Definition
Rolling resistance is the force that works against a wheel or round object as it rolls across a surface. It happens because the part of the wheel touching the ground gets squashed and then springs back, and energy of motion is lost in the process.
The Wheel Doesn't Slide, So Why Does It Stop?
When you pull a sled across ice, the runners rub directly against the surface, and sliding friction builds up. A wheel is different. It turns and steps onto the ground bit by bit, so it never scrapes along the surface.
Still, a bike or car rolling on flat ground slowly comes to a stop if you stop pedaling or take your foot off the gas. Even if you set aside air resistance from a headwind, something invisible is working against the rolling itself.
This hidden troublemaker isn't the usual friction that grips an object to keep it from sliding. It shows up even when the wheel rolls perfectly without slipping. It comes from tiny deformations in the wheel and the ground where they meet, and physics calls it rolling resistance.
Energy Leaks Out When Squashed and Released
Look closely at where a bike wheel meets the road. The rubber is pressed flat by the weight on it. As the wheel keeps rolling, the squashed rear part springs back to its round shape, while the new front part touching the ground gets squashed.
When something is pressed, it absorbs energy. When it springs back, it pushes energy out again. But materials like rubber and pavement can't give back all the energy that went in when they recover. The lost part becomes heat that spreads away and is wasted.
It takes a lot of force to squash the front of the wheel, but the push you get from the back springing back is smaller. That imbalance creates a drag pointing opposite to the way the wheel rolls. This is also why a soft bike tire feels so heavy to pedal: the tire squashes a lot and wastes energy.
A Little More Precisely: Is Harder Always Better?
To be a little more precise, rolling resistance depends not only on the wheel's material but also on how hard and smooth the surface is. A train's steel wheels on flat steel rails barely deform, so they have very low rolling resistance compared with most vehicles we ride.
But the asphalt and dirt roads we meet every day are full of tiny bumps and rough spots. If you used hard steel wheels on an ordinary road, both the wheels and the body would rattle and bounce up and down every time they hit the rough surface.
An air-filled rubber tire, on the other hand, flexes and swallows the bumps in the road. Even if the tire squashes and loses a little energy, it cuts the energy lost when the whole vehicle bounces. People often count that bounce loss separately from rolling resistance in the narrow sense, the kind caused by tire deformation. In the end, the right balance of air pressure and flexibility for the road is the key to lowering total energy loss.
Common misconceptions
Rolling resistance is friction caused by the wheel rubbing and sliding against the ground.
Unlike the friction of two surfaces sliding and scraping, rolling resistance happens even when a wheel rolls perfectly without slipping. The wheel and ground get squashed and spring back, and energy is lost as heat.
The harder a wheel is, the less resistance it meets on the road.
Hard steel wheels work well on smooth rails. On rough ordinary roads, though, a wheel that's too hard bounces up at every bump and wastes a lot of the energy meant to move you forward.
Where you meet it
On a smooth paved road, pumping your bike tires up firm means they squash less, and pedaling gets much easier.
A heavy train can travel a long way on little fuel because hard rails and steel wheels give it very low rolling resistance.
When you drive a car over sand or mud, the ground sinks under the wheels and they bury themselves, so rolling resistance shoots up.
In one sentence
Rolling resistance is the drag that appears because a rolling wheel and the ground get squashed and spring back, and not all the energy comes back. Some is lost as heat.