Oversteer, Understeer, Neutral Steer

[-Ferdinand-]

We had a lively discussion going on in another forum about oversteer, understeer, and neutral steer. It's surprising how many people don't understand the concept.

To demonstrate I hopped into my 1967 Lotus 49 F1 car, in the driving simulation 'Grand Prix Legends', and painted a few skid marks . The screenshots turned out pretty nice, so I thought I'd share them here as well.

Over-Under-Neutral Steer
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Following are three screenshots taken from 'Grand Prix Legends', of the Lotus cornering at high speed. The images illustrate Oversteer, Understeer, and Neutral steer.

Note that NONE of these three images actually represent the fastest way around a corner. If the tires are painting skidmarks on the pavement, it means the tires are being pushed too hard, beyond the point of best grip. It's not the quickest way.

The optimum grip and best cornering speed is reached well before the tires start to let go like this, but for these images I needed to lay down skidmarks to clearly show the line of travel being followed by the car.

It is important to realise that oversteer, understeer, and neutral steer occur equally at slower speeds, ie speeds and slip angles insufficient to produce skidmarks. The driver can still feel it happening. But, in a still photo, without the skidmarks it's more difficult for an outside observer to picture the vehicle's line of travel.

In each of these images the yellow arrows show the direction in which the tires are pointing, and the red arrows show the direction in which the car is actually travelling, as evidenced by the skidmarks trailing the tires. The angle between the yellow and red arrows is the 'drift angle' of the tire, the difference between where the tire is pointed and where it's actually going.

See next three messages...

[-Ferdinand-]

The yellow arrows show where the tires are pointed, and the red arrows show the actual direction of travel, as evidenced by the fat skidmark left behind by the outside rear tire.

That tire is overloaded and has lost grip relative to the other three tires. The combined level of reduced grip at the rear of the car causes the rear end to swing away. Even though the track curves to the right, the driver needs to keep the steering wheel cocked slightly to the left in order to prevent the car from spinning.

The car itself is still travelling where the driver wants it to go, in the direction of the red arrows, in a balanced drift around the corner. And the car is yawing slowly clockwise as it continues travelling around the corner.

However, you can see the slip angle between the yellow and red arrows is larger on the rear tires than it is on the front tires. The front tires still have more grip available, whereas the rear tires are already overloaded.

This is Oversteer.

[-Ferdinand-]

Again, as illustrated by the skidmark, the car is travelling at high speed in the direction of the red arrows. But this time the front end is suffering from insufficient grip. Look where the front tires are pointing relative to the rear tires, as indicated by the yellow arrows.

The slip angle of the front tires is larger than the slip angle of the rear tires.

In order to keep the car balanced and continuing to follow this right-hand corner, the driver is holding in more steering angle to the right.

A common rookie driving error when encountering understeer is to crank in way too much steering angle, in panic sometimes dialing the steering all the way to full lock. When the front tires are set at too large a drift angle, way beyond the angle of maximum grip, it only makes the understeer worse.

In this image the nose of the car is still pointing toward the inside of the turn relative to the car's line of travel, and the cornering forces are still 'balanced' making the car yaw clockwise as it follows the curve.

Nonetheless, this is Understeer.

[-Ferdinand-]

In this shot, the car is again travelling at high speed around a long right-hand curve, and the car is yawing (rotating) slowly clockwise at the correct rate as it travels around the curve.

The red arrows, and the skidmarks left by both outside tires, clearly show the direction in which the car is travelling. The yellow arrows show the direction in which the tires are pointing. Note the slip angles are pretty close to identical front and rear.

The steering wheel is centred with the front wheels pointing straight ahead. The car is following the line of the curve with no steering input at all. No more steering input is required through the front tires in order to hold the car balanced and tracking the intended line around the corner.

Again, it should be stressed that this is NOT the fastest way around a corner. The tires are being pushed too hard here beyond the point of optimum grip. The slip angles are too high, causing the tires to lose grip and create skidmarks. Peak tire grip, and best cornering speed, occur at a smaller slip angle than this.

Still this is a clear example of Neutral Steer, regardless of where the nose of the car is pointed in relation to its actual line of travel.

[CrashCymbol]

Great illustration Ferdinand. I've often had a difficult time explaining to people the difference between over/understeer. Neutral really wasn't much of an issue. I'll deffinately be sending people to see this for their own understanding. Its one thing to demonstrate IN a car, quite another to try and articulate it...well for me at least. I tend to learn/teach best through demonstrations etc.

[-Ferdinand-]

As an engineer, I like diagrams with angles, vectors, arrows, and stuff. But a friend came up with a much simpler and more eloquent description.

You're driving your car around in a circle at a steady speed. It takes a certain steering angle to hold the car on the circle. Now, what happens when you try to speed up?

If it requires more steering angle to hold the car on the circle, your car is understeering.

If it requires less steering angle, or maybe even opposite angle, your car is oversteering.

If you can continue holding the steering at the same angle as before on the circle, but at higher speed, then your car has neutral steer.

[JettaRacer]

[quote="-Ferdinand-"]If you can continue holding the steering at the same angle as before on the circle, but at higher speed, then your car has neutral steer.[/quote]

Or you're not going fast enough! ;)

[-Ferdinand-]

Understeer

Jackie Stewart, BRM, Spa Belgium 1967




Oversteer

Jochen Rindt, Cooper, Kyalami South Africa 1967




Neutral steer

With just a touch of oversteer.
This shot gives me goosebumps.

Jack Brabham, Brabham, Zandvoort Holland 1966

[Jaak]

Three labels but it is the transitions in between and what you can do with understeer, neutral and oversteer that amuse me. Even harder to explain.

There is a terrific mini-combination on the paved track at Mecaglisse. 2nd longest straight with a kink, then 100 degree hairpin, accelerate on short straight...then brake hard for a 90 degree left then brake even harder before a verrrry late apex right using throttle to understeer a FWD car to outside on exit then continue same arc without moving steering wheel to second apex then lift throttle and very quick brake to flick the car's rear end out to make the immediate very tight doubling back left.

From the "..." on you get to experience a pretty full range of attitudes.

It is very intense and you have to anticipate each action or you'll be late. Students are amazed by the busy-ness of this section and then thrilled because suddenly they're flying through and arriving at the next corner way faster which they inevitably go around with massive understeer...next lesson begins.