Handling balance/setup question
 

Hi,

I am running a widebody 3.6 powered 911SC at a variety of tracks here in Australia. The car is light ( all fibreglass panels etc - 2100 lbs ). It's currently running 21mm / 28mm tbars, stock SC sways, Bilstein sport rear shocks, and regular Koni adjustables front set about 3/4 to full hard. I'm running 225/265 Kumho V700's, -3 deg front camber, -2.5 rear, zero toe front, 1/8" toe in rear.

I'd like the car to have less turn in oversteer and more turn exit oversteer. To put it another way, less front grip on turn in, more front grip on turn exit.

Is what I'm chasing impossible? I am open to changing T bars, sways, or front shocks ( have koni sports and bilstein sports available ).

Thanks for any advice.

to decrease grip on the front you would either soften the rear sway or stiffen the front, because you are using stock non-adjustable sways i would try removing the rear sway and see how it drives. if it is in the right direction (probobly too much) then a front larger sway would do the job. another option is to put larger rear torsions (about 29-30mm). many racers end up with an 8mm difference in torsion sizes front to rear, stock factory 911s typically are 5-6mm difference.
hope this helps!
-matt

You say you want less bite at the beginning of a corner and less comming out.

Unfortunatly that is a 911's natural tendeacy but it can be managed to a degree.

Not an expert but have some thoughts.

Driving style has some bearing. As you probably know, under braking or throtle lift weight shifts to the front gaining more traction there. With this the rear losses traction. With acceleration (out of the corner) the rear beinfits from weight transver and gains traction and the fronts then is reduced and wants to push. Thus, for many drivers a 911 wants to over steer comming into a corner and under steer comming out.

If you trail brake into a corner or momentum drive a corner this will add to initual over steer. If you do you braking first and are on the throtle some as you enter a corner, this will lessen initual indersteer.

Of cource adjustable sways are the best way to change the front rear ballance. Worth looking into expecially with your set up.

You say you have adjustable shocks. You might try stiffening the front and softening the rear. This might transfer some weight to the inside rear tire for a bit at the begining of a turn and help a bit to increase rear hold into the corner at first turn.

21/28 is on the front soft side (but maybe not with your heavy motor) meaning the front should work a bit better or bite more. It is also on the soft side over all for a full out race car. Many run 911 race cars will run in the 23 or 24 front /33 rear.

Further, up front you are running -3 deg camber which would let the front bit hard at first also. There are two things you might try here.

First, you are probably running a bunch of Caster like most of us at about 6 deg+. With this much caster you are getting even more camber as you turn the wheel. Starting at -3 you might have as much as -4 deg or more camber in a turn which could start to reduce the front bite. As you know to much or to little camber hurts front traction.

If I had or could get that much camber I would try dialing caster back as much as possable or to 3 deg which ever is greater and see how that effects bite in different parts of the corner. The side beinfit is it may make your car less physicall to drive (less self centering force).

The second way would be to just dial back front camber to -2.5 or even -2. This might lessen front bit at first and increase it with the angle steering wheel in a turn. Play with the camber untell you have the most hold possable comming out of the corner. You might also try upping rear camber to -3 deg and that might help rear bite in total.

Last, a limited slip or lack of one can have effects on how a car handles a corner.

LOL your wish is your command...

A lot of what you describe can be done with the pedal on the right...

Set your car up with good balance.... and use the go-pedal to rotate the car etc...

There you go!

EarlySport,

One of our old Formula 2000s is now racing in Australia. (I have no idea what that has to do with your topic!)

A nice, wide variety of suggestions and things to try so far. And I agree with the others that it is a shame you do not have ADJUSTABLE anti-roll bars with which to work. On the other hand, A LOT can be accomplished with adjustable Konis!

Here are a few "guidelines" that we keep in mind when working on setup:
[1] If a car understeers AND oversteers, work to cure the UNDERSTEER first. Once that is satisfactory, then tackle the oversteer (if it still exists).
(HINT: One adjustment on your car that will help with BOTH will be increasing resistance to COMPRESSION on your Konis.)
[2] To correct corner exit understeer, you (generally) want to focus on the REAR suspension.
[3] To correct corner entry oversteer, you (generally) want to focus on the FRONT suspension.
[4] NEVER overlook the effect of tire pressures, and their resultant changes in effective spring rates.

Lastly, it is perhaps a matter of symantics, but you wrote, "To put it another way, [ I need ] less front grip on turn in, more front grip on turn exit", to which you received a couple of suggestions that will accomplish just that - LESS front grip! We would cast the problem in a slightly different light : You need MORE REAR grip on turn in, and more front grip on turn exit. We are never striving for less grip on either end.

Send us a PM if you want more specifics.
Ed LoPresti

Thanks for the quality feedback.

I do intend to fit adjustable sways, and have some TRG's ready to go on. I was also hoping to grab a tyre pyrometer and see if I can get some meaningful data from it regarding camber setting ( I'm aware of the difficulty in grabbing readings quickly enough ).

What I don't know much about, and Ed has hinted at, is shocks. I suspect my Bilstein sport rear and Koni ( non sport ) front are somewhat mismatched.

I am also aware that driving style plays a big part, and I can manage the turn in oversteer, but I can have the throttle wide open on corner exit and it'll understeer - the tail won't step out.

So I guess my highest priority is to try and increase front grip on corner exit.

(Oh, and just to clairfy some of my suspension settings - this is street/track car. See my thread on the build up by searching for 'Aussie IROC'. Hence the softer than full race setup).

Thanks,

Corner exit understeer will only be amplified by romping on the throttle. You are shifting weight back to the rear tires and that squat lightens the front end. Your front tires no longer grip and the rears dig in so there is no tail-out attitude.

wouldn't increasing the front rebound damping help him with this?
i am no expert and suspension tuning is indeed an art with so many variables to play with and (mess things up).
however, try changing one thing at a time and see what it does is key. Then move on to the next idea. always keep track of where youre starting form to go back if you make things worse.
Also you mentioned a tire pyrometer. this is something you should nail down first and get the camber settings and tire pressures nailed and take them out of the equation. once you find the camber required to keep the tires working to their potential, then move on to playing with other items like sway bars and damping.
at least you will be having fun while doing all this work....beats sitting behind a desk all day.

Well, fredmeister, you claim not to be an expert, but here you are most certainly giving expert advice! Bravo! Now we are getting somewhere . . . .

Sway bars
 

I have a similar setup (except for the alignment settings) on my car with the exception of an adjustable front sway bar from Tarrett Engineering. I adjust the front depending on how tight the track is, going looser for a tighter track (autocross, less understeer in the hairpins/Chicago boxes) and tighter (meaning stiffer) for a more open track, so the tale doesn't hang out exiting a fast corner. It takes like 10 minutes to do the adjustment.

T-bars are 21-29, sport Bilsteins, "aggressive" stock alignment, camber bar.

Side note, the rear is also a Tarrett adjustable, but I don't mess with it. I will probably have to once I put the LSD in :)

Scott
Lived for three glorious years in Castle Hill, worked at RAAF Base Richmond. Geez, I could use a slab of VB...

One thing that helped my car's front grip on exit was lowering the front ride height a tad.

Lowering the front effectively adds neg camber to the front.

911ST, you are right, but lowering my front also shifted more weight to the front and it changed the angle of the lower front a-arms changing the effective roll center of the front suspension. All of which helped my front grip.

Gordon,

The Porsche Race shop I take my 911 to notes the same effect from lowering the front.

Sense hearing it I have been interested in understanding why.

There might be some effect in that the lower front makes for less air getting under the car.

Lowering should effect roll center and might to a small degree have the effect of softening the front spring rate which might be like softening the front sway bar. Usually the effect on the front's roll center is seen as a negative and it increases roll /sway.

Plus, there is a small improvement with the A arm pushing the bottom of the strut out for a bit more neg camber. The offsetting neg being it can exaggerate toe change with compression (bump steer). At stock US delivery height I do not think one can even get -.5 deg of neg camber. At Euro-Style about -1. At race height up to about -1.25. I am thinking neg camber which gets the front contact patch closer to its best angle is the most significant.

Would not expect there would be any effect on static weight distribution or even weight transfer.

Just my guess.

The good news is your observation seems to confirm there is a real benefit.

My first thought on this is driving style. To get less turn-in oversteer, make your turn in transition smoother. Let the car settle a tad after your braking to get some weight back on the rear wheels. More oversteer on exit you'd have to go slow out (so you can throttle steer?)... Not really what we want to do right? If you up your rebound damping that should help reduce exit understeer.

Chris,

When you say up your rebound damping what do you mean?

Do you mean to do that on the rear, front, or both?

Would that not have the effect of stiffening the suspension. Thus if we did that on the front it would make the front hold less in transposition. Doing it the back might help during transposition. I would guess, only with first throttle when accelerating past the apex.

Just guessing/wondering?

I am certain Chris means increase the resistance to REBOUND on the front dampers. (Those are the only ones that are adjustable!)

But remember our point [2] above: To correct corner exit understeer, you (generally) want to focus on the REAR suspension. IF Early did have adjustable dampers on the rear axle, then we would be increasing resistance to BUMP there, instead of making the front even less compliant.

Now, some of you will ask, "Increase rebound resistance in front vs. increase bump resistance in the rear - Isn't the net effect the same?"

So, let's see where that leads us . . . .

Ed

Ed,

I am far from an expert and look to you for advice.

My thinking is that making a shock stiffer in any way makes that end stiffer for a short period.

A stiffer rebound is going to in effect increase the spring rate as it keeps the spring compressed longer in transition.

I thought that when you stiffen one end, the opposite generally gains traction.

At least for a short period with a shock.

Would not think a shock would help exit speed much unless doing so perfects the shock rate such that it increased traction from better modulation of the adjusted wheels.

Thus, if the adjustment is from a perfect setting to a stiffer one, is would hurt front traction in its own right plus transfer traction to the rear.

If it is from a less than perfect to a perfect, it might increase front traction.

Just guessing, this is out of my league.

I knew this topic had great potential for discussion!

911st - This stuff is certainly NOT out of your league, because you are thinking about it correctly, with perhaps one small point: the dampers do NOTHING to spring rate (pounds per inch). What they do (do) is seek to control the SPEED with which their respective springs compress and expand.

Now, with that different nuance in mind, here is how we would think about the points you are making -

" . . . making a shock stiffer in any way makes that end " slower to react.

"I thought that when you stiffen one end, the opposite generally gains traction. " Agreed 100%, especially for ROLL stiffness.

"Thus, if the adjustment is from a perfect setting to a stiffer one" , we wouldn't make that adjustment. But Early's handling is far from "perfect".

Going back to the car itself, it oversteers on corner entry, and understeers on exit - it lacks traction in the rear entering, and lacks traction in the front exiting. So, forgetting for a moment the 911's propensity to oversteer, it sounds like too much weight is SUDDENLY coming off the rear on corner entry, and too much weight is SUDDENLY coming off the front on exit - excessive PITCH.

Whenever faced with a car that does BOTH understeer and oversteer, we work on the understeer first. Now, if we can DELAY the transfer of weight away from the front, the front tires will enjoy a little more "time" to get the car pointed correctly.

And, controling the rate of pitch in this way will also DELAY the transfer of weight away from the rear tires under braking and turning - perhaps long enough to keep that tail more in line behind the car.

Are there other adjustments we would make if they were available? Probably. But the adjustable Konis are what Early has available right now.

Ed

911ST, I probably should have mentioned that my car has raised spindles (1") in the front and a strut brace. I am also running custom adjustable tie rod ends to minimize bump steer. I am able to get about 2 1/2 degrees negative camber in the front due to a couple of factors. The car is lowered below "euro" height, (both front and rear fenders measure about 24" above the ground) ,I have enlarged the adjustment holes for the upper shock mounts AND I have used the strut brace to JACK the shock towers together. My car is used mostly for autocrossing and so is most of my experience with my car's setup.
My car, like most older 911s tends to oversteer with the power off and understeer with the power on under most low and medium speed cornering. Yes, driving style makes a difference. I have, however, been working on reducing the amout of pitching I have to do to make the car corner.
I am running Koni classics which when adjusted primarily affect rebound. What has helped me is to soften the rear shocks, tighten the front shocks, while loosening the front sway bar. Keep in mind that shock settings mainly affect transitions, like entering and exiting corners. With the rear shocks set harder I found the rear too loose on corner entry. This was OK for REALLY tight corners, but wasn't very good for most others, especially faster ones. A softer rear also helps on corner exit. The reason for harder settings in the front is to control the weight transfer. It seemed to me like my car was picking up the front end on corner exit, which worsened the under steer. Stiffining the front shocks helped this and loosening the front sway helped maintain grip.
As I mentioned before I also tried lowering the front, which helped both turn in and exit noticably.
As for why lowering helped, well I imagine it is a combination of factors.
Which last week were again confirmed on a big track when I participated in a DE.
I had raised the front up about half an inch for more clearance on the street. I figured on the big track I wouldn't notice or need the extra grip in front.
Well after the second run sesson I was noticing that my front end seemed to be "wallowing" around. It felt as though the front end would roll over too much. So I got out my allen wrench and dropped the front end back down like I had it for the last couple of autocrosses. (I marked the adjustmenet screws the last time I corner ballanced the car, so I have some reference point to go on.) Wow, on my next two run sessions the front felt much more "planted" and secure.
Sorry for being so long.
I am by no means an expert on suspensions and handling, I am only trying to share my experiences with you guys.

Thanks for sharing. I love learning about this stuff.

I am a big fan of raising the spindle to try to restore some of the camber curve. Especially on a car that has not been set up extremely stiff.

Sorry but I do not recall most of the above discussion.

What seems to be the best for consistent handling as to the dynamics between throttle lift and acceleration is a good limited slip. Especially the one's that are stiffer on lift than on accel (60/40 or such). A great idea if in fender to fender where you do not want the car to get upset if you have to lift in a corner in traffic.

When one rases the front it effects toe. If you had your toe set at zero when lowered, when you raise the car it will induce tow-out. The will make the car faster to initiate the turn and make the car darty at speed. This might be part of what you are experiencing. Also, raising the front will reduce neg camber and at a faster rate if the spindle has been raised.

Stiffening the front will transfer more weight to the inner rear wheel for more traction up to a point when the other front wheel comes off the ground.

Most the fast track guys are running much stiffer rear suspension with a good LSD's and often a little more stager with something like 225 front and 255 rear.

Every time I see an inside front wheel off the ground I think how much it reduces the neg camber of the inside wheel. Thus, I think at some point stiffening up the rear will make the front stick better.

In my mind a 911 rides fastest on 2.5 wheels. This is because a 911 has 50% more weight on the rear to be contained and most do not have much stagger (225/245's). Thus, one outside front wheel, one outside rear wheel, and 50% of the inside rear wheel for two and a half wheels.

A stiff front is safer I suspect and alows for more room to come into a corner faster under lift or when trail braking. However, a stiffer rear is probably faster comming out of a corner with a good driver that can get on the gas earlyer and exit faster. Especially if that increasses one's exit speed on to the fastest straights. Going fast into a corner dose not seem to be as fast as exiting a corner at the most speed.

Not sure any of this rambling makes much sense. Just how I think.

Any one seen a spreadsheet (how about have and willing to share a good one or links leading to them) that helps with analysis of these variables; helps keep track of set up and set up changes and the corresponding effect. Something us mere mortals can use to tie together different set up charts. I expect such matrixes and variance analysis spreadsheets have been put together and are being used every week effectively by wining teams.

By using such tools the information may be better digested and understood faster, and therefore applied faster to improving the car, and decreasing lap times.

I know some guys can do this for a 911 in their head (as well as turn the wrenches properly and get the desired effect but these are guys that have ran their successful Porsche racing/service businesses for decades with out computers). Just a mere mortal here seeking knowledge of how to get faster, in order to help keep all four wheels of a race car planted and on the tarmac(s).

Regards,

Surely many of you have seen one NASCAR guys use, some others. I'm talking something that helps with Porsche 911, both, as something that helps with set up, and greatly improves teaching how to set up a 911 using a what if, then running at at least two tracks.

Regards,

Well, at the risk of taking Craig's original thread even further afield . . .


Bernard - It is somewhat unclear what you are asking. If the question is, "What changes will make a 911 faster, and which of those will be the most effective," then we can offer a few guidelines.

We can discuss the use of technology, data, spreadsheets, and many other tools, but there are some basics we need to get out of the way first.

[1] Most good teams treat the Driver+Car+Track+Conditions combination as a single entity, and we are tuning that "entity".

[2] Before much meaningful info can even be gathered, we need good consistancy - the driver must be turning laps within ½ - 1½ second variance, lap after lap. Here the use of some simple diagnostic tools, like a pyrometer, can be helpful.

[3] Assuming the driver and car are running near "their" maximun potential, someone needs to ask, "What is holding this car back? What will allow it to go faster still?"

[4] It is our long-held belief that the car "tells" the driver exactly what it "wants" in order to go faster. It is the driver's job to "listen", and interpret. Craig has obviously gone through this process to arrive at his original set of questions.

[5] Based upon what the Driver+Car "want" (at that Track, under those Conditions) in order to go faster, we make changes - less weight, more power, different gearing, better brakes, suspension modifications, different rubber compound, aero settings - the list is virtually endless.

[6] Naturally, as you suggest, each modification is noted, along with its resultant lap times. And after each "want" is basically satisfied, we pose the question again: "NOW, what is holding this car (and driver) back?"

[7] In this way, a team (or individual) can start to collect "data" in the form of two-way-trends: (A) To remedy mid-corner understeer (in the dry), we increased resistance on the front anti-roll bar. (B) When we increased rate on the front bar, we had less corner-entry oversteer.

[8] These trends can then become predictive. "The last time we switched to the 430 compound, we lost a second per lap. Can we gain that back by NOT having to stop for tires?" Maybe a good use for your spreadsheet, but each Driver-Car-Track-Conditions results will be different

Naturally, there are certain changes or adjustments that will be universally beneficial (like losing weight or gaining power), and many that will be a total waste of time and money. But generally speaking, this is a long, involved, trial-and-error process. And once the driver can no longer identify what the car "needs" (or it has reached its potential within the Rules), it might be time for data acquisition.

This may be too simple an explanation for what you are asking, but it is a process that most of us follow for as long as we race.

Ed LoPresti

What about chassis stiffness? I was surprised at how much the handling improved on my 84 Carrera when I installed Wevo motor and polyurethane trans mounts. I'm just a canyon carver, but the improvement was noticeable for me. It reduced my under steer (especially coming out of the turn).