Appunti del Corso Ground Vehicle Engineering A

Name: Appunti del Corso Ground Vehicle Engineering A
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Author: Polistudent

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Appunti del Corso Ground Vehicle Engineering A basati su appunti personali del publisher presi alle lezioni del prof. Mastinu, dell’università degli Studi del Politecnico di Milano …

Esame Ground Vehicle Engineering A

Facoltà Ingegneria industriale

Dal corso del Prof. Mastinu Gianpiero

Università Politecnico di Milano

A.A. 2021-2022

141 pagine

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Functions of car tires

Main functions

Cars tires carry vehicle load and provide a certain rifless. They act as a spring that filters out load irregularities and exchange forces with the ground. The tire Ft-z characteristic is non-linear, so it can be linearized and modeled as a spring, where ft = k.t ≈ s ≅ ^Ẑfat.

The compliance given the tire's deformability depends on the vehicle type.

Longitudinal force

Longitudinal force: fx = μ f_z. Breaking K = V - RW / V. Tractive K = RW - V / RW. K = longitudinal slip. μ = μ (environmental conditions) "μ = μ (speed)".

If k ≪ 1, I'm inside the friction cone in the static case, lower than the friction limit. All the waves merge. NB μ dry asphalt (μ > 1 → fx > fz), wet asphalt, snow, ice.

k^limox = 1.

Vehicle tire dynamics

Cars tires provide a certain rifflery, acting as a spring that filters out load irregularities (numerical) and exchange forces with the ground. Compliance → the tire Ft-z characteristic is non-linear, so it can be linearized and modeled as a spring, Ft = k * z ≈ 1/2 z_stat. The compliance gives the tire's deformability, depending on the vehicle type.

Longitudinal force Fx = μ x Fz.

Braking and traction

V > RW, K = (V - RW) / V

Traction

k = longitudinal slip. μ = μ (environmental conditions) "μ = μ (speed)". μ_l max = 1.

Tire behavior and elasticity

If perfectly stiff rim → L = 2πR for a 2π rotation. If braking deformer rim → LB > L → limit case LB → +∞. If traction deformer rim → LT < L → limit case LT → ∅. But working and measuring distance is hard.

Δt_L = L-LT = (L-LT)/Δt = ωR - V = ΔV / RW = k_T
Δt_B = LB-L = (LB-L)/Δt = V - ωR = ΔV / V = k_S

Tire wire characteristics

The characteristic tire wire depends on its elasticity and elastic behavior. The difference between static and dynamic friction coefficient varies and can be understood in the ideal (perfectly stiff wheel or hard ground) case. In the ideal case, if we are in the case of static friction, slip occurs when we exceed the friction limit.

Braked wheel, varying friction on wet and ice. Ice → @ T << 0^°C, μ ≅ μ_stat is good, but if T ≈ 0^°C under vehicle pressure, ice melts and μ_max ↓, creating a worst-case scenario.

Water film → @ V↑, the μ_max↓↓, if the film depth ↑ increases significantly. NB μ_max gives the wave peak/maximum.

Tire dynamics

If α in the limit are α=π/2 → Mt=Ø because the four wheel is ØMt. Quick aligning torque (max) is before the limit of fy, which may help the driver understand the limit role.

Turning over arm, fy, fx, braking, oblique, lateral force ⇒ fy = μy ft❌ = arc tg Vy / Vx❌ = lateral slip / side slip angle. The lateral force exchanged between the tire and the ground doesn't pass through the wheel axis. A self-aligning torque is generated due to lateral deformation causing distance Cα = dfy / dα. For Cα, the rule of thumb α/ft = 10÷20.

Combined slip and camber angle

Combined slip μx = μy = ○ circle, uneven μx + μy = 0 ellipse, higher fx in braking. Camber angle ⇒ increase or decrease the available fy shifting up or down the curve fy–α carpet plot.

The influence of internal pressure of the tire as a function of ft and α is notable. The bigger difference arises @ ft ↑ while @ ft ↓, the deflected tire could give a higher fy for the same α than the inflated tire. Tire wear also influences; the worn tire performs better because the stiffness of the tread pattern increases.

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