changing tyre pressures

[Steveh7]

A message on chitchat got me thinking about the subject of tyre pressures. Obviously there critical to get right, can anyone explain to me the exact effect of raising or lowering the pressures with respect to handling during a trackday. For example is it possible to dial out some understeer characteristics by lowering or raising the front tyre pressures? Can of worms I'm sure, I'm sitting comfortably

[beaufort]

Found this elsewhere, it covers the effects of changing tyre pressures. Put your feet up, you won't need a book tonight

 

Oversteer and Understeer

 

The weight bias of the vehicle determines its inherent oversteer/understeer characteristics. A vehicle that is heavier at the front will tend to understeer and one that is heavier at the rear will oversteer. A vehicle in which the weight is equally distributed between the front and rear axles tends to exhibit neutral steer characteristics. Although the inherent understeer/oversteer characteristics of a vehicle are determined by its weight distribution, the design of the suspension and the selection of wheel and tire size can enhance or moderate those characteristics.

 

Understeer results when the slip angle of the front tires is greater than the slip angle of the rear tires. A greater steering angle is then required in order to maintain the turn. When the steering angle reaches full lock and the turn cannot be maintained, the vehicle drifts to the outside. In an understeer condition, the driver is attempting to negotiate a turn, but the vehicle mushes ahead refusing to cooperate. Oversteer produces just the opposite condition.

 

During oversteer, the slip angle of the rear tires is greater than the front. Consequently, the turn-rate increases on its own and the driver therefore reduces the steering angle to compensate. During severe oversteer, the steering angle may reach full lock in the opposite direction while the vehicle continues on into the turn. The vehicle is then said to "spin out." A vehicle that understeers is considered safer in the hands of the average driver.

 

An obvious solution to the negative effects of understeer and oversteer would seem to be that cars ought to be designed for neutral steer. Neutral steer is the theoretical ideal in which the slip angle of front and rear tires increase in unison throughout the range of steering angles. Unfortunately, the factors that influence vehicle dynamics are not so precisely manageable. With the slightest encouragement, a car with neutral steer characteristics can easily cross over into an oversteering condition. Consequently, designers prefer to create some degree of understeer in order to avoid oversteer.

Tuning the Suspension of a Completed Vehicle

 

When the suspension is designed, certain handling characteristics are targeted. However, mechanical compromises, errors, or limitations of the art may result in a vehicle that does not handle precisely as intended. Even after the vehicle is finished, the suspension can be tuned for different cornering characteristics. The variables available for tuning the suspension include changes in tire and rim size, tire inflation pressure, and the stiffness and location of the anti-roll bar.

 

The anti-roll bar is essentially a transverse-mounted torsion bar designed to reduce body-roll during turns. It exerts no influence on the suspension when wheels bounce in unison. If vertical movement on one side exceeds the vertical movement on the other, the anti-roll bar exerts an opposing force. Along with its primary function of reducing body-roll, the anti-roll bar will also reduce the combined cornering force and the adhesion limits of the side-by-side tires that are being acted upon. Consequently, the location and stiffness of the bar can be modified to influence the oversteering or understeering characteristics of the vehicle.

 

An oversteering tendency will be reduced by locating the anti-roll bar at the front where it will reduce the cornering force and adhesion of the front tires. If the vehicle understeers, the anti-roll bar should be located at the rear. If an anti-roll bar is already required at both ends of the vehicle to achieve adequate roll stiffness, use an anti-roll bar of greater stiffness/diameter at the end of the vehicle where reduced cornering force is desired, and use a less-stiff/smaller-diameter bar at the other end.

 

Changing the tire's inflation pressure has a more limited effect on handling characteristics. Inflation pressure influences the slip angle of the tire. A softer tire will require a greater slip angle in order to achieve equal cornering forces. Also a lower inflation pressure will cause the tire to reach its limit of adhesion at lower g-loads. Consequently, increase the inflation pressure at the end of the vehicle requiring greater cornering forces (greater adhesion). Reduce the inflation pressure for reduced adhesion and cornering forces.

 

Tire/wheel size is another important variable. Larger diameter tires tend to ride more smoothly, and they also develop greater cornering forces. However, installing larger tire to improve cornering is not always practical. Larger tires can cause clearance problems if the vehicle was not design for them, and they also affect suspension geometry. An alternative approach would be to install the same tires on wider rims. This provides a wider cross-sectional base for the tires and thereby improves cornering. Wider tires also aid in cornering, but at the expense of a rougher ride. Tires with a lower aspect ratio (low profile tires) develop significantly greater cornering forces and therefore can be used to improve the handling of a vehicle with marginal handling characteristics. Within limits, varying tire-size, rim-width and inflation-pressure can adjust cornering forces to achieve the desired overall performance.

 

Cheers

[thinfourth]

Sorry htta reads to me like utter ****e the second last paragraph sayes as you increase tyre pressure you get more grip therefore if i put my tyres to 1000psi it will never ever slide 🙆🏻

 

Blatting round aberdeen wondering wether to buy a jumper or heater

[Graeme Smith]

'Tis true, although the full statement should be something like "grip increases with tyre pressure, until the maximum grip point then falls off"