10 THINGS YOU NEED TO KNOW ABOUT TIRE AIR PRESSURE
Having the right rubber on your bike for the dirt conditions is crucial to good performance, but you can sabotage even the sharpest new knobs by having the wrong air pressure. What is the correct pressure? Read on.
NUMBER ONE: HOW MUCH PRESSURE SHOULD I START WITH?
Typically, 12 psi front and 12 psi rear. Except in extreme conditions, a two pound adjustment up or down is the range you should stay within (from 10 to 14 psi).
NUMBER TWO: CAN ONE POUND MAKE A DIFFERENCE?
A one pound change in tire pressure is almost a 10 percent adjustment. The change in casing stiffness, knob footprint and the overall effect on traction is, however, exponential. Tire pressure is so sensitive that the factories adjust in quarter-pound increments. When you first make a pressure adjustment, stick to a one psi increase or decrease. If necessary, fine tune from there in 1/2 psi and even 1/4 psi increments.
NUMBER THREE: DOES RIDING A BIKE HEAT UP THE TIRE?
Whenever the tire is spinning, the rubber gets hot because of friction. But that’s not the only cause of tire heat. Sidewall flex and casing deformation also increase the temperature of a tire. Tire pressure increases more in the rear tire than the front, more on a hot day than on a cold day and on hard-packed track more than in loam. During the first three laps, tire friction will raise tire pressure by as much as six pounds (and the front by as much as three). The simplest solution to heat build up is to go to the starting line with less than optimum tire pressure. Start with 10 psi in the rear tire. That way the tire will top off at approximately 14 psi by lap three.
NUMBER FOUR: ARE FRONT TIRES MORE SUSCEPTIBLE TO FLATTING?
Yes. The front tire has a smaller cavity with less air volume. With the same pressure front and rear, it takes a lot less of an impact to hammer the front tire into the rim and pinch flat it.
NUMBER FIVE HOW CAN I TELL IF I HAVE TOO MUCH TIRE PRESSURE?
Lack of grip and excessive wheelspin are the most obvious indications of too much pressure. If the pressure is too high, there won’t be any “rim clean.”
NUMBER SIX: WHAT IS RIM CLEAN?
Rim clean is a phrase that refers to the sidewalls’ tendency to roll over the edge of the rim. A properly inflated tire will flex around the sidewalls and actually scrape the very top of the rim edge clean. If you don’t see a shiny strip of polished aluminum around the top edge of the rim, you have too much air pressure.
NUMBER SEVEN: HOW CAN I TELL IF I DON’T HAVE ENOUGH TIRE PRESSURE?
The bike will wallow through the bumps. Similar to too much pressure, a soft front tire also won’t stick in the turns. But instead of being too hard and sliding, the front goes away because the sidewalls are too flimsy and the tire is rolling over off the edge of the rim. If there is more than a 4mm swipe of “rim clean,” you don’t have enough air pressure.
NUMBER EIGHT: WHAT TIRE PRESSURE IS BEST FOR MUD?
If mud conditions reduce track speeds, a rider should reduce tire pressure down to 10 psi or lower in the back. If the mud isn’t very deep and the speeds are still high, stick with your standard air pressure. If the mud is so tacky that you’re fighting to keep the front end down, you might actually add air pressure to help the tire break loose easier.
NUMBER NINE: WHAT DO I DO ON ROCKY TRACKS?
Raise tire pressure on rocky tracks.
NUMBER TEN: WHAT ABOUT FOUR-STROKES?
Four-strokes transfer weight differently from two-strokes and tend to smash through more than skim over obstacles. This characteristic puts more of a load on the front tire than it does on the back. Where the standard pressure is 12 psi on a two-stroke, you should start with 14 psi in the front of your YZ-F or CRF. The rear tire can stay at 12 psi (but you might want to consider one of the new stiff sidewall four-stroke tires).