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Old 12-09-2008, 02:46 PM   #43 (permalink)
Geonerd
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Quote:
Originally Posted by tasdrouille View Post
Rolling resistance coefficient (RRC) literature review:

At moderate speed, 90%-95% of the tire RRC is due to material hysteresis, 2%-10% is due to surface friction or micro-slip and 1%-3% is caused by air friction.

Tire inflation pressure is the main factor affecting hysteresis.

Warm tires have lower RRC than cold tires not only due to increased pressure, but also because the internal damping of rubber decreases when its temperature increases.

Given equal pressure, the initial RRC of a tire at -20 C can be more than twice as at 40 C.

The RRC is almost the same from 0 up to 55 mph, when it starts to increase rapidly. At 80 mph, it is roughly twice as much as 55 mph.

Worn out tires can have a RRC up to 20% lower than new tires as a significant part of the hysteresis happen in the thread which is not so much affected by inflation pressure.

The relation between tire pressure and RRC is almost linear, and is still significant in excess of 100 psi.

As a side note, it is interesting to note that fuel consumption can vary by more than 10% depending on the smoothness of the pavement.

Effect of width, aspect ratio and diameter on RRC:

Note: This applies to RRC only, not rolling resistance in terms of force, which is more important than RRC in stop and go driving.

This aspect of RR is very hard to tackle as there are multiple variables working in opposite directions. Nonetheless, by looking at the variation in RRC of the same tire when one variable is modified at a time, and by looking at average RRC for each tire size, we can conclude that generally, all other things being equal:

- Larger diameter tires have a lower RRC than smaller ones.
- The lowest RRC tire width is the closest one to the tire diameter times 14 (ignoring units of measure). (At max sidewall pressure and max load)
- The lowest RRC aspect ratio is lower as tire diameter gets larger. It appears as the the lowest RRC Diameter/Aspect ratio are: 13/80, 14/75, 15/70. (At max sidewall pressure and max load).

Overinflated tires will wear more in the middle than optimally inflated tires. However, optimal inflation varies with load and tire size.

Blowout risks related to sudden pressure rises in overinflated tires are non existent as radial tires will not unbed under 150 psi.

Overinflated tires have higher risks of cuts, but reduced risks of rim damage.

The temperature increase delta decreases as initial cold pressure increases.

References:

TYRE ROLLING RESISTANCE ON ROAD

Passenger Tire Rolling Resistance

Rolling Resistance Testing: California Energy Commissionís Fuel Efficient Tire Program


Excellent, thank you for the writeup.
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