## The black art of motorcycle dynamics

flat tire   1 MW

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### The black art of motorcycle dynamics

So the subject is interesting: you have what seems to be a comprehensible system relying on "classical" physics that were iron-clad over 100 years ago. But then you find things like a wiki page lacking even basic information and universities web pages and books by physicists getting really basic stuff wrong: usually first that gyro forces are necessary to keep a bike upright and stabilized (wrong, bikes with counterrotating wheels that cancel the gyro forces demonstrate rake and trail are fundamentally responsible for stability) and second that either precession or "stepping out" is the sole means by which countersteering happens (completely implausible when you consider the forces involved and the extreme rapidity with which a motorcycle can be leaned by torquing the bars). Even Wikipedia's page on 2 wheel dynamics and countersteering leave it at "Because the forces in the contact patch are at ground level, this pulls the wheels "out from under" the bike to the right and causes it to lean to the left".

That sounds incomplete and I propose a more complete story of how countersteering ACTUALLY works: the rate the wheel tracks out from under the bike into a radius at a given steering angle has a linear relationship with the bike's speed but the lateral acceleration needed to take that given radius grows exponentially with speed. This explains how the bike can quickly lean at high speed with barely any lateral counter-track, and how at low speed there is significant travel the opposite way as the bike is in the process of being leaned over.

Please critique or correct what I have written and use this thread to share other mysteries and explanations of 2 wheel dynamics. My next question: how does whipping (like in MX) work? Riders give terrible and inaccurate explanations as always. Is it purely precession? That is plausible for the initiation but I'm having trouble figuring out how the bike gets straight again as the riders are twisting the bars in such a way that precession doesn't seem to be the culprit at that point.
Last edited by flat tire on Jun 24 2018 1:22pm, edited 1 time in total.

flat tire   1 MW

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### Re: The black art of motorcycle dynamics

Anybody?

Dauntless   100 GW

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### Re: The black art of motorcycle dynamics

So I got a perfect score on riding test for my motorcycle license. At that time I couldn't have told you anything about leaning or countersteer. I doubt you'll see serious comment.

https://www.cycleworld.com/2013/10/25/k ... -correctly

https://www.amazon.com/Motorcycle-Dynam ... e+dynamics
Any sufficiently advanced technology is INDISTINGUISHABLE FROM MAGIC!
- Arthur C. Clarke

John and Cecil   1 kW

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### Re: The black art of motorcycle dynamics

Most people probably do not know. Some very experienced riders have never studied or understood the principles of riding. They ride on instincts that they are constantly developing and adapting from all their previous rides. That is why new riders are much more likely to crash as they have no instincts to fall back upon when something unexpected happens.

Their are many, many laws of physics. If you wish to understand the underlying elements of some of your questions perhaps you would have more success asking your questions on a physics forum?

The more experienced riders love to ride, it is in their blood. They really don't want to study physics, they want to live physics

2WheelsMovesTheSoul   100 W

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### Re: The black art of motorcycle dynamics

this whip you speak of... I may have some info for you. Check out a few vids of guys doing freestyle.

Watch the rear wheel and exhaust tip for puffs of smoke very closely. You'll notice the rear wheel speed changes while in flight depending on the type of trick or desired effect. The rider whacking or pinning the throttle while mid air is not meant to thrill the crowd or piss off the treehuggers.

Seems counter intuitive but simply stopping a spinning wheel (at the right moment) while jumping is the easiest way to correct or change the angle at which you land. Say your front wheel is too high and you want to fix that mid air, stomp the rear brake, the all that momentum will now transfer all the way up front and bring the wheel down, leveling you out before landing. Newtons 3rd Law brah. The same works in reverse, WOT mid air, will help change your angle.

Think of it a 50lbs gyroscope that can spin up from 0-80mph in a fraction of a second. This produces tremendous amounts of force. Depending on where and when the spinning weight is stopped, started or just spun up to ridiculous speeds while in mid air makes a whole lotta different seemingly impossible tricks quite simple in principle but very difficult in practice.

flat tire   1 MW

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### Re: The black art of motorcycle dynamics

Keep the responses coming.
2WheelsMovesTheSoul wrote:
Jun 25 2018 10:34am
I may have some info for you.
You don't provide any info on a whip, so please come back with that info if you forgot to post it. The ability to pitch the bike up and down based on the wheels inertia wasn't mentioned in the OP but it is one of the few parts of motorcycle dynamics that is already completely understood outside the field, and it's not a mystery at all. Most MX riders could probably even explain how this works.

2WheelsMovesTheSoul   100 W

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### Re: The black art of motorcycle dynamics

I don't think I left it out, I just gave a basic description. The one thing that separates motorcycles from anything else is the abilty to use the built in fully adjustable gyroscope to alter the pitch, roll and yaw of an object in flight. There are some tricks you will never see on a BMX that can only be done with a motor driven cycle.

Seems like people think the gyro effect is limited to one plane. It can be utilized on all 3 axis. Pitch, roll, and yaw adjustments can be made mid air depending on exactly when, where and how much spin is produced by the rear wheel.

As for technique. Tail whips are pretty much 3 part, approach angle, body positioning and weight transfer.

You take off at an angle, almost turning off of the lip, while still on the gas. This gives you some lean to start with and since the rear wheel left later than the front, + was under power and left at a different angle it forces rotation and the tail end comes around faster.

Off the lip your body should be positioned on the high side of the bike, with you pushing down on the inside peg and all your weight pushing down on the bar. Once flat, level and rear end has come around, its time to bring it back.

You then lean again in the opposite direction, putting your weight on the outside foot peg + yank the bars the opposite way pulling your upper body weight over the COG toward the front wheel. Meanwhile your core should be rotating as well. With you knees dug into the tank, you can swing the body of the bike vs the handlebars. Eventually the bike rotates back and you land.

It's almost all moving your weight around the bikes COG and making last second corrections with the rear wheel if needed.

flat tire   1 MW

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### Re: The black art of motorcycle dynamics

Your first post mentioned literally nothing about whipping the second is a how-to not a physics analysis and furthermore it's a bad how-to that makes it clear you don't understand the process very well even if you have the skills to get totally upside down and sideways over the finish line. Body steering / leaning? In the AIR of all places? Yeah, no. Nice try, though.

Keep them coming, folks. There has to be someone on here who knows this stuff.

torker   1 MW

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### Re: The black art of motorcycle dynamics

2 wheels had very valid points .. Of course it is not a " black art " haha
Dave When I die I want to slide in sideways yelling WooHoo what a ride !

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2WheelsMovesTheSoul   100 W

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### Re: The black art of motorcycle dynamics

Cool... I'll just ignore my 20+ years of 2 wheeled motoring experience.
I'll give you this. I never addressed the whip specifically, just a generalization of the gyro effect. I had a 2 drafts going and accidentally submitted the rough before heading off the break. The final should have had several links to "how to Whip" vids as well as how the gyro can be used on all 3 axis to achieve various results, one of which is whips.
the second is a how-to not a physics analysis
I'm not claiming I do triple backflips on the daily here nor am I Physics professor so I don't have the exact vocab or lingo you're used to. But as someone who has some actual real world saddle time I thought I'd chime in here since the OP
I'm having trouble figuring out how the bike gets straight again as the riders are twisting the bars in such a way that precession doesn't seem to be the culprit at that point.
appears to have very little or ZERO saddle time. Anyone with some ACTUAL aerial experience, will tell you this twisting the bars is in fact what does it. As in every action has an equal and opposite reaction. Do you honestly think that bar yanking is just showing off or for theatrics?

All tricks, read again please.... ALL TRICKS including the whip that seems to have you perturbed, are performed using a combo of approach angle, speed, rotation, counter steering, weight transfer, using your legs and core to move the bike vs the bars, using the rear wheel gyro effect, which foot peg has weight on it when you hit the lip, ect...
furthermore it's a bad how-to that makes it clear you don't understand the process very well even if you have the skills to get totally upside down and sideways over the finish line
this statement is basically saying I don't use enough fancy technical lingo that you bookworms love so much... so read this professional instruction and get rekt https://www.dirtrider.com/features/prot ... fer#page-3
Body steering / leaning? In the AIR of all places? Yeah, no. Nice try, though.
Maybe if Deegan or McGrath don a chalky lab coat, present a nice power point packed with technical jargon, mathematical formulas that have more symbols and letters than numbers, purdy diagrams and at the end tell you flat out 95% OF THIS SHITS DONE IN THE AIR, the first 5% is done on the lip, maybe then you'll listen.

In any case, good luck finding credentialed physics professor who rides hard enough to answer any of this with some sort of authority utilizing the proper verbiage you seem to require in order for the learning process to take place.

"You are like this cup; you are full of ideas. You come and ask for teaching, but your cup is full; I can't put anything in. Before I can teach you, you'll have to empty your cup."
Last edited by 2WheelsMovesTheSoul on Jun 26 2018 2:10pm, edited 1 time in total.

81forest   100 W

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### Re: The black art of motorcycle dynamics

flat tire wrote:
Jun 25 2018 6:39pm
...and furthermore it's a bad how-to that makes it clear you don't understand the process very well even if you have the skills to get totally upside down and sideways over the finish line. Body steering / leaning? In the AIR of all places? Yeah, no. Nice try, though.

Keep them coming, folks. There has to be someone on here who knows this stuff.
Lol. 2wheels’ how-to makes sense to me. Seems like the physics rules would be completely different in the air vs. when the tires are in contact with the ground, correct?. “Body steering / leaning” are inputs that have a lot more effect on an airborn motorcycle than countersteering the bars, in my experience. Using the gas or brake to wind up or slow down the rear wheel’s gyro effect in the air also plays a huge role in getting the bike’s front up or down, but we all probably know that.

Interesting responses.

81forest   100 W

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### Re: The black art of motorcycle dynamics

flat tire wrote:
Jun 17 2018 12:36am
... universities web pages and books by physicists getting really basic stuff wrong: usually first that gyro forces are necessary to keep a bike upright and stabilized (wrong, bikes with counterrotating wheels that cancel the gyro forces demonstrate rake and trail are fundamentally responsible for stability) and second that either precession or "stepping out" is the sole means by which countersteering happens (completely implausible when you consider the forces involved and the extreme rapidity with which a motorcycle can be leaned by torquing the bars).
A bike with counter-rotating wheels? Huh?? Seriously interested in seeing a pic of that in action!

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### Re: The black art of motorcycle dynamics

flat tire wrote:
Jun 25 2018 6:39pm
Your first post mentioned literally nothing about whipping the second is a how-to not a physics analysis and furthermore it's a bad how-to that makes it clear you don't understand the process very well even if you have the skills to get totally upside down and sideways over the finish line. Body steering / leaning? In the AIR of all places? Yeah, no. Nice try, though.

Keep them coming, folks. There has to be someone on here who knows this stuff.
Oh man... I won't even try to explain that to you...
maybe go grab a medicine ball, run and jump in a pool, and trow the ball while you are in the air : get back to us if you landed at the same place than the ball...
or you take your electric bicycle hook it in the air and check where the handle bar move when you accelerate suddenly or brake suddenly...
maybe you will start to get some sens what inertia and momentum are... yeah, "black art", sorry

2WheelsMovesTheSoul   100 W

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### Re: The black art of motorcycle dynamics

Body steering / leaning? In the AIR of all places? Yeah, no. Nice try, though.
This right here... is some of the most mindless ignorant shit I've heard in years and Trumps spewing rhetoric from his soapbox everyday.

I can't imagine being trapped inside a brain that can't accept there are things I don't know or understand... yet or ever will.

liveforphysics   100 GW

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### Re: The black art of motorcycle dynamics

Plenty of mystery in motorcycle handling.

I used to think I understood motorcycle dynamics best before 25years of riding/racing dirt and asphalt (and still not a good rider).

I have learned its never the machine holding back whatever you wanted to achieve on the bike, always the rider is bottleneck.

Motorcycles are piloted and flown by pros to the limits of their imaginations and balls.
Each carcinogen vapor exposure includes a dice roll for cancer.

Each mutagen vapor exposure includes a dice roll for reproductive genetic defects in your children.

Each engine start sprays them into a shared atmosphere which includes beings not offered an opportunity to consent accepting these cancer experiences and defective genetics life experiences.

Every post is a free gift to the collective of minds composing the living bleeding edge of LEV development on our spaceship.

motomech   1 GW

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### Re: The black art of motorcycle dynamics

I don't think folks consciously countersteer, but there is the technique of 'loading" the inside peg that a rider can actually practice.
Those who want to read more on the dynamics of motorcycle riding can immerse themselves in the three volumes of Keith Code's very wordy Twist of the Wrist. When I switched from dirt to road racing, I forced myself to read and try an understand what he was trying to say. All I ever did take away from it was, brake hard to the apex, turn in late and hard and get out early, although knowing what I should be doing didn't help me from going in early and runing wide coming out, scaring myself and backing off the throttle
It did give me a new-found respect for "Fast" Freddy Spencer, arguably the first rider to purposely slide BOTH tires to turn in and "shorten" the corner.
But what does all this have to do w/ bicycles? Not much I would say, at least at the speed that I ride. The dynamics are just not the same and I suspect trying to counter-steer a bicycle would result in a painfull face-plant.
Motomech

'07 GT Idive 4 4.0, Q100H 201 frt. mounted, 14S Multistar LiPoly, elifebike 9-FET 20A controller. Mean Well HLG-320H-54A, Crazy Bobs on Alex DM32's 21 to 22 MPH. viewtopic.php?f=3&t=28151&p=1373714&hilit=Idrive#p13737

flat tire   1 MW

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### Re: The black art of motorcycle dynamics

So much emotion. Look I've talked to a lot of riders who I've SEEN throw some badass whips. But the problem is they give completely different and unconvincing descriptions of how it goes down. So it means nothing to me when someone who's been riding for 20 years describes how they THINK a whip goes down. The fastest and most beautiful riders usually know almost nothing about how their bike actually does what it does but many of them think they could write the book on it.

But the real problem is you're telling me how you DO a whip. Great. But we need to go a step beyond: HOW does what you do to the bike actually cause it to whip?

The whip is initiated and recovered solely / almost entirely by precession caused by the rider moving the bars. Yes or no.
motomech wrote:
Jun 27 2018 12:28am
I don't think folks consciously countersteer,
Wrong, a great many riders consciously countersteer including yours truly. The cause / effect is more direct the faster you go, with the physics behind this being explained in the OP.

motomech   1 GW

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### Re: The black art of motorcycle dynamics

Thinking about it slows you down.
Motomech

'07 GT Idive 4 4.0, Q100H 201 frt. mounted, 14S Multistar LiPoly, elifebike 9-FET 20A controller. Mean Well HLG-320H-54A, Crazy Bobs on Alex DM32's 21 to 22 MPH. viewtopic.php?f=3&t=28151&p=1373714&hilit=Idrive#p13737

81forest   100 W

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### Re: The black art of motorcycle dynamics

motomech wrote:
Jun 27 2018 10:15pm
Thinking about it slows you down.
Haha, so true!

I also really like the Keith Code books. He made a movie for twist of the wrist that is pretty funny. Lots of mullets. He is an interesting dude.

ulrichw   1 mW

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### Re: The black art of motorcycle dynamics

81forest wrote:
Jun 26 2018 12:35pm

A bike with counter-rotating wheels? Huh?? Seriously interested in seeing a pic of that in action!
http://www.rainbowtrainers.com/bicycle-science/

81forest   100 W

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### Re: The black art of motorcycle dynamics

ulrichw wrote:
Jun 28 2018 7:04pm
81forest wrote:
Jun 26 2018 12:35pm

A bike with counter-rotating wheels? Huh?? Seriously interested in seeing a pic of that in action!
http://www.rainbowtrainers.com/bicycle-science/
Mind blown!!

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