# Jumping tractor

Just an effect interesting to think about:

Delta2, Lnewqban, vanhees71 and 2 others

Homework Helper
Gold Member
Very interesting!
Reducing the reproducing speed of the video, it seems to me that there is certain initial pitch movement (left end up-right end down) that eventually becomes an oscillation.

There may be an angle of the chain connecting the tractor (higher) and the load (lower) that induces higher normal force and traction of the tires closer to the load and the opposite on the tires that are located farther from the load.

That condition makes the latter loose and regain traction while the up and down movement is amplified by the spring effect of the air inside the big tires.
Alternating less pulling force on the chain induces alternating less pitching moment.
The pitching oscillation transfers some of that effect to the tires located closer to the load, which ends doing something similar.

There is some kind of resonance at the point in time when the tractor is jumping more violently.

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jack action
Staff Emeritus
I agree that it looks like a resonance. I see the tires slipping and I see the treads on the tires, could that be what excites a resonance in the suspension?

My second thought was very close to what @Lnewqban said.

We can all speculate, but among us might be someone smart enough to give an answer that sounds definitive.

Lnewqban
Mentor
I saw the same thing at a truck pull. It was mostly the lightly modified street trucks rather than the off-road only trucks specifically built for pulling. A one ton crew cab dually jumped high enough to see simultaneous daylight under all six tires. Dually: One rear axle, each side has two tires.

Road vehicles have two primary natural frequencies:
Body / suspension is about 1.0 to 1.5 Hz.
Tire / unsprung mass is about 10 Hz.

I think that jumping happens when the shock absorbers are not tuned properly for pulling, but have not looked into it, so cannot say for sure.

Lnewqban and Nik_2213
Nik_2213
I used a similar trick to shift the heavy steel benches in our lab. Either I fetched three stout guys to help carry the beast, or I pulled firmly, then kicked one of the legs. At which, resonance set in, 'Godzilla Walked' and bench could be led as if on castors...

Lnewqban and Keith_McClary
Keith_McClary
I used a similar trick to shift the heavy steel benches in our lab. Either I fetched three stout guys to help carry the beast, or I pulled firmly, then kicked one of the legs. At which, resonance set in, 'Godzilla Walked' and bench could be led as if on castors...
Like this? :

jrmichler, Nik_2213, wrobel and 2 others
Nik_2213
Yes ! Sadly, ours did not come with hot pizza...

Twodogs
Perhaps this is a window on a very deep principle of a universe that exhibits cyclical dynamics over spatiotemporal scales that vary by many orders of magnitude and that occur in a myriad of diverse physical mechanisms. The common cause is yet to be appreciated.

I actually do not think that the tractor has any relation to the pizza

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Nik_2213
Twodogs
Quite possibly you are correct; however from a broader, more general perspective it is possible to view cyclical, iterative dynamics as a means of integrating opposing forces of change and constraint within a dynamical system. Accepting that paradigm the dynamics of tractor and the pizza vehicle are related but the integration takes different forms. The tractor is fully constrained and can only begin to bounce in place; the pizza vehicle is less constrained so moves with a wobbly gait.

Nik_2213

It is also useful to think about the following dry friction paradox.
Consider a wheel on the ground. The mass of the wheel is ##m##. The coefficient of dry friction between the wheel and the ground is ##\mu>0##. The axis ##O## of the wheel is attached to the ground by unstretchable wire ##OA##. A torque ##\tau## is applied to the axis and makes the wheel rotate with slipping. A normal reaction force ##N## and the friction force ##F## satisfy the equation ##F=\mu N##. It is easy to see that
$$N=\frac{mg}{1-\mu\tan\alpha}.$$
Thus if ##\mu\tan\alpha>1## then ##N<0## . That is a nonsense. So that for some values of ##\mu,\alpha## the described above motion is impossible. In real life the wire and the tires are elastic and when the values of ##\mu,\alpha## are like that , the wheel begins to jump

OCR
Just an effect interesting to think about:

In some cases, it can be a very difficult effect to mediate; it's actually called power hop .

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jack action, anorlunda and wrobel
Like this? :

Can a miniaturized version be made with brush bristles or something for the legs, and then maybe it can be put through a series of adventures like those complicated marble tracks that are so popular on youtube?