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Carroll Smith's Problem and Cause Guide Print E-mail
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, doesn’t 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 won’t 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.
 
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