Broke one of the most preached rules

[California]

what in world is a "wheelie bar" ?

Source of this diagram

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[MossRoad]

Now I am surely showing my age, but what in world is a "wheelie bar" ? Right up until I came back to this thread, I would have had visions of a drinking establishment, probably with girls my mother warned me about hanging out.

Haven't you ever seen a drag race car with wheelie bars on the back that keep it from going over backwards? Most tractor pulls at county fairs require them, from what I've seen.

I had a pair on my motorcycle when I used to drag race. Made me feel invincible. Full throttle and dump the clutch with no fear of the front end rising off the ground.

 

[tallyho8]

I have to disagree that the front end of the tractor can lift off the ground when hooked to an immovable object below the centerline of the rear axle.

See my image. The blue line is the chain hooked to an immovable object. The red line is the attachment point below the axle.

In order for the nose of the tractor to lift, the red line (attachment point) has to pivot forward. It can not come forward because it is chained to an immovable object. Thus either, the wheels must lose traction, the engine must kill, or the chain or attachment point must break. In none of these 3 scenarios will the front end lift off the ground.

If I am wrong, please explain how the front end can lift for the safety of all the readers of this forum.


EDIT: Off topic, but I noticed that the image I posted does not enlarge when you click on it. You must save it to your computer. I have posted hundreds of images that enlarge when you click on them on this forum. Why does this one react differently?

 

[SilverHawk]

As I read these blogs about high and low attach points, I'm remembering things my uncles did/ talked about down on the farm. Thanks for the memories!

Renze, we have Ubuntu on a few of our computers too for the same reason! Jaunty Jackalope just came out!!!

 

[hallbax]

Well, I guess one way to look at this is to consider if a tractor could flip if the rear wheels were solidly frozen to the ground. I suspect that it could. What makes the above diagram problematic is the assumption that the pivot point for flipping is the center point of the rear axle. I suppose that could be the case - but the pivot point could be the point where the rear tires contact the ground. In that case, the pulling force is above the pivot point, and the force could, if great enough, pull the front of the tractor up.

In my illustration, scenario A uses the axle as the pivot, and the rotational force pushes the front down. Scenario B uses the bottom of the tire as the pivot point, and the rotational force pushes the front up. Granted, the mechanical advantage in scenario B makes the required force to flip much greater, but it looks like the physics says it could happen.

 

[California]

In order for the nose of the tractor to lift, the red line (attachment point) has to pivot forward. It can not come forward because it is chained to an immovable object.

The top of the red line can move back toward the immovable anchor if you apply enough torque. Imagine parking a tire on the tow cable, chaining the cable to the tire, and letting out the clutch.

I'll bet someone in the world of pulling tractors can describe this better. They wouldn't need wheelie bars if a low anchor point would do the job. And I don't think there is a difference between a rolling start, and applying the torque standing still.

 

[Soundguy]

? Right up until I came back to this thread, I would have had visions of a drinking establishment, probably with girls my mother warned me about hanging out.

Hey, don't be hate'in on them girlz.. I work in a bar venue many nights a week and there is nuttin wrong with them girlz..

soudnguy

 

[tallyho8]

Well, I guess one way to look at this is to consider if a tractor could flip if the rear wheels were solidly frozen to the ground. I suspect that it could. What makes the above diagram problematic is the assumption that the pivot point for flipping is the center point of the rear axle. I suppose that could be the case - but the pivot point could be the point where the rear tires contact the ground. In that case, the pulling force is above the pivot point, and the force could, if great enough, pull the front of the tractor up.

In my illustration, scenario A uses the axle as the pivot, and the rotational force pushes the front down. Scenario B uses the bottom of the tire as the pivot point, and the rotational force pushes the front up. Granted, the mechanical advantage in scenario B makes the required force to flip much greater, but it looks like the physics says it could happen.

Yes, a tractor can flip if the rear wheels are frozen to the ground, but not if it is hooked to an immovable object as in my diagram. The engine can kill, the wheels can break loose and spin, there can be a drivetrain breakdown, the immovable object can move or the indestructible point of attachment can break.

In Scenario B the front can not come up if it is connected to the immovable object as in my diagram.

The top of the red line can move back toward the immovable anchor if you apply enough torque. Imagine parking a tire on the tow cable, chaining the cable to the tire, and letting out the clutch.

I'll bet someone in the world of pulling tractors can describe this better. They wouldn't need wheelie bars if a low anchor point would do the job. And I don't think there is a difference between a rolling start, and applying the torque standing still.

The red line can't move back unless the tractor is moving backward or the point of attachment is bending. (In my scenario, we have an indestructible point of attachment)

I believe in tractor pulls they allow the front wheels to lift a certain amount to transfer all the weight to the rear wheels for maximum traction. The attachment point is higher so the wheels will lift and the wheelie bars limit the amount of lift. And also for visual appeal to the fans.

 

[CurlyDave]

The pivot point for overturning is the bottom of the rear wheels. Any attachment above this will allow an overturning torque.

The lower the attachment point the better, but the only really safe place is the factory drawbar.

 

[Travelover]

Many years ago when I was in engineering school we had to draw free body diagrams to solve these types of problems. Basically the sum of all the forces has to add up to zero. See second page of PDF for the tractor example.

http://www.bae.ncsu.edu/people/faculty/mrburche/BAE200/Lectures/Wk11_Ellington_Tractor.pdf

It is true that you can flip a tractor even if the attachment is below the center of the axle, but it is a lot harder. Think of the example of the dragster that needs a wheelie bar to keep the front end down. All that is holding it back is its own inertia - no chain to a stump. When the torque around the axle shaft exceeds the torque generated by the weight of the chassis*, the front end lifts.

* this torque = center of mass of chassis x distance from rear axle