Rally Driving: the Basics. Andrew Comrie-Picard
Driving 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. But the similarity is only skin deep. 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. Curious as to how this all works? Read on.
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. 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. 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. 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. 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 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. So we dont just slide around to look cool sliding is, among other things, an effective traction tool under conditions of dynamic friction.
TURNING: On the street or the racetrack, you choose where you want to apex the corner and you steer smoothly through that point. 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. 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. 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. 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. 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. 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.
WEIGHT BALANCE: Herein lies the real secret of fast rally driving: the art of shifting weight between the front and rear axles. When youre sliding a car on a slippery surface, steering inputs are much less effective than they are on asphalt. 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). 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. 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. 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. Weight will transfer forward, the car will turn in, slide more, slow down, and tighten the line. Conversely, if you want to open the line, apply a little throttle and let off on the brake. Weight will transfer to the rear, the car will dig in on the rear, straighten out, and open the line. Steering is still important, of course, but the balance of the car is essential. 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. Snowboarders, mountain bikers, and motocross racers know this feeling too. Its all about balance.
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). You come in fast maybe 80kmh after the bridge, and turn the wheel to the left a little. A stab on the brakes suddenly shifts the weight to the front wheels and lightens the rear, pitching the car into a left-hand slide. Once youve got it sliding, you get off the brakes and straighten the wheel, so youre in a nice left-hand drift with the wheels clawing in towards the apex of the turn. Hold the throttle steady to keep the balance and ride out the long drift. If youre getting a little wide (watch those concrete blocks!) lift your right foot a little and press with your left foot on the brake a little to shift weight to the front. Once the line tightens get back off the brake and hold the throttle steady. As you begin to exit the corner, get on the throttle to straighten the car out for the exit and to accelerate. Whoo-hoo!! You did it! Youre probably still going 80kmh. And all the steering you did was a little left twitch before the corner began in order to start the slide.
GETTING FANCY THE PENDULUM: Some corners are really tight, and youre not going to get the car to yaw around them enough with the normal technique described above. If you try to turn in really hard youll probably understeer off the outside, and even a normal drift may not be tight enough. Dont reach for the handbrake. Instead, do a pendulum turn, or a Scandinavian Flick. By turning the car into a slide away from the corner and then snapping it around to slide in the right direction for the corner, you will transition the car more quickly than if you just turned in alone. Heres how you do it, for a left-hand pendulum: 1. start on the inside of the approach road, or at least with enough room to snap out wide. 2. turn right - away from the corner and touch the brakes to initiate a right-hand slide. Use this slide to scrub off some speed as we said above, use it as your braking. 3. turn the wheel left into the corner and punch the throttle for a second. This will transfer weight to the rear wheels and cause the car to snap (yaw) hard to the left. 4. straighten the wheel as the car snaps left and balance the throttle to hold your left-hand slide. As above, adjust with throttle and brake as necessary to shift weight. 5. As with a normal corner, get on the throttle as you want to straighten out and accelerate to victory!
Once you get really good, you will use these techniques together: the left hand drift through the sweeper at Camp Brule that I describe above is actually the first swing of the pendulum for the uphill right hairpin immediately afterward. When you start linking corners together like this youll be a pro, and Ill get out of your way.
REAR-WHEEL DRIVE: All of the above applies very well for front-wheel drive and all-wheel drive, and the weight balance issues apply to all drive layouts. But as everyone reading this probably knows, hard acceleration on a rear-wheel drive car usually causes it to throttle steer and slide out or turn on a sharper angle, overcoming the effect of weight transfer to the rear under acceleration. On gravel, this is a weakness of RWD cars, and youll notice that very few leading rally cars are RWD as a result. In the Good Old Days, of course, most rally cars were RWD, and they required a light touch to get the slides right. The technique is different it looks great, but its not that fast.
HANDBRAKES: The handbrake on the rear wheels is a destabiliser. I dont like it too much because it pitches a car into a slide without shifting much weight to the front wheels and so in a high-speed drift it can be unpredictable. However, if youre about to go off the road, or if you have a very tight corner and not enough speed or room to initiate a pendulum, then you can consider pulling the handbrake. If you have a hydraulic handbrake with a long lever, like mine, you can have a little more control. But its a blunt tool. Note that AWD cars with viscous centre differentials (i.e. almost all of them) will not allow you to use the handbrake for the rear wheels only the diff will lock and youll be braking all four wheels, but not with much force. Youve been warned.
LEFT FOOT BRAKING: Youll note that I mentioned using the left foot on the brake pedal. There are several reasons for this, but the most important is that you shave a fraction of a second off the transition time from throttle to brake, which is very significant when youre playing the throttle and brake off each other as described above for weight balance. When youre sliding, the two pedals are more like the rudder of an airplane than they are like throttle and brake (now you know why McGeer is so good!). Of course, if you have a normal gearbox then you may have to take your left foot away to do some shifting, although if youre very good at your rev-matching you may be able to get away with clutchless downshifts, which will improve your times and your safety margin a lot, as you can keep your left foot balancing the brake. Now you see the advantage of a dogbox that has no synchros and needs no clutch for shifting: although the shifts are indeed a little faster, the key is that you can keep your left foot on the brake all the time for more control. Once you dump the clutch on the start line youre done with it until the finish.
FRONT-WHEEL DRIVE: Ive left discussion of this until this point because you need to understand left-foot braking and handbrakes to understand an interesting aspect of FWD rally cars and why they can be very fast: with FWD, left-foot braking can work like a handbrake, without the disadvantage of not shifting weight forward. Confused? Think of it this way: using the throttle and brake at the same time on a FWD car is exactly the opposite of doing a brakestand with a RWD car: with FWD you can keep the front wheels spinning (with the engine overpowering the brakes) and slow down or lock up the rears. So as you enter a corner and want to initiate a slide, you can turn the direction you want to go and then apply the brakes while you hold the throttle the weight will transfer forward (better than with the handbrake, as the front wheels will slow down a little) and your left foot now has control over the speed differential between the front and rear wheels. You can pretty much make a FWD car turn around its nose this way, and this is the reason that no FWD rally car should ever have to worry about understeer. With my ex-factory Lada, you would basically never lift the throttle and just use the brake according to how tight any corner was. It was fantastic. And its why many front-wheel drive cars from the Saab 96 to the Mini Cooper to the VW Golf have been great rally cars, and on slippery surfaces far superior to their RWD competition. Now you know.
So to review some of the counter-intuitive things about rally driving under sliding friction: 1. Sliding is very effective for braking and for turning, 2. you steer with the throttle and brake, transferring weight rearward and forward (respectively) as necessary to control the slides, 3. you use your left foot on the brake so that you can play the throttle and brake against each other, like the rudder of an airplane, 4. if you have a FWD car you use the left foot on the brake to control the speed difference between the front and rear wheels, and 4. if youre getting faster than me, you slow down!
A couple of other little known techniques:
JUMPS: The biggest problem with jumps is that almost every car wants to fly nose-down. If youre slowing down or braking on the take-off, you will definitely land on your nose. So you want to be accelerating at the launch. For even the biggest flat-out jumps, like the big one at Baie, I stab the brakes and get back on the throttle just before the launch so that the car takes off nose up this is, as they say, the only way to fly. Also (and not many people know this), you want to land with each wheel at a fractionally different moment if youve ever seen a cat land youve seen this effect at work. In a car, you want all the shocks to be on a different harmonic rate if you compress them all at exactly the same moment they will all bounce back at the same moment and youll get a bonus second jump after you bottom out. I dont know why cats land that way, actually.
BUMPS AND DIPS: Mogul skiers get this principle: if turning is all about weight transfer, then the easiest time to turn is when the terrain gives you a natural transfer, as over a crest or bump. Try to plan your transitions for the moment that the car naturally unweights it will be almost effortless to get around. Conversely, in big dips or troughs, youll have a lot more traction, which means that compression areas are excellent places to use heavy brakes and more steering input.
WATER SPLASHES: You walk a tightrope with these. Like jumps, you want to be accelerating through them to keep the nose up. You want to go fast enough to have enough momentum to carry you to the other side. But you dont want to go so fast that you splash water into the air intake or onto the coils. You can try to waterproof these areas a little, but your best bet is to go into the splash medium-fast, nose up.