Yup, some good discussion from the group about Koni vs Illumina vs Tein.

Imagine the pavement suddenly stepping down by 1 inch. The wheels are extended down an inch and the body needs to come down. Bump damping damps this sprung weight body motion. Unsprung weight plays no role.

If bump damping is insufficient, the body will oveshoot in the downward direction, and come back up. The rears of Toyotas are very much like this. Keep an eye on them when you're following one.

No, you need more damping in both directions for rough roads - the knee point needs to be placed higher.

Good points and an excellent visual. Must ponder some more....

I do not agree. I think you would need a stiffer spring, along with more rebound. The spring is to control the body coming down, and the rebound to control the spring on release.

It all depends on the speed the suspension is moving. If you needed more compression it would be due to the higher compression velocity, so I'm not saying you wouldn't need more compression. All I have said is that you need the right amount of compression to control the unsprung weight. The right amount takes into account not only the unsprung weight, but also the speed at which it is moving. So, there are cases where you would use more compression. It is just not proper to use compression in place of spring rate.

The compression is only going to amplify the situation by trying to bounce the car back up, or keep it from coming down when "landing", the same thing. As the car is hitting the next bump the energy is still trying to come down when the road is trying to push it up.

You would agree that you would need a quick responding suspension on a rougher road, correct? Well, compression only delays this action.

I get this a lot. People are confused as why their suspension feels lazy on transition even though they have "a lot of compression". The compression is only going to slow things down. If you want it to get on the spring quicker, adding compression is the opposite of what you are trying to do.

A quick glance at FCM's site says HD's are better paired with stiffer springs up to 450lb/in F-350lb/in R.

In for thoughts on this as well. I too am considering Bilstein HD's and FM srings. At this point I have no need for adjusting any rebound or compression.

I disagree with FCM, I had that combo and it made me nauseous to drive; way too little rebound damping. I've driven with HD's using MSM springs and it was great aside from the jacked ride height. I think the FM springs would be great with the HD's.

Let's stir the pot a little today.

A lot of info out there says that Bilstein is superior due to them having a larger piston than others. They say this makes the shock more responsive due to a larger area. They also say it provides more resisting force, because of the larger size. It has been compared to a piston in a engine. Larger bore puts more force on the crank.

Is this correct? Why?

http://www.fatcatmotorsports.com/FCM...sion_Truth.pdf

Page #24

I think what he's saying is that larger pistons can start developing damping force with smaller motions and velocities because it moves more oil. Additionally you get less cavitation and hysteresis for a given damping force.

You sure the piston is moving through the oil, and not the separator piston pushing the oil through the piston?

Is there a significant difference between a piston being pushed through a static fluid (erg, that's an awkward phrase but you know what I mean) and a fluid being pushed through a static piston?

The fluid is not static on compression, it is on rebound.

Originally Posted by JasonC SBB
In the scenario I described, with critical damping the body will not overshoot in the downward direction. With zero damping, it will overshoot downward, then come back up. With zero damping, there will be greater tendency to slam into the bumpstops.

The spring applies delta F in both directions, whether the wheel is in bump or in droop. The damper damps the motion in both directions, whether coming back from a bump position or a droop position.

If you merely stiffened the spring in the scenario I describe, it will merely reduce the *distance* it overshoots, but it will still overshoot, and the peak downward velocity and peak downward acceleration will both increase.

The unsprung weight only plays a role when considering 2 scenarios:

1) when the tire catches air, in which case the spring force acts on the mass of the wheel/tire.

2) when considering the case of tire bounce frequency (imagine a heavy wheel with a tall soft sidewall tire), thus the wheel can bounce or "dribble" on the tire sidewall. SUVs do this a lot because of the heavy axle.

When considering the case of a lightweight wheel and relatively stiff tire sidewall, the damper doesn't do much for the unsprung weight.

In the case of the tire catching air, both bump and rebound damping can play a role. When the tire "launches" or flies off a bump, it is moving up relative to the body, and the bump damping reduces the distance it flies up. When the wheel then moves down before contacting the road, then rebound damping is reducing the peak velocity before the tire re-contacts the road.

I'm not saying this. What I do believe is that the bump damping needs to be appropriate for the spring rate for a given car.

No, the bump damping is arresting the downward motion, and by reducing the distance the body overshoots downward, reduces the stored energy in the spring that causes it to bounce back up.

I think several different effects can play a role.

Firstly only the first like 1-3 ips of damping plays a role - the definition of "a lot of compression" could mean "the knee point is high and I have a lot of high speed bump damping", vs what is relevant, which is "I have a lot of area under the curve in the compression damping from 0 to 2 ips)".

Second, when the car goes into roll at turn-in, being underdamped in roll can cause a roll overshoot - very bad for feel. The opposite problem, being way overdamped in roll, can also be detrimental because it actually takes extra time for the car to take a set. However it can also make the car feel more responsive because the energy being transferred into the car initially goes more into yawing it than into rolling it.

Third, with soft bump damping and stiff rebound damping (in the <3 ips range that matters for handling feel), it tends to lower the outside of the car quickly before the inside lifts up - this is momentarily favorable for the camber of the outside tire which helps turn in. because lifting the inside is bad for dynamic camber, but compressing the outside isn't, on a car with A arms). It is also momentarily favorable in terms of lower the CG.

Honestly I don't know which of the above effects is most significant, so I will defer to your vast experience, but I will ask you, if you only look at the 0-2 or 3 ips plots, is your above statement still valid?

The separator piston moves in relation to how much shaft is inside the shock. Therefore, the fluid in front of the separator piston is not static.

So, you are telling me I could take a stock suspension car to the Baja 1000 and as long I had stiff enough shocks it would be ok?

No, compression is not spring rate.

Again, I never said the above. I'm only saying that the bump damping needs to be matched to the spring rate.

I never said "instead of running stiffer springs, just increase bump damping".
I'm saying "stiffer springs need more bump damping".

Why would you need more compression damping if the spring is providing additional resisting force? Did the bump get bigger due to the stiffer springs? Is it trying to compress the suspension faster? No.

Look at it like this. Hit a speed bump at 50 mph with a stock car, now hit it with a race car running stiff springs. Which car is going to be more upset? Adding compression will not "smooth" things out for the car with stiffer springs, it will try to throw the car up in the air.

How is this for a real world scenario.

When working at Bobby Hamilton's in the NASCAR truck series, we made a transition from running a 300# spring in the RR, to running a 1100# spring. Not one time did we ever need to change the compression valving for the spring change.

One more thing, we ran less compression valving than a stock Bilstein NA HD.

...hency why I want more spring in my daily on the HD's, but not more than 250lb so I keep rebound in check.