Over-steer or Under-steer... that's the question...

[MNBoxster]

HI,

Both Topless and John V, while spot on, are describing the means with which to over come Understeer. But the Car, as set-up by the Factory, does have an inherent understeer.

The two most important factors that influence understeer and oversteer are Weight Distribution of the car and Roll Resistance which is varyable by selective spring rates. Having a basic design influence on these factors are front and rear Roll Center locations and Center of Gravity location (the height of the CG, or CGH) .

All factors affecting the car's handling act through the tires to the road surface. The most important tire characteristic is its development of a Slip Angle when lateral force (or accelerations) are applied. All pneumatic tires deform while cornering to some degree. They will follow a path between the steered angle and straight ahead.

Slip Angle is the difference between the line the tires are steered on and the line they actually follow. Slip angle is actually a drift angle. The angle of tire slip can depend on speed, lateral acceleration, vertical load on tire, coefficient of friction of the rubber, and tire pressure.

If the outside rear tire has a larger Slip Angle than the front outside tire, the car will oversteer. If the outside front tire has larger Slip Angle than the rear tire, it will exhibit understeer.

For example, a front heavy car will oversteer because the outside tire on the front will require a larger slip angle to handle the heaveir weight loading. Increasing the roll stiffness (with stiffer spring rates) on one end of a vehicle will yield a large Slip Angle on that end of the car. If the Roll Center is raised on one end, it will increase the load transfer and thus the Slip Angle at that end of the car. Varying tire pressure will also vary the Slip Angle. Lowering tire pressure will lower the load carrying capacity of a tire, so slip angle will be increased, conversely increasing the pressure will have the opposite effect.

Happy Motoring!... Jim'99

[John V]

Jim you always have to think in terms of dynamics and weight transfer. When driving spiritedly, you're always either braking into a turn or accelerating out of it. The overall balance of the car determines how it handles this combination of inputs.

If one were to create a car which was perfectly neutral during steady-state cornering, it would be so loose as to be dangerous. Nobody would even want to race such a car because it would be so difficult to drive.

So overall, I disagree in that I don't think the Boxster really has any inherent built-in understeer. More frequently, the driver does.

If my car exhibits any specific characteristic, I'd say it is more on the over steer side. Do you think this is due to my 19" tires? Front strut brace? Tire pressures? Driving style...or lack there of?

[Topless]

Originally Posted by bmussatti

If my car exhibits any specific characteristic, I'd say it is more on the over steer side. Do you think this is due to my 19" tires? Front strut brace? Tire pressures? Driving style...or lack there of?

All of the above can affect oversteer. Just curious... why the front strut brace? The 987S is an amazingly well balanced car right off the lot. There are a couple of guys around here who in stock trim regularly outrun all but the GT3's in the cones. On a tight course the 987S is near perfect.

Originally Posted by Topless

All of the above can affect oversteer. Just curious... why the front strut brace? The 987S is an amazingly well balanced car right off the lot. There are a couple of guys around here who in stock trim regularly outrun all but the GT3's in the cones. On a tight course the 987S is near perfect.

Hi Topless, I did it, just to do it. No real reason. It was a $350 gift to myself. And, a few guys on the Forum spoke about the added benefit to our cars.

[Topless]

Originally Posted by bmussatti

Hi Topless, I did it, just to do it. No real reason. It was a $350 gift to myself. And, a few guys on the Forum spoke about the added benefit to our cars.

As you may have guessed from my signature I am somewhat of a purist regarding Porsche performance. I am a firm believer in "If it ain't broke don't fix it." regarding these cars. They really got so many things right the first time. I am also keenly aware that I am in the minority on this board.

From this perspective If I were in your shoes I'd: Restore it to stock setup, and get to a track with someone who really knows how to drive these cars. I'd spend some time on a skidpad first and discover my traction limits and characteristics. I'd practice creating and correcting understeer/oversteer until I really understood its causes and cures. Then I would begin to work the course. Nice cleanly carved carousel turns or am I pushing and sliding... balanced planted slaloms or wild unsettled cone bowling... gentle controlable 4 wheel drift in off camber turns or dangerous fishtails and spins.

After spending some quality time with the car a picture should begin to emerge in the relationship between car and driver. Now I am in a much better position to decide if changes to the suspension and setup are in order. Maybe different tire pressures, maybe more neg. camber, maybe I do need that heavier sway bar up front. Now I have a really good baseline to work from, I really understand the car and how these changes will affect its handling on my way towards my ideal setup.

These cars roll off the lot far more capable than most of us will ever be as drivers. What I am suggesting is that by modifying the driver first, much greater performance gains are to be had than by attempting to modify the car blindly.

[MNBoxster]

Originally Posted by John V

Jim you always have to think in terms of dynamics and weight transfer. When driving spiritedly, you're always either braking into a turn or accelerating out of it. The overall balance of the car determines how it handles this combination of inputs.

If one were to create a car which was perfectly neutral during steady-state cornering, it would be so loose as to be dangerous. Nobody would even want to race such a car because it would be so difficult to drive.

So overall, I disagree in that I don't think the Boxster really has any inherent built-in understeer. More frequently, the driver does.

Hi,

Well of course I think in terms of Dynamics. Oversteer/Understeer is a dynamic phenomena, you cannot have it if the car is static.

But, you can have the car statically setup to produce either condition by the selection of Spring Rates, Tire sizes and pressures, Anti-Roll Bar size and deflection, etc.

The Boxster uses a larger Rear tire than Front as an OEM spec., this will create understeer. Higher Tire pressures in the Rear than the Front, again OEM spec and inducing understeer. It uses a smaller Anti-Roll bar in OEM spec than say the M030 suspension, again favoring understeer. Chassis Ride Height is higher in the stock setup than say the M030 one, again creating a higher CGH and thus greater understeer.

It doesn't matter that you think or feel the car doesn't understeer, it does, it must, that's the way it was set up. Chassis Dynamics is a well understood science with predictable results. The US Market suspension settings, in fact, including those of the Sport Pkg., favor understeer much more than those of the ROW and ROW Sport Pkg. models do, just look at the differences in the alignment specs.

I'm not saying you cannot defeat or overcome this tendency toward understeer by changing some of the variables, or by driving style. What I am saying is that if you don't, the car will understeer...

Happy Motoring!... Jim'99

[John V]

Originally Posted by MNBoxster

The Boxster uses a larger Rear tire than Front as an OEM spec., this will create understeer. Higher Tire pressures in the Rear than the Front, again OEM spec and inducing understeer. It uses a smaller Anti-Roll bar in OEM spec than say the M030 suspension, again favoring understeer. Chassis Ride Height is higher in the stock setup than say the M030 one, again creating a higher CGH and thus greater understeer.

I think you need to come drive my car, Jim. No understeer here. Maybe I got a "magic" one! Come to think of it, two of my buddies have (nearly identical) '03 3.2L cars, and neither of theirs understeers either.

Oh well, no worries.

[z12358]

Mine just goes where I point it. No over, no under, just perfect.
That is, when it's not stored for the winter.

[Sammy]

Originally Posted by John V

I think you need to come drive my car, Jim. No understeer here. Maybe I got a "magic" one! Come to think of it, two of my buddies have (nearly identical) '03 3.2L cars, and neither of theirs understeers either.

Oh well, no worries.

Try taking the car to a skid pad and see what happens...

[John V]

Originally Posted by Sammy

Try taking the car to a skid pad and see what happens...

I can go around a skidpad in steady-state understeer or steady-state oversteer. Does that mean the car oversteers AND understeers?

The point I'm trying to make is that there is a lot more to it than saying "this car understeers" or "this car oversteers." What are you doing when it over/understeers? Braking and turning into a curve? Going through the middle of the curve? Accelerating out of the curve? Going in a bit hot and decelerating / braking IN the curve? It's not as simple as people are making it out to be. With the Boxster (and several other cars I've been lucky enough to drive hard) you have so much control over what the car is doing, it's hard to just label the car as one thing or another.

[Sammy]

Originally Posted by John V

I can go around a skidpad in steady-state understeer or steady-state oversteer. Does that mean the car oversteers AND understeers?

The point I'm trying to make is that there is a lot more to it than saying "this car understeers" or "this car oversteers." What are you doing when it over/understeers? Braking and turning into a curve? Going through the middle of the curve? Accelerating out of the curve? Going in a bit hot and decelerating / braking IN the curve? It's not as simple as people are making it out to be. With the Boxster (and several other cars I've been lucky enough to drive hard) you have so much control over what the car is doing, it's hard to just label the car as one thing or another.

I'm with you John V in that what you feel on a track is dependent on what you are doing with the car; however, if you are doing a skid pad test correctly you will find one or the other. In every case I have driven a production level car on an actual skid pad I have found understeer.

[MNBoxster]

Originally Posted by John V

I can go around a skidpad in steady-state understeer or steady-state oversteer. Does that mean the car oversteers AND understeers?

The point I'm trying to make is that there is a lot more to it than saying "this car understeers" or "this car oversteers." What are you doing when it over/understeers? Braking and turning into a curve? Going through the middle of the curve? Accelerating out of the curve? Going in a bit hot and decelerating / braking IN the curve? It's not as simple as people are making it out to be. With the Boxster (and several other cars I've been lucky enough to drive hard) you have so much control over what the car is doing, it's hard to just label the car as one thing or another.

Hi,

You have made your point and no one disagrees with you at all. A Driver can, through the modulation of Throttle and Brakes change the dynamics of the car.

But the Boxster, or any other car can be setup to favor either understeer or oversteer. As is the case with virtually all street cars, the Boxster is setup (through the selection of Tires, Tire Pressures, Spring Rates, Anit-Roll Bar Thickness and deflection, all Alignment Settings, etc.) to understeer - period.

You can go on and on about how this effect can be mediated by the driver, but two points need be added. A driver must be experienced and knowledgeable in order to be able to overcome the car's natural tendency (as setup) in a controlled way, as you predict. Most drivers are so experienced or knowledgeable, nor does the manufacturer make any such assumption that they are. So an inherent amount of understeer is dialed-into the car to begin with. And, if an average driver takes the wheel, his control inputs will generally cause the car to understeer as was the Manufacturer's intent.

If you understand suspension setups as much as you appear to, just looking at the OEM suspension alignment settings will prove to you that what I'm saying is correct...

Happy Motoring!... Jim'99