Suspension Tech

[Kishin888]

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not sure how many people know how to tune their suspension, but I hope this is helpful


With toe-in (left) a deflection of the suspension does not cause the wheels to initiate a turn as with toe-out (right).



Caster



(TOP LEFT) Positive camber: The bottoms of the wheels are closer together than the tops. (TOP RIGHT) Negative camber: The tops of the wheels are closer together than the bottoms. (CENTER) When a suspension does not gain camber during deflection, this causes a severe positive camber condition when the car leans during cornering. This can cause funky handling. (BOTTOM) Fight the funk: A suspension that gains camber during deflection will compensate for body roll. Tuning dynamic camber angles is one of the black arts of suspension tuning.

Front spring rate increase:
More under steer; increase in proportional weight transfer to the front when rear wheel rate is not increased; reduces front traction when rear rate is not changed.
Usable adjustment: 150-600 lbs/in
Symptoms of too much adjustment: terminal under steer; front of car hops in corners; excessive wheel spin on inside front tire on FF cars.

Front spring rate decrease:
Less under steer; decreases proportional weight transfer to the front when rear wheel rate is not increased; increases front traction when rear rate is not changed.
Usable adjustment: 150-600 lbs/in
Symptoms of to much adjustment: Too much over steer; over steer then under steer if spring is so soft that the car bottoms out on lean, car bottoms out excessively with a jolting ride.

Rear spring rate increase:
More over steer; increase in proportional weight transfer to the rear when front wheel rate is not increased; increases rear traction when front rate is not changed.
Usable range: 100-600 lbs/in
Symptoms of too much adjustment: too much over steer; sidestep hop in corners; twitchy; pretty scary.

Rear spring rate decrease:
Less over steer: decreases proportional weight transfer to the rear when front wheel rate is not changed; increases rear traction when front rate is not changed
Usable range: 100-600 lbs/in
Symptoms of too much adjustment: car under steers; if way to soft car under steers then over steers as car bottoms out on lean; car bottoms out excessively with a jolting ride.

[Kishin888]

Front anti-roll bar stiffer: more under steer
Usable range: none to 1.25 inches in diameter
Symptoms of to much adjustment: terminal under steer; lifts inside front tire off the ground witch can cause massive wheel spin on FF cars; also not good for most effective tire usage as inside tire is now doing nothing.

Front anti-roll bar softer: less under steer
Usable range: none to 1.25 inches in diameter
Symptoms of to much adjustment: overstate scary; more like fun

Rear anti-roll bar stiffer: more over steer
Usable range: none to 1 inch in diameter
Symptoms of too much adjustment: Big-time over steer. Can cause inside rear tire to lift off the ground.

Rear anti-roll bar softer: less over steer
Usable range: none to 1 inch in diameter
Symptoms of to much adjustment: under steer; slow and boring

Front tire pressure higher: less under steer by reducing slip angels on most tires
Usable adjustment: up to 55psi hot
Symptoms of too much adjustment: no traction- tire crowned so more under steer; adds wheel spin in FF cars; jarring ride; center of tire wears out

Front tire pressure lower: more under steer by increasing slip angles on most tires
Usable adjustment: not less then 20psi
Symptoms of too much adjustment: edges of tire wear quickly because tire is folding over; feels mushy; tires chunk because low pressure means heat build up.

Rear tire pressure higher: less over steer by reducing slip angles on most tires
Usable range: up to 45psi hot
Symptoms of too much adjustment: no traction—tire is crowned so more over steer; bad wheel spin on FR cars; jarring ride; center of tire wears out.

Rear tire pressure lower: more over steer by incresing slip angles on most tires.
Usable range: not less then 20psi
Symptoms of too much adjustment: edges of tire wear quickly because tire is folding over; feels mushy; tires chunk because low pressure means heat build up

More negative camber front: less under steer because of better lateral traction as tread is flatter on the ground under side load.
Usable range: up to 3.5 degrees negative
Symptoms of too much adjustment: poor braking; car is road crown sensitive; twitchy; front tires wear on inside edge

More negative camber rear: less over steer because of better lateral traction as tread is flatter on the ground under side load. More rear grip
Usable range: up to 2.5 degrees negative
Symptoms of too much adjustment: more over steer; car feels twitchy in back; tires wear out on inside edge; less breakaway warning when limit is exceeded.

Ride height to low (typical beginner mistake): car is twitchy with unpredictable dynamics. Bump steer make you life miserable.
Usable range: usually 1.5-2.0 inches lower then stock unless car has been modified to go lower.
Symptoms of too much adjustment: everything that could possibly go wrong: sudden over/under steer; twitchy due to bump steer; very harsh ride; premature tire wear.

Toe in – front: car is stable going straight. Turn in is average
Usable range: 0-1/8th inch
Symptoms of too much adjustment: car has slow twitchiness under braking; feels odd; kills outside edge of tires

Toe out – front: Car turns in well; works pretty well on FF car as they tend to toe-in under load.
Usable range: 0-1/4 inch
Symptoms of too much adjustment: Car is really twitchy under braking; car wanders on straight road; kills inside edge of tire

Toe in – rear: car is less likely to over steer when the throttle is lifted
Usable range: 0-1/8th inch
Symptoms of too much adjustment: weird, slow, rocking movement in back; feels slow but still unstable; wears outside edge of tires.

Toe out – rear: Helps car rotate useful in low speed and slalom courses; very common on FF pro rally cars.
Usable range: 0-1/8th inch
Symptoms of too much adjustment: not to good for street driving; causes lift throttle over steer; makes violent side to side rocking motions in the rear; tie wears on inside more.

[Kishin888]

Positive front caster: helps stability; suspension will get more negative camber when turning; reducing positive caster reduces steering effort. (Negative caster is not usable)
Usable range: 4-9 degrees positive
Symptoms of too much adjustment: can increase under steer especially in cars with wide low-profile tires. Can increase steering effort.

Single adjustable shock stiffer: Better turn in; better transient response; causes slower onset of over/under steer by slowing weight transfer depending on what end of the car is adjusted.
Symptoms of too much adjustment: suspension becomes unresponsive; ride gets harsh; car skips over bumps, loosing traction; Causes a big delay in weight transfer resulting in strange handling like under steer then late corner stage over steer.

Single adjustable shock softer: slower transient response; quicker onset of over/under steer
Symptoms of too much adjustment: car oscillates due to under dampened spring motion, like a boat. Car gets twitchy in turns. Feels unstable.

Spring Rate Changes (def. important for those who dont pay att. to this)
Modification - Effect on Suspension

Increase front and rear rate - Ride harshness increases; tires may not follow bumps causing reduced traction. Roll resistance increases.

Increase front rate only - Front ride rate increases. Front roll resistance increases, increasing understeer or reducing oversteer.

Increase rear rate only - Rear ride rate increases. Rear roll resistance increases, increasing oversteer or reducing understeer.

Decrease front and rear rate - Ride harshness decreases; tires follow bumps more effectively, possibly improving traction. Roll resistance decreases.

Decrease front rate only - Front ride rate decreases. Front roll resistance decreases, decreasing understeer or increasing oversteer.

Decrease rear rate only - Rear ride rate decreases. Rear roll resistance decreases, decreasing oversteer or increasing understeer.

[Kishin888]

Antiroll Bar Changes (aka sway bar)
Modification - Effect on Suspension

Increase front rate - Front roll resistance increases, increasing understeer or decreasing oversteer. May also reduce camber change, allowing better tire contact patch compliance with the road surface, reducing understeer.

Increase rear rate - Rear roll resistance increases, increasing oversteer or decreasing understeer. On independent rear suspensions, may also reduce camber change, allowing better contact patch compliance with road surface, reducing oversteer.

Decrease front rate - Front roll resistance decreases, decreasing understeer or increasing oversteer. More body roll could reduce tire contact patch area, causing understeer.

Decrease rear rate - Rear roll resistance decreases, decreasing oversteer or increasing understeer. On independent rear suspensions, more body roll could reduce tire contact patch area, causing oversteer.



Shock Absorber Changes (aka your struts)
Modification - Effect on Suspension

Increase rebound and bump rates - Ride harshness increases.

Increase rebound rates only - On bumps, tires may leave track surface.

Increase bump rates only - Body roll resisted; outside tire loaded too quickly; car won't stabilize into a turn.

Decrease rebound and bump rates - Ride harshness decreases; car may float over bumps.

Decrease rebound rates only - On bumps, tires follow track surface more effectively; car may continue to oscillate after bumps.

Decrease bump rates only - Body rolls quickly; car is slower to respond to turn-in.



Troubleshooting Tire Temperatures
Reading - Handling problem - Reason

All tires too hot - * - Compound too soft for track and ambient temperature conditions.

Front tires too hot - Understeer - Front tire pressures too low.

Rear tires too hot - Oversteer - Rear tire pressures too low.

Inside edges too hot - Too much body roll - Too much negative camber or too much toe-out.

Outside edges too hot Too - much body roll - Too little negative camber, too little toe-out or too much toe-in or wheel width too narrow for tire width.

Center of tread too hot - * -Tire pressure too high.

Edges on too hot - * -Tire pressure too low.

All tires too cold - * - Compound too hard for track and ambient temperature conditions or car not being driven to limit.

Front tires too cold - * - Inadequate load on front tires.

Rear tires too cold - * - Inadequate load on rear tires



Solving Handling Problems
Problem - Manifestation *Solutions

Steady state understeer - All turns or low-speed turns only
*If front tire temps are optimum and rears are low, stiffen rear antiroll bar; *if front temps are too hot, soften front (most likely).
*If front tire pressures are optimum, decrease rear tire pressure.
*Increase if chunking occurs.
*Improper front camber.
*Too much body roll at front, causing excessive camber change.

Steady state understeer - High-speed turns only
*If front tire temps are OK, increase front downforce.
*If front tire temps are too hot, reduce rear downforce.

Steady state oversteer - All turns or low-speed turns only
*If rear tire temps are optimum, with fronts too low, stiffen front antiroll bar;
*if rear temps are too hot, soften rear antiroll bar (most likely).
*If rear tire pressures are optimum, decrease front tire pressure. *Increase if chunking occurs.
*Improper rear camber.

Steady state oversteer - High-speed turns only
*If rear tire temps are OK, increase rear downforce.
*If rear tire temps are too hot, reduce front downforce.

Corner entry understeer
*Front shocks are too soft in bump resistance.
*Too much front toe-in; use a small amount of front toe-out.

Corner exit understeer
*Rear shocks are too soft in bump.
*Front shocks are too stiff in rebound.

Corner entry oversteer
*Rear shocks are too soft in rebound.
*Rear ride height is too high (too much rake) compared to front.

Corner exit oversteer
*Rear shocks are too soft in rebound.
*Too much rear toe-in or any rear toe-out.

Straightline instability
*Tire pressure is too low in one or more tires.
*Too little positive front caster.
*Too much front toe-in or any toe-out in rear.

Straightline speed too slow
*Too much overall downforce.
*Too much toe-in or toe-out.
*Ride height is too hight.

Excessive steering effort - All turns
*Too much positive caster.
*Front tire pressures are too low.

Chassis or suspension bottoms
*Spring rates are too soft.
*Shock absorber bump rates are too soft.
*Inadequate suspension travel.
*Inadequate ride height.