As a follow up to this post: Blitz Benz Velomobile? , I worked on a design of a chassis. My role model for a chassis is a kayak. I fetched some ideas of how to build the chassis by reading Paddelmaier’s blog on building the MKG Mannheim.
When building this kind of kayak a wood frame is formed by layered plywood ribs and birch laths that are tied together and then covered with a skin. Eskimos didn’t have plywood but wood or whale bone for ribs and the skin was made out of seal hide. Paddelmaier went with ballistic nylon instead of seal hide and coated it three times. Paddelmaier managed to build a 550 cm long and 57 cm wide kayak that weighed 15 kg. How heavy would my design of a 280 cm long and 50 cm chassis be?
Views of the replica model.
I started out by drawing a CAD model of the concept from the previous post and adjusting the form of the chassis to resemble the original Blitzen. The rear is stretched higher than the original to give a single seat more support. The replica may be only 58% the size but a driver would not shrink. Because of this, the seat opening is also longer, extending further forward.
The model in its individual pieces.
My first concept had the chassis in three sections, front, centre and rear. The front had 5 ribs, centre 4 ribs and rear 5 ribs, making a total of 14 birch 9 mm plywood ribs. In the model above, I combined the front and centre into one section and reduced the ribs to 10 pieces, with a 30 cm to 40 cm spacing. I wonder what forces this chassis would have to withstand. Would 9mm plywood be enough for the ribs? Or would 19 mm plywood be necessary? Are 6cm thick ribs too thick? Can they be reduced to 4.5 cm? One could either calculate it through or test a version to understand what one be left away to reduce weight.
The chassis with ribs and laths in place as well as a chair concept.
Bellow is the further developed model of the main and rear chassis section. The ribs are 19 mm and 4.5 cm thick, with 1 cm by 2 cm cut outs for laths. The rib number #1 has no central cut out as does the very small #10 also. The rib #2 #3 and 4# would have protrusion from the topside into the pedal room for the steering system. Rib #5, by the drivers hatch is in U shape. The rib #6 of the main section and #7 of the rear section would be placed next to each other and support together the seat of the velomobile. Probably the hardest part of forming the chassis would be the transition between a rib #7 and #8 when the chassis goes from a square with rounded edges to an ellipse. The rear of a Bugatti Type 35 would be much easier form.
The volume of 10 ribs together would be about 0.0156 m³ made out of multilayered birch plywood with a density of 640 kg/m³. That would translate into a weight of 9.95 kg. If they were 9 mm instead of 19mm they would weigh 4.71 kg.
Laths are the part of the construction that runs from front to back. The role model kayak has 6 of these and an extra 2 in the centre, so up to 8 in the centre. By the kayak, because the laths are up to 540 cm long, they are spliced in the centre. By the main chassis section I figure 10 are necessary and by the rear section 8. None would have to be spliced because their maximum length is 183 cm. The laths would be fastened with pre drilled screws to the ribs and each rib would have 1 cm deep 2 cm long cut outs for the laths. I pondered if the amount of laths could be reduced; let’s say 8 or 6 that are 2 cm by 2 cm or with 2 cm by 1 cm laths that are reinforced in between the ribs with 1 cm by 2 cm laths that do not further cut into the ribs. This would reduce the amount of cut outs required by ribs. But my notion is this would have little advantage (my design is all notion, no calculations or experience in velomobiles). Instead of birch I would use beech wood (2 cm by 1 cm) which is slightly denser (680 kg/m³) and has better bending properties. The sum of all laths would be 0.0042 m³ in volume and weigh 2.84 kg.
Close up of the ribs and laths by rib #4 and #5.
The Blitzen was built with steel and the chassis surface was painted white or the steel polished. I estimate that the surface area of this velomobile would be about 5.498 m². Covering the chassis with 0.5 mm sheet metal with a density of 7900 kg/m³ would weigh 21.72 kg. That is a lot. I researched stainless steel sheet metal that is available as thin as 0.4 mm, weighing 17.37 kg (if there is metal as thin as 0.3 mm available, then it would weigh 13.03 kg). Aluminium has a density of 2710 kg/m³, thus a 0.5 mm aluminium sheet metal would weigh only 7.45 kg. If instead of metal, 1.5 mm birch plywood is used the weight could further reduced to 5.28 kg.
Weight of chassis
Without adding the weight of screws, rivets, seat, pedals, a ring or side boards by the drivers hatch, the weight of the chassis with 9mm birch plywood and 0.4 mm stainless steel body would be 24.92 kg, with an aluminium body 15.00 kg and with plywood 12.83 kg. Is it worth the extra 10 kg to have shining surface?
This doesn’t include the ladder frame, gears, chain, wheels or steering wheels, which I will estimate now weighing at least 18 kg. A velomobile which you can buy, weighs anywhere between 25 kg and 40 kg. Can a replica car reach 30 kg? Maybe, if the design would cut further back on the number of ribs and laths, or finds another material (material combination?) to cover the body.