One of the things velomobile riders fear most is a high speed roll over. Anything that causes the rear tire to lose grip at high speed can lead to a roll over. Riding with the rear tire on a roadside rumble strip can induce a roll over. Here’s an example of that:
Another common cause of a velomobile roll over is a rear tire flat. It seems to be even worse if the tire breaks free of the rim. Vittoria has come out with an interesting product, called the Airliner, that could bring a new level of safety to the rear flat situation. The Airliner is a light weight foam ring inserted into a tubeless tire/rim that compresses to a fraction of its volume when the tire is inflated, then expands to fill the tire if the air escapes the tire. Under these circumstances, the tire behaves as though it has a pressure of 25-30 psi. This should be enough to keep the tire on the rim. The fact that it is compresses out of the way when the tire is inflated makes the tire behave like a normal tubeless tire. You can read about it here… or watch this video.
I decided to give Airliner a try. This is my first time working with tubeless technology. The tubeless wheel consists of a tire with a special bead that mates to a rim with a compatible tubeless profile. The rim is taped with a special tape. Also a special valve stem is required. After all of that is put together a small volume of a sealant liquid is introduced into the air space of the tire. Once the tire is seated on the rim, the bead locks into the rim’s hook profile, making it very difficult to separate the tire from the rim. There’s a good overview of tubeless technology here…
Rear Wheel Build No. 1
Neither of my velomobiles had a rear wheel with the required tubeless rim so I set out to build one. I found a source for a new DT Swiss 559 tubeless rim on eBay. I had a conventional rear hub and a wide selection of spokes in the drawer. Since the Milan accepts a conventional rear hub, it was chosen to be the test mule.
This first wheel didn’t work out as I had hoped. I built the wheel with the DT Swiss tubeless rim and mounted the Schwalbe Pro One tubeless tire containing the Airliner. Surprisingly, the Airliner Road version fit the 559 tire without cutting it even though it’s sized for a 700c wheel. I applied soapy water to the bead before inflating it with an air compressor. The tire seated on the rim and held air just fine. However, when I then deflated the tire, simulating a flat, the tire’s bead separated from the rim, making it possible for the tire to escape the rim. This combination of tubeless tire and rim were not going to work.
Rear Wheel Build No. 2
Next I bought a 559 Alex tubeless rim on line at Universal Cycles. I disassembled the wheel with the DT Swiss rim and re-assembled it using the Alex rim. I mounted the same tubeless tire with the Airliner and soapy water and filled it using the compressor. The tire’s bead seated perfectly on the rim. This time the tire remained firmly locked to the rim when I deflated the tire. So I now felt confident enough to inject about 30cc of liquid Vittoria sealant through the valve stem with the valve core removed.
Testing the Airliner at zero pressure
I inflated the tire to 85 psi then mounted the wheel in the Milan and set off for a ride. At this pressure, the Milan’s handling felt normal. I stopped near a bike path and deflated the tire. The Airliner expanded within the tire to give it enough shape that the rim didn’t contact the pavement. So far so good. I set off for a ride with zero pressure. Vittoria says that the Airliner alone should provide the equivalent of 25-30 psi of air pressure. That’s what it felt like to me. The rear end of the Milan felt a bit squishy of course but it was manageable. I rode for about a mile on a combination of smooth bike path and rough pavement on an adjacent road. I kept the speed below 15 mph. I took some turns at a reduced but reasonable speed just to complete the test. The tire remained locked to the rim as I had hoped. I called that a big win.
Testing the Airliner’s Effect on Performance
There’s a hill near my house that I have used to do coast down tests to measure maximum speed with an onboard wired cycle computer. I do these tests to compare the effects of modifications and adjustments that I make to the velomobiles. I was curious to see if the presence of the Airliner would adversely effect the maximum speed of the Milan. I did a couple of runs with the original Pro One/latex tube wheel that I normally use and a couple of runs with the Pro One/Airliner wheel. All of the runs showed the same maximum speed within a few tenths of a mile per hour.
The Airliner performed as advertised once I got the combination of rim and tire correct. The fact that I was able to ride a mile on zero air pressure in the rear tire without the tire leaving the rim convinced me that the Airliner will work in the event of a normal flat. I’m also convinced that there is no performance penalty to pay running the Airliner.
On the negative side… I wasn’t able to simulate a blowout so I don’t know how well the Airliner will perform in that case. Another potential concern is that I will have a difficult, almost impossible, task of removing the tire from the rim to repair a flat or replace a damaged tire while on the road.
I’m convinced that the extra precaution against a roll over makes the use of the Airliner worth while. I will leave it on the Milan and build a tubeless rear wheel for the DF to run an airliner on it.