Thoughts on designing and making a bicyle

As outlined on my previous page, I had found a system of making a hollow wooden bicycle frame, which, though not common, did seem to be a system used by several makers. This would involve laminating wood to make two separate halves of the bike frame, hollowing out these halves then gluing them together. Into the bike frame would be bonded short sections of steel tubing to take the seat tube, steerer tube and bottom bracket. The seat stays and chain stays would be made up and fixed to the main bike frame afterwards. I was making things a little more complex by the addition of a curved top tube, but in essence I was going to follow this system. 

The main bike frame is a triangle, and is therefore, made up of three pieces - the seat tube, top tube and down tube. My frame was to be 52mm wide; each half being 26mm. To make up each 26mm thickness involves the use of laminating layers - typically three. The use of a constrasting wood in the centre layer for aesthetic reasons is also pretty typical and looks great. I rummaged through my workshop and found some nice lengths of Brown Oak and Sycamore Maple. I think I just about had enough, so using these was the plan. It was left over timber from paid furniture projects, so I considered it free wood!

Oak and maplebrown oak

 

 

 

 

 

  

 

 

 

Ultimately, I wanted to make myself two bicycles - one summer and one winter. I felt the machining and cutting of the timbers would involve fiddly setup so I decided to take the risk that my design and my method of construction would work, and decided to cut and machine the timber for two frames at the same time. So long as I had sufficient timber, then I planned that the winter bicycle would be the same woods but layered in the opposite order. I still have quite a bit of old English Elm in my workshop, and access to plenty of English Walnut, so if I ran out of the Brown Oak or Sycamore I'd likely revert to Elm and/or Walnut.

The three tubes require fixing at the joints - down tube to seat tube at the bottom bracket, seat tube to to top tube and top tube to down tube. Some makers made these joints using a finger joint cutter on a router whilst others layered the laminations in an overlapping manner to create the joint. I decided to do a bit of both in order to gain strength and minimise the visual effect of the joints - I hoped. 

Once glued up, each bike frame half would be hollowed out using a router. I would leave a wall thickness of 5mm. I knew this would be fiddly and require a good deal of accuracy when hollowing the inside and also rounding the outside of the frame. The design of accurate jigs would be invaluable so I set about considering just how I would put all this together and began designing some jigs to help me. I also had to consider my top tube because in my case this was not going to be straight so could not simply be cut from lengths of timber in the same way as the seat tube and down tube.

The Curved top tube

For the wood to maintain its strength it is very important not to simply cut a curve out of a straight piece of wood as you will end up with sections of short grain, which would create a significant weakness.

short grain

It meant, therefore, that I would need to bend wood in order that the grain runs along the length of the curve. I have made several curved wood forms when making pieces of furniture. I have used both steam bending and laminating techniques. It is possible to steam bend quite substantial pieces of timber - boat builders bend some really hefty stuff. When laminating it is necessary to saw the wood into thinner pieces that can bend when dry, then fix and glue them in a form to create the curve. Provided the pieces are cut from the same stock and then glued back together in order the glue lines can be quite minimal. My plan was to laminate, but I would have to do some playing around with some scrap to see what section thickness I could get away with for the curve I ultimately planned.

Frame Geometry

I read up on bicycle frame geometry. In truth, if you want a decent, ridable bicycle and not a funky art project then there is not huge scope for fiddling with the geometry. Head and seat tube angles and fork rake, which are critical to good handling, vary only a degree or two between bicycles. I had recently bought a Giant Defy1, which was more compact than any of my previous bicycles. It was a bit twitchy, but I really liked it. I found a table showing its geometry and uploaded a picture of the Giant as a background reference image to my CAD package. I planned to stay fairly close to its geometry whilst managing to  change the frame sections to accommodate my curve and the fairly large tubes required to get the strength I needed from hollow wood. I began to draw and tweak.

© Christopher Thompson