When driven under acceleration with no brakes applied, a front wheel drive car will understeer when the front wheels are turned. To overcome the understeer, a technique called left foot braking is used to change brake bias (balance) to the rear. By using a combination of throttle and brake you can change from understeer to neutralsteer to oversteer. How does it work? Lets find out....
When you stand on the throttle, which way does your body try to go? Toward the rear wheels! Right? Now what happens when you brake? Your body tried to go forward! This boys and girls is weight transfer! By using the basic rules of weight transfer and a combination of throttle and left foot braking, you can be in control when the unexpected happens on a rally (which always happens)
How Does It Work?
Brake bias in normal street cars is toward the front wheels for safety reasons. If you have ever pulled up your hand brake when turning on a wet road and spun out, you know why they build cars this way. What you did was bias the brakes to the rear, and this is not what Grandma wants in her Buick coming home from the supermarket. But as rally drivers we can use a means of biasing the brakes to the rear. Some of the things that can be done include using a brake bias control valve or installing more powerful brakes on the rear wheels. That works wonderful on a track, but on a rally it's hard to change your brake bias in the middle of a turn that suddenly changes from dry dirt with good grip, to wet slime with no grip.
Enter Left Foot Braking!
Once your have mastered LFB, when the road surface changes, you can change the brake bias by either adding power or brake pressure ( this is also called "modulating"). Example: You are in a turn and the rear of the car starts to slide out more then you want. You could release some brake pressure and increase the throttle. This will change the oversteer to understeer. The reverse is also true. If the turn tightened, you can change to oversteer by increasing brake pressure against the throttle. Also by balancing the brake and throttle you can get a neutralsteer.
Why Does It Work?
A tire has 100% of it's possible traction when it is rolling straight with no braking or acceleration. When you change any of these factors the tires affected will have less available traction and break loose earlier.By applying brake against the throttle in a front wheel drive car, two things happen. One: The front wheels keep turning. And two: The rear wheels try to lock up. If you are in a turn, the back of the car will start to slide toward the outside of the turn. To control or stop the slide, apply less brake and more power ( this changes the oversteer toward understeer and stops the rear wheels from sliding toward the outside of the turn ).
Putting Theory Into Practice:
Now comes the fun part, driving the car. Find a safe place to practice, a gravel parking lot with no trees will do.( If you live up north and it's Winter, try a frozen lake, this is ideal because everything happens at low speed ). Set up some traffic cones to make a Tee intersection. Now get in your car and practice driving the turn without left foot braking, but try doing the normal braking with your left foot.(What did he say??) Yes, try braking with your left foot! You can't learn to LFB until you train your left leg to brake! If you have never tried using your left leg, your will find it's like trying to write with your left hand if you are right handed. This is very important. Until you can drive doing the braking with your left leg, you can't do LFB. Once you can brake with your left leg, now let's try something new. First think about what you want to do, then start your practice turn. After turning in, apply the brake against the throttle. If you did it right you should feel the rear of the car start to slide to the outside of the turn. When you are pointing in the direction you want to go, let pressure off the brakes and increase the throttle. If you did it right you will be amazed at how the car seemed to pivot on the front wheels. So wipe the silly grin off you face and try it again, and again, until you can start or stop the slide at will. Congratulations, you now know what LFB is!
Practice Makes Perfect:
The first few times you try the exercise above, you will probably find the car slowing down in the turn but not much else. Try using less brake. You will be amazed at how little pressure is needed. Also remember to turn in before you apply the brake.I don't mean try to turn before you have slowed down for the turn, I'm talking about after you have braked and selected the proper gear for the turn, you must let some pressure off the brakes before turning in.After you get bored with just making a left or right, try setting up an "S" turn and play with that until you can keep up your speed through both turns. Remember to treat each turn separate, and lift off the brake and turn in each time. When you have mastered Left Foot Braking you will find that you will have control of your car like you never thought possible! It's both faster and more important safer. So don't get discouraged if it seems impossible to master, keep trying and all of a sudden it will seem easy!