actually lift part of its tread. Too low on the air pressure will also decrease ground pressure and less of that can decrease grip. We’ve talked about tire pressure in detail in prior Finesse episodes.
The rock crawler’s preferred tread design is an open tread mud-type tire. It fulfills the “lots of edges” requirement mentioned above, but adds a couple of other elements. The widely spaced blocks of tread are very flexible. Not only that, they have a high void ratio. With more gaps between the tread blocks, the parts that actually hit the rocks gets more contact pressure, which equates to more grip.
Ledges are part and parcel of rockcrawling. A double ledge where the front and rear tires are both hitting the obstacle at the same time provides the most challenge. A bit of forward momentum will provide some “horizontal gravity” to put weight on the forward edge of the tire to momentarily stick it to the ledge. With a judicious blip of the throttle at the right moment, the Blazer should climb right up.
Weight transfer is a major variable in the tire grip area. When you climb, for example, vehicle weight is transferred from the front to the back. That tends to increase grip on the rear tires and decrease it up front. On a steep climb, the front tires don’t have enough weight on them to have much grip, and the rear tires alone can’t provide enough to push the vehicle up the slope.
There are more complications than just an uphill, front to back weight transfer. There’s side-to-side weight transfer as well. And the weight transfer that comes from an articulated axle, where the compressed side has most of the weight on it and the dropped side has little or none. At times, you might have a combination of weight transfer, such as being slightly diagonal on a slope and thus putting most of the weight on that single low tire.
Short wheelbase rigs transfer a lot more weight on a steep slope and they don’t get much shorter than an old Jeep flatfender. In some cases, the front gets so light on a really steep slope that a rig like this can go over backwards. In any case, the front end is so light that it’s not helping very much. This is a dangerous situation and on this obstacle, Pritchett Canyon’s notorious Rockpile, a strap is often hooked to the front of super-SWB rigs for safety. Seldom do the shortest of the short make it under their own power. Too much momentum is the kiss of death in a precarious situation like this.
Vehicle characteristics play the largest part in weight transfer situations, with narrower/shorter wheelbase rigs transferring the most weight and wider/longer wheelbase rigs transferring the least at any given angle. A heavy engine up front also tends to add some grip to the front tires by keeping more of that weight uphill. Unless you build specifically to achieve a specific weight balance, all you can do for tuning the vehicle is to load the vehicle creatively, keeping as much weight as possible forward of the rear axle, and with tire pressure adjustments.
Torque multiplication comes into play two ways, either when the amount of torque applied exceeds the grip available or the amount of torque required to haul your rig up a given slope exceeds the grip available. In the first case, the situation is controllable by the driver. He can either shift up a gear to reduce torque multiplication, or back off on the throttle to reduce torque delivery. In the latter case, one of the traction equation elements must be changed; either find some way to acquire more grip, reduce vehicle weight or increase momentum.
On the same obstacle as the flatfender, this longbed Chevy is at a shallower angle and the front tires have a bunch more weight on them. While not exactly a piece-o-cake, the Rockpile is a bunch easier for the Chevy than it is for the Jeep. The only danger might be if the truck didn’t have enough breakover angle to clear the lip with it’s midsection. Momentum can be used relatively freely by this truck.
Another grip-reducing fact of physics involves gravity. Gravity always puts the weight of the vehicle directly perpendicular to the surface of the earth, or more accurately from the surface of the earth downward. When on level ground, that places the weight directly on the footprint of the tire. When climbing a slope, the point where the tire grips the ground is forward of there and the tire doesn’t get the benefit of all that weight directly on the footprint. The steeper the slope, the farther ahead the weight is from the contact patch of the tire.
Momentum
A tire in the air is a tire not doing it’s job. Obviously, there may not be a choice in the matter, but the driver should pick lines that keep the tires on the ground as much as possible. Even a tire with reduced contact pressure can provide some traction. Obviously, in a case where the choices of line are down to something like this one, a locker on the axle with the lifted tire is necessary. A more flexible suspension could help keep the lifted tire on the ground.
Momentum is the great equalizer of the ‘wheeling world. It’s a tool drivers can use when some other part of the traction equation isn’t working. Newton’s first law of motion says that a body in motion tends to stay in motion unless acted upon by an outside force. We can use that! It takes substantially less grip and torque to push a vehicle up and over an obstacle when it’s already in motion than it does when completely at rest. Momentum isn’t always possible to safely use, or use in large amounts, but even a 1/2 or 1 mph roll with a 5,000 pound rig can be enough to make a big difference.
Momentum has some direct applications in the grip area as well. When an abrupt change in ground contour is encountered, such as a ledge, grip can be reduced enough to stop the vehicle. In the ledge scenario, the weight of the tire is on the bottom of the tire, not up at the edge of the ledge where it’s needed. That can be changed momentarily by using momentum as “horizontal gravity,” if you will, and pushing that patch of rubber hard into the ledge. If the driver times his application of throttle right, that momentary grip can be enough to pull the rig up and over.