If you’ve been involved with motorcycles or racing for any length of time it’s almost certain you’ve come across people talking about chatter. At the same time you were probably treated to a lot of hand gestures. These probably looked like someone miming mowing the lawn with a machine gun, accompanied by much pursing of lips and many frowns.
That’s because from the top of the MotoGP tree, to a Sunday afternoon club meeting at Mallory Park, if someone is pushing their bike hard, chatter is probably about to make an unwelcome appearance. But getting a straight answer about what chatter really is, is almost as hard as trying to get rid of it. There are a lot of definitions, explanations and solutions depending on who you talk to – and the trouble is they’re all people who know what they’re on about.
But the one caveat everyone seems to pin to any explanation, is that they’re not sure. And that’s probably because chatter is one of those things that turns up and disappears without warning. It’s like the Spanish Inquisition, only less well received. So what is chatter?
To understand that you need to put yourself in a riding frame of mind, so picture yourself with a wide-open throttle hurtling down a straight. At your given marker you pop up, grab a handful of brakes and control the back-end of the bike as your turn-in point approaches. When you get there, you give the bars a nudge to turn and start to reduce the front brake pressure and it’s at that point this otherwise-beautiful display of riding turns to trash.
You’re not sure when it happened, but somewhere around the point of full-lean and letting the brakes off, the bike just went nuts. It wasn’t a tank-slapper sort of instability; it was more like the wheel weights had slipped round the wheel rim. So not only was the front wheel no longer balanced, it was actively unbalanced –like a brick-filled tumble dryer on a trampoline.
In fact the vibration from the front end was so bad it travelled up your arms and affected your vision slightly. Any confidence you had in the front end vanished immediately, of course. It was like there was far less grip – except the front didn’t tuck, it simply walked away from you. Then at some point, when the apex and correct line were just a distant memory, everything went back to normal, and you were able to finish the turn and get back to the pits.
That is what chatter feels like. By definition it’s much less scary. Chatter is a self-excited vibration of the front or rear wheel. In simple terms, that means it’s a vibration that starts of its own accord, tries to get bigger and wouldn’t want to exist if the wheel wasn’t chattering in the first place.
Think of it this way – when you ride over a series of bumps, the wheel has no choice but to move because the bumps deflect it. In the case of chatter, which can happen on a flat surface, the alternating force that drives the vibration is created by the motion of the un-sprung mass itself. A vicious circle if ever there was one.
Of course bikes vibrate all the time. In fact as soon as the engine runs there are vibrations running all through the chassis. But the thing
about chatter is it happens through a fairly narrow frequency range of 15 to 20Hz – although it seems to peak at about 17Hz. That means your suspension is moving up and down 17 times per second. It doesn’t move through the whole stroke though. The amplitude for most chatter is generally between 0.5mm and 3mm.
To help visualise that, think about the pneumatic hammers used to dig up roads. They cycle at roughly 25Hz, so your suspension is moving up and down by about 3mm, at nearly the same frequency as a road drill. Of course the real question is why it happens in the first place. Sadly you’re not going to find the answer to that here, because different people give different answers. If there is a consensus, it’s being kept quiet.
The more plausible suggestions we’ve heard have come directly from tyre and suspension types. To understand the tyre explanation you need to recognise that tyres form part of your suspension system. Ask a bike and watch what happens to the tyres as they do – they compress then return to their original position. In other words they behave like springs.
The strength of a spring is given by its rate, which is simply a way of describing how much force it takes to compress the spring a given amount. For example, if the force a 10 kilogram mass exerts on a spring compressed it 10 millimetres, then it seems obvious the force a one kilogram mass exerts will compress it one millimetre. We can say the spring has a ‘rate’ of 1 kg/mm (one kg per mm).
For reference, a 1 kg mass exerts 9.81 newtons of force, so we should say the spring has a rate of 9.81N/mm. But if we include our tyre in the explanation (picture a spring balanced on top of the tyre), we will get a different spring rate. We know the 10kg mass compresses the spring 10 mm, but that force also acts to squash the tyre too.
In this case it may squash it by 0.5mm. So to work out the spring rate of the total system (spring plus tyre), we look at the change in height versus the force applied. In this case the spring compressed 10mm and the tyre by 0.5mm. So applying a 10kg mass compressed the entire system by 10.5mm. That now makes the overall rate 0.95kg/mm (10kg ÷ 10.5mm = 0.95kg/mm), which is a five per cent change.
Much of the spring force created in a tyre is of course from the air pressure inside, but some of it comes from the rubber construction too. In fact the rubber influences the damping characteristics of the tyre more, and that is apparently one of the things that influence chatter. Take a look at a photo of any bike when it’s leant over and you’ll see the sidewall of the tyre sort of folds in on itself under heavy load.
The tyre engineer explained that when the sidewall collapsed in such a way that it almost formed a straight path for the forces to pass
from the rim to the contact patch, the tyre behaved as though there was infinite damping. Regardless of the spring force in the tyre therefore, it behaved as though it were solid, which made it rather skittish. Of course as soon as that skittishness made the suspension move, it also changed the load on the tyre, which changed the path the force could take.
The end result was that the suspension was now moving, but the driving force for that movement had disappeared. The suspension got it under control, and everything started to move back to its original position. Except for whatever reason, the tyre folded in the same way and the process repeated itself.
That’s the explanation from a tyre man’s point of view, what about from the suspension engineer’s? They don’t seem to worry much about why
chatter happens, only how to stop it getting out of hand. Speak to any suspension technician about chatter and depending on how the
bike is currently set they’ll suggest one of several options. This might involve changing the spring rate in the fork, rebound damping, air-gap or even the tyre pressure itself.
In some cases they may even suggest adding or removing weight from the unsprung mass (the parts that don’t appear to move when you bounce on a bike). In each case the idea is simply to change the spring or damping characteristics of the system, or at least make it vibrate at a
different frequency from the 15 to 20Hz that seems to cause problems for the rider.
From an internal point of view, the one thing suspension people do seem concerned with is hysteresis in the damping circuits. To understand this think about what happens when the suspension compresses. Oil is pushed around the system or a piston is pushed through the oil. In either case, oil will have to squeeze past some shims, and the harder the oil pushes on these shims, the more they bend out of the way.
When the oil stops moving and flows in the other direction the shims in the compression stack return to their original position, and the rebound shims start to open. But what if the suspension changed direction again before compression shims had returned to their original position? In that case, there would be so little restriction to flow for the current velocity, that there would effectively be no damping force.
Equally, the same may be true if the suspension quickly changed direction again. This high-speed changing of direction should sound familiar of course – it sounds like chatter. So some systems can struggle to control chatter if the damping circuits aren’t able to respond quickly enough.
So we know riders don’t like chatter for the reasons described above. Chassis technicians don’t like it either because it makes the riders
moan a lot. And it’s a problem that seems to affect all kinds of bikes, tyres and riders (except perhaps the steady ones). But that doesn’t mean it’s something you really need to worry about as a road rider. Unless you’re really pushing on hard, chatter is something you’re highly unlikely to come across, and if you do, it can probably be tuned out with some suspension tweaks. If people are talking about it on the road, it’s highly likely there’s something wrong with their bike, they’re mistaken about what they’re feeling or they’re
just lying. In short, don’t worry about chatter – leave that to racers.