Forcednduckshn

Gents, I'd like to hear from both the successful and experienced autocrossr's and track guys on this forum. To 65% critically damp, critically damp or over damp is the question. I've done quite a bit of research these past 6 months on shock design, shock setup/valving to achieve particular damping rates and curves and now it's on to the real world practice of shock tuning. I feel like understanding the "what" is meaningless without the "how" and "why."

Let me make it clear than I am relatively new to shock tuning. Although I have a mechanical engineering degree, I'm a victim of "don't use it, you lose it." Without a technical background, I think this stuff would be more challenging to come to grips with. So I want to hear you comments: Am I right, am I wrong on my assumptions and/or conclusions? What tuning strategy works for you in setting up shocks? Are you a "feel" guy, a strictly "data analysis" guy or both? What are your tips and secrets?

I purchased a set of Lee Grimes spec "B-Stock" shocks from Tom and Andrew at Performance Shock at Infineon. They are great guys. Installed them 3 weeks ago, ran them 2 weeks ago for this first time. Lucky for me, they have already went through the exercises of calculating the natural frequencies of the S2000, the motion ratio (to a not a perfect but high level) and all of the corresponding information necessary to calculate the damping coefficients of the shocks and accordingly, what damping rates are needed to achieve critical, 65% critical etc. damping ratios. So with the 4.3 kg Front and ~4.0 kg Rear of the MY2002, (I've seen figures of 3.9 or 4.1 so I took an average) Andrew gave me the damping forces needed at the specified shaft velocity intervals to achieve both 65% Critical and Critical damping across the range. I overlayed these against my dyno plots and here is what I found:

EDIT: These dyno curves show the full soft and full stiff setting for both compression and rebound. C on the top and R on the lower axis.

EDIT: Just to clarify, these are the DOUBLE ADJUSTABLE versions of the shocks.

EDIT: Let me know how you feel about zero gas pressure shocks. Is it worth it for the subtle drop in ride height for stock class purposes? Have you had issues with cavitation in an autoX environment? Does cavitation create a lot of inconsistency in performance? How so?

Fronts:



Rears:



Thoughts? What I take from this is that Lee Grimes clearly felt that through his own testing, for autocross the fastest setup was one that tended to be more critically or even overdamped. This is opposite of what my research has told me for TRACK use - where shocks are typically setup in the 65%-70% critical range. This idea of "more damping than track setups" follows what a lot of fast guys in the s2000's do - run crazy high damping rates (both Compression and Rebound). Sure this translates to a more stable "feel," but won't this lead to less ultimate grip? There will be less weight transfer and less "ability for the suspension to work" and especially with stiff high speed compression, won't the car become quite unsettled over bumps and surface undulations? Because autoX is so slalom and transition intensive, has your testing found that biasing your setup toward this "transitional" stability ultimate nets a faster time than a more grip focused setup? At the end of the day, I feel like the setup game is just a compromise between ultimate steady-state grip and transitional/slalom stability. What is your particular mindset?

At the event, on 140 run Hoosiers and a bumpy lot in 50 degree weather, running high compression damping in the front was NOT working. It was skating and very pushy just after I began turn in. Going from 9 C front to 4 C front helped tremendously and I laid down my fastest time of the day on a run that was "conservative." (Rebound F/R was 1/2 turn from Full Stiff front, 3/4 turn from Full Stiff Rear) It seems very obvious to me, but are you guys that run crazy high compression for the front only using these on the flattest surfaces? Again, I want to hear how you guys go about tuning your shocks initially, and then how you fine tune them at different events. My thoughts seem to be this at the early stage: get a good baseline for the compression settings. Start low than work up higher until I get skating or a pushy sensation at turn in. Then, start with conservative rebound settings and work slightly stiffer until the car feels like it's loading up. Back it off, then make subtle adjustments to get balance changes, but always running as little rebound as possible (on these shocks anyway) to maximize grip, with a bias towards more rebound for "heavier slalomed" courses.

Congrats if you have read this far. I've posed alot of theoretical questions regarding shock setup, and practical questions regarding shock tuning. I hope to get some good info from the gurus here. Thanks in advance.

Nick

Antonov

I'm pretty sure you want underdamped. Even though you get a bit of overshoot, this is the fastest way to reach your steady state while maintaining maximum control over the system. I think.

Definetly like where this going.

mavm86

This is interesting. I think one of the reasons people tend to run so much rebound on stock-class cars is because of the relatively soft spring rates we are stuck with. As I understand it, running more shock force acts as artificial "spring".

Dennis Grant has an interesting bit in the "shocks" section of his autocross to win website (http://farnorthracing.com/autocross_secrets6.html) where he discusses shock forces. He claims he used to run way to much damping force on his shocks, and when he backed it down to 65% critical it "felt" worse but was actually faster.

I'm not an engineer, I'm just adding what I've read to the discussion

TheNick

They didn't do any calculations to come up with the valving. Lee didn't do anything (he's a nice guy but he's not an engineer).

They don't have a magic spreadsheet that pops out damping curves. All shocks are based on experience and knowing what works and what doesn't from doing hundreds of different valvings.


Those shocks are the SA versions of the TC Kline DA shocks. TC Kline did all the testing and research with their cars, and developed the package. The SA are similar but not the same - they have less compression damping (which I hate).


The biggest problem with those shocks is that in order to get the compression you want, you have to jack up the rebound way too high.

And you'll run overdamped. This is based on experience, not theory.

glagola1

IMO:

Rebound and especially large amounts of low speed rebound is the devil. It essentially neuters the shock. When a wheel hits a bump or the body rolls, the shock compresses and before it extends it goes though a change from compression to extension. At the transition between the two, it stops and accelerates. Between being stopped and extending, the shaft is traveling at low speeds. If there is a lot of low speed dampening, the shock doesn't allow the wheel to extend back down to the road surface and the tire spends a brief time not contacting the ground. You know what happens when the tire doesn't make contact with the ground. On bumps, the car floats and braking is weakened as well as corner grip if the car is side loaded.

It's the convention that rebound is the way you control a spring and that springs and sway bars control compression and body roll. When this convention is applied to auto-x where transition is the name of the game, things get funny. Auto-xers start demanding large amounts of shock force so that the car transitions quickly. Here in lies the problem as explained above.

.... and really, does anybody think the weight of the unsprung mass on the inside of a car in a turn is going to hold down the mass of the chassis as it rolls to the outside tire? It's only going to be lifted off the ground and minimize traction.

In my experience, only enough rebound should be used so that extension of the shock and the resulting upward acceleration of the sprung mass is kept under control and doesn't disrupt traction and car dynamics. This is probably what is called 65% of critical.

Here's some anecdotal evidence from the Texas Tour this last weekend: The site is bumpy especially when traveling across the direction of paving. On day 1, I ran my shocks with a rebound damping setting of 4 which is what I used on the smooth asphalt of Dixie. The car had a hard time braking in the slalom across the bumps and was generally bouncy and difficult to keep on line. For a video reference of this see the day 1 video in the Texas Tour Results thread.

On day 2, I remembered this and decided to start with the rebound set on position 3. Sure enough, the car was a good deal less bouncy and the braking was much improved. I then noticed that the rear of the car took a little longer to transition so I added a click of compression which solved the problem with no ill effects.

Now, my thoughts on compression:

High speed bad, low speed good... especially when applied to transitional nature of auto-x. I run a ridiculous amount of low speed compression. I feel it loads the tire faster and does all of the chassis control we demand but doesn't lift the unsprung mass of the ground like rebound. It provides that feel of stability and instant tire loading that we're looking for in transition but with the right blow off force, it allows the sprung weight to still accelerate quickly when faced with a road irregularity. This ability to "blowoff" is crucial and is why I was still able to run high amounts of compression on the bumpy Texas site. It's only when a shock either has a hard time reaching that blow off force or if the high speed compression is too high that we run into problems maintaining traction.

The temporal nature of shocks also allows large amounts of low speed compression to act as a "disappearing sway bar". On entry, the shock is resisting roll as a sway bar or spring might do but a fraction of a second later during mid corner, the shock is finished with it's influence over body roll and allows the springs and sway bars to dictate a roll bias. This effect can be utilized to achieve a car that is stable in slaloms but balanced in sweepers.

Now, putting these concepts to use means you have to have a shock capable of allowing a smooth blow off of the low compression. That's the trick.

Forcednduckshn

Ok good feedback, but just to clarify Nick - these shocks are NOT the single adjustable version, they are the double adjustables. The shock dyno plots you are looking at show the full "stiff" settings for both Compression and Rebound together, then the full "soft" settings together, i.e. for the graphs you are looking at, 1 shock dyno run was made using the max setting adjustment for compression and rebound and making a plot, then a second run was made using the full soft settings for compression and rebound.

Also understood that they don't have a magic spreadsheet, that's why I'm curious to hear feedback from guys like you that have more "practical" setup knowledge. I want to hear your thoughts on under vs over damping for stock class autoX. From your last paragraph, you are of the mindset that "overdamping" or "trying mimic more spring rate" is bad for an autox setup, correct?

Forcednduckshn

Agreed, that's what the research shows- guys try to mimic spring rate. Some love it, some hate it, both show examples of fast cars/drivers that are in separate schools of thought. Thanks for the link, I'll check it out.

mavm86

This page of Corey D's shock thread on SCCAForums has some interesting stuff in it (mainly the shock dyno sheets from his Penskes, Drew V's shock dyno sheets from his Penskes, and some interesting comments from Jason Collett, Steve W, and Andy Howe).

Interestingly, both Drew and Corey's Penskes had a TON more rebound than the above posted Konis, although Steve W commented that he thought this was more rebound than ideal.

alvanderp

65% critical is almost completely irrelevant on a stock class auto-x car. As others have noted, adding a bunch of low-speed rebound can compensate for the lack of spring rate in a stock car. Wynveen and I had this discussion back when I was still running in Stock, and we both ended up with rebound settings that were well north of 100% critical.

I think Nick said it in another thread. The only way you are going to figure this out is to set a baseline, make some runs, turn the knobs and make some more. I've found vastly different setups on different cars and it all has a lot to do with what a driver wants. The calculations can get you in the ballpark, but don't be surprised if you end up a ways away from what some people might say is "optimal"

And yes, as noted in the above thread, my shocks had way too much rebound, I never got anywhere near the upper end of the adjustment range. I'd like a bit more front compression though.

To the OP - did you get a dyno for all 4 shocks? I think you'll find that they vary a good bit, and you'll need to adjust them differently to get the same amount of force left to right. At least that's been my experience with Koni Yellows, even ones they have rebuilt to the "RACE" version.

TheNick

You are going to be overdamped. Thats what is necessary to make the cars work. Is it good? If it makes the car faster - then yes.

Overdamped rebound CAN lead to a loss of traction on the inside tire - will it? Thats a question you have to answer with your testing. Overdamped low speed compression CAN lead to skipping and a lack of grip from the outside tire....will it?