Remember: other factors that affect the mechanical trail are tire and rim size, inflation pressure and tire wear. The caster angle in turn determines another important parameter, namely mechanical trail. Still looking at the vehicle from the side, the mechanical trail is the distance between the center of the contact area and the point of junction between the ground and the steering axis. To explain this, we introduce another parameter: the mechanical trail. Why can a caster angle be good for one car but may not be ideal for another? On Assetto Corsa Competizione we can in fact find cars like the Porsche 911 GT3-R that require a low caster angle, equal to +6.7 degrees and other cars that require a higher angle, such as the Ferrari 488 GT3 Evo and the Mercedes AMG GT3 Evo, greater than +12 degrees, and then there are cars like BMW with a caster angle of about +9.4 degrees. In common road cars the caster angle value is usually between +3 and +5 degrees and is usually more pronounced on rear-wheel drive cars. In fact, on those with front-wheel drive and all-wheel drive it tends to be more contained, because there is already a self-aligning effect caused by the traction, which produces a forward displacement of the center of the contact area of the tire, which generates a moment which tends to bring the wheels parallel to the direction of travel. These cars have a low positive caster angle as the steering needs to be light and manageable and to reduce vibration. In sports and competition cars, the caster angle can assume much greater values (for example between +12 and +13 degrees). This force is called camber thrust and is used to increase cornering abilities since it's much greater on the heavily loaded, outside tires.The first purpose of the caster angle is to give stability to the steering wheels, which would otherwise be unmanageable. Since the center of the contact area is some distance away from the steering axis extension, a realignment moment (torque) is generated every time an attempt is made to deflect the wheel from straight travel. This self-aligning moment is felt every time the steering wheel is turned, noting a certain opposition of the same, which tends to return to the “center”, that is to the zero angle. If the angle is small, the realignment will be slow and bland, vice versa with higher angles there is a more marked return of the steering and a greater hardness of the same. Camber also creates a force that pushes the car inward. Dialing in negative camber helps combat tread lift and wheel tilt. Since AWD cars have a large percentage of weight over the front wheels, they require more negative camber up front when compared to other drivetrain types - about the same amount that a FF sedan needs. Keeping the tires flat on the road is the primary reason to add negative camber and combat tread lift. As a car rolls into a corner, the chassis roll tilts the tire onto its outside edge, worsening the problem. If you drive hard, you've probably noticed the outside edge gets chewed up faster than the rest of the tread. When a tire is subjected to side load its sidewalls flex, digging the outside tread into the ground and lifting its inside into the air. Thanks to problems like tire distortion and production car-compromised suspension geometry, this rarely happens. A tire must use its contact patch efficiently to generate its maximum grip potential. Negative camber is when the top of the tire tilts inward. Positive camber is when the top of the tire tips outward. For track use, EVOs and DSMs should be fit with slightly stiffer rear springs, while Subarus require fairly equal rates at both ends erring slightly stiffer up front.Ĭamber is the inward or outward tilt of the tires when viewing them from the front. If you're a capable driver participating in many track events, you're better off ordering stiffer rear springs than what's typically offered. Such companies also seem to do the same thing concerning FWD cars. The ratio of front and rear stiffness offered is more appropriate for street driving. For serious autocrossing, track days and racing, the standard rear suspension spring rates usually offered by Japanese coilover makers tend to be too soft in proportion to the front, however this remains debatable by many suspension experts. AWD vehicles require more rear spring and/or rear bar, pretty close to what works well in a FWD car. Since most AWD cars exhibit a front-weight bias, they tend to understeer and as such need a disproportionately larger amount of rear roll stiffness to help counter this when compared to a typical FR sports car. The Basics: Springs, Anti-Sway Bars and ShocksĪ brief suspension primer is in order.
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