Ever had a tank slapper? It's common on a motorcycle, so what can we learn from motorbikes?

One of the scariest moments you can experience on a bicycle is head shake or a ‘tank slapper’ as it’s more commonly known. 

On a bicycle it tends to happen when descending quickly. I know lots of cyclists dread it. On a motorcycle, with all the extra speed and mass it can be catastrophic.

So I thought I would look at the problem from the point of view of the key chassis dynamics and see what ideas we can perhaps borrow from motorcycle set-up if your bike shakes its head more than you’d like.

You will have all read about how to physically handle the dreaded ‘tank slapper’, and how you might give rise to them through gripping the bars too tight too.

So what’s going on, when your bike tries to kick you off like this? 

Well, a good example can be found down the local play park. If you push a swing at the right moment, you can stop it. If you push in the ordinary way, however, just a little energy applied at the right time to the push can send the swing ever higher. 

As we ride along, our front wheels make thousands of tiny movements in operation. It’s imperceptible, and happening all the time. The magic that keeps it in line is Trail Force. 

The correcting movement is constant, and divots, dips and stones in the road lie in wait to contact the tyre and give it the shove we don’t want, in the wrong direction at exactly the right time. 

We can’t judge the ‘swing’ in our steering in the same way as we can a Child’s swing and we can’t time the push, or resist it, so any input we provide tends to make the situation worse.  That’s why all the advice says ‘let go’ and let it settle on its own.

Whether on a motorcycle or a bicycle, the physics is the same, and Trail Force is our friend, so the motorcycle experience should be informative. 

Rake and Trail 

Stability in the steering system on a bicycle or motorcycle is delivered through Positive Trail, generating Trail Force. This means the vehicle can make relatively straight progress. It’s essentially a ‘self-centering’ system. 

Push a bike by its saddle, and you’ll see this system in action. When you’re riding, take your hands off the bars and with practice you can just plough on. That is our friend Trail Force again.  

Trail forces combine with precessional or gyroscopic forces which (in order to initiate a turn) must be overcome through roll forces, generated by counter steering, then accelerated by gravity. In a well-set-up performance bike, we design the frame geometry so that the moment is short, and almost imperceptible, and this gives rise to intuitive turn-in. 

The bike seems to go where you want it to without much effort. We call this intuitive handling. It just seems to go where you want it to go. This is desirable of course. 

But it can also give rise to certain undesirable characteristics, like a propensity to shake its head. 

To counter this, we (as bike designers) need to design the suspension system of the bike (through the frame design, wheel and tyre choice) to ensure we minimise this impact. When frame designers ignore the need to balance the suspension system in a performance bicycle, it spells trouble. 

So what can we learn from motorcycles? Surely their suspension systems are totally different? Well, yes, in a way. But once the frame and chassis decisions have been made, the suspension system can be set up in an almost unlimited number of configurations, which could, if you wanted or needed to, help you manage head shake. So why do we see it at all? Even on pretty ordinary motorcycles? 

Well, manufacturers have a lot of different people to please, so they take a call that most riders aren’t going to rag the granny out of their sporty commuter every day on their way to work. To deliver a front end that feels secure on the brakes and when cornering at normal speeds, then they tend to spec overly stiff forks. To deliver an overall set-up that’s comfortable, they might also specify a relatively softer rear suspension. The result is a bike that feels great round town or on a test ride, but that when really pushed may have some pretty unpleasant issues when you open the taps. 

Because of the way they happen, tank slappers are more likely on any bike when you are pressing on. On even humble bikes the compromise in the spec sheet becomes more obvious, and the suspension usually needs set up to eliminate the head shake, understeer and other issues.

But what's that got to do with my bicycle?

You may or may not realise it, but you do have a fully adjustable suspension system on your road bike. And in that respect, the physics are the same.

Ok, it’s not Ohlins, but your frame designer chose materials or a lay-up that gives the chassis the right balance of damping, and stiffness. Combined with sensible wheel and tyre choices designed for the riding you want to do. This is your adjustable suspension system. We hear words like ‘compliant’ used in reviews all the time, but what is the ‘right balance’ of damping and stiffness? Unsurprisingly, it is not just about comfort.

Let’s look at an example to demonstrate good or neglectful design in terms of its suspension system:

Your road race frame is made of inherently inflexible material, then it also most likely has an enormous head tube, accommodating massively oversized bearings to hide those cables away, alongside a wonderfully aero-optimised fork blade (also oversize) with an oversized carbon steerer then it is probably also, delivering an unavoidably stiff front end. 

If that’s the case though, you’d better hope that the frame designer considered some similarly stout componentry in the bottom bracket and chain stay area of the bike, leaving ride and comfort damping to the seat stays. Balancing out the ‘suspension’ at both ends. 

If there’s undue flex at the rear, and it doesn’t serve to balance the stiff front end, you’ll have an imbalance like the example we discussed in the shock set-up in the motorcycle context. And it’s this that I believe can give rise to otherwise avoidable head shake.

Regardless of how well-made your frame is, don’t be dismayed as the main component of your suspension is your tyre. So don’t neglect tyre set up if you’re trying to solve head shake issues. 

“But, tyres aren’t suspension are they?” Yes, they are!

If you need some convincing of this fact, try riding without a tyre for a while, and see how well the rims cope with potholes or stones on the road. 

The suspension system here is fully adjustable for preload through tyre pressure; Spring rate via tyre volume; and compression and rebound is innate in the sidewall and pneumatic action of the tyre. 

Lastly, consider weight distribution. When the suspension is set up nicely, it’s desirable to have a motorcycle feel like it’s 'sat on its nose’, with a wickedly connected front end and level feeling in the bike. We’re stretching the link here deliberately, as weight distribution is far more important in bicycle design where the rider accounts for some 90% of the weight in the system, but anecdotally we know we all feel a little bit faster when we do ride an aggressive fit. But overdo it with excessively long stems, or by slamming the bike when your position doesn’t need it and you’ll struggle to maintain speed on descents whilst fighting needlessly with the steering. 

If you’ve got steering issues or are terrified of the next tank slapper, or perhaps have a helpful comment about how you’ve managed the issue in the past, comment below.

My old Yamaha at Brands Hatch about 13 years ago. The bike has a steering damper which can stop things getting too out of hand.