Originally Posted by
Leisesturm
I would too. That is one sweet bike. 406 rear wheel would NOT improve aero! Only if YOU weren't there, but you are, and the rear wheel is in your wind shadow so the full size rear wheel is not an issue and your gearing and tire choices improve. Where do you find oval chainrings bigger than 50T? How much do they cost? Short cranks are for if you have a problem with heel strike against the front wheel which should not be the case with a 406 front wheel. IF you shorten the cranks then you should also lower the gearing, most people don't do that. They just use 155mm cranks with the same gears that the rest of us use 170mm cranks to turn and then wonder why their knees still hurt, or why they are still so slow. BTW a standard bike can be out of UCI compliance for various reasons. A recumbent isn't UCI compliant. Period. Once you are on a recumbent you are dead to the UCI. Most of us don't care. The Stringbike drivetrain still has rotary motion of the cranks. It has all the disadvantages of being way more complex and fragile than a bicycle drivetrain needs to be, heavier too, probably. But that is just first impressions. It looks interesting. I am going to study it some more. Linear pedal motion is useful if you need your foot motion to stay inside the confines of a fairing. Consider however: year after year, the fastest recumbent in the world is what amounts to a drop dead simple Easy Racer (LWB) recumbent inside a carbon fiber shell. You wouldn't kill yourself if you tried to ride it around town but if you fell over for whatever reason getting back up again would not be quick, or easy. But have you seen a
Peregrin on Birk streamliner? More information
available here. I don't think there are that many on the road and they don't look cheap but the idea of a two wheel recumbent in a shell being ridden on public roads is intriguing to some.
I know a smaller rear wheel won't change much aerodynamics, but the builder said the seat is fixed to the frame at 25 degrees, so a smaller wheel lowers this angle, then making the rider more aero.
I also know that shorter cranks require a smaller chainring, in the same proportion as the cranks are shortened, so you keep the "pedal speed" the same. I actually got 120mm Sinz cranks at hand, and 38t chainring that would be equivalent to 54t on 170mm cranks. I was about to try that on a TT build, but might now save it for the recumbent. Or maybe just try before on the TT bike to decide if chainring size is right, because...
I also know that smaller wheel requires bigger chainrings, so if I go with 406 rear wheel, and if I expect my CdA to drop, and if I'm able to hold the same power... Then probably a chainring around 50t, being equivalent to 65t on 170mm cranks, would do the trick. But you can find up to 60t on aliexpress, look for "Stone 110 BCD oval chainring", the biggest one $70 shipped.
Then the UCI thing. I care about UCI for racing upright road bikes. I won't care about it while building a recumbent.
The StringBike has a bit more than only disadvantages, take a better look. I'd ride that on my upright race bike... if UCI didn't state bikes have to have a chain drive... (!!!!!!!!) I guess they made it with rotary motion just because that's what people are used too, but I intend to build something with linear motion some time. Being out of UCI anyways, that should be a recumbent too.
Then the linear pedal motion. Yes, the fastest HPV is circular motion so far. Could be for a few reasons: 1) nobody until today has trained long enough to have peak performance on a linear systems 2) Elite athletes are trained in circular motion 3) Linear drivetrains are underdeveloped. I believe all 3 play a role. Graeme Obree recently built a linear motion machine to go for HPV top speed record, but failed. His "Beastie" is backyard build with no wind tunnel data, made of cheap steel. It's fixed gear. He's now much older and less fit. He only trained a few months on it. But I still dream with linear motion. The human body is not made to pedal circles, it's that simple, we can theoretically generate more power linearly.
Finally, the Birk bikes. I like the Comet idea, tail fairing. I'd add a windshield at front to it. It's as close to full fairing you can get while still being able to put your feet on the ground, and getting up. Probably then you can add an stretched fabric/canvas on the sides when you wish, like the Aerowing stuff. I like their idea, looks like it helps.