Technique: lateral and circumferential forces

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Technique: lateral and circumferential forces
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Technique: lateral and circumferential forces

Technique: lateral and circumferential forces
The physics of motorcycling

Even if you have nothing to do with physical terms, you won’t get anywhere without them. Accelerate, brake, lean happily – without the driving and supporting forces, nothing works when riding a motorcycle.

Werner Koch

08/17/2011

To make it short, we could now use the time-honored representation of Wunibald Kamm: the Kamm’s circle. A structure with which every driving school, every physics teacher and every motorcycle instructor makes it clear to their proteges how leaning (lateral force) and acceleration or braking (circumferential force) get along – or not. However, since the Kamm’s circle is not really of any help to us when it comes to pleasurable inclines, we try to understand the whole thing on the basis of the inclination and the acceleration or deceleration that we feel.

The concept of circumferential force is derived from the construction of the wheel. There is a diameter, a width and a circumference, i.e. the rolling length of the tire rubber. If you turn the accelerator vigorously or boldly grasp the brakes, forces arise that drive or decelerate the motorcycle: the peripheral forces.

When used one hundred percent (at about a G acceleration), they run along the direction of travel and transmit the driving forces of the motor on the rear wheel or the braking forces.

If the circumferential force is greater than the possible transmission to the road through static friction, the wheel spins or locks when braking. You then come into the so-called slip area of ​​the tire, which loses its grip and thus the motorcycle can come into a dangerous, unstable position.

However, this slip is not only caused by accelerating or braking too hard, but also by too great a lean angle. Because the inclined pleasure is only possible because the centrifugal force of the vehicle masses can be supported by the tire grip. The side force then acts on the tire contact area, which builds up with one hundred percent use, under optimal conditions there are 45 degrees of inclination, transverse to the direction of travel.

If Wunibald Kamm hadn’t invented his circle, very few of his contemporaries would know that circumferential and lateral forces do not distribute the possible liability over 100 percent, but that the sum is significantly higher depending on the incline. Anyone who demands 50 percent lateral grip, which corresponds to about 35 degrees of lean angle, can still decelerate with 85 percent of the possible braking force.

For a better understanding: 85 percent braking force corresponds to a braking distance of still 43.7 meters, which can be achieved at 35 degrees incline. Of course, only if this is initiated on the front wheel by means of a specifically designed dynamic axle load change (pressure build-up on the front brake within about 0.7 seconds).

The three phases of cornering


Technique: lateral and circumferential forces


MPS photo studio

Red: cornering forces. Green: circumferential forces.

Red phase:
Adaptive braking when turning. This creates what is known as the righting moment, especially on machines with wide tires, due to the contact area of ​​the front tire being off-center to the steering axis (sketch on the left). The driver has to compensate for this phenomenon with a counter-steering force (blue arrow). During the driving tests, a counter-steering force of up to 250 Newtons (corresponds to a weight of approx. 25 kilograms) was measured at a tilt of around twelve degrees.

Yellow phase:
Lean in the roll phase. In this driving state, the peripheral forces on the front wheel are minimal, while the driving force acts on the rear wheel depending on the speed – at 100 km / h around eight hp. The tires can now transfer high lateral forces and thus enable an enormous lean angle. If this is covered, the narrower front tire usually loses its grip first. You should therefore try to accelerate slightly as early as possible to relieve it.

Green phase:
Accelerating from an incline. At the exit of the curve, the gas is gently drawn up, causing the motorcycle to stand up and the curve radius increases. If this process is to be accelerated, additional pressure on the end of the handlebar on the outside of the curve helps. Depending on the acceleration, a more or less strong circumferential force acts on the rear tire, which is why it can cope with less lateral forces, i.e. leaning, than the front tire, which only has to transfer minimal circumferential forces in this phase.

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