[python] Re: frame geometry

  • From: Peter Oliva <kaznjammer@xxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Fri, 23 Jan 2004 21:41:00 +0100

Maybe Peter can explain this better because he is more
familiar with english.

Well, physics are not my thing, and I also don't know very much bicycle terminology. But I'll have a go anyway.

Attached to this mail there is a illustration of a Python frame. The center pivot and the axles are in red.

When the a python bends in the middle the center pivot rises. For this to happen the rider's weight must be lifted, since he's sitting on top of the pivot. Which means: The weight of the rider stablilizes the frame by pushing the pivot down.

The pivot must be at an angle. If the pivot were vertical (to the ground), then the frame could bend without lifting the weight of the rider, and the Python would be a flip-flop instead of a HPV.

The other important thing is for the pivot to be lower than the axles. Otherwise the weight would force the frame to bend in the middle. That would make the Python a merry-go-round, though admittedly human-powered.

The height of the pivot is not bound to the height of the seat, so the seat doesn't have a direct effect on the stability of the frame. The more important question determining the sitting position is: Can you reach the ground with both hands? (Handling the Python is more fun if you can.)

This leaves a number of interesting questions:
Do both axles need to be higher then the pivot?
Does it matter whether the pivot "leans" to the front or the back? (On which side the angle is more narrow?)
By how much can you change the angle without losing stability?
Does altering the angle in one direction make steering easier and in the opposite direction increase stability for riding in straight lines?

Greetings,
Peter

PNG image

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