Velomobile development drawings: Part 4

In Part 1 of this write up I said the steering geometry is based on the invention of Jurgen Mages and his Python recumbents:-

http://en.openbike.org/wiki/Main_Page

There has been a great deal of discussion and argument about the best steering angles for any size wheel in any application of the bicycle, road racing, track, mountain bike, shopping bike. All have their subtle variations in angles and trail, all to do with where the tyre meets the ground and pivots. Then along comes Jurgen who places the steering head behind the wheel, does not attempt any of the known rules and it works.

What Jurgen discovered was that at a steering head angle of +/- 65 degrees as the steered wheel is turned the frame rises. The weight of the rider pushes back down stabilising the system.

I refer to Python Projects Survey  http://www.python-lowracer.de/projects.html   where I find the trail figure for a 20 inch wheel is +/- 140 mm and a steering pivot angle of 57.5 – 71 degrees. Now I know this is for two wheeled vehicles and I am designing a three wheeled vehicle but you have to start somewhere.

At this point in this project I have developed a simple idea. I have looked at wheel sizes, axles, airflow, body shape by profile stacking, rotation, and extrusion. I have enough toys to play with.

Now is the time to measure the movement in the body structure to give a reasonable turning radius. To do this I use the original Chassis Bounded Volume set up with 20 inch wheels. The file is smaller and takes less generating time. It also allows me to check I am not entering any clearance borders.
Importantly it will also show where a chassis will have to reach to tie it all in to a structure. I take the axle and wheels and add a rotation block with its axis at the point where the 65 degree steering angle meets the ground.

I then place a circle at the origin (the centre of the 3 axiis) and make it a Component. The axle, which is a separate Component is then placed with the trail point at the origin, and tilted forward. Both components are then made a Group. When the Group is tilted back so the axle is level the handling circle is now at 25 or 65 degrees. When the rotation tool is applied the handling circle the axle rotates at 65 degrees to the horizontal. I then position the axle to the correct point on the body and rotate the axle. Axle and body are now combined as a Group. When the Group is rotated to get the axle level the body leans away from the turning direction.

At this point, I reversed the direction of the steering pivot, everything else remains exactly the same. The body now leans into the turning direction. By the findings of Jurgen the geometry is self centering so to pull the body back upright release the steering.

 

I repeated the process with the rendered body and the 26 inch wheels and straight axle to check for clearance, and this is what you get. I still have to carry on with the development of the one piece axle/nose and body bending design, but this is a good indication the geometry might work.

When I started out to design something I did not ‘see’ before, I did not expect this, but that is why you do it.

Velomobile development drawings: Part 3

In various drawings you will see blocks and cubes dotted around. These are for handling, they allow you to manipulate the component accurately, Sketchup rotation tools have a hard time handling curved surfaces.

Now I have a quick-and-dirty knock up drawing. some parts are accurately placed but nothing is fixed or final.

The only restriction I have placed on this design is that the maximum width is 800 mm, to go through a door in my apartment.

The first thing I checked for was the sight line, it was way too long.
So I redesigned the body with a straight line nose profile from just below the eyes and clearing the front axle.
It was so ungainly I am not allowing it out in public. Maybe the theory is sound but it does not always produce pleasing results as I found.
Maybe there is work to be done improving visibility without scaring people.

So I went back to body 1.
I had a 20 inch BMX wheel lying around while i was drawing. The smaller diameter rims have a hard time with the condition of the road and cycle track around here. I drew 26 inch 559 wheels and spats.
The increase in surface wetted area and cross sectional area is offset by the decrease in rolling resistance and tyre availability.

The down side of the larger wheels means the air flow between the spats and nose is becoming more restricted and looks like it will generate higher air pressure and therefore drag.
I act on niggles. I am not very smart and it takes time for things to sink in. Then they start to niggle at me, then I have to do something about it. The problem was always there, it is just exaggerated with larger spats.

The thing I tried was to increase the size of the axle fairing and placed so the air travels up the profile increasing in pressure. However when it goes over the hump on both fairing and spats the pressure will be negative by the time it hits the nose which has effectively pushed back.

Body 3.1 is the same but with an enclosed head fairing and a larger rear wheel spat, but this will cause grounding problems.

Body 3.2 Is another variation on the head fairing.

Body 4 gH2Ost Rerun:  is the beginning of thinking about the engineering of bending the body to steer the wheels.
I have lying about some 100 mm diameter Tumble Dryer hose, 25 mm compressed length stretches out to 120 mm, while retaining it’s circular form. I also have 80 mm hose.
I needed a semi circular cross section round about the trailing edge of the spats. At the same time I took the opportunity to split the body into two zones.
The upper section for the width of the arms and shoulders and the lower for the rib cage, hips and feet. I used E 817 for the upper profile, angling it in from the spats through clearance at the shoulders to the tail.
I went back to Loft Along Path and this time got it to work. This essentially the same system as ‘gH2Ost’ a tapered central flat section with circular rotated profiles on the edge. The air flow underneath needs more work. The air flow on top is improved and the overall shape is simplified.

With my new found success with LAP I went back to body 1 and rendered all the E 214 profiles.

Body 5. With the body 1 chunk I reversed the trike setup and put the spats at the back. This has the chance of excellent airflow well down the form. I can also reduce the overall width down to +/-600 mm.

Velomobile skin on frame body from Cezar

Recently Cezar Totth from Romania contacted me with this message, and I have included it and the images here, as it is similar to what I am doing.

Hi Ken,

I just started a sketch for a simple and lightweight skin-on-frame velomobile hull.
Not aiming for that gorgeous streamline you made, I’m more into building simplicity and comfort, that means fatter & shorter, open cockpit, with fewer longitudinal tubes.

My current target is a light electric commuter delta trike with pedal assist.

I started a 1/2 scale model frame made of 11mm pvc tubest.

Thanks for your inspiring effort. As I mentioned, I find it gorgeous.

Keep having fun,
Cezar

v1

v2
v3

v3

v4

v5

IMG_20151219_203423

IMG_20151219_203446

IMG_20151219_203550

Yelomobile, velomobile, first trial run

 

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This was the first test of the Yelomobile, the first time the trike, and velomobile, had moved in over 2 years.
We had previously spent some time aligning the body to the frame for cycling clearances.
However between then and arrival on the track the adjustments had shifted again.

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All the data was collected on a Garmin sat. nav. and a heart rate monitor. Cadence could not be monitored, today.
Charles completed a couple of shake down laps and then started a flying lap of the 400 m cycle track in Bellahouston Park, Glasgow.
The same gear was used throughout.
He tried to maintain a speed he would normally use for long distance cycling of around 18 kph, +/-11 mph, for 10 laps.
This pace would be the target for +/- 50 miles a day for 100 days, +/- 5,000 miles.

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We then fitted the body.
Charles started from a standing start and then concentrated on producing the same cadence and effort.
He found for the same effort he was spinning out and his heart rate elevated trying to keep up.
This suggests a larger outer chain ring may need to be fitted, or reduce the effort for the same speed.

Yelomobile Test Results1

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Comparing the information at lap 5 the duration was reduced from 08:32 to 06:52 (-18%).
However the heart rate was 14% higher. Charles reported it just felt easier.
We then had to leave to meet an appointment.

It is way too early to early to draw conclusions but initial results are encouraging.

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Velomobile: Coated and painted

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The bare Dacron is sealed with Golden Gel Mediums : Self Leveling Clear Gel, mixed 1:1 with water by volume.

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The gel stops the paint from bleeding through the skin, and adds the first layer of waterproofing.

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Two coats of Golden Cadmium Yellow Medium Hue were then applied. The paint was mixed 1 volume paint to 2 volumes of water

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Charles getting fitted for a new velomobile. I used approximately 474 ml of neat paint to cover the outside and edges.

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The Lexan polycarbonate sheet comes with 2 protective skins. These were peeled away to allow gluing, using Evo-stik contact adhesive, and pop riveting.

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The wheel discs are held on from the back.

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The Dacron and protective skins were left in place on the windscreen until all coating and painting was completed. Three longerons were left in place to maintain structural strength. They are narrower than the distance between your eyes so they do not register.

Velomobile: Skin shrunk on

Front elevation

Front elevation

The skin was cut using the pre- shrunk pattern. I had to learn how to use a sewing machine to do this, so it is NOT perfect. It now has to be sealed and painted with acrylic mediums and paint. I use Golden acrylics.

Side elevation

Side elevation

The wheel covers are PVC Foam rims and hubs with Corex spokes covered withe Dacron.

Three quarters above

Three quarters above

The tail is covered by sewing the two halves along the spine,

Nose and wheel well

Nose and wheel well

The Dacron is heavy duty polyester sourced from Kudzo Craft. It is used for covering kayaks and small boats. I did not think aircraft fabric would hold up to being handled 200+ times on a long ride.

Velomobile body pattern wrap

The pattern is shrunk bubble wrap.

The pattern is shrunk bubble wrap.

This is the body frame covered in bubble wrap which has been shrunk to fit with a heat gun. The bubble wrap is about the same width as the final covering material, so this is a very accurate pattern. This gives a quick and cheap way of giving a surface to sketch ideas on to before committing to the final and harder-to-come-by Dacron. The red/brown lines are electrical tape which have been stretched on to give the cut lines.

Velomobile body construction progress

First position of the diagonals and the wheel well jig

First position of the diagonals and the wheel well jig

The forward half leaving the floor and the tail. Shut faces are PVC foam.

The forward half leaving the floor and the tail. Shut faces are PVC foam.

The diagonals interfere with the drivers knees on the first trial fit.

The diagonals interfere with the drivers knees on the first trial fit.

Checking for sight-lines

Checking for sight-lines

The floor is supporting the weight of the body but there are many clearance problems

The floor is supporting the weight of the body but there are many clearance problems. The diagonals have been moved. The hinges are tried in place.

The shut faces have been replaced by plywood for durability. PVC Foam is stiff but a little soft

The shut faces have been replaced by plywood for durability. PVC Foam is stiff but a little soft

One of the front body hinges attached to a 3D printed attachment point

One of the front body hinges attached to a 3D printed attachment point

Frontal aspect

Frontal aspect

Velomobile: Zip-Tie Connections

test pieces velomobile construction

test pieces velomobile construction

The great advantage to using zip-ties for joining is they can be adjusted as you build. They are light and incredibly strong. I used them with PVC conduit tube and ply bulkheads to build the “”Blimp”. This is where I started again, this time with PVC tubing and PVC foam bulkheads.

IMG_2838

I knocked up a test to see if the foam bulkhead could take the strain of the nylon zip-tie which can be sharp. The tube took up a tilted position. I redid the test three times using different wraps until I got a self locking result with final straight pull through.

IMG_2841

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Using tubular stations brought another development line. Each solution also showed a weakness, and by trying to solve it, brought another knot. This is called ‘praxis’, learning by doing.

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The final process involves temporarily holding the frame together to get the spacing correct. A 5mm hole is drilled through and through the longeron into the frame. All frames and bulkheads are done at the same time. The longeron is then turned 90 degrees. Two zip-ties are used, starting on opposite sides, The 5mm hole in the frame is enough to locate the longeron. Using a single 200mm tie is possible, but it can be clumsy in confined spaces and time consuming. The extra block weighs 0.165gm. which is reasonable for all the avoided frustration.