Rolling friction
Rolling resistance, sometimes called rolling friction, is the resistance that occurs when an object (e.g a wheel or tire) rolls. It is much smaller than sliding friction except for special cases like ice skating. 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 friction than steel. Also, sand on the ground will give more rolling friction than concrete. A vehicle rolling will gradually slow down due to rolling friction, but a train running on a steel rail will roll much further than a car or truck on rubber tire.
It is worth noting that for all vehicles that travel upon wheels (such as cars and bicycles), the sum of rolling friction 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 First Law 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 Third Law. Rolling friction can be compared to sliding friction, as when the brakes "lock up", they 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 ABS braking systems, and cadence braking, a manual technique which achieves something similar.
Several factors affect the magnitude of rolling friction a tire generates:
- Material - Tires with higher sulfur content tend to have a lower rolling friction. This is one strategy that most hybrid car vendors use to improve fuel efficiency.
- Dimensions - rolling friction 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 friction. This friction in the sidewalls increases resistance and can also lead to overheating and may have played a part in the infamous Ford Explorer rollover accidents.
- Hard rail steels last longer but may also have lower static friction. They may also suffer fatigue cracking because the cracked area is not worn away by the passing trains.
Rolling resistance is generally measured in Crr, and ranges from 0.0028 for a bicycle with Michelin Ecorun bicycle tires[1] up to around .05 for earth-moving equipment[2].
Physical formula and tables
The force of rolling resistance is given by:
- <math>F = C_{rr} N_f \ </math>
- where
- F is the resistant force,
- Crr is the rolling resistance coefficient or coefficient of rolling friction (CRF), and
- <math>N_f</math> is the normal force.
- where
In usual cases, the normal force on each tire will be the mass of the object (wheels plus what they're supporting) times the gravitational acceleration (9.81 m/s² on Earth) divided by the number of wheels (if the wheels have the same rolling coefficient).
Table of Crr examples:Template:Citation needed [3]
| Crr | description |
| 0.001 to 0.0025 | train steel on steel with tatz-mounted electric traction |
| 0.005 | tram-rails standard dirty with straights and curves |
| 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 teer/asphalt |
For example on the earth a car with 1000 kg on asphalt will need a force of 300 N for rolling.