WorldSBK commentator Steve English delves deep into every rider’s nightmare: chatter!
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 that can upset everything for them.
WHAT IS CHATTER?
“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, with it starting at a lower frequency.”
With a rider hanging on for dear life, they’re 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 14,000rpm 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 and even when we see a smooth, progressive slide it’s actually a continuous sequence of slipping and gripping again through the corner. This weekend at Assen, we have a series of fast corners where this action will be repeated lap after lap.
This slip-grip sequence creates a vibration at a frequency which travels from the tyre into the motorcycle, and usually gets damped by the suspension components on the bike. Chatter will develop for the rider if the frequency of this vibration coincides with the frequency of the complete bike, and then resonance will and both frequencies will 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.
NO EASY FIX
Chatter is the enemy of this compromise and therefore 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 for music as a WorldSBK bike is on track. If you randomly select notes across the guitar fretboard, you’ll hear the differences between one note 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 because of these vibrations.
A motorcycle is the same as a musical instrument. The vibrations caused by the engine or the environment all create their own notes. The key for a manufacturer is avoiding the frequencies that cause a dangerous resonance that unsettles the bike. 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,” continued Bom. “The glass will start to vibrate, and that creates an audible high tone. On a 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.
“Chatter happens when there’s relatively little 'tension' in the bike,” explains Bom. “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 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. The riding style can make a big difference and it’s why a rider like Jorge Lorenzo would have more issues riding around chatter than someone like Casey Stoner.”
Like playing a guitar and moving up through the fretboard, you’re increasing the tension on the string and changing the frequency of the bike as you open or close the throttle. 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.
How do you solve it in WorldSBK?
In the current era of aerodynamics in WorldSBK, the latest key to solving issues 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 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, 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 don'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.” In summary, 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.
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