Saturday, August 23, 2014

Longtail construction (2/3) Rear rack

In the first post about building the Longtail frame, I covered extending the chain stays and putting in a platform for the battery box to sit on.  The next step was adding on the rear rack and then tying the top and bottom horizontal tubes together to form a truss structure.
After installing the wheels, the frame was propped up along a line drawn on a flat and level floor.  (You can do this work at any angle if you are willing to do a lot of measuring and calculating offsets, but it's much easier and more foolproof to just hold up a level.)  I used an oxy acetylene torch to heat the tubing while bending human friendly curved ends on the rack, as the radius was too tight for any cold bending forms I had available.  It was then rested on top of the battery box with 3/8" thick spacers to create space for a tie down rope to fit through, leveled, propped in place at the rear, and then notched to fit to the seat tube.  There are wedges centering the front of the tire in the chain stays so that the tire tracks straight, and the boards clamped on each side of the tire are the right width to hold the rear of the rack centered.  At this point the old seat stays are not connected to anything at the top.

The sequence for connecting the chain stays to the rack was based on working from the fixed point at the seat tube towards the tail end, and fixing any errors from welding heat warping as I went along:
1.  Tack weld the front of the rack to the seat tube to stop things from moving around.
2.  Cut and tack on uprights that just clear the back of the battery box.
3.  Readjust the rack to be square and level, and add diagonals from the drop outs to support the back of the rack.  The upper ends are notched as usual with a grinder to fit, but the lower ends were slit for an inch and squeezed in to make a taper on the tubing, and then hammered square to fit over the existing luggage rack mounts on the drop outs.
4.  At this point the assembly is starting to be self supporting, and cutting tools can now be used on the top of the old seat stays to fit them to a new cross stay inside the rack tubes.
5.  A diagonal is then run from the cross stay to the main chain stay junction.

All of the fitting work had to be done inside on the flat floor, which is OK for one project, but if I were going to build several bikes I'd make a table or jig in a better space or outside.  Each diagonal was briefly tack welded in place with many plywood shields around it to contain sparks, and when enough triangles had been formed to stabilize the structure it was moved outside for more complete welding.  Tack welding an assembly first also helps control warping from welding.

The cross stay that connects the top of the old seat stays to the rack tubes.
It also links to the diagonals running down to the chain stay extension junction,
and holds the rack tubes at the right width.

After making several mock ups, I decided that the board on top of the rack should be about 5" to 6" wide.  If it was wider the corners cut into the insides of a passenger's thighs, and if it was narrower it wasn't as useful for supporting a load.  Since standard 1"x 6" lumber is 5.5" wide, I used that width so it could be an option even though I planned on using thin plywood, as 5.5" would also allow strips to be cut from a sheet of plywood with a margin for a kerf but not too much waste.  The rack tubes are 5 5/8" wide, so that the tubes stick out 1/16" further than the plywood on each side to act as a rounded bumper.

With diagonals welded in to form triangles, the rear section of the frame was pretty stable, and I went back to finish up the front of the rack.  (Although some parts look yellow in this photo, there was no brazing, only welding.)  I added two strips for attaching the top of the battery box, and finished the seat tube welds.  Unfortunately the rack landed on the original seat tube exactly at the top, so I removed the old seat clamp lugs, and then welded a few inches from the top of another tube on top.  

I try to work as cold as I can to avoid changing the properties of the steel, while still achieving a good weld penetration.   But even though I also used a piece of aluminum tubing (it doesn't stick to a weld) inside the seat post to align the top extension, all the welding in this area deformed the seat tube enough that a seat post would not go in.  I don't have the proper size reamer to fit this bore, so I taped a 1" wide strip of coarse grit belt sander cloth on a 3/8" rod, wound it around to build up the width until it just fit inside the tube, and spun it with a drill for about 20 minutes inside the problem section to open it back up.

Looking at the rack attached to the lower chain stay assembly from the rear.

I prefer priming the work after I've finished a section, mainly because it splits hours of sanding and cleaning for paint preparation into smaller sessions, but also because it immediately protects the newly worked metal from oils and salt from my hands and dirt, so there is less prep needed.  With this method the primer needs only a light cleaning with a degreaser, a scuff sanding, and then a quick tack wipe when I'm ready to paint.  I've always preferred a self etching primer for automotive work as it adheres a bit better, but I found out on the Bakfiets that the chemicals in it do not work well with an acrylic latex top coat.  The water based paint reacted through the primer and rust spots bled through.  For this Longtail I used a regular plain lacquer style primer and had no problems.

A side view of the rack assembly.

The weak spot in this design is the hole for the battery box, as it has only a short height diagonal brace under it that lands in the center of a seat tube section.  However overall this design is overkill, as the loads in a truss are carried mainly in the top and bottom chords, which in this frame are well braced straight tubing spaced 17" tall.  To give you a rough idea of the strength, compare this to a 10" or 12" high floor joist in your house- the load carrying capacity of a beam increases proportionally to the square of the height.  Without considering the effect of width, a 17" high beam is 2.9 times stronger than a 10" high beam.  A stiffer frame will also give better handling (more precise steering).

No comments:

Post a Comment