Talk:Rolling friction

Either this article is incorrect or there are two kinds of rolling frictions. How can the deformation of a wheel cause a car to slow down? Andries 18:27, 26 Apr 2004 (UTC)

This article is confusing, it seems to be refereeing more to rolling “resistance" than friction. As the tire rolls the shape is deformed and some of the energy that causes deformation is lost as heat. As result, the car must overcome this energy loss, or it will slow down.

Deformation

I made a few changes to clean up the article as far as it itself goes, but I'm still a bit confused as to what the significance of the tire deformation is to the entire equation. The external article linked at the bottom seems to imply the deformation isn't nearly as important to the concept as this article seems to state (though it should be noted anyway, as it is not insignificant). In fact, this article may be considerably rewritten with data available from the link (though not to the point of copying everything in it, of course).

Regardless, the changes I made simply clean up the article assuming the data is correct. The issue with the Ford Explorer rollover accidents, however, seems somewhat irrelevant to the article, as that could be easily disputed to shift the blame on Firestone's manufacturing. CaptainSpam 00:09, 11 Apr 2005 (UTC)

Deformation of tyres does slow you down

The rubber of the tyre behaves as a spring, so as it squashes under the load some energy is stored in it. As the centre of the wheel passes over the part that is right now on the road, that part begins to expand again, relasing its energy. But some of that energy is lost, as with any spring. (To test this, try hanging a load on a spring and let it bounce up and down - it doesn't continue forever. Not even if you do it in a vacuum.)

So some energy is lost.

Actually, no. The energy is just tranformed into heat. Well, most of it. A small part of the energy is used to slowly destroy the tire (this only becomes significant when the air pressure in a tire gets too low).

Try slipping your car into neutral on a level road. Does is slow down? Yes it does. You'll probably find that the deceleration is greater when the car is rolling at 100 km/h that at 20 km/h. That difference is accounted for in part by the mechanical rolling resistance, which is linked to the square of speed, and the wind resistance, which is linked to the cube of speed. But a surprising amount of that slowing is caused by the resistance of the tyres to being repeatedly squashed.

You can get another estimate of the work required to squash the tyres just by trying to push the car on a level road.

Finally, try checking the temperature of the tyres after driving a few km - even on a cold day, you'll find they're warm. They've been heated up by the internal friction of all those rubber molecules moving around as the tyre squashes and expands. The work required to do that is what slows you down.

Trains run on hard tyres over a hard road. They can roll for tens of kilometres without the loco doing any work. You might find that it's not much harder to push an 80 tonne freight car than a 1 tonne saloon car.

Include friction in bearings?

Should the friction in the bearing also be included? It is related to the wheel and the rotation of that...

Stuff (and nonsense)

This was added to the article by an anon contributor:

***Want to flag this article 1) it is a known fact that the size/width of a tire DOES NOT have any effect on the friction between such tire and the surface it is rolling upon

2) the forces at work to bring a mechanical wheeled device (bike or car) to rest are the following three types of friction: drag or air resistence, rolling friciton between the tires and raod surface, and friciton between all of the associated moving parts.

First, if you want to raise an issue with the article, the place to do it is here, on the talk page.

Second, your first point is simply wrong. Obviously tyre size affects rolling friction, as any cyclist will tell you. That's why racing bikes have extremely narrow tyres. A simple thought experiment along reductio ad absurdum lines will also show you this is false - what if you had a tyre of infinite width? Would it still have no more rolling friction than a tyre a few inches wide? Of course not, it would be immovable. Likewise an infintely thin tyre would have zero rolling friction. The tyre diameter also has an effect because larger diameter tyres have a larger contact area than a smaller tyre. That's why performance cars often have larger wheels - it increases the grip at the expense of an increase in rolling friction.

The second point is correct, but of limited relevance to this article, which is about rolling friction, not about what brings a vehicle to a standstill.

Hope that's clear. Graham 23:25, 22 December 2005 (UTC)[reply]

Maybe before you label something as nonsense you should examine it a little more carefully. I added that in hopes of a article rewrite, unaware I could comment on this page. Anyway, I am a former physics and current Applied Mathematics major, and frequent Wikipedia user and fan; we deal with problems like this all the time. The size of contact area IS NOT a factor in the equation for friction (see main article on friction or any first year physics textbook). Formula is as follows: Rolling Friction = (coefficient of rolling friction) * normal. This is the same as other formulas for friction: static and knetic. It is a common misconception that the size of the contact area affects friction. For instance, a pyramid-shaped object will be just as hard to push across a rough surface whether it makes contact along the square base or simply the point at the top. Further, it is a misleading fallacy in this article when it states that the ONLY force to slow down a bike is rolling friction, it should clearly state that three forces act on a bike or car to slow it down, drag, rolling fricion of wheels, and friction of moving parts. When you pedal at a constant speed you are negating the sum total of these forces combined. Drag is of particular importance as it is what determines the maximum velocity one can go on a bike. BTW, thinner tires are useful because they are more aerodynamic, and reduce drag. I would like someone else with more expertise in this area to clean up this article.

P.S. The idea that disc brakes are not responsible for slowing down a body is ridiculous... get a front-wheel drive car, jack up the front, floor the gas, the the brake, and tell me if they play no part in stopping the motion of the wheels. The brake are external because the are externally applied by the operator, and law of interia doesn't apply. just an afterthought

The article wording is a little confusing on this point, though I think I understand what it's trying to say. The vehicle slows because the braking force is transferred to the road. You are also right, a jacked up car's wheels will slow when braked. But imagine a car flying over a hump-backed bridge, so that it has lost contact with the ground. Applying the brakes in this situation will stop the wheels but it won't slow the car down! Contact with the ground is necessary for the brakes to work, so therefore rolling friction must be playing a key part in allowing the brakes to slow the vehicle. The article isn't claiming that the brakes don't slow the vehicle, just that contact with the ground is needed for them to have the desired effect. I think some better, clearer wording would help. Graham 04:41, 19 January 2006 (UTC)[reply]

My guess is that the 'coefficient of rolling friction' is directly related the width of the tyre. As it is to the pressure, the temperature of the tyre and the roughness of the road. Please do not turn this encyclopedia into an 'I'm bigger 'n better than you'-forum. -- regards, CFM

I think I'd tend to agree with that. I'm not a physicist, but it seems obvious that tyre width must be a factor, and like many physical equations, the coefficient probably includes this factor. It's like the way the coefficient of lift embodies both the shape and the angle of attack of a wing into a single number, which "hides" the AoA from the lift equation. However, nobody would claim that AoA has no effect on lift! Intuition isn't always reliable when talking about physics (far from it), but in this case I think it has something to say. Thin bike tyres do reduce aerodynamic drag too, but the main benefit of them is reduction in rolling friction. Ask a cyclist! The pyramid example is interesting, but it's only a first approximation of a real physical system. A real tyre on a real surface behaves very differently from an inelastic solid, so the basic equation is probably inadequate to fully describe it. A further thought - if surface contact area has no effect on riolling friction, why is it so much harder to push a vehicle with flat tyres than one with fully inflated tyres? According to the basic first approximation (like the pyramid), this shouldn't happen. If you get too detached from the real world and start to believe equations more than you do your own everyday experience, it's time to take a break! Graham 04:18, 19 January 2006 (UTC)[reply]

A further thought - the reason so many physics equation fall back on "coefficient of... x some other stuff" approaches is because the phenomena are so damned difficult to understand. It's much easier to simply measure the effect of a whole series of models, measure the results and then hide all the difficult stuff in a single multiplier so the curves come out to fit observation. It's true of aerofoil sections and I suspect it's true of rolling friction. So I guess what I'm saying is that whenever you see a formula containing a "coefficient of.. " anything, think what that number REALLY means. It means that there is stuff going on that's too complicated to separate out into more meaningful parts. Graham 04:30, 19 January 2006 (UTC)[reply]

"Rolling resistance" as article title

I've never seen rolling resistance called rolling friction before, although the simple equation Fro=f.m.g is of the same form as coulomb friction. I think that is the main source of confusion here. Can the page be renamed? I will do some edits. I'll probably quote from the Bosch Automotive Handbook (Is this fair use?) jimbowley 19jan2006

Rolling resistance redirects here. I've added the other name in the first sentence. — Laura Scudder ☎ 01:34, 19 January 2006 (UTC)[reply]

I concur with Jim- it should be the other way around. --Nil0lab 00:48, 3 April 2006 (UTC)[reply]

I agree too - though to swap the pages will require an admin's intervention. Graham 13:06, 3 April 2006 (UTC)[reply]