Vehicle Dynamics - Cause and Effect Guide

From Trackpedia

This information can help drivers and mechanics to accurately link the behavior or "feel" of a race car to specific components of the car's suspension and braking systems, or to the driver themselves.

Furthermore, it can be used to confirm that changes to the car being considered will have not only the desired effect, but will not create unwanted side-effects.

RIDE AND ROLL RESISTANCE

SPRINGS

Too much spring: overall

Harsh and choppy ride
Much unprovoked sliding
Car will not put power down on corner exit – excessive wheel-spin

Relatively too much spring: front

Understeer – although the car may initially point in well
Front breaks loose over bumps in corners
Front tyres lock while braking over bumps

Relatively too much spring: rear

Oversteer immediately on application of power
Excessive wheel-spin

Too little spring: overall

Car contacts the track a lot
Floating ride with excess vertical chassis movement, pitch and roll
Sloppy and inconsistent response
Car slow to take a set – may take more than one

Relatively too little spring: rear

Excessive squat on acceleration accompanied by excessive rear negative camber, leading to oversteer and poor power down characteristics
Tendency to fall over on outside rear tyre and ‘flop’ into oversteer and wheel-spin

ANTI-ROLL BARS

Too much anti-roll bar: overall

Car will be very sudden in response and will have little feel
Car will tend to slide or skate rather than taking a set – especially in slow and medium speed corners
Car may dart over one wheel or diagonal bumps

Relatively too much anti-roll bar: front

Corner entry understeer which usually becomes progressively worse as the driver tries to tighten the corner radius.

Relatively too much anti-roll bar: rear

If the imbalance is extreme can cause corner entry oversteer
Corner exit oversteer. Car won’t put down power but goes directly to oversteer due to inside wheel-spin
Excessive sliding on corner exit
Car has a violent reaction to major bumps and may be upset by ‘FIA’ kerbs

Too little anti-roll bar: overall

Car is lazy in response, generally sloppy
Car is reluctant to change direction in chicane and esses

Relatively too little anti-roll bar: front

Car ‘falls over’ onto outside tyre on corner entry and then washes out into understeer
Car is lazy in direction changes

Relatively too little anti-roll: rear

My own opinion is that on most road courses a rear anti-roll bar is a bad thing. Anti-roll bars transfer lateral load from the unladen tyre to the laden tyre – exactly what we don’t want at the rear. I would much rather use enough spring to support the rear of the car. The exception comes when there are ‘washboard ripples’ at corner exits, as on street circuits and poorly paved road circuits.

SHOCK ABSORBER FORCES

Too much shock: overall

A very sudden car with harsh ride qualities, much sliding and wheel patter
Car will not absorb road surface irregularities but crashes over them

Too much rebound force

Wheels do not return quickly to road surface after displacement. Inside wheel in a corner may be pulled off the road by the damper while still loaded
Car may ‘jack down’ over bumps or in long corners causing a loss of tyre compliance. Car does not power down well at exit of corners when road surface is not extremely smooth

Too much bump force: general

Harsh reaction to road surface irregularities.
Car slides rather than sticking
Car doesn’t put power down well - driving wheels hop.

Too much low piston speed bump force

Car’s reaction to steering input too sudden
Car’s reaction to lateral and longitudinal load transfer too harsh

Too much high piston speed bump force

Car’s reaction to minor road surface irregularities too harsh – tyres hop over ‘chatter bumps’ and ripples in braking areas and corner exits.

Too little shock: overall

Car floats a lot (the Cadillac ride syndrome) and oscillates after bumps
Car dives and squats a lot
Car rolls quickly in response to lateral acceleration and may tend to ‘fall over’ onto the outside front tyre during corner entry and outside rear tyre on corner exit.
Car is generally sloppy and unresponsive

Too little rebound force: overall

Car floats – oscillates after bumps (the Cadillac ride syndrome)

Too little bump force: overall

Initial turn in reaction soft and sloppy
Excessive and quick roll, dive and squat

Too little low piston speed bump force

Car is generally imprecise and sloppy in response to lateral (and, to a lesser extent longitudinal) accelerations and to driver steering inputs

Too little high piston speed bump force

Suspension may bottom over the largest bumps on the track resulting in momentary loss of tyre contact and excessive instantaneous loads on suspension and chassis

Dead shock on one corner

A dead shock is surprisingly difficult for a driver to identify and/or isolate
At the rear, that car will ‘fall over’ onto the outside tyre and oversteer in one direction only
At the front, the car will ‘fall over’ onto the outside tyre on corner entry and then understeer.

WHEEL ALIGNMENT

Front toe-in: too much

Car darts over bumps, under heavy braking and during corner entry – is generally unstable
Car won’t point into corners, or if extreme. May point in very quickly and then dart and wash out

Front toe-out: too much

Car wanders under heavy braking and may be somewhat unstable in a straight line, especially in response to single wheel or diagonal bumps and/or wind gusts
Car may point into corners and then refuse to take a set
If extreme will cause understeer tyre drag in long corners

Rear toe-in: too little

Power on oversteer during corner exit

Rear toe-in: too much

Rear feels light and unstable during corner entry. Car slides through corners rather than rolling freely

Rear toe-out: any

Power oversteer during corner exit and (maybe) in a straight line
Straight line instability

Front wheel caster or trail: too little

Car too sensitive (twitchy?)
Too little steering feel and feedback

Front wheel caster or trail: too much

Excessive physical steering effort accompanied by too much self return action and transmittal of road shocks to the drivers hands
General lack of sensitivity to steering input due to excessive force required

Front wheel caster or trail: uneven

Steering effort is harder in one direction than in the other
Car will ‘pull’ towards the side with less caster – good on ovals, bad on road courses

Camber: too much negative

Inside of tyre excessively hot and/or wearing too rapidly. At the front this will show up as reduced braking capability and at the rear as reduced acceleration capability. Depending on the racetrack and the characteristics of the individual tyre, inside temperature should be 10°-25° hotter than the outside. Use a real pyrometer with a needle rather than an infra red surface temperature device.

Camber: not enough negative

Outside of tyre will be hot and wearing. This should never be and is almost always caused by running static positive camber at the rear in an effort to avoid the generation of excessive negative camber under the influence of aero download at high speed.
A better solution is improved geometry and increased spring rate. Dynamic positive camber will always degrade rear tyre performance and if extreme, can cause braking instability and/or corner exit oversteer.

Bump steer, front: too much toe-in in bump

Car darts over bumps and understeers on corner entry

Bump steer, front: too much toe-out in bump

Car wanders under brakes and may dart over one wheel or diagonal bumps
Car may understeer after initial turn in

Bump steer, rear: too much toe-in in bump (same as solid axle steer on outside wheel)

Roll understeer on corner entry
Mid phase corner understeer
‘Tiptoe’ instability when trail braking
Darting on power application on corner exit

Bump steer, rear: too much toe-out in bump (same as solid axle steer on outside wheel)

Instability on acceleration
Good turn in followed by a tendency to oversteer at mid-phase and exit

TYRES

Too much tyre pressure

Harsh ride, excessive wheel patter, sliding and wheel-spin
High temperature reading and wear at the centre of the tyre

Too little tyre pressure

Soft and mushy response
Reduced footprint area and reduced traction
High temperatures with a dip in the centre of the tread

Front tyres ‘going off’

Gradually increasing understeer – Enter corners slower, get on power earlier with less steering lock

Rear tyres ‘going off’

Gradually increasing power on oversteer – Try to carry more speed through corner and be later and more gradual with power application

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)

HANDLING

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.

LIMITED SLIP MALADIES

Limited slip differential wearing out

Initial symptoms are decreased power on understeer or increased power on oversteer and inside wheel spin. The car might be easier to drive, but it will be slow
When wear becomes extreme, stability under hard acceleration from low speed will diminish and things will not be pleasant at all

Excessive cam or ramp angle on coast side plate (clutch pack) limited slip differential

Corner entry, mid-phase and corner exit understeer. Incurable with geometry changes or rates – must change differential ramps. In 1998, virtually everyone is running 0/0 or 80/80 ramps.