Damper Tuning

[JeffYoung]

Ok, so I just dropped a buttload on a set of DA Konis for the rear of my car, and single on car adjustables for the front.

I've never had tuneability like this, and have been doing some reading. Right now my car, live rear axle, has slight to significant oversteer. I have trouble getting the power down in high speed transitions, like VIR's Hog's Pen, and slower corners, like the Auto-X section of CMP.

My reading would suggest that on the rears, I want a very low compression setting to let the rear compress and a high rebound to keep it down and planted for corner exit.

For the fronts, on SA shocks, my understand is that I should set it to the softest setting and slowly work it up until the nose starts to slide.

Spring rates are 600 front, 300 rear if that matters.

Any thoughts appreciated.

[JohnRW]

You made the right choice for the rear of a live-axle car. DA (regardless of brand) is a huge tuning aid back there.

Most race shock adjustments (IME) are VERY non-linear. You might have "20 clicks" soft>hard, but you'll find that you really only use the 'hardest' 1/3-1/2 of those settings. Since you can adjust them on-car, bring a capable friend to an all-day test day, and just turn all the knobs, one at a time, from softest to hardest. You'll learn a LOT.

I had Truechoice build me front and back Koni DA's for the big noisy V8-powered, live-axle vintage car a few years ago. Arrived at all my settings thru on-track fiddling. Having independant rebound adjustment on the rear of a live-axle car is HUGE. I run moderate bump, but a whole load of rebound back there. Really makes a big difference in braking/corner-entry, according to my sensitive but uncalibrated heinie-dyno.

I've recently gone to a short-body Penske single-adjustable in the front, as I kept blowing out foot-valves in the Koni's by trying to run the front end too low. My fault...any shock would explode if it got bottomed hard on a regular basis. Not sure if I'd pay big extra money for a separate rebound adjustment in the front.

[dj10]

You made the right choice for the rear of a live-axle car. DA (regardless of brand) is a huge tuning aid back there.

I've recently gone to a short-body Penske single-adjustable in the front, as I kept blowing out foot-valves in the Koni's by trying to run the front end too low. My fault...any shock would explode if it got bottomed hard on a regular basis. Not sure if I'd pay big extra money for a separate rebound adjustment in the front. [/b]



I agree with you about the rear but I believe the DA in the front are just as important when fine tuning the car. Aren't Penske's expensive? It couldn't be much more for DA could it and last question what kind of car did you put thes Penske's on?

[JeffYoung]

Koni said they couldn't DA the fronts on my existing strut tube. This was before I realized that the strut tube could be machined -- next time, I'll do something more custom and open up the strut tube a bit.

John RW -- thanks for that.

[John Herman]

+1 on the test day. When you do the test day, FIRST, just play around with the settings. Make MAJOR adjustments to the shocks/struts so you can really feel the difference (e.g. full soft front, full hard rear, then switch). When in this stage, there is no need to stay out for 20 minutes. You should be able to feel the difference in just a few laps, if not a few corners.In the beginning, you want to learn what some of these knobs will do and how the car will react. Major adjustments will help you do that. Be prepared for some surprises as you learn. The car may want to understeer/oversteer severely in some corners or parts of corners. Once you have a bit of a feel for the adjustments, start your fine tuning phase like Koni recommends. Besides just for the learning, one of the reasons for experimenting with the major adjustments, is for the time when you head out on the track and the car is misbehaving. You'll have a reasonable idea of what you'll need to adjust and by how much(e.g for between qualifying and a race). After all, the reason for the adjustments is to fine tune the car for changing track conditions. There are many times I've had to qualify in the morning on a cold track, but by the time we raced, the track temperature had increased significantly. If you want to be fast, you'll need to have a gut feel on the adjustments priot to hitting the track.

[JohnRW]

Aren't Penske's expensive? [/b]


Yes. In spades.



It couldn't be much more for DA could it and last question what kind of car did you put thes Penske's on?[/b]

DA Penske's are cubic dollars. They're little "works-of-art", but cheap...they ain't. For the application ('65 Mustang), Joe Stimola provided me a good ratio of bump/rebound, as he had loads of experience building shocks for these cars. As far as having separate bump/rebound controls in the front...while I could obviously feel and "tune" them independantly on the Koni's, the final settings I arrived at feel an whole lot like the Penske's currently installed. I would have "tuned" myself to the same place, and spent a whole lot more money to DA-diddle myself to exactly the same place.

I still have the Koni DA's in the back of the car. I worked with Phil Harris at Truechoice to build the Koni DA's. They hadn't built them before, but made me something that worked and I was happy with. Just didn't have a short-enough body front shock to keep from blowing them out at the ride-heights I eventually arrived at.

[lateapex911]

Great thread!

[Quickshoe]

Ohlins is coming out with a new shock that is more economical than their typical stuff. Supposed to be similarly priced to a 8700 series Penske.

[dj10]

Ohlins is coming out with a new shock that is more economical than their typical stuff. Supposed to be similarly priced to a 8700 series Penske. [/b]



Just from my experience I've found the 3 things you don't skimp on if you want to go fast is:

1. Good suspension, at least DA, and most companys make good stuff now.

2. Good tires

3. Drivers Seat Time

[Tak]

I've got Koni DA on both ends of my Gen 1 Rx-7. Truly wonderful, but it took me the better part of two seasons to figure out the tuning... You didn't say if your fronts are rebound only, or hybrid compression and rebound adjustment. You said they are Koni, so I suspect rebound only.
Here is the simplest advice I can give:
Use compression damping to get the car to handle over bumps and kerbs. Axle hop = too much compression damping.
Use rebound damping to adjust corner entry and exit understeer/oversteer balance.
More rear rebound = more oversteer.
Less front rebound = more oversteer.
Less rear rebound = more understeer.
More front rebound = more understeer.
Remember that dampers only work when the car is in TRANSITION. If you car doesn't handle correctly in the middle of the corner, dampers will not fix that.
What I can not explain, is how to recognize that you have too much rebound damping at BOTH ends. The balance is right, but the car is not as fast as it could be...

Good luck!

Tak
#29 ITA
SFR SCCA

[Quickshoe]

What I can not explain, is how to recognize that you have too much rebound damping at BOTH ends. The balance is right, but the car is not as fast as it could be..[/b]

Car feels too "nervous" or "twitchy" ????

If the car is balanced soften the rebound at all corners and see if the car is either a ) faster, b ) easier to drive as fast or c ) combination of both :D

[kthomas]

I've got Koni DA on both ends of my Gen 1 Rx-7. Truly wonderful, but it took me the better part of two seasons to figure out the tuning... You didn't say if your fronts are rebound only, or hybrid compression and rebound adjustment. You said they are Koni, so I suspect rebound only.
Here is the simplest advice I can give:
Use compression damping to get the car to handle over bumps and kerbs. Axle hop = too much compression damping.
Use rebound damping to adjust corner entry and exit understeer/oversteer balance.
More rear rebound = more oversteer.
Less front rebound = more oversteer.
Less rear rebound = more understeer.
More front rebound = more understeer.
Remember that dampers only work when the car is in TRANSITION. If you car doesn't handle correctly in the middle of the corner, dampers will not fix that.
What I can not explain, is how to recognize that you have too much rebound damping at BOTH ends. The balance is right, but the car is not as fast as it could be...

Good luck!

Tak
#29 ITA
SFR SCCA
[/b]

Generally true. Too much rebound usually manifests itself as the car "jacking down" (which of course without shock pots connected to your DAS you won't be able to tell unless you start scraping the oil pan for no apparent reason) or overall loss of grip, which again you might need to look at your DAS to see.

Before I go into details, what exactly do these shocks adjust? High spped bump, low speed bump, high speed rebound, low speed rebound, and are they digressive valving or other? My guess is the high speed stuff is set by the valve stack and all you have is low speed adjustments. Confirm and we'll take it further.

[JeffYoung]

I believe that is correct.

[Tak]

Quickshoe--Excellent suggestion of softening both ends when it feels good. I'm going to try that!
Do note that there is a fair sized damping range between "optimal" damping and jacking the car down.
A very simple method to identify jacking (or bottoming out) is to put a zip tie on your damper shafts and drive. If the zip tie gets wedged into the bump rubber, you are either jacking down or need stiffer springs. It is a good test that everyone should conduct when playing ride heights, spring rates, or damping.

With regard to what is adjusted on the dampers...I believe that the Koni DA adjust rebound by changing orifices on the piston, and high speed compression by adjusting orifices on the foot valve ( I havn't taken one apart, so I'm not sure about that). I believe that changing the orifices on these valves has two effects: it changes the amount of fluid that can move through the piston when the shim stack opens, AND it changes the area of the shim stack exposed to pressure, changing the amount the shim stack moves. Both of these are what I would consider high speed damping.

I believe you need a remote reservoir for low speed rebound, and IIRC, those are illegal in IT. I Havn't got a clue what the fourth adjustment on 4 way dampers is. Anyone care to explain?

A quick word on high speed vs low speed: Tires and chassis are "spring mass systems". They have a predictable response frequency that is dependant on the mass that is moving, the spring rate, and amount of damping. Since the tire and chassis share the same spring, there are two distinct frequencies (speeds). The first is the tire frequency, which is high speed (the tire, wheel, upright, and brakes have much less mass than the chassis). The second is the chassis frequency, which is low speed. These apply very nicely to compression damping. For example, a car traveling around a corner hits the apex berm, and the tire compresses the suspension 1 inches--this is high speed compression damping, as the tire is moving. Similarly, the same car turns into a left hand corner, and the chassis rolls 5 degrees, compressing the right side suspension 1 inch--this is low speed compression damping, as the chassis is moving. At the same time, the left side suspension extends, or rebounds, 1 inch. This is rebound damping. You could call it low speed rebound, but it is the only rebound damping speed that matters.
The fourth scenario I do not personally understand...The car is airborn, and the wheels rebound (extend) until the tires touch the ground or the suspension is fully extended. Sure you could call this high speed rebound because the tires are moving, but who cares? The car is IN THE AIR and no amount of damper tuning is going to change how it handles until the wheels touch the ground (at which point it becomes low speed compression damping as the chassis compresses the suspension)...


Tak

[JeffYoung]

Extremely helpful thread, thanks guys. This type of information is what made IT.com such a great place.

Thanks.