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How to Steer a Motorcycle (?!)

It's funny, the first bicycle was invented in the 1800's or before, and with it, the first person to be able to operate it.  Yet it is far from obvious why you can ride, balance or control a bicycle.  To prove this to yourself, you only have to listen to people try to explain it.  There are at least three theories, and each is typically presented as the way it works.  Discussions get heated, minds get closed.  Physicists have made pretty good attempts, but anything approaching a complete explanation is so math-heavy that we cannot use it to inform our riding. 

Even the physicists find it too difficult to be worth it to model the entire chassis/motor/suspension/aerodynamic/rider matrix which is the real-world operation of a motorcycle.  They typically break it down into individual systems, with simpler models, knowing full well that the dynamic state of each system affects the others in certain ways, and ignoring that fact so they can get their job done in a reasonable amount of time.

We have argued with each other back and forth, and the lay public has not yet been graced with a clear, obviously correct explanation that everyone can agree on.

I'm here to take my shot.

There are three basic theories:

-You lean in the direction you want to go, and the bike follows.

-You turn the handlebars and the gyroscopic action of the front wheel leans the bike over.

-You steer the bars the opposite way from your intended direction of travel for a moment, to get the bike leaned, then you steer the right way, to go around the corner.  This is called, "countersteering."

In this article I will:

-Acknowledge the role and use of each technique, the role of each explanation as you ride, and the reason why it won't do to think about it just one way.

-Do away with the fixation on countersteering as a concept, not because it is wrong, but because it confuses people

-Offer a new way of thinking about it, which involves the physics behind all three theories in one tidy package.

The simple explanation of, "Lean and the bike will follow," and other explanations involving shifting one's weight to control the bike, are often promoted by skilled, competitive riders, especially those from long ago.  Bicyclists also find that explanation credible.  It's hard to say to a guy who can beat you in a race that he doesn't have a correct understanding of how to control his vehicle, so I won't.  The fact is, at some level they know very well how to operate their vehicle.  Problem is, the doing of the riding is managed by the Cerebellum and brain stem, parts of the brain that we have in common with reptiles, and which have nothing to do with language or theory.  The understanding in the lower parts of the brain is not immediately accessible to the parts of the brain that can explain things.  So the knowledge they have, in its usable form, is private and non-transferrable, and there are few who can fit such understanding into language that we can read and understand correctly.

This is how I think it is best to look at steering.  By the way, here I am making an assumption, that the vehicle is not sliding.  If the vehicle were sliding, the situation would become much more complicated.  More on that later.

When you steer any vehicle with wheels, it is most precise to say that you steer the places where the rubber meets the road, the contact patches.  Since no vehicle is without height, the contact patches are always below the center of gravity of the vehicle, so the top of the vehicle tends to go straight, even though you steered the contact patches to the side.  Thus the vehicle tends to lean out of the turn. What happens next depends on how the vehicle is constructed.

When you turn a car, you point the wheels in the direction of intended travel, and, to be precise, the contact patches go in the direction that the front wheels are pointed.  The fact that the places where the rubber meets the road define a plane rather than a straight line means that the car is resistant to leaning, because most of the car, and thus the center of gravity, would have to lift up for it to lean.  The effect is the same as if the car were motionless, and you pushed it from the side.  It would lean away from you, but unless you were a bison or something, the wheels would remain planted.  Thus it is easy to get away with saying you are steering the car, rather than the contact patches, because it follows the contact patches, almost always. 

On a bike, you also steer the contact patches.  As with a car, the top of the bike tends to go straight, while the contact patches move to the side.  But a bike, whose contact patches are in a line, responds differently.  When a bike leans, every part above the contact patch gets lower to the ground, so the center of gravity falls.   This is just as self-perpetuating as it is when the bike is standing still.  Push it to the side at rest, and it falls and rotates on its contact patches, till it hits the ground.  A bike has a small tendency to lean into a turn on its own so it won't fall over, based on the rake and trail of the front suspension, but it is a gentle force that cannot be counted on to cancel the lean exactly.  So you can steer the contact patches, but you cannot really steer the bike, because only the contact patches themselves answer to the directional control of the bars.   The rest of the bike just tries to go straight, every time.

So how do you make a bike corner?  Using the above explanation as a model, you use a strategy to "fool" the bike into cornering.  You want to turn right.  Turning the bars to the right will make the contact patches go right, but the rest of the bike will go straight.  In a second, you'll fall on your left side.  That won't do.  So instead, you steer the contact patches to the left, but only a little.  The contact patches go left, the rest of the bike goes straight.  You start falling to the right, and the center of gravity is now to the right of your contact patches, because you are leaning.  Your contact patches need to follow your center of gravity to the right, to keep your lean under control.  So you steer the contact patches to the right.  Voilá, you and the bike and your contact patches are now turning.

What you are doing is reminiscent of balancing a baseball bat vertically on your hand.  Making the bat lean in one direction always requires moving its bottom in the opposite direction for just long enough to initiate a fall in the direction you want.  That is followed by an aggressive move to catch the bat before it falls too far, by moving the bottom faster than the top is going, in the direction of the lean. 

Likewise, you need to steer the contact patches of a bike out from under it, so it falls, then arrest the fall by steering the contact patches to follow the top of the bike around the corner, then steer them even harder in the direction of travel than the top of the bike is going, in order to make the bike stand up again.  It is often said that you recover from the lean by accelerating.  Accelerating does widen your corner, thus making you go straighter.  It does this by increasing the distance over which the sideways force of your lean acts.  The physics adds up to the rule that a certain degree of lean, if you stay still on the bike, will result in a certain angular velocity (rate of directional change) at a certain forward speed.  If you increase that speed by accelerating, it will not automatically change your degree of lean, but it will spread the force of the lean over a longer distance, thus diluting your ability to change direction, and you will go straighter.  This is a natural and sensible thing to do when coming out of turns, but it is not sufficient to stand the bike up completely.  It won't get the job done.  You still need to finally steer the contact patch back under the bike. 

 

Here's what acceleration in a corner does for you.  It shifts weight to the rear, extending the fork, thus increasing rake and trail, which increases the self-correcting tendency of the steering.  To the degree that you are leaning, this makes the bike stand up more.  But it won't take you all the way, because that self-correcting force goes to zero as your lean goes to zero.  Also, the faster you go, the more effective are your inputs to the handlebars, so it takes less and less motion of the bars to bring the contact patches under the bike the more you accelerate out of the turn.  That reduces the trouble you need to go to in order to cancel the lean, but once again, you have to deliver that last nudge, to get the contact patches back under the center of gravity.    Unfortunately, the contact patch takes more and more handlebar motion to steer, as speed approaches zero.

It is understandable why they wanted to call it countersteering, but get that out of your head.  You just need to remember that you are only steering the contact patch, and you have to use that direct control to indirectly influence the direction of the bike.

So what about just leaning into the corner?

Strangely enough, just as steering the contact patch has two different effects depending on if your wheels are in line or in a plane, so throwing our weight around has two different effects depending on how long you do it.

 Just as you throw your body to the side, the bike leans (or moves, if you turn the bars at the same time) in the opposite direction.  This is a great way to avoid really nasty road features suddenly, and is the quickest way to make the path of your contact patch move to the side, without changing your direction of travel.  Offroad riders learn this one quickly, as a way of avoiding rocks and ruts without using any traction.

But of course, you have to pay the piper.  You just leaned way off the bike, and although it moved in the opposite direction momentarily, it is going to follow you now, by leaning toward you.  If you are quick, you can steer the contact patch back under you, and let your weight counterbalance the bike, till you and the bike line up again.  If you are less quick, you will have to use the handlebars to steer the contact patch to balance your lean, and you will turn.  If you persist in leaning off the bike as you turn, the chassis will balance to a less leaned-over attitude than it would have done if you stayed in your seat.  This effect is used to great advantage on the road, where a low chassis stance helps maneuverability and stability, and is practical because the road is smooth.  It allows you to corner much harder than the ground clearance of a bike would otherwise allow.  It also lowers the overall center of gravity between you and the bike, further enhancing stability.  Leaning off also loosens the connection between rider and bike, allowing the bike to bounce around a little, and even drift, without upsetting the bike/rider system.  That is why roadracers do it so much.

So leaning or throwing your body around has its place.  You can push your contact patch quickly and briefly to the side, to fine-tune your line, on a sub-1-second time scale, and take the move back without changing direction, if you do it fast.  On the 3+-second time scale, you can initiate a turn.  If you lean your body into a turn, you bias your bike to turn more readily in the direction of your lean, enhancing control by reducing the inputs necessary to make the bike lean over.   This is the centerpiece of Lee Parks' Total Control Riding curriculum.  In summary, throwing your weight around is both the quickest and the slowest way to relocate your bike to the side.  And it works at any speed.  Problem is, the quick moves don't change direction, they just move the bike sideways briefly.  And the slow move is great to optimize the bike/rider system for a turn, but it doesn't act fast enough for you to precisely control the initiation of the turn. 

And what about gyroscopic forces?  They increase with wheel and engine speed.  They are almost nil at walking speed, but if you have heavy wheels, like the cast aluminum mags on late-'70's Yamahas, they have a significant effect at 60MPH.  Their overall effect, if you don't turn the handlebars, is to resist changing the lean of the motorcycle.  Keep in mind, though, that only the front wheel's gyroscopic forces will make the motorcycle lean, since that is the one that you can turn to the side.   There are also gyroscopic forces present in more parts of the motorcycle than just the front wheel.  I just went out to my garage and took a motorcycle wheel, spun it on its axle, held it, and turned it different directions.  Here are the results:

Regarding front wheel motion I have these observations.  If you steer it left, it leans right.  If you steer it right, it leans left.  But it only leans farther as long as you steer farther.  If you hold your steering angle, it holds its lean.  If you steer straight again, it goes vertical.  So the gyroscopic force has the same effect on lean as countersteering, but that force disappears as soon as you stop moving the handlebars, and reverses as you bring them back to center.  This means that in that initial stage when you get the bike leaned over, gyroscopic force works for you in the front wheel, but as soon as you try to steer around the corner, it works against you, tending to stand the bike up.  Obviously, we overcome that standing up force, because we can stay leaned.

Regarding rear wheel motion, I have these observations.  If you lean it over to the right, as would happen to the rear wheel, it steers left.  If you lean it over to the left, it steers right.  This works against directional change of the chassis in the turn.  This means that the gyroscopic force in the rear wheel fights us as we try to balance a lean by pushing the chassis to steer straight, and it fights us as we try to straighten up, by trying to steer us the opposite way.  The rear fights us all the way, and we overcome the gyroscopic force of the rear wheel every time we change lean or direction.

Obviously, gyroscopic force has very limited utility in steering the motorcycle- it is working for another master, not us.

That is where steering the contact patch comes in.  It can get you leaned over faster than anything, unless you are going very slowly.

So what about sliding?  Offroad bikers have noticed that you can save a front wheel slide by sliding the rear wheel.  And you can learn to maneuver in deep sand, where the idea of contact patch doesn't really seem appropriate.  And that the fastest, most secure way around a corner is with the rear wheel walking to the outside of your line, just a little.  What's with that?

First, how can you change direction by sliding?

Basically, in the case of a rear-wheel slide, we are talking about a form of rear-wheel steering.  In rear wheel steering, the rear wheel initiates motion of the contact patches.  You can get a feel for rear-wheel steering by sitting on a bicycle backwards and going down a mild hill, and trying not to fall.

If you do this, you will quickly learn that when you use the rear wheel to change direction, it leans the bike over automatically.  There is no need to countersteer to initiate a lean.  However, you will find that to hold a line, you need to back way off your initial steering input, once you are changing direction.  Strangely enough, holding the bars behind you, the resulting handlebar motion is roughly the same as with front wheel steering.  To turn right, you twitch the bars briefly to the left, then back off and turn them more to the right.  The difference is, with rear steer the bars never cross the centerline during the turn- you are still steering slightly to the left, till you want to stand up again.  With front steer, the bars cross the line from left initially, to right during the turn.  But the sequence of motions is the same.

Second, why is it ever better than not sliding?

Many road bikers and car drivers have learned firsthand that once you are sliding, you have a great deal less traction, and it is a challenge not to go off the road or fall over.  So how can it ever be helpful?

It can be helpful in cases where there is enough dynamic friction between the tires and the road surface to be useful to control the vehicle.  What is dynamic friction?  It is the force one thing can exert on a thing it is sliding against, to resist the slide.  This is not to be confused with static friction or traction, which is the force one thing can exert against another because they are pressed together, but not sliding.  For most things, static friction is much greater than dynamic friction, so if you want to maneuver your vehicle, the best bet is usually, but not always, not to slide.

In motor vehicles, there are two cases where dynamic friction is greater than or equal to static friction, so sliding with control is possible. 

The first is with tire compounds that are specially designed to mold themselves quickly to the microscopic details of the road surface.  They are typically very gummy and short-lived.  They include racing tires, and especially qualifying tires.  You can see footage on the web of GP racers like Valentino Rossi, Casey Stoner and others, taking turns at triple-digit speeds with both the front and rear tires sliding, and making blue smoke.  This works because those tires mold themselves to the road constantly, even as they creep outward in the turn. 

The second is on loose surfaces with tires that are designed to penetrate the road surface continually.  This is the case of knobby tires.  Loose road surfaces are constantly nudging tires back and forth as the tires bounce off of rocks and things.  But as the tires slide, they also dig into the surface, with their knobs.  The more they slide, the more they dig, up to a point.  The digging makes it so they are always braced against the next piece of dirt to resist further sliding.   This generates fairly consistent force in a slide, so you can still make it around the corner and balance the bike.

Ok, so sliding with control is possible.  When is it helpful? 

It is helpful when you cannot count on sufficient traction from the front wheel.  This happens on loose or unreliable road surfaces.  Steering the contact patch with the front wheel depends on the front wheel being able to exert a reliable level of sideways force on the ground.  If that force fluctuates enough, it becomes hazardous to rely exclusively on steering the contact patch with the front wheel.

I would point out that steering the contact patch with the front wheel is still the main way to operate a motorcycle, even offroad.  You can see on YouTube various helmet-cam videos of people riding in the desert or on straight trails.  If you watch the handlebar angle change, it is clear that they are constantly using the handlebars to keep the contact patches under them as the front wheel darts this way and that on the uneven surface.  It is also clear, due to the duration of some of their corrections, that the front wheel is sliding at times.  But at an upright angle, the control is still sufficient, as long as you loosen up your reflexes enough to allow the intermittent sliding.

The problem develops if you are trying to turn.  Once a lean angle is introduced, and you are counting on a reliable sideways force to balance it, the normal way of riding is way too invested in tight control over the sideways force exerted by the front wheel.

Enter rear-steer by sliding the rear wheel.  As I discussed, rear-steer initiates lean on its own.  However, unlike on a backwards bicycle, the rear-steer of a motorcycle is done by sliding the rear wheel.  The great gift of rear steer by sliding is that it depends on loose rather than tight engagement between the contact patch and the ground.  Therefore, it is not important if your front wheel is sliding or not, to maintain both lean angle and your chosen line through a turn.  Just as long as your chassis is leaned into the turn, giving it more gas will tighten your line, and letting off the gas will straighten you out.  If your front wheel is sliding, your line straightens out, but your lean angle does not, resulting in a fall.  If you respond by sliding the rear wheel more, it tightens your line back up, keeping the lean angle under control, and you go around the turn looking like Ricky.

It's kind of like diversifying your investments.  On the road, there is enough general order to things that you can throw all your eggs in the steer-the-contact-patch-with-front-wheel basket.  You can spice it up a little with a sprinkle of body English at low speeds and extreme lean angles, for fine tuning at no expense to traction.

Offroad, the order of things is patchy and unreliable.  The on-road method is still the easiest to use, so you use it when you can.  But you have to put some eggs in the rear-steer-by-sliding basket.  That way you have the way of steering that works upright and when you are not sliding, and also the way of steering that works when you are cornering and sliding, since you can't avoid it sometimes.  And an even healthier dollop of body English will be needed to help you through or around ruts and rocks and over logs, and to help slide the bike right, and to keep you up in the super-slow sections.

The fact is, offroad riders do all these things at once.  For the ultimate example, watch dirt-trackers.  This is the most orderly possible loose-traction environment.  They are walking the rear wheel whenever they are turning, which is most of the time.  But you will see that they are still operating the handlebars, constantly nudging the front wheel this way and that, in the front-steer mode.

Traction environments and cornering strategies

Speaking of order, the dirt-track situation illuminates another factor of steering a motorcycle.  On an orderly surface, it is a good strategy to spread the turn over the longest distance, to maintain speed through the corner and minimize peak traction requirements.  This is what they tell you to do on the road with cornering lines.  It is also true in dirt-track, and occasionally in desert or deep sand, where the traction situation is consistent through the turn.  But in less orderly environments, like woods, trails, and motocross, spreading your turn over the longest distance increases your risk that your traction will change as you go through the turn, requiring very quick thinking from you, to get out the other end wheels-down.  So you will see the best offroad riders surveying their options for cornering environments, and picking just one.   

These are the basic options that might present themselves. I discuss them assuming inconsistent traction:

The flat surface.  Here, the best riders slow down to a speed where they can slide the rear wheel and navigate the corner in the flat space available to them.  They tend to select a small site for their corner, sliding the rear wheel with the brakes on the way in to slow and initiate rear steer, then power-sliding the rear wheel to exit.  Gary Semics, in his instructional materials, calls this, "Squaring the corner," because there is a point where you slide the rear wheel to make the bike almost stop as it rotates about 90 degrees, as the main part of a turn.  By selecting a small site to execute the turn they minimize the chances of the traction situation changing during their turn, thus can execute more simply and smoothly.

The rut.   Here, the best riders have to carefully calibrate their entry speed and direction, because mid-corner correction is impossible.  However, insane lean angles and cornering speeds are possible.  Ricky Carmichael and James Stewart are masters of this.

The berm.  This is like a flat surface, but with the advantage of the surface itself leaning into the turn.  It is handled sort of like a real flat surface, but faster.  The disadvantage is that it is typically the longest line through the corner.

The sand.  This is a flat surface, but very loose, where rear-steer is paramount.  Its disadvantage is that it is slower than most turning environments.  You don't choose it unless the rest of the track is really chaotic.

The mud.  This is like sand, but with horrible levels of resistance to forward motion.  People don't typically choose this, if they can help it.  Mud in a race tends to equalize things- The best racers don't handle it much better than the more casual ones.

Switching traction environments in mid-corner is very difficult and risky, because, as I have discussed, the rules change.  So you will see people adjust their speed and line to stay in a single traction environment through the turn.  This tends to make offroad riding more of a point-and-shoot affair, because sometimes a traction environment is very small.  You will see this the most in MX racing, where there are many tight, technical turns, which typically have at least a rut or two, a berm, and a flat surface.  You will see the likes of Ricky and Bubba doing normal front steer coming into a corner, as they aim for the cornering environment they like best.  But the moment when they are changing direction the most is usually very brief, as they arrive at their chosen cornering environment, and change direction within that environment, then shoot out of it in a more-or-less straight line.

So there you have it.  My attempt to explain the unexplainable.  Hope you like it.