Q: Why is the line different for rear-wheel drive versus front-wheel, or high horsepower versus low horsepower, or is there a difference?
A: A car has a fixed amount of grip, and although we can change which corner(s)/tire(s) have more grip at any dynamic moment by braking, accelerating or turning, we cannot increase the total amount of grip available. That grip may be spent 100 percent on turning, accelerating or braking. For example, if we are turning and accelerating at the same time, that total grip must be shared by various tires. These laws of weight transfer and of grip sharing as seen on friction circles, apply to all cars, front- or rear-wheel drive.
In a low-horsepower car, acceleration uses a small amount of grip. Thus we use most of our grip for turning, keeping the corner speed as high as possible all the way through the turn. This means we turn in and hold nearly a constant radius though the turn, apexing only slightly late. If we slow down 5 mph more on the turn-in and make a very late apex, our low-horsepower car takes a long time to recover that extra 5 mph that we slowed, even though we have grip to spare on the exit. It’s safe and comfortable, but not fast.
Conversely, in a high-powered car, if we slow that extra 5 mph to set up for a late apex, then we can gain that 5 mph back rapidly once we reduce steering input and in fact will be back to 100 percent throttle much earlier than trying to hold a high mid-corner speed on a constant arc. Both cars run a traditional outside-inside-outside line with a late apex, but the later apex is more accentuated in the higher horsepower car.
Front-wheel-drive cars are generally prepossessed of weight over the front wheels compared to rear-wheel-drive cars. When accelerating out of a turn, weight is transferred to the rear of the car, taking away the front tires’ ability to grip, while the tires already have dual duties of turning and accelerating. This is why front-wheel-drive cars tend toward understeer on corner exit. Thus a front-wheel-drive car likely will favor the later apex, like a higher horsepower rear-drive car to minimize its turning duties during the acceleration phase of the turn. The tradeoff is that with all that weight up front (read that as “grip” up front) the front-wheel-drive drive car brakes very well and can be rotated very quickly at mid corner so it is pointed and ready for full throttle relatively soon.
The basic late-apex line is the same starting point for all types of cars, but you can see that the line will need subtle changes if you want to optimize your lap time in a particular car. I have oversimplified the physics at work here, but just think in those simple terms about what is happening in each phase of the turn, and what is being asked of each tire. The key is optimizing the available grip at all times, and you can see that different cars demand different lines and techniques to do that.
Charlie Putman is a driving instructor with the Ford Performance Racing School. He also is a professional driver in the IMSA Tudor United Sports Car Championship, Continental Tire Sports Car Championship, FIA Blancpain GT Series, Creventic Endurance Series and the ADAC GT Series.