Chassis Systems and Components Flashcards
Tire
The most critical component for vehicle dynamic behavior in the longitudinal lateral and vertical directions
What characterizes tire behavior
Local effect at the contact patch (friction)
What are the two main forms of friction between tire and road way
Adhesive (intermolecular adhesion - parallel) and hysteresis friction (perpendicular).
Types of Tire Loads and forces
Longitudinal force Fx
Lateral Force Fz
Wheel load Fz
Aligning torque Mz
Tire Slip
Slip between tire and roadway due to the elasticity of the tire’s tread profile and structure, as well as the momentary coefficient of friction between tire and roadway.
What is slip made of
Deformation and Relative Movement
Driving Slip K_A
(V_tire- V_vehicle) / V_tire
Braking Slip K_B
(V_vehicle -V_tire)/ V_vehicle
Relationship between K (tire slip) and Friction Coefficient
Coeff. of friction onto Slip %
1. Initial Linear Region: At low slip values, the coefficient of friction 𝜇 increases nearly linearly with 𝜅. This is because a small amount of slip allows the tire to generate more grip.
2. Peak friction: As Slip increases μ reaches a peak. This peak represents the maximum grip the tire can provide, which is critical for optimal braking and acceleration.
3. Declining Region: Beyond the peak, further increases in slip lead to a reduction in 𝜇. This is because the tire begins to lose grip and transitions into a sliding regime where friction decreases.
What does the friction coefficient depend on?
Vertical wheel load
Tire Pressure
Driving Speed
Slip
Condition of the road surface
When can force transfer occur
Only when the tire is elastically deformed
Tire Slip Formula
S_a = V_wheel sin(alpha)/ V_wheel cos(alpha)
Pneumatic trail meaning
A pneumatic trail is the distance between the point where the lateral force (also known as the cornering force) acts on the tire contact patch and the geometric center of the tire contact patch. This distance creates a moment arm that generates a torque, helping to align the wheels with the direction of travel.
Can pneumatic trail change?
Yes, it is influenced by tire pressure, speed, load on the tire
What happens to the pneumatic trail if the slip angle increases
Pneumatic trail decreases
Lateral Force meaning
The force exerted by the tire on the road surface in the lateral direction (perpendicular to the direction of travel). This force is responsible for changing the direction of the vehicle during cornering.
Relationship between lateral forces and side slip angle
At low slip angles, the lateral force increases linearly with the slip angle
As the sideslip angle increases the relationship becomes a nonlinear decreasing slope. Nearly linear for values < 3deg
What is the aligning torque
The torque around the vertical axis of the tire that tends to align the tire with the direction of travel by reducing the sideslip angle of the wheel. The pneumatic trail (Mz/Fy) acts as a lever arm for the lateral force and causes a moment Mz.
What is a combined slip
When a tire experiences both longitudinal and lateral slip simultaneously.
Kamm’s circle
It represents the relationship between longitudinal slip (x-axis) and lateral slip (y-axis).
How many axles does a passenger vehicle have
2
What do axles do
They connect the wheel carriers to the body via the suspension linkage
What do most axles feature
A stabilizer bar which connect the wheels to each other, and helps reduce body roll
Are vehicle axles preassembled
Modern passenger vehicle axles are pre-assembled to a subframe and attached to the body using four bolts in a final assembly step
What are the subcomponents of the front axle
The steering gear. It helps control the motion of the front wheels.
How are the wheel carriers connected?
They are connected to the stationary axle via the suspension components
Where is the suspension located
The suspension should be exactly symmetrical about the vehicle’s centerline
How many d.o.f do the wheels on a rear suspension system have?
One d.o.f, which is vertical, relative to the vehicle’s body in order to allow rebound and compression travel
How many d.o.f do the wheels on a front suspension system have?
Two d.o.f, a vertical one allowing rebound and compression travel and a rotational about the pivot axis to allow steering motion
Types of axles
- Rigid (dependent wheel control)
- Independent Suspension System
- Semi-rigid axles (twist beam sus. sys.)
A rigid axle…
Connects rigidly two wheels to one another, the motion of one wheel is dependent on the motion of the other.
Are rigid FRONT axles common?
No, they are only found in off-road SUVs
Are rigid REAR axles common?
Yes, they are generally used in heavy vehicles like SUVs, vans, and light trucks. But those vehicles have reduced comfort.
Semi-Rigid Axles…
Provide a mechanical connection between the two wheels, and also features an elastically deformable central connecting element, which allows a certain amount of targeted relative motion between wheels.
Independent Suspension…
It has a collection of joints and rigid elements that connect the vehicle’s body to the wheel carrier.
Must allow the wheel carrier to move in the vertical direction with respect to the vehicle’s body. Second D.o.F required for steering motion
What does the McPherson Strut consist?
Wheel carrier, damper, spring, steering tie rod, control arm
What is the purpose of a spring system?
Ride comfort, and overall safety, protect the vehicle structure, road, and vehicle’s passengers from impacts.
Types of springs
Leaf springs, helical coil springs, air springs, torsion bar springs
Principle of stabilizer bar
Reduce body roll, change over/under steer behaviour no effect to comfort when both wheels are equally compressed or extended
Purpose of damping
Dampen oscillations and vibrations of the vehicle’s body caused by uneven roads or driving conditions.
How are dampers arranged?
Parallel to vehicle suspension
Monotube shock absorber Pros and Cons
Pros: Cooling of the fluid, Increased piston diameter for an equal tube diameter, less foaming and cavitation of the oil due to inner
pressure, any installation position possible
Cons: Cost
Types of dampers:
Monotube Shock Absorber
Twin Tube shock absorber
Twin tube shock absorber pros and cons
Pros: Simple production, low friction
Cons: Vertical installation only, less cooling, foaming of the oil possible
What are the requirements of a steering system
Accurately guide the vehicle
Must be robust, sensitive, and precise enough to inform the driver about vehicle condition parameters and their changes
