cgfen

I’ve read a lot of information regarding suspension tuning and chassis setup, probably too much to really understand the dynamics involved.

I keep reading about using ARB’s to “fine-tune” a balanced car for specific applications.
My specific application = How do I optimize front / rear grip for AX by adjusting ARB’s

My car;
1. 96 2S, weighs ~ 3300 pounds with me in it
2. 70% street car / 30% AX / DE not a daily driver, more of a weekend toy.
3. Lowered to RS ride height + 6 mm
4. Good street tires 245 – 35 front 295 – 30 rear on 8.5" and 11" wheels
5. Bilstein P’10’s with P’10 springs ROW Turbo 22mm front ARB stock 17mm rear ARB
6. Running -2 degrees +/- 0.2 degrees camber front and rear toe set at effective zero

What I’ve learned;
1. Moving from a 17mm to a 21mm rear ARB caused dramatic oversteer at an AX, my perception, typically caused by my poor technique, (lift-off oversteer), I went back to stock 17mm bar till I learn to drive better and stop scaring myself.
2. Changing front coilovers from stiff to softer generally reduces turn in understeer.
3. Changing rear coilovers from soft to stiffer generally increases oversteer, (easier to get rear to rotate).

I know that the biggest change I can make to my car’s setup to make it better is to buy the driver more seat time, but like many of us I get caught up in the buying-new-hardware syndrome. I’m currently thinking of buying 993 RS adjustable ARBs. Front = 21mm 5 way and Rear = 19mm 3 way.

Questions;
1. Is adjusting ARB drop link anchor point roughly equivalent to adjusting bump setting on P’10s?
2. How? Does more bump = stiffer ARB and less bump = softer ARB?
3. Is / are adjusting ARBs more effective that diddling with coilover bump? I would think yes because ARBs affect roll couple and coilovers control wheels in dynamic vertical deflection.
4. What is your go-to resource for Suspension Tuning Theory? I’ve read Fred Puhn’s book and many many web pages, (including this forum).

I’ll stop now. Looking for some interesting / informative discussions

Thanks
Craig

Bob Rouleau

I think you're over thinking this one. Softer means more grip. So if the car understeers you should soften the front by some means, easiest is the take advantage of the sway bar setting. Looking at it simply, the car understeers because there is too much grip at the rear compared to the front. So add grip at the front by softening the front sway bar.

If the car still understeers, make the rear stiffer so that the back end will have less grip - this will help balance the car.

Not sure what you mean by bump settings. Setting compression higher , i.e. takes more force to compress the shock can help tune out under/oversteer but adjustable sway bars generally do a better job unless you want to start changing spring rates to match the shocks. Not sure if your reference to "bump" is the same as compression. If so, it might help but the sway bars should be your first step. Oh, and learn how to come off the gas gradually

Keep in mind that even a well balanced 911 will understeer into slow corners. That's why God invented trail braking ... but you need to come off the brakes very progressively to avoid a spin.

cgfen

Thanks for the comments and help.

Craig

Larry Herman / Veloce Raptor / Jgordonsenior / GaryR / Others more comments?

Bill Verburg

you are fighting yourself
the big rear wheels and tires increase understeer/decrease over-steer, i'd leave the fronts alone but go to a 285/30 in back and have the car as low as you can live w/.

making the front shocks stiffer makes for more under steer/ less over steer
making the rear shocks stiffer makes for more over steer/less under steer

adjustable sways are great because then you can experiment at no additional cost, JMO, but RS sways are worth it.

the biggest change will come from springs, what rates are you using?

Larry Herman

You know that one can write a book just answering your questions. In a nutshell:

1) You pick your springs to control acceleration squat and brake dive
2) You then size your sway bars to add in the amount of roll stiffness that you want
3) You set your camber to maximize the grip of the tires
4) You adjust the roll bars to give you the overall balance that you are looking for
5) You then adjust your shocks (if they are 2 way) to fine tune the balance on corner entry and exit

One important note here specifically about your 993. Make sure that your Kinematic toe is set to no more than 1.5 degrees, and you do need to run a little static toe-in in the back. I run 1.5mm per side on ours as measured from front of rim to back of rim on an 18" wheel; that is about .2 degrees per side. If the Kinematic toe is set higher, it will constantly be trying to toe itself in under acceleration and you will be battling that until you add so much rear bar that it snap oversteers. I would suggest that you do that and try it out before you continue to chase your setup.

pontifex4

This doesn't answer your question, but in my case, when I started having trouble with lift-off oversteer with a big rear bar, I found that I was able to prevent it by consciously looking for the corner exit as soon as I could. I'm still inexperienced, but it did cure me of the "panic and jerk" reflex.

obsessedone

Think of sway bar as the primary or macro adjustment and adjustable shocks as micro or fine tuning - get the sway bars right first. With the oversteer you described, softening the rear sway bar helps by helping keep both rear wheels on the ground for more traction (stiff bar tends to lift inside wheel on hard cornering) - either by going to a softer bar as you did, or by changing to softer setting on adjustable bar.

M758

Larry is correct. Step 1 is spring rates. Those need to be correct to get the balance you need. However sway bars work great for fine tuning. Ideally you use adjustable bars. That is when they come in to their own. You can make many small adjustments between sessions to fine tune the balance. If you find that you max out a bar by doing this you need more spring rate.

Remember that sway bars are like springs. They do not control squat or dive, only side roll. Also remember that very stiff bars with soft springs can cause odd handling. When you corner with stiff bars the inside wheel tends to lift because the bar is actually pull it up to compensate for roll. So roll stiffness from sway bars is not the same as roll stuffness from spring rate. Even so adjustble bars are handy since you can adjust them very quickly. I once found 0.5 seconds on a 1:14 lap with a small sway bar adjustmet.

Now if you have not fully learn to feel balace in corner entry ,mid corner and corner exit you will not have a good chance to tune the car. Plus you need to seperate driving style from chassis balance. In some places a sway bar adjustment makes the car slower overall until you adjust your techique/line to compensate. Once you do go faster overall.

jrgordonsenior

Craig there's lots of good advice here from some very experienced drivers....
No one's mentioned it, but setting up a car for AX is different than setting one up for a high speed track. You need lots of mechanical grip up front and experience to keep that rear end from jumping out. I too think you're probably fighting the limits of the PSS10 spring rates which are around 250f/375r IIRC. Those dampers are a significant improvement over the PSS9's, but your 3300lb. 993 diving into AX type corners needs more spring to adequately transfer weight and keep the car stable. I woudl also suggest more neg camber if this isn't a daily driver, and definiely some toe-in at the rears. You need to find someone who really understand's the impact of the kinematic toe adjustments on a 993. I'd suggest Jae Lee at Mirage...

cgfen

Thanks for all the thoughtful replies.
sorry, can't reply at length, lucky you, due to poor connection while traveling.

It seems the consensus =
drive more
look at correct spring rates for my application
fine tune using ARBs
i really like the "consciously looking for the corner exit as soon as I could" advice.
Jae @ Mirage does my alignments and set K toe $$$$ last summer.

cheers

Craig

Larry Herman

Interesting. Jae charges $$$$ to set the K toe?

To anyone in the Northeast, I will do it for only $$$.

bobt993

Larry, everyone knows $$$$ is faster than $$$

ausgeflippt951

Personally, I would focus on becoming a more experienced driver before really diving deep into suspension stuff. It can get ridiculously hairy, ridiculously quickly.

For the full-on race team, communication between the race engineer and the test driver are absolutely crucial; if the test driver can't communicate what he is feeling from the vehicle chassis (or the engineer can't coax the info out of him properly), then the car will remain a poor performer.


For the privateer (be it autox or high $$$ club racing), the race engineer and test driver are often one in the same person. This means that you, as the vehicle owner, need to be able to smoothly driver the car at its limits and identify areas in need of improvement. From there, you must have a solid understanding of vehicle dynamics in order aptly make adjustments.


My point is this: I've seen more than a few car suspensions improperly tuned because the driver tuning it was not suitably experienced. Additionally, a truly good driver can make more out of a poorly-tuned chassis than a poor driver can. And through learning to be a better driver, you automatically learn many of the principles surrounding chassis dynamics! It really helps, when you are knee-deep in your suspension tuning book, and it mentions something and you say, "Hey, I know what the author's talking about because the same thing happened to me at the DE last weekend."



I just read through this post, and realized that it may come across as a bit disparaging and/or negative. Absolutely no offense meant; I'm just running on little sleep today...eesh.

Veloce Raptor

Sorry, you specifically asked for my opinion & I just saw this thread now.

Larry's comments are generally what I would say. In addition, his #1 should also mention that spring rates also predicate to some extent how muuch body roll the car will experience in corners. This is why many full race cars have ridiculously high spring rates compared to what you are likely running. This will also affect grip, and can be an especially important issue in conepacking (AX) where quick transitions are key. You have to be careful, running a staggered tire set up, that you don't overadjust your settings. You will never get rid of entry understeer in a 993 in conepacking IMO. The key is adjusting things (and adjustments also include tire pressures, which no one mentioned) so that getting the rear to rotate (ie, stiffening it up) does not make it so stiff to prevent you from getting back to hard power quickly.







Professional Racing and Driving Coach

cgfen

just re-reading this thread once again.
even though I've learned a bit in the past 3 years, this info reinforces what I've been experiencing.

cheers

Craig