Interesting Reading

Here are excerpts from a study by Hunter, the company that makes suspension and tire test equipment. In this study they measured the effect of tire pressure on tire spring rate, and road adhesion over bumps.

Hunter study
Adhesion is the minimum percentage of remnant vertical tire contact force between the tire and the road surface during vertical oscillation of the wheel

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Tire spring rate is measured as the number of pounds required to compress the tire one inch. Each 1 psi of pressure above 32 increases the spring rate about 35 lb/in. Inflating to 50 psi gives tire a spring rate of 1730 lb/in...a 57% increase.

Note below how the 2 performance tires at the top of the graph go from spring rates around 1100 lb/in at 30 psi to 1600+ lb/in at 45 psi.


Figure 14b: Tire Spring Constant vs. Tire Pressure



Why is the tire spring rate important? Because the tire spring and the suspension spring act in series. Increasing the tire spring rate transfers it's bump absorbsion to the suspension spring. The suspension assembly compresses faster and the tire contact patch loses more adhesion with the road. You can see this in the next graph. The K2 in parenthesis is the tire spring constant in lb/in. The middle line at K2(1040) lb/in (28 psi) strikes the bump and immediately drops to about 80% adhesion. Then, as the suspension oscillates it recovers and drops further to 65% adhesion. Adhesion between 61 and 100% is considered "excellent" dynamic tire contact.


Figure 15b: Analytical Effect of Tire Spring Constant on Adhesion



Now look at the spring constant line for K2(1560) lb/in. This represents a tire over-inflated by 15 psi. Adhesion drops to <40%. Adhesion between 21 and 40% is "fair" dynamic tire contact. You can extrapolate that a tire at 50 psi, or K2(1730) lb/in would drop adhesion exponentially to <20%. This is the range of "poor" dynamic tire contact.

Hunter study
The relationship of tire pressure to adhesion is an inverse relationship in the proper range of inflation and load on the tire.

The tire spring constant is greatly effected by the inflation pressure. A higher tire spring constant allows more of the road disturbance to be transferred to the sprung mass and increases the acceleration of the unsprung mass at wheel hop frequency. A higher tire pressure increases road noise and causes more disturbances to be transferred to the sprung mass. A change of one p.s.i. of tire pressure can change the minimum adhesion by 0.5% to 2.2%. From testing over 100 different vehicles, an increase of one p.s.i of tire pressure decreased adhesion by an average of 1.2%.



Additionally, higher pressures create a longer and narrower tire FAR (footprint aspect ratio). The front of the tire footprint is the contact zone and the rear is the slip zone. It is now agreed that a long narrow footprint has a larger slip zone than a wider short footprint. This is why wider tires have better grip and why over-inflation reduces grip and degrades safety.

Some interesting data and something people should think about before over-inflating their Prius' tires....