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Rolling Resistance & Drag Coefficients

 

Rolling Resistance

There are a number of sources of information about appropriate values of Coefficient of Rolling Resistance Cr, not all are online so I've summarised some below.

J. Carvill in "The Mechanical Engineers Data Handbook" suggests the following for pneumatic tyres.

Drive Surface Cr
New Asphalt/Concrete 0.01
Worn Asphalt/Concrete 0.02
New Small Cobbles 0.01
Worn Large Cobbles 0.03
Rolled New Gravel 0.02
Loose Worn Gravel 0.04
Medium Hard Soil 0.08
Sand 0.1 - 0.2

Wikipedia has an online article with more data - some of which is:

Type & Drive Surface Cr
Very Low Resistance Radial Tyre 0.0025
Standard Dirty Tram Rails 0.005
Car Tyres on Concrete 0.01- 0.015
Car on Stone Plates 0.02
Car/Bus on Asphalt 0.03

Obtaining data for rolling resistance on grass is a bit more difficult. There are some references. BICICLETTA has published data for mountain bike tyres here. They summarise that rolling resistances on grass are about 6 (six) times those on good road conditions, this may suggest Cr values of around 0.06 - 0.08. Resistance on gravel is about twice that on the road.  Wider tyres and lower inflation pressures are reported to reduce rolling resistance on off-road conditions.

Sauer-Sundstrand the hydrostatic drives manufacturer provides some interesting data for commercial vehicles in their Applications Manual:

Drive Surface Cr
Sand 0.25
Wet Soil, Mud 0.20
Fresh Deep Snow 0.16
Loose Soil, Gravel 0.12
Grassy Field, Dry Croplamd 0.08
Packed Soil, Dirt Roadway 0.02
Pavement 0.08
Steel on Steel Rails 0.004

Clearly the various off-road conditions can give rise to much larger rolling resistances than hard, smooth road driving. This will be reflected in vehicle power consumption.

 

Aerodynamic Drag

J. Carvill in "The Mechanical Engineers Data Handbook" suggests the following for Drag Coefficient Cd for different vehicle shapes:

Vehicle Cd
Sports Car with Sloping Rear 0.2 - 0.3
Saloon with Stepped Rear 0.4 - 0.5
Open Top Convertible 0.6 - 0.7
Bus 0.6 - 0.8
Truck 0.8 - 1.0
Motorbike & Rider 1.8
Flat Plate (face on to flow) 1.2
Long Streamlined Body 0.1
Sphere 0.47

NB, Cd and Cr are not the same thing - don't confuse them.

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