![]() ![]() GETTING PLAYFUL: So lets take a high-speed left hand sweeper (like the section from the bridge to the uphill hairpin on Camp Brule at Baie-des-Chaleurs). Snowboarders, mountain bikers, and motocross racers know this feeling too. If you ski you get this feeling naturally more weight on the balls of your feet causes the skis to turn in quickly and slide out at the tail, and more weight on your heels causes the tails to dig in and the skis to want to go straight ahead. Steering is still important, of course, but the balance of the car is essential. Weight will transfer to the rear, the car will dig in on the rear, straighten out, and open the line. Conversely, if you want to open the line, apply a little throttle and let off on the brake. Weight will transfer forward, the car will turn in, slide more, slow down, and tighten the line. So if youre sliding through a corner and you want to tighten the line a little, the thing to do is not steer into the corner, but lift a bit on the throttle and apply a little brake. So the only way to increase traction on a given tire is to increase the weight on it, and the way to do that is with the throttle and the brake. The technical reason for this is that you are always outside of the static friction circle when youre sliding, and the coefficient of dynamic friction is constant. But transferring the cars weight from one end of the car to the other has a very large effect on the direction the car will spin (or yaw). When youre sliding a car on a slippery surface, steering inputs are much less effective than they are on asphalt. WEIGHT BALANCE: Herein lies the real secret of fast rally driving: the art of shifting weight between the front and rear axles. Under static friction (on a race track) the lateral grip of the tires does this work, while under dynamic friction we use the clawing of the wheels in a forward direction to do some of this work. For the scientists: you want the linear accelerative vector to point inside the arc of the car through the corner to compensate for the momentum drawing the car to the outside. ![]() Third, and especially with an AWD car, you want to use the engines power through the wheels to claw your way in towards the apex of the turn if youve even seen a dog try to take off on a shiny floor or on ice you get the idea. Second, on a slippery surface a car that is already sliding is easier to control (see below) than a car that is gripping but may break into a slide at any moment. We want to slide during a corner for several reasons: first, as in braking, the static friction circle is very low and we want to use the sidewalls of the tires to take some of the lateral forces of the corner. In rally, we want the car to follow essentially that same path, but we want to do it while sliding with the wheels pointed more or less straight ahead relative to the car. TURNING: On the street or the racetrack, you choose where you want to apex the corner and you steer smoothly through that point. So we dont just slide around to look cool sliding is, among other things, an effective traction tool under conditions of dynamic friction. On gravel, it is much more effective to throw the car sideways, use the sidewalls of the tires as one part of the braking force, and feather the brakes for the other part of the braking force then youre using the tires in two directions simultaneously, and youre misaligning the rear wheels from the fronts as you slide so that they get more bite on a fresh surface too. But on gravel the static friction circle is very small (theres not much traction) and threshold braking in a straight line is not very effective. On asphalt with gripping friction, the best technique is to brake as hard as possible while not locking up the wheels, using the tire within its (static) friction circle and maintaining directional control. For example, if you stand on the brakes on a gravel road or on asphalt, a lot of the cars weight will transfer to the front wheels and all four wheels may lock up. This means that the fast way to drive on a slippery gravel or snowy road is very different than on a sticky race track, or even on the street. SLIDING FRICTION: The thing that sets rallying apart from most other types of driving is that we are racing under conditions of dynamic (sliding) friction instead of static (gripping) friction. Curious as to how this all works? Read on. For example, I do a lot of steering with the throttle and brake (sometimes simultaneously), a lot of braking by making the car slide, and I use my left foot on the brake pedal and only use the clutch once per rally stage. Andrew Comrie-Picardĭriving a rally car is, of course, a lot like driving a normal car: steering wheel for left and right, throttle for faster, brake for slower, clutch for shifting and all that.
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