How do bikes go round corners

Photography: Kardesign

For a bike to go round a corner it needs a) a corner, b) grip, c) steering and d) velocity. Take any of these away and you’ll be in trouble. But also, each of them is interdependent – the radius of the corner, the amount of grip available between the bike and the road, the bike’s operational steering design and its speed all interact with each other and all have to be operating within a physical limits.

So the short answer is bikes go round corners because the rider is balancing a large number of physical forces to a high degree of accuracy – at least, accurately enough not to crash, hopefully.

To explain further may break the internet or your attention span. But here goes:

You are approaching a corner – let’s make it a left hander. We’ll assume you’ve done your braking, the bike is travelling at the speed you want. The first thing you have to do is steer (we’ll assume your bike has sensible steering geometry – rake, trail and wheelbase; how they effect steering is a whole other novel). If you’re travelling above 5-6mph, you will need to turn the front wheel a very small, yet significant amount to the right. It depends on your speed, the radius of the corner and the point at which you initiate the turn, but it’s usually up to around 4 degrees and for around half a second. This is called counter-steering. The tiny movement is enough to start a process that sends 300kgs of bike and rider sweeping majestically across the road, heading for the apex.

How it works is like this: imagine something with a high centre of gravity and a small contact patch – like an upside down garden shovel. If you stand the shovel with the handle in the palm of your hand, balancing, you’ll notice you constantly have to jiggle your hand around to maintain balance. Ignore that for a moment; the important thing is this: if you want the shovel to topple to the left, you have two choices – hit the top of the shovel to the left (which is cheating), or move your hand to the right. You displace the shovel’s centre of gravity, and it falls.

Now, this is only slightly analogous to counter-steering in the sense that a bike has a high centre of gravity and a small contact patch – and to make the big heavy bit of it fall in one direction, you have to displace its centre of gravity by moving the only bit you can move – the steering – in the opposite direction (funnily enough, humans are rare in the animal kingdom as being lanky bipeds with a small, narrow footprint relative to a high centre of gravity. Most animals have a low centre of gravity and four widely spaced footprints. You could say, therefore, we are biologically predisposed to balancing motorbikes. Ostriches would be natural motorcyclists, too. Badgers, not so much).

Anyway, you’ve now initiated a turn. In terms of steering effort, as the bike begins to lean, you now go back through 0 degrees to a state of a few degrees of left steering – in the shovel analogy, you’re moving your hand to the left to ‘chase’ the centre of gravity and balancing the topple (except in extreme cornering, when steering remains counter to the direction of travel – a bit like oversteer; watching Marc Marquez drifting through the top left hander at Valencia with his wheels pointing in opposite directions springs to mind – although technically this could be slip angle; see below).

The shovel analogy, which was fine, now falls apart because a bike also has velocity (and two contact patches, one of which is steered). But this is just the start. By initiating the turn, we’ve now set up a bunch of other forces that are interdependent on each other.

The bike is steering into the corner. Using our natural, evolutionary skill at balancing, we blend velocity, steering effort and body position (the only things we can change; friction, bike mass, steering geometry and gravity are all supplied as standard) to balance the following forces:

1. Centripetal acceleration acts towards the inside of the corner. Centripetal force is generated by tyre friction (camber thrust, see below) and pulls the bike towards into the corner at the intersection of imaginary lines drawn through each wheel spindle. The force is proportional to the bike’s mass, velocity and the cornering arc radius (mv²/r) and appears at ground level.
2. Centrifugal force, sometimes called a fictitious force because it doesn’t arise from a physical interaction – but which nonetheless exists as a force so that’s what we’ll call it. For our purposes, it’s generated by the bike’s inertia and acts at the height of the bike’s centre of gravity in the opposite direction to centripetal force.
3. Camber thrust is a friction force generated when a tyre contact patch is forced to follow a straighter path than its natural course. The contact patch of a leaning, rolling tyre will naturally try to follow an arc – as if it was a rolling cone (Mick or Keith). The radius of the arc will be tighter than the desired corner radius, so the resulting force appears to pull the bike down into the corner, as a component of centripetal force.
4. Slip angle is the difference between the direction a wheel is pointing and its actual direction of travel – the friction generated at the tyre is also a force, called, imaginatively, cornering force, and is also a centripetal component. And if this difference in slip between the front and rear tyres is great enough, the bike will oversteer into the turn – can you see Marquez waving?
5. Gyroscopic precession - the level of contribution of this force to steering is a bit contentious; some people say it’s negligible and point to the fact you still need to countersteer a ski-mobile, with a ski at the front instead of wheel. But the forces exist and, as anyone who’s tried to twist a spinning bicycle wheel in their hands will know, it has an effect. Thus trying to countersteer a large, rapidly spinning wheel to the right to turn into a lefthand bend will result in a force at 90 degrees – to the left, helping the bike turn left (if you still think gyroscopic precession has no effect on steering, MotoGP engineers will tell you even the direction of crank rotation has an effect).

That’s enough forces to be going on with (there are plenty more, especially generated by the tyres, but we need to be getting on with cornering). By now the bike is mid corner and in a steady state – that is, the forces that need to be in balance are in balance, and the rider is doing the balancing. The bike has adopted a cornering ‘set’, and the rider can adjust the balance of the ‘set’ to either maintain it or alter it – he can move his body position (shifting the centre gravity), change velocity or change the steering angle.

To come out of the corner, the usual condition is to accelerate the bike – change the velocity and, at the same time, steer (we’re only talking a few degrees) back to 0 degrees (or even beyond it). This has the effect of reversing the forces balancing the bike in the corner (back at the shovel, we’re regaining a static balance).

And off we go...

Biog: Simon Hargreaves