Talk:Rolling resistance

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[edit] Additions

I have added phisical formulas and a table with some examples and in the text correlation with emergency brake. --138.89.56.7 18:14, 16 May 2006 (UTC) Dieter

I removed the paragraph about the "correlation" because it had no source and I don't think it's correct. Rolling resistance is fundamentally different from simple friction, as described in the article. Something that's firm but rough has a high sliding friction but low rolling resistance, and something that's squishy but smooth has a low sliding friction but high rolling resistance. —Keenan Pepper 21:27, 16 May 2006 (UTC)
I don't that thats right either, the smoother a surface, the lower the rolling resistance. Think about the difference between riding a bike on smooth concrete vs rough asphalt - or even riding a bike on a bed mattress. The bed mattress would have the highest rolling resistance because its "squishy". Fresheneesz 20:17, 27 May 2006 (UTC)
...Right, the mattress has a high rolling resistance because it's squishy, not because its surface is rough. It doesn't matter how smooth it is for rolling resistance. Even if you put the smoothest silk sheets on that mattress, it would still be hard to ride your bike on it, because most of your energy goes into compressing the mattress and heating it up. —Keenan Pepper 21:07, 27 May 2006 (UTC)

[edit] g vs normal force

I'm going to change "g" in that equation to FN for normal force. I think that would be more general. Fresheneesz 20:17, 27 May 2006 (UTC)

[edit] Temperature

That external link at the bottom is very interesting, is Crr's relationship with temperature not well documented? This would be something that should be on this page. Fresheneesz 23:05, 27 May 2006 (UTC)

It is well documented. see for example SAE paper 800090. Reading off the graph .025 at -20 deg C, .012 at 30 deg c, .009 at 80 deg C Greglocock 22:44, 23 September 2006 (UTC)

[edit] Definition of 'slip'

The term 'slip', referred to road vehicles, is equivalent to 'creep' of railway wheels (see adhesion railway), it appears that automotive engineers don't talk to railway engineers.

It is a misnomer because the region of contact is not subjected to actual sliding, but elastic distortion, the figure of 11% is indicative of the condition where the tyre forces arising from the shear stress in the region of contact are sufficient for gross slippage to occur. Wheel forces arise from the elastic distortion of both the wheel and supporting surface at the region of contact.

This cyclic distortion results in energy loss, particularly in materials which have high internal friction such as rubber, so rubber tyres on asphalt have higher rolling loss than iron tyres on steel rails, because the internal losses are higher, and a much larger volume of material is affected by the distortion.

There is some partial slippage at the edges of the region of contact, which adds further to the losses, but the region is effectively stationary, so that adhesion is governed by static friction, which is usually greater than sliding friction.

I think what is meant in the article is the fundamental kinematic property of the wheel to transfer the motion of the vehicle relative to the surface from the region of contact to the hub, where greater control of the friction loss is achievable, hence roller bearings and lubrication.

Despite its superficial simplicity, the humble wheel is quite a sophisticated machine when studied to any depth. Gordon Vigurs 10:46, 4 June 2006 (UTC)

[edit] Table of Rolling Friction Coefficients

This table does not cite an original source. If it is known where the values originated, please cite.

[edit] Rolling resistance independent of speed?

From the formula given it seems that the rolling resistance is independent of the speed of the rolling object. Is that correct?--129.70.14.127 12:07, 11 May 2007 (UTC)Quixy

Yes, pretty much. At very high speeds things get more complicated, but to within the normal sort of testing accuracy it doesn't change significantly with vehicle speed. That is, in a fuel economy model they don't normally have a speed sensitive contribution to rolling resistance. Anything small would be masked by the aerodyanmic drag from the wheel, which is quite significant above 80 mph. Greglocock 07:25, 12 May 2007 (UTC)

[edit] Rename to "Rolling friction"

Shouldnt this article be renamed to "Rolling friction". That s what its called in physics. Also the rename is needed to be consistent with Friction article. Firestonetireguy 17:22, 9 September 2007 (UTC)

'Opoose'Since it is mainly concerned with vehicles, rolling resistance is the usual term. Why not add a redirect if you want to cover both in the same article? Alternatively start a physics based page rather than an engienering one. Greglocock 23:09, 9 September 2007 (UTC)

[edit] Totally incorrect statement regarding static vs sliding friction

The person who said kinetic/sliding friction is greater than rolling/static friction is totally wrong. The coefficient of sliding friction is much less than static/rolling friction. That is why good car brakes doesn't skid to a stop when you hit the brakes. Furthermore, sliding is much more efficient on a lubricated surface, hence rocket sleds use sliding sleds, and not wheels. Edited and fixed this portion by adding this:

"Rolling friction/static friction is much greater than sliding friction. The coefficient of rolling/static friction is greater than that of sliding/kinetic friction. Static friction prevents motion. This is why car wheels roll to a stop instead of skidding in order to stop as soon as possible. Also, rocket sleds using sliding sleds instead of wheels since sliding friction is much less. ...However, rolling/static friction does not create heat, since it prevents motion.

"Rolling friction depends on the coefficient of rolling friction between the two materials (µr) and the normal force (N) of the object. The force of kinetic friction depends on the coefficient of kinetic friction between the object and the surface on which it is moving (µk) and the normal force (N) of the object. For any pair of objects, the coefficient of kinetic friction is usually less than the coefficient of static friction." -MSN Encarta"

Edit it if you want to make it more concise.

Intranetusa 05:13, 9 October 2007 (UTC)

(Note: moved from top of page to bottom of page, per convention.) Intranetusa, I'm fairly sure you think "static friction" is another term for "rolling friction." They mean very different things. The confusion seems to permeate all of your changes. If you want a more detailed explanation, I can provide it, but I would suggest checking your understanding of the terms with other sources. For anyone else trying to follow this, note that Intranetusa's quotation from MSN Encarta took sentences from different paragraphs and reordered them.
Your fundamental dispute seems to be over the sentence that says rolling friction "is much lower than sliding friction...." It may not be the best wording, but the essence of its meaning seems indisputable; as the MSN Encarta source you cited put it, "Wheels and other round objects will roll along the ground much more easily than they will slide along it." Ice skating being an obvious general counter-example.
I'm going to revert your recent edits, for reasons above. I will also add a citation for the statement that rolling friction "is much smaller than sliding friction...", since that seems to be a point you're challenging. A quick google search finds hundreds of reliable sources that could be cited. Examples:
-Agyle 11:24, 9 October 2007 (UTC)

[edit] "in braking"

The section "in braking" seems to me a tangent at best, more appropriate for an article on the physics of braking. I think it should be at least moved later in the article if not moved to the brake article (which is rather thin at the moment). Any thoughts?Ccrrccrr (talk) 03:50, 14 December 2007 (UTC)

[edit] Rolling resistance value for steel.

What sources do you have for the steel wheel/steel rail values?

I have found specific ranges in between 0.0002 and 0.0005 for c. 30" diameter wheels. That's 2e-4 to 5e-4, mainly from the following two sources:

-Gordon, David W. Bicycling Science. Cambridge, Mass. : MIT Press, c. 2004. (They computed a range of 0.0002-0.0005)

-Williams, John A. Engineering Tribology. New York : Cambridge University Press, 2005. (This one has a value of 0.0004, but doesn't specify a size but it strongly implies a regular-size railroad wheel)

It is my belief that the latter value in the first source, 0.0005, is often rounded to 0.001. Just my two cents, hope it's correct.

[edit] Small wheel size?

The LGV-Est world speed record ini 2007 (the one which achieved 357 mph) was done specifically under special conditions, and one of those conditions was that they used, in fact, larger-diameter wheelsets for the train than usual. —Preceding unsigned comment added by Facial (talk • contribs) 06:57, 2 March 2008 (UTC)

[edit] Rolling friction depends on the wheel radius!!!

The correct formula for the rolling friction is:

F = C_{rr} N_f / R \

where R is the radius of the wheel. The wheel and the coefficient of friction are measured in meters. (Sometimes both the wheel and the coefficient are measured in millimeters.) The fact that the rolling friction depends on the radius of the wheel is just common sense. If you make a simple sketch of the forces it will become obvious to you.83.6.39.246 (talk) 04:39, 7 March 2008 (UTC)

Unfortunately, the sources cited in the article disagree with you. The "Special Report 286" from National Academy of Sciences, Transportation Research Board, on page 17 states:
"Rolling resistance coefficient (RRC). The value of the rolling resistance force divided by the wheel load. The Society of Automotive Engineers (SAE) has developed test practices to measure the RRC of tires. These tests (SAE J1269 and SAE J2452) are usually performed on new tires. When measured by using these standard test practices, most new passenger tires have reported RRCs ranging from 0.007 to 0.014."
There is no mention of radius, and RRCs are given without units.
Wilson in Bicycling Sicence, Third Edition, on page 217 states:
"The ratio FR/FV is defined as the coefficient of rolling resistance, CR." Where FV is the "downward force on wheel" and FR is the "propulsive force".
Again, no mention of radius, and all example coefficent values are given without units.
Without one or more comparable sources backing your claim, it appears to be untrue or original research. -AndrewDressel (talk) 15:19, 7 March 2008 (UTC)

These sources do not mention any formulas.

These sources do, in fact, mention formulas: specifically the one currently presented in this article and the one that corresponds to all the unit-less sample coefficients currently listed in this article. I even provide the page numbers. -AndrewDressel (talk) 20:56, 7 March 2008 (UTC)

Why don't you look it up in any engineering textbook?

As the introducer of new material, might you consider that the onus lies on you to provide a source for that material? -AndrewDressel (talk) 20:56, 7 March 2008 (UTC)

The Wikipedia articles in other languages: German: http://de.wikipedia.org/wiki/Rollwiderstand Polish: http://pl.wikipedia.org/wiki/Tarcie_toczne have the correct formula. 83.24.44.201 (talk) 16:41, 7 March 2008 (UTC)

Unfortunately, I am not skilled enough in German or Polish to read these articles. Never-the-less, I submit to you that it is incorrect to describe any formula as 'correct'. There appear to be several models that, to varying degrees and under varying circumstance, inexactly match the physical phenomenon. In fact, Hibbeler, on pages 441-442 of his Engineering Mechanics: Statics & Dynamics, Eleventh Edition, describes "a simplified method ... to explain one way engineers have analyzed this phonomenon." After deriving the formula and coefficient you are espousing, he goes on to explain "Experimentally, though, this factor is difficult to measure, since it depends on such parameters a the rate of rotation, the elastic properties of the contacting surfaces, and the surface finish. For this reason, little reliance is placed on the data for determining a."
Thus, it appears that there are several models of rolling resistance, each with adherents, benefits, detractors, and drawbacks. I suggest we update the article to reflect that fact. Merely replacing the current formula for another and adding units to the existing coefficients, all without citing new sources, doesn't help anybody. -AndrewDressel (talk) 20:56, 7 March 2008 (UTC)