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Experimental Delta Recumbent Trike

Background

Recumbent = lying down (or at least sitting and leaning back). This results in increased comfort and reduced air drag and pedalling effort.

Delta = a tricycle with 1 wheel at the front and two at the back.

Most pedal recumbent trikes follow the "Tadpole" scheme, with two wheels at the front and one at the back. The downside for a homegrown construction is the difficulty of building the steering mechanism and getting effective (ie, front-wheel) braking. Still, it can be done, eg, Tim Smith's trike. Standard bike wheels are designed to be supported on both sides, and do not have axles stiff enough for single-side mounting as used on commercial tadpole trikes and all cars. Tim Smith gets around this by using 20 inch rear wheels from BMX bikes, as these have a thicker, stiffer axle.

The tadpole also has the disadvantages (from my reading, not from experience!) of a very long chain run and a tendency to scrub the front tyres.

On the other hand, the simple delta type, with rear-wheel steering, has a reputation for getting unstable in corners at speed, and downhill at speed, which rather tends to offset its simpler construction. Google for "jouta trike", "raven trike", "Gthun" and "forelle trike" to see some.

A Delta with a Twist

Well hopefully if I ever get to build it, the frame will be straight, not twisted, but my idea for a design has two twists that are not in the typical deltas

Here's a (very) rough sketch plan view and a side view

The twists are

1. the pedals are behind the front wheel, not in front of it like the Jouta, Forelle, Gthun and Raven. This gives a longer wheelbase and hopefully greater stability. Downside is that there will probably be some wheel-spin when accelerating or climbing steep hills. Still, you can always pedal gently, and I just discovered a trike that has this arrangement. Google for "Schumacher Mechanik"
2. the 2nd twist is in the steering. By far the easiest is rear-wheel steering with the a raked pivot behind the rider's seat. However, at speed the mass of the rider will tend to force the trike into a more acute turn on tight corners, ie, it will over-steer strongly, and possibly jack-knife with the resulting crash and tumble. My solution in the sketches would give a much stronger leverage (mechanical advantage) to the rider to resist these forces. Sadly, I realised after doing the sketches that it won't work! 8-( The steering would certainly be resistant to side forces, because it would be locked in the straight-ahead position! Any suggestions are very welcome. Best I've come up with is a cross-shaft under the driver's seat, with a steering wheel on each end, and connected to the rear, pivoting, frame via cables. Imagine something like small wheelchair wheels, locked to the shaft. Rotating forward at the top (using either or both hands) would steer in one direction, and rotating rearwards the other. Here's another sketch. Possibly something could be done by way of a steering wheel, set flat like a go-kart's so that it did not interfere with the legs while pedalling.

Model Tests

While on holiday earlier this year I made a cardboard model as an amusement. Here are some photos

The wheels are made of 2 shallow cardboard cones taped and glued together. The tyres are slices from a yellow balloon.

The motor is actually the chassis of a $2 toy model car I picked up a few years ago. Stuck on with U-Tac (sticky putty - think of Blu-Tack). Rubber band for drive to the front wheel.

Here is a short movie of it running in a wide circle at speed. You can see how the lean steering works. Hopefully on a real one, the moving of the rider's mass into the centre of the turn will offset the over-steering tendency (along with a powerful enough steering mechanism, and probably a steering damper. Trike Model at speed

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