Oversteer & Understeer

[Halski]

I did another IAM drive yesterday - very good it was too, did motorways and so forth; and spoke about oversteer and understeer. The instructor suggested that I should try to get onto a skid pan which is what I shall do.

I have never really been sure about what over/understeer is/are - what I think they are is (i) understeer: although the wheels are pointing left or right due to steering wheel offset, the car keeps going in a straight line and (ii) oversteer: when steering, the car steers disproportionately to the amount of offset of the steering wheel, i.e. the back swings around in the direction of the steer apropos a rally car on a bend - anyway - would anyone care to contribute a few lines about either or both and how to control them?

[VinnyP!]

I think your definition means you understand the differences well enough. Understeer you go through the hedge forwards, oversteer you go through the hedge backwards :D

But there are degrees, most modern cars understeer, its built in as for 90% of drivers its much safer. Usually all it means is the car runs a bit wider than intended. Avoid the temptation to wind on more lock as it runs wide as it just makes matters worse. There are a few solutions, but what you are looking to do is restore grip to the front wheels OR get in a position where the rear wheels and/or engine can help. The former is easier and has the side benefit that even if you do still have an accident you will be going slower when you do.

The reason you are understeering is that the front wheels are skidding, usually caused by too fast an entry speed for the grip available. So the easy way is to reduce this speed. EASING off the throttle or even dipping the clutch are the usual methods. This moves more weight over the front wheels to help grip; as the speed falls they regain grip and you get round. The downside is that the front wheels are doing even more work and it causes unweighting of the rear wheels. So if its overdone you can end up with "lift off" oversteer which is usually sudden and very difficult to control as the more the front tyres grip the less the weight over the rear wheels. In theory re-applying some power and easing off the steering will counter this, but in most cases its happening so fast that by the time you get the wheel round the cars chassis has compensated and you are off in the opposite direction.

The other methods depend more on the type of car you are driving. If its got clever things with three letters like DSC, TCS etc etc then keeping the power on the car will do a pretty good job of sorting things out. The more Tescos car park techniques are not really things you can learn by reading, and if you know them its unlikely you'd get in this situation in the first place. Left foot braking (most effective in front wheel or 4 wheel drive car) is more progressive and controllable, it also lets the rear wheels do some of the work. BUT it takes a lot of practice and the road is not the place to practice it. Its good for going fast because it keeps the power available especially in turbo cars. Similarly but less controllable is using the handbrake. In a rear wheel or well set up four wheel drive car you can correct understeer if you have enough room by using power to make the rear wheels come round and point in the direction you want to go so the engine power is working for you. This is the crowd pleaser, but if it goes wrong its also good for the hospitals and bodywork repairers. :eek:

Oversteer is easier on paper I've talked about oversteer on front wheel drive cars already but in any car, steering wheel twiddling is the answer. You need to wind off the lock and steer in the direction of the skid, this needs to be done quickly but not overdone, you need to anticipate the correction and wind off the lock as its happening or even a touch before not once it has happened, this leads to the fish tailing usually followed by a spin often seen at track days. You also need to keep the car balanced by small throttle corrections, some crash courses (pun intended) advise no matter what to dip the clutch and for the unpracticed its not a bad idea at all. One key secret is to keep looking and facing in the direction you want to go it seems to make everything easier.

The best and smug answer is not to get in a situation where you need any of this but like first aid its good to know if it goes wrong. Slow in fast out really is the key. The only way is to practise somewhere safe. Skid pans are good for learning the techniques but the skid control cars most use these days are often better once you have the basics because they can make you deal with it when it happens unexpectedly.

Oh just noticed you have an MG Rover avatar if you drive one of these its more likely your under/oversteer means either a front or rear wheel has fallen off respectively. :p

[7db]

You haven't bought Milliken yet, have you?!

Understeer / Oversteer gets used in two confusing ways, so pay attention at the back.

The first is the behaviour of a vehicle, characteristic to the vehicle -- loading, spring stiffness, tyre grip, weight distribution all taken into account -- which means that when you turn the wheel a certain distance, the vehicle when travelling at speed, either turns more (oversteer) or less (understeer) then you would expect it to at very low speed (the Ackerman steering angle is the turn at low speed where yaw of the vehicle can be ignored and is characteristic of the neutral steer car).

This is derived from the geometry of the vehicle, and the tyres maintaining good linear grip (ie cornering force proportional to stiffness and slip angle).

Nearly all cars manufactured are understeer cars in this sense. Since front wheels tend to turn the car more, and back wheels turn it less; a simple way to think about it is that they have more grip at the back than the front.


The second are more properly call "final understeer" and "final oversteer". This is where one or more tyres reaches the limit of adhesion and the car "ploughs-on" (understeer where front wheels give up) or "spins out" (oversteer where back wheels give up).



On the public road, one ought never to be encountering the latter forms of u/s; o/s. But a skidpan will certainly develop these.

The former type is most useful to understand. Since power and weight distribution (whcih changes a lot under power and braking) are critical parts of the U/S;O/S characteristc, it is one of the things that can alter the way that you go round corners, the stability in the corner to perturbations, and the effort that the tyres need to make.

I'm not going to do the maths here, but the right amount of power in corners can kill off the understeer, increasing centripetal grip and cornering force and reducing the required steering angle. It makes the car feel lovely too. :)



I haven't had breakfast yet, so that might not be the most coherent I've ever written. My driving instructor is considering a lecture on this stuff -- might you be interested?

[7db]

Damn beaten to it by Vinny.

[Halski]

thanks for the comprehensive replies Vinny and 7db.....


I haven't had breakfast yet, so that might not be the most coherent I've ever written. My driving instructor is considering a lecture on this stuff -- might you be interested?

I would bite your hand off

I could offer in return a lecture on Science and the Law for PC's/Avoiding Death in Custody, and curiously, Operation Market Garden, the Battle for the Bridge at Arnhem....

BTW these are all available at no charge to Forum members who can organise an audience - esp the first one, for Feds if you have a training day for the relief/sector team. Wazza came to one of these and found it relevant and topical.......

[7db]

Ok - will let you know when (if?) he pull his finger out and books it up. He's an ex polacc investigator, and rather good with vehicle dynamics.

You really will have to get Milliken if you want to keep up...

Where are you based?

[Halski]

Ok - will let you know when (if?) he pull his finger out and books it up. He's an ex polacc investigator, and rather good with vehicle dynamics.

You really will have to get Milliken if you want to keep up...

Where are you based?

I live in Redhill and work in Brighton.....

[7db]

This would probably be up towards St Neots, but all tba. Could give you a lift from West Brompton...

[Andi]

With understeer, you see the tree that kills you ;)

[7db]

Definition from Cadence:-
""Understeer" is when you go into the hedge forwards and "oversteer" is when you go in backwards!"

[Meridian]

StNeots is right near me and I might be interested.


M

[7db]

Ok - Will let him know that there might be a few takers.

Lovely roads up around Kimbolton, don't you think? (B660)

[Meridian]

I often do the B660 from just south of Norman Cross and then go round the south of Grafham Water to Buckden. As you say, a nice road.


M

[VinnyP!]

Wow just looked at Race Car dynamics 1000 pages :eek: :hail: A few years back I waded through Thomas Gillespie's Fundamentals of Vehicle Dynamics a piddling 500 or so pages, and although good not exactly a page turner. I am tempted to go with the Milliken but its a bit big to read in the car, is it worth it as I don't race cars. Will it make me more attractive to women? ;)

[VinnyP!]

Halski you have PM

[Halski]

Bonza - good plan....

[Meridian]

Will it make me more attractive to women? ;)


Only if you stick it down your trousers.

Although it may only scare them...



M

[18wheeler]

oversteer= keep it nailed,steer in and smile:D



understeer=back off,and clench once buttocks:D

[7db]

I am tempted to go with the Milliken but its a bit big to read in the car, is it worth it as I don't race cars.
You think it's hard in your car - try reading it in a race car...I keep losing my place in corners.

It is the bible of car dynamics, racing or otherwise. Race cars are just very simple road cars which are taken to the limit of ride and handling (which is where interesting stuff happens).

I don't race cars either - but I do like to understand what is happening when I use the controls. The biggest accusation is that it's hard to follow all the explanations without a reasonable (A-level) grasp of Maths and/or Physics.


Will it make me more attractive to women? ;)
Nice package?!

[7db]

oversteer= keep it nailed,steer in and smile

Only if you're looking for a good way to see what's behind you without using the mirrors.

[Meridian]

Perfectly valid recovery technique for AWD vehicles though. Steer in the direction you want to go (as opposed to the one you are actually going in) and give it a tiny bit more gas. Or so I'm told, which is why I'd be interested in some training.


M

[7db]

It all does rather depend on how the skid develops (assuming we are talking about final oversteer, rather than oversteer in the linear phase). Usually the way to get out of a skid is to undo what you just did to get it into the skid.

This is hardest for lift-off oversteer...

[18wheeler]

Perfectly valid recovery technique for AWD vehicles though. Steer in the direction you want to go (as opposed to the one you are actually going in) and give it a tiny bit more gas. Or so I'm told, which is why I'd be interested in some training.


M
training would be good,i could do with some too;)
can we get a 5ive-0 skidpan???

[7db]

I'm off to play on Lincolnshire plod's skidpan in November.