Bike Tilting: How Racers Produce Centripetal Force

[Himal kharel]

I noticed recently that serious bike racers(like that of moto gp) tilt their whole body and bike while taking curves. I know that they do so to produce necessary centripetal force. But I wonder how can they tilt their bikes?
The biker will exert certain force on bike to tilt it. By Newton's third law, the bike will exert same reaction force on biker. So, net force on system of biker and bike is 0. Then, how is the biker able to tilt bike?

 

[A.T.]

http://en.wikipedia.org/wiki/Countersteering

 

[Andrew Mason]

I noticed recently that serious bike racers(like that of moto gp) tilt their whole body and bike while taking curves. I know that they do so to produce necessary centripetal force. But I wonder how can they tilt their bikes?
The biker will exert certain force on bike to tilt it. By Newton's third law, the bike will exert same reaction force on biker. So, net force on system of biker and bike is 0. Then, how is the biker able to tilt bike?

You are forgetting about gravity. The biker in shifting his position by leaning to one side, creates a torque on the bike about the point of contact of the tires with the road. This torque creates a change in angular momentum of the wheels which causes the bicycle to turn in the direction of the tilt.

The purpose of leaning is not to provide centripetal force (this is not like banking). The centripetal force is provided entirely by friction between the road and the tires.

AM

 

[rcgldr]

The biker in shifting his position by leaning to one side, creates a torque on the bike about the point of contact of the tires with the road. This torque creates a change in angular momentum of the wheels which causes the bicycle to turn in the direction of the tilt.

That ceases to work in high speed (100 + mph 160 + kph) turns. The bike will lean, but turn so little that it's imperceptible to the rider, due to gyroscopic reactions at both front and rear tire interfering with the lean senstive steering that trail would otherwise (at slower speeds) produce . At high speeds, direct countersteering by applying opposite torque to the handlebars is needed.

Hanging off to the inside allows the bike itself to lean a bit less in turns, allowing for more cornering clearance. This isn't needed on the skinny two stroke racing bikes, but the riders still hang off. Hanging off may make the situation more forgiving if the tires start to slide.

One of the newer techniques done by some riders is to swing the inside leg away from the bike during approach to a corner, then put the foot back on the peg as they lean the bike over. I'm not sure why they do this, but it's something to do with the transition.

 

[Andrew Mason]

That ceases to work in high speed (100 + mph 160 + kph) turns. The bike will lean, but turn so little that it's imperceptible to the rider, due to gyroscopic reactions at both front and rear tire interfering with the lean senstive steering that trail would otherwise (at slower speeds) produce . At high speeds, direct countersteering by applying opposite torque to the handlebars is needed.

Hanging off to the inside allows the bike itself to lean a bit less in turns, allowing for more cornering clearance. This isn't needed on the skinny two stroke racing bikes, but the riders still hang off. Hanging off may make the situation more forgiving if the tires start to slide.

One of the newer techniques done by some riders is to swing the inside leg away from the bike during approach to a corner, then put the foot back on the peg as they lean the bike over. I'm not sure why they do this, but it's something to do with the transition.

I was assuming "bike" meant bicycle, but I now see the reference to "moto gp" means we are talking about a motorcycle.

Yes. I suspect that at some speed, the ability of the rider to cause the bike to tilt by leaning would be lost. So to turn left and get the bike to lean to the left the rider would countersteer to the right, providing a momentary torque to the bike about the centre of mass pushing the wheels to the right. Inertia would put the centre of mass out to the left (ie. left of the point of contact between the tires and road) and create the desired left lean.

The purpose of the tilt is really to balance the torques on the bike. If the bike was upright while turning, the centripetal force on the tires would cause the bike to flip away from the turn. The lean is needed to keep the torques on the bike balanced: ie the gravitational torque of the centre of mass about the tire/road contact balances the opposite torque of the tire/road about the centre of mass.

AM

 

[1mmorta1]

As a motorcycle rider, I know the precise answer :) it had nothing to do with leaning...you do that for balance. Look up "countersteer"

 

[1mmorta1]

I was assuming "bike" meant bicycle, but I now see the reference to "moto gp".

Yes. I suspect that at some speed, the ability of the rider to cause the bike to tilt by leaning is lost. So to turn left and get the bike to lean to the left the rider would countersteer to the right, providing a momentary torque to the bike about the centre of mass pushing the wheels to the right. Inertia would put the centre of mass out to the left (ie. left of the point of contact between the tires and road) and create the desired left lean.

AM

Actually, leaning doesn't accomplish anything at any speed. I've tried all sorts of crazy things on my bike, and countersteering is physically the only way to turn a motorcycle. At higher speeds, you have to counstersteer through an entire turn to not high side

 

[Andrew Mason]

Actually, leaning doesn't accomplish anything at any speed. I've tried all sorts of crazy things on my bike, and countersteering is physically the only way to turn a motorcycle. At higher speeds, you have to counstersteer through an entire turn to not high side

I have never driven a motorcycle but I think that it would depend on the mass of the motorcycle and the size and weight (moment of inertia) of the wheels. Leaning might have an effect on a light bike at low speeds. If the angular momentum of the spinning wheels is great enough, leaning out would not cause the bike to tilt at all.

I do ride a bicycle. I don't think I countersteer at all. To turn left I just lean left and maybe turn the wheel slightly left. At least that is what I think. I will check a little more carefully.

AM

 

[1mmorta1]

I have never driven a motorcycle but I think that it would depend on the mass of the motorcycle and the size and weight (moment of inertia) of the wheels. Leaning might have an effect on a light bike at low speeds. If the angular momentum of the spinning wheels is great enough, leaning out would not cause the bike to tilt at all.

I do ride a bicycle. I don't think I countersteer at all. To turn left I just lean left and maybe turn the wheel slightly left. At least that is what I think. I will check a little more carefully.

AM

It feels that way doesn't it? But that's not how it works. When you turn your wheel to the left, your bike tilts over to the right. The only reason that leaning left turns you left is that, when you lean left, you accidentally push the left handlebar forward. Thus, you countersteer. If you focus very hard on not changing your pressure on the handlebars, you will note that when you lean left, your bike either does nothing or tilts over to the right slightly and continues to go straight.

Some misguided people(not you) will tell you that countersteering is only necessary at high velocities. It is actually only VERY NOTICABLE at high speeds.

 

[1mmorta1]

NOTE: You have the impression that you turn your wheel to the left to turn left. Which is correct, AFTER you countersteer. To turn left, you lean left, which(for someone who doesn't race motorcycles) means you push the left handlebar forwar for a fraction of a second. THIS is what causes your bike to lean over. After that brief fraction of a second, if you continued to countersteer on your light bicycle you would low side(fall over to the inside) so you immediately push on the right handlebar. This keeps you from falling to the inside, and doesn't work on a fast motorcycle.
So turning left looks like this:
1. Turn the wheel to the right for a fraction of a second
2. This causes your bike to flip to the left, which is where you are leaning
3. Turn the wheel to the left to counter balance your fall to the inside shoulder(this is the part you notice, because it lasts long enough)

The whole process can be completed without any leaning at all, and cannot be done if the front wheel is locked into place.

From this you can see that leaning has no effect on turning, and that turning has everything to do with steering(countersteer, specifically.)

I hope that example is clear :)

 

[rcgldr]

On some motorcycles, you can do a mild turn at moderate speeds (40 or less mph 64 or less kph) by leaning. The rider leans inwards, the bike leans outwards, the trail on the bike causes the front tire to steer outwards in response to the bike leaning outwards, an indirect method of counter steering, and now the center of mass is offset inwards so the bike can lean left and make a gradual turn. This doesn't work on motorcycles with a small amount of trail, such as road racer replicas. If there's a steering damper, that will also interfere with the trail effect. As mentioned before, this doesn't work at high speeds on any motorcycle that I'm aware of.

Keith Code runs a motorcycle racing school, and he has a test 600 cc road racer motorcycle with a second set of handlebars fixed to the frame that don't steer. The bike is stable, but while holding on to the second set of handle bars, rider leaning will only produce a tiny amount of turn response on that bike. However Keith also mentions that his dirt bikes (these have a lot of trail), can be easily turned by leaning.

 

[berkeman]

Keith Code runs a motorcycle racing school, and he has a test 600 cc road racer motorcycle with a second set of handlebars fixed to the frame that don't steer. The bike is stable, but while holding on to the second set of handle bars, rider leaning will only produce a tiny amount of turn response on that bike. However Keith also mentions that his dirt bikes (these have a lot of trail), can be easily turned by leaning.

I use body steering pretty exclusively on my liter sportbike, both on the street and at track days. I originally learned body steering at a CLASS track school by Reg Pridmore, and have also attended STAR school with his son Jason Pridmore. I prefer body steering over countersteering because it works so much better in emergency situations. Relying on your arms to do something useful when you are tensing up because something bad is happening in front of you, is not generally a good thing. But moving your body and shifting your weight on the pegs (only 20% of your weight is on the seat when steering) is very natural, even when things are getting crazy in front of you.

The problem with Keith's "No BS Bike" is that you still sometimes use small corrections with the handlebars, even if you mostly use body steering to turn the bike. I still remember climbing the hill toward the Corkscrew at about a buck at Laguna Seca at one track day, and laughing to myself how I had to turn the handlebars into a turn (the opposite of what you would do for countersteering) because of the 30mph crosswind across the track. Goofy.

BTW, here is a video of Reg Pridmore, celebrating his 72nd birthday on the track at VIR. Reg uses bodysteering pretty exclusively. I want to celebrate my 70th birthday like that (still plenty of years to go...)! GOOSEBUMPS!



EDIT -- BTW, in the video you can see that bodysteering works just fine at 165mph+

 

[JeffKoch]

I suspect that at some speed, the ability of the rider to cause the bike to tilt by leaning would be lost.

On a motorcycle this speed is very low, like walking speed. Tony Foale has a thick book explaining some of the dynamics involved, but above about a walking speed you have to force a countersteering input to turn the bike. You can force this directly with the bars, you can force this by leaning to one side, or you can force it by leaning to one side and cutting the throttle a bit.

Incidentally I rode Keith Code's bike like 10 years ago at Streets of Willow, and you can indeed turn it but it's a challenge to reorient your brain and just get a feel for what's happening as you move around and play with the throttle. It's still slow to turn, but you can do figure-8's in the parking lot with practice.

Some bikes do require constant steering input through a turn, but it depends on the bike and tires - many bikes, if set up correctly, are neutral. You don't highside unless you give the bike too much throttle while leaned over, it actually has nothing to do with mid-turn steering inputs. Yes, I've highsided and lowsided many times when I used to roadrace...

 

[1mmorta1]

The thing to keep in mind is that its not the "leaning" that's turning these bikes. Its the countersteer. There are two ways of countersteering: Mechanically, using your handlebars, or by thrusting your weight one direction or another. When you thrust your body, its more than just a lean, and it is a physical chain reaction(equal and opposite the whole way through): the motorcycle tilting slightly in the opposite direction of your thrust, thus the tire turning in a direction opposite that of the motorcycles lean, causing the entire body to be flipped the direction opposite the tires turn. In either case, the action of your front wheel turning one way causes your motorcycle to turn the other.
Once your in this lean, your back to using your handlebars.

 

[berkeman]

The thing to keep in mind is that its not the "leaning" that's turning these bikes. Its the countersteer. There are two ways of countersteering: Mechanically, using your handlebars, or by thrusting your weight one direction or another. When you thrust your body, its more than just a lean, and it is a physical chain reaction(equal and opposite the whole way through): the motorcycle tilting slightly in the opposite direction of your thrust, thus the tire turning in a direction opposite that of the motorcycles lean, causing the entire body to be flipped the direction opposite the tires turn. In either case, the action of your front wheel turning one way causes your motorcycle to turn the other.
Once your in this lean, your back to using your handlebars.

I'm not tracking what you are saying, but that's okay. Bodysteering is weighting the pegs in the direction you want to turn, and getting your butt on the edge of the seat toward the inside of the turn. You don't touch the bars other than to work the controls. The bike front end finds the right angle of turn-in to match the balance and lean angle. You make mid-corner corrections to tighten up the turn by weighting the inside peg more, moving your body more to the inside, and pulling in a little bit on the outside of the tank with your outside thigh.

There is no initial outward motion of the front tire at body steering turn-in, and you don't use the handlebars for either turn-in or to straighten back up at turn exit.

It is true that you can quicken your transitions by using both body steering and countersteering, but because of the issues with using countersteering in emergency situations (that I mentioned above), I do my best to never use yaw inputs to the bars for steering.

 

[1mmorta1]

With all due respect, I have to disagree with your assertion. Do you feel you could use body steering to lean over a bike with a fixed front wheel? The answer is no. The reason for this is that the front wheel MUST slightly turn the opposite direction, for the most brief fraction of a second, in order for the bike to tilt the opposite way. Its simple physics, nothing more than equal and opposite reaction.

 

[A.T.]

The thing to keep in mind is that its not the "leaning" that's turning these bikes.

Probably because when you lean one way, the center of mass of you and bike still stays centered, and the bike tilts the other way.

Its the countersteer.

Yep. People don't realize it, because it's minimal at low speeds.

http://en.wikipedia.org/wiki/Countersteering]At[/PLAIN] low speeds countersteering is equally necessary, but the countersteering is then so subtle that it is hidden by the continuous corrections that are made in balancing the bike, often falling below a just noticeable difference or threshold of perception of the rider.
...
Countersteering is indispensable for bike steering. Most people are not aware that they employ countersteering when riding their bike any more than they are aware of the physics of walking. They have learned to apply the required countersteering without thinking.

 

[berkeman]

With all due respect, I have to disagree with your assertion. Do you feel you could use body steering to lean over a bike with a fixed front wheel? The answer is no. The reason for this is that the front wheel MUST slightly turn the opposite direction, for the most brief fraction of a second, in order for the bike to tilt the opposite way. Its simple physics, nothing more than equal and opposite reaction.

I could certainly turn it in, but would then promptly low-side. When body steering, you lean the bike with your weight shifting on the pegs, and the bike's front end turns itself into match the balance and lean angle.

I learned body steering at Laguna Seca at my first racetrack CLASS day. I had been using countersteering for many, many years, and thought I was pretty good at it. But I could not manage to steer through the Corkscrew no matter how hard I tried, and how quick I tried to correct my lines.

And then in one of the classroom sessions (you alternate classroom-track sessions all day), Reg was talking about how if you used your balance to aim the bike where you wanted to go, the front end would find its own turn angle to keep the bike stable and going where you wanted it to go. So the next time out on the track I stopped using my hands and started using body weight shifts, and nailed the fall line through the Corkscrew time after time. It was a true epiphany.

If you haven't tried using body steering at speed (and especially at a racetrack), you probably shouldn't be trying to convince others of how it works by some form of countersteering. It doesn't.

Try this. With the help of a friend, walk your sportbike in a straight line, not holding the bars. It will probably track straight pretty well. Now lean the bike to the right about 10 degrees while still walking it and not touching the bars. See what happens to the front end? It goes from straight to turned, without any counter-turn involved...

 

[1mmorta1]

"It is important to distinguish between countersteering as a physical phenomenon and countersteering as a conscious rider technique for initiating a lean (the usual interpretation of the term). The physical phenomenon always occurs, because there is no other way to cause the bike and rider to lean short of some outside influence such as an opportune side wind, although at low speeds it can be lost or hidden in the minute corrections made to maintain balance.

At the same time, the rider technique of applying pressure to the handlebars to initiate a lean is not always necessary, since, on a sufficiently light bike (especially a bicycle), the rider can initiate a lean and turn by shifting body weight, called counter-lean by some authors.[1][2][3] Documented physical experimentation shows that on heavy bikes (many motorcycles) shifting body weight is less effective at initiating leans.[4]"

That's from the wiki page on countersteering. Note how it explains that it is impossible to turn without the physical phenomena of countersteer taking place.

 

[berkeman]

That's from the wiki page on countersteering. Note how it explains that it is impossible to turn without the physical phenomena of countersteer taking place.

Don't believe everything you read at wikipedia, especially if it involves the word "impossible".

Please try the experiment I outlined above, and tell us what the front wheel does.

 

[1mmorta1]

Sir, I recognize that you see the the tire turn in the direction you expect :) But just because it appears so, doesn't mean it is so. I have spent a lot of time experimenting with my ninja, and I'm well aware of how motorcycles behave.
What you DON'T see, is that JUST as the motorcycles tilts to, say, the right, the wheel is slightly(less for lower speeds, and for a VERY brief time period) pointed to the left.
It is physics.

 

[1mmorta1]

Please note: I confess I am reluctant to use wikipedia. However I feel comfortable in this instance because I know that the information quoted is correct, and there are many sources cited at the bottom of the page ;)

 

[berkeman]

Sir, I recognize that you see the the tire turn in the direction you expect :) But just because it appears so, doesn't mean it is so. I have spent a lot of time experimenting with my ninja, and I'm well aware of how motorcycles behave.
What you DON'T see, is that JUST as the motorcycles tilts to, say, the right, the wheel is slightly(less for lower speeds, and for a VERY brief time period) pointed to the left.
It is physics.

So we agree to disagree. That's fine.

I think a more accurate way of describing the "physics" of body steering is that by weighting the inside peg and moving your weight to the inside (not weighting the seat much at all), you are leaning the bike to a position that is inconsistent with the front end still pointing straight. For any lean angle and balance, there is a natural turn angle for the front end that keeps the bike balanced. The front end turns into match this angle as you lean the bike with peg weighting.

So the wheel never has to turn the opposite way. As soon as you weight the inside peg with your butt off the seat, the front end is pulled to point into the turn until it balances the lean angle.

Again, since you appreciate physics, I think you will find the walking experiment enlightening.

Above all, ride safely and well. And watch out for the cages.

 

[1mmorta1]

Above all, ride safely and well. And watch out for the cages.

Aren't they the worst? Thank you, and you as well. I would encourage the original poster then(since I in know way agree with your conclusion that the wheel never points the other direction) to do some research of his own :) He's heard both arguments.

 

[Andrew Mason]

The thing to keep in mind is that its not the "leaning" that's turning these bikes. Its the countersteer.

I would suggest that it is the lean that allows the bike to turn (ie. in the direction of the lean). The countersteer allows the bike to lean.

It is interesting that you cannot turn a bike without leaning it. I think there are probably two reasons for this. The first is that the lean balances the torque on the bike (gravity balancing the centripetal torque) on the long horizontal axis so the bike does not flip to the high side. The second is that the lean changes the direction of the angular momentum of the wheels and the gyroscopic reaction to this is to rotate the bike about its vertical axis in the direction of the turn ie. the change in angular momentum of the wheels results in a torque on the bike about its vertical axis in the direction of the turn. Try holding a spinning bicycle wheel by the axle and leaning it to the left or right.

AM

 

[JeffKoch]

One of the things Code's bike, or just riding up an off-ramp no-handed with a cruise control of some kind on, teaches you is that while you can turn at constant throttle by shifting your weight, it doesn't really work unless you move off quickly - and that quick motion comes from weighting and pushing off the outside peg. That motion acts as a countersteering input, moving the front wheel off-center towards the outside peg while you move your weight onto the inside peg. You can help make this happen by rolling off the throttle at the same time, reducing trail and making the bike easier to turn with smaller inputs.

This is what Freddie Spencer's school used to teach, bodysteering and throttle steering. They work smoothly in conjunction with bar inputs by making your whole body motion and all your control inputs directed in the same way, and they also help put your focus on something other than blasting the bars so you tend to be lighter on the bars, enhancing control. It really works, but not because you're replacing direct bar inputs with something else - if you do all these things with zero bar input (Code's bike), you find that the bike turns like an ocean barge. It turns, but only very slowly.

Watch out for physicists who ride sportbikes.

 

[rcgldr]

It is interesting that you cannot turn a bike without leaning it.

You can turn just by steering, at least for a brief moment until the bike ends up on it's side. What turns a bike is the angle between the direction the front and rear tires are pointed. If you vision extending the axis of the front and rear wheels, they will cross somewhere under the pavement (if the bike is leaning inwards). If you then note the point on the pavement directly above where the axis lines cross below, that will be the center of the circular path that the motorcycle would tend to follow for a given amount of steering input and lean angle, absent other forces. The actual radius will be slightly larger due to deformation of the contact patches under cornering loads.

So back to basics, countersteering steers the tires out from under a bike in order to lean the bike for a coordinated turn, then normal steering is used to actually turn.

 

[berkeman]

Sir, I recognize that you see the the tire turn in the direction you expect :) But just because it appears so, doesn't mean it is so. I have spent a lot of time experimenting with my ninja, and I'm well aware of how motorcycles behave.
What you DON'T see, is that JUST as the motorcycles tilts to, say, the right, the wheel is slightly(less for lower speeds, and for a VERY brief time period) pointed to the left.
It is physics.

So I experimented some on the ride home tonight. I paid a lot of attention to the bars as I body steered the bike in turns. As far as I can tell, there is no homeopathic outward turning of the bars or front wheel when unweighting your butt from the seat and transferring your weight to the inside peg to turn the bike in. Are you folks really trying this? There's the walking experiment to show that it works, and there's the riding experiments to show to yourself that there is no homeopathic countersteering required to ride a sportbike at speed through corners.

I know that the debate between countersteering and body steering is a bit contentious in general, but the physics angle shouldn't be too hard to address.

Maybe we should all ride the Corkscrew for fun...and talk physics afterwards.

 

[JeffKoch]

As far as I can tell, there is no ... outward turning of the bars or front wheel when unweighting your butt from the seat and transferring your weight to the inside peg to turn the bike in.

It's hard to see but it's there, and this is why you turn without touching the bars at all. Very, very, very slowly. One way to get a feel for this is to experiment with how rapidly you move your butt off to the inside - if you blast off quickly, you turn more quickly, which supports the notion that the turning input is the blasting, not the weighting per se.

 

[Andrew Mason]

You can turn just by steering, at least for a brief moment until the bike ends up on it's side.

Which is why you have to lean the bike in order to turn.

What turns a bike is the angle between the direction the front and rear tires are pointed. If you vision extending the axis of the front and rear wheels, they will cross somewhere under the pavement (if the bike is leaning inwards). If you then note the point on the pavement directly above where the axis lines cross below, that will be the center of the circular path that the motorcycle would tend to follow for a given amount of steering input and lean angle, absent other forces. The actual radius will be slightly larger due to deformation of the contact patches under cornering loads.

Does it also depend on the forward angle of the forks and the distance between the extended line from the forks to the road and the contact point of the front tire with the road? In other words, is there a difference between a regular bike and a chopper in the amount you have to turn the wheel?

I didn't mean to suggest that you don't have to turn the handlebars. It is just that, for a motorcycle, if not for a bicycle, the lean of the bike produces a gyroscopic torque on the bike that tends to turn the bike in the direction of the lean.

AM