Two Wheel Blogs

Someone struck up a conversation with me last night during a ride about this post. They were inquiring about power savings with tire set up X versus tire set up Y, or Z, or...

I hesitate to discuss this much since I do not even have tools for measuring anything performance wise on my bike. I report based on feel which is ridiculous for any power/resistance discussion. I do want to discuss a few concepts that may help some understand the difficulty of quantifying this stuff even with special equipment…and why I think even that special equipment may fail to some degree.

For the record, tires are REALLY complex creatures and are often poorly understood in terms of power consumption. That is right, your tires are holding you back...and you really *do* want that!...at least in some regards. Traction is necessary to keep you on the road and in control. If nothing else is clear, we should all understand that some power needs to be diverted towards getting traction and staying safe.

There are three main avenues for tires to consume power: Hysteresis which is basically the rubber flexing to form a contact patch with the road surface (and you want that!), friction which is basically how rough the surface that the tire is rolling on is, and wind resistance which is basically how much air the tire has to push out of the way.

Hysteresis loss
Hysteresis loss results from tires going through their continuous deformation and recovery cycles. This loss is energy being converted to heat. Hysteresis loss is the one thing that is tested most in labs. If you see a report of rolling resistance for a tire (commonly called "Rolling Resistance Coefficient" or Crr), there is a strong chance that the data was collected in a lab with a tire rolling on a large steel drum and basically equates to hysteresis loss (because they have removed the other variables of friction and wind). Therefore all this data is prefaced on the assumption that the tire will contact a nearly perfectly smooth and hard surface. It is not very "real world" but it is pretty hard to nail down other variables so it is usually the only data we see posted.

Hysteresis loss can be reduced by using materials that are highly resilient. This is done in tires through the clever use of rubber compounds and certain types of threads in the fabric of the tire. In general the thicker the tread, the higher the hysteresis loss, but it is this tread that helps tire "stick" to the road. Most of us like to have a decent amount of tread rubber to help the tire last longer.

Friction
Friction is fairly straight forward, but highly variable. It is simple enough to envision a tire experiencing low rolling resistance on a wooden velodrome where the friction is low, and a tire that has greater resistance to rolling on a broomed concrete surface. In my world, I want a tire that does fairly well on both extremes. Is that possible or practical?

Wind Resistance
Dimensions are the biggest and usually the only factor impacting wind resistance on a bicycle tire. A 700c tire that measures 23mm wide is going to have about the same wind drag as any other tire this size and width...so it is not pertinent to most conversations discussing resistance from tires...though it should be considered because narrow tire may not be best for everyone, but very wide tires may have a rider suffering unnecessarily.

So, what if we minimize hysteresis loss (discussed above) by tactics such as minimizing tread thickness, compounding the tread for minimal hysteresis loss, using very resilient materials (perhaps steel) and extremely high inflation pressures? A tire of this sort would score well on the drum test by having a hysteresis loss of almost nil, but when subjected to a rough concrete surface the tire deflects wildly (oops, this is diverging into another duty of tires…suspension).

I have seen data that suggest that the testing on smooth surface yields results similar to testing on a rough surface (i.e. the set up with the lowest Crr on a smooth drum will tend to produce the lowest Crr on a rough drum), but Crr may not be the only thing to consider. Most riders can tell you the importance of fatigue from lack of suspension (be it springs on a MTB or cushion from a supple tire).

As you may infer, I hope to reduce hysteresis loss through the use of a more resilient (latex) tube in the tire. I can not do much about friction and wind losses, but if I can make a tire (that I like) more resilient without doing undesirable things like cranking up the operating pressure, or shaving off the tread, I will be happier. Can I really hope to improve the “suspension” effect that a tire offers with a latex tube? That is *really* the only thing I will be able to report on…and I have no idea how anyone could meaningfully quantify that concept.