Dynamics System Parameters Research
- Tania Lai
Camber Angle
The general consensus is that a positive camber is good for keeping a vehicle stable (greater stability in a straight line), while a negative camber allows high-performance vehicles to achieve better grip on the road when cornering.
Most street cars have a slight negative camber on all wheels, with more negative camber in the rear than the front to reduce oversteer (loss of grip in rear). Why? The outside front wheel gets about that much additional negative camber when the front wheels are steered during cornering; while the non-steering rears have a constant camber angle. Front wheel caster also increases the load on the outside rear tire in a turn, requiring additional negative camber to balance it.
For a normal car, the negative camber can range from -0.5° to - 2° for a good balance of cornering grip. Some race cars can have a negative camber as much as -4°.
How much is too much?
Generally speaking, the inside of the tire should be 10-15° F hotter than the outside, depending on the track and the temperature measuring points. Excessive heating on the inner or outer side of the tire can be indicative of improper camber angle. If the inside is exceedingly hotter than the outside, there is too much negative camber which will lead to faster wear.
** Lateral tire scrub is increased when you try to optimize camber angle in the suspension. For solar cars, minimizing scrub is arguably more important than optimizing camber for efficiency reasons.
Caster Angle
Caster is the measure of how far forward or behind the steering axis is to the vertical axis, viewed from the side.
Positive caster helps with straight line tracking if the driver lets go of the steering wheel. It helps to resist unintentional turning of the vehicle. Negative caster reduces steering effort because the weight moves forward of the spindle vertical center but also detracts from high-speed stability. It is usually not preferred.
Adding a small caster angle encourages the wheels to track in a straight line and stabilizes the steering. Camber does not matter for handling purposes, but if we had an excessive amount, the wheel could rub on the carbon fibre fairing.
Most road cars have positive caster. The angle value is usually between +3° and +5°. In sports and competition cars, the caster angle can assume much greater values (e.g. +12° and +13°).
A safe starting point for caster is 2° to 3° of positive caster.
**Caster angle affects the camber of the wheel during steering. Positive castor angle induces a positive camber angle on the inside tire and produced a negative camber on the outer tire.
Toe Angle
The toe setting is typically used to help compensate for the suspension bushings compliance.
Toe-in enhances straight-line stability. Toe-out responds quickly to steering. Toe-out encourages the initiation of a turn, while toe-in discourages it. So street cars are generally set up with toe-in, while race cars are often set up with toe-out.
Commonly, a rear-drive vehicle would likely require a front wheel toe-in setting, and a front-wheel-drive vehicle would likely require a slight toe-out setting.
The toe settings for production cars are usually less than ~0.1°. Typical toe-in specs vary from 1/32 to 1/8 inch.
Some toe-in can help improve stability of vehicle at the cost of increased tire scrub.
Zero static toe maximizes performance and efficiency. With an inward or outward toe angle, both energy and performance would be lost due to friction and any toe angle could cause bump steer. Bump steer makes the car less efficient, so we want to minimize that.
Scrub Radius
The scrub radius is the distance in front view between the king pin axis and the center of the contact patch of the wheel. Scrub radius affects the tire's tendency to change the dynamic toe angle during braking and acceleration.
Positive scrub causes a toe-out force which helps to keep rear-wheel-drive vehicle straight. It is useful with certain racing setups. It’s also a standard setting with many double-wishbone suspension designs. During braking, if braking effort is greater on one side of the vehicle than the other, positive scrub radius will cause the vehicle to veer towards the side with the greater effort.
Negative scrub causes a toe-in force. It has stability benefits for the split braking. When one half of the brake system fails, the vehicle will pull up in a straight line. Negative scrub is commonly seen in front-wheel-drive vehicles.
In summary, most road cars have a slight negative scrub radius. Zero to positive would be preferable for race cars.
For solar cars, zero scrub radius is the most efficient, it minimizes bump steer. However, zero scrub radius, under hard braking, causes the suspension to be skittish because varying road conditions created varying amounts of torque (both positive and negative) around the steering axis.
Therefore, some amount of scrub radius, positive or negative, is preferred. Remember to keep scrub radius small, around 0.25 – 0.75 inch.
Kingpin Inclination
Due to a structural limitation, there’s no way to cram your control arms, steering knuckle, and brakes into your wheel, there’s just not enough space. So we have to rely on an inclined steering axis to achieve a vertical steering axis and zero scrub radius at the same time.
Kingpin inclination produces a lifting force that has a self-centering effect on the steering wheel just like the caster angle does. This is because the kingpin angle makes the wheel axle trajectory travel in an upside-down arc, as opposed to traveling in a perfectly horizontal plane. So when the wheels turn, they dig into the ground and actually lift the car up slightly.
The tendency of the steering wheel to self-centre when a vehicle is driving is called returnability. Returnability increases with increased kingpin angle. Increased kingpin inclination also contributes to increased steering efforts.
In general, kingpin angles between 6° - 14° are seen on production vehicles. Double wishbone suspensions usually have values closer to the lower end.
**Kingpin inclination can add unwanted camber angles on the outside wheels. This effect can be reduced by adding additional caster angles.
http://www.suspensiondesigner.com/design-with-race/suspension-key-performance-indicators-kpi/
https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1379&context=mesp