Why compression?

[kcbrown]

Why compression damping?

I've been giving the question of springs and dampers a bit of thought, and there's one thing I haven't been able to understand:

Why do dampers have any amount of compression damping at all?

I can think of some downsides to having it, namely that you wind up with an effective variable spring rate on compression that is dependent upon the speed at which the spring is being compressed, thus reducing the ability of the suspension to follow sharper changes in the surface, increasing the "harshness", etc. In addition, you wind up needing to use less spring rate for a given set of conditions, when maximizing the spring rate almost always results in better overall handling.

But what are the upsides? I can't think of any.

Hence, my question. It seems to me that the ideal setup would have a damper that operated in rebound only, with spring rates being used to tune the compression side of things. What am I missing here?

 

[sheizasosay]

"Why do dampers have any amount of compression damping at all"

To control the rate of weight transfer while in compression.

 

[dontlifttoshift]

Sam broke it down best, compression controls unsprung weight. Not enough compression and the tires leave the ground over bumps and the car gets sketchy. Rebound controls weight transfer, or sprung weight.

 

[VTXFrank]

If your suspension didn't compress, the first time you went into a high speed, sweeping turn with an elevation change at the top that leads into a direction change, you'd fly off the track instead of sticking to it and just having a little hop at the top going into the change of direction.

Also, if your suspension didn't compress, then when you got onto your brakes hard, weight wouldn't shift forward as much. Too much forward shift and your rear end can get loose. None at all and the fact that around 70% of your vehicles braking strength is dependent upon the front compressing a little bit, which is why the brakes are larger in front than out back, and your braking distances would be much greater. Without that bit of compression up front to take on the weight of the sudden braking, there wouldn't be that weight shift. With the rear of these cars being so much lighter than the front, if there wasn't some shifting to the front to take advantage of that weight up there, you'd loose effective breaking power and take longer to come to a stop.

Mid or rear engine vehicle setups still depend on front suspension compression for effective breaking. It's simple physics. You take a vehicle and try to slow it down, the mass of said vehicle pushes forward. With some compression up front, you can use than forward and down movement to help the car stop faster by putting larger brakes on the front.

If you do not control that compression rate, then you'd have your ass end up in the air under hard braking conditions and lose that 30% breaking effectiveness from the rear tires.

 

[kcbrown]

Sam broke it down best, compression controls unsprung weight. Not enough compression and the tires leave the ground over bumps and the car gets sketchy. Rebound controls weight transfer, or sprung weight.

But don't spring rates also control unsprung weight?

The question I'm trying to get at here is: why would you use compression damping instead of additional spring rate, given the benefits of using stiffer springs? Obviously the suspension has to be able to compress, so you have to tune the compression appropriately, but why would you do that tuning through damping instead of through the spring rate itself?

 

[sheizasosay]

Metaphorically, damping would be related to acceleration and spring rate would be related to top speed.

Spring rate is definitive . A 100in/lb spring will compress 1" per every 100"lbs....until it runs out of travel.

Damping will slow this process down, but ultimately the weight will still compress that spring 1" per 100lbs given the weight remains applied ie...you haven't changed direction and are steady state.

 

[SoundGuyDave]

Long story short: for a given load transfer, spring rate determines distance of suspension travel, while the damper characteristics control therate of that travel. With no compression damping, a brake application will let the nose slam down, and since mass in motion defines inertia, you WILL overshoot as the suspension sets. Until it rebounds to a steady state (where it has"set"), you will have variable load on the tires, and thus variable grip. In addition, unless the tires are perfectly balanced, with no control over the spring oscillation, you'll have either massive wheel tramp (with low rebound), or the suspension will "jack down" (with high rebound).

 

[NoTicket]

In addition, one simple example is going over a bump. Your car will go through several repeated steps of almost instantaneous suspension compression, followed by a damped rebound, followed by almost instantaneous compression. This would feel awful, and would make the car unstable.

Remember, springs don't just compress and then go back to steady state. Mass on a spring travels with harmonic motion. Every bump will result in several cycles of compression and rebound. With effective control of both, you can make the travel less instantaneous. The shock will also bleed off the forces of the spring in the form of heat, resulting in fewer rebound and compression cycles.

When thinking of suspension, always think in terms of bumpy roads, because they will always be there, no matter what track or what street.

 

[frank s]

I know of a Cobra that sat for a few years without any kind of use or movement. When the owner decided to revive it he went through an effective routine with the engine so it started and ran healthy. He was delighted and decided to bring it over and show it off. Unfortunately there was no appropriate ritual for the dampers, and the jounce failed as he motored down the freeway: the suspension "jacked down" and his big fat, old tires burned holes in the aluminum rear fenders. It wouldn't un-jack, so it had to be dragged onto a flatbed.

 

[sheizasosay]

I know of a Cobra that sat for a few years without any kind of use or movement. When the owner decided to revive it he went through an effective routine with the engine so it started and ran healthy. He was delighted and decided to bring it over and show it off. Unfortunately there was no appropriate ritual for the dampers, and the jounce failed as he motored down the freeway: the suspension "jacked down" and his big fat, old tires burned holes in the aluminum rear fenders. It wouldn't un-jack, so it had to be dragged onto a flatbed.

I would have been emotional if that Cobra was mine.

 

[Department Of Boost]

I know of a Cobra that sat for a few years without any kind of use or movement. When the owner decided to revive it he went through an effective routine with the engine so it started and ran healthy. He was delighted and decided to bring it over and show it off. Unfortunately there was no appropriate ritual for the dampers, and the jounce failed as he motored down the freeway: the suspension "jacked down" and his big fat, old tires burned holes in the aluminum rear fenders. It wouldn't un-jack, so it had to be dragged onto a flatbed.

Sounds like the rebound "stuck".

 

[Department Of Boost]

Long story short: for a given load transfer, spring rate determines distance of suspension travel, while the damper characteristics control therate of that travel. With no compression damping, a brake application will let the nose slam down, and since mass in motion defines inertia, you WILL overshoot as the suspension sets. Until it rebounds to a steady state (where it has"set"), you will have variable load on the tires, and thus variable grip. In addition, unless the tires are perfectly balanced, with no control over the spring oscillation, you'll have either massive wheel tramp (with low rebound), or the suspension will "jack down" (with high rebound).

Good reply^^^^^

I'll add that maybe the OP doesn't understand how the compression circuit works (or is supposed to). Compression is speed sensitive. At slow shaft speeds (rolling bump) there is very little compression damping/force taking place. In some cases almost none. When jumping on the brakes or making a fast transition you get high shaft speeds which will give you more compression damping/force....a lot more. And on the far end of the spectrum lets say you hit a curb, better dampers will "blow off" which reduces compression damping to almost zero despite having very high shaft speeds. Compression damping is very complex in its tuning, the kind of tuning where you are getting inside the dampers and playing with the valving.

To repeat Dave:

Springs are linear in rate and "load sensitive".

Compression damping is variable in rate and "speed sensitive".

You need both.

 

[csamsh]

Good reply^^^^^

I'll add that maybe the OP doesn't understand how the compression circuit works (or is supposed to). Compression is speed sensitive. At slow shaft speeds (rolling bump) there is very little compression damping/force taking place. In some cases almost none. When jumping on the brakes or making a fast transition you get high shaft speeds which will give you more compression damping/force....a lot more. And on the far end of the spectrum lets say you hit a curb, better dampers will "blow off" which reduces compression damping to almost zero despite having very high shaft speeds. Compression damping is very complex in its tuning, the kind of tuning where you are getting inside the dampers and playing with the valving.

To repeat Dave:

Springs are linear in rate and "load sensitive".

Compression damping is variable in rate and "speed sensitive".

You need both.

I had always wondered why 3-way dampers typically give you 2 compression adjustments but only one rebound, when 1-way just gives you rebound. It would seem that rebound is the "more important" adjustment if you can only have one. This seems to answer that question, thanks!

 

[kcbrown]

Long story short: for a given load transfer, spring rate determines distance of suspension travel, while the damper characteristics control therate of that travel. With no compression damping, a brake application will let the nose slam down, and since mass in motion defines inertia, you WILL overshoot as the suspension sets. Until it rebounds to a steady state (where it has"set"), you will have variable load on the tires, and thus variable grip. In addition, unless the tires are perfectly balanced, with no control over the spring oscillation, you'll have either massive wheel tramp (with low rebound), or the suspension will "jack down" (with high rebound).

Okay, this makes sense. So, essentially, the idea behind the compression setting is to minimize overshoot for those situations in which the steady state differs from that of the default steady state.

Seems to me, then, that you'd basically want to reduce compression dampening as the spring rate goes up, while you'd want to increase rebound dampening as spring rate goes up, because the amount by which you'd overshoot the steady state point is smaller with larger spring rates.

 

[sheizasosay]

KC, it might help to take a look at page 5 to get an idea of tuning application. http://www.fordracingparts.com/mustang/fr500sownerinfo/Sachs_Damper_Guide_FR500S_v1.pdf

 

[c_reber]

Good info! Now can anyone tell me where/how to adjust the rebound on the rear of the KW V3 coilovers? The fronts there is an adjustment on the top (compression) and the bottom of the strut (rebound). No adjustments on the bottom of the rear shocks!

Sorry to hijack the thread....

 

[sheizasosay]

If the V3's adjust anything like the KW clubsports, and I imagine they do, then the adjustment is at the top accessible via the trunk.

 

[Department Of Boost]

It would seem that rebound is the "more important" adjustment if you can only have one.

Yes, if I could only have one adjustment I would want rebound.

Okay, this makes sense. So, essentially, the idea behind the compression setting is to minimize overshoot for those situations in which the steady state differs from that of the default steady state.

One of many ideas.

Seems to me, then, that you'd basically want to reduce compression dampening as the spring rate goes up, while you'd want to increase rebound dampening as spring rate goes up, because the amount by which you'd overshoot the steady state point is smaller with larger spring rates.

Things are rarely so simple but you have the theory right.

 

[Department Of Boost]

Good info! Now can anyone tell me where/how to adjust the rebound on the rear of the KW V3 coilovers?

If you link me a picture I can tell you.

The fronts there is an adjustment on the top (compression) and the bottom of the strut (rebound).

I think you have that backward. The Clubsports are rebound on the top, compression on the bottom. If you link a picture I can probably tell you for sure.

 

[c_reber]

Jeez, now I'm really confused! I will start a new thread in a couple days with some Pics, asking the same questions. Don't have access to my car right now...

Thanks for the quick replies though!