Oversteer increases as your 911 goes faster...

[moneymanager]

Does it? Or is it only that your ability to deal with it declines? My guess is it's real and that it's all driven by aero, not suspension considerations.

[Dave at Pelican Parts]

It's real, and it's not all aero. All cars tend to understeer more at low speeds and oversteer more at high speeds, not just 911s.

--DD

[chrismorse]

The mechanics of handling balance are dependent, primarily, on the traction available at each corner and the forces acting through the center of gravity.

Recently,I recall seeing a chart listing the front and rear lift/down force for various chin spoilers, dams and rear scoops/wings. It was also pointed out that at highway speeds the net effect was about zip, but at 120 or 150 the lift/downforce balance was a serious concern.

I probably don't have enough HP to get in trouble with my stock engined 74 coupe, but just in case, I might go with the chin spoiler under the front valance and a scoop on the back :-)

chris

[gliding_serpent]

Many factors at play. Lift is a big one that just lightens the car more at high speed and makes it more likely to rotate. Next is less reaction time. Finally, rotation happens when you try to change directions... I.e a corner. The faster you go, the closer your tires are to their limit of adhesion. Thus the less margin of error there is for bonehead driving. driving mistakes at low speed (lots of excess grip to save you) will be hidden, but you have less of a buffer at high speed.

[winders]

Originally Posted by gliding_serpent

Many factors at play. Lift is a big one that just lightens the car more at high speed and makes it more likely to rotate. Next is less reaction time. Finally, rotation happens when you try to change directions... I.e a corner. The faster you go, the closer your tires are to their limit of adhesion. Thus the less margin of error there is for bonehead driving. driving mistakes at low speed (lots of excess grip to save you) will be hidden, but you have less of a buffer at high speed.

You can be going slow and be near the limit of adhesion or going fast and be near the limit of adhesion. Availability of excess grip is not speed related only.

[gliding_serpent]

Quote de gliding_serpent

Many factors at play. Lift is a big one that just lightens the car more at high speed and makes it more likely to rotate. Next is less reaction time. Finally, rotation happens when you try to change directions... I.e a corner. The faster you go, the closer your tires are to their limit of adhesion. Thus the less margin of error there is for bonehead driving. driving mistakes at low speed (lots of excess grip to save you) will be hidden, but you have less of a buffer at high speed.

You can be going slow and be near the limit of adhesion or going fast and be near the limit of adhesion. Availability of excess grip is not speed related only.

Many factors at play. Including tire temp, pressure, age, compound, tread design, pavment, bumps, suspension, alignment, gravel, pickup on tires, ice, slush, snow, water, oil, cats, etc etc

[winders]

Originally Posted by gliding_serpent

Many factors at play. Including tire temp, pressure, age, compound, tread design, pavment, bumps, suspension, alignment, gravel, pickup on tires, ice, slush, snow, water, oil, cats, etc etc

Fine. But this statement is just plain wrong:

Originally Posted by gliding_serpent

The faster you go, the closer your tires are to their limit of adhesion.

[gliding_serpent]

"rotation happens when you try to change directions... I.e a corner. The faster you go, the closer your tires are to their limit of adhesion."

Sure, i simplified things, maybe left my explination a bit vague. Point is, everything else equal, when cornering at a constant angle, the faster you go, the more centrifugal force is applied on the car. Thus, the closer your tires come to their maximum limit of grip. Go too fast and they brake free (centrifugal force greater than tire grip). The faster you go, the closer you get to the limit of grip. Thus less room for error.

Friction circle.

Same relationship also applies to forces generated from acceleration or braking, with or without turning at the same time.

Now in my original statment of a car going faster has less grip is true for a 911 due to more lift. Faster speed, more lift, less grip, all else being equil. Heavy aero can certinally change this relationship so it is not universal, and aero lift only comes significant above certain speeds... But i think we all get the point here.

[winders]

If you mean:

"The faster you go in any given corner, the closer your tires are to their limit of adhesion."

Then say that. But that is not what you said.....

[Flieger]

There is a fundamental decrease in yaw damping from the tires that occurs with increasing speed. This is because the slip angles that result from a small yaw (rate) disturbance are reduced. It is best to visualize it as a vector triangle at the contact patch. One leg is forward velocity and the other leg is lateral velocity. The angle of the resulting velocity vector for a given magnitude of lateral velocity (yaw disturbance) gets smaller as the forward velocity increases. Therefore the slip angle that creates the restoring force (stability) is smaller and so is the restoring force. Thus, all cars become less stable as speed increases.

When you start with a car with reduced directional stability to begin with (like a 911 with such a rearward weight distribution, short wheelbase and relatively high moment of inertia) it makes things even more interesting as the yaw damping decreases. The pendulum swings further and further before coming back into line. Eventually you could theoretically reach a speed (critical speed) where the car is no longer stable and any small perturbance will result in a spin. That speed varies with vehicle parameters and should be quite a bit higher than the car can reach on its own power.

[moneymanager]

Finally a response that actually addresses the question.
Now if I can just understand it... Could this be graphed so that different combinations of torsion bars and sway bars could be tried to see when they might take oversteer to some empirically knowable point?

[Flieger]

Originally Posted by moneymanager

Finally a response that actually addresses the question.
Now if I can just understand it... Could this be graphed so that different combinations of torsion bars and sway bars could be tried to see when they might take oversteer to some empirically knowable point?

This is not a suspension phenomenon (though suspension can have second order effects). This is purely a matter of tire cornering stiffness, wheelbase, and inertia. The stability you start with is based on tire cornering stiffness and the cg position within the wheelbase.

If you want to increase the yaw damping: get tires with greater cornering stiffness (lateral force per slip angle), reduce the inertia or increase the wheelbase within the same overall length.

To increase the static directional stability you want tires with higher cornering stiffness only on the back and also move the cg forward.

Realistically what you can do on a 911 is put wider tires on, especially in the rear, and also remove weight from the back and move things from the back to the front when possible (such as oil tank).

You can also use aerodynamic downforce to increase the effective cornering stiffness (so add a tail or rear wing).

[Flieger]

If you have a copy of Race Car Vehicle Dynamics by Milliken and Milliken it is in there.

[gliding_serpent]

Originally Posted by winders

If you mean:

"The faster you go in any given corner, the closer your tires are to their limit of adhesion."

Then say that. But that is not what you said.....

Tough crowd.

[ganun]

Originally Posted by Flieger

There is a fundamental decrease in yaw damping from the tires that occurs with increasing speed. This is because the slip angles that result from a small yaw (rate) disturbance are reduced. It is best to visualize it as a vector triangle at the contact patch. One leg is forward velocity and the other leg is lateral velocity. The angle of the resulting velocity vector for a given magnitude of lateral velocity (yaw disturbance) gets smaller as the forward velocity increases. Therefore the slip angle that creates the restoring force (stability) is smaller and so is the restoring force. Thus, all cars become less stable as speed increases.

When you start with a car with reduced directional stability to begin with (like a 911 with such a rearward weight distribution, short wheelbase and relatively high moment of inertia) it makes things even more interesting as the yaw damping decreases. The pendulum swings further and further before coming back into line. Eventually you could theoretically reach a speed (critical speed) where the car is no longer stable and any small perturbance will result in a spin. That speed varies with vehicle parameters and should be quite a bit higher than the car can reach on its own power.

Great answer flieger, needed a bit of help to see it thru from my son in college with their SAE race car and who consider RCVD their bible.

[ttweed]

Many of the factors that effect the dynamics of steering balance (aero, suspension and tire characteristics, etc.) have been mentioned, but it seems to me the one fundamental variable that has been left out of this discussion is steering angle.

Leaving aside the other factors that effect handling balance in cornering (weight transfer from throttle and brake application, etc.), in steady-state cornering the simple reality is that slower corners are sharper, so more steering angle is necessary to negotiate the tighter turn. Faster corners (bends or sweepers) are less sharp and require less steering angle. The front wheels do all the steering (unless you have one of the new hi-tech cars with rear-wheel steering integrated) so their grip relative to the rear wheels is compromised more in tight corners by the higher steering angle, since a greater percentage of available front grip is being used to change the direction of the car. Thus the handling balance tends more to understeer in a slow corner than in a high-speed turn, where the front wheels have much less steering angle applied, and thus a higher percentage of grip available to resist lateral forces in relation to the rear wheels (which changes the balance towards oversteer).

This dynamic change is best balanced with aero, since the effectiveness of aero is also dependent on speed. A rear wing will develop greater downforce in fast turns, giving the rear tires more grip right when you need it (as the car is tending towards oversteer) while allowing the suspension to be tuned for more rotation (towards oversteer) for the slower corners, which can be advantageous in lowering lap times.
That's my $.02,
TT
(not a race engineer, but can play one on the internet as well as anyone)

[Flieger]

Originally Posted by ttweed

Many of the factors that effect the dynamics of steering balance (aero, suspension and tire characteristics, etc.) have been mentioned, but it seems to me the one fundamental variable that has been left out of this discussion is steering angle.

Leaving aside the other factors that effect handling balance in cornering (weight transfer from throttle and brake application, etc.), in steady-state cornering the simple reality is that slower corners are sharper, so more steering angle is necessary to negotiate the tighter turn. Faster corners (bends or sweepers) are less sharp and require less steering angle. The front wheels do all the steering (unless you have one of the new hi-tech cars with rear-wheel steering integrated) so their grip relative to the rear wheels is compromised more in tight corners by the higher steering angle, since a greater percentage of available front grip is being used to change the direction of the car. Thus the handling balance tends more to understeer in a slow corner than in a high-speed turn, where the front wheels have much less steering angle applied, and thus a higher percentage of grip available to resist lateral forces in relation to the rear wheels (which changes the balance towards oversteer).

This dynamic change is best balanced with aero, since the effectiveness of aero is also dependent on speed. A rear wing will develop greater downforce in fast turns, giving the rear tires more grip right when you need it (as the car is tending towards oversteer) while allowing the suspension to be tuned for more rotation (towards oversteer) for the slower corners, which can be advantageous in lowering lap times.
That's my $.02,
TT
(not a race engineer, but can play one on the internet as well as anyone)

I think you should look up ackermann steering angle (not to be confused with ackermann steering which is the difference between inside and outside steer angles to avoid scrubbing) in Race Car Vehicle Dynamics and that will show you why your explanation does not really make sense. You are correct that in tight turns the rear wheels tend to track inside the front wheels which some may consider to be understeer. But steering angle itself does not compromise grip. It is all about slip angle. True, if you do not have correct ackermann then the slip angles are not optimal and you will have more drag, and there is a greater component of the front tire lateral force directed rearwards in tight turns because of the slip angle.

Technically, understeer/oversteer refers to the steering angle required to maintain a certain turn radius being more than or less than, respectively, the ackermann steering angle . The reason for this difference from the geometrically correct angle is because the front and rear tires require different slip angles. If the rear requires more slip angle than the front you have oversteer. A neutral steer car will have the rear wheels on a smaller radius than the fronts and this effect is more pronounced the smaller the turn radius. An oversteer car has the front tires on a smaller radius than a neutral steer car, and the rears on a larger radius. You can even have so much oversteer that the front tracks inside the rear.

Note that this is completely independent of limit oversteer or understeer, which has to do with the maximum grip available at the front or rear, independent of the slip angle. You can have a car that oversteers that pushes wide at the limit on a skidpad. Of course, drive or braking forces can change the handling balance as well.