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Explanation of Rolling Resistance
The two major factors that oppose the motion of an EV are Aerodynamics and rolling resistance. Power from the batteries is needed to overcome these forces to travel down the road. Rolling resistance is not a measure of the grip or traction of the tyres, it is simply a measure of how easily they roll down the road. As the tyres of the car roll along the road the surface deforms to become flat as it comes into contact with the ground and then changes back to its normal shape as it leaves the road. This constant deforming of the tyres takes energy. There is also some adhesion forces as the tyres stick slightly to the road and then are peeled off. The combination of these two forces gives the rolling resistance of the tyres, other factors which effect the overall RR of the vehicle include wheel alignment and tyre pressure. The force due to rolling resistance is equal to the mass of the vehicle times the acceleration due to gravity times the coefficient of Rolling Resistance (Frr=mgCrr) or rolling resistance coefficient
(RRC). For those less mathematically inclined that just means that the force increases with the weight and rolling resistance of the vehicle. This following section will describe how to decrease this parasitic force.

How to minimize the Rolling Resistance of your EV:
  • Buy Low RR Tyres: The Californian Energy Commission (CEC) found that fitting low RR tyres gave average fuel savings of 1.5-7%, that is a pretty decent increase in range and means less DOD for your batteries. This Article compares the RR of 77 equal size 195/65R15 tires in June of 2009. California has done lots of reports and studies on tire rolling resistance and have passed some laws (Assembly Bill 844) to require more efficient tires like their California Fuel Efficient Tire Program . Rolling Resistance Force (RRF) is a newer rating term CEC uses instead of rolling resistance coefficient (RRC). Most new passenger tires have reported RRCs ranging from .007 to .014, the lower the better. Smaller tires have lower resistance. An increase of 7% in a EV would mean a 7% increase in power or distance.
  • (SAE J1269 and SAE J2452) being performed on new tires
  • Check and Maintain Air Pressure: Low air pressure means there is greater deformation in the tyres, increasing the RR dramatically. It is recommended that tyre pressure be checked weekly and kept at the maximum rated pressure for those tyres to maximize efficiency (and handling). You would be surprised how much difference it makes, A tyre thatís under-inflated by 20 percent increases fuel consumption by around 5 percent.
  • Check and Maintain Wheel Alignment: Bad wheel alignment means the tyres scrub and scrape against the road and slow the car down, having it checked and corrected regularly will maximize your EV's efficiency.
Use Lightweight Synthetic Lubricants: Use of synthetic oils is a great way to cut down drivetrain loss. In most cases with electric conversions, the drivetrain will not see the maintained high levels of torque the internal combustion engine would deliver. In which case a lighter weight oil can be used.

Contributors: atzi, mattW, lazzer408
Created by mattW, 12-23-2007 at 01:53 AM
Last edited by atzi, 08-19-2009 at 01:38 PM
2 Comments , 20028 Views

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