[python] New Vython frame design -- Using rectangular tubing, closer to Python design

Hello All,

Josh & I started working on the Vython today, but after beginning to cut up the 24" mountain bike frame, we decided to change the frame design!

I'll make a graphic tomorrow, but I wanted to quickly describe it to see if anyone had any advise or feedback.

It will be made almost completely out of rectangular tubing (mild steel I think), 1.5" x 0.5" (38.1 mm x 12.7 mm), 16 gauge, which means a wall thickness of 0.065" (1.651 mm), and wow it's really heavy! :( I just looked it up, looks like it weighs 0.5852 lbs./ft (0.8709 kg/m)! Oh well this is a prototype I guess, I shouldn't worry about the weight... about 2.3 kg (5 lbs) of rectangular tubing, plus the four bar linkage, plus the BB. Seat, drivechain, wheels. We'll see.

There will be two 690 mm (2.26 ft) long sections of tube connecting the rear dropouts to the ground link (G). G will be 110 mm (4.3 in) from pivot to pivot (center to center), which is the same as the rear wheel's dropout spacing, which means that the two 690 mm (2.26 ft) tubes will be parallel to each other.

The seat will be just like the Python hammock seat.

The four bar linkage will be made out of the rectangular tubing or out of some recycled bike tubing, not sure yet. Four shortened headsets will be used for the pivots, I think we're going to attach the plates to the ground link (G) & to the coupler link (C), and weld short pieces of headtube to the ends of the side links (S). See http://www.freelists.org/archives/python/08-2004/msg00048.html (Jürgen's post on using headsets for the pivots) for more details.

Any reason to do the opposite, that is, use the plates for the side links, and weld the short pieces of headtub to G & C? Josh and I were thinking that it would be weaker that way, any thoughts on this?

The dimensions of the four bar linkage:

G = 110 mm = 4.3 in
Y = 180 mm = 7.09 in
S = 181.1 mm = 7.13 in
C = 70.4 mm = 2.77 in

The front end will much almost exactly like a Python, except that of course the coupler link (C) will replace the steering BB, making a boxy ' [ ' shape instead of a capital ' A ' shape, which will allow the trailing edge of the front tire (TEFT!) to be closer to the coupler link (C). The same rectangular tubing will be used to construct the front end.

I will short the distance FWaP to BB by 10 mm from 420 mm to 410 mm.

I will 'gain' an additional 26 mm between the FWaP and the coupler link, due to the use of 507 mm rims instead of 559 mm rims ([559 mm - 507 mm]/2 = 26mm). There should be enough room to use 44-507 (24" x 1.75") tires, but I am planning on using 32-507 (24" x 1.25") tires.

The distance from the centerpoint of the coupler link (C) to FWap will be 320 mm.

Basically since my inseam is 81 cm instead of 86 cm like Jürgen, I need to shorten the front by about 5 cm, but I'd like to allow the bike to be ridden by people that are slightly shorter and taller, so I'm going to try to make the seat slightly adjustable (by moving forward and back).

I'm falling asleep! Any feedback would be most appreciated, since I have to finalize everything by Saturday, and tomorrow (Wednesday) is my day off it would be nice to fix up all of the details.

Oh, Jürgen, in regards to the computer drawing of p4 on the Python homepage ( http://www.python.tieflieger.net/pics/p4.gif ), is that pretty accurate? If I wanted to take measurements from it, can I use "seat height: 25 cm, wheelbase: 113 cm" to determine the scale?

Michael
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