airiscool

I am trying to determine if there is a rule of thumb for damper adjustments in relation to spring rate changes. Let me present a scenario.

Well tuned set up = 300lb spring with the damper set to 4 bump and 6 rebound.

If the spring rate is increased to 400lb I assume the rebound needs to be increased but what happens to the bump setting? Does the bump get reduced because the spring now supports the car better in transition and turns or...?

I am not asking about how to tune for understeer, oversteer or neutral handling, what I am trying to find out is how the damper setting needs to be changed relative to the spring rate.

I hope you are able to understand my question

Gasser

Larry Herman

Actually the bump (compression) gets increased too. The purpose of the shock is to dampen and control the movement of the suspension. As spring rates increase, the movement of the suspension gets quicker, and shorter. The dampening of the shock (in both directions) must increase too to keep control of the suspension.

924RACR

Please don't dampen your dampers unless they're protected against rust...

Actually, I talked to Bilstein about this; they (Bill Hindorff) said the general need was to soften compression and stiffen on rebound when going to stiffer springs.

That said, with such a modest change in spring rate, you're probably best off to just drive it and see where you're at, and how much you wish to change...

kurt M

I am in the less bump as spring rate goes up camp too but I am not an ex=purt on such things. More rebound control less bump. Dampers are there to keep the tire on the road.

Larry Herman

This doesn't make any sense. Are you certain that is what he said? By logic then, the stiffer your springs, the less compression damping you should have? I guess that I shouldn't be running any dampening on compression then.

FredC

From the little i know...

1. Spring rates will set the overall stiffness of your suspension

2. Dampers will modulate the behavior of your suspension

It follows that the stiffer the springs, the more dampening you need in both directions

924RACR

Yep, that's precisely what he said - as a rule of thumb.

Given that more energy will be absorbed on compression by the spring for a given displacement, less energy will need to be absorbed by the damper to maintain the same dynamics, right? Seems plausible to me, anyway.

Going to test it out next week, anyway - had the sense that my fronts were too stiff on compression after bumping my (spring) rates 50%, so I've eased the front compression setting. We'll see...

Tom Tweed

FWIW, this is quoted directly from the Penske adjustable shock technical manual:
"...the rule of thumb is a stiffer spring requires lower compression and higher rebound. A softer spring requires higher compression and lower rebound."

TT

Larry Herman

No, I feel that it is the exact opposite. If you have more energy absorbed for a given displacement, you will need more dampening to control that energy.

Springs strictly control movement - the more force that you put on a spring, the more it moves. Shocks control the timeframe of that movement. Softer springs move farther for a given force input, and so the dampening occurs over a longer time interval, and therefore can be softer as well.

Larry Herman

Are you certain that they do not mean respective of each other, not absolute like I think that some are assuming? For example if you are running soft springs, you may have the compression set at 5, and the rebound set at 3. If you change to very stiff springs, you may raise the rebound up to 10, but raise the compression only to 8. That would make sense to me, and still fit with what Bilstein and Penske say.

Geoffrey

I've done a lot of work with multi adjustable dampers with MoTeC data acquisition and shock sensors. In general you want to have an equal time spent at compression as you do rebound from a data perspective. However, the reality is that there are other considerations into the decision making process such as how and where in the corner the car is rolling. The damper and spring is more for controlling pitch and roll than recovering from the bumps. If you concentrate on the former, you will likely have good bump management too.

Here is an example of what you might look for from a data acq system view of damper position. This is from my car after a day's worth of setup and adjustments with triple adjustable dampers at WGI. The adjustments were all different on the dampers and compression and rebound differed by a number of clicks on each corner.

Tom Tweed

Exactly, but the key word you use here is that the energy is being absorbed by the spring during compression, Larry. There is little work for the damper to do during that part of the process. It is when the spring releases the stored energy during the extension cycle that the damper has to do a lot of work to absorb it and prevent the energy from being released too quickly, causing handling problems from changes in tire loading and grip.

Claude Rouelle is a very well known designer and engineer and these are his words:
"When the suspension is compressed, energy is being stored in the spring, and during rebound energy is being released from the spring. Since the job of a damper is to absorb energy for the purpose of controlling resonance, less damping force is required by the damper during compression due to the energy going into the spring. Similarly, more damping force is required by the damper during rebound, as it has to control resonance and the energy being released by the spring."
Ref: http://www.optimumg.com/ -- in "Tech Tips" section

I do not currently believe that the "relative vs. absolute" explanation you give is correct, but I am open to having my mind changed. I am certainly no expert on the subject, and only know what I have read about it. Please explain to me what type of energy absorption (or resonance and oscillation control) you think it is necessary for the shock to perform during the spring's compression cycle, exactly, and why? Perhaps this will help me understand why you think it is necessary to increase compression damping as well as rebound damping when spring rates are increased.

Thx,
TT

Larry Herman

Thanks Tom, now you made me spend the last hour thinking about this.

When you put a load on a spring it will compress to a new length relative to the load, i.e. put a 500 lb weight on 250 lb/in springs and they will compress 2 inches. If you place the weigh carefully onto the spring, it will settle into its new height. If you just drop it onto the spring (assuming it was just touching it) the momentum of the weight will cause the spring to overshoot its height, and then rebound up. This oscillation can go through several cycles until the momentum dissipates. The job of a shock absorber is to dampen the momentum so that the spring compresses to its correct height, and no more. If you are under-damped on compression, you will overshoot the mark, and then have to rely on the rebound to control the momentum on the way back. With cars, the load is caused by the body "leaning" on the suspension. And that load is usually dumped onto the suspension.

As your spring rate increases, the time it takes for the spring to "set" is lessened, because the amount that it has to travel is reduced. Now the shock has less time and less movement to control that compression and prevent it from overshooting. Therefore, the shock dampening has to be increased.

As an empirical answer, my car is sprung very stiffly, and my Motons are set well up in their range for both compression and rebound. In trying to find a better setup for bumpy tracks, I reduced the amount of compression significantly. The results were not pretty. After some experimenting, I found that my best bumpy setup was one click softer on the compression, and 2 clicks softer on the rebound from my smooth track setup.

Tom Tweed

Thanks, Larry. That "overshoot" explanation is not something I have thought about. Wouldn't that be most prevalent in high-speed damping, though, and not low-speed, i.e, more applicable to running over berms or bumps than in smoother transitions like cornering and braking, which would primarily be low-speed damping? Are you running triple adjustables or double?

All I have ever heard from the easily accessible sources has indicated to me that stiffer springs require less damping in bump and more in droop. All the statements do leave open the fact that some compression damping is necessary, though--certainly, no one says you can do without it entirely. How much, exactly, seems to be a well-guarded secret, subject to individual analysis of each car's setup. Too much damping on compression, though, would certainly mean that the shock would end up doing some of the spring's work, and actually increase your overall initial spring rate. That couldn't really be a good idea, I don't think, although people do it in Stock class Solo racing, where you aren't allowed to change springs but shocks are free.

Some single adjustable shocks will adjust rebound only, although others will make a smaller change to compression when rebound is increased, as you suggest. I have only recently started driving a car with double-adjustable Ohlins, and am still trying to get my head around how to optimize them. When you get into triple adjustable setups, with separate adjustments for high and low speed damping, that's when my head begins to explode. It is a very complicated subject, for sure, and depends on a lot of variables, including the tires, swaybars, suspension geometry, chassis stiffness, aero effects and driver preferences, among other things, I'm sure. That's why the race teams spend the big money on the 7-post shaker rigs, wind tunnels, and the engineers to interpret the data, I guess.

Sorry I made your brain spin for an hour.

TT