Why does Rolling Resistance matter in the real world?

Some 13% of the power produced by the engine to move a long haul rig along a smooth road at 65 mph is “lost” due to Rolling Resistance. What is it, what causes it, and how can you help to reduce it.

What is Rolling Resistance?

Rolling Resistance is the term used for the energy required to roll a tire over a particular road surface.

It can be viewed as a Parasitic Power Loss, accounting some 13% of the total energy required to move a long haul tractor-trailer combination over a smooth road at 65 mph.

What causes Rolling Resistance?

Rolling Resistance is caused by the following factors

  • The deformation of the tire in the contact patch and the sidewalls as it rolls (this accounts for some 80% – 95% of Rolling Resistance – Michelin)
  • Aerodynamic drag of the rotating tire
  • Friction (microslippage) between the tread and the road surface

Rolling Resistance is also greatly impacted by condition of the road surface.

Why does it matter?

Because it is energy LOST, it is equivalent to a power loss. Power made by the engine is not fully usable, and inhibits forward movement of the vehicle.

Fuel consumption is therefore increased because of this parasitic loss.

How can you reduce Rolling Resistance?
  • Keep tires properly inflated. Properly inflated tires minimize deformation of the contact patch and side walls, and keeps energy loss to a minimum.
  • Use “green tires”, with rubber compounds and treads designed to minimize this energy loss. As much as 35 HP may be “saved” at 65 mph at 80,000 lb.
  • Use of single wide-base tires in place of dual assembly tires. Two dual tires have four sidewalls as opposed to two for a single tire, thus reducing the side wall flex and energy loss.
  • Larger diameter tires have slightly lower energy loss due to the fact that there is less bending of the tire as it enters and leaves the contact patch.

Source: Michelin

Calculating Rolling Resistance

The standard baseline basic tire is the bias ply type, which is given a Rolling Resistance factor of 1 running on smooth concrete. This is given as a resistance of 10 lb per 1000 lbs gross vehicle weight, which is equivalent to 1% grade.

Most tires used today are radial tires, which have 25% – 30% less rolling resistance. This means a factor of 0.70 – 0.75. The most recent design low-rolling resistance tires have factors in the range of 0.5 to 0.6 depending on the specific design and application.

Power lost to Rolling Resistance can be calculated using the following formula:

HP
Vehicle Weight x (6.1+(.06 x MPH)) x CP x MPH
375,000
HP = Horsepower to overcome rolling resistance
MPH = Miles per hour
CP = Tire factor = 1.0 – Bias
CP = Tire factor = 0 .75 – RadialExample:
Vehicle weight = 80,000 lb
Speed = 65 mph
Power lost to Rolling Resistance
80,000 x (6.1 +(0.06 x 65)) x 0.75 x 65
375,000
= 104 HP

 

Note that at higher speed, the aerodynamic forces begin to overtake Rolling Resistance as the major contributor of power loss.

HP required to overcome drag and rolling resistance/accessories relative to top speed for a typical Class 8 truck trailer combination.

U.S. Dept. of Energy 2000 “Technology Roadmap for the 21st Century Truck Program, a government-industry research partnership” Technical Report 21CT—001 December.

Road surface factors

These factors can also be expressed as HP.

A bias ply tire on a smooth concrete surface has a factor of 1.0, and a radial tire 0.7 to 0.75.

These “multipliers” must be used to adjust the rolling resistance power loss to the specific road surface.