After our previous discussion on what we can teach teenagers about driving that hasn't also been figured out by rats (you can still catch that one online in the Life & Style section by clicking on my name in one of my earlier stories), let's bring things back to basics and explain oversteer and understeer.
First off, let's assume we're in a car together on a wet skidpan, and you're a novice wanting to learn more nuanced car control than simply what each pedal is for.
To give things their comical definitions, understeer is when you hit the wall with the front of the car, and oversteer is when you hit the wall with the rear. Further to that, power is how fast you hit the wall, and torque is how far you take the wall with you.
Power and torque actually get taught in high school physics, but here's a very simple summary. Torque is the work being done (how much force is being applied, hence the wall is shoved further), and power is the rate of work (how quickly the work is being done, hence moving faster when you hit the wall).
Another more important thing we need to take from physics is the principle that any moving object wants to go in a straight line unless some other force is being applied. In cars, that other force we have control of comes from the tyres, if they have any grip.
Anyway, since we'd really rather not hit the wall at all, understeer is when the front tyres don't have enough grip to turn the car as much as the steering wheel is asking them to. In mild cases you'll run wide in a corner. In really bad cases like locking the front tyres under brakes, you'll go straight ahead.
Oversteer is when the rear tyres begin to lose grip. To avoid a spin, you usually need to apply just the right amount of steering correction, with precisely the right timing (because overcorrection is bad too).
Also note that different cars can react rather differently to lifting off the throttle.
A front-wheel-drive (FWD) can have lift-off oversteer, so don't attack a corner and just leap off the throttle, ease off instead (reapplying throttle can correct FWD oversteer too, but using that depends on how much clear space you have). Many mid- and some rear-engined cars also have lift-off oversteer, as can some tall 4x4s, or even any cars with a locked rear diff (especially in the wet).
Otherwise, in most RWDs (and many all-wheel-drive sedans) you can often reduce understeer (or reduce power oversteer, which we'll get to in a second) by lifting off.
Either way, all cars can begin to understeer when a bit too much power is applied. However, RWDs can also snap into (power) oversteer if way too much power is applied.
Pay attention to the steering wheel too. In steady-state turning (big circles), the wheel should normally get slightly heavier with grip. If we exceed that grip and get understeer, the wheel should feel a bit lighter. On hard surfaces it can feel like the wheel has a small lump to get over as it goes from grip to understeer. (Note that if a FWD has torque-steer, which is throttle application turning the wheel a bit, that can interfere with this feeling.) If we get oversteer, the wheel often wants to turn itself the other way.
If it's a brake lock-up that caused the understeer (ABS would prevent such lock-ups), ease off the brake pedal so you still slow down but you get the wheels rotating again.
If a change in surface friction (maybe a puddle or oil patch) causes understeer, the trick in this instance is to resist the urge to turn the wheel further, because that'll make things worse. It will probably only last for a moment, so hold your nerve, apply a tad less steering lock to get the grip back, and when they regain grip (you may feel the grip return in the wheel, as well as feel the car turning again) you can then deal with pointing the car where it needs to actually go (having hopefully not run out of clear space, which is why we're practicing this on a wet skidpan).
Now, let's practice losing and regaining grip (you can do this at home if you have a sim rig, or even just do the old licence tests in the Gran Turismo series with a controller).
That was just the basics to get you started. In a more advanced lesson we can look at the effect of load transfer on grip levels.
Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.