Bicycle design trials

I knew the build of my bicycle would not be straight forward. In large part this was because of the curved nature of the top tube and seat stays, which complicated everything a good deal. I was also extremely short of Brown Oak. I have had a couple of brown oak planks kicking around my workshop for about 15 years just waiting for a project. I measured up and made up a cutting list but realised it was going to be very tight. As I couldn't risk a failed trial with the real wood, I decided to use scraps or MDF to try out techniques, specifically for the laminated curve.

On other projects I had used both steam bending and laminating techniques. As I didn't want to have to cut very thin laminates I thought that pre-steaming the parts might help. Parts taken off the steaming clamp always relax somewhat, but I felt that this pre-bending would leave them close to shape, so less force would be needed to later bend and glue them. In order that it fit the bike frame accurately, it was important that it remained true to the jig once unclamped, otherwise it would be a nightmare joining the bike frame properly and the two halves might not match.

Instead of making a steam box then whipping the wood out and quickly clamping it before it cools, I thought I would try a technique I had seen a boatbuilder use. He got a long, flexible polythene tube, like a plastic bag. The advantage of this technique is that you can bend the wood whilst it's still steaming.

 

bike steam jig trial

I liked the polythene tube technique. It allowed me to keep adding pressure to the clamps and feel when it was ready to bend. I clamped it up, still in the tube, and left it to cool. Unclamped, the timbers did relax a little as expected, but as I could now clamp and glue them with far less pressure to make them take shape, I hoped the final form woud more likely stay true. For the trial I used 5 laminates, but as the timber was still a bit hefty I realised that I would be better to use more, thinner laminates, but in principle it worked.

 bicycle bent mahogany trial

I decided that I would use 8 laminates instead of 5, the thinner sections being easier to bend. I also realised that because the bike's top tube narrows from head tube to seat tube then the laminates needed to be tapered in order that all the laminates run the full length of the top tube. This was important to maintain strength. Each piece would taper from a thickness of 8mm down to 5mm over the length of 750mm. I made a jig and cut these out on my table saw. Each bike frame half was to be made up of three layers. Inner and outer layers would be Brown Oak and the centre layer Sycamore Maple. As the frame was to be hollowed out in due course, and because I had so little of the timber to play with I decided the two inner layers would be made with shaped filler pieces inside so I could reduce the amount of timber used and save myself some hollowing out work. I did a glue up trial with some pieces of MDF and mahogany spacers just to check all was good to go. I also made some shaped jigs for the down and seat tubes, and finger jointed these three together to check it all fitted.

bike frame trial

MDF versions of the three bike frame tubes finger jointed together and indexed into my jig with locating pegs. The MDF down and seat tubes  would be used with a flush cutting router as a pattern for my final pieces. All this seemed a bit of a faff, but all things considered it was definitely worth the trials to give myself confidence it would work with the final timber. There was precious little room for error, given how tight I was making things.

© Christopher Thompson