Wheels - Driving dynamics 1
It is always explained from the perspective of the driver, but here it is considered from the perspective of the tyre. How does he feel when cornering? No, not emotionally, just factually. The front ones notice something first
because someone turned the steering wheel. They suddenly find theirselves slightly twisted to their previous driving direction.
Again no, this one does not change completely in relation to the new direction. No car in this world follow directly the newly set direction, indeed rather, but also not complete, one with hard rubber tires. Because there,
together with the tendency to drive straight ahead, there is a certain amount of pushing on the contact area between the tires and the road.
With pneumatic tires, the contact area of the tire moves inwards, the more the more the steering is turned. If there were no humps (picture below) on the rim, it could even be forced so far inwards under large lateral forces
that it lifts off the rim well, loses air and then even jumps off the rim completely.
The tire is said to need a certain amount of slip in order to generate any power transmission through friction. The rims of the drive wheels also turn meticulously when starting off. The tires deform towards their contact points
and the one with the greater friction with the road then ultimately powers the car.
So when cornering, the direction of the wheel is different from the actual direction of movement of the wheels. One speaks of a slip angle (picture below). If the ahead is larger than the rear, then you have to force the car into
the curve with a lot of steering effort until it is overwhelmed at too high a speed and, in relation to the vehicle, decamps straight ahead.
They are also called understeerers, very often due to more mass on the front axle. Conversely, of course, there are also oversteerers, i.e. cars with more mass at the rear, whose rear tends to swerve away in the curve. These
are harder to catch again because you have to act very quickly and people are not used to steering outwards in a curve.
However, the reaction of rather inexperienced drivers is not an option either, because you usually don't catch an understeering vehicle by turning harder and thus overtaxing the front tires even more. But at least you don't get
spinning motions like in the case of wrong oversteer.
|The problem of weight distribution is why there are mid-engines and transaxles.
But it would be too easy to base the question of understeer or oversteer solely on weight distribution. A lot of gas makes the front axle more unsafe with front-wheel drive and the rear axle with rear-wheel drive. The trick with
the pulled handbrake is well known, which, if it acts on the rear axle, can cause oversteer, regardless of whether you have front or rear wheel drive.
|Production vehicles are usually tuned to understeer.|
A tire only has a certain amount of power transmission available to the road. This can be used longitudinally for acceleration or braking, or laterally when cornering. It can also be divided between the two, but it doesn't
become more as a result.
And since it is easier to provoke wheel spin through drive or braking forces than through safely controlled lateral driving, the longitudinal dynamics seem to confuse the system a little more often, especially with sporty drivers.
High-torque vehicles often choose wider tires on the drive axle for propulsion.
This is much more difficult to counteract constructively with front-wheel drive or understeerers. Here there was at most a significantly larger track width at the front, which is at least useful for lateral dynamics. The front-wheel
drive is always somewhat disadvantaged because it relieves itself when accelerating hard, and even more so when driving uphill with increasing gradients.
But it doesn't have the best prerequisites for braking either, because the heavier it is, the more mass there is on the front axle. A really well-distributed mass on both axles can only be found in vehicles that brake heavily only
with a rear engine. But they in turn tend to oversteer as a habit when cornering quickly. However, there is now a wealth of countermeasures.
For lateral dynamics, the imaginary axis running vertically through the center of the car plays an important role. One speaks of the vertical axis and in relation to the vehicle of 'yaw'. It basically describes the additional turning of
the vehicle, here in the curve.
Finally, one question: Let's assume the hypothetical case that you used almost all your money to buy a somewhat older vehicle in order to participate in races or rallies. Where do you invest the rest? You will probably
mention engine performance at this point, but professionals in the field recommend the tires.