#TBT: Unlock faster autocross times in your front-wheel-drive car | Articles

Keeping the Front Wheels Connected

Picture any car under acceleration. What happens? Easy: The front wheels unload and the rear wheels become more heavily loaded due to weight transfer. In extreme cases, such as drag racing, the front wheels may actually leave the ground.

On a rear-wheel-drive car, this weight transfer is desirable under acceleration, as the heavily loaded rear tires provide great traction (even though the fronts can’t steer!). The harder the car accelerates, the harder it plants the tires, allowing them to put more power to the ground.

If they should begin to slip, however, acceleration is reduced, weight begins to transfer back to the front, and traction can only be regained by reducing the torque applied to the wheels. This is why drag racers try not to spin their tires off the line. You often see a driver shut down completely if he happens to smoke the tires, knowing that his race is hopeless.

In the case of the front-driver, when acceleration causes weight to transfer off the front tires, further acceleration is limited. But even if a tire loses traction, the resulting reduction in acceleration tends to reduce weight transfer, putting weight back on the front wheels and reestablishing their traction capabilities. The problem self-corrects. For this reason, front-wheel-drive cars are generally easier to launch and less sensitive to driver technique than their rear-drive counterparts.

Weight transfer doesn’t only happen during acceleration. Under braking, weight transfers to the front wheels, which also works against the front-wheel-drive car, as it is already likely to have most of its weight on the front wheels. Under braking, the rear wheels become lightly loaded and susceptible to lockup.

Nose-heavy rear-wheel-drive cars face the same problems, but mid-engined or rear-engined cars can benefit from the forward weight transfer, achieving a good balance of load between the front and rear wheels, so that all four tires contribute to braking.

Under steady-state cornering forces, weight transfers from the inside to the outside tires, affecting front- and rear-drive cars similarly. The essence of suspension tuning is to try to balance the loads on the four tires, and to keep each of the tires pointing in the right direction.

Once you see how weight transfer tends to hurt front-drive cars more than rear-drive cars under acceleration and braking, it is easy to see why most pure racing cars are rear-drivers: The higher the g-force, the greater the effects of weight transfer.

This also explains why front-drivers perform well under slippery conditions. A lack of traction reduces weight transfer—the front driver’s worst enemy. If a rear-wheel-drive model and a front-wheel-drive model are equally quick in typical dry conditions, the front-driver will suffer less and gain the advantage when the track gets slick. Conversely, the rear-driver gets the upper hand when the track surface and tires get really sticky.

Other Factors to Consider

The front-wheel-drive car’s inability to transfer power through the inside-front wheel is probably the greatest equalizer since Samuel Colt’s .45-caliber “Peacemaker.”

Many high-powered front-drivers can produce stunning acceleration numbers in drag strip tests, but a little bit of lateral g-force sends all that horsepower up in smoke from the inside front tire. A limited-slip differential can make a huge difference, but few car manufacturers are willing to absorb the added expense of a feature that only the most vigorous driver will appreciate. Thus, limited slips for most front-wheel-drive cars remain an expensive aftermarket item, but one that is essential for a serious autocrosser.

Roll stiffness is an important parameter in the setup of any sports or competition car, whether front- or rear-drive. Cornering forces act on the car to cause lean or “roll,” and this is resisted by, you guessed it, roll stiffness. Body roll is not necessarily a bad thing, since it gives the driver a feel for what the car is doing, but too much body roll can use up suspension travel, disrupt the suspension geometry, and allow the outside tires to lean over, reducing the tires’ contact patches.

Roll stiffness is determined primarily by spring stiffness and anti-roll bar stiffness. In competition classes where springs may be changed, they can be used to adjust the roll stiffness. As a car gets lower and has less suspension travel available, it is necessary to increase the spring rate to prevent bottoming.

Anti-roll bars, also known as “sway bars” or “stabilizer bars,” are auxiliary springs that are free to move up and down as the car travels over bumps, but are forced to twist as the body leans, thus resisting the lean. The amount of roll stiffness at the front of the car, relative to that at the rear (roll couple distribution), determines how weight transfers to the outside wheels during cornering.

More rear roll stiffness (heavier rear bar or springs) causes weight to transfer to the outside-rear tire and helps to equalize load on the front tires. More front roll stiffness causes weight to transfer to the outside-front, while distributing the load on the rear tires more equally.

Front-wheel-drive cars are necessarily nose-heavy, and have a natural tendency to overload the front tires in hard cornering leading to understeer. To compensate for this, manufacturers provide ample rear roll stiffness, so much that the inside rear wheel may unload completely under high cornering forces, rising into the air in the now-familiar “fire hydrant salute” that has become the hallmark of front-wheel-drive racers.

At that point, the outside rear tire is carrying 100 percent of the rear weight, and load on the front tires is distributed as evenly as possible, so understeer is minimized. Once that inside-rear wheel is in the air, the rear anti-roll bar has no effect. Like the wheel itself, it could theoretically be unbolted from the car in the middle of the corner.

Under SCCA Stock category autocross rules, front anti-roll bars are free—meaning they can be changed, removed or added—but rear bars cannot be touched. On a front-wheel-drive car, more front roll stiffness reduces roll and keeps the outside front tire from leaning over, but at the same time it increases the loading on that tire, and decreases load on the inside front, resulting in increased wheel spin.

Whether the car ends up faster or not will depend on the individual car and perhaps the course. It is not uncommon for a car to “feel” better, but actually run slower when front roll stiffness is increased.

If you are not sure whether your car would benefit from roll stiffness changes, you may be able to test it by loosening the end link bushings to make the existing bar less effective, or by installing harder bushings to increase its effect. If you are dialing-in the car for competition purposes, do this only when you have the opportunity to make back-to-back timed comparisons, so you will not be misled. Remember, the clocks don’t lie.

Shock absorbers (or “dampers,” as the British more properly call them) can be used to fine-tune handling just like anti-roll bars, but keep in mind that they only function while they are moving—that is, when the car is going through some sort of transition, causing suspension movement.

During steady-state cornering on a smooth surface, shock absorbers have no effect (except where they are part of a structural link, like a MacPherson strut, and even then, the damping characteristics make no difference). The faster the suspension moves, the greater the effect of the shock absorber.

Stiffer rear shock absorbers act like a stiffer rear anti-roll bar during transient maneuvers. They may help a car turn in better, but can make it tricky to drive through slaloms. Stiffer front shock absorbers can make the car respond quicker to steering inputs, but may also momentarily overload the outside tire, unload the inside tire, and provoke wheel spin.

A complete discussion of shock absorber characteristics is beyond the scope of this article, but I would urge restraint in the use of stiff shock absorbers to “tighten up” a car’s handling. For every car that is enhanced by the proper application of aftermarket shock absorbers, there are probably two more that are turned into evil-handling beasts by excessively stiff shock absorbers.

My personal approach is to keep the shock absorber settings fairly light and let them simply serve their original purpose, which is to damp unwanted spring oscillations. This is another area where what feels best may not be the fastest, and back-to-back time comparisons are essential.

Driving Techniques

Although front-wheel-drive and rear-wheel-drive cars may differ greatly in their designs, the basic principles involved in racing them are similar. There is not as much difference as some people think.

In either case, the driver must take the car to the cornering limit, then balance the car at that limit through delicate use of the throttle, steering and brakes—the only controls available. Subtle differences in technique can be used to extract maximum performance, and each car will respond differently to the inputs of the driver.

Correct Line: In all race cars, on all race tracks, it is essential to be on the correct line, which is generally the most gentle curve that can be fit through the maneuver. Precision is vital in autocrossing, even more than in road racing since the size of the track is scaled down.

If a pylon is keeping you from straightening out a maneuver even more, then you know that you will have to nearly touch that pylon as you pass by it. To leave room next to it is to give away valuable time.

Aggressiveness is important, but only up to the point at which precision is compromised. The winning driver is the one who can coax the car around the course on that ideal line while maintaining the maximum possible speed, just shy of sliding off-line.

A common habit of front-wheel-drive pilots is the tendency to turn the steering wheel too much. As simple as it sounds, this is a major limitation on the competitiveness of many drivers.

Front-wheel-drive cars nearly always understeer, and the driver may tend to crank in more and more steering as the car strays from the intended line. The poor, overloaded outside front tire is already operating at an excessive slip angle, and more steering just makes things worse. It’s usually better to back off on the steering, which gives the tires some relief, and translates into higher g-force and lower lap times.

Along with too much steering, drivers tend to apply too much power to their front wheels while cornering. Give those poor front tires a break. If you want them to deliver maximum cornering load, then you had better not ask for much acceleration or braking force at the same time. Easing off the throttle will generally allow the car to turn better, although in those rare front-drivers with limited-slip differentials, application of power can help pull the car around a corner.

Try this experiment: The next time you reach the limit in steady-state cornering, lift off the accelerator and see if you can feel the car turn in sharper. By gently “rolling” on and off the throttle and delicately balancing the steering input, maximum cornering force can be coaxed from your car, and that is the key to quick times. Hand Position: Much has been said and written about steering technique: “10 and 2” vs. “9 and 3;” shuffle steering vs. hand-over-hand; one hand vs. two, etc. All methods have their pros and cons, and their proponents and opponents.

Personally, I don’t worry too much about it, because I figure the car doesn’t know how I’m holding the wheel. The important question is not “What is the proper way to hold the wheel?” It is more like, “Is my method of steering causing me any problems? Are there times when I can’t move the wheel fast enough? Times when my speed is limited by steering, not by cornering force? Do I tend to steer too much? Do I lack the strength to turn the wheel fast enough at times?” By contemplating the answers to these questions, and how the hand positioning might be improved at that critical moment, the driver will be able to determine for himself what changes, if any, should be pursued.

Trail Braking: Trail braking gained acceptance among drivers in the ’70s, and is practiced by most race drivers today. Simply put, it is the practice of turning into a corner while still braking, then easing or “trailing” off the brakes as steering is increased. The goal is to gently transfer vehicle loading from braking to cornering. This technique contrasts with “classic” driving technique, which demanded braking in a straight line prior to turning.

Ideally, trail braking enables later braking, and the heavily loaded front tires will respond well to initial steering input. The transition from braking to cornering can be tricky for the driver to master, however, and the additional braking forces may simply overload the outside-front tire. Also, the lightly loaded rear wheels are susceptible to lockup, a condition that can start the back end sliding around. That may be good if the driver is expecting it and uses it to get the car started around the corner, but can be very bad when the car ends up backing into the turn.

For various reasons, some cars respond better than others to trail braking. Our 1986 Honda Civic Si, for example, loved a little trail-brake to get it started around a corner, while our A2-chassis VW Jetta never seemed to care for the extra load on the front tires. Each time I drive a new car, I make a point of trail braking into a corner (one with runoff room) to see if it makes the car turn better. If the car seems to like it, I make it a standard practice for that car.

Trailing throttle can be thought of as a kinder, gentler form of trail braking. Instead of applying braking force, the driver simply “trails off” the throttle to transfer weight to the front wheels and off the rears. As discussed previously, most cars turn in more sharply under lift-throttle conditions, whether front- or rear-wheel drive.

Left-Foot Braking: Left-foot braking has become popular among autocrossers in the last decade, but it is not a technique that I practice or recommend. In theory, a left-foot braker can make a quicker and smoother transition from acceleration to braking, and vice versa.

In practice, my observation has been that most left-foot brakers use the brakes far too much, resulting in slower times, loss of steering precision, and overheated brakes. Personally, I have tried the technique, and simply not found it to be beneficial, but I know several top level drivers who practice left-foot braking religiously. The only type of car in which I believe it would be valuable is a turbocharged car with an automatic transmission, where it would allow the driver to maintain boost and eliminate turbo lag by keeping a load on the engine.

Slaloms: Slaloms are the playground of front-wheel-drive cars, one place where they can make up time on the rear-drivers. Typically, the rapid transitions of a slalom promote instability of a car, in a manner that I call the Pendulum Effect. (It has a technical name that sounds more sophisticated.)

You know the feeling: The tail swings a little bit one way, then farther the other way each time the car turns, wagging its way through the slalom until the car either scrubs off speed or spins out of control.

The front-wheel-drive car, with its forward weight bias, tends to be more stable here. If the tail swings out, the driver need only apply power and keep the front wheels pointed the way he wants to go, to pull the car out of the incipient spin. In fact, an aggressive front-wheel-drive racer can power his way through slaloms with abandon, rolling off the throttle a bit if understeer takes over. The Pendulum Effect offsets the front-wheel-drive car’s natural tendency to understeer.

Lines: Lines through a corner are really very similar for front- and rear-wheel-drive cars. Every basic driving principal that applies to rear-wheel-drive cars also applies to front-wheel-drive cars. Since the front-drivers have trouble accelerating while under any cornering load, the importance of a late apex and straight exit from a corner is greater.

A rear-wheel-drive car can accelerate while “unwinding” from a corner, but the front-driver needs to get rid of cornering force as early as possible to enable it to accelerate. Once again, many front-wheel-drive drivers tend to apply too much power and too much steering, when backing off on both would relieve the front tires and free the car to achieve a higher exit speed. Of course, those fortunate front-drivers with limited-slip differentials benefit from power application, as discussed earlier.

Downshift: Nearly every autocross course includes a tight corner where the driver must decide whether or not to downshift to first gear. The rule of thumb has always been to try staying with the higher gear, and only plan to downshift on the next run if the rpm proved to be too low at the exit corner. The distraction of the downshift, the potential for missed shifts, the engine braking effects and the likely wheelspin all tend to offset the theoretical performance advantage of downshifting.

For front-wheel-drive cars, the drawbacks of downshifting are greater, and the benefits less. First gear wheelspin is likely to much worse, and when a front wheel start to spin, steering precision goes away, making it difficult for the driver to maintain his intended line.

Torque steer can also wrest the steering wheel away from the driver. Most front-wheel-drive shift linkages are not as precise as the typical rear-drive linkage, so more of the drivers’ concentration is needed for the shift, and missed shifts are even more likely. Also, the application of power detracts from the cornering power of the front driver.

All things considered, a front-wheel-drive car can stand to chug around a corner at low rpm in second gear and still be quicker than it would be in first. The extra acceleration that seems attainable through downshifting simply isn’t, due to the nature of the front-wheel-drive car. Through the years, I have seen drivers achieve big improvements in time simply by resisting the temptation to downshift.

New World Order

Front-wheel-drive cars are here to stay, and will continue to dominate the marketplace, even though rear-drivers have natural advantages for high performance and racing applications.

Going fast in front-wheel-drive cars involves understanding and accepting the differences, but applying the techniques that race drivers have always used.