|
Carroll Smith's Problem and Cause Guide |
|
|
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)
RESPONSE
Car feels generally too heavy and unresponsive
Tyre pressures too low
Insufficient ride and/or roll resistance (springs and bars)
Excessive aerodynamic download, or insufficient spring for the amount of download
If high speed acceleration is sluggish, the culprit is often too large a rear wing Gurney lip
Car feels sloppy, is slow to take a set in corners, rolls a lot, doesnt want to change direction
Insufficient tyre pressure
Insufficient damper forces
Car too soft in ride and/or roll
Car responds too quickly has little feel slides at the slightest provocation
Excessive tyre pressure
Excessive bump force in shock absorbers
Car too stiff for inexperienced driver
Excessive ride or roll resistance
Excessive front or rear toe-in
Insufficient aerodynamic download
Understeer
Corner entry understeer: car initially points in and then washes out
Excessive toe-in or toe-out (car is usually darty)
Insufficient front droop travel (non droop limited cars only)
Incorrectly adjusted packers (car rolls on to packers)
Insufficient front damper bump resistance (similar to roll stiffness example)
Insufficient front roll stiffness car may feel like it is pointing
in but may actually be falling over onto the outside front tyre due to
insufficient front roll stiffness or diagonal load transfer under heavy
trail braking. Initial understeer can often be cured by increasing
front roll resistance, even though doing so may increase the amount of
lateral load transfer.
Non linear lateral load transfer due to spring and/or bar geometry. Or to non-optimal roll axis inclination
Corner entry understeer: car wont point in and gets progressively worse
Driver braking too hard, too late
Relatively narrow front track width
Excessive front tyre pressure
Excessive front roll stiffness (spring or bar)
Relative lack of front download (excessive rear download)
Incorrectly adjusted packers or bump rubbers (car rolls onto packers)
Insufficient front toe-in
Insufficient Ackermann effect in steering geometry
Front roll centre too high or too low
Insufficient front damper bump force
Insufficient front toe-out
Insufficient front wheel droop travel (on non droop limited cars only)
Nose being sucked down due to ground effect
Excessive Ackermann steering geometry
Can also be caused by unloading the front tyres due to rearward load transfer under acceleration cures include:
Increasing front damper rebound force
Increasing rear damper low speed damper rebound force
Increasing rear anti-squat
Droop limiting front suspension (will also make turn in more positive and will reduce overall understeer)
Mid-corner (mid-phase) understeer
Excessive front tyre pressure
Excessive relative front roll stiffness
Excessive front toe (in or out)
Excessive Ackermann steering geometry
Insufficient front dynamic camber
Relatively narrow front track width
Insufficient front wheel travel (car rolls onto packers or bottomed shock)
Insufficient droop travel (on non droop limited cars)
Corner exit understeer: slow corners
Often a function of excessive corner entry and mid-phase understeer
(whether driver induced or car induced) followed by throttle
application whilst maintaining the understeer steering lock. The first
step must be to cure the corner entry and mid-phase understeer. If this
is impractical, then corner entry speed should be reduced slightly in
order to allow earlier throttle application. Sometimes we have to be
patient.
Corner exit understeer: fast corners
Relative lack of front download often caused by negative pitch
angle (squat) due to rearward load transfer on acceleration. Can be
helped by increasing rear anti-squat and/or by increasing rear low
speed bump force, increasing front droop force and by limiting the
front suspension droop travel.
Relatively narrow front track width
Excessive ramp angle or pre-load on clutch pack or plate type limited slip differentials.
Understeer stronger in one direction than in the other
Uneven corner weights
Uneven caster
Uneven camber (especially front)
OVERSTEER
Corner Entry Oversteer
Excessively heavy trail braking
Excessive rearward brake bias
Severe rearward ride rate/roll resistance imbalance
Rear roll centre too high
Diabolical lack of rear download
Severely limited rear droop travel
Broken or non-functioning outside rear damper
Broken or non-functioning front anti-roll bar
Note: A slight feeling of rear tiptoe type hunting on corner entry
can be due to excessive rear toe-in or excessive rear damper rebound
force.
Mid-corner (mid-phase) oversteer
Driver threw the car at the corner to get through initial understeer
only cure is to educate the driver and/or decrease understeer
Excessive rear tyre pressure
Excessive relative rear ride and/or roll stiffness
Rear suspension bottoming in roll
Insufficient rear droop travel (non droop limited cars only)
Very loose rear anti-roll bar linkage
Corner exit oversteer: gets progressively worse from the time the power is applied
Worn out limited slip differential
Excessive anti-squat geometry
Excessive rear ride and/or roll stiffness
Insufficient rear spring, bar or shock (low piston speed bump force) allowing the car to fall over onto outside rear tyre
Excessive rear negative camber
Too little dynamic rear toe-in
Relatively insufficient rear download
Note: If car feels as though it is sliding through the corner rather
than rolling freely, reduce the rear toe-in and see what happens.
Corner exit oversteer sudden car seems to take a normal exit set and then breaks loose
Insufficient rear suspension travel (lifting the inside wheel on non
droop limited cars or bottoming the outside suspension due to lack of
bump travel)
Incorrectly adjusted packers
Dead rear damper
Sudden change in outside rear tyre camber
Too much throttle applied too soon often after the drivers confidence has been boosted by the car taking a set.
Car does not put the power down smoothly on the exit of smooth corners
Worn out limited slip differential
Excessive rear ride/roll resistance
Excessive anti-squat geometry
Excessive rear tyre pressure
Tyres gone
Excessive rear damper low piston speed bump force
Excessive rear dynamic camber either from download or from camber change on squat
Relative lack of rear download
Car does not put the power down on the exit of bumpy corners
Any or all of the above for smooth corners
Excessive rear damper high piston speed force
Excessive rear damper rebound force (jacking down)
Insufficient rear droop travel
TRANSITIONS
Understeer in, snap to oversteer on power application
The most common complaint of all ! Usually caused by too little roll resistance car falls over on entry and then snaps.
Increase front bar and/or spring and/or front damper low piston speed
bump force. Stiffening the bar will also transfer some load on to the
inside rear tyre on acceleration.
If the suggestion above cures the understeer but the car still snaps,
the culprit is almost always the car falling over on the outside rear
tyre on longitudinal plus lateral load transfer. Add rear bar or
spring. Bar will transfer load away from the inside rear tyre. Spring
will not. Spring will, however, decrease traction over exit bumps while
bar will not.
Loose anti-roll bar linkage/blade sockets can have the exactly same effect
Car is slow to change directions in chicanes or esses
Insufficient ride/roll stiffness, especially at front.
Relatively narrow front track width.
Insufficient front damper low piston speed bump force.
BRAKES
Brake pedal gets soft, spongy and/or long during session or race
Fluid boiling in calipers. Not pad fade ! Upgrade fluid and/or cool calipers.
Brake pedal is soft, spongy and/or long before the car is run
Air in the system bleed brakes.
Brake pads badly taper worn replace
Reduced stopping power with normal brake pedal
Pad fade due either to unbedded new pads or to temperature beyond pad capacity. Upgrade pads.
Long pedal with little effort required
Master cylinder(s) too small or pedal mechanical advantage too great.
Rough braking pedal vibrates under pressure
Organic pickup on discs clean discs with garnet paper (not aluminium oxide sandpaper) and upgrade pads
Warped (not grooved) rotors. Grind (or, if you must, turn) rotor surfaces
Insufficient axial float on floating discs
Uneven braking car pulls to one side
Stuck piston(s) rebuild calipers
Brake bias changes during application
Excessive clearance between master cylinder push rod clevises and bias bar bearing housing.
Rod end bearings used instead of clevises on master cylinder push rods.
Bias bar incorrectly adjusted. Bar must be perpendicular to vehicle
longitudinal axis with full foot pressure applied. Contrary to popular
opinion, relative length of master cylinder pushrods is immaterial.
|
Home 
