Stupid Mistakes #1

Dang!

Anyone else trying this, learn from my supid mistakes. When attaching the rear fork into the bike frame, do it with the wheel in the fork. I just put the wheel into the fork, and it's all but impossible to keep it from rubbing up against either the bamboo, the brake, or both.

Now I have to cut off the rear fork and put it back on again.

Splicing in the crank supports

This is a close-up of the joint for the crank support poles. The one that joins to the lower pole is cut on the angle and butted together. A slot is cut in both poles, sufficient to accept a 3/16" ply, which is carefully cut to the precise angle required, making a long, skinny triangle. Epoxy is applied to the edges of the slot and the butt joint, it is assembled and bolted near the joint. A wooden pin (not shown) will be inserted near the points of the triangle. The wire wrapping is to prevent the bamboo from splitting at the cut.

If I do it again, I will cut a slot through both sides of each pole, and have the triangle pass all the way through. As it is, the ply can wiggle slightly inside the poles, and depends on the epoxy to prevent this.

The upper bracing pole is cut on a very careful angle (took me several tries to perfect), and rounded out a bit with the dremel for a firm fit. I also cut into the pole it connects to, so that it can butt up against a solid edge, but not so far as to open the cavity of the pole. I single bolt holds it very firmly, although I will probably add epoxy as an additional measure.

Metal-bamboo joints

Well, I've finally done it. I've joined bamboo to metal. Someday maybe I'll tell the story of my adventures trying to drill hardened steel, and how I wound up with three 3/16" drill bits. But I slid one end of a 5/8" dowel into a bamboo pole, and the other into the tube of the bike, where it fit snuggly. Drilled twice through both ends. Took it apart, smeared the dowel with JB Weld (epoxy), put it back together and bolted it fast. It's permanently joined, now.

What I actually connected was the rear half of the rear bike triangle with the support poles.

Third Pole Photos

Below I talk about how I added the third pole by making a triangle and insterting the point into a mortise in the third pole. Here are pictures.

How much bike to use

Today's question is: do I cut off the entire rear triangle of the salvage bike, or do I only use half of it? That steel is heavy! Moreover the connection into the main bike is tricky, since I foolishly did not leave a long tail on the lower pole, and the bracing poles interfere with connection parts.

I have long tails on the upper two bars. This is what I leaning toward: I will use half the lower bar of the triangle, connecting the cut-off ends to a bamboo fork I can splice into the main frame. I will use the entire rear bar of the triangle, to have the mounting space for brakes and other items. The tails of the upper bamboo can make nice lap joints with the rear triangle bar. The bamboo, with a little shaving, can fit into the cut ends of the lower bar. A little diagonal bracing and this should be a strong mount.

Progress

As you can see in the photo below I have added the supports for the third pole so I will have a strong, triple poled frame. I built these braces a little differently than the first, as you can see from the close-up below.
I cut each brace pole on a 30deg angle, and drilled their mortises at a similar angle around the main poles, so that they meet in the middle snuggly. Only one of them required any real trimming to make snug. Then I make three oval holes in the third pole, set it down over these braces, and secure with a single pin through both braces. This oval hole is on the large side. To keep it from getting too large, and weakening the pole, I notched the peak of the braces.

It all worked very nicely. I don't have pictures of the main frame assembled, yet, but it is almost done. Next will come building the support for the front wheels.

Bracing the frame

I've been adding diagonal braces to the frame poles. It took some experimenting to determine the best way to do this, but this is where I settled:

1. I use a mitre saw or coping saw to cut two 45deg cuts opposing each other on the inside of the poles, at each end of each brace. These cuts go down only a 1/16th of an inch, not even enough to go through the bamboo wall.
2. I use a 1/2" chisel to cut out the opening.
3. I use a dremel or rotary cutting tool with the drum sander to shape one side to the shape of the brace pole. The other side, which takes the end of the brace, is kept flat.
4. When I have two good openings like this at the proper places on the two poles, I measure and cut the diagonal brace from 1/2" bamboo.
5. I firmly hold the brace in place, and drill through the frame and brace at a slight angle.
6. I insert a 2" machine screw (bolt) through the hole and bolt it together.

The use of bolts unfortunately adds a noticeable amount of weight, but appears required. I suppose I could mortise this joint as well, and secure with a pin, but drilling at a 45deg anle into the bamboo would be challenging, and there would be complications, such as the difficulty of getting the brace in between the poles, or of passing it through the poles.

The frame is started!

I have constructed the first piece of the frame!

This is the two lower poles that for the main part of the frame, which will make a triangle-beam with a third pole. The 1" bamboo poles are joined by short lengths of 1/2" bamboo, mortised and pinned, as shown in this photo:

The wire is 18 gauge galvanized steel I wrapped around the bamboo to prevent/repair splitting when the snug crosspole is inserted (an issue). The pin is a 3/16" hardwood dowel that prevents the crosspole from coming out.

This entire construction has nothing to do with my plans, and indeed contradicts advice I gave elsewhere: a speedbore can be used successfully on bamboo, if the bamboo is prevented from vibrating with a firm parrallel clamp. I decided to go with this joinery option, thinking that I would use several half-inch bamboos for triangulation anyway. Other sites described this sort of motising as a strong joint, and it appeared to be the simplest and least fussy option. Here I go!

Joinery Experiment

I just tried both the methods listed below (Joinery 2 & 3). For the hanger-bolt joint (joinery 2), I omitted the epoxy, since I hoped to reuse the materials. For the mortise-and-tenon joint (joinery 3) I used a piece of 1/8" ply cut in a modified rectangle for the tenon and bamboo skewers for pins. The modification was essentially a tail, an extention of the rectangle at a width slightly less, so that it could slide inside the cavity of the crosspole, giving more pinning length while requiring the bamboo to be cut less.

Hanger-Bolt Joint
Joining caused splitting of the hardwood dowel, and subsequent stress and splitting of the crosspole. Joint was not square. No problems with compression of the bamboo from the bolt. There was minimal wiggling in the joint, but when I applied pressure the screw of the hanger-bolt pulled out of the dowel, due to its split.

The non-squareness of the joint was likely due to the difficulty I had centering the hole for the screw-side of the hanger-bolt, and making the hole perpendicular. Also, I did not perfect the curvature of the crosspole cut to fit the main pole. The splitting dowel was encouraged because of the difficulty getting a snug fit between the dowel and the bamboo cavity.

Also, screwing in the hanger-bolt is challenging.

Mortise and Tenon Joint
Fitting the mortise-and-tenon to each other was easily done with a dremel. Getting a tight fit was not difficult (the hanger bolt pulls the parts together in the other joint, in this joint this must be done manually). Pins held without glue. The resulting joint was square with minimal wiggling. However, when force was applied, the wiggling gradually increased. Eventually, pins cracked, one failed entirely, and the tenon tail split off.

Conclusions
I like the mortise-and-tenon joint, despite its failures. 1/4" ply and larger diameter pins (bamboo chopsticks? wood dowels?) will improve strength. Epoxy in the joint will not only give adhesion, put will also fill gaps and minimize the wiggling. Triangulating joints will also minimize wiggling. If wiggling can be eliminated, the progressive growth in wiggling should also disappear.

Most important, of course, was the ease with which I was able to get a good-looking joint. Precision is not a strength of mine, so any method that doesn't require me to hold my drill perfectly vertical is a better method.

Bamboo Joinery 2

Making a T joint:

1. Cut the end of the side pole into a curve closely fitting the cross-pole, using a coping saw.
2. Take a 1" length of a hardwood dowel that fits snugly into the bamboo cavity, apply a strong, weatherproof glue such as Weldbond Outdoor wood glue, and insert into the end of the side pole.
3. Bore a hole into the dowel in the side pole (use a bit the diameter of the shank of your hanger bolt screw-side) and another through the side of the cross-pole (use a larger bit, the diameter of the threads of the hanger bolt bolt-side).
4. Drive the screw end of the hanger bolt into the dowel, and insert the bolt end into the cross-pole.
5. Secure the bolt with a washer and nut.

Trike: one in front or one in back?

I knew I wanted a trike so I felt safe on slippery roads, so it felt more car-ish. I bike is simpler but I wanted a trike. However, trikes do have a stability problem. The traditional trike design, called a "delta," with one wheel in front, can easily flip at high speeds. This illustration shows why:

A. Trike moving forward. Blue arrow indicates interia keeping it moving forward.

B. Front wheel is turned, creating a deflection force (pale blue) for the front of the trike.

C. The trike starts to turn. Inertia still wants the trike to move forward, the wheel is still deflecting the trike front to the right. Notice that inertia is now moving across the line between the front wheel and the rear left wheel. This line become a fulcrum for the bike to tip over. In a car, the weight of the car is shift to the front left wheel. In a delta trike, this wheel doesn't exist.

D. Inertia keeps the bike moving forward, by adding a simple torsion to the bike mass, known in highways safety lingo as a roll-over, or just a crash.

In a trike with one rear tire and two front tires (called a "tadpole"), the left front tire does exist, and the rollover is prevented.

Other factors, pro and con:

• A delta trike requires a more complex power train, some sort of differential to allow the rear tires to spin at different rates in a turn, and or a single axle to transmit power to both tires.
• A tadpole trike requires a more complex steering mechanism, so the front wheels turn in unison.
• One poster has said that tadpoles are more inclined to fishtail.
• The majority of trike designs available from more technically informed people are tadpoles.
• Tadpoles look cool.

I'm gonna make a tadpole.

Frame Design

I really know very little about bamboo as a building material. Exactly how thick a pole of what species do I need? My cursory research didn't reveal "how strong is bamboo" so I did what Idle Dads do, which is to get some and see. I need to keep costs down, but I figured that, depending on what I found, I could probably adapt the design. I ordered 1" dia. tonkin poles from http://bamboofencer.com/, which doesn't have surcharges for small orders and ships from nearby Massachussets.





Here are the three basic frame designs:

The Single Pole, or cross design, is what to use if the bamboo proves very strong and sufficiently rigid. In the picture above, the rear wheel is on the long end of the cross, the front wheels on the ends of the crossbars, and the crank and pedals on the bent tip (the crank needs to be above the seat, for various reasons).

The Double Pole design should be much stronger, and stiffer in the vertical dimension. Not shown in this sketch are short lengths of bamboo connecting the two principal poles. The weakness of this design is that there is no stiffening in the horizontal direction, nor from twisting. This means loss of some of the shock absorbency of bamboo, and some wiggle in steering. It is possible to put the two poles side-by-side instead of over-and-under, which would restore the shock absorbency and stiffen the steering, but the trade of would be loss of strength in the vertical direction. Since this is the direction of stress from a pothole hit at 30mph with 180lbs of Idle Dad aboard, it would seem to be the primary need for extra reinforcement.

The Triple Pole design is stronger in all directions. I can't imagine that it won't be strong enough. It loses some shock absorbency, and it is significantly more complex to build than the others, but is the assurance that my 1" poles will be adequate. The picture above may be tricky to see, but I am finishing a model that I will post soon.