Reflections on the potential of human power for transportation

Friday, September 4, 2015

Make My Velomobile a Quad!




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
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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.


Hephaestus

P.S. Thanks to long time friend and HPV'er Jerry Onufer for this link to a very well executed cycle car, the Podride.

http://www.jmk-innovation.se/?lang=en

P.PS. When I wrote this post I was unaware that  the people at Velomobiel.nl have a single-rider four-wheel  velomobile nearing production.



http://en.velomobiel.nl/quattrovelo/

Clearly, I am not the only engineer who was inspired by the visionary work of Miles Kingsbury and his Quatro.



 

4 comments:

  1. This comment has been removed by a blog administrator.

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  2. I think You make mistake assuming that adding extra wheel to 3-wheeler to make it 4-wheeler) adds 1,5x rolling resistance. Rolling resistance depends upon weight on the wheel. In case of 4 wheels weight is distibuted across more wheels so less per one wheel. Let's say 3 wheels 120 kilograms means 40 kilogram per wheel. 4 wheels 120 kilograms means 30 kilograms per wheel. Summarize each rolling resistance and You will get almost the same. Of course adding extra wheels means more weight. But looking at whole unit (velomobile + rider) it is tiny amount of extra weight for an extra wheel. There is a catch: in case of velomobile it is rare to get even weight distribution across front and rear wheels. So it means more mathematics. Let's say 40 weight on front and 60% at rear. Adding extra wheel in this case lessens rolling resistance but not "in linear" way I described above (by simply dividing weight per number of wheels). I hope You get it..

    Now the "perceived problem" with adding extra whels for every biker is that it means more inertia. Rotating inertia and non rotating inertia. In case of 4-wheel vehicle it will be slower to accelerate such vehicle than 3-wheeler. It also means that 4-wheeler (4 wheels spinning instead of 3, each rotating wheel has rotational inertia) will be harder to slow down. So what riders feels as "more resistance" when addinhg extra wheel is not rolling resisatnce multiplied 1,5x but more (rotational and non rotating) inertia.

    I wonder if You know that Quest already produces 4-wheeler:
    https://velomobiel.wordpress.com/2015/10/28/quattrovelo-prototype-rides/

    There is also another 4-wheel velomobile being developped, Sunrider:
    http://mooigeelisnietlelijk.blogspot.com/2015/11/in-het-duister-tasten.html

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    Replies
    1. Dear Mr. Dlugosz, thank you for the lecture on polar moments of inertia and rolling resistance. I am aware of the phenomena. Unfortunately, you misinterpret the crux of the post. My concern is will the resistance to flipping the vehicle during turns, not rolling resistance. Call it cornering ability. Any rider who has flipped a trike over in a turn would be more concerned with those effects that a bit more rolling resistance. And the polar inertia effects are conservative (as opposed to disapative). I was aware of the four-wheel Quest but I thought it was a two-seater.

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    2. I was thinking about rolling resistance and not rolling over resistance. My mistake. Sorry for that.

      It's obvoius thing that flipping trike is much easier than quad. You don't need any calculations for proving that.

      Apart from being more stable there is more things to consider: cargo space. In delta configuration You might be able to carry more cargo (than in tadpole) but due to the more weight of the whole setup (cargo gives extra weight) You need to have very good brakes. Two braking wheels in front vs one braking wheel give tremendous difference:

      https://picasaweb.google.com/lh/photo/hUnrXP1EuvIQfyC7l23sfNMTjNZETYmyPJy0liipFm0?feat=directlink

      Braking distance from 100 km/h to zero is shortened by 1/4 (40m vs 30m). This is huge advance. Stable, cargo-capable, good braking vehicle ... should be quad.

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