What is the physics behind performing a wheelie on a motorcycle?

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The discussion focuses on the physics of performing a wheelie on a motorcycle, emphasizing the mechanics involved, such as the free-body diagram and the forces at play. Riders typically squat low, push down on the front of the bike, and throttle forward to lift the front wheel, but excessive force can lead to losing control. Safety concerns are highlighted, with anecdotes about reckless attempts leading to accidents. The conversation also touches on the importance of understanding the dynamics of both initiating and maintaining a wheelie, including the role of the center of mass. Overall, the thread serves as a blend of technical analysis and cautionary advice regarding motorcycle stunts.
mike_241
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trying to find out the mechanics involved in performing wheelie.
I need to understand the mechanics involved in performing a wheelie. That includes analyzing the free-body diagram. And considering all the factors involved in performing this manoeuvre in motorbilkes.
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Generally, the rider will squat low, push down on the forward part of the bike hard to propel himself upward, pull the forward part of the bike up with him, and then throttle forward for quick acceleration to propel the front wheel further up. Obviously, if he's too energetic, he can end up on his back.
 
.Scott said:
Generally, the rider will squat low, push down on the forward part of the bike hard to propel himself upward, pull the forward part of the bike up with him, and then throttle forward for quick acceleration to propel the front wheel further up.
LOL, not on my 450 MX bike or my liter superbike. :smile:

mike_241 said:
I need to understand the mechanics involved in performing a wheelie.
Welcome to PF. Can you say why you need to understand this? Is this question for schoolwork? Or are you looking to buy a dirtbike/MX bike/sportbike and try it out? We're kind of big on safety here at PF, but plenty of us ride...

EDIT: Safety Spoiler Added

This thread may be closed soon because of safety issues, but one of the most important things to be practiced and skilled at is covering the rear brake pedal if you are going to be lifting a wheelie. You need to be able to pull in the clutch and tap the rear brake pedal any time you are up in a wheelie, to bring the front wheel back down. Try not to overdue this bring-down maneuver, BTW.

Similarly for a stoppie, you need to be ready to rev up the engine and rear wheel to bring the back end down if you end up too high at the back during a stoppie.

Professional riders on a closed course only please.
 
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Please do not follow the example of my friend Bob. Back in the 1970's, he bought a Kawasaki 500. That bike was a three cylinder 500 cc high performance machine. Bob had never driven a motorcycle, but had watched Evel Knievel movies.

He started the engine, revved it a few times, revved it harder, and popped (suddenly released) the clutch. The front wheel went straight up, the entire bike went up, he fell off, the bike fell straight down on the rear wheel, then fell over. Negligible damage to the bike, and Bob was unhurt. But after that, it was known as Bob's Killasaki Murdercycle.

And the process of analyzing starts by making a free body diagram. Start with the machine stationary with both wheels on the ground.
 
jrmichler said:
Please do not follow the example of my friend Bob. Back in the 1970's, he bought a Kawasaki 500. That bike was a three cylinder 500 cc high performance machine. Bob had never driven a motorcycle, but had watched Evel Knievel movies.
What could possibly go wrong?

jrmichler said:
He started the engine, revved it a few times, revved it harder, and popped (suddenly released) the clutch. The front wheel went straight up, the entire bike went up, he fell off, the bike fell straight down on the rear wheel, then fell over.
Oh, well there's that.

jrmichler said:
Negligible damage to the bike, and Bob was unhurt.
Whew, good news and no Darwin award. Great.
 
.Scott said:
Generally, the rider will squat low, push down on the forward part of the bike hard to propel himself upward, pull the forward part up with him, and then throttle forward for quick acceleration to move the front wheel further up. If he's too energetic, he can end up on his back.
Thanks for your reply. Can you please explain this concept using a free-body diagram so that this concept can be understood better.
 
berkeman said:
Can you say why you need to understand this? Is this question for schoolwork? Or are you looking to buy a dirtbike/MX bike/sportbike and try it out?
@mike_241 -- You did not answer my questions...
 
  • #10
mike_241 said:
Thanks for your reply. Can you please explain this concept using a free-body diagram so that this concept can be understood better.
It would be good for you to respond to @berkeman 's question.

This sounds very much like a homework problem to me - or at least close enough to treat it as one.
For a homework problem, you need to show your own contribution to this problem before anyone here guides you further along.
With that in mind, I suggest you draw a free-body diagram of an accelerating motorcycle, complete with center of gravity and the loading on each wheel. Then we can talk about what kind of tweaks you will need to get to the next step.

In the meanwhile, please refrain from doing anything especially daring on a motorcycle. A torn up free-body diagram is quickly discarded. A torn up human body is kept for life.
 
  • #11
However you do it, PLEASE be careful...

Been a few years, but my wife was watching TV when a motor-bike went past the window upside down, back to front and rider-less. A moment later, the rider followed, also upside-down and back to front.
No helmet.
Fire & Rescue hosed his brains into the drains.

Perp had hot-wired bike outside a nearby 'Pub', warily ridden it around local side-roads until confident.
Opened the throttle--
It was a high-tuned machine, a limited edition, 'Street-Legal' version homologated from a 'Racing' model.
A genuine 'Pocket Rocket'.
'Darwin Award' ensued.
In fact, Perp was dead before owner noticed the bike had been stolen and called it in...
 
  • #12
berkeman said:
Berkman@mike_241 -- You did not answer my questions...
I was trying to figure out how this works. If we look at FBD of this. and balancing the center of mass on one wheel at high speed. Doest seems right. I have tried to figure it out. But was to avail.
 
  • #13
Nik_2213 said:
A genuine 'Pocket Rocket'.
Around here, in the Southern part of the state, the term is "Crotch Rocket."
 
  • #14
mike_241 said:
I was trying to figure out how this works. If we look at FBD of this. and balancing the center of mass on one wheel at high speed. Doest seems right. I have tried to figure it out. But was to avail.
Is this for schoolwork? Or just a personal interest and want to understand the Physics behind them?

In either case, show us your FBD so far. You can use the "Attach files" link below the Edit window to upload PDF or JPEG files.
 
  • #15
I am trying to understand the dynamics of performing a wheelie. Please correct me if I am wrong. And help me out
wheelie.jpeg
 
  • #16
  • #17
Think of it from the point of moments being applied on the free body.

One moment is because of the graviational force multiplied by the perpendicular distance to the rear wheel contact, as that is the fulcrum about which the bike is rotating.

The second moment is because of the frictinoal force, multiplied by the rear wheel radius. Or you can take this as the force the bike applies on the rear axle multiplied by the rear wheel radius, which is the perpendicualr distance from the force to the same fulcrum mentioned before.

A third moment will be because of the force exerted by the front wheel on the ground, multiplied with the distance between the wheels in contact.

Try to construct the free body diagram from this.

So when the second moment is greater than the first one, there will be a net moment that will make the front of the bike rise up. Here the third moment will not matter because when the front is rising up, the force exerted by the front wheel will be zero.

When the first moment is greater than the second, then the third moment will come to play and the net moment will be zero, causing no rotation of the body.

Does this make sense @mike_241 ?
 
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  • #18
mike_241 said:
I am trying to understand the dynamics of performing a wheelie. Please correct me if I am wrong. And help me outView attachment 328958
Your FBD is wrong (and you do NOT need two: one when it is moving and one at rest).

Lifting the front wheel up will happen when you are accelerating. So put the forces involved with acceleration on your FBD in addition to the ones related to the weight. (A vehicle can accelerate even if its velocity is zero.)
 
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  • #19
  • #20
Welcome, Mike!

Initiating a wheelie is possible because the combined center of mass of rider and machine is vertically located way higher than the contact patch of the rear wheel and pavement or dirt.

As the contact patch is forced to accelerate forward by the engine, that center of mass has inertia or resistance (both linear and rotational) to be moved in that forward direction and to be rotated higher up.

zggPj.png


Please, see:
https://en.wikipedia.org/wiki/Wheelie

https://www.scienceabc.com/pure-sci...nal-normal-force-torque-angular-momentum.html

https://www.animations.physics.unsw.edu.au/jw/wheelies.htm



 
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