Why does the Motorcycle does A Wheelie when you Clutch-Wheelie

[ProgressNation]

I personally don't ride a motorcycle, but I want to know if that relates to Physics.
When someone holds the Clutch and leave it fast or Clutch the motorcycle to make a Wheelie, What is that thing in the Clutch which makes the Motorcycle pop up? I think I even saw cars do wheelies in videos, so what's the reason in Physics that makes it pop up while clutching?

 

[berkeman]

I personally don't ride a motorcycle, but I want to know if that relates to Physics.
When someone holds the Clutch and leave it fast or Clutch the motorcycle to make a Wheelie, What is that thing in the Clutch which makes the Motorcycle pop up? I think I even saw cars do wheelies in videos, so what's the reason in Physics that makes it pop up while clutching?

When you pull in the clutch, that allows you to rev the engine, so that when you let out the clutch, the engine is at a high RPM which provides more torque. It is the extra torque that helps to lift the front wheel.

 

[berkeman]

BTW, if your bike has enough power (or more specifically, a high power-to-weight ratio), you don't need to use the clutch to lift a wheelie. Just roll on the power and the front end comes up...

PS -- I don't do wheelies on purpose on public roads (illegal). I keep them on the racetrack and in the dirt.

 

[Danger]

I would further recommend using caution when experimenting.
I love bikes, but I was always a bad rider (vertigo problems) and quite timid. The only time that I popped a wheelie was by accident. I ended up flat on my back with a couple of hundred pounds of steel on my chest. There was no injury, but it could very well have resulted in a broken back, broken ribs, or a concussion.

 

[rcgldr]

During heavy acceleration, at the point of contact, the rear tire pushes backwards against the pavement and the pavement pushes forwards against the tire. The forwards force from the pavement is below the center of mass of the bike, which experiences a backwards reaction force due to the acceleration. The forward force from the pavement and the backwards reaction force at the center of mass create a torque that results in a wheelie if the acceleration is strong enough.

There's also the issue of the torque generated on the rear tire. If the rear tire was locked in place, unable to turn or move with respect to the pavement, then with sufficient torque being appied to the rear tire from the engine (and clutch), a bike could wheelie. Under normal circumstances (the rear tire rotating and accelerating), you have torques related to the engine and rear tire accelerating, but these would be relatively small.

In the case of a chain drive, you have a tension in the upper part of the chain, and a compression along the swing arm. This would generate internal torques, but I'm not sure if this contributes to a wheelie. For a shaft drive, there are internal torques applied to the shaft drive and whatever is holding the shaft drive in place, but again, I'm not sure if this contributes to a wheelie.

Also as mentioned by the previous posters, most high end sport bikes will power wheelie without using the clutch or sudden application of the throttle (to produce a jerk) if in first gear at full throttle. Leaning forward will reduce the tendency to wheelie, but it may not eliminate it.

 

[physwizard]

if you understand the physics behind it well, you can be more confident about doing the wheelie. you can also do power wheelies if your bike engine is powerful enough. since your question is about clutch wheelies, i am restricting my answer to those.
as per Newton's third law, just as the chain tries to rotate the back wheel, the wheel also exerts a reaction torque which will try to rotate the chain, and hence the whole bike, in the opposite direction. under normal riding conditions, this reaction torque does not exceed the torque produced by gravity, in other words the weight of the bike prevents it from wheelieing.
the force of kinetic friction between the clutch plates should be greater than the engine torque for normal operational torques. otherwise, you would never be able to get the clutch plates to lock at normal operational torques. when you abrupty drop the clutch, the normal reaction is maximum, so the force of kinetic friction which you face is maximum. this force can be enough to counter gravity and lift the front wheel. if it doesn't work, try giving full throttle or increasing the rpm. this will increase the time for which the frictional force acts, thus increasing the time for which the wheelie lasts. or else try sitting a little bit back on your seat. remember you can stop the wheelie at any time by pressing the back brake, this will provide enough counter torque to stop the wheelie. but be careful as sometimes wheelies can happen in a fraction of a second not giving you enough time to control it.

 

[Danger]

sometimes wheelies can happen in a fraction of a second not giving you enough time to control it.

That's what happened to me. Even worse, the first thing that happened was that my torso was forced rearward. In my instinctive reflex to hold on for dear life, that opened the throttle more.

By the bye... is your shift key broken?

 

[sophiecentaur]

I have often thought that the throttle control is the wrong way round on motorbikes. Positive feedback is never a good thing in control loops.
Also - the Hand control adaption on motorcars is daft. You push to stop (unlike bicycle brakes) and pull to go faster - same problem as with a bike throttle, especially for a driver with not much body strength.

 

[rbj]

I have often thought that the throttle control is the wrong way round on motorbikes. Positive feedback is never a good thing in control loops.

it seems to me that the throttle control is the correct way for negative feedback.

if i am riding the motorcycle and popping a wheelie, and you are on my right observing my right flank, the sense of rotation of the bike is the same as the sense of rotation of my right-hand grip. but given Newton's third law, i would expect that the rotational jerk of the bike popping up would tend to cause my hand to twist in the opposite sense.

 

[rcgldr]

What the clutch does is add the torque related to angular decleration of an engine to the torque produced by the engine through combustion, increasing the overall torque for a brief period.

 

[berkeman]

it seems to me that the throttle control is the correct way for negative feedback.

if i am riding the motorcycle and popping a wheelie, and you are on my right observing my right flank, the sense of rotation of the bike is the same as the sense of rotation of my right-hand grip. but given Newton's third law, i would expect that the rotational jerk of the bike popping up would tend to cause my hand to twist in the opposite sense.

If you are supporting your upper body position with your abdominal muscles, then your statement is true. But if you get surprised by a quick acceleration, your upper body rotates back with respect to the morotcycle, which pulls back on the top of the throttle grip. That can result in a bad positive feedback loop.

So good body position and good throttle mechanics are important in performance riding. At least for riding a dirtbike or a sportbike, you tend to stay in a leaned-forward position and bend your elbows, so that a quick acceleration will not result in pulling the throttle open more (instead, your elbows just extend a bit until you can pull your upper body back into the forward position). You also operate the throttle grip by having your forearm more in-line with the axis of the grip, and twist your wrist like you are opening a doorknob. You don't have your forearm at a right angle to the grip and open the throttle by pulling/rotating back on the grip -- that's bad form. Think "twist the doorknob open" and lean forward and keep your elbows bent, and you will have a lot fewer problems with unexpected accelerations...

(Glad you weren't hurt, Danger!)

 

[Danger]

(Glad you weren't hurt, Danger!)

Thanks. Boy, but I wish that someone had told me that "posture" advice 40 years ago!
Luckily, I was barely moving when the accident happened. It was on my friend's farm, and I was just idling up a bumpy trail (with a passenger, which upset the load distribution). The front wheel hit a rock and that's all she wrote.

 

[berkeman]

Good way to lose a girlfriend. In more ways than one!

 

[Danger]

Good way to lose a girlfriend. In more ways than one!

Reminds me of one of my favourite poems (I've rewritten two words to modernize it from the original 1923 version):

Ruth rode on my Harley
On the seat in back of me.
I took a bump at 95
And rode on Ruthlessly.

 

[physwizard]

By the bye... is your shift key broken?

lol no. just avoid unnecessary punctuation.

 

[physwizard]

it seems to me that the throttle control is the correct way for negative feedback.

if i am riding the motorcycle and popping a wheelie, and you are on my right observing my right flank, the sense of rotation of the bike is the same as the sense of rotation of my right-hand grip. but given Newton's third law, i would expect that the rotational jerk of the bike popping up would tend to cause my hand to twist in the opposite sense.

remember torso is also rotated during a wheelie. so body weight can stretch your hands. I've mostly heard this problem from beginner bike riders and never faced it myself. but I've never really tried to wheelie too high though.
sudden braking is really more of a concern than sudden acceleration. normal bike riders are not expected to suddenly accelerate anyway.
another thing which has bothered me about bike design is the fuel tank hump right in front of your privates. in case of an emergency if you have to brake suddenly you could end up banging your privates on the fuel tank. scooters appear better off in this respect.

 

[sophiecentaur]

it seems to me that the throttle control is the correct way for negative feedback.

if i am riding the motorcycle and popping a wheelie, and you are on my right observing my right flank, the sense of rotation of the bike is the same as the sense of rotation of my right-hand grip. but given Newton's third law, i would expect that the rotational jerk of the bike popping up would tend to cause my hand to twist in the opposite sense.

That was an interesting response. On first reading it I though "The man's mad" but then I thought a bit. . . . . It's actually quite complicated,
There are two senses in which the feedback acts. Supporting your idea; if, when you rev the engine, you wrist dips down and the bike surging forward will tend to leave your body behind, raising the wrist and reducing the acceleration. This is certainly negative feedback and it would suggest that it's the 'proper' way to hold the throttle.

Supporting my idea, if you hold the throttle with your hand 'over the top' , with your wrist elevated, then acceleration of the bike will drag your wrist down and increase the acceleration. If the acceleration is hard enough to cause loss of grip then as your body moves back, your hand will move further back over the throttle and produce further acceleration. This is positive feedback and would be the 'wrong' way to hold the throttle.

Then there is the issue of what happens when the front wheel starts to lift. You are right about the initial negative feedback due to body rotation vs bike rotation but, once again, if you start to lose grip, the top of the throttle will still be pulled backwards, giving more positive feedback. There must be a temptation to loosen grip when a wheelie starts and this can only make things worse - the last bit of contact with the bike will be as your hand rolls back over the top of the throttle, putting it to Max!

"Proper posture" and throttle grip are clearly very important

 

[rcgldr]

I've witnessed a few incidents where new riders allow their upper bodies to get pulled back and in turn pull back on the throttle, causing it to go full throttle. On some bikes, you have to twist the throttle a lot in order to go full throttle which help, and one tip for a new rider would be to position their right wrist below the throttle before grabbing it to limit how far they can twist the throttle, and if they pull back and straighten their wrist, they shut off the throttle, unless they open their grip and their fingers pull the throttle open due via friction.

Where the direction of rotation of the throttle makes sense is when you're blipping the throttle while downshifting and braking at the same time. I can blip the throttle just using my palm and thumb while squeezing the brake lever with all 4 fingers, while others only use 2 or 3 fingers on the brake lever (as long as the fingers remaining on the throttle don't restrict how far back you can pull the brake lever). If I'm lane splitting (California), I'll keep all 4 fingers on the brake lever and use my palm and thumb for throttle control so that braking reaction time is reduced.

 

[Danger]

What on Earth is "lane splitting"?
One other thing that should be mentioned, primarily for people who have not yet had any experience at all with bikes, is that there's a huge difference between 4-stroke and 2-stroke engines when it comes to throttle response. Most street bikes are 4, and most dirt bikes are 2. A 4 acts more or less the same way that a car does. A 2 is like a Tasmanian devil in a paper bag; you open the throttle a bit and it moves—open it a bit more and there's no change—open it a bit more and there's no change—opBLAMMO!

 

[OCR]

What on Earth is "lane splitting"?

Lol... "lane splitting"? ... AKA, whitelining.




OCR

 

[rcgldr]

What on Earth is "lane splitting"?

Wiki article:

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

California may be changing the law to limit the speed differential (10 to 15 mph?) between motorcycles and cars while the motorcyclist is lane splitting, and to also not allow lane splitting if car traffic is moving above some specific speed (40 mph?).

 

[Danger]

That's nuts!
That is so totally illegal here that no one would ever think of doing it.

 

[rcgldr]

That's nuts! That is so totally illegal here that no one would ever think of doing it.

In heavy stop and go traffic, it's a tradeoff of a motorcyclists getting rear-ended by a car versus getting side-swiped while lane splitting. The accident rate related to lane splitting is a small percentage of the overall accident rate for motorcycles. If the motorcyclist is lane splitting "safely", only going 10 mph or so faster than traffic, he/she can either slow or swerve into another lane if a car ahead or to the side swerves. There was at least one time, perhaps two times, that I was about to get rear ended by a car behind me and I avoided the accident by lane splitting to get around the car in front of me. On one of those occcasions, I think the car behind me ended up rear ending the car that was in front of me (I heard a loud clunk as I went by the car that was in front of me).

Also, most of the lane splitting done on California freeways is between the "car pool" lane and the next lane over. The car pool lanes mostly have double yellow lines meaning cars can not change lanes into or out of the car pool lane except for the occasional entry / exit points which are marked with striped white lines. The distance between the car pool lane and the next lane over also tends to be wider than the rest of the lanes.

 

[ProgressNation]

Thank you guys for answering my question, I really Appreciate it! Good luck and take care!

 

[Danger]

You're welcome. Sorry that I sort of side-tracked your subject. When my mind works at all, it tends to wander...

 

[ProgressNation]

You're welcome. Sorry that I sort of side-tracked your subject. When my mind works at all, it tends to wander...

No problem dude ;) It's enough for me that you shared your answer. You're welcome at anytime!

 

[ProgressNation]

BTW, if your bike has enough power (or more specifically, a high power-to-weight ratio), you don't need to use the clutch to lift a wheelie. Just roll on the power and the front end comes up...

PS -- I don't do wheelies on purpose on public roads (illegal). I keep them on the racetrack and in the dirt.

Thank you man for answering my question and explained to me in a perfect way!
And oh! Thank you Physywizard for answering and special thanks to all the people that shared their answers! Good luck!

 

[Danger]

Stick around, kid; we have another show scheduled for the 3rd week of August.

 

[Dr. Headholio]

Hi,
I'd like to revive this thread to get some clarity on the physics of wheelies. My understanding agrees with what was posted in this thread here:

During heavy acceleration, at the point of contact, the rear tire pushes backwards against the pavement and the pavement pushes forwards against the tire. The forwards force from the pavement is below the center of mass of the bike, which experiences a backwards reaction force due to the acceleration. The forward force from the pavement and the backwards reaction force at the center of mass create a torque that results in a wheelie if the acceleration is strong enough.

I think the wheelie can be simply explained through these 2 horizontal forces (at the contact patch and at the center of mass). However, some explanations I have seen also consider the torque of the engine on the rear wheel, and an opposing torque that lifts the front end as has also been posted in this thread here:

There's also the issue of the torque generated on the rear tire. If the rear tire was locked in place, unable to turn or move with respect to the pavement, then with sufficient torque being appied to the rear tire from the engine (and clutch), a bike could wheelie. Under normal circumstances (the rear tire rotating and accelerating), you have torques related to the engine and rear tire accelerating, but these would be relatively small.

And here:

as per Newton's third law, just as the chain tries to rotate the back wheel, the wheel also exerts a reaction torque which will try to rotate the chain, and hence the whole bike, in the opposite direction. under normal riding conditions, this reaction torque does not exceed the torque produced by gravity, in other words the weight of the bike prevents it from wheelieing.

In my thinking this is not true. The torque on the engine creates chain tension which turns the rear wheel. This chain tension is a horizontal force between the rear wheel sprocket and the engine sprocket. It is an internal force and creates no torque about the COM.

Am I right or wrong?

On the other hand if the engine shaft was connected directly to the rear wheel axle (1 continuous shaft with the engine off to the side of the motorcycle) and the engine was connected to the frame, then engine torque would create an opposite torque on the frame.

rcgldr went on to express a similar thought to mine, here:

In the case of a chain drive, you have a tension in the upper part of the chain, and a compression along the swing arm. This would generate internal torques, but I'm not sure if this contributes to a wheelie. For a shaft drive, there are internal torques applied to the shaft drive and whatever is holding the shaft drive in place, but again, I'm not sure if this contributes to a wheelie.

So, what is the truth here? Is there a reaction torque counter to the engine torque that lifts the front of the bike, or is it just the moments created from the horizontal forces at the contact patch and at the COM?

 

[A.T.]

So, what is the truth here? Is there a reaction torque counter to the engine torque that lifts the front of the bike, or is it just the moments created from the horizontal forces at the contact patch and at the COM?

There is no contradiction here, just different ways to divide the system into bodies, that determines which forces/torques are external/internal. If you model the back-wheel and rest-bike as two separate bodies, then it's the torque from the chain force that rotates the rest-bike. If you model the entire bike as one body, then it's the torque from the ground reaction.