That's a good start for your drawings, but if you give that to the
welder he will laugh at you. I went to the machine shop I'm working at
with similar plans. They weren't to scale, had nearly NO dimensions
marked, and didn't really give much of an idea of what was going on. I
started actual machining of my frame 2-3 weeks late because I had to
continue modifying the drawings until the guy who ran the shop was
satisfied. It payed off in the end.
A quick list of things you need to do to the plans before you go to the
welder:
1. Decide on the diameter if your seattube. 22.2mm will work, but
basically whatever size you can get should work. The Thomson seatpost
website has the full list of sizes, so you can just compare that list to
a tubing source list. 1"=25.4mm. It's actually 25.399..., but that level
of precision is unneccessary. Tubin is sold by the OD (outside diameter)
and wall thickness. For example, 1.25" tubing with .090" wall thickness.
The way to calculate ID, which is the important dimension on a seattube,
you do 1.25"-2(0.090). That equals 1.07"ID, which you then multiply by
25.4mm. 1.07x25.4=27.178mm. This is *just* under 27.2. Sadly this won't
work for you, so you should find something else. Remember OD-2xWT(wall
thickness)=ID. Then muntiply by 25.4 to get the ID in mm.
2. Decide on the diameter and wall thickness of the tubing in the crown.
If you are using chromoly, don't go over 1" diameter, and you don't need
very thick walls.
3. Consider whether or not you want a reinforcing plate under the
tubing, a-la KH frames. This really strengthens the crown, and is worth
consideration. 1/16" (0.0625"😉 chromoly plate would be plenty.
4. Measure the angle of the crown pieces relative to the seattube, so
that instead of being a 20 degree angle it will be a 110 degree or 70
degree angle. Mark the axis you are measuring that angle from.
5. Decide on how wide you want the crown to be. You will need at least
1/8" of clearance on each side, and 1/4"-3/8" would be far better. But
on the other end of things, a crown wider than about 4.5-5" will be in
the way (I don't know about cokers, but when riding muni and trials wide
crowns [censored] me off). The clearance, crown width, and tubing OD arte all
related, and should be considered with the others in mind.
6. Give the length of each of the branching crown pieces, and define the
angle you want them to interface with the fork leg.
7. Define the length of the fork leg. This should be at least 18"
(36"/2), but 19-20" would be better. I would weld up the crown, cut the
angles for the interface on the crown, and then taper the legs with an
estimated length. Then weld on the bearing holders. After welding on the
bearing holders, put the legs on the frame and see how the crown fits on
top of it all and decide if you would like to shorten the legs any.
8. Define how much taper there will be over the course of the leg. Does
it taper from 1" round to .5" wide at the bottom? You will need to know
this, and should know if the shop you'll be at has a press before you
buy anything. It isn't easy to taper steel tubing without a good
hydraulic press.
9. Decids how long you want the seattube to be. Cut it 1-2" longer than
what you expect to need, and then cut it if it's too long. You can't add
tubing, you can always remove it.
The rest of the design looks good. Good luck.
--
gerblefranklin - Trials Unicyclist
http://gallery.unicyclist.com/Trials-Muni
Nick's main man.
"Democracy is just a word when the people are starving"-Immortal
Technique
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