Axle PathKingpin Inclination Angle makes the axle travel in an upside down U-shape when th
Increasing positive caster projects the Dave Point (the point where the steering axis meets the ground) further in front of the tire's contact patch. This distance is called caster trail. When the tire's contact patch is behind the Dave Point, the tires want to stay centered behind the Dave Point the same way a shopping cart's casters naturally align its wheels in the direction of travel.
Like the shopping cart caster, the distance between the Dave Point and the tire's contact patch creates a torque reaction, which causes the steering to self-align. The driver perceives this reaction as greater stability and on-center steering feel. More positive caster means a bigger torque reaction as well as increased stability and feel.
Unfortunately, this virtual lever arm also increases torque steer on front-wheel-drive and all-wheel-drive cars because the force is reversed when the wheel is driven. This is why most front- and all-wheel-drive cars don't have as much caster as rear-drive cars.
Lots of positive caster causes the outside wheel to gain camber in a turn when you need it most. Think of a parked chopper with the wheel flopped to the side. That's an extreme example of negative camber gain with positive caster. Too much positive caster can increase tire loading and understeer.
KIA increases stability by making the axle path travel in an upside down, U-shaped arc (when viewed from the side) as the steering wheel is turned. The axle is at the apex of the arc when the steering wheel is centered (see graphic, pg. 154). As the wheels are turned, they actually lift the front of the car. This lifting effect increases effort the more the wheel is turned, which contributes to steering feel and straight-line stability. KIA also tilts the wheels outward in a turn, which reduces camber.
Positive caster and KIA are both huge considerations for design engineers. Balancing positive caster's ability to increase camber in a turn with KIA's ability to decrease it is critical to achieving the right combination of stability and steering feel.
Caster can be adjusted with tension rods, adjustable arms and universal (racing) camber plates like those from Ground Control. These parts are available for almost any car that's been raced.
Here are some basic guidelines for adjusting caster:
Positive Caster
Just Right
Improves straight-line
stabilitySharpens turn-in Improves traction everywhere in the turn
Too Much
Very high steering effortProvides sharp turn-in but increases understeer from midturn onward Increases torque steer in front- and all-wheel-drive cars Caster adjustment guidelines
Caster Adjustment Guidelines
Degrees Positive
FWD/AWD: 3-4RWD: 4-10
In the next installment, we'll continue our discussion on suspension geometry, and begin an in-depth look at dampers.
In the next installment, we'll continue our discussion on suspension geometry, and begin an in-depth look at dampers.
Other Installments:
Making It Stick Part 1: Four basic steps to better handling
Making It Stick Part 2: Four more steps to better handling
Making It Stick Part 3: It's all in the geometry
Making It Stick Part 4: More lessons in suspension geometry
Making It Stick Part 5: Damper fundamentals
Making It Stick Part 6: More advanced dampers
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Ground Control
530-677-8600
www.ground-control-store.com
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By Mike Kojima
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