# The Naked Scientists Forum

### Author Topic: Why does leaning a motorcycle make it turn around a corner?  (Read 41675 times)

#### CliffordK

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##### Re: Why does leaning a motorcycle make it turn around a corner?
« Reply #25 on: 24/12/2012 23:49:32 »
With a bicycle, it is certainly possible to steer it with leaning alone.

I think part of it has to do with the fork rake.  If the headset was perfectly vertical, and the forks had no rake, then leaning the bicycle would do nothing.

Having the forks raked backwards, like on a shopping cart, works well for the wheels to follow the turn when the cart is kept vertical.

Likewise, on a tadpole trike, one may choose a rearward rake as the trike doesn't lean, and thus one would choose wheels to follow the direction of motion.

On a bicycle, I think the forward angle of the headset and rake of the fork acts to make the wheel turn towards the direction it is leaning which may, in fact, help with balance.

Rake, of course, can be achieved with bending forward anywhere below the headset, or even with merely the bolt location on the forks.

It may be a combination of things, so that the round tire may work in conjunction with the fork geometry to turn the wheel.
« Last Edit: 25/12/2012 02:17:55 by CliffordK »

#### Pmb

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##### Re: Why does leaning a motorcycle make it turn around a corner?
« Reply #26 on: 25/12/2012 03:30:09 »
Why does leaning on a motorbike cause it to turn?
Don't know. Great question though. It has to do with gyroscopics. The wheels act like gyroscopes.

And is it true that counter-steering - turning the handlebars in the opposite direction to the turn - can actually make you steer around the corner better?
Yes. That little fact saved my life many moons ago.

#### David Cooper

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##### Re: Why does leaning a motorcycle make it turn around a corner?
« Reply #27 on: 26/12/2012 21:52:36 »
Why does leaning on a motorbike cause it to turn?
Don't know. Great question though. It has to do with gyroscopics. The wheels act like gyroscopes.

I suspect you didn't read through the thread. Out of interest though, if you don't know the answer, why do you then immediately go on to state an answer as if it's a truth? This idea of it being due to gyroscopic effects may be nothing more than a meme.

#### CliffordK

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##### Re: Why does leaning a motorcycle make it turn around a corner?
« Reply #28 on: 27/12/2012 13:29:41 »
Ok,
I started thinking about some of the bicycle geometry a little bit more.

The weight vector comes essentially straight down from the hub, going through the center of the point of contact between the tire and pavement when the bike is vertical.

The headset is tilted forward, creating a tilted line which is the axis of steering rotation which lies in front of the weight vector.

When you lean the bicycle, say leaning it to the right, that creates a crosswise force on the tire to the left.

Because this crosswise force is being applied to the tire behind the axis of steering rotation, it tends to kick the point of contact between the pavement and the tire out to the left, turning the forks to the right, and causing the bicycle to turn to the right.

Raking the forks forward brings the weight vector forward, towards the axis of steering causing the bicycle to be less responsive to leaning.  One should be careful to make sure it is not pushed in front if the axis of steering rotation (aligned with the headset), otherwise it would make the bicycle steer away from the lean, and make it very unstable.

I think there is another benefit of raking the forks forward in that it creates an upward vector (I think) that tends to self correct the steering, and make the bike tend to ride more in a straight line.

You can create an equivalent fork rake with any method to bring the hub to the same spot in front of the axis of steering through the headset.

The round tire merely gives good surface contact and maximizes grip throughout the turn which is especially important as both the lean and centrifugal force tends to cause the tire to slide away from the lean (in the above example, to the left).

The advantage of this complex geometry is that it when turning, one leans the bike in the direction of the turn so that the centrifugal force vector of the turn plus the weight vector equalize to go through the axis of the frame.  In fact, if you lean too much, it would tend to cause the bike to turn towards the lean, and increase the centrifugal force vector to right the bike.  If you don't lean enough, it would cause the bike path to straighten.

#### ReyD

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##### Re: Why does leaning a motorcycle make it turn around a corner?
« Reply #29 on: 07/05/2014 01:42:04 »
I've noticed this is a really old convo but since it looks like the answer hasn't really been found and I was searching through Google a bit myself to find the answer when I found this site, I'll tel you why I think people tell you to counter steer motorcycles.

First, you counter steer into the turn to tilt the bike in the correct direction ( let's say you want to go right), because as the wheels turn left, the center of mass tries to stay where it is meaning that you are now tilted right (you can see it in cars taking very tight turns as well, the wheels on the inside of the turn even leave the ground sometimes) You didn't actually go anywhere, but the bottom of the bike pivoted around you and is now to your left, meaning you are leaning to the right. you do this to lean because most bikes are much heavier than you are, unlike bicycles where you are the heaviest thing on it. Lighter motorcycles don't really need to be counter steered since you can still force the bike into a lean

Now, why does leaning make you turn? It does this because unlike cars, motorcycle tires are shaped like donuts, and if you cut a donut into slices like a pizza, you will notice that the farther you lean the slice you cut to one side (let's say right), the shorter the surface in contact with the road on the right is compared to the left, because the closer you get to the place where the "pizza" slices meet the more the slice tapers to a point. Now what this means is each turn of the wheel makes the left side of the tire travels further than the right side, since the right side is basically a smaller circumference because of the way the tire is shaped. So as the right side lags behind, the bike turns right.

OK, that's cool, but why do people say you have to counter steer IN the corner as well? Now THIS is the part where the gyroscopic effect comes in. Because the gyroscopic effect of tires increasing the bigger they get, you start to notice on motorcycles, especially bigger ones, that the bike is trying to level itself throughout the turn. On smaller bikes the effect isn't too noticed because the tires ase lighter and have less effect and the bikes themselves are lighter so you can just use your own strength and weight to keep it in the lean. On bigger bikes you continue to counter steer, which just like when you started the turn, pulls your tires further left out of the turn causing you to lean more right and cancel the gyroscope trying to level you out. Once you're done turning, you either keep the bars straight and wait for the gyroscopic effect to straighten you out or turn right into the turn which brings your tires back under you and then straighten out.

You turn like this on bigger bikes instead of just turning right to go right because doing that at the speeds you're going with a vehicle much heavier than a normal bicycle heavily increases your chances of flipping the bike, and unless you're strong and heavy enough to stop the bike from leaning the way I described above, you'll just end up going the wrong way anyway

Hope this helps someone who finds this topic two years from now like I did, and hopefully you know what to do with my example to make the bike turn left as well ;P

#### Slartibartfast

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##### Re: Why does leaning a motorcycle make it turn around a corner?
« Reply #30 on: 08/12/2014 13:19:44 »
I see this thread has been left to die several times only to be jolted back to life time and time again, so allow me to get the defibrillator out once more.

This question I have been searching for the answer to for over 10 years and I have invested a significant amount of thought and research onto trying to understand it over this time and I have to say I am not even remotely convinced by any of the suggestions posted in this thread thus far.  In fact, after considerable investigation, I am inclined to think it is simply a mystery :O

I mean no offence to previous posters but I will first go through and dispute each of the theories proposed so far...

Before I rip though let me clarify a little of the discussion about counter steering.
Firstly I have to assert that counter steering is absolutely, definitely, 100% a thing!  On occasion people will try and say it does not exist, or at the least assert that they do not use it but this is simply not the case.  Absolutely everyone who has ever ridden a single tracked, two wheel bike has used it.  It makes no difference if the bike is a motor bike, a pushbike or even a childs scooter.  Counter-steering is fundamentally the mechanism through which a turn is initiated (and also concluded).  It also matters not how fast nor slow you travel nor does it matter what the mass of the vehicle may be.  Counter-steering, fundamentally, is the means by which the lean of a bicycle is controlled.

To illustrate, let us imagine a rider making a turn to the right.  To make the turn one must first push a little with the left hand causing the front wheel to turn to the left which in turn causes the front wheel to track to the left resulting in first the front but then the rear wheel tracing out to the left.  This causes the center of mass of the rider and bike to fall entirely to the right-hand side of the wheel track which invariably causes the bike and rider to lean (or fall) to the right.  You can think of tracking the wheels to the left as being like slipping on a banana peel or sweeping out an old mans walking stick.  If your feet are swept out from underneath you to one side you will fall to the opposite side.  Once the lean has been initiated (through counter-steering) the rider must turn into the corner a little in order to "hold the lean angle" and prevent the bike from falling over.  Note the amount by which the rider steers into the corner is very minimal and the deviation in steering angle is much less than that of the angle of curvature around the circumference of the turn itself.  Finally at the end of the turn the rider will actually turn more strongly into the turn causing the wheels to once again track underneath the rider thus standing the bike and rider upright again like an inverted pendulum.  So, in summery a right turn is initiated with a counter-steer to the left, it is held with a slight right turn and ended with a sharper right turn to stand the bike upright again.
There are countless examples on YouTube, but here is one of the better ones:

If anyone doubts the existence of counter-steering on a pushbike I invite you to try lightly holding the right handle grip with the fingers of the left hand.  Although most of us are unaware of the minor corrections we give a bike to remain stable once you reach across and hold with your opposite hand you'll very quickly become aware of them, and once you are aware of them you quickly realize all these adjustments are in fact "counter-steering" inputs.  I was most surprised when I first tried this myself.  Be advice though it is very easy to get confused so only do it at a relatively slow speed and be prepared to let go and grab with your normal hand the moment things get uneasy.

But on with the main question at hand:
First point I will refute is the argument of gyroscopics.  Gyroscopics is a frequently misunderstood and magical seeming force so not surprisingly all manor of bizarre effects are often attributed to it.  While it is true that gyroscopic precession will cause the axle of a rotating body to precess or turn when the wheel leans and this indeed has an affect on bicycle dynamics this is not the principle means by which a bicycle turns.  For one, precision would only be present while the bike leans in and leans out, and would not be present through the main course of the turn when the lean angle is relatively constant.  Also, if the turning of a bike were governed by the procession of wheels the wheels would precess one way while leaning in and precess in the opposite direction by the exact same amount when returning to vertical, which would result in an 'S' track but would always end with the wheels heading in the same direction they started.  Note that while gyroscopic forces are also the favorite high school physics explanation as to why a bicycle stands up when you ride it but falls when your stationary, unfortunately gyroscopic forces really don't explain it at all.  In school they then love to show you the experiment of spinning a wheel then dangle it from a piece of rope attached from one end of the axle and show how it 'magically' stays upright.  This is an impressive and dramatic experiment but unfortunately it really doesn't have much to do with bicycle stability at all.  In fact the "gyroscopic" explanation for stability was conclusively dis-proved in the 1970's when a couple of physicists built a bike with counter rotating weights designed to completely cancel out the gyroscopic effects of the wheels and preceded to ride the bike with relative ease.  In fact the reason bicycles are self stabilizing is actually still a mystery, and anyone who tells you otherwise is misguided.

For anyone interested here is an excellent paper by some egg heads at Cornel that systematically disproved all known theories for bicycle stability: newbielink:http://bicycle.tudelft.nl/stablebicycle/StableBicyclev34Revised.pdf [nonactive]

In regard to the "conical profile" argument I again say it seems like a good argument at first but doesn't really hold up under further scrutiny.  I can give several counter arguments but two simple ones are that the camber of roads does not much affect steering and ridged sharp edge disks (like coins) still turn when they lean.

If you look closely you will notice most roads are not flat at all and are in fact significantly higher in there center than at their edge.  This is to facilitate water run off.  As a consequence, when riding in a straight line you are rarely riding on the center of the tyre.  In fact every time I replace my front tyre it is always noticeably more worn right of the center line than it is on the left (I assume those from the land of the free and home of the brave wear the left side of there tyres).  If this "conical profile" theory were the main course for a bike to turn than I would constantly be fighting my bikes natural tendency to pull over to the other side of the rode.  I in fact tried an experiment one time where I lent my bike over to a significant angle and was able to keep it tracking straight by hanging significantly off the other side.  It wasn't an overly scientific test and I felt really silly doing it but I was able to demonstrate to my self that a bike will indeed still travel straight even if leaned at a significant angle if the center of gravity is still essentially upright.  Also, if the "conical profile" theory were indeed the reason bikes turn than the camber often built into road corners would in fact have a adverse affect on bikes rather than assisting them, which is clearly not the case.  If bikes were to perform better on inverse cambered corners velodromes would be designed with outward sloping berms.  Can you imagine it?

The other counter argument I'll mention is that of a coin rolling across a table.  While the coin is upright the coin rolls straight but if the coin leans over the arc of travel curls around in the direction of inclination.  Obviously the edge of a coin is fairly sharp and unless you argue that the corner is rounded at a microscopic level there really is no difference in the rolling diameter of the inside edge and outside edge of the contact patch, yet still the coin curves.

Wow, this post really is getting long and it's now getting late here so I think I'll call it a night.

It's been a good read fellers and hopefully someone will one day work it all out ;)
« Last Edit: 12/12/2014 06:57:09 by Slartibartfast »

#### PmbPhy

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##### Re: Why does leaning a motorcycle make it turn around a corner?
« Reply #31 on: 08/12/2014 14:37:28 »
ReyD - Welcome to the forum.

If you're interested in this subject then see - http://en.wikipedia.org/wiki/Countersteering

I know from first hand experience just how important this is since this knowledge saved my life once.

#### chasarms

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##### Re: Why does leaning a motorcycle make it turn around a corner?
« Reply #32 on: 11/09/2015 23:25:52 »
Newbie, this one is still not answered clearly, IMO.

The reason a MC turns when you lean is because of the laws of physics. if you roll a coin, flat washer or any other disc-shaped object across a table, what happens when it starts leaning? It turns in the direction of the lean. Nothing to with the shape. If coin or washer flat on the bottom, it will stand on its edge, and turn! You have one force moving it forward and one force (gravity) trying to pull it over. The torque created causes it to turn.

A MC going down the road at speed has two gyroscopes on it called wheels. Those spinning wheels want to keep the MC as it is just like gyros stay as is even if the pivot point of the axis is moved. It's the same reason it's easier to balance a bike at faster speeds. It takes more force to overcome the gyro effect. It's also why all the cool kids could ride bicycles with no hands on the handlebars. As long as there is not force to change it, physics demands that goes as it is going.

As to counter steering, yes it is absolutely VERY real. A simple experiment: Stand straddle of a bicycle and balance it with single finger on the goose neck, what would happen if you pushed the left handlebar forward? Yes, it would fall and hit your left knee. Pushing only the slightest bit forward would cause this. With a single linear axis of wheels like a bicycle or motorcycle, the slightest tip off center starts the bike tumbling to the ground. Counter steering is simply producing that effect. By varying the forward pressure on the handlebar, you are controlling the balance between the gyroscopic effect of the wheels in keeping it as is and it falling over. As it leans, it turns as described above.

The lean also keeps the CG such that you are not flung off the machine.

Why counter steering verses leaning? Safety. As a 25+ year veteran of motorcycling, I can firmly attest to the fact that a quick forward thrust on the bar at speed will cause the motorcycle to make an adjustment in direction FAR more quickly than you can attempt to lean your body weight and turn the bike. Approaching a curve, this probably doesn't matter, but dodging a car or obstacle in the road it absolutely does. Training your brain to think like this and developing your riding skills with counter steering in mind will make you a MUCH more capable operator.

FWIW, Motorcycle tires are rounded to maintain surface contact to the road WHEN the bike leans to turn. You could ride a bike with tires made like car tires and turn it just fine. It just wouldn't be very safe.

« Last Edit: 08/11/2016 22:47:42 by chasarms »

#### The Naked Scientists Forum

##### Re: Why does leaning a motorcycle make it turn around a corner?
« Reply #32 on: 11/09/2015 23:25:52 »