Читать книгу Extreme and defensive driving - Dmitry Aleksandrovich Liskin - Страница 5

Steerability is one of the most important characteristics of car. In many cases behavior of the vehicle can be described by the term of steerability. Steerability – is speed of change the longitudinal direction of the body when the steering wheel is turned while driving. Steerability shows how car will behave while moving after turning the steering wheel.

Steerability may be low, neutral, or high. Steerability affects the behavior of car in corner and, as a consequence, the technique of passing corner.

Let us run a couple of tests. We will use two cars. The first is a retro car, a 1970 rear wheel drive sedan. Like many cars of those years, it has a big and heavy engine, which is located in the front. The second is a modern 2011 front wheel drive hatchback with a lighter engine, which is also located under the hood in front of the front axle. Fuel injection on the hatchback engine is carried out by injectors controlled by an electronic engine control unit. Torque of this engine is lower and shifted towards higher rate (compared to the sedan engine), but the engine develops more power at high rate.

The tests will be carried out on a single corner. We will enter the corner at over speed, at which a car does not “build in” the corner a little.

At the entry to the corner depress the throttle pedal and press the clutch pedal, after which we quickly turn the steering wheel all the way to the corner and keep the steering wheel in the full turned position. When speed drops below 10 km/h, press the brake and stop. First, we test the retro car.

After turning the steering wheel the car did not move along trajectory of desired radius. The front wheels began to slide. You can see the tyre trace of the left front wheel unloaded in the corner. Eventually, the car flew out of the corner. When trajectory is straightened, there is slide of the front axle and car cannot move along trajectory of desired radius, it is said that car shows low steerability or understeer. Car with low steerability when maneuvering is disposed to sliding of the front axle. When driving car with understeer, the body unwillingly turns after turn of the steering wheel.

Now we mount the hatchback and repeat the same experiment.

We got trajectory with smaller radius. If you compare the final positions of the bodies, the sedan body turned at angle less than the hatchback body.

In the modern car, in addition to sliding of the front axle, began drift of the rear axle, after which the car lost speed in drift and went inside the corner. This means that the car’s steerability is high. High steerability is accompanied by oversteer and sliding of the rear axle. When you turn the steering wheel the body of car with high steerability begins to turn quickly in the direction of corner, and there is drift of the rear axle.

Let us compare the characteristics of the cars. The old sedan front track is less than the rear track, the hatchback has the conversely. The sedan has engine at the front, like the hatchback, but the base is longer, which means the center of mass is moved away from the rear axle. The automobiles have different body types – sedan and hatchback. Thus, the steerability is affected by width of the front and rear tracks, mass and its spacing. Let us look at some examples.

In race cars prepared for drag racing, the front track is less than the rear, and the heavy engine is at the front, at large distance from the rear axle. All this creates desired very low steerability. Besides, on the front axle of these monsters – are very light wheels with small diameter compared to the rear. The huge wheelbase allows you to accurately adjust trajectory.

In the rally hatchbacks with front and all wheel drive are popular. The front track is usually wider than the rear. The steerability of the rally cars with these types of drive is often high.

Cars prepared for oval racing – are front engined sedans with large wheelbase and less the front track than the rear. Their steerability – is low.

In circuit racing cars with different steerability are used, depending on type of race track, driving style and preferences of racer. Beginning drivers who participate in circuit races usually prefer stable and predictable car behavior and tend to choose low steerability. High steerability allows you to enter corners faster, but driving car with high steerability requires the racer’s driving skills and special car settings.

Let us sum up the results of our tests and reasoning.

• on car with low steerability it is easier to fly out of the turn;

• hatchbacks have higher steerability than sedans;

• if the motor is distanced from the rear axle we get less steerability;

• low steerability is priority for drag and speed racing, such as oval;

• high steerability is well suited for non-speed tracks with lot of corners.

Settings that result in reduce of steerability:

• narrow of the front track and widen of the rear track;

• increase the front clearance and decrease the rear clearance;

• increase deflection rate of the front suspension springs and decrease deflection rate of the rear suspension springs;

• increase the bump and rebound resistances of the front shock absorbers and decrease the bump and rebound resistances of the rear shock absorbers;

• increase the stiffness of the front antiroll bar and decrease the stiffness of the rear antiroll bar;

• use of more hard front tyres and soft rear tyres;

• increase pressure in the front tyres and decrease pressure in the rear tyres;

• increase camber of the front wheels and decrease camber of the rear wheels;

• increased toe in of the front and rear wheels;

• increase aerodynamic downforce of the rear of car and decrease aerodynamic downforce of the front of car;

• increase side aerodynamic drag of the rear of car and decrease side aerodynamic drag of the front of car;

• move the center of mass to the front axle (distance the center of mass from the rear axle);

• decrease inertia moment (mass or diameter) of the front wheels and increase inertia moment of the rear wheels.

Opposite actions lead to increase of steerability.

Influence of the aerodynamic elements on car behavior strongly depends on speed of movement. At low movement speeds effect of the aerodynamics on behavior of car is negligible. But at high speeds, when heavy air flows, influence of the aerodynamics becomes significant.

In practice, as a rule, it is difficult to talk about neutral steering – this term exists in theory and mathematics. After tests, you can only tell what kind of steering appropriate of car – understeer or oversteer and how pronounced oversteer is. Only theoretical calculations (we are talking about so-called steerability factor), which take into account large number of variables, can accurately tell what kind of steerability car has. Oversteer can be estimated qualitatively by angle of drift or by distance between traces of the unloaded in corner wheels of the front and rear axles.

Besides drift angle, steerability can be evaluated by passing the following test.

Test of steerability. Gain certain speed, press the clutch pedal and quickly turn the steering wheel to fixed angle. After vehicle is rotated 180 degrees, determine the distance between the starting and ending positions of car.

Let us run this test on the hatchback and measure distance between initial and final trajectories.

As result the distance was slightly more than four lanes. If the greater distance obtained in the test, therefore, lower steerability. After changing vehicle settings (for example, spring rates, stiffness of antiroll bars), you can repeat the test and compare the results. Initial speed and turn angle of the front wheels must be the same. Steerability, which we have determined by methods described in this chapter, we will call steerability or steerability of coasting motion.

In the tests we turned the steering wheel against stop (angle of turn of the wheels is about 40 degrees), the front wheels slide. If you turn the front wheels at a smaller fixed angle (10—15 degrees), drift angle (steerability) will be higher, since load on the front tyres will be less.

With help of the controls, you can increase or decrease steerability. For example, if you press hard on the throttle pedal on rear wheel drive car while cornering, the rear axle can easily slides. That is pressing the throttle pedal on rear wheel drive car leads to acceleration not only, but also increases steerability. On front wheel drive car, on the contrary, pressing the throttle pedal causes car to straighten trajectory – this is effect of reducing of steerability. Oversteer (high steerability) may be created by using the handbrake: it is often possible to see drivers in rally races enter the hairpins with sliding of the rear axle, locking the rear wheels.

Steerability created with help of the controls will be “added” to the vehicle’s steerability. Let us say if car has very low steerability, it will be more difficult to send it into drift due to traction on the rear axle than car with high steerability.

Exercise. Try to determine steerability of your car. For this purpose you may place cones on ground that simulate a corner and do the test. Do not forget about safety: car can show very low steerability, or it can start slide and turn around, so there should be enough space on the ground.

Be careful! Incorrect “tuning” of car (it may be suspension parameters, track, ground clearance) can make your car unsafe. Manufacturers design passenger cars in such way that they are stable and do not roll over during sudden maneuvering. If there is a risk of overturning in a corner, turn the steering wheel against the direction of the corner to return vehicle to the four wheels.

It is worth noting that electronic drive control systems, electronic control of torque distribution on the wheels, dynamic electronic motion stabilization systems can distort perceived steerability of car.