|Caster, Camber & Toe, Part I: CASTER
If everything in suspension comes down to the contact patch, then it makes sense that how the contact patch is placed against the road is pretty important. Changing the orientation of the wheel/tire will change the interface of the rubber and the road and will change how the car feels to the driver, especially when steering.
The three planes of orientation for the wheel relative to the chassis are defined by caster, camber and toe, as follows:
· Caster, fore or aft (positive or negative), is an angle describing the difference between the point on the road where the force of effort of the suspension is being applied on the wheel and the center of the wheel in the contact patch. · Camber, positive or negative, defines whether the top of the tire is leaning into the centerline of the car or out, away from the centerline. · Toe is, as it sounds, the "pigeon-toe" effect. Toe-in causes the front face of the tires to be angled toward each other. Toe-out causes them to be angled out away from each other. Toe affects the angle at which the tire tread intersects with the road.
What you do with the interplay of caster, camber and toe will affect how the tires greet the road surface, which will affect how they handle when cornering and how they respond to bumps and dips in the road surface. They also affect whether the car understeers or oversteers and determines some of the MANY baselines which must be considered before you get into shock and spring modifications.
The next three sessions of the LEMD University, will look at each of these areas.
Caster The most familiar example of caster is the caster you find on an office chair or dolly. These wheel assemblies demonstrate the fact that when you assemble the superstructure so the rolling tire can pivot freely, the tire is most comfortable trailing the applied force. The wheel/tire spins around the assembly axis to follow the superstructure. If you very carefully push a caster, it will roll forward, but at the slightest excuse to pivot, it will - usually quite abruptly! A similar thing will happen to a wheel/tire whose suspension arms cause the force which travels down the suspension geometry to intersect the road at a point behind the centerline of the wheel/tire. This effect is what's called "negative" caster and causes the car to be nearly uncontrollable. While it may be theoretically possible to steer through negative caster, just as you can very carefully push the caster on a dolly from behind -- with wheels ahead of the assemblies - the fact is that the wheels will pivot abruptly when that opportunity arises because there is no force to counter-balance the rotation. You have the majority of the mass in the wheel/tire ahead of the pivot point, carrying along the momentum of the turn. Without going too extensively into human anatomy (because the analogy diverges pretty quickly), the impact of negative caster is like walking with your weight on your heels. Your ankle geometry and the muscle mass of your legs is predominately already "behind" the pivot point. If your ankle turns on a stone when you're walking on your heels, you will be thrown off balance. When you're walking with your weight balanced on your foot, and you turn, you can control the motion more smoothly (and you also have better play of your ankle dynamics). If you carry your weight on your toes (positive caster), as you do when you're running, you can turn more nimbly. However, if your weight is distributed too far forward, (too much caster) you'll have a hard time turning because you have reduced the leverage on the pivot point and momentum will tend to carry you forward in the direction you were already traveling. Caster is modified by changing the alignment of the upper and lower outer pickup points (in an A-arm assembly it's the upper and lower ball joints in your suspension; with a strut it's the lower ball joint and upper strut bearing which is the upper pickup point). The imaginary line which runs through the middle of the ball joints defines what's called the Kingpin axis. The force of your suspension is directed along the Kingpin axis. The resultant angle of the Kingpin axis relative to vertical line through the center of the wheel/tire is caster (parallel to the front-to-back centerline of the car.) If the force falls ahead of the center, drag causes the wheel/tire to be pushed back into line if it starts to stray. This is because the greatest mass of the wheel/tire is behind the kingpin axis point. If the force falls on the center-line of the wheel/tire, you have zero caster - the wheel is balanced on the contact patch and can turn fairly easily out of or back in line. When the force falls behind the center of the wheel/tire, that mass outweighs what's behind the pivot point and the wheel/tire wants to hare off at every excuse. What happens in turns with these different angles of caster? With positive caster (suspension force ahead of the center of the tire), the steering (change in the angles of the steering arms and the resultant force) is translated directly into a closely parallel motion in the wheel/tire; but you do have to work against the drag - the wheel/tire's tendency to want to come back in line. Too much positive caster makes for heavy handling in turns. Neutral caster means the wheel/tire will follow the directions the steering arms supply with little or no resistance. Negative caster means the wheel/tire will say, "Oh boy! He's let go of the leash!" and will cause that corner to turn RIGHT NOW. Of the three, caster has the more noticeable effect on how a car feels to the driver, and less effect on tire wear than either camber or toe. We'll get into the interplay and results of changing caster, camber AND/OR toe after we've looked at Camber and Toe individually.
A large part of caster's job has to do with steering feel and recentering. Camber and bump steer affect tracking (where it often is in direct conflict with caster and toe), keeping the contact patch in contact.
The least understood by most people is caster and what it really does. Caster is primarily (90+%) is about having the front wheels re-center themselves whenever a driver wants to increase the radius of any turn or straighten the car out. The mechanical trail in the steering system, created in large part by the caster, is the main source of the re-centering forces exerted on the front wheels. As a general rule: the more caster the more re-centering force and the more sluggish the feel and response of the car. The average road driver wants, and needs because of virtually nonexistent driving skills, a car that "goes straight", caster does this. Caster does help a lot in keeping a car from being "darty" under braking (where toe can make this problem worse.)
Toe-in also adds to the "goes straight" tendency of the car. The front tires are constantly trying to cross the car's centerline and these opposing forces assist in the car tracking straight despite bumps, holes, and road crown. Negative camber also does the same thing in a straight line. Many cars come with some positive camber in the front, which promotes understeer, which scares people, which slows them down helps keep the car builder from getting sued. Positive camber does not promote good handling, unless you have a beam axle kingpin front end and a solid axle rear with no shocks at all.
Caster, Camber and Toe are a team used to create stability, turn response, and feel in the front end of a car.
Bump steer is basically a toe change dynamic. A certain amount of toe change can be caused by caster and camber changes caused by the loads put into the front suspension in corners. The movement of the upright in the caster and camber arcs can cause changes in the effective length of the steering links. Bump steer caused by bumps is a toe thing. Bump steer is caused by changes in the effective length of the steering links as they move up and down through their arcs. Toe is by far the simplest of the three to understand, however bumpsteer problems are often some of the most difficult to fix. As with everything in the front end, packaging is a major consideration when trying to solve bump problems. We frequently use bump steer to help a car into and/or out of the corner. Bump steer and Ackermann are tied very closely together as are spindle angle and caster, in how they affect the toe, camber dynamics.
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