Steel

• the bike has to be fast
  - I'm thinking to use the bike on-road
• I want to be visible in traffic
  - I can't consider "low" solution "safe" as in my area there are not so many streets dedicate to bicycling
• easy to find commercial components
  - it means components used mainly on standard bicycles
• easy to build
  - frame with less parts as possible and without curves
  - take in consideration the use of front and rear fork from conventional bikes

• easy to transport
  - I just want to be able to put it easily inside my car; I don't want to consider foldable bikes
  
• riding must be easy to learn and not so much challenging
  - the Flevo Racer experience teaches that only very motivated people can learn to ride this very unique kind of bent; you need a lot of time (months); this reduces a lot the possibility to involve sceptical people giving them the opportunity for an occasional riding test
  - in other words: I would like to involve other people to this different & clever way to pedal

The 3rd requirement depends a lot from how much it's easy to find special components for bent; living not so far from a "recumbent guru", this could be not a big problem; in other words: I could accept to only partially follow this requirement if the solution I'll prefer needs special components.

Straight, mono-boom frame seems to be the best choice to respect the 4th requirement; fortunately there are so many blazoned bents to drawn inspiration from. In order to improve torsion stiffness, a round section tube will be used (here after consideration about this topic).
Shock absorber will be not considered due to the consequent complication in the frame construction, not forgetting weight & cost; even if it's really simple, the rubber cylinder used on the Flevo Racer is not the best solution, due to undesired damping effects.

About the material, even if I'm not able to weld at all, steel seems to be still the best solution.

Even if I'm really fascinated by the centre steering bent concept with front wheel drive, to satisfy the 6th requirement I'll follow a more conventional steering solution.

Having these specifications in mind, it's almost defined that the new "monster" will be:

• SWB (Short Wheel Base)
• FWD (Front Wheel Drive)
• Medium Racer (2 x 20" wheels)
• Straight mono-boom (circular section)
• Steel frame
• No front and rear suspension
• Dimensions good also for a twelve years old kid, like my first son
  

• Fig.1 - Bacchetta (models Aero, Corsa, Giro)
• Fig.2 - CarBent (model Dual 650)
• Fig.3 - Velokraft (model VK Highracer)
• Fig.4 - Volae (model Team)
• Fig.5 - Rans (model Force 5 XP)
  

• Toxy ZR
• ZOX
• XPORT 4
• Dragon Bike CTZ
• Fast FWD
• HHS/1 (download .pdf manual)

As you can see from the data on the left, the round section has:

• almost the same weight (a little bit lighter): -1%
• an higher bending stiffness: + 15%
• a very considerable higher torsion stiffness: +55%

(Obvious) conclusion: round section is better than square.

• two "bushes" (machined with a lathe) are welded (just three stitches each side of both bush) to the original headset tube of the fork
  
  
• the inner diameter of each bush is slightly smaller then the outer diameter of the headset tube in order to have a sort of shrink-connection and a precise fitting; ~0.02÷0.05 [mm] are more than enough; heating the bush on a camping-gas enable to insert it very easily, then you have to keep it in position till the temperature will go down (be careful, use thick gloves!)
  
  
• the outer diameter of the bushes is machined in order to fit the inner diameter of the mono-boom tube
  
  
• the bushes total length is ~20 [mm] in order to avoid stress concentrations on the headset tube of the fork
  
  
• in order to reduce the weight (some grams!), the bushes are machined keeping ~10 [mm] for the centring between the bush itself and the inner diameter of the mono-boom tube
  
  
• the rear tip of the mono-boom tube is then shaped with a Dremel to fit with the fork
  
  
• after this fitting, it will possible to define the precise position of the 10 [mm] centring band of both bushes where to drill three 5 [mm] holes [*] at 120° around the outer diameter
  
  
• [*] in order to keep the fork in position for welding, one hole for each bush can be treated M5 (different hole!) ; two screws will complete the job
  
  
• these 3 + 3 holes will permit "welding spots" to fix the bushes to the mono-boom tube (here after detailed pictures of the welded frame)
  
  
• then the rear fork will definitely fixed with a continuos welding around the two arms of the fork
  

This is a special fork designed to accept a 20" wheel with 9 gears. I decided to use a disk-brake for the front wheel too; this is not the best solution to reduce weight, I know, but disk-brakes are really cool and my children love them ...

Advantages:

• chain is shorter than standard rear driven systems ' lighter and cheaper (anyway at least 2 std. chain are needed)
• no undesired effects due to forces coming from chain tension along the bike frame
• rear wheel can be easily disassembled giving transport advantage (reducing length)
• lay-out design is easier as it's not necessary to take in consideration the chain path from the bottom bracket to the rear wheel

Disadvantages:

• a special front fork is necessary (but a std. front fork can be used for the rear wheel)
• steering angle is a little bit limited (you could feel this limitation only at low speed)
  

Here after some pictures that show the solution used on LOSNA. As you can see two pulley are necessary. The position of the rear one is a little bit critical in order to reduce at the minimum the limitation of the steering angle; the best position depends on the number and size of gears and on the shape and geometry of the front fork; generally speaking, the best position is the one that keep the chain as close as possible to the steering axle, but the chain must not hit the front fork arm. Some practical lay-out test moving the chain on all the gears will permit to easily define the rear pulley position. Usually the rear pulley has a bigger diameter than the front one, but the front pulley can have the same "big" diameter. Once defined the rear pulley position, the position of the front pulley will be easily found: as close as possible to the rear pulley, a little bit lower to avoid interference and with space enough to enable to mount the chain.

The first test were done with a preliminary version of pulleys; this to check if they worked properly.

Then I decided to move to a solution based on a M10 bolt welded trough the mono-boom for the main pulley (as close as possible to the steering axle), plus a carbon fiber support plate for the second pulley (same item used on the shifter).