Tire Pressure

Tire deflection is the most important variable concerning the area of contact between the tire and roadway. If inflation pressure and load are simultaneously varied so as to maintain constant tire deflection, the contact area of the tire will remain relatively constant.

The contact patch of a tire is acted upon by a force vector which can be expressed as having two components. One is perpendicular to the contact surface, and the other is tangential. The tangential component can be further broken down into a component parallel to the central plane of the tire, and another perpendicular to the central plane. These components collectively are called the shear components.

The vertical component at any point is equal to the inflation pressure of the tire plus the bundle of (tire structural characteristics, tire driving, braking torque, tire side forces, tire velocity, etc.)

Experiments have supported the postulate that the net pressure distribution at any point depends primarily upon the inflation pressure, while subsequent experiments have found that the primary factor is the bundle of (tire structural characteristics, tire driving, braking torque, tire side forces, tire velocity, etc.)

Extensive experimentation on rigid surfaces has demonstrated that tires with tread patterns using many kerfs (or tires with small tread patterns) exhibit complex pressure distributions. There are often marked variations in pressure over a small block. Tires with smooth patterns differ greatly from tires with tread patterns, or [in turn] tires with small tread patterns when tested at varying tire pressures.

Maximum pressure points occurred in the patterned treads, slightly toward the "toe" of the tread blocks, in the direction of vehicle travel, due to the increased stiffness of the sheared and compressed rubber in that area.

I will not at this juncture get into the difference that tire pressure makes on a wet surface, but it would make an interesting future discussion.

The contact pressure distribution on a flat surface, flat inflated membrane means that no matter what the tension in the membrane, the fact that it is in geometric contact with the flat surface means that the contact pressure distribution is exactly equal to the inflation pressure inside the membrane. The primary component of tire vertical pressures would be the inflation pressure.

However, if the tire was instead in contact with a cylindrical road wheel, this would no longer be true, since the cover tension now plays some role in defining the net contact pressure. Therefore, we must examine the structural components which affect the vertical contact pressure, to compare with the importance of tire pressure.

A radial tire is essentially an elastic band, supported by the tire side walls [which are under tension - see "how works a tire" above]. The structural components of most importance with respect to tire pressure interaction are:

1. Elastic support of the tread by the sidewall;
2. Bending of the tread;
3. Shear deformation of the tread;
4. "Snap through" buckling of the tread, defined as the tendency of the tread to seek a deformed equilibrium position due to membrane compression; and
5. Normal compliance or stiffness of the tread.

Isolating some of these, we can see that, for example, deflection of the tire would result in buckling, which would decrease the contact pressure near the center. The effect of stiffness is not well understood, except to say that its effects on contact pressure act in the manner of a slow buildup. Velocity generally increases vertical contact pressure at the forward edge of the contact patch, with a decreasing value at the rear portion of the contact patch.

In automotive tires, there is a relatively high normal contact pressure in the shoulder area of the tire tread, due to the heavy tread shoulder. This is, of course, less pronounced on motorcycle tires.

The tire carcass does not freely deform in use, because the friction in the contact patch of a moving and/or turning vehicle causes tangential forces. As a result, the friction of rubber/roadway friction coefficients are available which exceed those normally observed in vehicle use. This is largely believed to be due to the secondary slip caused by these tangential forces.

The effect of sideways shear forces is quite different than the effects of tangential shear forces.

In the case of yawed rolling, the tangential stress distribution is associated with forcing the elastic tire against a flat roadway, plus the additional effects due to yawed rolling, braking or acceleration. Cornering force intensity is obtained by integrating the lateral component of tangential stress across the width of the tire contact patch. This results in asymmetric measurement of force due to yawing.

Returning to the experiments regarding vertical pressure, there is little change in vertical pressure due to yaw, but a measurable amount due to braking. Rotational torque in acceleration compresses the tread elements in the zone immediately before the contact, and decreases the pressure as the tread is released from this compression.

In summary, then, tire pressure affects primarily the vertical pressure on the tire. The structural components of the tire, and its manner of use, determine the remaining component of contact pressure.

An unrelated, but potentially critical note, is that the vertical pressure secondarily effects the range of tire carcass motion, which in turn can dramatically affect tire temperature. For example, low tire pressure can result in the rubber turning hard, and blowing out the entire sidewall. This can happen in a remarkably short time on an underinflated tire operated at high speeds.

What I conclude from this is that there are usually at least an equal number of other factors affecting handling which are interacting with vertical tire pressure to produce ride and handling characteristics. Theoretically we could postulate the tire is the same in terms of characteristics, therefore, tire pressure can be changed independently with predictable results. Experimentally, however, it is found that the relationships interact in a far more complex and definitely non-linear fashion.

While as a rule of thumb, lowering pressure for the track (or for more aggressive street riding), might give better handling on a flat surface in dry weather, this may not be advantageous for braking, or for maneuvers combining different surface conditions.

I don't begin to claim enough knowledge to make this call. For what it's worth, I keep my tires at 40/42, carefully checked with an accurate gauge. I do not find particularly substantial advantage, even on a track, with lower pressures, although I do find some.

Mike Padway

Last update: 06:31 PM Sunday, September 26, 2004

• Venture Model Numbers
• Yamaha Online Manuals
• Oil Filter Autopsy
• Yamaha Online Parts Catalog
• Common Motorcycle Tire Wear Patterns
• Gyroscopic Precession
• How Tires Work
• How To Read Tire Codes
• General Tips
• Removing Stuck Bolts
• Motorcycle Motor Oil
• Snake Oil!
• Oil Filter Alternatives
• Anatomy of Oil Filters
• Whistles and Whitetails
• Deer Whistles - Do they work?
• Winterizing Your Bike
• Heated Hand Grips
• Drill Bit Sizing Chart