Rolling resistance

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Scrolling resistance , sometimes called swipe friction or friction resistance , is a force that rejects movement when an object (such as a ball, a tire, or a wheel) On surface. This is mainly due to non-elastic effects; that is, not all the energy required for deformation (or movement) of wheels, trails, etc. can be recovered when pressure is removed. These two forms are hysteresis losses (see below), and the permanent deformation (plastics) of the object or surface (eg the soil). Another cause of rolling resistance lies in the slippage between the wheel and the surface, which wastes energy. Note that only the last of these effects involves friction, therefore the name "rolling friction" is a misnomer.

In analogy with friction shearing, rolling resistance is often expressed as the coefficient of normal force times. The coefficient of rolling resistance is generally much smaller than the shear friction coefficient.

Each wheeled vehicle gradually slows down due to rolling resistance including those on the cushion, but a train car with steel wheels running on a steel rail will roll farther from the bus with the same mass as the rubber tires running on the tarmac. Factors contributing to rolling resistance are the number (deformation) of the wheel, the deformation of the road surface, and the movement below the surface. Additional contributing factors include wheel diameter, speed, load on wheels, surface adhesion, shear, and relative micro shear between contact surfaces. Losses due to hysteresis are also highly dependent on the material properties of the wheels or tires and surfaces. For example, rubber tires will have higher rolling resistance on paved roads than steel railway wheels on steel rails. Also, the sand on the ground will provide more rolling resistance than concrete.

Video Rolling resistance

Primary cause

The main cause of pneumatic tire rolling resistance is hysteresis:

Characteristics of the material can be deformed so that the deformation energy is greater than the recovery energy. Rubber compounds in tires show hysteresis. When the tire rotates under the weight of the vehicle, it undergoes a cycle of deformation and repetitive recovery, and removes the loss of hysteretic energy as heat. Hysteresis is a major cause of energy loss associated with rolling resistance and is associated with viscoelastic characteristics of rubber.

- National Academy of Sciences

This main principle is illustrated in a rolling cylinder figure. If the same two cylinders are pressed together then the contact surface is flat. In the absence of surface friction, normal contact pressure (ie perpendicular) to the contact surface. Consider a particle entering the contact area on the right side, moving through the contact patch and leaving on the left side. Initially vertical deformation increases, which is opposed by hysteresis effects. Therefore, additional pressure is generated to avoid interpenetration of the two surfaces. Then the vertical deformation decreases. This is once again opposed by the hysteresis effect. In this case, it reduces the pressure required to keep the two bodies separate.

The resulting pressure distribution is not symmetrical and shifts to the right. The vertical force action line (aggregate) no longer passes through the center of the cylinder. This means that sometime happens that tends to inhibit the rolling motion.

Materials that have large hysteresis effects, such as rubber, bounce back slowly, show more rolling resistance than materials with small hysteresis effects that bounce back faster and more fully, such as steel or silica. Low rolling resistance tires typically combine silica as a substitute for carbon black in the tread compound to reduce low-frequency hysteresis without sacrificing traction. Note that railroads also have hysteresis in the road structure.

Maps Rolling resistance

Definition

In a broad sense, the specific "rolling resistance" (for vehicles) is the force per vehicle weight unit required to drive the vehicle on flat ground at a constant slow speed where aerodynamic resistance (air resistance) is insignificant and also where there is no traction ) force or brake applied. In other words, the vehicle will slide if not for the force to maintain a constant speed. An example of use for railroads is [3]. This broad meaning includes wheel bearing resistance, energy dissipated by vibration and oscillation of both road and vehicle, and slide wheel on the road surface (sidewalk or rail).

But there is a wider sense that will include the energy wasted due to wheel slippage due to the applied torque of the engine. This includes the increased power required due to an increase in wheel speed at which the drive tangential speed (s) becomes larger than the speed of the vehicle due to slippage. Since power is equal to the speed of time and wheel speed has increased, the required power has increased.

The pure "rolling resistance" for trains is the case due to deformation and possible small shear in the wheel-drive contacts. For rubber tires, analog energy losses occur across tires, but are still called "rolling resistance". In a broad sense, "rolling resistance" includes wheel bearing resistance, energy loss by shaking both roads (and the earth below) and the vehicle itself, and by shifting wheels, rails/contacts. Train text books seem to encompass all these resistance powers but do not mention their number of "rolling resistance" as is done in this article. They simply summarize all resistance forces (including aerodynamic pull) and call the amount of basic train resistance (or the like).

Because the rolling resistance of trains in the broadest sense may be several times greater than just the value of the pure rolling barriers that are reportedly likely to be in serious conflict as they may be based on different definitions of "rolling resistance". The train engine must, of course, provide energy to overcome this vast rolling barrier.

For tires, rolling resistance is defined as the energy consumed by tires per unit of distance covered. This is also called rolling friction or rolling drag. This is one of the forces that act to oppose the motion of a driver. The main reason for this is that when the tire moves and touches the surface, the surface changes shape and causes tire deformation.

For motor vehicles, there is obviously some energy lost in shaking the highway (and the earth beneath it), trembling the vehicle itself, and slipping the tires. However, in addition to the additional power required due to torque and friction of the wheel bearings, the non-pure glide resistance seems unexplored, possibly because the pure rolling resistance of the rubber tires is several times higher than the neglected obstacles..

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The scrolling resistance coefficient

"Koefisien hambatan bergulir" ditentukan oleh persamaan berikut:

${\ displaystyle \ F = C_ {rr} N}  ÂÂ$

di mana

${\ displaystyle F}  ÂÂ$ adalah gaya tahanan bergulir (diperlihatkan pada gambar 1),

${\ displaystyle C_ {rr}}  ÂÂ$ adalah koefisien hambatan bergulir tanpa dimensi atau koefisien gesekan gesekan ( CRF ), dan

${\ displaystyle N}  ÂÂ$ adalah gaya normal, gaya yang tegak lurus dengan permukaan tempat roda berputar.

Di atas menunjukkan resistensi sebanding dengan ${\ displaystyle C_ {rr}}  ÂÂ$ tetapi tidak secara eksplisit menunjukkan variasi apa pun dengan kecepatan, beban, torsi, kekasaran permukaan, diameter, inflasi/keausan ban, dll. karena < math xmlns = "http://www.w3.org/1998/Math/MathML" alttext = "{\ displaystyle C_ {rr}}">                                    C                         r              r                                      {\ displaystyle C_ {rr}}    itu sendiri bervariasi dengan faktor-faktor tersebut. Mungkin tampak dari definisi di atas ${\ displaystyle C_ {rr}}  ÂÂ$ bahwa tahanan gelinding berbanding lurus dengan bobot kendaraan tetapi tidak.

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Pengukuran

There are at least two popular models for counting rolling resistance. Rolling resistance coefficient (PRC) Rolling resistance coefficient (RRC) value Rolling resistance force divided by wheel load Society of Automotive Engineers (SAE) has developed practice test to measure tire PRC These tests (SAE) J1269 and SAE J2452 ) is usually done on new tires, and when measured using this standard test practice, most new passenger tires have reported the PRC ranging from 0.007 to 0.014. "In the case of a bicycle tire, a value of 0.0025 to 0.005 is achieved. This coefficient is measured on a roller, with a power meter on the road surface, or by a beach trial. In the latter two cases, the effects of air resistance should be reduced or the tests performed at very low speeds.

The rolling resistance coefficient b , which has a long dimension, roughly (due to the small-angle approach ${\ displaystyle cos (\ theta) = 1}$ ) is equal to the value of the rolling resistance force times the spokes of the wheel divided by the wheel load.

ISO 18164: 2005 is used to test rolling resistance in Europe.

The results of these tests can be difficult for the general public to get as manufacturers prefer to publish "convenience" and "performance".

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Physical formulas

Koefisien rolling resistance untuk roda kaku yang lambat pada permukaan elastis sempurna, tidak disesuaikan dengan kecepatan, dapat dihitung oleh

${\ displaystyle \ C_ {rr} = {\ sqrt {z/d}}}  ÂÂ$

di mana

${\ displaystyle z}  ÂÂ$ adalah kedalaman tenggelam

${\ displaystyle d}  ÂÂ$ adalah diameter roda yang kaku

Rumus empiris untuk ${\ displaystyle \ C_ {rr}}  ÂÂ$ untuk roda mobil tambang besi cor pada rel baja.

${\ displaystyle \ C_ {rr} = 0,0048 (18/D) ^ {\ frac {1} {2}} (100/W) ^ {\ frac {1} {4}}}  ÂÂ$

di mana

${\ displaystyle D}  ÂÂ$ adalah diameter roda di dalam.

${\ displaystyle W}  ÂÂ$ adalah beban pada roda dalam lbs.

Sebagai alternatif untuk menggunakan ${\ displaystyle \ C_ {rr}}  ÂÂ$ seseorang dapat menggunakan ${\ displaystyle \ b}  ÂÂ$ , yang berbeda koefisien hambatan bergulir atau koefisien gesekan gesekan dengan dimensi panjang. Ini ditentukan oleh rumus berikut:

${\ displaystyle \ F = {\ frac {Nb} {r}}}  ÂÂ$

di mana

${\ displaystyle F}  ÂÂ$ adalah gaya tahanan bergulir (diperlihatkan pada gambar 1),

${\ displaystyle r}  ÂÂ$ adalah jari-jari roda,

${\ displaystyle b}  ÂÂ$ adalah koefisien hambatan bergulir atau koefisien gesekan geseran dengan dimensi panjang, dan

${\ displaystyle N}  ÂÂ$ adalah gaya normal (sama dengan W , bukan R , seperti yang ditunjukkan pada gambar 1).

The above equation, where resistance is inversely proportional to radius r. seems to be based on a discredited "Coulomb law" (not Coulomb's inverse square law or Coulomb's friction law). See dependence on diameter. Equates this equation with the force per coefficient of rolling resistance, and the settlement for b, gives b = C _rr Ã, Â · r. Therefore, if a source gives a coefficient of rolling resistance (C _rr ) as a dimensionless coefficient, it can be converted to b, has a unit of length, by multiplying C _rr by wheel radius r.

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Scrolling examples of barrier coefficients

Table example of scrolling coefficient of resistance: [4]

For example, in Earth's gravity, a car weighing 1000 kg on the asphalt would require a force of about 100 newtons for rolling (1000 kg ÃÆ'â € "9.81 m/s ² ÃÆ'â €" 0.01 = 98.1 N).

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Dependency on diameter

Stagecoaches and railroad

According to Dupuit (1837), rolling resistance (wheeled car with wooden wheel with iron tire) is approximately inversely proportional to the square root of the wheel diameter. This rule has been verified experimentally for cast iron wheels (8 "- 24" in diameter) on steel rails and for 19th century train wheels. But there are other tests on the wheels of the hopper that do not agree. The cylinder theory revolving in the elastic path also gives the same rule. This contradicts earlier (1785) test by Coulomb's rolling wood cylinder in which Coulomb reported that rolling resistance is inversely proportional to wheel diameter (known as "Coulomb law"). This is disputed (or incorrectly applied) - "Coulomb's Law" is still found in the handbook, however.

Pneumatic tires

For pneumatic tires on hard sidewalks, it was reported that the diameter effect on rolling resistance was negligible (within the range of practical diameter).

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Dependence on applied torque

Torsi penggerak ${\ displaystyle T}  ÂÂ$ untuk mengatasi hambatan bergulir ${\ displaystyle R_ {r}}  ÂÂ$ dan mempertahankan kecepatan stabil pada tanah datar (tanpa hambatan udara) dapat dihitung dengan:

${\ displaystyle T = {\ frac {V_ {s}} {\ Omega}} R_ {r}}  ÂÂ$

di mana

${\ displaystyle V_ {s}}  ÂÂ$ adalah kecepatan linear dari badan (pada poros), dan

${\ displaystyle \ Omega}  ÂÂ$ kecepatan rotasinya.

Patut dicatat bahwa ${\ displaystyle V_ {s}/\ Omega}  ÂÂ$ biasanya tidak sama dengan jari-jari tubuh yang berputar.

Semua roda

"The applied torque" may be the driving torque applied by the motor (often via transmission) or the braking torque applied by the brake (including regenerative braking). This kind of torque produces energy dissipation (above that because of the basic rolling resistance of the rotating, unmoved, unmoved) wheel. This additional loss is partly due to the fact that there are some slipping from the wheels, and for pneumatic tires, there is more flexing of the sidewall due to torque. The slip is defined in such a way that the 2% slip means that the steering wheel's steering speed exceeds the vehicle speed by 2%.

A small percentage slip can result in a much greater percentage increase in rolling resistance. For example, for pneumatic tires, a 5% slip can be translated into a 200% increase in rolling resistance. This is partly because the traction force used during this slip is many times greater than the force of rolling resistance and thus more power per unit of speed is being applied (memory = force x speed so power per unit speed is just a force). So only a small percentage increase in circumferential speed because the slip can be translated into a loss of traction power that can even exceed the power lost due to the basic (ordinary) rolling resistance. For railroads, this effect may be more pronounced due to the low rolling of steel wheels.

Steel railway wheel

To apply traction to the wheel, it takes several wheel slippage. For Russian train climbing classes, this slippage is usually 1.5% to 2.5%.

Slip (also known as creep) is usually roughly proportional to the traction effort. The exception is if the traction effort is so high that the wheel is approaching a substantial slip (more than just a few percent as discussed above), then slipping rapidly increases with traction effort and is no longer linear. With a slightly higher traction effort, the wheels spin out of control and the adhesion decreases so the wheel spins faster. This is the kind of slip that can be observed by eye - slip says 2% for traction is only observed by the instrument. Such fast slips can cause excessive wear or tear.

Pneumatic tire

Rolling resistance greatly increases with applied torque. At high torque, which applies tangential force to the road about half the weight of the vehicle, rolling resistance may be threefold (200% increase). This is partly due to a slip of about 5%. Rolling resistance increases with applied torque not linear, but increases at a faster rate when torque becomes higher.

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Dependence on wheel load

Steel railway wheel

The rolling resistance coefficient, Crr, significantly decreases as the weight of the rail wagon per wheel increases. For example, an empty Russian freight car has about twice the Crr as a loaded car (Crr = 0.002 vs. Crr = 0.001). This same "economic scale" appears in the testing of rail cars. Theoretical crr for rigid wheels that roll on elastic roads shows that Crr is inversely proportional to the square root of the load.

If Crr itself depends on the wheel load per reversed square rule, then for a 2% load increase only a 1% increase in rolling resistance occurs.

Pneumatic tire

For pneumatic tires, the direction of change in Crr (rolling resistance coefficient) depends on whether or not tire inflation increases with increasing load. It was reported that, if the inflation pressure increases with the load in accordance with a "schedule" (unspecified), then a 20% load increase decreases Crr by 3%. But, if inflationary pressures do not change, then a 20% increase in the load results in an increase of 4% Crr. Of course, this will increase rolling resistance by 20% due to increased load plus 1.2 x 4% due to the increase in Crr resulting in a 24.8% increase in rolling resistance.

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Dependence on the curvature of the highway

General

When a vehicle (motor vehicle or railway train) runs around the curve, rolling resistance usually increases. If the curve does not bend to exactly match the centrifugal force with the same centripetal force and the opposite because of the banking, there will be an unbalanced balanced sideways force on the vehicle. This will result in an increase in rolling resistance. Banking is also known as "superelevation" or "cant" (not to be equated with railroad tracks). For railroads, this is called curve resistance but for that path (at least once) has been called rolling resistance due to cornering.

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Sound effects

Scrolling friction produces the sound (vibration) of energy, as mechanical energy is converted into energy forms due to friction. One of the most common examples of frictionless friction is the movement of motor vehicle tires on the highway, a process that produces sound as a by-product. The sound produced by tires of cars and trucks as they roll (especially noticeable at highway speeds) is largely due to tire tire percussion, and compression (and subsequent decompression) of captured air while in the tread.

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Factors contributing to tires

Several factors influence the magnitude of the rolling resistance produced by the tire:

• As mentioned in the introduction: wheel radius, forward speed, surface adhesion, and relative micro shear.
• Different fillers and polymers in the tire composition can increase traction while reducing hysteresis. The replacement of some black carbon with silica at higher prices is one of the common ways to reduce rolling resistance. The use of exotic materials including nano-clay has been shown to reduce the rolling resistance on high-performance rubber tires. The solvent can also be used to swell the solid tires, lowering the rolling resistance.
Dimensions - rolling resistance on the tire is related to the flexibility of the sidewall and the tire contact area For example, at the same pressure, wider bicycle tires are less flexible on the sidewall as they roll and thus have lower rolling resistance (though higher air resistance).
• Inflation rate - Lower pressure on the tire produces a wider stretch on the side wall and higher rolling resistance. This energy conversion on the side wall increases resistance and can also lead to overheating and may have played a role in the infamous Ford Explorer rollover accident.
• More bulging tires (such as bicycle tires) can not lower the overall rolling resistance because the tires can jump over and jump over the road surface. Traction is sacrificed, and overall frictional rotation may not decrease as wheel rotation speed shifts and skid increases.
• The sidewall deflection is not a direct measure of frictionless friction. High-quality tires with high-quality (and flexible) casing will allow for more flexibility per energy loss than cheap tires with rigid side walls. Again, with bikes, quality tires with flexible casing will keep rolling easier than cheap tires with rigid casing. Similarly, as noted by Goodyear's tire tires, tires with "fuel-efficient" chassis will benefit the fuel economy through many tread life (ie retreading), while tires with "fuel-efficient" tread designs will only benefit until the footprint wears into under.
• In the tire, the thickness and shape of the tread have a lot to do with rolling resistance. The thicker and more contoured the tread, the higher the sliding barrier. Thus, the "fastest" bike tire has very few footprints and heavy duty trucks getting the best fuel savings when the tire tire runs out.
• The effect of diameter seems to be negligible, as long as the sidewalk is difficult and the diameter range is limited. See dependence on diameter.
• Almost all the world speed records have been set on a relatively narrow wheel, probably due to its high speed aerodynamic superiority, which is much less important at normal speeds.
• Temperature: with solid and pneumatic tires, rolling resistance has decreased due to temperature increase (in the temperature range: ie there is an upper limit for this effect) For temperature rise from 30 ° C to 70 ° C rolling resistance decreases from 20 to 25 %. It is said that the drivers heated their tires before the race.

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Railway: Rolling resistance component

In a broad sense, rolling resistance can be defined as the number of components):

1. Loss of wheel bearing torque.
2. Pure roll resistance.
3. Slide the wheel on the rails.
4. Loss of energy to the road (and earth).
5. Loss of energy for rolling stock rail oscillations.

Wheel torque losses can be measured as rolling resistance on the rim wheel, Crr. Railroads usually use cylindrical (Russian) or pointed (cindy) roller bearing (United States). The specific rolling resistance in Russian bearings varies with both wheel loading and speed. Rolling resistance of the lowest wheel bearings with high axle loads and speeds between 60-80 km/h with Crr 0.00013 (axle load 21 tons). For an empty freight car with axle load 5.5 tons, Crr rises to 0.00020 at 60 km/h but at low speed 20 km/h rises to 0.00024 and at high speed (for freight trains) of 120 km/h this is 0.00028. The crr obtained above is added to Crr from the other components to get total Crr for the wheels.

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Compare the rolling obstacles of vehicles and trains on the road

The resistance of rolling steel wheels on steel train rails is much less than the rubber tire wheels of a car or truck. The weight of the train varies greatly; in some cases they may be much heavier per passenger or per ton of cleaner goods than cars or trucks, but in other cases they may be much lighter.

As an example of a very heavy passenger train, in 1975, Amtrak passenger trains weighed slightly above 7 tons per passenger, which is much heavier than the average slightly above one ton per passenger for a car. This meant that for the Amtrak passenger train in 1975, most of the energy savings from lower rolling resistance were lost due to greater weight.

One example of a very lightweight high speed passenger train is the Shinkansen Seri N700, which weighs 715 tons and carries 1,323 passengers, resulting in weight per passenger of about half a ton. This lighter weight per passenger, combined with the rolling resistance of lower steel wheels on steel rails means that the Shinkansen N700 is far more energy efficient than an ordinary car.

In the case of freight shipments, CSX ran an advertising campaign in 2013 claiming that their freight trains moved "a ton of goods 436 miles on a gallon of fuel", while some sources claimed the truck moved a ton of goods around 130 miles per gallon of fuel, fire as a whole more efficient.

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See also

• Friction coefficient
• Low-spiral Ban
• Maglev (Magnetic Levitation, rolling and rolling resistance)
• Rolling scrolling element

References

• ??????? ?.?. (in Russian) "???????????????????????????????????????????????????????????????????????????????????? ???????????????????????????????????????????????????? Resistance to rolling stock railroad movement) ????? ???? ??? (ISSN 0372-3305). ?????? 311 (Vol 311). - ??????: ?????????, 1966. - 178 pp. Perm. record at UC Berkeley (In 2012, full text is on the Internet but US is blocked)
• ???? ?.?., ????? ?.?., ?????? ?.?. (in Russian) "???? ???????" (Train train)

??????? ??????? -?.: ?????????, 1987. - 264 p.

• Hay, William W. "Railway Engineering" New York, Wiley 1953
• Hersey, Mayo D., "Scrolling Friction" ASME Transactions , April 1969 pp.Ã, 260-275 and Journal of Lubrication Technology , January 1970, pp. 83-88 (one article shared between 2 journals) Except for "Introduction to History" and literature survey, especially about the lab. testing of cast iron mine wheels with a diameter of 8 "to 24" was performed in the 1920s (nearly half a century delay between experimentation and publication).
• Hoerner, Sighard F., "Dynamic fluid drag", published by author, 1965. (Chapter 12 is "Land-Borne Vehicles" and includes rolling resistance (railroad, autos, trucks).
• Roberts, G. B., "Waste of power on tires", International Rubber Conference, Washington, D.C. 1959.
• US National Bureau's Standard, "Mechanism of Pneumatic Tires", Monograph # 132, 1969-1970.
• Williams J A, "Engineering tribology" Oxford University Press, 1994.

External links

• Rolling Resistance and Fuel Savings
• temperature vs. rolling resistance
• Simple scrolling test to measure Crr on cars and bikes
• Threshold Scrolling Threshold

Source of the article : Wikipedia