[python] Re: Lean Steer Python/Hipparion Trike

  • From: Henry Thomas <whpthomas@xxxxxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Fri, 05 Jan 2007 01:05:04 +1000

dirk.bonne@xxxxxxx wrote:

Hi Henry,

Following things I noticed (I could be mistaken!)

The forces on the pads of the swing arm (item 39) and thus the rod links are very high. There is a lever between swing arm and pad. And the force on the pad will be about 360 / 60 = 6 times as high as the force on the wheel. Just to keep the swing arms in place the rod links will have to work hard.


Yes, this is a function of the tilt geometry. If I increase the length of these rod end links, it reduces the maximum tilt quite significantly. If I scale it larger and make everything bigger, it adds weight. But I will have another look at it.

As far as load, the rear wheels each have a load of @25 kg, so the rod ends end up with 150 kg each, they are rated at 700kg so they certainly should be strong enough.

Even if this is not a problem strength wise, it could be a problem because there can occur low frequency oscilations (is it called shimmer? because of the sloppy feel), and may the effect on speed stability.

This is a good point, which I hadn't considered. I was planning to let the rocker arm flex to give the rear end a bit of spring. But I hadn't considered oscillations. Although the two rear wheels are connected, they bounce independently of each other, and average the total deflection to the frame between them. On my first prototype it gave quite a smooth ride over uneven ground (even down stairs and across rail way tracks! particularly on the diagonal). At speed the bike seemed to handle well in a straight line (I probably got it up to 25-28kph max) but cornering was unstable, so I never went out on the road to test it at high speeds.

It is my theory that the reason why we hear reports of non tilting python trikes experiencing shimmy at higher speeds is a function of the static forces on the bike caused by road camber. I imagine that on a perfectly flat road there wouldn't be a problem, but on a normal road shoulder the trike is on a slight lean, so the weight of the rider is always forcing the steering to turn up towards the road center. At higher speeds this would be the likely cause of these oscillations. My expectation is that my trike design will eliminate this shimmy, because the rider and steering forces will remain vertical and thus neutral to the road.

Did you consider using an aluminium seat, you can save weight there. The seat is now 2.6 kgs. A alu seat is about 1kg.

That was my mistake, I didn't notice that I had selected the 2.0mm wall thickness, when really I should have selected 1.2mm -- it cuts the weight by half (1.5kg).

Alu would be nice, but I am not set up to weld it. I looked into getting a MIG that had dual pinch rollers to do Alu, but the lightest wall thickness they recommended was 3mm. An Alu TIG welder is a little pricey for me right now.

otherwise a very nice design! :-)
best regards,
Dirk

Thanks, I will keep you posted ;-)

Henry

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