Hi,

as it`s almost 20 years now since I graduated I`ve managed to forget pretty much entire Mechanics of Materials course and I kinda struggle to understand how do we change car`s behavior with sway bars tuning.
Very simplistic approach shared pretty much by everyone is that stiffer front sway increases understeer, stiffer rear increases oversteer.
But by how much exactly and with what side effects?

To look at any sway bar we have essentially an adjustable lever.
So does anybody know - say, if we take stock non-adjustable C2 or C2S 997 bar and compare to stock GT3 bar, TPC bar, GMG bar - how exactly stock C2S bar stiffness compares to those bars when they are in 1st hole (softest) position and stiffest position? What is the actual range of values there? Is it even disclosed anywhere? Everybody knows about springs stiffness - is same info available about sway bars? Or is it even relevant?

I am not sure how TPC sets their bars but I have heard GMG bars are 1.5 stiffer than GT3 stock ones - is it correct? If it is correct - so, how exactly it translates into car`s behavior - I was always under impression that sway bars stiffness has to be precisely matched with springs as it is a same single system, so to say, and then in case of after market bars we simply take it out of balance. I just want to believe Porsche engineers actually had done some computations and modeling when they created their suspensions, and all elements are configured to compensate each other.

Is there anybody here who knows this subject well enough to explain all this? As most typical responses from tuners are - trust us blindly, we race those cars forever and have a tons of empirical knowledge of what exact stiffness we need. It is all nice, but what actually happens when we take out an element from a well balanced system and replace it with 1.5 stiffer one?

also, more practical question. gt3 front bar has 5 holes, rear one has 3. If I have rear one now at middle hole and want to move it to hole 3 (stiffest) - to what hole should I move front bar considering it was on 2rd hole (from softest) now if I intend to keep same or as close as possible understeer/oversteer balance as it was - should I move front 1 hole or 2 holes closer to stiffest?

 

Bars are selected for intended situation so one size fits all is not the case. Just stiffing bars are not always the answer. They also eliminate some of the effects of independent suspension by transferring some of the effects to the opposite tire, thus a stiffer bar makes for a rougher ride and could even reduce traction on a bumpy surface. Ideally a car would be best without any bars if it didn’t have to turn.

Lowering the center of gravity reduces the needs for sways.

Stiffening a bar actually reduces traction on the front or rear so stiffing the front gives it less grip. Increasing spring rate can be more effective in many cases.

Porsche sets the suspension for what they feel has the least compromises for intended purpose. This is why the GT3 has stiffer bars springs and dampeners. They are also adjustable.

The one place where modification is usually in order is, IMHO, all cars tend to have more under-steer dialed in at the factory to compensate for safety reasons. I personally prefer my cars closer to neutral with just a little under-steer.

In general I find the 911s to be set too much toward comfort and not enough for performance so increasing spring, shock and sway rates are in order. GT3s are very close, IMHO to ideal for most roads and work well enough on most tracks. Handling can be changed a great deal by changing ride height, front and rear bar settings, and alignments.

If you were to spend more time on the track vs the street it may then justify changing components.

 

i could be way off, but i was under the impression that stiffening the front sway bar and softening the rear bar setting reduces understeer, but with the side effect of increasing the chance of the rear end sliding out (oversteer). the OP indicates the opposite.

at any rate, since each car is different based on tires, weight reduction measures taken, ride height changes etc., on track testing is the only way to truly determine correct settings for you and your car.

 

I see the only practical way is to try different settings until a some sort of optimal setup is found.
So essentially what I am trying to understand here as well is this: we have 2 variables from sways - a ratio between front/rear sways that controls to some degree understeer/oversteer PLUS we have absolute numbers of front/rear stiffness, as we can dial up at least 3 different combinations using diff. holes in rear bar with corresponding holes in the front bar.

So probably real question is - what setup works better and when, and what are primary factors to make such a decision. I am probably trying to put a theoretic determination for an issue that requires experimental results only to be solved... may be. Still it is kinda sad that there is no actual stiffness numbers for those bars nowhere.

I can see how my car`s behavior changes when I use 2nd from softest front hole or 2nd from stiffest hole in combination with middle hole in rear bar. Still I do not really understand what sort of experiment i should conduct to figure out if I should dial out rear end into softest sway position with corresponding softest front bar setting or go into opposite setup with stiffest rear bar settings and play with front to adjust it for neutral understeer/oversteer.

Plus tires sizes between street and r-comps also change this a lot and I do not like to re-adjust sways to often.

 

stiffening sway bar on any axle reduces grip on that axle. that is what I was told and what I have read about. if that is incorrect it would be a totally new info.

but again, it is not only about ratio between front/rear bars stiffness, absolute stiffness number also plays some role and that is whatI am trying to understand - like, why exactly do people replace stock gt3 bars with, say, GMG bars and how can it actually work well if those bars are not 'matched' with stock springs.

it does not apply to my suspension as it is totally custom but i hope same rules work there as well, so those generic questions are kinda relevant.

 

If referring to given track, then on track testing is correct.

Balance between front and rear is the thing. Due to weight shift stiffing both bars may in fact increase overall grip, as it spreads more of the weight to all four tires. It is true that stiffing reduces grip in relation to the other; front to back. Optimum grip however may be achieved at a stiffer setting.

You can learn a lot by watching F1. On some tracks softer setting are faster. Im not sure if F1 cars are running sways due to low CG, however the principle is the same. Finding the best balance is often difficult. They have a starting setting for each track they are going to and make adjustments from there.

 

I see.
So theoretically optimal track setup procedure would be to stiffen up rear axle sway bar to its max and then check what front bar settings will result in optimal neutrality and if it`s not achievable (what is kinda unlikely) then try to reduce rear sway stiffness to mid-hole and try again, right?

right now i have -3f deg -2.5r deg camber and with stock gt3 sways front is set at second from stiffest hole (just did it this morning as rear end felt very loose before), rear at mid-hole and car has some understeer now but it is way more predictable on street tires than a lot of oversteer so i hope if I can simply alternate rear sways to stiffest position to change it into neutral or slight oversteer mode it would solve the problem perfectly as front sway droplinks are kinda difficult to reach.
still i will need to try this setup on r-comps at auto-x first as on street i cannot really say how it will behave in a turn at speed.

 

NO.....the first thing to determine is whether your car is overall loose (oversteers) or tight (understeers) with current settings and address the end that is the problem...trying to adjust both ends at the same time is counter-productive and you will chase your tail all day...just going full stiff in the rear and adjusting the front from there will likely get you severe power on oversteer not more traction

 

There is no optimal setup, just setups for a particular driver and a particular track. The general rule is to adjust the end of the car experiencing the "problem." For example, if you are oversteering you could reduce rear bar, and if you are understeering you could reduce front bar. Alternately, you could do the opposite to the other end, but doing this doesn't mean you are improving the balance of the car, rather, just unimproving grip on the wrong end so the car is manageable. This is an option if you run out of corrective movement on the affected end.

Not to add another variable to the equation, but [depending on the track of course] if you do a lot of trail braking, you might consider softening the rear bar and stiffening the front. You will still get the required rotation under braking, but you will be able to get to power earlier with a more planted rear from transition to exit. That being said, this setup inherently induces understeer, so if you are not trail braking, it will not be faster.

All of this really depends on the driver's ability to read the car and interpret where the car is misbehaving. It's not enough to say your car understeers, you really need to know where - i.e., corner entry, midcorner, exit. AND you have to be sure it's not driver input causing the issue. I've had students complain their car understeers on entry and oversteers on exit - only to find out that they are dive bombing the corner causing understeer and then sawing the wheel under power to create oversteer.

-td

 

So, well, the deal is - I feel I can adjust it more or less neutral using either rear mid-hole or rear stiffest hole. I am not sure I can adjust it well with rear bar in softest mode ever, it is a contant oversteer even with front at softest setting.

But I also feel that I like car a bit better when rear sway bar is set to a middle.

Real dilemma is that I need to adjust car to 3 different modes - street driving on street tires with some understeer, auto-x mode on hoosiers a6 with some slight oversteer and to learn track I need slight understeer as I do not want to loose my *** in a turn there ever. Not sure if what I write here makes much sense but it is what it is. I am trying to understand what is the 'proper' procedure is to make this tuning so I could later apply it to my personal situation.

so you suggest starting with rear bar at stiffest and adjusting front one is wrong - i mean, for initial setup? mechanic left sways in middle one rear and second one from softest in front and on street tires it was quite a severe oversteer.
so i adjusted front bar up to the second from stiffest hole. should I instead have left front at second from softest and reduce rear to softest one?
it is really a bit problematic to understand - I see how one can soften up this stuff to improve grip, but what are initial settings one should start from? and how this 'softening' approach correlate with aftermerket sway bars which are by default stiffer than stock - so it sort of assumes stiffer is 'better'?

 

that is totally true, but i could not really express it all in a single question.

plus i do not really know even a half of an answer to be able to ask that question properly.

 

where you got off base was when you went to changing the front when the rear was the problem...best to start at the end that is the problem until you run out of adjustment on that end...if you went full soft on the rear and still had oversteer then you would have had no choice but to stiffen the front...if you run out of adjustment you might need more toe in for the rear...but as suggested above there is no one solution...different drivers will require slightly different setups to suit their style and street, autox and track will all require slightly different setups and alignments...you also should be much more aware of tire pressures now that you are tracking...street and autox will not get the tires pressures up like track driving will and you need to determine what your level of aggressivness and driving style will do to temps/pressures and balance the car that way too...you could have reduced your rear tire pressures 2.5 pounds and the oversteer might have gone away without any sway adjustment...there are many variables and you need to get a view of the big picture and not be so narrow in your focus...attached is a basic chassis tuning chart that gives you more approaches to reducing oversteer

 

i see. thanks.
i think i understand the general idea now a bit better. still a kinda confusing step is - how to determine what original front bar setting should be used and why? it has to be some starting point, right? or just arbitrarily set both bars to the middle holes and then start tuning?

 

well, to determine your starting point use your own criteria...you said for track driving you would like understeer so set the rear one softer than the front (middle in front, one softer in rear)...just to be sure you are going the right way..holes closer to the bar are stiffer...if it has excessive understeer move the front one stiffer until you get balance or run out of adjustment (all during this process you are checking tire pressures cause if your pressures get too high they can throw off the handling)...keep in mind that all this assumes a proper corner balance and alignment suited to your driving style to begin with because camber and toe adjustments that are not suited to your style can prevent obtaining balance by tire pressures/sway adjustments...it's a long, tedious process but once you find it for a particular track you will know it...be sure to log all your changes/adjustments so you can use them in the future and then mostly only worry about tire pressures when you arrive at a track..

 

Softening the front bar transfers the grip balance to the front end, inducing oversteer. If you're trying to reduce that, you need to stiffen.

In general:

Corner entry push, soften front bar.
Corner exit push, stiffen rear bar.
Corner entry loose, stiffen front bar
Corner entry loose, soften rear bar.