Bending Lower Rails for a
Wide Tyre HD Frame I
A lot of people seem to get
confused by this, so I thought I’d have a look at it and see if I could
figure it out. I spoke to Alex about it and he agreed to come over and bring
his set of plans along and help me out with it.
The first thing we noticed was
that the bends shown on the plans don’t make sense. The two shown in the plan
view don’t correspond to the one shown in the side elevation. So it isn’t
awfully surprising that people get confused! Seeing as neither Alex or I needs
a 280 frame, and I was funding this out of my own pocket, I decided to bend
the rails out of some 1” OD electrical conduit I had kicking around.
Before I get too involved in
this, a quick explanation of the method I use for marking out bends might be
in order. I use a nominal “bend line” which is nothing more complicated than
the edge of the former, for this article I’ve used yellow insulating tape to
mark the bend line for clarity.
Fig.1
Figure one shows a piece of
tube in the bender and the positioning of the bend line. What you can’t see
because of the paint on the conduit is that from the bend line the tube is
marked every inch with a felt pen though a scribed line would have been better.
Get a couple of feet of tube Mark whatever you use for a bend datum and then
every inch (EXACTLY one inch) along the tube for about 10 or 12 inches. Then
bend the tube to around 90 degrees. This is going to be the tool you use to
do the tricky bit, so do it carefully. What you should end up with is
something like this.
Fig.2
The yellow tape is the bend
line and the tube is marked every inch past the bend line. By lying this test
bend along a straight line we can determine how far from the bend line the
actual bend starts. So if your plan calls for a bend to start at 30” from a
given point, and you now know that your bend line is 2 3/8” from the actual
start of the bend, you need to put the bend line 27 5/8” from that point ( 30”
– 2 3/8” = 27 5/8”).
The next thing we did was to find a piece of board to
draw some layout lines on. We took the measurements for these directly form
the plans using the internal measurement between the rails as opposed to the centreline
measurement, as its easier to line a rail up beside a mark than on it. If we
were doing this for real we’d have probably taken the time to cut some 2”x2”
timber into blocks and screwed them to the board to locate the rails. Fig. 3
shows the board we used with a centreline and lines for the inner edges of
the lower rails under the motor, I’ve no idea how big it was as it was just a
random piece I pulled out of the stack in the corner.
Fig.3
Next we cut two pieces of tube (I know it’s conduit, but
tube is quicker to type, so I’ll call it tube from here on) and bent the
front end of them up and placed them on the board. We bent them exactly the
distance in the plan from the end of the tube, next time I do one I’m going
to leave an inch or so extra there as I suspect the dimension is the vertical
separation between those points and not the linear length of the tube. Seeing
as the tube is angled at 11 degrees this means your tube comes up short if you use
that length.
Fig.4
PART 2
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