[Subscribe to Daily Digest] |
Dean,
Forgive me now, this is going to be rather long. It's going to be long because the answere is not simple, and I have not yet seen a comprehensive discussion of the purpose of the dampers on any of the Saab forums I visit yet, with the exception of some of the competition specific based forums.
I feel the need to do this because it took be many many other external resourced to learn about this outside this performance forum, and I want to share that back.
This will be a primarily performance based explanation, however the ride quality differences fall out of the discussion and can be analyzed from this perspective.
I am going to discuss the purpose of the dampers as a component of the suspensin system, not in a particularly Saab specific way, but it all applies. I drive an '85 900 Turbo with Koni one-way adjustables and B&G springs, and so do have some Saab specific knowledge (and so can be trusted!!!) <--- You'll have to decide for yourself on that one.
but in my time designing the suspension for our Uni's Formula SAE car I have come to understand the function of the dampers as follows:
The dampers serve two basic functions aside from their structural requirement in a strut layout.
1) They dissipate the energy induced by oscillation of the sprung mass of the vehicle when the vehicle encounters a road irregularity, smoothing the ride and keeping the occupants from bouncing around. This is where the 'shock absorber' term comes from. (In actuality the spring is the portion of the suspension system that acts as the 'shock absorber,' allowing the vehicles wheels to move over road irregularities, while the damper is merely a dissipator). The sprung mass is thus allowed to remain steady relative to road irregularities. Early 'shock absorbers' were friction based, and used on carriages to keep the occupants from getting so motion sick over rough terrain before paved roads came into wide existance. The purpose of the 'shocks' maintained that basic requirement until some time in the late '70s when gas charged hydraulic dampers came into more common use.
2) The dampers provide instantaneous and temporary stiffness to the suspension system due to movement of the suspension linkage, and acts to stabilize the load pushing the tire into the ground. NOTE that road surface irregularity does not play into this portion of their function, that is what No. 1 was for (in actuality it does a little bit, but in a road car application they can be pretty completely separated).
The springs provide force to the wheel based on how much the spring is compressed, a distance relationship. The dampers provide a force to the wheel based on how fast them move, a VELOCITY relationship, so when the suspension is stationary relative to the car, the dampers are not providing any force input to the tires (gas charged dampers do produce a little, but compared to the spring it's negligible).
COMPRESSION DAMPING:
The compression damping is what dissipates some of the energy of the road compressing the suspension upon the wheel encountering some road irregularity and causing the suspension to move. This situation is when the wheel moves up, commonly called bump, compression, or jounce. During wheel bump some of the energy used to push the wheel up is dissipated through the damper, some is used to compress the spring (and is thus stored in the spring) and basically the rest is lost due to friction in the system. (The tire itself is also a damped spring, and so dissipates and stores some energy, but I'll ignore it for now - it's frighteningly complicated). It is compression damping combined with the stiffness of the spring that largely determines ride quality over bumps. Since the damper and spring both provide force to push the wheel back down against a ride in the road surface their forces add.
If the force input of the spring is much greater than the damper then the spring will prevail - and the spring is said to be 'under-damped.' This condition will cause the car to bounce. The spring compresses, springs back down and overshoots the initial static ride height, then the car falls compressing the spring again, and oscillates. This is one of the symptoms of worn out dampers - they don't damp! The force input to the tire from the suspension is primarily from the under-damped spring and so the force input varies with suspension motion. The car moves down and increases force on the tire, which causes the spring to push back up against the car through the spring perch which lessens the load on the tire, which causes the car to come back down ..... etc. Load on the tire oscillates from small to large as the car bounces up and down.
THIS IS IMPORTANT BECAUSE: Tires produce grip based on how much load there is on the tire. Said another way, How much weight pushes the tire into the ground determines how hard the car can corner / accelerate / brake. When the load is varying (like when the car is bouncing up and down and 'pulsing' how hard the tire is pushed into the ground) the force that the tire can supply to accelerate the car (cornering / accelerating / braking) also varies. If you are hard on the brakes and encounter a road irregularity that sets the sprung mass oscillating then the car cannot brake at a constant rate - it cam brake harder when the nose is bouncing down into the ground pushing down on the tires than when the nose bounces up and unloads the front tires. This makes the car very hard to drive, because the driver input has to vary with the motion of the sprung mass.
If the force input of the damper is much greater then the spring (think of it as infinite) then the wheel is essensially locked to the mass of the car and the suspension will not move. The damper is locked solid when something tries to compress it. This situation is known as 'over-damping.' Over-damped systems are undesireable for two reasons. Ff the vehicle encounters a road irregularity, the suspension cannot move to absorb the motion of the irregularity and the car is forced to follow the irregularity exactly, and the ride is jarring and extremely hard, and unconfortable (the only thing able to absorb the shock is the tire compressing). It is also bad from a performance aspect for a similar reason why the under-damped case is bad, because the load on the tire varies again! If you are hard on the brakes and hit a bump then the tires are momentarily pushed hard into the ground and are then unloaded by the inertia of the car trying to continue allowing the car to move up. Think of pushing a hand truck over the weatherstripping of your front door fast - the whole truck is pushed up to go over the threshold but if you go fast enough the wheels will fly off the ground over the bump and not even be touching the ground! If this happens in the car under the brakes and you crest a bump and the tires come off the ground then you can't generate any braking grip at all!
What you need is a combination of the spring and damper working with each other, which is where the second part of No. 2) above came into play, about stabilizing the load on the tire. A properly (ideally actually) set-up spring / damper combination will allow the wheel to move over a bump, compress the suspension, and allow the wheel to come back down over the back side of the bump without varying the load on the tire, allowing you to maintain exactly the same braking acceleration as before you hit the bump. The damper minimizes oscillation of the car, but has to be compliant enough to allow the suspension to move.
REBOUND DAMPING:
Completely different purpose from compression damping above, although they are related. Rebound damping comes into play primarily to stabilize the spung mass of the vehicle. Rebound damping comes into play when the wheels move down relative to the body of the car, called droop, or rebound. Yes, suspension droop happens when we go over a bump and the wheel has to follow back down the back of he bump. BUT, when encountering a bump, the car then has to come back down onto the wheel, and compression damping comes into play again to minimize oscillations, so we're back to compression damping!
(If the rebound damping is way too hard it will cause the wheel to fly off bumps like we talked about above, but usually that also means compression damping is way too hard as well [they are coupled to each other] and that needs to be remedied anyways.)
WHAT WE CARE ABOUT FOR REBOUND DAMPING:
We care about rebound damping because it's primary purpose is to slow the roll (cornering) or pitch (braking / accelerating) of the sprung mass of the car during a maneuver!
Cornering: When you turn into a left hand corner the weight of the car shifts to the two right side (outside) tires, and OFF the two left (inside) tires. As the weight shifts the body of the car rolls over to the right. The outside suspension goes into compression, and the INSIDE suspension goes into DROOP. What the rebound damping does is resists the motion of the wheels trying to go into droop.
As the car rolls, if the inside tires are prevented from going into droop the left (inside) side of the car is kept lower to the ground for a period of time. If the inside is kept lower to the ground the car rolls less.
So DURING TURN IN, the rebound damping SLOWS THE ROLLING OF THE CAR.
This provides several advantages to handling, namely it keeps the center of gravity a little lower to the ground and more weight on the inside tires a little longer (improving turn-in). It also keeps the inside tires from cambering out (independent suspension) quite as fast on turn-in, which lets the tires make a little more grip, which ALSO IMPROVES TURN-IN.
The same thing also happens during braking and acceleration, but with one end or the other of the car, rather than one side or the other.
If the rebound damping is too soft then the inside wheels will droop faster, if it's too hard then the inside tires will 'skitter' around as the load is suddenly taken off of them, making the car unstable and slidey in corners.
So, THE DAMPERS MOMENTARILY RESIST MOVEMENT OF THE WHEELS RELATIVE TO THE BODY OF THE CAR.
More damping slows the motion of the car, but during a very long corner / long distance of braking etc., the car will eventually settle at the position dictated by the stiffness of the springs, it just takes longer to get there.
Less damping allows the car to shift faster but at the risk of being so soft that the sprung mass can bounce around (faster usually not better in this case!).
So, for the Saab specific application with the Koni adjustables, the compression damping comes fixed, which means the ride quality is basically fixed, and rebound is adjustable, for tuning vehicle response.
The adjustments come into play based on relative vehicle weight, how severe the maneuver is and at what road speed it is performed, and road surface quality.
Stiffer dampers will ride harder on a street car, especially in urban areas or where the road quality is not great (like where I am to the North of Chicago). Stiffer dampers also require stiffer springs so that the damping force input does not dominate the suspension response, and the suspension becomes over-damped. Softer dampers will ride smoother over rough road, and need softer springs so that the system does not become under-damped.
I simplified this greatly, but from there you can sort of decide what qualities of the suspension are important to you. Generally performance minded drivers do not mind the slightly harsher ride of stiff dampers and springs over not-so-perfect-road, but someone concerned about ride quality might well think that the Konis and Eibachs will be too stiff.
When I put the Konis and B&Gs on my 900 the ride actually improved because the stock dampers were pretty worn out and the car bounced around over bumps more, and now the Konis control the car over bumps much better in addition to providing better transient response in corners.
To further complicate things the Konis as you rightly mentioned are rebound adjustable only, or are called 1-Way Adjustable.
A 2-way adjustable damper would allow you to tune how stiff the compression and rebound damping are independent of each other.
Most racing dampers alloy you to also separetely tube how stiff the damper is in relation to road bumps compared to how stiff it is when the car turns into a corner, such as a 4-Way Adjustable damper. The 4-Ways give two adjustments for rebound, two for compression. The two ways are a 'low-speed' adjustment, for low damper extension speeds like cornering and hitting the brakes, and a 'high-speed' adjustment, for when you hit bumps in the road. 6-Way dampers give an intermediate between the high and low speed to tune as well.
That's how I understand things, hopefully that helped instead of hurt. I typed this all out at once, so please forgive any typing errors, and there are bound to be contradictions in there as well, so please let me know if there is anything that didn't come off very clear.
Let me know what you think!!
Best,
Drew
posted by 165.124.166...
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
