Tire load sensitivity
Encyclopedia
Tire load sensitivity describes the behaviour of tire
s under load. Conventional pneumatic tires do not behave as classical friction theory would suggest. The load sensitivity of most real tires in their typical operating range is such that the coefficient of friction decreases as the vertical load, Fz, increases.
Coulomb friction
theory says that the maximum horizontal force
developed should be proportional to the vertical load on the tire. In practice, the maximum horizontal force Fy that can be generated is proportional, roughly, to the vertical load Fz raised to the power of somewhere between 0.7 and 0.9, typically.
Production car tires typically develop this maximum lateral force, or cornering force
, at a slip angle
of 6-10 degrees, although this angle increases as the vertical load on the tire increases. Formula 1 car tires may reach a peak sideforce at 3 degrees
The same sensitivity is typically seen in the longitudinal forces, and combined lateral and longitudinal slip.
Tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
s under load. Conventional pneumatic tires do not behave as classical friction theory would suggest. The load sensitivity of most real tires in their typical operating range is such that the coefficient of friction decreases as the vertical load, Fz, increases.
Coulomb friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
theory says that the maximum horizontal force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
developed should be proportional to the vertical load on the tire. In practice, the maximum horizontal force Fy that can be generated is proportional, roughly, to the vertical load Fz raised to the power of somewhere between 0.7 and 0.9, typically.
Production car tires typically develop this maximum lateral force, or cornering force
Cornering force
Cornering force or side force is the lateral force produced by a vehicle tire during cornering.Cornering force is generated by tire slip and is proportional to slip angle at low slip angles. The rate at which cornering force builds up is described by relaxation length...
, at a slip angle
Slip angle
In vehicle dynamics, slip angle or sideslip angle is the angle between a rolling wheel's actual direction of travel and the direction towards which it is pointing...
of 6-10 degrees, although this angle increases as the vertical load on the tire increases. Formula 1 car tires may reach a peak sideforce at 3 degrees
Example
As an example, here is data extracted from Milliken and Milliken's "Race Car Vehicle Dynamics", figure 2.9:| Vertical load | Fy/Fz | Slip Angle |
| (lbf) | max | degrees |
| 900 | 1.10 | 5.6 |
| 1350 | 1.08 | 6.0 |
| 1800 | 0.97 | 6.7 |
The same sensitivity is typically seen in the longitudinal forces, and combined lateral and longitudinal slip.