Devon TRF Group Forum

Yep I recall force is a product of mass and acceleration so,subject to sufficient grip, the back wheel is being forced past the front and whilst your life preserving response will be to stop that rear wheel forcing itself past the front by pushing the front down a wheelie will occur. When the product of the rear wheels mass and acceleration overcomes the mass you can exert in keeping the front wheeNd own and the rear retains grip an embarrassing wipeout will occur.

-- Edited by Phaedrus on Tuesday 20th of March 2012 09:22:26 PM

In my mind its the engine force pulling against the chain vs the friction between the rear tyre and the road.

So..out of your 2 options i'd go for No. 1 :)

The torque of the engine is being driven by the chain to the back wheel.  The bike is operating as a lever with the fulcrum being the back axle.

The torque required to do a wheelie will be the lever arm length from the rear wheel axil to the center of mass. The center of mass can be found a couple of ways. I usually use this: x_{cm}=\frac{m_1x_1+m_2x_2+...+m_nx_n}{\sum m}

which can be rewritten as:

x_{cm}=\frac{\sum^n_{i=1} m_ix_i}{M}
y_{cm}=\frac{\sum^n_{i=1} m_iy_i}{M}
z_{cm}=\frac{\sum^n_{i=1} m_iz_i}{ M}

Set the origin at the rear wheel axil, and assume the motorcycle is 2D thus elliminating the z.

The x and y component can now be used to find the moment about the rear axil \tau=(Mg)r where Mg is the weight of the bike at the center of mass (total wieght) and g is the gravitational acceleration and r is the radius from the moment axis to the point of force.

where r= \sqrt{x_{cm}^2+y_{cm}^2}

You now have a radius from the rear axil to the center of mass and the center of mass is found by knowing the weight distribution of the bike (distance between the wheels, engine placement, rider placement/orientation, rider weight, weight of wheels).

In your problem, the y component would be the height (.88m) and the x component would be the distance along the x (1/2 of 1.55m). All you need is the force (weight of the bike and rider). Think about what is going on. The engine via a set of sprockets and a chain/belt must produce enough torque to cause the center of mass to rotate about the rear axil. Its just like using a torque wrench to tighten a bolt. You have a lever arm and a force.

This doesn't set the torque at the rear axil but torque is a free vector (able to move) so it will work.

http://www.physicsforums.com/archive/index.php/t-17759.html

delvey91 wrote:

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Wack the throttle back and hope for the best :S

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 I find initially accelerating gently in second or first, quickly shutting the throttle and wacking it open, catching the forks on the rebound a reasonable method. What takes the bottle/skill is getting to the balance point and keeping it there whilst covering the back brake . Not there yet .

Uphill slopes make it much easier.

Craig

RichT4 wrote:

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what do you guys think?

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 I think what with this and your chain thread you need to actually ride your bike or buy a copy of the Karma Sutra - you have far to much time on your hands

RichT4 wrote:

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rest of the advise - don't need the manual, I wrote it!

-- Edited by RichT4 on Tuesday 20th of March 2012 09:03:33 PM

Co-wrote would be more convincing

RichT4 wrote:

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What's actually happening

Is the back wheel being accelerated under the front due to weight shift..

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So this is the consensus then?

The main effect is the shift...

That's a big word for you brownie - you should try it sometime.

Dirk Diggler wrote:

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It's the rider that pulls the 'wheely' on the bike without one it won't so forget all the physics just go out and do it.........practice will help perfect.... Simple as that!

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 Without a bike a rider can't pull a wheelie all he can do is wee wee.

Bad lad. I'm going to try that on my new pink bike !

RichT4 wrote:

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That's a big word for you brownie - you should try it sometime.

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 4 kids proves i have, wheres your proof???

mind you i have moved 4 times and got the same milkman everytime.

V Twin Funster wrote:

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delvey91 wrote:

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Wack the throttle back and hope for the best :S

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 I find initially accelerating gently in second or first, quickly shutting the throttle and wacking it open, catching the forks on the rebound a reasonable method. What takes the bottle/skill is getting to the balance point and keeping it there whilst covering the back brake . Not there yet .

Uphill slopes make it much easier.

Craig

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 The key on any bike is being in the right gear at the right speed, which varies between bikes.

My Firestorm: 30mph in first, open throttle smoothly and the front comes up nicely until you hit the limiter at 70mph. Whack it open and the bike will flip, so do this carefully and progressively.

XR4: Best in 2nd gear - 1st runs out of revs too quickly, and 3rd (on mine) required a bit more commitment, although possible especially on a slight rise.

CRF250x: 1st lifts much too quickly to easily control it. 2nd is ideal.

If you want to make it easy, you need to get the revs up a little, just before the main torque band starts. Stay seated, and keep your body position still in relation to the bike. Make sure you've got clear, straight, flat area in front, including time to slow down afterwards, then wind the throttle open steadily and see what happens! If nothing, then try it again and open the throttle a bit faster, or change the amount of revs when you start the attempt. The instinctive reaction is to push forward as soon as the front starts to lift, but you've got to resist this as that is exactly what you are aiming for!

Ideally, you will reach the balance point, then roll off the throttle a little to keep you at this point. This is the real key to maintaining a decent wheelie and it takes a good feel and some guts (I'm not there yet!)

Some people cover the back brake so that if it all goes **** up, a tap on the brake will bring the front wheel down again. If you have a reasonable amount of engine braking, then throttle control will maintain the height of the wheelie.

Start small, or you'll find yourself picking yourself and your bike off the ground and putting the bits in your pocket.

Oh, and don't do this on a public road, as it's frowned upon!

Note that repeated 'poor' wheelies, ending with slamming the front wheel down hard, will eventually notch your head races, so best not to do this if you can help it.

 

This guy seems to have it perfected

Big T wrote:

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Ian, were you working all this out when you were going up the hill at Whiteway last year?

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Yeah, it came to me just before I hit the ground - HARD!

Lever, back axle, fulcrum.  Job done, next question?

For me the main sensation around balance point is of forcing the rear wheel under the bike.

Ive not managed to hold balence point since I was six on my Raliegh Chipper ,when my dad started shouting at me. The one and only time. Never thought I'd have issues but I'm scarred for life

No gears was probaly a good thing!
As some of the chopper boys had to have their bags stiched up

my little bro had one of these..