Suspension Gurus - What Happens When You Over-Dampen?

So I know what happens in extreme cases - if the bound is overdamped relative to the rebound, the suspension will lift itself over a series of bumps and you end up with a trophy truck. If the rebound is overdamped relative to the bound, the suspension will lower itself over a series of bumps and you will beat the crap out of your bump stops. 

What I don't know is why you wouldn't run the factory shock valving off an F350 on your Escort for better handling and faster transient response. It seems that as long as the damping isn't so extreme it prevents the tires following the road, there would be no performance drawback to running a stiffer shock, regardless of the spring rate. (Obviously it would wreck the ride quality, but I'm just wondering about performance.)

In reply to DaewooOfDeath :

The Corrado I bought from Jason Saini in the early '00s was setup with custom valved Koni Sports that would pull the rear of the car down onto the bump stops on the autox course, and it worked amazing. 

Bound? I've only heard that referred to as "compression" or the old-school "jounce..." But it's clear what you're talking about!

TL;DR: I suspect that the F350 dampers would in fact be too stiff under "high (shaft) speed" conditions and would prevent the tires following the ground even if they did land in the ballpark for "low speed" damping. I think this because the forces an F350 needs to deal with are much larger than an Escort under the same "hitting bumpy pavement" circumstances.

The rambly version:

I think the crux of the matter here is that for any kind of performance damping, you're not talking about one rate/orifice.

When you hear about dampers that are adjustable for "high speed" and "low speed" that has to do with the shaft speed of the damper. To some extent, you're looking for stiff low-speed damping for responsiveness and to start load transfer before the springs and sway bars have traveled enough to do that job once you're fully into the corner (or braking, or accel...)

But in terms of "high speed" a gross oversimplification is that (and here it will become obvious that I should've put a "not a guru!" disclaimer at the start) you want the least compression damping that will control the wheel getting launched upward, and the least rebound damping that won't dribble the tire. "Soft" dampers make for better traction by letting the tire follow the road.

What we have in reality is a damping curve, which is what all those stacks of washers and shims you hear about are doing. If you just had a piston with a couple of holes in it (like a cheap old motorcycle fork) it would be underdamped in low speed motion just to avoid breaking your wrists when you hit a sharp bump at 50mph, but you would already be approaching an asymptotic level of damping at that speed. Instead, what we have is that the more quickly and forcefully that wheel is shoved upward by a bump (or downward by a spring), the more the force in the hydraulic system lifts open the washers and shims, so that the effective "hole in the piston" gets bigger the faster the shaft is moving, so that we have some amount of body control in roll and pitch, while also not punching the shock mounts out of the body over sharp pavement changes.

The upshot with regard to your actual question, I think, is that the low-speed damping might actually not be far off, but the forces the F350 shock needs to "open up" are probably massive compared to what the Escort needs, just because the forces are so much bigger with the weights involved. That said, it might not be nuts, might work some of the time, or there might be a less severe thing that would work (e.g. dampers from a heavier car rather than a truck with three times the mass).

I feel like I'm forgetting a guideline here in terms of something like compression damping being related largely to masses and rebound related to spring rate, but... Now I'm really dropping packets coming back from the memory bank. In your example, I believe by the time you're either jacking up or packing down, you're BADLY overdamped relative to masses/springs, I think on the high speed specifically. Another fragment: Broadly, low speed adjustment is often an adjustment of the basic orifice that oil flows through freely, while high speed adjustment is going to affect that shim stack's stiffness.

Anyhow, that's my best guess. Look for descriptions of "digressive" damping curves. I want to say there was some good reading on QA1's site, but surely any quality damper mfr has some info. I wonder whether with some digging you could find some shock dyno info for representative applications.

EDIT for semi-pedantic pet peeve (essentially mandatory for anyone geeking out enough to have a stab at damper questions): "Damping" is what we're talking about here, "dampening" is when something is made wetter.

Being that digressive damping was mentioned, I'll mention that I personally hate it.  A linear or progressive curve is much better for most things, IMO.  Digressive damping can easily lead to a setup that's harsh, jittery and has too much low speed damping but just blows right into the bumpstops if you hit a bigger bump.  About the only thing it's good for is making poorly set up suspensions feel "sporty" to people who don't understand what sporty is. 

My Locost uses 1975 Corvette rear shocks on the front (Monroe blue), and 1985 Chevy sprint rear shocks on the back (Gabriel Guardian).  They are "about" the right valving for the spring rates I'm running (340/140). It's been on the street since 2006, long highway trips, and a number of autocross seasons on streets, R's, and slicks.  I'm very happy with the valving - but honestly, I kinda lucked out really well in picking the shocks I did.

I'd say your thinking is sound.

The right answer is "things get wet" but overlooking the mis-terminology to provide an actual answer:

Basically a little bit of overdamping doesn't do much but ruin the ride quality. If you're driving on a smooth surface (that's a big if for anything other than track driving) then yes it does essentially act as extra spring rate. Street/stock class autocrossers have been doing roughly that for years, and also the super rebound damping to jack the car down thing. It's been less of a phenomenon lately as manufacturers have started putting more reasonable spring rates on cars from the factory. Well...other than mazda.

But go too far and/or bring bumps into the equation and you lose tire contact control pretty quickly. So nobody plays that game unless they are limited by rules or budget into doing so. Since it was kind of referenced, just turning up the damping across the board is generally how OEs inform the customer that the car they're driving is sporty. It doesn't really help performance much but does ruin the ride, and being uncomfortable is how you know something is sporty.

On digressive shocks...basically everything made in like the last three decades is at least lightly digressive, the only thing I know of that is still produced with purely linear valving is koni circle track dampers, and they're pretty awful if you try to use them on a road application unless it's something with a really good motion ratio and even then they're not great. It is a balancing act, low speed damping is good for control and handling but hurts ride. Most OE dampers are low on super low speed damping but ramp up pretty quickly into the middle of the damping curve and then have a pretty significant knee in the curve at some point.

They're also leaning more heavily on the bump stops for ride control, and one feature of elastomeric springs is that they have damping all of their own.  So the sharp hits blow through the kneepoint in the digressive curve and into the bumpstop, which acts as both progressive spring and a bit of last-minute compression damping.

I never really thought about the damping aspect until this post.  Hmm...

In reply to dps214 :

It's true that digressive valving is very common.  I will say that the best thing I ever did to the Jeep suspension was the Bilstein 7100s (which have linear valving).  I've never gotten the damping 100% dialed in on it, but with it at least decent and no digressive curve, it rides better than it has any right to for having something like 60% more than stock spring rate up front and more than double stock in the rear.  Combined with big, cushy bumpstops up front, hitting a nasty dip over some train tracks at 50 is nothing more than a thump that you hear more than feel.  A digressive compression curve would require even bigger, more progressive bumpstops to absorb that hit, as the bumpstops would have to do more of the work to control the suspension's motion in a fast hit (as the shocks would do less). 

As another note, progressive spring rates can be harder to match shocks to.  And if you don't get the shock valving curve right, those progressive springs can ruin the ride completely.  When I first played with progressive springs in the rear of the Jeep, they were great from a load handling perspective and made the behavior more consistent with vs without extra weight in the vehicle.  But they were harder to damp.  If the damping wasn't right, you could easily get a situation where they rode great most of the time, but if you hit a dip right and compressed the rear suspsension far enough, the spring rate would exceed what the shocks could control and it would basically pogo back and then settle. 

rslifkin said:

Digressive damping can easily lead to a setup that's harsh, jittery and has too much low speed damping but just blows right into the bumpstops if you hit a bigger bump.

Anything done poorly can easily lead to undesirable results. This is no more true with digressive than linear valving.

It seems to me that a big part of linear vs digressive is about the amplitude of high-speed movements you're tuning the suspension for. I might be inclined toward linear for large amplitude, but digressive for small amplitude. Beyond that, low-speed could even be tuned similarly between the two if you really wanted.