What you need to know about your suspension

[bimmerd00d]

This is a great writeup from Palentirion@BMS from the E39 board i frequent. I figured you guys might want to give it the old college try.

From time to time it's come to my attention that many BMW owners do not really know their suspension - that is to say, how components interact, how they affect a vehicle's handling, etc. This can lead to confusion when it comes to determining what to spend you hard-earned $$$ on, and can also lead to detrimental choices at the track. Several people have asked me recently what terms like "bump" and "rebound" actually mean, so I figured there are probably more that can benefit from a brief crash course in suspension terminology and cause-and-effect.

General terms
Axle: Whether a old truck solid axle or a modern independently sprung suspension, an "axle" is the centerline of a car's front or rear wheels and their related components.

Spring: A piece of flexible metal that holds up the car. They come in many varieties, but the only ones you'll come into contact with on a modern BMW are coil springs. They are a coiled round bar of steel, engineered to resist a specific amount of force, measured often in pounds of force needed to compress the spring for each inch of travel. IE: 600lb springs require 600lbs of force to compress 1", or 1200lbs to compress 2".

Coilover: A shock or strut that has a coil spring located cocentrically around it, most often with an adjustable spring perch used to change ride height.

Shock, dampener: The shocks we're concerned with are hydraulic. They use a liquid and valves to dampen the actions of the spring to which they are attached. Without dampening a spring that is compressed and released will expand and contract in a violent manner. The job of the shock is to control how fast the spring expands after compression is stopped, and to modify how quickly the spring compresses in the first place.

Strut: The same as a shock, but mounts in a way that it also provides longitudinal location and stiffness for part of the suspension, in lieu of control arms.

Anti-sway bar, anti-roll bar, sway bar, roll bar: These are all the same thing. They are a bar of tubular metal that connects the two ends of the front and/or rear axle. They are a torsion spring, that is to say they resist twisting motion. Because they are anchored at the left and right of the vehicle, and to the frame in between, they receive input from both sides. As one side compresses, whether by a bump in the road or by body roll, it twists the sway bar. The sway bar resists this twisting motion, and thereby reduces the effect of the bump or body roll. Because of the way they mount to the vehicle's frame, sway bars can rotate freely if both sides of their axle contact a bump at the same time. This means that you can have better ride comfort and the same degree of roll resistance by stiffening a sway bar than by simply stiffening the related coil springs.

Strut bar: This is a bar, solid or tubular, that connects the top of the shocks or struts in the front and/or rear of a car. It does not move. It is effectively a frame reinforcement, specifically reinforcing one of the weakest points in most cars. It stiffens the frame of the car, increasing its torsional rigidity. This is a very good thing! The frame of your car IS for all intents and purposes a giant, very stiff spring. When you hit a bump or turn the wheel the frame of your vehicle bends a little. When it bends, it stores energy. When the load goes away, that energy is released and will affect, ultimately, your tires' adhesion. The stiffer your car's frame, the less it will be deflected by outside forces, and the more accurately your car's suspension and tires will work.

Tires: The black things you drive on. These ARE springs, in addition to the rest of their complex properties. The important thing to remember is that they are undampened springs, controlled only by tire pressure. The higher the pressure, the higher the effective spring rate. High means more resistance to the tire deflecting during a lateral load (cornering), but also means less deflection to vertical loads (bumps).

Understeer, push: A condition where the vehicle wants to go straighter than your steering input.

Oversteer, loose: A condition where the vehicle wants to turn more than your steering input.


Adjustment points
Bump: This controls the stiffness of a shock relating to compression. Sounds just like a spring, and it pretty much is. The difference being that while a spring's stiffness always has the job of holding up your car, the shock's bump setting only acts on temporary forces likes bumps and roll. Adding more bump will act very much like stiffening the spring on the same corner.

Rebound: This controls how quickly the spring descends after compression. This is a VERY important setting when tracking a car. Without enough rebound a spring will hop and skip along the ground, making it hard to put down power, braking or cornering force. If you have too much rebound, the spring will not be allowed to expand quickly enough to return to its proper ride height before the next input. Too much of this will lead to "jacking down", which is a term for when a vehicle lowers itself until it is riding on it's bump stops (hard rubber, used as a last-ditch safety feature to keep tires from rubbing on very hard bumps). You can use rebound to control roll. By increasing rebound you decrease the velocity of the spring's expansion. If the spring cannot expand, it cannot push that side of the vehicle up. So the vehicle must either not roll as much, or lift its inside tire. Obviously you don't want to set so much rebound as to end up with the latter.

Sway bar settings: Increasing a sway bar's stiffness will reduce roll, but also increases the sprung tension on that axle. Granted, most of the increase in tension is apparent only when there is body roll - but even statically there is an increase. This means you must modify your rebound settings when you change your sway bar settings (just as you must whenever you add spring tension).

Camber: This is a tire's difference from vertical when looked at from head-on. + means the bottom of the tire is in more than the top. - means the bottom is out more than the top. Remember, this setting is only accurate when the car is NOT moving. As the wheel moves with the suspension, the camber will change depending on the geometry of the suspension. What is important is observing how the tire wears, and by taking tire temperature readings on the outside, center and inside to make sure that all of the tire is being used when it is at full load.

Castor: This is the difference between a wheel's centerline and the axis of suspension travel on that corner when viewed from the side. This is rarely adjusted on a street car. The more forward the wheel's centerline relative to the suspension's axis, the better the car will soak up bumps. And the more it will nosedive when you hit the brakes. The reverse is also true.

Toe: This is the difference between the direction a tire is pointing and straight ahead, when looking down from above. + means the tire points in. - means the tire points out. Positive toe makes a car more stable. This is useful at higher speeds and for general steering comfort. Negative toe makes a car more darty. This is useful to initiate quick directional changes, but can also reduce your total level of grip with excessive tire scrub (more on that later).


Options
Reduce understeer:
-Enter turns slower! Then power out.
-Reduce front tire pressure, and/or increase rear tire pressure.
-Reduce front spring tension, and/or increase rear spring tension. This can be done by swapping springs, or by changing sway bar settings/diameters.
-Reduce front bump, and/or increase rear bump.
-Reduce rear camber, and/or increase front camber.

Reduce oversteer:
-Do the opposite of the above "reduce understeer" options.

Reduce roll:
-Increase tire pressure all around.
-Increase bump settings.
-Increase rebound settings.
-Swap springs for stiffer ones.
-Increase sway bar settings/diameters.

Increasing traction for applying power:
-Reduce rear tire pressure.
-Reduce rear spring tension.
-Reduce rear bump.
-Increase rear rebound.
-Reduce rear sway bar settings/diameters.

Increase braking traction
-Reduce tire pressures.
-Increase front spring tension.
-Increase front bump.
-Reduce rear rebound.
-Reduce all sway bar settings/diameters.

[rob101]

CARROLL SMITH’S CAUSE AND EFFECT GUIDE

RIDE AND ROLL RESISTANCE-SPRING

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-our: 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

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.

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)

[rob101]

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, 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.