Mike Padway <MMxyzp@aol.com>
Date: Fri, 15 Oct 1999 18:57:11 EDT
I am thumbing though a DOT publication on pneumatic tires that is close to 1,000 pages long and has some interesting things in it on tires. I am passing along some parts of it that were interesting to me.
It is abundantly obvious that this material has limited interest. I hope it does help explain some of the choices made by the tire manufacturers. If you can stomach this post, it may help you make some decisions next time. (or, you can do what I do - ask the dealer for advice).
'Nuff warning, here goes:
In an undeflected tire the cords are tensioned by the excess pressure of the inflating gas over the external or atmospheric pressure. The tire casing takes up its equilibrium shape, which is determined primarily by the cord paths, somewhat modified by the other components of the tire. As the tire is pressed against a flat roadway, the tread rubber is compressed and at the same time the tire casing locally loses its axial symmetry and takes on a substantially flattened contact patch. If there were no tread rubber, the casing would be flat to the ground in the area of the contact patch.
The bottom sidewalls of the tire do not "push upward" to keep the wheel rim off the ground. The wheel rim HANGS in the bead coils, which in turn hang in the tensioned casing cords, which have lower tension in the contact patch than elsewhere. The sidewalls of the tire are in tension. The contact patch has reduced tension because of the weight of the vehicle.
The tire contact area's shape depends on the tire cross section shape and structure. The relationship between tire contact area and tire deflection is nearly linear, as you would expect. Tire deflection is the most important variable governing the area of contact between the tire and the roadway. If inflation pressure and load are varied simultaneously, the contact area will remain effectively constant. There are few studies on the effect of velocity on contact area, but experimental data seems to indicate that velocity slightly increases contact area. Fluid contaminants, such as water, oil, slush or mud reduce the contact area due to the persistence of a a fluid film in portions of the formerly dry contact area. This resistance of the fluid to expulsion is tied to the viscous and inertial forces of the fluid as it is wedged and/or squeezed between the tire and the pavement. This results in a loss of dry contact area.
Tests of tires on wet surfaces using photographs taken through glass in the "pavement" surface have analyzed the variables in determining the effect of the fluid on the tire. The extent to which the fluid will persist along the centerline of the contact patch is determined by whether the fluid inertial forces are large enough to bend the tire surface inward along the footprint centerline. Tires with rigid carcass construction, high inflation pressures, and large tread element size will tend to work better in wet conditions. Tires with flexible carcass constructions, low inflation pressures, and narrow, widely spaced grooves, will tend to lose contact along the centerline in wet conditions.
Even when fluid is insufficient to flood the major groove network, it can reduce traction by persisting in individual areas. Contact is established first around the perimiter of the contact area, trapping the fluid in the central area under pressure. The sipes of the tread element and microtexture of the surfaces of the tire and pavement allow the fluid to escape (if it does so).
Contact area on dry areas is still not completely understood, and there are different conflicting theories. Experimental data indicates that the net pressure distribution at any point in the contact patch depends primarily upon tire pressure. Small blocks of tread design often demonstrate markedly different distributions on different areas of the same block. While in theory the smooth tire gives the best traction, pressure levels in blocked patterns exceed those in smooth areas by about 25%. Maximum tire pressure (on the contact area) occurs in the middle of the projection, being slightly shifted to the "toe" of the blocks, in the direction of vehicle travel, due to the increased stiffness of the sheared and compressed rubber in that region. Uneven tire wear and moulding irregularities including seams, can result in quite different distributions at different points in the life of a tire.
The tire structural elements which most effect the amount of pressure in the contact area are: Elastic support of the tread by the sidewall; bending of the tread; "Snap through" buckling of the tread, defined as the tendency of the tread to seek a deformed equilibrium position due to membrane compression; and normal compliance or stiffness of the tread. Slip between the tire and the roadway is important with reference to the contact patch. When a tire is brought into contact with a flat roadway, it can only form a flat contact area by simultaneous bending and compression of the tire surface. This means that tread elements in contact with the roadway will generally undergo a small amount of deformation in the plane of the carcass as such tread elements pass through the contact patch of the rolling tire and exit out the trailing edge. This process takes place in every tire.
It is possible to minimize the membrane stretching and contraction by making the carcass structure of the tire as rigid as possible. This is generally the principal behind radial tire construction. It is also possible to minimize such membrane distortions by use of extremely stiff materials for the tire carcass, such as steel wire. Since the membrane deformations actually take place in the carcass of the tire, it should be possible to prevent motion of the tread on the contact surface by choice of an appropriately soft tread compound.
This won't solve the tire pressure dilemma, but it should shed some light on the relative importance of tire pressure in the ride, handling, braking, mix.
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:
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.
The source for the factual (non-opinion) information above is taken almost exclusively from Alan Browne (General Motors), K.C. Ludema and S.K. Clark (Dept. of Mechanical Engineering - U of Mich. Ann Arbor), as reported in "Contact Between the Tire and Roadway", Chapter 5, MECHANICS OF PNEUMATIC TIRES, U.S. Department of Transportation - August 1981.
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