Throttle steering is the technique of rotating the car with throttle. It is most often used in rear wheel drive cars, although some principles carry over to all wheel drive and front wheel drive cars as well.
When a car turns, it effectively rotates about its center of gravity in the horizontal plane. Usually this rotation is accomplished by turning the front tires, but it can also be accomplished by slipping the rear tires or transfering weight from rear to the front.
Throttle steering alone normally cannot initiate a turn. The car must already be turning, at which point throttle steering can be used to rotate the car more or less. Throttle steering may be used during corner entry, but it normally follows a steering input: first the driver initiates rotation with the steering wheel, then they fine-tune car trajectory with throttle using throttle steering.
Sliding Rear Tires
If a car in a turn loses rear tire grip completely, the rear tires will slide in a straight line while the front tires will continue turning. The result will be additional rotation of the car which can become a spin if not controlled.
Sharply applying additional throttle in a high power rear wheel drive car produces a similar effect. The rear tires break loose and continue in a straight line, typically sending the car into a drift. This initiation technique in drifting is called "power oversteer" or "power over".
In road course driving we usually don't want the car to go into a drift, hence we need to slip the rear tires instead of sliding them.
Slipping Rear Tires
This is actually an advanced driving technique in that it requires a specific car setup and the driver practiced and comfortable with a slightly loose car. A rear wheel drive car with a limited slip differential is required. Low power cars are easiest to execute this technique in, and Spec Miatas use it all the time. Higher power cars can still execute it but may require only partial throttle which takes more driver skill and practice.
The idea here is that, as throttle is applied while the car is still cornering, the rear tires will exceed the grip available to them but not to such an extent as to send the car into a drift. Instead what happens is the rear tires lose adhesion and slide laterally for a bit, then regain grip which stops the rotation and allows the car to accelerate forward. Several of these cycles may happen consecutively in a corner, for example due to track surface not being perfectly flat. Each time the rear tires lose grip and slide the car rotates a small additional amount.
If the car has too much power, it may go into a drift, in which case the driver must apply partial throttle to keep the rear tire slips small. If the car has an open differential it is likely to spin the inside rear tire while the outside rear tire remains planted and not doing much; a limited slip differential is probably required for both tires to slip. Lastly, the car will often begin accelerating while on a trajectory that would appear to send it off track; however, additional rotation due to slipping rear tires would fix the trajectory while in mid corner on throttle.
Slipping Front Tires
In a front wheel drive car, the front tires will break loose under excessive throttle, causing understeer. The practical application of this phenomenon is intentionally plowing the car.
When a front wheel drive car is in a corner under power with front tires slipping, removing throttle would permit front tires to regain grip. As a result, the car would rotate more rapidly.
If a car is under power and the driver lifts, weight is transfered from the rear tires to the front. If the rear tires were close to the limit of adhesion, resulting unloading can break them loose. The result is similar to power oversteer. When rear tires lose traction due to throttle reduction, the process is called "lift-off oversteer" or "trailing throttle oversteer".
Three principal differences between power oversteer and trailing throttle oversteer are:
Trailing throttle oversteer requires the rear tires to be close to their grip limit, whereas power oversteer can still happen when the car is not at the grip limit if it has enough rear wheel torque.
Trailing throttle oversteer, once mastered, can be applied more smoothly and to a lesser extent than power oversteer. Power oversteer typically results in a drift even in skilled driver's hands, whereas trailing throttle oversteer does much less so.
Power oversteer can often be recovered from simply by reducing throttle. With trailing throttle oversteer, reducing throttle does nothing. The car must physically decelerate to where the rear tires grip again. For this reason trailing throttle oversteer is often regarded as more dangerous than power oversteer, even though power oversteer can result in loss of control of the vehicle as well.
Weight transfer induced by lifting does not only rotate the rear around the front, it also increases the amount of grip available in the front. When properly timed, lifting during turn in gives more grip to the front tires and rotates the rear of the car around the front to turn the car much better than it would otherwise. Trailbraking exploits the same weight transfer principle to an even larger degree.
Weight Transfer To The Rear
Just like throttle reduction moves the weight from the rear tires to the front, increasing throttle moves the weight from the front to the rear. Abruptly adding throttle can result in power oversteer if the car is powerful enough and uses rear wheel drive or rear-biased all wheel drive. On lower powered cars, abrupt weight transfer can unload the front tires without spinning the rear tires. The result will be reduced car rotation, or understeer - the car will stop turning as much as it did previously.
If done sufficiently smoothly and in an appropriately balanced car, yet another phenomenon will happen: weight will move to the rear faster than added torque requires extra grip, and the rear tires will grip more while accelerating harder. The car will typically be turning somewhat less at this point, because the front tires would have less grip.
The fastest way through a corner in a balanced rear wheel drive car, then, is something like the following:
- Come into the corner and either brake, with trailbraking, or lift just after turn in to move weight forward and achieve maximum possible rotation of the car;
- Wait until the car just begins to oversteer, indicating that it starts to rotate too much;
- Smoothly get on power, thus stopping the rotation, and apply maximum throttle, unwinding steering at the same time.
In theory, the car would be going faster than a neutral (coasting) car could in each stage.
Alternating between on-throttle and off-throttle modes allows the driver several attempts at taking the car through the desired weight transfer transition. For example, in a low power car rotation would come from trailing throttle oversteer rather than power oversteer; therefore, when the driver lifts they would lift abruptly, whereas they might get on power reasonably smoothly not to unload the front tires too rapidly. In a high power car the driver might get on and off throttle abruptly.
Tagged: weight transfer, advanced