I I would like a better understanding of friction and hysteresis

[fog37]

TL;DR Summary

understand rolling friction and hysteresis

Hello,

In reviewing friction, I realized a couple of things: the coefficient of static friction can be larger than 1 (always thought it would be smaller than 1), that the surface area does not matter for static and kinetic friction ONLY for simple solid, rigid objects ( for materials like rubber, which are viscoelastic, the surface area matters a lot: the more the contact area the larger the friction).

Rolling friction really exploits static friction since the point (area) of contact is momentarily at rest.

In regards to rolling friction for a rubber tire (always smaller than kinetic friction), the rubber tire deforms as it rolls (the surface must deform too a little bit). As the front portion of the tire deforms while the wheel rotates, elastic potential energy $PE$ gets stored inside the tire in that specific compressed tire area under the load of the vehicle. As that same tire area relaxes, after moving past the contact patch, potential energy is then released. However, the released potential energy is less than the stored potential energy because of some of the original $PE$ energy is lost/converted to thermal energy inside the tire.

That said, assuming it is correct, what does hysteresis mean and what does it have to do with this process?
My interpretation: looking at the graph Deformation vs Loading force (Strain vs F), as the tire area is loaded, the area under the curve represents the stored potential energy. Assume the loading force is RED. As the tire relaxes the loading force (GREEN) decreases but the force is lower...Why? What is the area under the decreasing loading force (GREEN)? I think the area between the compressing force (RED) and relaxing (GREEN) force is the energy that gets converted into thermal energy, correct?

Hysteresis simply mean that we go from a specific state $A$ to a state $B via a certain path. But when we return to state$A$from state$B##, the path is different...Is that correct?

Thank you!

 

[Lnewqban]

... In reviewing friction, I realized a couple of things: the coefficient of static friction can be larger than 1 (always thought it would be smaller than 1), that the surface area does not matter for static and kinetic friction ONLY for simple solid, rigid objects ( for materials like rubber, which are viscoelastic, the surface area matters a lot: the more the contact area the larger the friction)...

When cornering, racing motorcycles equiped with special tires (very soft and resilient rubber) can lean around 60° respect to vertical, which implies a practical coefficient of friction way over 1.0.

The area of the contact patches helps friction some, but not a lot.
I would call that bonus friction (besides the pure static type) “macroscopic grip”, as it is more instantaneous mechanical inter-lock of a semi-fluid rubber mimicking the macro irregularities and imperfections of the road surface than interaction at molecular level of the pure static friction.

That “macroscopic grip” varies much under the influence of the suspension movement and weight transfer during strong acceleration and braking.

The hysteresis is what forces a racer to be smooth on the controls, as the contact patches have certain reaction inertia, needing certain time to “adapt” to new loads and demands.
Grab the front brake handle suddenly and you go down inmediately after; apply gradual increasing pressure and the machine can do a “stoppie”.

That overall coefficient of friction is also dynamic; hence, irregularities or undulations of the surface, as well as camber of the road, combined with the speed of the bike and the response of the suspension mechanism is constantly changing both, the normal force and the area of contact.

The combination of flexible rubber, deformable carcass, rotating tire and lateral forces create a crabbing effect; therefore, the tire cannot follow the path that a rigid wheel would follow.
That is an additional twisting effect on the contact patch that makes it less complying with the roughness of the surface: less available traction when that is happening.

 

[Baluncore]

"Stick and slip" is a chattering motion, referred to as hysteresis. It occurs when the area of the contact surface rapidly switches between static and dynamic friction.

Chatter magnitude tends to increases as the area of the contact is increased.

As an extreme example, you can think of gear teeth as being regular asperities on friction rollers. With a contact angle of about 20°, the friction coefficient will be significantly greater than one. "Skipping a tooth" is an example of "stick and slip".

 

[jack action]

In reviewing friction, I realized a couple of things: the coefficient of static friction can be larger than 1 (always thought it would be smaller than 1),

I saw the following video and thought of this comment. Watching the video between 1:30 and 2:00, I think you have proof that the coefficient of friction can be larger than 1.

(source)​

 

[anorlunda]

I saw the following video and thought of this comment. Watching the video between 1:30 and 2:00, I think you have proof that the coefficient of friction can be larger than 1.

That is interesting. Where does adhesion end and friction start?

 

[jbriggs444]

That is interesting. Where does adhesion end and friction start?

When the relationship between frictional force and normal force can no longer be reasonably approximated as linear?

When normal force can go negative?