If, since 2016 and the arrival of unique electronics, Michelins and wings, MotoGP has mainly focused on the search for traction when exiting turns, they have not, however, obscured the chattering phenomena which limit performance, just like their little sisters in Moto2 and Moto3.
Dribbling and chattering are the two words that often come up in riders' statements to explain the problems encountered on their motorcycle. We therefore also often find these names in press articles which happily confuse them…
On the eve of the resumption of the season, we wanted to take stock with several engineers, some of whom are involved in the world circuit, to clarify these different phenomena and know exactly what we are talking about. And the subject is all the more topical as it could well have a connection with the famous “black box” fixed under the Ducati saddle…
Warning: despite all our efforts to popularize the subject, the reader is nevertheless asked to have two Aspirin tablets and a glass of water near their computer or smartphone...
Dribbling or chattering, what is the difference?
“Dribbling is the rebound of one wheel, often the rear, while chattering is a vibration which concerns both wheels, therefore the entire motorcycle, but it is its effect on the front wheel which is the most annoying. »
Dribbling is quite simple to understand and visualize (See Ben Spies in Qatar in 2012, at the 23rd second of this video) but what is chattering?
“Schematically, it is a vibration of the two wheels (unsprung masses) in opposition to phase, on the elasticity of the tires and suspensions, which explains that the grip fluctuates (this is the perceived problem ) and that the suspension debates (this is what the acquisition shows).
The movement is transmitted from one wheel to the other by a small pitching movement of the suspended mass.
The amplitude is very low, the frequency between 16 and 20 Hz, the travel speed around 60 mm/s, the vertical accelerations at the wheel axis around 1g. Chattering is therefore characterized by the fact that the movements of the front wheel and the rear wheel are essentially in opposition to phase.
This does not mean that the amplitudes are necessarily of the same value at the front and at the rear but, even when the amplitude is very low for one of the wheels, its movement still remains in phase opposition with that of the wheel. other wheel.
This vibration therefore reproduces more than 16 and 20 times per second and, depending on its amplitude, is more or less felt by the pilot.
If the amplitude does not exceed 0,4 millimeters, this is not too bothersome for the pilot, but it can go up to 1 or 1,2 millimeters and there, it is really handicapping for the pilot. »
What is the origin of this chattering?
“The horizontal forces of the tires (braking and turning) are the cause: these forces fluctuate (ground relief, irregularity of pilot control, etc.), but these fluctuations induce (with delays linked to the inertia of the masses) variations in vertical forces (adhesion), which themselves will cause these forces to fluctuate.
Simplified example, the rear wheel dribble in engine braking: the engine brake requests the braking force of the rear tire, which will brake the motorcycle, then via the load transfer we lose the rear load, which will induce more pronounced sliding of the tire, which will then generate less braking force, reducing deceleration and reversing the transfer, and we start again in the other direction of the oscillation of several things (degree of freedom) at the same time (which is 'calls a mode).
Multiply that by two and put the phenomena in phase opposition and you have a small idea of what chattering is…”
When chattering appears, how do you react?
“Above all, in concrete terms, it depends on the driver because it remains partly a phenomenon linked to comfort. For example, Marc Marquez often suffered chattering in Moto2 but didn't care much about it. Certainly, the bike was not really "comfortable" but a competition bike is not made to be comfortable and the ideal still remains to "make do" because all the competitors are in roughly the same boat regarding chattering…
The most disabling chattering is that which occurs on the front axle, from releasing the brake and aiming for the apex until the moment of re-acceleration and the start of lifting the motorcycle. There, the pilot then sees his sensations disturbed by the vibrations and often loses time by using a trajectory that is not perfect.
For riders who do not have this ability to ride with this problem, we are going to find tricks which will degrade a little the effectiveness of the compromise represented by the bike adjusted to the best, to solve this problem of comfort and grip. »
How do you combat this?
“The means arise directly from the description of the phenomenon and its equation: unsprung masses (but we have very little latitude in this area), stiffness of the tire (at least via inflation), stiffness and damping of the suspension .
If we stiffen the damping too much in the area concerned (60 mm/s at the wheel) the suspension will move less and we will switch to the associated mode (lower frequency) of pitching on tires with locked suspension, which is not better. The “valve” of the shock absorber in this speed range will therefore be decisive.
Without forgetting the internal cushioning of the tire, but there, only the manufacturer can do something.
In fact, the important thing is to “break” this resonance or, at least, to try to move it to a less sensitive area of the piloting but, concretely, we are quite limited by the very restricted range of our settings, even if In MotoGP we have acceleration sensors on the wheels (editor's note: most teams have them in MotoGP, very few in Moto2 and none in Moto3) in addition to suspension sensors, to clearly observe the phenomenon. »
So it's not easy to satisfy the pilot?
“Regarding the rider, it is important to remind him that chattering appears more easily with a rider who will hold the handlebars very tightly than with a more relaxed rider; Sometimes it's a solution in itself to tell the rider to be less stressed on the handlebars.
The rider in fact represents a significant mass whose connections with the chassis (his arms, his feet, his buttocks or his thigh placed across the saddle, depending on the sway) present elasticity and damping.
Its action is therefore anything but negligible. »
Regarding tires, do you receive technical data regarding these resonant frequencies?
“Not directly regarding the frequency, because it does not only depend on the tire, but also on the value of the unsprung masses.
What depends on the tire is its stiffness and its internal damping, which result from the construction of the tire (rubber composition and carcass structure) and also, of course, from the inflation pressure.
Measuring these two quantities requires specific testing means, which the teams do not have, but the manufacturers can communicate the values; with more or less good will depending on whether or not they consider them confidential.
Then, the resonance frequency is deduced directly from the “stiffness/unsprung mass” ratio.
Finally, why today does the word chattering always come up in pilots' statements?
“Because for many years, motorcycle and chassis manufacturers have strived to refocus the masses around the center of gravity.
This obviously offers many advantages in terms of handling but it makes them more sensitive to vertical forces, which themselves are greater because of the very significant increase in tire grip.
On the ice, there is never chattering! »
And if, currently, we mention a manufacturer who (perhaps) would seek to fight against this chattering by decentering a mass in relation to the center of gravity, it is indeed Ducati that we are talking about, as we will see in there second part of this article...
