Causes of tire rolling resistance, and our protocols when testing it

Editor’s note: We recently lab-tested 15 Paris-Roubaix tires and 24 gravel tires for rolling resistance on a rough surface at Wheel Energy in Finland. Below is an explanation of the what and why of our testing.

When tested on a smooth lab drum, the rolling resistance of each tire will continue to decrease with increasing inflation pressure. But we don’t usually ride on smooth, lab drums.

A graph of tire pressure on the horizontal scale and rolling resistance on the vertical scale would show a line starting on the left at very high rolling resistance and very low tire pressure trending downward to the right: higher tire pressure = lower rolling resistance. An unconscious belief in this downward curve may explain why so many of us used to race in the 1970s and 1980s with our tires pumped up to over 130psi, which, as I have explained in many rolling-resistance articles over the past couple of decades, is energy-sapping folly on anything other than glassy-smooth surfaces.

Hysteresis is energy lost in the tire structure due to the lag in rebound after deflection. When you drop a ball onto a sidewalk, each bounce will be lower than the last, even with a highly elastic “super ball.” This is hysteretic energy loss—the material of the ball absorbs the energy of the impact and does not return all of it; some of it is instead converted to heat. The same thing happens with a bicycle tire as it rolls along: the elastic tread and tire carcass deflect as it rolls, and some energy is constantly lost through hysteresis and given off as heat.

Rolling on the smooth drum, the primary energy loss is hysteresis: the section of the tire carcass at the bottom deflects as it rolls, and some energy is lost as heat when that section of carcass rolls past the bottom and returns to its prior shape. You can minimize this energy loss with a casing that flexes more easily and with a tread compound engineered for less hysteretic energy loss, but primarily you will do it by pumping the tire up harder so that it deflects less at its contact patch with the drum.

Casing flex also explains why wide tires roll fast on smooth surfaces. At a given air pressure and load on the wheel, the surface area of the tire’s contact patch on the road will be the same, no matter the size of the tire. Thing is, a skinny tire will have to deflect more deeply to flatten that same amount of area onto the road as a fat one will, hence lower hysteresis loss in the fat tire.

A bike with super-hard tires feels lively and fast, bouncing along over the road surface, but that bouncing is energy lost from propulsion. In suspension terminology, the harder the tires on a road bike are, the greater the “unsprung weight” that moves up and down the full height of each bump the tires encounter. If there is no give at all in the tires — i.e., they are rock hard — the entire mass of the bike and of the rider will be fully deflected upward by each bump.

Of course, the rider will automatically, consciously or not, convert much of his or her mass to “sprung weight” by absorbing some of the bouncing, primarily through bending the elbows, and, when it gets really bumpy, by standing up on the pedals and absorbing with the knees. Further, the shaking and vibrating of muscles, skin, and even organs further damps the motion.

Absorbing bumps in the body saps energy from the rider that could be used to propel the bike, resulting in early fatigue. And the energy to vertically lift the bike and the body is converted directly from the energy that the rider and bike had going forward.

The trick to riding fast with minimal energy expenditure over bumpy surfaces is to have as much of the mass as possible of the bike and rider become sprung weight, meaning it is supported by springs and goes up and down less than the height of the bump or, ideally, not at all. In this case, the “spring” we are talking about is the tire itself.

The beauty of using a tire as suspension is that the unsprung weight can be very low. If the pressure is low enough and the tire large enough, the entire bump can be absorbed by the tire, so that the rim does not go up at all. In this case, the unsprung weight — the mass that goes up the height of the bump — is only the amount of mass in the tire (and inner tube, if present) that is deflected as it rolls over the bump. This is a lot less weight than, say, the front wheel, tire, and outer legs of a suspension fork that might otherwise have been recruited to deal with this particular bump.

Suspension members built into the head tubes, seat tubes, and/or top tubes of some bikes for Paris-Roubaix assume that the tire won’t absorb the entirety of impacts with cobblestones. Those suspension pieces are designed to isolate the rider from the portion of the bump force that does come up to through the fork or seat tube — to make only the bike be unsprung weight and the rider be (ideally) sprung weight.

As contrasted to the downward-trending curve of rolling resistance vs. tire pressure on a smooth test drum in a lab with a static mass on the wheel, the curve of rolling resistance vs. tire pressure on a bumpy surface with a human atop it instead trends upward after a certain pressure is reached. At some point, higher tire pressure = higher rolling resistance.

As the tire pressure increases, the amount of unsprung weight — the weight that is deflected on each bump — increases, as does how high that unsprung weight is lifted. Lifting weight takes energy. So does flexing of the tire tread and carcass; that results in much greater hysteretic energy losses than on a smooth surface.

And, more important than either the lifting or the additional tire-flexing losses, there are energy losses resulting from energy being transmitted into the “floppy human” and dissipating as heat.

Our 2018 test of riding on a set of rollers with an 8mm-tall ridge welded to two of the three rollers showed thousands of vibrations of around one “g” going into the rider’s seatpost per minute, with the magnitude increasing with higher tire pressure. The U.S. Army has studies that quantify the amount of energy absorbed by vibrating humans, and it can be quite a bit.

Cycling Analytics founder Tom Anhalt Silca CEO and Josh Poertner are engineers who think and publish frequently about rolling resistance. They refer to the increasing energy loss with increasing pressure in the tire as “impedance.”

Rolling over the cobblestones of Paris-Roubaix, the base rolling-resistance characteristics of the tire seen on a smooth test drum are still there; however, impedance plays a much bigger role than it does on smooth pavement. There, the higher the pressure (and the rougher the road), then the more rolling resistance you will have.

The rougher the surface, the more quickly the rolling resistance line will transition from sloping downwards to sloping back up. On polished glass, the rolling resistance slope would continue downwards with more pressure, approaching zero. On smooth pavement, the rolling resistance slope would trend downwards for longer before the inflection point; on cobblestones, the rolling resistance line would change much sooner and bank up more steeply, with increasing pressure.

Why we use Wheel Energy

We don’t see this characteristic of the impedance causing the curve to turn upward after a certain pressure in a lab that does not have a way to simulate the squishy human riding the bike and damping its vertical motion.

Wheel Energy is the world leader among independent rolling-resistance testing labs by virtue of the huge diameter of its test drum that the tire rolls on, the method by which it measures energy lost in the tire that is rolling on the drum, and the way the load is applied on the spinning wheel. For this test, Wheel Energy used a cobblestone-simulating surface for the drum that the tires could bounce along over.

In this test, after an initial 15-minute warmup period at 2 bar, Wheel Energy increased the pressure in each tire by half a bar (7psi) after each test run and then ran the test again (and only recorded data after an additional 5 minutes of warmup after each pressure change). The technician repeated this process with each tire until reaching a point where the readings of energy consumed through rolling resistance reversed their downward trend and started going back up.

Wheel Energy is not the only independent lab testing bicycle-tire rolling resistance. Its funding comes from those who pay for running a test, and the data belongs to whoever paid for the test to do with as they see fit (and we are publishing our data).

Another rolling-resistance lab publishes bicycle tire rolling-resistance data and funds itself by selling subscriptions to more detailed results. It uses a smaller-diameter test drum than Wheel Energy’s. The smaller drum pushes more deeply into the tire, thus less faithfully simulating rolling on a road. Perhaps more importantly, the load on the wheel consists simply of a dead mass on a vertical slider sitting on the wheel-holding apparatus. This latter item is easily overlooked and deserves particular attention.

When riding over a rough surface, the human body is not like a pile of weights sitting on the saddle and handlebar, waiting to be slammed around on every bump. Rather, the arms, legs, and entire body damp a lot of the bike’s vertical acceleration, thus converting some of the kinetic energy of the bike and rider into heating of the rider’s body. This is one reason why cyclocross riders stay warm while racing in shorts at low wintertime temperatures.

Wheel Energy’s apparatus attempts to approximate this damping with a pneumatic shock absorber with which it pulls down on the frame holding the wheel axle, thus converting some of the wheel’s kinetic energy into heating the shock. By contrast, the dead mass on the slider in the subscription lab’s apparatus has so little hysteresis (it doesn’t get heated up much since there is so little damping in the system) that the tire’s hysteresis will completely overshadow it no matter the tire pressure; results from such an apparatus will show continually decreasing Crr (coefficient of rolling resistance) with increasing tire pressure; it won’t show the increase in Crr due to impedance.

By increasing air pressure in this test until rolling resistance started to increase, Wheel Energy identified the inflection point, within a half bar of tire pressure, where the smooth-rolling and impedance curves cross. The difference in the energy lost over time due to rolling resistance, measured in watts, before and after the inflection point, however, varied from 0.1 to 2.2 watts. After the inflection point where those lines cross, the rolling resistance will increase rapidly on, say, cobblestones, since the impedance graph’s slope is steeper the rougher the surface. Thus, it makes sense for a rider to race at the inflation pressure in the tire that is just before its rolling resistance trends back up rather than just after.

If we accurately measure the hysteresis in the tire while simulating the hysteresis of the human body, we can determine with a high level of confidence a good estimate of how much extra power a rider has to produce to overcome a tire’s rolling resistance.

Interested to see the results of our Roubaix and gravel tire tests? The detailed findings are available to all VeloNews and Outside+ members.