Tech Briefs: Chatter, Or When Harmony Overwhelms A Motorcycle

Jack Miller, Ducati, Misano 2019 MotoGP round

If you eavesdrop on a rider and his team debriefing in a garage during a session you’ll invariably hear a comment about chatter. It’s the oldest enemy for a rider because it robs confidence. With the bike moving underneath them they can’t open the throttle and the problem exaggerates the longer it continues.

But what exactly is chatter? It’s a harmonic imbalance created by vibrations and frequencies on a motorcycle. Man and machine need to be in perfect harmony to go racing but sometimes it’s the imperfect harmony of frequencies can upset everything for them.

"The word chatter gets used a lot, but a lot of the time, it's not strictly accurate," says former Moto2 crew chief and technical expert Peter Bom. "It's applied to everything which shakes and vibrates, but in the original sense, chatter was caused by the front or rear tyre (or both). Improved tyre construction has eliminated a lot of what used to be called chatter."

"There seem to be a wider variety of types of chatter now. The newer forms of chatter are at a lower frequency, and can sometimes be an old-fashioned bouncing movement. The video of Pol Espargaro from Thailand is a good example, where he had horrible chatter and had to hold the bike up on his elbow."

Bad vibes

With a rider hanging on for dear life they are doing all they can to ensure that while they are on track, they’re extracting every last ounce of performance from their bike. The only problem is that their bike is a box of vibrations that houses a 15,000rpm engine running on a bumpy circuit with a 70kg rider bouncing from one side to another.

The rider is pushing as hard as they can, trying to go fast. But that takes them deep into oscillation territory. "The harder you push a tyre, the more it slides instead of just rolling through the corner," Peter Bom explains. "Even a smooth slide is actually a continuous sequence of slipping and gripping again, slipping and gripping."

"This slip-grip sequence creates a vibration at a frequency which travels from the tyre into the motorcycle, and usually gets damped there by the frequencies of the other components of the bike, especially the suspension damping and springs. If you're unlucky, this frequency coincides with the inherent frequency of the motorcycle (the sum of many individual frequencies for each component of the bike) and these two will start to resonate in sympathy and amplify one another."

These vibrations and movements all have an effect on performance and if they’re not dealt with correctly they can wreck a weekend or indeed a season. Racing is all about compromises. Bikes need to be stiff but supple. They need to be forgiving yet also razor sharp.

Resonant frequency

Chatter is the enemy of this condition but something that needs to be constantly thought about. Every object in the world has a frequency when struck but musical instruments give us the best example of how these can affect a racing motorcycle. If you pluck a guitar string it releases a musical note. The guitar can release frequencies over a huge range, approx 1300 hertz, and this makes as powerful a weapon as a MotoGP bike is on track.

If you were to play a guitar and randomly select notes across the fretboard you can hear the differences between one and the next. You can feel it too because the vibrations cause the pitch of the note. Beethoven was deaf but could still “feel” the music from these vibrations.

A motorcycle is the same as an instrument the vibrations caused by the engine or the environment all creating their own notes. The key for a manufacturer is avoiding the frequencies that cause a dangerous resonance. This is the natural effect of harmonics with their frequency amplified.

"It's a bit like when you wet your finger and glide it around the edge of a wine glass," explains Peter Bom. "The glass will start to vibrate, and that creates an audible high tone. In our motorcycle, the rider can feel a high-frequency vibration in his backside or in his hands, while the bike takes a wider radius through the corner, in other words, it runs wide."

Every object has natural resonance frequencies that can cause them to amplify the frequency. Bridges and buildings have collapsed due to this phenomena - it’s why armies will walk out of step across a bridge for instance - and the destructive power of this has the same effect on a motorcycle. It causes it to vibrate and move underneath the rider.

"It starts in the tyre as a vibration, and this has to resonate with the other frequencies in the bike to create chatter," Peter Bom says. "The reason that not everybody suffers from chatter with the same tyres is because they are all riding different bikes."

Keeping everything taut

"This chatter happens when there is relatively little 'tension' in the bike. It often starts just after the rider releases the last bit of brake pressure, and disappears as soon as they open the throttle again. So especially during the rolling phase. It costs lap time, but it doesn't make you crash."

The easiest cure for this on a bike? Change the frequency by winding on the throttle or adding a touch of brake. "Riders can influence it themselves," Peter Bom explains. "Keeping tension in the bike for longer or using a different line can make a big difference. Remember the photos of Rossi, where you can see him still holding the front brake on while he is already starting to open the throttle? A rider like Lorenzo would have more issues than someone like Stoner."

Like playing a guitar and moving up through the fretboard you are increasing the tension on the string and changing the frequency. The goal on a race bike is to tighten the string by accelerating and forcing the centre of gravity to the rear of the bike. Easier said than done though for a rider with the bike bucking underneath them due to the forces of chatter, and pushing them off line.

Opening the throttle could make things even worse, though, and lead to a crash. The cure can be worse than the disease... Sometimes, just riding out the chatter and sucking up the loss of time is the better option.

Ten thousand piece puzzle

In the current era of aerodynamics in MotoGP and WorldSBK the key can come from the fairings of bikes. The ultra light carbon fibre shrouds are crucial to performance but depending on how the wind hits the fairing it can have a big effect on what happens with the bike. Is that air moving cleanly over the bike or is it causing a downstream effect to another part?

How can you reduce the effects of chatter? There’s lots of ways around it ranging from riding style and body positioning on the bike to teams adding weights to different parts of the bikes to ensure that the frequencies from certain parts can be eliminated. This equates to adjusting your style on the guitar. Instead of a harsh strum, you can reduce the force and suddenly the notes are the same but they come out cleaner.

There are no guarantees, however. "In terms of solutions, I have seen every suggestion under the sun," Peter Bom says. "You wouldn't believe it. The trouble is that one solution might work today but not tomorrow. You want to reduce the chance of resonance starting, but there are hundreds of components involved."

"Lead-filled axles, kilos of wheel-balancing lead stuck on all over the bike, extreme damping settings, you name it. If you had the choice, you would struggle on and then use a different tyre and try to get the best out of that. Sometimes you didn't have that luxury, though. A tyre that was sensitive to chatter would always start chattering more as it wore, a new one was OK for a little while."

A crew chief is in a constant battle to try and make the notes sing from their instrument. Sometimes it means harsh words with the rider but in most cases it’s about finding a compromise.


This is part of a series of articles published in partnership with RacingLowdown.com, run by MotoMatters.com contributor Steve English.

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year: 
2020

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Comments

When I raced, conventional wisdom on chatter said "go faster". Easier said than done. An alternative solution: increase tire pressure (reduce traction).

Salad box

Anyone have information to share about Ducati's contraption and how it relates? And is Honda's bento box the same? Assuming it is offsetting unwanted frequencies that disharmonize.

(Jinx, still there? How about you?)
:)

The seat mast box contains the Ducati electronics, they put them in there in 2017 to make more room for aero in the fairing.

Matt Oxley wrote a article on mass dampers a while back when it was first proposed as a reason for the Ducati seat unit box.