Jig saw puzzle connection !

paper model

I think have found a new way to make (or approximate) compound curved shapes with flat sheet. Plywood material is flexible and can be deformed to a compound curvature. On my computer I have modelled a spherical wheel cover consisting of 7 pieces. They will be connected with jig saw puzzle joints. I have deformed the piece on the computer to a flat plate. The picture shows a paper model. I hope to present a model in plywood soon. The jig saw puzzle joint can be produced easily with laser cutting machines and I expect the assembling with some glue is easy too.

Easy connecting with the jig saw puzzle connection. The curved sides will deform the flat plate into a sphere segment.

Summerlab was OK

I presented the plywood velomobile to on the Summerlab in Nantes. Watch the video. We tested the machine. The seat, the front suspension, the soft top and the chain guidance were failed or considered inadequate and than repaired and improved. Thank you guys!

We have thought about how the plywood sheets can be attached. Below you see a sample.

provisionally attached with staples...

glued with wood construction glue (foaming PU)

will we ever get these staples out again?

To get experience we tried to make a hard top too.

The chain is guided by four toothed chain wheels to reduce the losses up to the max. But it would easily derail from the rear sprocket because it could not be tensioned properly.

Jacques and ? (sorry I forgot your name) are adapting the wheel mount so the chain can be tensioned properly

Joris wanted to make a one wheeled trailer (remorque). I showed him my plywood design. Building a simple trailer was a fine way to learn (developing the sheet, bending the alu tube... )

Joris has made a mock-up to evaluate the shape and generate the development of the sheet

unwrapping the sheet

The edge at the top of the trailer will be reinforced with a tube. Bending it is not so easy. Because we did noty compensate for the elastic relaxation the tube did not get its definitive form directly...

bending aluminium tube Dxt 18x1

for the last turn we fix at a new position the tube so we can release the first par

oops ! We should have filled it with sand...

But not only details were studied. Scale models and a 1:1 mock-up was made too:

 

On the wiki you will find some more !

The summerlab was a very pleasant gathering of interesting people and thoughts. Although not presented at the summerlab and already written in 2004 to me the most intriguing was the philosophy on cyborgs of Natasha Roussel.

S Eco Marathon

Today I visited the S Eco Marathon in Rotterdan. 3000 students united in 200 teams trying to get as far as they can on one liter gasoline...  It was a great pleasure to discover that the winner of the 2003 competition was a wooden monocoque. It was a team of the St Joseph La Joliverie from Nantes, France. They reached 3103 km. Come to Nantes this summer

The 2003 winner LPTI St Joseph La Joliverie

The 2003 winner LPTI St Joseph La Joliverie

detail of the wooden body

Summer lab à Nantes

Check this out: July 23 to 28 plywood velomobile summerlab and this too in Nantes.

Please let me know if you are coming to build your own plywood velomobile. Provided enough people come I will be there and bring my proto.

The second proto of a plywood velomobile

K-drive

The HPTeam Delft and Amsterdam  choose the K-drive to improve their machine. This video shows the K-drive. I found this redesign on the web. I like the toothed belt: light and clean.

LucaBike redesigned the K-drive

On mass and aerodynamics

Is there a trade off between mass and aerodynamics? A streamlined velomobile may reach a higher speed at the same power input because of the decreased aerodynamic drag but the fairing adds extra mass to the vehicle. Is there a speed penalty in adding mass? The rolling resistance of the wheels will increases but also the time and energy needed to accelerate the vehicle. While setting a new hour record only the first minutes are used for increasing the speed. But in normal every day traffic we may have to stop every km...
I calculated the total trip time for a 5 km trip and varied the number of starts. In the next diagram you find the total trip time and the maximum speed for different vehicles. The coefficient of rolling resistance was held constant at 0.005. The QuestSL is a Quest with the mass of an MTB. For the calculation of the plywood velomobile I estimated the effective area to be equal to that of the Versatile. The mass of the Plywood Velomobile was very optimistic chosen to be 18 kg (The current proto is 23 kg).

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I assumed a cyclist that delivers 75 W power. At that level the MTB would reach ~20 km/h which is more that the most of us do... We can conclude that at 7 starts in 5 km the Quest and the Plywood Velomobile perform equal. The mass reduction compensates for the compromised aerodynamic quality. At an increased cyclist power the break even point moves to the right. 7 starts in a 5 km trip is quite high but not unrealistic for an urban trip. 

All in all we can conclude the most apparent difference exists between the MTB and the other vehicles. The differences between the velomobiles are small. It would be interesting to see what happens when a small slope is taken into account. Another interesting thing is recuperative breaking and start assist.

On second thougth: After a 5 km trip in a Quest with one start only the total work done is 48.2 kJ. The Quest reaches a maximal speed of ~42 km/h after 3.5 minutes. The kinetic energy of the vehicle and rider at 42 km/h is ~7 kJ. Now it becomes very clear what happens when we have to stop and accelerate again: we loose the kinetic energy of 7 kJ which is 7/48=15 % of the energy needed with one start only. Would we have to stop 7 times our energy usage more than doubles ! The MTB uses 104 kJ for the 5 km trip with one start only.  His kinetic energy reaches 2 kJ only (2/104= ~ 2%). An extra stop doesn't bother him to much... 
Depending on the number of starts in our trip, recuperative breaking and start assist would give a significant increase in performance of the Quest...

Remark: decreasing the mass of the velomobile is not helping much as it is dominated by de rider.

Cycloid drive

Miles Kingsbury reports a 90% efficiency on his cycloid drive:

"We have also found that although the drive is mechanically very efficient, physiologically it was not so good. We did a number of tests and found it was only about 90% as efficient as a circular drive. In the end we decided this inefficiency was caused because the main leg muscles were performing a 50% duty cycle compared with only about half that on a circular drive. Although it seems that this higher duty cycle should help efficiency, it doesn’t allow the muscles to recover and get rid of lactic acid build up, causing a severe burning sensation! ".

I will try to make a multi body analysis of this drive. It may be the problem is the kinetic energy of the thigh, calf and foot at the extreme positions. The cyclist has to do work for decelerating and accelerating his legs at the returns.

A page of Human Power, the technical journal of the IHPVA issue Volume 8 No2, spring 1990