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7 Jun 2010

Some thoughts on motorcycle stability.

Stability in general and motorcycle stability in particular is one of those subjects that is so little understood that pretty well every misinterpretation of what's going on usually ends up as gospel. According to Roadcraft, “When you corner, your machine loses stability” which at first glance might appear to be a perfectly valid statement, but sadly it’s not entirely true and here’s why.

First of all, it might help to understand what stability or perhaps the lack of it, actually is. There are broadly two types of stability, static and dynamic and it’s quite important to understand the subtle difference between the two.

Simply put, if something has static stability and you disturb it, it will tend to try and return to its original state of equilibrium, but just because it is statically stable doesn't mean that it will ever get there. It may overshoot by further and further each time until catastrophic failure. For something to be dynamically stable it must actually be able to return to its state of equilibrium. It must therefore be statically stable in the first place otherwise it wouldn't even try to return at all. So while something that is dynamically stable may still overshoot, the overshoots will get smaller each time till there is nothing to correct. So you can end up with various combinations of the above

Statically unstable - gets disturbed and just disappears off somewhere. A good example of this is trying to balance a marble on a football. You can be certain that the marble will simply roll off the instant you let it go.

Statically stable + Dynamically unstable - gets disturbed, disappears off somewhere, but will keep coming back before disappearing again. A great example of this is a classic tankslapper when the front wheel shakes violently to left and right getting worse and worse with each movement.

Statically and Dynamically stable - gets disturbed but in time will return to its original position. A suitable example of this is putting the marble we used before into a fruit bowl. If you roll it up the side of the bowl and let it go, it will return to the centre, overshoot a bit and return to the centre and so on until it comes to a rest.

Now we know what stability is, let us see how it relates to our motorcycle. All bikes can exist in one of three states, moving, stationary and leaning against something (side stand, wall or the ground). As you might expect, the type of stability in each case is different. When the bike is stationary all we have to do to find out what sort of stability it has is to get off it and let it go. It then exhibits the classic properties of static instability in that it falls over with a mighty crash. Put it on its stand however and when we let it go it stays there, an example of static + dynamic stability.

It’s when the bike is moving that it starts to get interesting and this is where most of the confusion about stability arises.

The instant we drop the clutch and move away, we can happily put our feet on the footrests and enjoy the ride. Unless we are Dougie Lampkin however, we would find it next to impossible to keep our feet up whilst the bike was stationary. It is obvious then that the type of stability that our bike exhibits changes from being statically unstable to statically and dynamically stable as we open the throttle and move off.

This static plus dynamic stability is generated in a number of ways. Firstly our forward movement invokes Newton’s first law of motion in that something at rest wants to stay at rest and something moving wants to stay moving in the same direction, unless acted on by an outside force. Once we get the mass of the bike moving forward, it wants to stay moving forward and not to the side. Secondly, our wheels exhibit something called gyroscopic rigidity in that once they are spinning they resist being moved in any direction other than the one they are currently taking. Thirdly, the bikes steering geometry is set up so that if the bike leans to the left, it displaces the centre of gravity also to the left. This in turn sets up a steering action to the left which brings the centre of gravity back between the contact points of both wheels, thus restoring stability. The final piece of the jigsaw is how the rider subconsciously uses control inputs and body weight to keep the bike on its chosen path. For most of us, these control inputs were not learnt on a motorbike, but were probably discovered many years earlier, when we first wobbled off on our push bikes.

So if our bikes are stable when they are moving, is there any time when they are unstable? We touched on this a little earlier when we learnt how the steering geometry of the bike (rake, trail etc) acts to restore stability if the C of G becomes displaced from being between the tyre contact patches. At this crucial moment of instability, our bike is beginning to fall over, just like it would when it was stationary. Other words we can use to describe this falling motion are leaning and banking, the essential elements of cornering.

The crucial point here is that in order for us to steer our bikes, we must make the bike temporarily unstable, otherwise it would not lean and no lean means no turn.

We usually get our bikes into an unstable state by countersteering which displaces the C of G to one side of the bike. This unstable state persists until something acts to restore stability (if it didn’t our bikes would just crash to the ground every time we wanted to go round a corner). This restorative force comes from the steering geometry, the actions of the tyres on the road and the cone or camber steering force of the wheels all working together to make our bikes want to move in the direction of lean, cornering in other words.

These restorative forces continue to act until stability is regained when the C of G is once again immediately between the contact patches of the tyres. At this point, our bikes are banked over and cornering, but now they exhibit exactly the same amount of static plus dynamic stability as they did when they were travelling upright in a straight line. This is called the “steady state” lean angle and with an unlimited amount of petrol and with unlimited tyre life, our bikes could whizz round and round like this for ever.

So Roadcraft is sort of half right when it says “When you corner, your machine loses stability” but what it should say is “In order to go round a corner, your bike must first become temporarily unstable so that it starts to fall (lean) in the direction of the turn. Stability is regained once the steady state bank angle has been achieved and the cornering forces have restored the centre of gravity directly between the tyre contact patches.”

Stability is a mightily complex subject and I can hardly do it justice in this little Blog entry, but if you do want to know more about stability then I must recommend the book by Tony Foale called Motorcycle Handling and Chassis design. You can buy it online at and although it is quite expensive, you will find a wealth of information within its pages about how your bike works.

A Total Control Riding Clinic will show you how to manage the moment of instability to make it work for you so that you can corner better and safer than you might at the moment. We still have some places left on the upcoming courses, so why not do yourself a favour and get yourself booked in?

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