Rolling Resistance Made Simple

Of the things you can change on your vehicle to improve its fuel efficiency, rolling resistance is one of the easiest. Rolling resistance can be explained in very complicated mathematical formulas and theory, but we are going to break it down into simple terms that are easy to understand. Hopefully this easy explanation will allow anyone to improve their rolling resistance, and therefore their fuel efficiency.

What is rolling resistance?

Rolling resistance, in simple terms, is how much force it takes to roll an object. In our case, the object is a vehicle, or more specifically, a vehicle wheel. This resistance is one of the many forces that your engine has to overcome to move your car. By reducing this drag on your engine, you burn less fuel.

rolling resistance graphic

3D CGI model of what causes rolling resistance

What causes this resistance?

The main cause of rolling resistance is the softness of the tire. If you look at your tires, you’ll notice they look flatter on the bottom. It’s true; they are flatter and wider on the bottom than they are anywhere else around the wheel. This is called deformation. The tire deforms at the bottom from the weight of the vehicle. The pressure from the weight of the vehicle is exerted between the bottom of the wheel and the road. As the tire turns, the part of the tire that gets the pressure applied to it moves to whatever part is at the bottom of the wheel.

The reason the tire deforms is because it’s soft. If your tires were hard and had no air in them, they would contribute almost nothing to your rolling resistance.

An easy way to visualize this is to think of a half-flat tire.

If you were to push a wheelbarrow with a tire that is fully inflated, it is easy to push. When you add weight to it, the tire flattens a little on the bottom and becomes a little harder to push. If you let some air out of the tire, it becomes even harder to push. If you make the tire almost flat, it will become very hard to push. The amount of deformation that the tire has determines how much resistance it causes to movement.

This is exactly the same thing that happens with an automobile. There are obviously a couple differences between a wheelbarrow and a car, but the tire deformation resulting in resistance to movement is the same. Each of the four wheels on a car adds to the rolling resistance. If even one tire is low on air, it causes additional drag on the engine.

Race LRR

Race car showing the tire deformation that causes rolling resistance

Is rolling resistance due to the weight of the vehicle?

Part of the cause of rolling resistance is the vehicle weight, but the majority of it is caused by the softness of the tires. A light car with soft or underinflated tires will have more drag on the engine than a heavy car with hard tires. This is the reason that road racing bicycles have narrow hard tires. The narrow hard tires have less area in contact with the road, and they have less deformation. Because they deform less, they cause less resistance and roll more easily.

LRR graphic

LRR tires help you save fuel

How can you improve your rolling resistance?

  • Use LRR tires

Luckily, tire companies have started to take rolling resistance into account. They have developed LRR(low rolling resistance) tires, and list the resistance numbers of most of their offerings. These numbers will give you the option of picking a tire that has lower resistance numbers than the tire you are currently using.

  • Use narrower tires

A narrow tire has less contact area with the road. This narrower contact area has less opportunity to deform. It also has less area to deform to. Narrower tires will generally run at higher pressures and will be harder because of that.

There are many stories of auto manufacturers equipping certain models with narrower tires and marketing them as “high efficiency”, “eco”, or “green” versions. In many cases, the only difference between these versions and the standard model is narrower tires.

Honda did this with the CRX “HF” version. The HF stood for High Fuel efficiency. This model was about 100lbs lighter and had narrower tires. These were basically the only differences between this model and the standard model. This car achieved over 50 MPG highway.

  • Keep your tires inflated to the max PSI

Look on the sidewall of your tires for the listed maximum PSI rating. This is the max amount of air pressure that the tire manufacturer feels is safe for this tire. Add air to your tires up to this amount.

Usually the car manufacturer will recommend an air pressure below what the maximum for the tire is. This isn’t for safety reasons; this is to give the vehicle a softer ride. The car maker wants the tires to be a little soft to help the suspension absorb some of the bumps. This does work, but it also adds rolling resistance and decreases fuel economy.

Rolling resistance isn’t the only factor affecting your fuel economy, but it’s an easy one to improve. Some people may go all out and buy the lowest resistance and narrowest tires they can for their vehicle. Others may simply choose to make sure their tires are properly inflated more often. Either way, being aware of the impact that your tire resistance has on your automobile is the first step in reducing it.

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