I was talking to my son Kyle the other night. I told him I had finished the testing of my leaning-trike proof-of-concept commuter vehicle and that I had met the cruise-speed goals I had in mind for the project. I also told him about the latest lean-lock mechanism I had incorporated to ease stopping and starting with its faring in place. He listened patiently and after I have finished explaining, he pointed out that this was to be a commuter vehicle and the requirement to lock and unlock the leaning for stopping and starting was too complicated to expect riders to have to cope with. And I couldn’t argue with him.
Now Kyle says he doesn’t read my blog, too much information, even though he is a road cyclist and an accomplished mountain biker. He has had the misfortune of riding and crashing one of my ill-fated rear-steering leaning-trike prototypes. With misplaced fatherly devotion, I was more concerned with the bent-up trike than his welfare. So he has a right to mistrust his old man’s hair-brained contraptions
Sometime after that I was reading Miles Kingsbury’s account of his riding his Quattro on the Roll Across America velomobile tour and how his four-wheel vehicle handled road imperfections better than the trikes with whom he was riding. Below is another picture of the Quattro in its convertible guise.
And another with the top in place.
So I had to ask myself, am I avoiding a four-wheel layout for my human-powered commuter vehicle because it is too simple to satisfy the gizmologist in me? Since I have no plans to use powered assist, the vehicle-classification reasons for not using four wheels is not an issue. So rethinking my assumptions is in order.
The historically significant velomobile, the Pedicar had four wheels. However its 38” width made it difficult to fit thru some barriers and its exposed wheels gave it poor aerodynamics. And of course it had the linear-pedal drive that had some efficiency issues.
It probably is not a good starting point to speculate on an efficient quad velomobile design.
The Quattro, on the other hand, IMO is too close to the ground for commuting in traffic. I do like the fact that the steering wheels are enclosed. Driving and steering the front wheels does make things complicated. There are the velocity fluctuations associated with single-universal joints. There does not appear to be any differential to account for the different wheel speeds during cornering.
Another candidate the design of a commuter quad is the Quest velomobile. It also has enclosed steering wheels and I have always been impressed by the level of refinement in its implementation. And its overall width of 30” is narrow enough to fit through bike-trail barriers.
Now my assumption is that the Quest’s current rollerover resistance is adequate for most situations. That being the case, the addition of a fourth wheel should increase that resistance by about 1.5X. Since rollover resistance is a function of vehicle track (spacing between the paired wheels) divided by the c.g. height, one could use that 1.5X to increase the c.g. height and maintain the three-wheel rollover resistance.
Referring to the dimensioned drawings,
the faring behind the rider’s head is about 34” off the ground. That puts the riders head about 36” off the ground. If this could be raised to 48”, I would feel a lot more confident riding the vehicle in traffic. If the c.g. is raised 48/36 of 1.33 times, this is within the 1.5X rollover margin that the fourth wheel contributes.
Raising the rider and making the rear faring wider to accommodate the fourth wheel will make the vehicle less aerodynamic, but this should be acceptable considering that the original vehicle had a racing heritage and a commuter does not need to be as fast. One benefit to raising the rider's position is that ease of entry and exit can be greatly improved.
Since the rider is up higher, the steering can be located beneath or alongside the seat. This will allow the faring to be lower between the rider’s knees and his head and provide better forward visibility,
So the front end construction of the Quest quad can remain the same.
Behind the seat, the drive chain has to drive two wheels instead of one. The easiest way to do this is attach the cassette to an intermediary jack shaft. At the ends of the jackshaft, at the same width as the rear wheels, attach single-speed freewheels. Then connect the wheels to the jackshaft freewheels with two additional chains. The two-freewheel drive approach produces a posi-traction-like drive. That in combination with at least 50% of the vehicle weight being on the drive wheels (as opposed to 33% with a tadpole trike) provides exceptional traction for slippery-road conditions.
Depending on which side the rear wheels are being driven from and how the freewheels are threaded, one of the freewheel ratchets, its threading and the cog threaded on one of the rear wheels may need to be left handed. Left handed freewheels are available as BMX components. The rear-wheel suspension can be based on the wheels pivoting around the jackshaft, so chain tension is not affected by suspension movement.
Yea, the quad conversion may not be that simple as I just proposed, but the Quest Quad would make a very attractive commuter vehicle.