Technical > Four Wheel Finesse
4x4 Lite
by Jim Allen
Here’s an example of less is more. Back in the mid-1990s, this ordinary-looking flatfender earned nods of respect at Moab. It was simple and basic to the point of being powered by a Ford 2.3L four-cylinder. The sound four-banger inspired the rolled eyes of the thought: “Yeah, he brought a knife to a gunfight,” but the light weight of this rig allowed it to go just about anywhere.
The term “Lite” has been applied to a bevy of food products, from beer to salad dressing. The term often has the connotation, “Less filling, less flavor, less pizazz.” Well, if your belly has been absorbing your belt buckle, you might have to choke down those things, but you dang sure don’t want the four-wheel-drive equivalent.
Rather than gag down the term “4x4 lite,” lets switch from “lite” to “light” — as in less weight, fewer pounds. ORA’s files are full of letters from wheelers wanting more power, better fuel economy, and better trail performance. They’re from people willing to spend a lot of money to achieve those goals, money that’s often necessary and vital. Still, there is a lot of free performance to be found via the simple expedient of lightening up.
Want a wake-up call? Put all your normal wheeling gear in your rig, fill the fuel tank, and take it to a place with scales and weigh it with yourself on board. My reaction was “whoa!” when I weighed my old diesel Blazer some years back. Manufacturer listed curb weight for this optioned-out Scottsdale: 4,936 pounds; actual weight: 6,610. That’s without 200 pounds of me in it. Sheesh! The rated GVW for the truck was only about 6,300 pounds. It didn’t have all that much gear bolted on compared with a lot of rigs, and I didn’t think it was overloaded. That was my wake-up call.
The Calculated Effects of Weight
What effect does weight have on performance? I rounded up some rule-of-thumb formulas to demonstrate. Let’s start with one of the many Web-based quarter-mile calculators: http://www.web-cars.com/math/index.html. We’ll enter my Blazer’s 6,610 weight and estimated 200 horsepower. Then we’ll drop the weight 2,000 pounds and see. At 6,610 the ET was 20.27 seconds at 72.9 mph. At 4,410 pounds, the ET was 17.7 at 83.45 mph. It would take 300 horses at the higher weight to match the performance of lighter weight. Dropping the 2,000 pounds was like gaining 100 horses.
Can we run similar calculations on fuel economy? Yes, though you can see them yourself by comparing solo mileage with towing mileage. Government EPA studies have yielded that every 100 pounds over a vehicle’s normal weight can cut fuel economy by approximately .09 mpg. Stock, the 4,936-pound Blazer could get about 21 mpg (it was a diesel with 3.08 gears and overdrive). At 6,610, some 1,674 pounds more, the fuel economy calculates a loss at 1.5 mpg (1,674/100 = 16.74 x .09 = 1.5066). Dropping to 19.5 mpg from 21 certainly doesn’t hurt as much as dropping from 15 mpg to 13.5. Another government rule of thumb says a six-to-eight-percent decrease in weight can result in a 10-percent increase in economy.
What about trail performance? That’s more difficult to calculate on paper, but let’s look at it a couple of ways. There is a formula for calculating the grade-climbing ability of a vehicle. Since a lot of hard four-wheeling involves climbs, perhaps this is a useful yardstick.
Using our weights above, the 6,610-pound, 200-horse truck can climb a 15.8-percent grade (9.08 degrees. FYI, a 100-percent grade is 45 degrees) in high range with 34-inch tires and 4.10:1 axle ratios. The same truck at 4,410 pounds can handle a 23.7-percent grade (13.6 degrees) with the same gears. Don’t bother to calculate these numbers in low range. You can generate the theoretical ability to climb straight up, but due to traction limits and other incidental laws of physics, such as gravity, such ability isn’t possible. All of these figures are minus rolling resistance.
Rolling resistance increases with weight and a rougher or softer ground surface. Four inches of snow will generate 75 pounds of rolling resistance per ton of vehicle weight (the equivalent of a 3.75-percent grade or 2.15 degrees). Mud will generate a wide variance of rolling resistance numbers, with about 300 pounds per ton being in the middle (15-percent grade/8.6 degrees). The rolling resistance would be 990 pounds with the 6,610 rig and 660 pounds with the 4,410-pound rig. These numbers would be subtracted from your truck’s tractive effort in pounds of pull, a.k.a. drawbar pull.
Even on the street, more weight increases rolling resistance. Smooth concrete will generate 20 pounds of rolling resistance per ton of vehicle weight on level ground, roughly the equivalent of a one-percent grade (.57 degrees). Good asphalt will generate 25 to 35 pounds per ton. An engineer using regular truck tires as models generated these numbers, so the typical mudder tire would probably generate even more resistance.
How will this affect fuel economy on the street? We can use a formula to calculate the horsepower required for a certain speed. This works as a test model because it takes more fuel to make more horsepower. Let’s plug in our numbers for 60 mph. We did not account for air resistance in this formula. A typical 4x4 would generate lots of aerodynamic drag, but let’s not depress ourselves any more. It takes 15.55 horses to drive the 6,610-pound rig at 60 mph and 10.37 horsepower for the 4,410-pound unit. That 2,000 pounds requires nearly 50 percent more power and correlates nicely with our acceleration numbers above.
Here’s an example of how to lighten up for the trail. Remove doors: 180 pounds. Replace hardtop with soft-top: 250 pounds. He’s got minimal bumpers and material removed from the rocker area. This is a pre-1968 unit (no side market lights), so we can assume its weight would be somewhere similar to a V-8-powered Roadster model’s (U-130) at around 3000 pounds.
The Observed Effects of Weight
Weight has some good effects on traction. Put more weight (ground pressure in pounds per square inch) onto a tire on a hard surface, and it will generate more traction. That’s something people have figured out by observation as well as on paper. Rock crawlers use this knowledge to perform extreme acts of derring-do, but they also know there are limits. More weight requires more traction. Also, you can get to a point where something in your drivetrain breaks before the tires break loose. You need to know whether the axles and drivetrain can handle all the torque generated by the engine, multiplied by the gears up to the point where your tires can deliver grip. You’ve seen it on the trail; heavy rigs with big tires and weak drivetrains break parts.
Weight has bad effects on floatation. Soft ground can only support a certain amount of weight. It varies widely according to the type of ground. Your vehicle spreads its weight on the ground over the area of those four tire footprints. More weight or fewer square inches of tire footprint yield higher ground pressure. More ground pressure on soft ground means you sink in further. The more you sink, the more rolling resistance is generated. The more rolling resistance, the more engine power and traction is required. Eventually you run out of something, and you’re stuck.
The footprint and ground pressure of the tires vary by the amount of air pressure in the tire and the weight on the tire. A 35x12.50-15 ProComp Mud Terrain with 1,500 pounds of weight will yield various ground pressure numbers from 74.25 psi at 30 pounds of air pressure, 17.6 psi at 23-pounds tire pressure and 10.7 psi at 10 psi tire pressure. Bottom line is that heavier rigs need bigger tires to stay on top of soft ground.
What Can You Do?
Sixty-plus years ago, when the immortal Jeep was being designed, they learned that you need as much weight as it takes to make the vehicle strong and capable. That will aways be true. Needlessly adding to that weight decreases performance.
The first easy steps involve looking at how much extra gear is added. Don’t whine about lost fuel mileage when carrying around 800 pounds of superfluous tools and spare parts on an everyday commute. Do you really need the range generated by an auxiliary 40-gallon tank (300 pounds of fuel)? If you need it only sometimes, leave it empty except when you do. If you don’t need it, lose the tank.
Are you overbuilding fabricated items, adding hundreds of pounds of unneeded extra metal? Why use half-inch plate steel when 1/16 inch will do? Why use a section of railroad iron when a piece of tube will do? Do you really need that locomotive-style cattle-catcher bumper?
When building a rig from scratch, think holistically. Do you need dual exhausts when a big-bore single system will offer the same or nearly the same performance at 40 pounds less? Headers are lighter than cast-iron manifolds. Aluminum intakes are 50 percent lighter than cast-iron units. Automatics are at least 30 percent lighter than those big, cast-iron manuals. Chain-driven aluminum transfer-cases are 50 percent lighter than cast-iron gear-driven ones. Do you need a 580-pound 14-bolt when a 350-pound Dana 60 will do? All this stuff adds up, and you need to make many small and some large decisions.
Get the point? Now go out there and shed some pounds!