Understeer and oversteer kept simple
Whilst all sports car drivers are familiar with the terms understeer and oversteer, there appear to be many misconceptions as to quite what each term means or just what causes such issues, so here’s a quick introduction. Quite simply, understeer is when a vehicle turns less than the angle requested through steering input, i.e. the front tyres struggle for traction, causing the vehicle to under-rotate and push wide through a turn. Conversely, oversteer is when a vehicle rotates more than the angle requested by the driver through steering input, most commonly understood as the rear tyres sliding sideways in a bid to overtake the front tyres.
Pretty much all modern cars are deliberately engineered to understeer before they oversteer, including high performance sports cars. This is due to understeer being relatively easy and largely intuitive to control and correct, hence safer, whereas oversteer requires considerably more skill; not to mention very quick reactions. Another reason for cars being engineered to understeer is that, if the worst does happen and control is lost, it’s generally safer to crash head-on than sideways; benefitting from larger airbags and significant impact absorbing crumple zones in the vehicle, plus we are physically better able to withstand a frontal impact than one from the side. The downside to this tendency toward understeer for the high performance driver is diminished dynamic handling and enjoyment at the edge of the grip envelope, but this can be readily compensated for by adapting our driving style; and is something frequently covered in Dynamic Handling Masterclass courses here at Total Car Control.
So, what causes understeer? Quite simply, it’s the driver. Turning the steering wheel harshly, abruptly or just too much can exceed the front tyres’ traction, forcing them to slide across the road surface in understeer. Cold or wet conditions make this all the more likely of course, whilst a lack of weight over the front tyres when accelerating too much too early mid-corner also triggers the problem.
Oversteer is a result of one of three driver actions: either applying excessive, sudden throttle in a powerful gear whilst steering (in a rear wheel drive car), or lifting off the throttle suddenly and steering, or excessive ‘trail braking’ (more of this in a forthcoming blog post). In the first instance, the harsh application of too much power can overcome the rear tyres’ traction and create wheelspin, with the lateral inertia from the steering input causing the spinning tyres to slide sideways. In the last two instances, excessive forward weight transfer leaves the now light rear tyres with little grip, resulting in an oversteer slide when the heavily laden front tyres turn in to a bend. ‘Lift-off’ oversteer can affect all cars, whether with rear, front or all-wheel drive.
Next time, we’ll look at the key principles of responding to and correcting both understeer and oversteer skids. In the meantime, enjoy your driving.