Lineair drive

The conventional pedal rotates around the crank axle. In a velomobile with rotating pedals the feet are not only moving in driving direction but also up and down. Should the up and downwards motion be done away with the nose of the velomobile could be much lower. That may improve the aerodynamics of the body.

multi body of a human leg

Instead of a rotating crank we could use an oscillating crank. The pedals would move for and backwards while the crank rotates 60º only. The cranks could be connected via 2 sprag clutches to the wheels. But than the pedal speed would be almost constant. Only at both extreme positions the pedal speed would have to change very fast to the opposite direction. Is that comfortable and efficient?

motion of center of gravity of thigh, calf and foot [m]

To get an impression about the losses that could be created I calculated the kinetic energy of the thigh, calf and feet of a human leg while driving the lineair drive. At the returning positions they are 2 and 5 Joule. The last value is when the leg is stretched. Humans (animals too) are able to optimize their motion. It could be that we would stretch out our foot in the last phase to reach the most stretched leg position. In that case the kinetic energy of the thigh and calf would be recuperated. I think this is what we do while we are running.

kinetic energies Et: translation, Er: rotation

But in the most pessimistic approach all kinetic energy would get lost: 7 Joule at every stroke. At a normal frequency of 1.5 Hz that would amount up to 2*7*1.5=21 Watt. That is an enormous  amount. A normal bicyclist produces 75 to 150 Watt! It may be that the foot motion has to be decelerated and accelerated in a controlled way at the returns. This kind of loss may exist in the rowing bike too. Is there anyone out there who has done experiments with lineair drives? Paul Jaray developed a similar drive in 1920. Miles Kingbury developed an interesting alternative: the K-drive. The Human Power-team is experimenting with it too.

CFD with OpenFoam

I'm going to resume my CFD studies again. A few year ago I found a fantastic open source CFD package: OpenFoam . The learning curve is quite steep for a dummie like me but I was able to simulate flows with low Reynolds numbers (Re). A study of high Re flow around a velomobile is something completely different:

1. I have to learn more about fluid dynamics
2. I have to learn to use the OpenFoam snappyHexMesh tool to create a mesh.
3. I have to learn to use an model that can describe turbulent flow (Reynolds Averaged Simulation).

I 'm not sure about this but it could be that a report on how I tackle my beginners problems are useful to you. You will find my report here

The motor

This is the kind of motor I would like to use in the velomobile wheels. It is a cam drive. The pistons make multiple strokes per revolution. This is ideal because the velomobile wheel rotates at low speeds (~ 500 rpm). The Hagglunds has to be scaled down a little. The smallest type weighs ~800 kg has a displacement of 15100 cc (15.1 liter) and produces 530 kW.

The Hagglunds Compact hydraulic motor

Efficiency of this machine is high but should be improved for the velomobile. Options for improving the efficiency may be found in reducing leakage, friction and optimizing the commutation. The velomobile motor may have a cam with 3 or 4 waves, 4 or 5 pistons and a total displacement of ~2 cc per revolution. The transmission ratio may be controlled by switching on and off the pistons with valves (see Artemis intelligent Power and this video). See also this page

 

Hydraulic drive

check valves allow pedaling and free wheeling

Why not choose a hydrostatic transmission for the velomobile?

When I see a drum brake I think: In the same volume a hydromotor fits with about the same mass. With that motor one could drive and brake the velomobile. While braking energy may stored in a nitrogen volume to be used for starting again. A small electric motor could easily be added to the system.

Hydrostatic transmissions are used when high torques, power and control are needed. The efficiency of these drives use to be smaller compared to gears or chains. But the efficiency can be improved a lot. Innas has developed machines with pump efficiencies up to 97%. I think a hydrostatic transmission developed for the velomobile may compete with the derailleur en planetary gear systems.

Imagine two pedal cranks that are swinging about 120º. Each connected to a hydraulic cylinder. Both front wheels of the velomoble are fitted with a small hydro motor. The scheme shows the basic system. Questions left: How to make it a variable transmission? How to realize a rear driving option?

A new sketch with Pro Engineer

Today I found a way to unroll (develop, expand, unfold, unwrap) the body in ProE (with the insert/advanced/Flatten quilt command). I did not find an exact description about what it really does. It does works on surfaces with single and compound curvature so I have to guarantee that the surfaces are of single curvature only. Modeling surfaces with the style feature in ProE is not that easy. I'm having a trouble with connecting the surfaces and making the connections tangent (smooth). Also I don't know how to guarantee a surface is developable in ProE. If you can help me with this please drop me a mail.

Add study in ProE of a velomobile with front wheels inside. The gap between
the grey top cover and the red plating is a modeling fault.

Red plating is developable. The nose is not and could be thermo formed. The grey top and tail have compound curvature and can be made of cloth. The sketch shows some ideas I'm thinking about: Wheels inside, a tail of textile, a nose with compound curvature. Wheels inside will reduce the aerodynamic drag but limit the steering angle. An even better alternative would be to have the wheels covered with doors that open at sharp turns. A textile tail is light, easily removed, makes no noise and can have a beautiful saddle like compound curvature.

I am very happy I have (almost) found a way to get total control about the design. The last proto was modeled in Autocad (R14). Developing the sheets was done manually...

Paddy makes his maiden ride

Paddy climbs into the velomobile. (The packing tape over the nose holes reduce the ventilation.
With open holes it gets to draughty when its cold)

Today Paddy Milford paid a visit. He made a short one hour trip. When he returned a fillet at the mount of the seat was failed. I have to strengthen this place. Exhilarating was the decent of the Moerlaken bridge : 50 km/h without much pedalling ! We discussed the project and hope to build at least 3 body's this summer. If you would like to join us this summer drop me a mail.

Wind and speed

Today I went for a short (10 km) test trip again. I improved the chain guidance with a long polyethylene (plastic shopping bag) tunnel under the floor of the body. Works great. No friction, no noise. At least until the packing tape fails... Than the chain springs of the rear chain wheel easily.

But what fun it is in a T-shirt only on a cold windy day driving on a dike. Catching the wind, sailing.The feeling that you are pushed aside. It was really drifting ! I have to get a speedometer.

I saw the nose flexing due to the load of the crank axle. I think this can be eliminated by closing or stiffening the little nose holes. Lots of other little nuisances: The brake cable is not mounted right, in a sharp right turn it brakes. The soft top comes of because the velcro is not strong enough. The Rohloff hub sometimes fails. The front suspension is too stiff.

But all in all I should be very happy with the results so far.