Air Free tires are designed to fit snugly into your rims. This means that the construction of the airless tire has several constant stretch and compression ratios. Unlike rubber tires, the Air Free can be stretched on to larger diameter wheels but will not work if the rim diameter is too small.

Figure 1 shows the internal safety cords and govern the diameter wheel or rim that the airless tire can fit. These cords are made from nylon and have a pre-programmed stretch factor. Our tire calculator takes this into account so when you measure your rim, you could be off by a little bit due to the bow of the spokes, but the calculator takes care of that for you. With a 5% stretch present, it is possible to install a 700c Air Free tire on a 27" wheel which is 8mm larger in diameter.

Since the cords only allow for a small stretch, usually a tool is required to leverage them into place. Once the cords have been stretch their allotted amount, they will not stretch any further. With over 10 million wheelchair tires made annually in this fashion, you have our assurances on this. The image to the right represents a typical 20" (iso-406) double wall rim and an Air Free tire The actual diameter of the rim is 420mm or about 16 1/2". The inside portion of the Air Free where it would come into contact with the top of the rim walls is sized smaller to allow for the pre-programmed stretch. When sizing your wheels or rims don't be alarmed if the calculator tells you a different size tire may fit your rim when you were looking for something else. For example you need a 27" tire but our calculator says that our 700c tire can fit the dimensions you entered. If our calculator says it will fit, then you can take that to the bank.
 

The width of your rim plays an important part in determining which Air Free tire will fit your rim. What follows may be a bit too technical information but it does explain how part of our tire calculator works and may assist you in getting the best possible fit.

Figure 2 shows how the tire is mechanically affixed to the rim. The hook beads present on the rim to prevent a rubber tire from rolling off do the same thing for the airless tire. Our urethane will allow the tire to be squeezed into a rim up to 15% thinner than the actual width of the portion of the tire that must be fitted into the rim. Our calculator takes that into account so if your rim exceeds the 15% compression allowed by the tire, then you would have a difficult time getting the tire into the rim. 

The opposite is true if the rim is wider than the tire portion that has to snap into the rim and would cause the airless tire to be unstable. This is why airless tires just can't fit all rims. If we don't have a tire to fit your rim now because of width, we will soon.
The diagram to right clearly shows what we would consider to be a very good fit. The tire would compress approximately 7% of its' width into the rim and would result in a very snug fit. The delta area or the portion below the hook beads is compressed 7% as well. The tire shown is the Open Road A and is designed to fit rims between 16 and 17.8mm wide. 
If you are going to get your tires customized to a higher psi, the denser tire would then not be able to compress as much. Our standard density which imitates 50-60 psi can compress up to 15%. Tires with a psi equivalency of 80-100 can compress 10% and tires with a psi of above 100 can compress only 8%. Our high resilient material means just that and does not take kindly to being squeezed into a channel or rim that is truly too small. 

To determine if the higher psi tire will work in your rim just multiply the bead width of our tire by 10 or 8% and subtract that from the bead width and if your rim falls between those two values, then you will have a fit. For example; you have a rim that measures 17mm wide. This is your Minimum Bead Width allowed. This means any Air Free tire with a bead width of less than 17mm would fit loosely and would flop around.

Take your rim width of 17mm and times that by 15% and add it to 17mm and you will have your Maximum Bead Width allowed, which would equal 19.5mm. Any of our tires that are between 17mm and 19.5mm wide at the bead width would fit nicely. This is what our calculator does for you. Now for a higher psi you would use either 8% or 10%.

Your rim width x 8% (100 psi plus) or 10% (80-100 psi) or 15% (50-80 psi) plus your rim width equals the min/max bead with that an airless tire will be able to successfully seat itself into your rim.

The depth of your rim is another important aspect of proper tire fit. Many rims today are much shallower than they were even five years ago. Measuring the interior depth of rim is sometimes difficult but it should be done.

Figure 3 illustrates the depth issue of the airless tire. Because the airless tire relies on a height/width aspect as it relates to the rim to prevent it from becoming leveraged off, it is very important that the bottom of the tire does not come into contact with the bottom of the rim. If your rim is too shallow for our tolerances, you would not be able to install one on your wheel as the balance or leverage point would be altered and the leverage needed to wrest the tire off of the rim is substantially decreased.
When measuring your rim, get to the deepest part of the rim or a the top of a spoke nipple. The extrusion points on either side of the rim are not important as the airless tire has been beveled inwards to avoid them.

The illustration to the right shows the tire depth in this example to be 7.5 mm deep and the rim that it is to fit is10.4mm deep. This means that the tire would not come into contact with the bottom of the rim and the tire in essence is suspended by the tops of the rim walls. The hook bead height as shown does vary from manufacturer to manufacturer but all Air Free tires have a corresponding notch large enough to accept them all. 

As with the width, the depth of the tire is important to height/width aspect ratio because when the tire is inserted into the rim, the compression of the urethane against the inside of the rim walls causes the urethane to want to expand back out wards, just like a rubber tire that has been inflated with air. There does need to be a certain amount 'meat' or 'filler' in the rim to resist the effects of leverage caused by cornering.

Now the tire is properly fitted to the rim and will be snug and not come off and give years or thousands of miles of reliable consistent service. How long the tire will last for you will depend on your style of riding, weight and road conditions.

The picture to the right shows a properly fitted tire and rim. Of course not all rims will look like the Alex DA 16 shown here but all rims will have similar features. There will be a small flat area where the tire will rest on top of the rim shoulders. There will also be an interior cavity where a portion of the tire must fit and to a degree hold it in place. but the main safety feature is the interior cords present inside all airless tires which once leveraged into place will not become elasticized. 

To summarize, the airless tire needs to be matched up to an acceptable range of diameters. The tire will only compress a predetermined amount inside the rim walls and the tire should not come into contact with the bottom of the rim. By browsing our database of rims or using our calculator, you will more than likely find an Air Free tire waiting for you.

Hopefully our technical explanations have helped. If you would like more information about airless tires, please browse the website as there is a lot more information than meets the eye.

Please tell us what you think about this section and how we might make better use of the English language to explain the technical aspects.