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Design of front suspension for recumbent tadpole trike

October 17, 2023 — BarryK

Continuing the custom solar recumbent trike project. The last couple of posts:

The trike has coil suspension for the rear wheel, but no suspension for the front wheels. I want to be able to ride the trike on very rough roads, even heavily corrugated gravel roads like this:


...there are thousands of kilometres of roads like this in inland Australia. Without front suspension, the trike would be destroyed. Well, even if I ride very slowly, it will not be pleasant.

I bought a very cheap go-kart/buggy/ATV front suspension kit:

Will post more details about that, such as how it will be fitted to the trike. For now, some preliminary designing...

This kind of suspension is known as "double-wishbone" or "double A-arm". It is a complicated science to get a vehicle suspension to work satisfactorily. This webpage explains "camber", "castor" and "toe":

I think it is good to have slight negative camber on a tadpole trike (top of wheel will be tilted inward). At least, that is my understanding after a couple of days reading. Trikes have bicycle tyres, with rounded tread, so they handle camber OK. Furthermore, I think that it would be good if the camber becomes more negative when the shock absorber is compressed.

Consider that corrugated road. Both front wheels will hit the corrugation at the same time. The shock-absorbers will compress, the wheels will move up, increasing the negative camber. But, at the same time, the distance between the wheels in contact with the road must remain the same.

If both wheels move up, and the distance between them on the road surface increases, that will greatly increase tyre wear.

I used Solvespace and worked out proportions that satisfy both of these requirements. Here is the Solvespace file, with a false ".gz" appended to the filename (that is, it isn't really compressed):


Here it is exported to PNG:


...the circle is the central frame of the trike. The 200x200 square is a frame that I plan to manufacture, that will attach to the tube frame. The rectangle is the wishbone-suspension. Immediately to the right is a 90mm line which represents the spindle for the wheel. The 250mm line at 90 degrees to the spindle represents a 20" wheel.

I played with the coordinates and got it to work quite well. That 319.67 distance which is wheel contact on the road, remains within about 2mm over the full range of the suspension. Maybe could tweak it slightly more.

Another design consideration is cornering. The rider will be able to lean into corners, and the wheels will also lean. This greatly aids stability. I have designed it to have overall width of just under 730mm, so as to be able to fit the trike through my front door. Consequently, it is narrower than most trikes, that are over 800mm.

Being narrower, there is more risk of tipping when going round corners. Except, this design allows the rider to lean into the corner. Here is a Solvespace drawing showing leaning to the right:



And leaning left:



Look at the first diagram, it shows a distance from centre to wheel of 319.67mm. Multiply by two gives 639.34mm.

Now look at the last two diagrams. 322.83 plus 316.78 gives 639.61mm. Good!

This means that the rider can tilt into a corner, and the wheels will also tilt, while keeping the separation between wheels on the road surface the same.

Looking good. One parameter that I do not know how to handle is caster. It is neutral caster. I will rely on a dampener to reduce any tendency of the wheels to wander from side to side. Not going to ride any faster than 25km/hr anyway.

Toe will be handled using mechanisms already provided with the trike.

I am a suspension novice, so there is going to be an element of surprise in how this turns out. The above design is preliminary, and highly likely to change as get closer to implementation, which is probably a month or two away.    

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