Air 'em Down

By: Rick Boiros

Your vehicles tires are the link to the ground, and four wheeling is usually all about TRACTION. For most of the New England trails and aggressive all-terrain or Mud type tire in either a radial or bias ply is the appropriate tire. The current trend is towards 32-35"(+) tires on club vehicles.

A vital tool and fine-tuning point for the terrain at hand is lowering tire pressure. In sand, and mandated by the rules of running some Massachusetts beaches with a 4x4, tire pressure MUST be lowered. The reason is a large contact patch or footprint and flotation.

The same rules apply to the trails we do here, though there is no club rule. Street pressure does not cut it! Not only will the tire's traction be less effective, but the vehicle will bounce more, ride hard, be harder to control, and at it's worst parts can break from shock or lack of give in the drivetrain.

Rock crawling of any type demands low tire pressure. It is amazing what a 5 PSI difference will make when attempting stair steps and loose rocks. Low pressure allows the tire to flex over or even envelop obstacles. Hard tires will bounce and hop and have less rubber in contact with the trail.

Everyone has an idea of ideal tire pressure. Factors that must be considered are tire type and size, rim width and proximity to an air supply. For starters, carry a tire gauge! Everyone. For one thing, locking differentials like the same tire pressure side to side, part time 4 wheel drive systems like consistent pressure front to back, reason being, the rolling radius of the tire changes with pressure.

Some general rules: Radials will be less tolerant to extremely low pressure (4-7 PSI), the more flexible sidewalls will not support as much weight with low pressures.

Narrow wheels (7-8" wide) will hold the bead better. A 10 " wheel runs a higher risk of popping a bead since the sidewall and bead itself are not forced against the rim as hard as the narrower wheels. 15" wheels have a safety bead and will tolerate low pressure, 16.5" wheels have no safety bead and low pressure will easily unseat a bead.

Tall tires can be aired down lower. The further the rim is from the ground, the less chance of hitting it, or pinching the aired down sidewall between the ground and wheel.

Some examples for challenging trail use, though results will vary and experimentation is necessary:

33x12.5x15" Super Swamper bias ply on a Jeep TJ with 8" wheels: 5-7 PSI will result in excellent flex for rocky or loose trails. (Bias ply is stiffer so mandates lower pressures)

33x12.5x15 BFG Radial Mud TA on a Jeep TJ with 10" steel wheels, safe low limit of around 10-12 PSI (10" wheel risks unseating a bead)

35x12.5x15 BFG Radial Mud TA on a Jeep CJ w/ 8" wheels 8 - 10 PSI for maximum tire flex. ( taller tire allows lower pressure)

So AIR THEM DOWN and hit the trails!

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