Quite often we at Air Free get asked as to whether or not we have any technical data on our tires. Starting in February of 2001, we contracted with a firm in England to begin testing our tires so that we may improve the design and performance. Since our target market is the avid cyclist, the performance needs of that demo are more acute than the recreational/occasional cyclist. As more data becomes available we will make it available here. In this special section we will attempt to explain the testing procedures and interpret the results.

The chart to the right represents a sample of popular high end rubber tires which have been tested. All tires, rubber and urethane ones are tested exactly alike so that the results are relative to one another. The columns displayed are the Tire Name, Tire Size, psi and crr. The Tire Name and Tire Size are self explanatory. The psi of the rubber tires given are at the point where the tire had the lowest rolling resistance except for the Air Free tires. The crr is the technical term for the coefficient of rolling resistance and the lower the number the better. 
HR - is our High Resilient Formula
Std - is our Standard Foam

The above chart shows that the crr of the LeMans Sprint is on par with the high quality rubber tires. Even though some rubber tires have lower rolling resistance a good many more have higher rolling resistance. One should not conclude that one tire is better than the other simply because of rolling resistance crr numbers as the watts required to propel the bike are very close to one another and can be seen on the watts chart. Other factors affecting performance are discussed below and in the 'Ask Hugh' section. 

The High Resilient material allows us to experiment with differing shapes.

The tire we are using for our example is the Air Free LeMans Sprint made with our High Resilient material at about 45-50psi and the tire was loaded with a 112 pound weight. We can see that the unmodified tire (171-A) has a crr value of .0118 which is very good for a tire of such low psi. The next step was to shave off the center groove which resulted in a wider contact patch and the crr decreased. The lowest crr achieved came from tire 171-C. Tire no. 171-D had its' crr value increase by altering the shape to a point. This caused an increase in the hysterisis (inefficient movement) which doubled the rolling resistance. The last tire 171-E was virtually even with 171-C but has a better footprint. When we examine the footprint of a tire we are looking to see if an oval emerges. This oval would show that more of the load is being distributed across the tire which decreases the hysterisis and improves performance. It is not unusual to see wider tires out perform thinner ones in the crr value.

For example, the tire to the right is a Michelin 20" tire that is 44mm wide and recorded the lowest rolling resistance ever by our contracted lab. Notice that the oval is much more pronounced at 50psi than it is at 90psi. Just by looking at these two foot prints, one could easily assume that the 50psi is much faster than the 90psi one. Our future designs will incorporate an oval foot print by widening the tire and making it lower in profile. With a slight curvature in the middle, the more weight that is applied, the wider the foot print will be.

The second aspect of a tires performance as it relates to speed is how responsive it is. We have seen that low psi tires can have better rolling resistance, such as when coasting, but in a sprint or hill climb situation, lower psi tires will sap energy. Looking at the chart to left, the red line is an Air Free tire made with our standard foam. Notice that it doesn't fully recover after the last set of weight is taken off of the tire. Even though the crr of that same tire is better than some rubber tires, its inability to fully recover has led to assertions that airless tires can be sluggish. On the other hand, our tire made with the High Resilient material at 45 psi very closely resembles the deflection rates of high pressure rubber tires as seen by the yellow and blue lines. 

The Michelin tire tested is fairly representative of the high quality rubber tires in that high quality rubber tires for the most part, will want to fully come back to the original position. However the deflection rates increase sharply when lowering the psi of rubber tires. The more the tire travels back and forth, the more energy is required to overcome this and is the reason why avid cyclists would keep higher psi in their tires. While the Air Free HR tire has technically a slightly higher rolling resistance value, it is plain to see that when stressing or loading the tire, it will be as easy to propel as the Michelin racing tire at a much higher psi. This bodes well for us because this means as we increase the density of the HR material, the crr value goes down and the deflection curve will show that the HR material will exceed the rubber tire in its ability to totally reconform itself after loading. These deflections are measured by taking predetermined weights and placing them in on the tire in increasing increments and then removing the weights and measuring the deflections each step of the way. To see a partial list of tires tested,  please click here