At the turn of the century, we published an article about rolling resistance on bicycles works. The article points out three main things:
- Rolling resistance is a valuable enough performance variable that you should pay attention to it.
- Narrower and higher pressure tires are not always better.
- Suspension and compliance help reduce rolling resistance.
While we have yet to see a lot of emphasis on the benefits of suspending the rider when it comes to rolling resistance, there is a lot of focus on wider tires and lower tire pressures in the past few years. With this in mind, revisiting the topic of that old article seems worthwhile.
Bicycle Tire Rolling Resistance Summary:
For those interested in a short read, here is the synopsis:
Rolling resistance is affected by friction caused by the weight of the vehicle (bike and rider) and how much of that weight has to be absorbed by the tires while riding. Bicycle weight, tire tread, tire width, compliance/construction of casing, tire pressure, the texture of the surface, and the vertical compliance of the components and frame are all variable. If you are currently riding equipment that has a high level of rolling resistance (stiff, high flat protection tires for example) and you average 20 mph over 100 miles, you can cut 3-4 minutes off your time by minimizing your rolling resistance. If you average 15 mph, you can cut 4 to 5 minutes off. All the small pieces add up to the whole…
In order to minimize rolling resistance, consider how vertically compliant and supple the products you are using are and how rough the road surfaces you ride are. Use high quality tires with supple sidewalls (usually this means a higher thread count) and do not over-inflate tires. Almost all tires should be run well below their maximum recommended psi. If you are interested in finding a good starting pressure, Silca’s Tire Pressure Guide or SRAM’s Tire Pressure Guide can be good references.
Especially if you ride particularly rough pavement or a lot of gravel, look to maximize vertical compliance through more compliant frame designs or with suspension. Compliant frames effectively reduce the amount of weight on the tires and lower the rolling resistance. The goal is to find the balance between torsional stiffness and vertical compliance for your weight and use. Of course, always make sure that any bike you are considering fits you properly in advance of buying it.
What is Rolling Resistance on a Bicycle?
Rolling resistance is the amount of energy required to overcome the friction between the road and tire. It sounds simple, but what effects it and how it works defies common thought. The key to understanding rolling resistance is to understand that it is determined less by size of tire contact patch than consistency of tire contact patch. Many variables from the bicycle’s tire pressure, tire width and tire construction, to its weight, to how its frame design affects sprung vs. unsprung weight all play a part.
We’ll discuss each variable individually, from less complicated to more. Just keep in mind that the real thing to understand from all this is that consistency of tire contact patch is what really counts in minimizing rolling resistance. The methods of how to keep the tire contact patch as consistent as possible is where it can become difficult to understand.
Tire Pressure, Width and Construction:
Unless you ride on very smooth roads (think Velodrome…), narrower tires and higher tire pressures are not always better. In fact, the rougher the surface you ride, the more important wider tires and lower pressures become for rolling resistance. Here is why:
A narrower tire casing compresses and deforms under load (rider weight, adapting to road imperfections….) more than a wider tire. With a narrower tire, there is simply less surface contact area to absorb the shock from the road and thus will deform more than a wider tire. Like most anything, spreading the impact across a greater area reduces the effect of the overall impact. This is a big part of why 29″ MTB wheels roll over things better than 26″. Likewise, 700c wheels with their longer contact patch will have lower rolling resistance than their 650c counterparts. More surface area results in more absorption with less deformity. When it comes to rolling resistance, you should pick a tire based upon the quality of the casing and its ability to maintain its shape and choose other components based upon their ability to absorb shock so that the tire doesn’t have to.
How Does Bicycle and Rider Weight Factor into Rolling Resistance?
Weight’s relationship to rolling resistance is indirect. On two completely rigid vehicles, the lighter vehicle (bicycle and rider) will have less rolling resistance because it will not put as much pressure on the tires as the heavier vehicle (bicycle and rider) and thus will be easier to lift up and over variances. However, vertical compliance in the wheels and frame changes this completely. The only way to explain how this works is by describing the somewhat complex difference between sprung and unsprung weight, which is found below.
Sprung vs. Unsprung Weight on a Bicycle:
For those looking to minimize rolling resistance, and understand how a vehicle reacts to the ground beneath it, the vehicle’s weight needs to be broken down into sprung and unsprung weight. Over twenty years ago, I wrote a description of how sprung and unsprung weight works, so I hope you won’t mind me plagiarizing from myself in an attempt to explain what is not an easy concept.
“…resistance on a bicycle is determined by how much energy is required for it to move over the road. Even fresh pavement is riddled with surface imperfections that slow a bicycle down. Without suspension (vertical compliance), both the rider’s and the bicycle’s weight (an average of 175 pounds for both) is ‘unsprung’ and must be lifted up and over these imperfections for the vehicle to move forward. With suspension, the majority of the weight is ‘sprung’ and imperfections are absorbed by the suspension. On a ‘sprung’ vehicle, only the unsuspended portion (wheel and lower frame) and a small amount of the rider’s weight needs to be lifted (about 35 pounds for both). It takes far less energy to lift 35 pounds than 175. Thus, to the road, a suspended vehicle feels significantly lighter than an unsuspended vehicle and will have less rolling resistance.
‘Sprung’ weight also directly reduces tire rolling resistance by keeping the tire contact patch more consistent. Tire rolling resistance is not as much about tire width or tire pressure as it is about consistency of tire contact patch. The more consistent the tire’s contact patch is with the road, the less rolling resistance the vehicle will have. Without suspension there is less vertical compliance and the majority of the vehicle’s weight is ‘unsprung’ and the road imperfections must be absorbed by the rider and tires. Therefore, the vehicle will be slowed as the tire deflects and deforms in an attempt to absorb the shock. Suspension, on the other hand, increases the portion of ‘sprung’ weight the vehicle has. By redirecting the load into the suspension system, the tires are kept from having to deflect as much. The more consistent the contact patch, the lower the rolling resistance and the less energy the rider will have to use to overcome the resistance.”
Big Picture: What Really Matters When It Comes to Rolling Resistance on a Bicycle?
More vertically compliant frames (especially suspended frames), wheels and components have lower rolling resistance than less vertically compliant frames, wheels and components. The bottom line is that you need to know a lot if you really want to analyze the finite details of rolling resistance on a bicycle. The terrain you are riding, your weight and riding style, your rim width (which changes your tire profile), your frame construction, tire size, casing construction, tire type (tubeless, tubed, tubular) are all variables.
This being said, taking one step back, if you think in more general terms (the rougher the roads, the more lower pressure and tire width benefit), addressing rolling resistance well on a bicycle isn’t too hard and is well worth doing. If you are riding on New England pavement and can put a 28-30mm tire on your road bike, you should likely do so. If you are riding gravel, it really depends on how rough it gets.
Contact us if you would like specific recommendations for your use and goals.