Technical FAQ: Searching for the ideal tire width
Over the last couple of years, there’s been a lot of talk about tire width. It seems nowadays the 25mm width has become the de facto standard. While I understand the concept physically, I’ve only read qualitative articles. These articles don’t shed much light on the entire subject.
I have not read one article which correlates road surface conditions vs. tire width vs. rider weight. Can you shed some light on this? If you weigh so much and the roads are in such and such condition, you should be on an XXmm tire.
I have noticed in the Euro pro ranks you’ll see some riders on different width tires, depending on conditions. Apparently, so it appears, they know more about the subject.
While it may be that 25mm, or (?), is the best compromise, I have not really seen this question broached.
Both tire width and tire pressure (as well as tire construction) should ideally be optimized to minimize rolling friction on a particular road surface and under a certain rider weight.
I continue to do a lot of rolling-resistance testing of tires of various sizes and at various pressures on a variety of surfaces. It is a long-term goal of mine to come up with a complete and quantitative answer to your question, namely a table that a rider could use to select tire width and pressure (and construction) based on the conditions. Over time, I will publish more and more of these tests in VeloNews. A big issue with producing a reference table is that some parameters are not as quantifiable as rider weight or tire width or pressure, like categorizing a road surface or tire construction method so everybody is in agreement about what it means.
You have likely seen my recent gravel road tire test looking at rolling resistance as a function of tire pressure. Comparing the test results on the gravel road with those on a roller, both with and without a bump welded on it, points out the need to do it in the real world. Testing a tire on a roller is simply not the same as on the road — the deflection into the tire is deeper on a roller than with the same bike and rider on a road.
Why does the depth of tire deflection matter? It’s because of hysteresis, or internal friction in the rubber compound.
Consider a graph of the extension of a rubber compound versus the force stretching it. As the rubber is loaded under increasing force, the curve starts out steep and then levels off, meaning the rubber initially holds its shape but then gives in to increased force. When the load is released, the rubber again tries to hold its shape. It is initially slow to respond and then rapidly returns as the last bits of the load are removed.
This lag in the deflection response is called hysteresis, and it results in a graph that looks like convex lens tilted up to the right. The amount of energy loss in internal friction is equal to the area in the center of this “hysteresis loop.” The faster the tread compound, the smaller the hysteresis loop and the faster the tire. Some deflection is necessary for traction and control, as well as for reduced sprung weight on rough surfaces. All tires lose some energy in going from being deflected by the surface to not being deflected as the tire rolls.
When it comes to internal friction, wider tires have shorter contact patches than do narrower tires, and a wider, shorter contact patch will have less vertical depth of deflection (resulting in lower internal friction/hysteresis loss). This is because, if the tire pressure is the same, the area of the contact patch must be the same to support the same load (a certain number of pounds of rider and bike weight spread over a given number of square inches of contact patch must, by definition, equal the pounds per square inch of pressure measured in the tire when the rider is sitting on it).
Since both the load on the tire and the pounds per square inch inside it remain constant, the area in contact with the surface will also be the same. The wider the tire, the wider the contact patch, so to maintain the same area as the contact patch of the narrower tire, the contact patch for the wider tire must be shorter. This means the deflection into the tire is less, and, hence, the hysteresis loss of energy to internal friction is less with the wider tire than the narrower tire at the same pressure. This is why wide tires can have lower rolling resistance than narrower ones.
When testing tire rolling resistance on a roller, the smaller the roller diameter, the deeper the deflection into the tire, and hence the greater the hysteresis loss.
Reducing the pressure in a tire literally means reducing the number of pounds of pressure on each square inch of the tire. Since, with the same rider and bike, the tire supports the same load at lower pressure, the contact patch will be larger. For example, Wheel Energy Oy in Finland measured the footprint area of a 700x23mm tire inflated to 112 psi with 50kg of weight on it to be 75mm long and 15mm wide. The footprint of the same tire with the same weight on it, but inflated to 84 psi, became 82mm long and 14mm wide. The longer contact patch means that the tire deflection will be deeper, resulting in more internal friction and hysteresis within the tire’s layers. That makes the case that higher pressure reduces rolling resistance — but only on a smooth surface.
On a rough surface, the hysteresis loss due to the larger contact patch is counterbalanced by the fact that deflecting the entire bike and rider on each bump costs more energy than does absorbing the bump into the tire. This is because much less mass is moved up and down if the tire can absorb the bump, rather than the entire bike and rider being lifted. This is the sprung weight vs. unsprung weight argument that explains why vehicles with suspension are faster on rough surfaces than ones without suspension.
These articles shed some more light on this discussion:
Thanks to you and your writing on this subject, both the professional and amateur pelotons have gone to wider and wider tires, never to go back. Each time I’ve gone up in width, I’ve been sure there would be some tradeoff, and each time it’s been like something for nothing. I’m maxed out on my current ride, so I’ll be folding my hand at 28/30mm, but that brings up a question. There must be a limit to how big one can go before efficiency finally drops — where a bigger tire finally just does not make sense. Any guess where that limit might be, for the usual combination of smooth asphalt, chip seal, and rough rural roads?
You’re obviously correct, but I can’t answer that yet. I keep doing tire rolling resistance tests. One of these days, I’ll have an answer.