So what exactly goes into building an ultra light recumbent bicycle? The soul of the bike is the main tube, and ours is light, stiff and unbreakable!
There are several ways to determine the ultimate strength of the tube you are using for your bicycle. Federal regulations give minimal requirements, but they are just that, minimal… Some companies will use cycle testing, a process that involves loading the bike thousands and thousands of times with a given weight. When I think of this, I think of the strength tests they use on IKEA chairs! Another way to do it is with a three-point break test where two ends of the tube are held in place and a lever/weight system is used to determine the static weight at which the tube breaks.
When Dave Karcher first started building recumbents in 2002, he used an off-the-shelf tube for the main tube. He measured the static breaking weight of approximately 1500 pounds. These first few bikes rode well if the rider was under 180 pounds, but tended to get whippy with heavier riders. So he set out to design a better tube.
The results of multiple lay-ups and testing resulted in the tube that was used in all of his remaining bikes and the first bikes that we built. A beautiful filament-wound tube, this tube broke consistently at 1860 pounds of weight, and had a weight limit of 230 pounds. Many riders are still enjoying these bikes today – they provide a comfortable, compliant ride for heavier riders, and a stiff but damped ride for lighter ones.
Within the first few months of acquiring Carbent, we lost the supplier that Dave had been using for filament-wound tubes. In our search for a new vendor, we found Carl at Edge (now Enve) Composites. He was the engineer who originally worked with Dave to formulate the filament-wound tube, and he was delighted to be working with us again to build a better bike!
After several iterations and tests, we ended up with the lay up pattern that we use to this day. Our roll-wrapped tube consists of 13 layers of prepreg unidirectional carbon fiber. Break tests have shown our tube to be unbreakable! Using the same fixtures, we were able to load it up to 3500 pounds before the jig actually broke! We have never been able to break one of these tubes! Amazing!
Our most recent tubes feature a layer of twill woven carbon fiber over the top. It just looks great and it is also a great weave to resist zippering tears if the bike gets dinged.
A tube is not just a tube, especially when it is a Carbent…