Tire (Magic Formula)
Tire with longitudinal behavior given by Magic Formula coefficients
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Simscape / Driveline / Tires & Vehicles
Description
The Tire (Magic Formula) block represents a tire with longitudinal behavior given by the Magic Formula [1], an empirical equation based on four fitting coefficients. The block can model tire dynamics under constant or variable pavement conditions.
The longitudinal direction of the tire is the same as its direction of motion as it rolls on pavement. This block is a structural component based on the Tire-Road Interaction (Magic Formula) block.
To increase the fidelity of the tire model, you can specify properties such as tire compliance, inertia, and rolling resistance. However, these properties increase the complexity of the tire model and can slow down simulation. Consider ignoring tire compliance and inertia if simulating the model in real time or if preparing the model for hardware-in-the-loop (HIL) simulation.
Tire Model
The Tire (Magic Formula) block models the tire as a rigid wheel-tire combination in contact with the road and subject to slip. When torque is applied to the wheel axle, the tire pushes on the ground (while subject to contact friction) and transfers the resulting reaction as a force back on the wheel. This action pushes the wheel forward or backward. If you include the optional tire compliance, the tire also flexibly deforms under load.
The figure shows the forces acting on the tire. The table defines the tire model variables.
Tire Model Variables
Symbol | Description and Unit |
---|---|
rw | Tire rolling radius. |
Vx | Wheel hub longitudinal velocity. |
u | Tire longitudinal deformation. |
Ω | Wheel angular velocity. |
Contact point angular velocity. If there is no tire longitudinal deformation, that is, if , then . | |
Tire tread longitudinal velocity. | |
Wheel slip velocity for a tire without compliance. | |
Contact slip velocity for a tire with compliance. If there is no tire longitudinal deformation, that is, if , then . | |
Wheel slip for a tire without compliance. | |
Contact patch slip. If there is no tire longitudinal deformation, that is, if , then . | |
Vth | Wheel hub threshold velocity. |
Fz | Vertical load on tire. |
Fx | Longitudinal force exerted on the tire at the contact point. |
Tire longitudinal stiffness under deformation. | |
Tire longitudinal damping under deformation. | |
Iw | Wheel-tire inertia, such that the effective mass is equal to |
τdrive | Torque applied by the axle to the wheel. |
Tire Kinematics and Response
The equation for translational motion of a non-slipping, non-compliant tire is . When tires experience slip, they respond by developing a longitudinal force, Fx.
The contact patch slip velocity is . For a tire without compliance, u = 0. The block defines the contact patch slip as
where the square root expression provides numerical robustness when Vx = 0. For example, in the case of a non-translating, spinning tire, while is finite.
When the tire deformation is negligible and the wheel hub longitudinal velocity is sufficiently large, the equation for wheel slip approaches
For this equation, a locked, sliding wheel has k = -1. For perfect rolling, k = 0.
If the tire is modeled with compliance, it is also flexible. In this case, because the tire deforms, the tire-road contact point turns at a slightly different angular velocity, Ω′, from the wheel, Ω, and requires, instead of the wheel slip, the contact point or contact patch slip κ'. The block models the deforming tire as a translational spring-damper of stiffness, CFx, and damping, bFx.
If you model a tire without compliance, that is, if , then there is no tire longitudinal deformation at any time in the simulation and:
Tire and Wheel Dynamics
The full tire model is equivalent to this Simscape™/Simscape Driveline™ component diagram. It simulates both transient and steady-state behavior and correctly represents starting from, and coming to, a stop. The Translational Spring and Translational Damper are equivalent to the tire stiffness CFx and damping bFx. The Tire-Road Interaction (Magic Formula) block models the longitudinal force Fx on the tire as a function of Fz, and k′ using the Magic Formula, with k′ as the independent slip variable. Here, Fz is the input signal at port N.
The Wheel and Axle radius is the tire rolling radius rw. The Mass value is the effective mass, . The tire characteristic function f(k′, Fz) determines the longitudinal force Fx. Together with the driveshaft torque applied to the wheel axis, Fx determines the wheel angular motion and longitudinal motion.
Without tire compliance, the Translational Spring and Translational Damper are omitted, and contact variables revert to wheel variables. In this case, the tire effectively has infinite stiffness, and port P of Wheel and Axle connects directly to port T of Tire-Road Interaction (Magic Formula).
Without tire inertia, the Mass is omitted.
Assumptions and Limitations
The Tire (Magic Formula) block assumes longitudinal motion only and includes no camber, turning, or lateral motion.
Tire compliance implies a time lag in the tire response to the forces on it. Time lag simulation increases model fidelity but reduces simulation performance. See Adjust Model Fidelity.
Ports
Input
Output
Conserving
Parameters
Model Examples
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References
[1] Pacejka, H. B. Tire and Vehicle Dynamics. Elsevier Science, 2005.
Extended Capabilities
Version History
Introduced in R2011a