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Bound, rebound and more



 
#1

Shock absorbers



BRIEF HISTORY

In the early 1900's, cars still rode on carriage springs. After all, early drivers had bigger things to worry about than the quality of their ride - like keeping their cars rolling over the rocks and ruts that often passed for roads.
Pioneering vehicle manufacturers were faced early on with the challenges of enhancing driver control and passenger comfort. These early suspension designs found the front wheels attached to the axle using steering spindles and kingpins. This allowed the wheels to pivot while the axle remained stationary. Additionally, the up and down oscillation of the leaf spring was damped by device called a shock absorber.
shock absorber, history
Early shock absorbers

These first shock absorbers were simply two arms connected by a bolt with a friction disk between them. Resistance was adjusted by tightening or loosening the bolt.
As might be expected, the shocks were not very durable, and the performance left much to be desired. Over the years, shock absorbers have evolved into more sophisticated designs.


Despite what many people think, conventional shock absorbers do not support vehicle weight. Instead, the primary purpose of the shock absorber is to control spring and suspension movement. This is accomplished by turning the kinetic energy of suspension movement into thermal energy, or heat energy, to be dissipated through the hydraulic fluid.
You want more technical terms? Technically they are called dampers. Even more technically, they are velocity-sensitive hydraulic damping devices - in other words, the faster they move, the more resistance there is to that movement. They work in conjunction with the springs. The spring allows movement of the wheel to allow the energy in the road shock to be transformed into kinetic energy of the unsprung mass, whereupon it is dissipated by the damper and heat. The damper does this by forcing gas or oil through a constriction valve (a small hole). Adjustable shock absorbers allow you to change the size of this constriction, and thus control the rate of damping. The smaller the constriction, the stiffer the suspension. Phew!....and you thought they just leaked oil didn't you?
Shock absorbers are basically oil pumps. A piston is attached to the end of the piston rod and works against hydraulic fluid or gas in the pressure tube. As the suspension travels up and down, the hydraulic fluid is forced through tiny holes, called orifices, inside the piston. However, these orifices let only a small amount of fluid through the piston. This slows down the piston, which in turn slows down spring and suspension movement.
The amount of resistance a shock absorber develops depends on the speed of the suspension and the number and size of the orifices in the piston. Because of this feature, shock absorbers adjust to road conditions.
As a result, shock absorbers reduce the rate of:
- Bounce
- Roll or sway
- Brake dive
- Acceleration squat

Shock absorbers work on the principle of fluid displacement on both the compression and extension cycle. A typical car or light truck will have more resistance during its extension cycle then its compression cycle. The compression cycle controls the motion of a vehicle's unsprung weight, while extension controls the heavier sprung weight.

Shok absorber, compession cycle

Compression cycle or Bump

During bump, the dampers and springs absorb the upward movement from cornering or road irregularities (the springs store some of it). Acceleration, braking or cornering in this state with also vary due to the various download rates, so it is important to have enough bump stiffness to be able to deal with uneven surfaces.
If there is too much damping, then it is effectively like running no suspension and any upward motion will be transmitted directly to the chassis. Over damping will result in a increase in the loads acting on the suspension and the tires. The handling will feel very harsh and hard, this will effect street driving in terms of comfort levels, so might not be desired for a daily drive.
This is undesirable in both under and over damping settings as it will reduce the handling of the car and will affect acceleration, braking and cornering loads.

At the piston, oil flows through the oil ports, and at slow piston speeds, the first stage bleeds come into play and restrict the amount of oil flow. This allows a controlled flow of fluid from chamber B to chamber A.
At high speeds, the limit of the second stage discs phases into the third stage orifice restrictions. Compression control, then, is the force that results from a higher pressure present in chamber B, which acts on the bottom of the piston and the piston rod area.



Shok absorber, exstension cycle

Extension cycle or Rebound

During rebound (following the bump compression phase) the dampers extend back to their original positions, using up the stored energy from the springs. The rebound stiffness needs to be set at a higher value then the bump setting as the stored energy is being released. If there is not effect damping on the rebound, the wheel will quickly return through the static level and start to bump again, with the bouncing effect unsettling the suspension with little control.

If there is too much rebound stiffness, then the wheel could hold longer in the wheel arch then needed, effectively losing contact with the road as the force to push the wheel back down is slower to respond to the changing surface level. This state is again far from ideal and it is best to make sure a good level is set for optimal tire/tire contact with the road.

As the piston and rod move upward toward the top of the pressure tube, the volume of chamber A is reduced and thus is at a higher pressure than chamber B. Because of this higher pressure, fluid flows down through the piston's 3-stage extension valve into chamber B.
However, the piston rod volume has been withdrawn from chamber B greatly increasing its volume. Thus the volume of fluid from chamber A is insufficient to fill chamber B. The pressure in the reserve tube is now greater than that in chamber B, forcing the compression intake valve to unseat. Fluid then flows from the reserve tube into chamber B, keeping the pressure tube full.
Extension control is a force present as a result of the higher pressure in chamber A, acting on the topside of the piston area.

Shock piston

Piston is attached to the end of the piston rod and works against hydraulic fluid in the pressure tube. As the suspension travels up and down, the hydraulic fluid is forced through tiny holes, called orifices, inside the piston. On the picture left is modern design for use in road car dampers.

Damper
http://www.carbibles.com/

The image above shows a typical modern coil-over-oil unit for long time in use with sports cars and motorcycles. This is an all-in-one system that carries both the spring and the shock absorber. The adjustable spring plate can be used to make the springs stiffer and looser, whilst the adjustable damping valve can be used to adjust the rebound damping of the shock absorber. More sophisticated units have adjustable compression damping as well as a remote reservoir. Whilst you don't typically get this level of engineering on car suspension, most motorbikes do have preload, rebound and spring tension adjustment, and this adjustments are normal in racing.

Shock absorbers work in conjunction with springs and stabilizers. Dampers provide a resistance for the spring to work against. The purpose of this is to prevent the spring from oscillating too much after hitting a bump. Ideally, the spring would contract over a bump, then expand back to its usual length straight afterwards. This requires a damper to be present as without one the spring would contract and expand continually after the bump, providing a rather horrible ride!
Modern F1 and racing shock absorbers can be regulated for bound and rebound but only before race. Shock absorber does not absorb impacts, but damp the motion of the car and oscillations of the spring after traveling over bumps and dips. When weight transfers from back/front and side/side (roll), or when you go over a bump on the road, the wheels/tires compress (bound), and when you are past the bump the wheel returns to equilibrium after the compression (rebound). That is basically the suspension movement.


Shock absorber in parts

Shock absorber

Bound is the rate at which the shock compresses.
Rebound is the rate at which the shocks decompress.

Bound damping affects how far and fast the suspension travels up. When the suspension is on its way back down, rebound damping affects how far and fast it goes the other way. More precisely, bound damping affects the compression rate, while rebound damping affects the expansion rate.

If you make your bound damping too stiff, your car will be skittish over rough surfaces. Rebound damping also affects your steering as you transition into and out of corners.

In general, stiffer absorbers are better suited for flat tracks with sharp turns. They prevent your springs from coiling too quickly, which decreases the dip you have when cornering and braking. Softer adjusted absorbers are better suited for winding, coiling tracks, but they'll also lengthen your braking distance.

So having bound at (for example) value of 9 and rebound at value of 2, make the car stiffer when absorbing a bump, compression is harder. The suspension on rebound will not return as fast. This suppresses weight transfer. Not very good because the tire won't make contact with the ground fast enough causing slip, that induce oversteer.

On the other hand, bound at 2 and rebound at 9, absorbs more bumps, but returns the shocks the opposite way to fast. You'll find the car literally jump over small bumps. This is also undesirable, as the tire is not in contact with the road. Bound at 7 and rebound at 6, keeps the tires stiff and return to the ground slower. Having bound at 6 and rebound at 7, will result in a good stiff compression of shocks and a higher bound means the tires return a bit faster to the ground but not too fast. This is the ideal configuration, a slightly higher rebound.


LINK (cu mai multe detalii si cu sistemul de amortizoare din F1) : http://www.formula1-dictionary.net/dampe...orber.html



TUNING

The goal you are seeking is getting your car to react to the ground, so you must remember that suspension tuning is actually making your tire work harder and more efficiently. Realize that a very soft suspension can give the tire too much motion to do its job, and a very stiff suspension can give too little.

An example of working the tires in a different way is a test we did last year with one of the North American Touring Cars. The track was smooth, and the suspension was plenty firm. In successive tests and adjustments, we slowly raised the rebound until good balance was achieved, but then a hot lap produced a nasty hopping motion.

Although the pavement was smooth, Touring Cars have a tendency to use curbing and berms to their greatest advantage. After firmly popping a berm, the car launched slightly and then hopped on landing. We realized that the hopping motion wasn't from spring bounce (which would mean it needed more rebound), but was actually from the tire's sidewall flexing because the suspension was firm enough that the only compliance to dissipate the energy came from the tire. A softer tweak on the rebound let the suspension and tires do their own jobs, permitting the car to stay on the ground and the driver on the throttle.

The initial setup was good for smooth driving, but when the berm variable was introduced, an adjustment needed to be made. By the way, the driver, Randy Pobst, won the North American Touring Car championship on those shocks.

The rule of thumb says that greater rebound damping loosens that end of the car, so a front-drive car that won't turn in can use some more rear rebound. Couple that with enough front rebound to slow body roll, but not so much as to cause inside wheel lift, and you are on your way.

A tail-happy rear driver could probably use more front rebound (to loosen the front) and less rear rebound (to reduce rotation) in the pursuit of balance.

Your other thumb tells you that if you can isolate handling responses to corner entry and corner exit, then you know which end to work on. In a decelerating corner entry situation, the rear suspension is extending and transferring its load to the front, so adjusting the rear rebound can control the transfer rate. On accelerating at the corner exit, the front is extending as the weight is transferred to the rear (usually more subtle unless you have big power or soft springs), so the front rebound will be adjusted.

Increasing compression damping will also affect how quickly the other end of the car accepts that weight transfer. Too little compression can overwork or literally stun the contact patch, while too much can give too little input and also start acting like added spring rate.

If you are allowed to change springs, do so and let them do their job and share the work. If your rules mandate that you can't change springs, consider more compression, but remember the other compromises involved. Ride quality and skittishness on intended and unintended bumps must be factored in.

Manufacturers can alter the different valving tools in the adjustment procedure to get their desired effect. Some use bleed holes in the rod to make the changes and therefore vary the amount of oil missing the piston valves. The clue for this style is if it adjusts both compression and rebound in one motion. Other manufacturers (usually more racing oriented) will adjust valving independently, either by making only rebound adjustable and using an optimized, preset compression for many situations, or with a double-adjustable unit that allows independent adjustments. This style usually effects changes with rod bleed and orifice and valve stack spring preload pressure, and therefore can make changes over the more possible piston speeds.

The days of the old 50/50 (same rebound damping as compression damping) and 90/10 drag race shocks have gone by. Today a 50/50 shock would have either way too much compression or, more likely, too little rebound. A 90/10 design just isn't paying attention to the evolution of suspension design and aerodynamics.

Today, street performance shocks have rebound damping rates that are two or more times greater than compression damping rates. The single action of adjusting bleed to affect bump and rebound is, by definition, a 50/50-style change, so the overall damping proportion will change as more bleed is dialed in. Independent adjustments allow the alteration of one characteristic while not affecting the other; this is therefore more precision and involves less compromise.

Rebound and sprung weight adjustments will cover 90-plus percent of most autocross and grassroots racers' needs. Making compression adjustments of the unsprung weight has traditionally been the realm of more hard-core race tuners, but as the stakes in the pro and national levels of autocross and club racing go up, so does the need for more tweaking and tuning ability.

As you can see (and probably know from firsthand experience), simply jumping into a car and counting on your heroic driving abilities to carry you to the front is the stuff of daydreams. Proper research and use of your suspension system is a safer spot to place your bets. Some of the most pivotal yet much misunderstood parts of your suspension package are the dampers.

If your goal is a favorite road or competition class, maximizing your dampers' capabilities will take you far and fast. Autocross is a great example-it is vehicle transitional control at the limit. A nationally-recognized autocrosser recently confirmed this by stating that suspension control is everything, and handling gains get you seconds whereas horsepower gains usually just get you to the next corner. Road or oval track racing is not as extreme in transition, but the vehicle speeds are higher and the necessity for control at the limit makes damper understanding critical.

Your car manufacturer probably didn't have you in mind when they chose the original dampers, so it is up to you to select and tune the best performance set for your unique needs.
[Image: grm_shock2.gif]
[Image: grm_shock3.gif]

mai multe info la http://www.se-r.net/suspension/shock_tech.html

alt link cu info multiple despre suspensie: http://www.trackpedia.com/wiki/Suspension


Springs

Run as soft as practical for maximum grip

Hard:
Reduced Grip
Increased reaction
Soft:
Increased Grip
Decreased reaction
Soft Front, Hard rear = Oversteer
Hard Front, Soft rear = Understeer

Anti-Roll

Primary effect in slow turns

Front
Hard = Steering precision in slow turns, understeer, bad turn-in, more precise handling
Soft = Grip in slow turns, oversteer, better turn-in, less precise handling
Rear
Hard = Reduce Understeer in slow turns, oversteer, better turn-in
Soft = Reduce Oversteer in slow turns, understeer, worse turn-in
Large effect on relative L/R tire temp, Soft front/hard rear may balance

Run zero or low in wet conditions
Harder increases tire ware
Dampers (Shocks/Anti-shock)

Dampers work in speed rates (Fast (low numbers) and Slow (high numbers)) not Soft or Hard

Slow Dampers
Slow F+R = Slow weight transfer, Corner stability
Fast F+R = Fast weight transfer, Good grip
Slow F, Soft R = Corner entry+exit oversteer
Fast F, Hard R = Corner entry+exit understeer
Fast Dampers
Fast Rebound will normally need to be faster than Fast bound.
Slow F+R = Reduced bounce over bumps and curbs
Fast F+R = Good grip over bumps and curbs
Slow F, Soft R = Bump understeer
Fast F, Hard R = Bump oversteer

Bound/Rebound (Mainly for Slow dampers (weight shift)):

Front
Bound
Slow = Slower weight shift under Brake, Reduce oversteer on Turn-in
Fast = Faster weight shift under Brake, Better Turn-in response + Reduce understeer
Rebound
Slow = Slower Nose Lift under Accel
Fast = Faster Nose Lift under Accel
Rear
Bound
Slow = Slower weight shift under Accel
Fast = Faster weight shift under Accel
Rebound
Slow = Slower Nose Drop under Brake, Better Turn-in response + Reduce understeer
Fast = Faster Nose Drop under Brake, Reduce oversteer on Turn-in

Slow dampers
. Left turn Oversteer Right turn Oversteer
Corner Entry Increase FR Bump, Decrease RL Rebound Increase FL Bump, Decrease RR Rebound
Corner Exit Increase FL Rebound, Decrease RL Bump Increase FR Rebound, Decrease RR Bump
. Left turn Understeer Right turn Understeer
Corner Entry Decrease FR Bump, Increase RL Rebound Decrease FL Bump, Increase RR Rebound
Corner Exit Decrease FL Rebound, Increase RR Bump Decrease FR Rebound, Increase RL Bump
Differential

Power
High = Good propulsion out of corners, power understeer, Snap oversteer with overpower
Low = Poor propulsion out of corners, power oversteer, easier but more frequent traction loss with overpower
Coast
High = Stable braking, lift off understeer
Low = Unstable braking, lift off oversteer
Pre-load (low throttle range)
High = Nervous transitioning from braking/acceleration, easier to balance turns with throttle, harder to keep stable accelerating from slow turns, easy to spin under power, easier to break traction with rapid power lift-off
Low = Less responsive transitioning from braking/acceleration, easier to accelerate from slow turns but tendency to understeer

http://carsetup.wikidot.com/



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#2

un articol bun de la australieni despre FWD suspension improvement:

http://www.autospeed.com/cms/article.html?&A=1273



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#3



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