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Rolling resistance, sometimes called
rolling friction or
rolling drag, is the
Friction that occurs when an object such as a ball or
tire rolls. It is caused by the deformation of the wheel or tire or the deformation of the ground. It depends very much on the material of the wheel or tire and the sort of ground. For example,
rubber will give a bigger rolling resistance than steel. Also,
sand on the ground will give more rolling resistance than
concrete. A
vehicle rolling will gradually slow down due to rolling resistance, but a
train with steel wheels running on steel
Rail tracks will roll much further than a automobile or
truck with rubber tires running on pavement, even when differences in mass and momentum are accounted for. The coefficient of rolling resistance is generally much smaller for tires or balls than the coefficient of sliding friction.Peck, William Guy. (1859.) "Elements of Mechanics: For the Use of Colleges, Academies, and High Schools". A.S. Barnes & Burr: New York, page 135. Retrieved on 2007-10-09.
In braking
It is worth noting that for all vehicles that travel on wheels (such as cars and
bicycles), the sum of rolling resistance and static friction is what causes the vehicle to slow when the
brakes are applied. The actual force applied in braking (for example, clamps applied to
disk brakes) is internal, and by
Newton's laws of motion cannot cause a change in the vehicle's motion. Therefore the slowing is caused by contact between the road and the car's tires; the static friction force between road and tire is the "equal and opposite reaction" specified in
Newton's Laws of Motion. Rolling resistance can be compared to sliding friction, as when the brakes "lock up", the wheels slide upon the driving surface and do not sufficiently slow the car. Maximum braking force occurs when there is about 11% slip between the wheel's speed and the road - this is used to advantage in Anti-lock braking system braking systems, and
cadence braking, a manual technique which achieves something similar.
Factors that contribute
Several factors affect the magnitude of rolling resistance a tire generates:
- Material - different fillers and polymers in tire composition impact rolling resistance. The replacement of some carbon black with higher-priced silica–silane is one common way of reducing rolling resistance. "Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance -- Special Report 286." National Academy of Sciences, Transportation Research Board, 2006. Retrieved on 2007-08-11.
- Dimensions - rolling resistance is related to the flex of sidewalls and the contact area of the tire. For example, at the same pressure wider bicycle tires have less flex in sidewalls and thus lower rolling resistance (although higher air resistance).
- Extent of inflation - Lower pressure in tires results in more flexing of sidewalls and higher rolling resistance. This energy conversion in the sidewalls increases resistance and can also lead to overheating and may have played a part in the infamous Ford Explorer Firestone vs Ford Motor Company controversy.
- Over inflating tires (such a bicycle tires) may not lower the overall rolling resistance as the tire may skip and hop over the road surface. Traction is sacrificed, and overall rolling friction may not be reduced as the wheel rotational speed changes and slippage increases.
- Sidewall deflection is not a direct measurement of rolling friction. A high quality tire with a high quality (and supple) casing will allow for more flex per energy loss than a cheap tire with a stiff sidewall. Again, on a bicycle, a quality tire with a supple casing will still roll easier than a cheap tire with a stiff casing. Similarly, as noted by Goodyear truck tires, a tire with a "fuel saving" casing will benefit the fuel economy through many casing lives (i.e. retreading), while a tire with a "fuel saving" tread design will only benefit until the tread wears down.
- Tread thickness has much to do with rolling resistance. The thicker the tread, the higher the rolling resistance. Thus, the "fastest" bicycle tires have very little tread and heavy duty trucks get the best fuel economy as the tire tread wears out.
- Hard steel rails last longer but may also have lower static friction. They may also suffer fatigue (material) cracking because the cracked area is not worn away by the passing trains.
- Smaller wheels, all else being equal, have higher rolling resistance than larger wheels.{{cite web
| title = VREDESTEIN Bicycle Tires| url = http://www.vredesteinusa.com/index.html?lang=en-us&target=d16.html| accessdate = 2006-08-14-->
Physical formula and tables
The force of rolling resistance is given by:
F = C_{rr} N_f \
::where
:::
F is the resistant force,
:::
Crr is the
rolling resistance coefficient or
coefficient of rolling friction (
CRF), and
::: N_f is the
normal force.
In usual cases, the normal force
on a single tire will be the mass of the object which the tires are supporting
divided by the number of wheels,
plus the mass of the wheel,
times the
gravitational acceleration (9.81 m·s−2 on Earth). In other words, the normal force is equal to the weight of the object being supported.
Table of Crr examples: {]
radial tire used for solar cars/eco marathon cars as specially made by Michelin|-|0.005 || tram-rails standard dirty with straights and curves|-| 0.0055 || Typical BMX bicycle tire used for solar cars|-| 0.006
to 0.01 || low rolling resistance car tire on a smooth road
and truck tires on a smooth road|-|0.010
to 0.015 || ordinary car tires on concrete|-|0.020 || car on stone plates|-|0.030 || car/bus on tar/asphalt|-|}
For example on the earth a car of 1000 kg on asphalt will need a force of 300 N for rolling.
Effects of
Rolling friction generates heat and sound energy, as mechanical energy is converted to these forms of energy due to the frictional contact. One of the most common examples of rolling friction is the movement of
motor vehicle tires on a
roadway, a process which generates heat and roadway noise as by-products. C. Michael Hogan,
Analysis of Highway Noise, Journal of Soil, Air and Water Pollution, Springer Verlag Publishers, Netherlands, Volume 2, Number 3 / September, 1973 The heat generated raises the temperature of the frictional surface; moreover, this temperature increase typically has an effect upon the coefficient of friction itself.Gwidon W. Stachowiak, Andrew William Batchelor,
Engineering Tribology, Elsevier Publisher, 750 pages (2000) ISBN 0750673044
See also
References
External links
- physics tutorial
- temperature vs rolling resistance
- how tires support a car
- Simple roll-down test to measure Crr in cars and bikes
Rolling resistance, sometimes called
rolling friction or
rolling drag, is the
Friction that occurs when an object such as a ball or tire rolls. It is caused by the deformation of the wheel or tire or the deformation of the ground. It depends very much on the material of the wheel or tire and the sort of ground. For example,
rubber will give a bigger rolling resistance than
steel. Also,
sand on the ground will give more rolling resistance than
concrete. A vehicle rolling will gradually slow down due to rolling resistance, but a train with steel wheels running on steel
Rail tracks will roll much further than a automobile or
truck with rubber tires running on pavement, even when differences in mass and momentum are accounted for. The coefficient of rolling resistance is generally much smaller for tires or balls than the coefficient of sliding friction.Peck, William Guy. (1859.) "Elements of Mechanics: For the Use of Colleges, Academies, and High Schools". A.S. Barnes & Burr: New York, page 135. Retrieved on 2007-10-09.
In braking
It is worth noting that for all vehicles that travel on wheels (such as cars and
bicycles), the sum of rolling resistance and
static friction is what causes the vehicle to slow when the brakes are applied. The actual force applied in braking (for example, clamps applied to
disk brakes) is internal, and by Newton's laws of motion cannot cause a change in the vehicle's motion. Therefore the slowing is caused by contact between the road and the car's tires; the
static friction force between road and tire is the "equal and opposite reaction" specified in
Newton's Laws of Motion. Rolling resistance can be compared to sliding friction, as when the brakes "lock up", the wheels slide upon the driving surface and do not sufficiently slow the car. Maximum braking force occurs when there is about 11% slip between the wheel's speed and the road - this is used to advantage in Anti-lock braking system braking systems, and cadence braking, a manual technique which achieves something similar.
Factors that contribute
Several factors affect the magnitude of rolling resistance a tire generates:
- Material - different fillers and polymers in tire composition impact rolling resistance. The replacement of some carbon black with higher-priced silica–silane is one common way of reducing rolling resistance. "Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance -- Special Report 286." National Academy of Sciences, Transportation Research Board, 2006. Retrieved on 2007-08-11.
- Dimensions - rolling resistance is related to the flex of sidewalls and the contact area of the tire. For example, at the same pressure wider bicycle tires have less flex in sidewalls and thus lower rolling resistance (although higher air resistance).
- Extent of inflation - Lower pressure in tires results in more flexing of sidewalls and higher rolling resistance. This energy conversion in the sidewalls increases resistance and can also lead to overheating and may have played a part in the infamous Ford Explorer Firestone vs Ford Motor Company controversy.
- Over inflating tires (such a bicycle tires) may not lower the overall rolling resistance as the tire may skip and hop over the road surface. Traction is sacrificed, and overall rolling friction may not be reduced as the wheel rotational speed changes and slippage increases.
- Sidewall deflection is not a direct measurement of rolling friction. A high quality tire with a high quality (and supple) casing will allow for more flex per energy loss than a cheap tire with a stiff sidewall. Again, on a bicycle, a quality tire with a supple casing will still roll easier than a cheap tire with a stiff casing. Similarly, as noted by Goodyear truck tires, a tire with a "fuel saving" casing will benefit the fuel economy through many casing lives (i.e. retreading), while a tire with a "fuel saving" tread design will only benefit until the tread wears down.
- Tread thickness has much to do with rolling resistance. The thicker the tread, the higher the rolling resistance. Thus, the "fastest" bicycle tires have very little tread and heavy duty trucks get the best fuel economy as the tire tread wears out.
- Hard steel rails last longer but may also have lower static friction. They may also suffer fatigue (material) cracking because the cracked area is not worn away by the passing trains.
- Smaller wheels, all else being equal, have higher rolling resistance than larger wheels.{{cite web
| title = VREDESTEIN Bicycle Tires| url = http://www.vredesteinusa.com/index.html?lang=en-us&target=d16.html| accessdate = 2006-08-14-->
Physical formula and tables
The force of rolling resistance is given by:
F = C_{rr} N_f \
::where
:::
F is the resistant force,
:::
Crr is the
rolling resistance coefficient or
coefficient of rolling friction (
CRF), and
::: N_f is the
normal force.
In usual cases, the normal force
on a single tire will be the mass of the object which the tires are supporting
divided by the number of wheels,
plus the mass of the wheel,
times the gravitational acceleration (9.81 m·s−2 on Earth). In other words, the normal force is equal to the weight of the object being supported.
Table of Crr examples: {] radial tire used for solar cars/eco marathon cars as specially made by Michelin|-|0.005 || tram-rails standard dirty with straights and curves|-| 0.0055 || Typical BMX bicycle tire used for solar cars|-| 0.006
to 0.01 || low rolling resistance car tire on a smooth road
and truck tires on a smooth road|-|0.010
to 0.015 || ordinary car tires on concrete|-|0.020 || car on stone plates|-|0.030 || car/bus on tar/asphalt|-|}
For example on the earth a car of 1000 kg on asphalt will need a force of 300 N for rolling.
Effects of
Rolling friction generates heat and sound energy, as mechanical energy is converted to these forms of energy due to the frictional contact. One of the most common examples of rolling friction is the movement of motor vehicle tires on a
roadway, a process which generates heat and
roadway noise as by-products. C. Michael Hogan,
Analysis of Highway Noise, Journal of Soil, Air and Water Pollution, Springer Verlag Publishers, Netherlands, Volume 2, Number 3 / September, 1973 The heat generated raises the temperature of the frictional surface; moreover, this temperature increase typically has an effect upon the coefficient of friction itself.Gwidon W. Stachowiak, Andrew William Batchelor,
Engineering Tribology, Elsevier Publisher, 750 pages (2000) ISBN 0750673044
See also
References
External links
- physics tutorial
- temperature vs rolling resistance
- how tires support a car
- Simple roll-down test to measure Crr in cars and bikes
