Tag Archives: tilting trike

Velomobile development drawings: Part 7

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I thought it best to build a prototype to test the steering geometry, as wobbles have been reported. This to be built with aluminium section and composite rear triangle.


This is the geometry involved. The idea of the tilt axle is from Vi Vuong, aka Futon Express, from the USA.

The rear axle uses a bottom bracket system, 100 mm cranks, 15 mm axles and the chain wheel represents a disc brake to stop the tilting.

Chassis with drive structure, longerons, windscreen frame and ‘’gills’’ for bending.

These are the intersections to define body parts and joints.

The final iteration with 26 inch wheels all round.

Velomobile development drawings: Part 6

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I looked first at the volume of the space taken up by rotating the feet on 175 mm cranks, and compared that to 140 mm cranks. Being so far forward it makes a difference in the volume of the nose and it is easier to push through air.

Also with this I wanted the following:-

  • A single simple form with no jointed surfaces, spats and head-farings. All joints on exterior surfaces of aircraft have fillets between them to reduce vortex drag.
  • The simplest drive line.
  • A faired recumbent bicycle that did not fall over when you try to get in or stop.
  • I wanted it to tilt into a curve to reduce the loads and stress on the wheel system and chassis so they can be lightly built.

!75 mm compared to 140 mm crank frontal areas

140 mm crank rotational volume.

The chassis bounded volume with 26 inch front wheel and 20 inch rear wheels. The chassis will be built from 3.4 mm Corex and 11.5 mm honeycomb board hot-glue gunned with 12 mm conduit.

Velomobile development drawings: Part 4

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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.