Geometry & Instability

[jam_rs]

Geometry & Instability


SUSPENSION GEOMETRY

Excessive front scrub radius (steering offset)
■ Excessive steering effort accompanied by imprecise and inconsistent "feel" and feedback

Excessive roll centre lateral envelope: front or rear
■ Non-linear response and feel to steering input and lateral "G" (side force) generation

Rear roll centre too low (or front r/c relatively too high)
■ Roll axis too far out of parallel with mass centroid axis, leading to non-linear generation of lateral load transfer and chassis roll as well as the generation of excessive front jacking force.
■ Tendency will be towards understeer

Rear roll centre too high (or front r/c relatively too low)
■ Opposite of above, tending towards excessive jacking at the rear and oversteer

Front track width too narrow relative to rear
■ Car tends to "trip over its front feet" during slow and medium speed corner entry, evidenced by lots of understeer (remember trying to turn your tricycle?)
- Crutch is to increase front ride rate and roll resistance and increase the camber curves in the direction of more negative camber in bump (usually by raising the front roll centre)

INSTABILITY

Straight line instability: general
■ Rear wheel toe-out, either static due to incorrect (or backwards) setting, or dynamic due to bump steer or deflection steer
■ Vast lack of rear download or overwhelming preponderance of front download
■ Wild amount of front toe-in or toe-out
■ Loose or broken chassis, suspension member or suspension link mounting point
■ Dead shock absorber

Straight line instability: under hard acceleration
■ Malfunctioning limited slip differential
■ Insufficient rear toe-in
■ Deflection steer from rear chassis/suspension member or mounting point
■ Rear tyre stagger (car pulls to one side)
■ Dead rear shock absorber
■ Wildly uneven corner weights

Straight line instability: car darts over bumps (especially one wheel bumps)
■ Excessive Ackermann steering geometry
■ Excessive front toe-in or toe-out
■ Uneven front caster or trail settings
■ Insufficient rear wheel droop travel
■ Dead shock or uneven shock forces or incorrectly adjusted packers/bump rubbers
■ Wildly uneven corner weights
■ Front anti-roll bar miles too stiff

Instability under hard braking: front end wanders
■ Excessive front brake bias or uneven corner weights or excessive front damper rebound force

Instability under hard braking: car wants to spin
■ Excessive rear brake bias
■ Insufficient rear droop travel
■ Wildly uneven corner weights
■ Excessive rear damper rebound force
■ Unbalanced ride/roll resistance - too much at rear
■ Insufficient rear camber (usually in combination with one or more of the above)