Wheel alignment is an important design factor for the vehicle, In addition to allowing the vehicle to be turned, the steering system must be set up to allow the vehicle to follow straight path ahead without steering input from the driver.
How to make it possible ?
Four parameters are set by the designer to make proper wheel alignment , and these must be checked regularly to ensure they are within there limits as decided by designer.
Wheel Alignment Parameters
Camber
In wheel alignment Camber is the angle of the tire/wheel with respect to the vertical as viewed from the front of the vehicle.
Camber angles usually are very small, Positive camber is defined as the top of the wheel being tilted away from the vehicle. It is measured in degrees . Most vehicles have small degree of positive camber
Whereas negative camber tilts the top of the wheel toward the vehicle also viewed from front of the vehicle. However, some off-road vehicles and race cars have zero or slightly negative camber.
What is the Point of adding Camber ?
The point of adding camber in racing is , let take example of car standing stationary and having zero camber therefore having perfect rectangular contact patch.
while on other hand vehicle taking corners , the Centrifugal Force is experienced by the Tyre, therefore the contact patch becomes imperfect like a triangle , which means it deforms the contact patch resulting the more wear of tire from outer edge of Tyre
So the point of adding Negative camber is that to tilt the Tyre one way so that it can counter the deformation of the contact patch . Therefore the patch becomes perfect rectangle rather than triangle as shown in fig below.
Downside of adding the camber is that you are compromising grip while straight line , means while straight line the contact patch becomes triangle rather than nice rectangle. By giving camber you are actually optimizing grip while cornering rather than in straight line . In conclusion Negative camber is useful for circuit racing cars.
Steering Axis Inclination (SAI)
In wheel alignment Steering axis inclination (SAI) is the angle from the vertical defined by the center line passing through the upper and lower ball joints. Usually, the upper ball joint is closer to the vehicle center line than the lower.
The advantage of combining positive camber with an inclined steering axis. If a vertical steering axis is combined with zero camber (left side of Fig.), Any steering input requires the wheel to scrub in an arc around the steering axis. In addition to increasing driver effort, it causes increased tire wear.
While the combination of SAI and positive camber reduces the scrub radius (right side of Fig.). This reduces driver effort under low-speed turning conditions and minimizes tire wear. An additional benefit of this system is that the wheel arc is no longer parallel to the ground.
Suppose vehicle has SAI or KPI then it tries to lift the tyre resulting heavy steering, because the ground is not movable. thus, the weight of the vehicle tends to turn the wheel back to the straight ahead position called as self centering .
This phenomenon is very evident on most vehicles-merely turning the steering wheel to full lock while the vehicle is standing still will make the front end of the vehicle rise visibly.
Although the stationary the weight of the vehicle may not be sufficient to rotate the wheels back to the straight-ahead position,
As soon as the vehicle begins to move, the wheels will return to the straight-ahead position without driver input.
Caster angle also contributes to this self-centering torque.
Toe
In wheel alignment Toe is defined as the difference of the distance between the leading edge of the wheels and the distance between the trailing edge of the wheels when viewed from above.
Toe-in means the front of the wheels are closer than the rear; toe-out implies the opposite. For a rear-wheel-drive vehicle, the front wheels normally have a slight amount of toe-in.
When the vehicle begins to roll, rolling resistance produces a force through the tire contact patch perpendicular to the rolling axis.
Due to the Resistance of the scrub radius, this force produces a torque around the steering axis that tends to cause the wheels to toe-out. The slight toe-in allows for this, and when rolling, the wheels align along the axis of the vehicle.
On other hand , front-wheel-drive vehicles require slight toe-out. In this case, the tractive force of the front wheels produces a moment about the steering axis that tends to toe the wheels inward.
In this case, proper toe-out absorbs this motion and allows the wheels to parallel the direction of motion of the vehicle.
Caster
In wheel alignment Caster is the angle of the steering axis from the vertical as viewed from the side. Positive caster is defined as the steering axis inclined toward the rear of the vehicle.
With positive caster, the tire contact patch is aft of the intersection of the steering axis and the ground. This is a desirable feature for stability.
When the wheel is turned, the cornering force acts perpendicular to the wheel axis and through the contact patch. This creates a torque about the steering axis that acts to center the wheel. Obviously, negative caster results in the opposite effect, and the wheel would tend to continue turning about the steering axis.
The most common example of positive caster is a shopping cart. The wheels are free to turn around the steering axis,
And when the cart is pushed straight ahead, the wheels self-align to the straight-ahead position.
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Originally published at https://mechmind.website on March 21, 2020.