Handling balance/setup question

[EarlySport]

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.

[gestalt1]

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

[911st]

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.

[TimT]

I'd like the car to have less turn in oversteer and more turn exit oversteer.

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...

[911st]

There you go!

[RaceProEngineer]

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

[EarlySport]

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,

[Flieger]

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.

[fredmeister]

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.

[RaceProEngineer]

Originally Posted by fredmeister

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).

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

[sbmackie]

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...

[Trackrash]

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

[911st]

Lowering the front effectively adds neg camber to the front.

[Trackrash]

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.

[911st]

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.

[cstreit]

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.

[911st]

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?

[RaceProEngineer]

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

[911st]

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.

[RaceProEngineer]

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