Valvoline.com

Off-Road Tire Tech, Part I

When bigger is better

Strangely enough, when you buy a rugged new 4x4 from a dealer, often the tire installed by the vehicle manufacturer is for street use. Designated as a "P" (for passenger, not pavement) rated tire, it's installed because most of the driving done on any truck or SUV is on asphalt. Truth be told, for many 4x4 owners, a typical off-road excursion might consist of driving illegally across the median strip of an interstate. So manufacturers consider the actual use of the vehicle, not the "climbing Mt. Everest" images conjured up by commercials.

Not all SUVs come with "P" rated tires; some wear the "LT" (light truck) designation. Whatever type of tire on your rims, you should keep a few things in mind before you run out and upgrade your rig to those big knobbies. Aside from the macho look, the reason to put larger tires on a 4x4 is to add more ground clearance and achieve better traction (those median strips can get real slippery!).

Change is Good

Most tires on stock vehicles are about 29 inches in diameter; but, on any trail, you will often see 4x4 tires ranging from 31 to 35 inches in diameter. You can find even bigger tires out there, but that's getting into the monster truck realm, not the average 4WD owner.

It's important to realize that bigger tires often require changes to the suspension and gearing because they can have an effect on steering geometry, driveline angles, and power transference from the transmission/transfer case. However, you may be able to get away with going up just a size or two from the stock tire, depending on the stock ring and pinion ratio and the overall final drive ratio that is already installed.

When you throw on a set of 33s or 35s, though, plan on lifting the suspension and/or body, and possibly trimming the fenders as well. You should also change the gear ratio in the axles (the ring and pinion); and, while you're at it, consider installing air lockers, too.

Side-Effects

What about the effect of bigger tires and wheels on other components such as the wheel bearings, drivetrain, axle housing, engine and brakes? Make sure you've got enough reserve power (read: V-8 instead of 4-banger) to handle them. If the tires and axles are done right, usually the powertrain can handle the extra weight, assuming it's still in good running condition.

What will big meats do to your performance on pavement, though? Well, you can always turn up the audio system to drown that high-pitched whine. Just keep in mind that if the vehicle rides higher, it is inherently less stable and requires some restraint when cornering. Also, make sure the caster angle is corrected on the front end to help the steering tires return to center.

With an increased drop rate of the suspension, the steering can be affected by all this movement and may need to be strengthened with a stabilizer, support brackets, and possibly a steering gear box support. In addition, some 4-wheeling enthusiasts also cut off the ends of the bumpers to allow the tires to climb easier over obstacles instead of having the bumper hit the rocks.

When going up a size or two in the tires, you can usually stay with the stock rims if they are between 7.5- and 8-inches wide, which can hold a 31/10.50x15-size tire. You should even be able to mount a set of 33/12.50x15 tires on an 8-inch rim—and there's nothing wrong with having the sidewalls bulge out a bit since they can help protect the rims. If a tire fits the rim flush, it'll get scuffed. Not only that, on some trails, a flush rim can catch on rocks or tree roots, which bend may it up, releasing air. So don't let the tire store sell you rims when you may not need them.

Next, part two of this off-road tech article will deal with tips on picking the right tread design.