Tacking the Frame

Posted: 22nd June 2010 by Nigel in Bamboo Bike, The Build

Once all the sections of bamboo have been heat treated, cut to length and mitred, and once the metal sections have been prepped, then you are ready to prep the frame for a wet layup. A wet layup takes several hours, and requires the frame to be held at different angles and loads to be applied as the carbon is laid up. To ensure that you end up with a nice straight frame, you need to create solid bonds at each joint.

First we had to figure out how to get the frame to be straight in all the appropriate dimensions. If we end up building more in the future, then we will build a jig, however I think for a one-off a jig is a bit overkill. That being said, a using a jig is really the ideal way to do this step, so the following is not so much a set of instructions as it is an explanation of how I tacked my frame together. Please take that into consideration if you make your own bamboo bike.

I had decided to use the steel chain stays from the donor bike partly because of the tension, compression and torque loads that are applied on them when riding, and also to help with getting the whole bike to be straight and true. It was a trade-off because I had to compromise with a longer than desirable wheelbase, and I would have liked to build the whole bike out of bamboo (I even treated, cut and mitred bamboo chain stays), but in the end I think I made the right choice.

We (Jame Kay, who was invaluable in this phase of the build, and I) started by tacking the seat tube the bottom bracket shell, which was conveniently already attached to the chain stays. To get it to set straight, we put on the rear wheel, put a spacer between the wheels and the seat tube, and sighted from behind the wheel to ensure it was straight up. Once we were satisfied this would work, we popped the seat tube off, slapped some 5-minute epoxy on the joint, and then taped it all in place.

Next we attached the seat stays. Once problem we noticed was that a compression force between the seat stays and the dropouts (which is the load they will experience when the bike is riden), creates an outward force on the bottom end of the stays. As such, they will at best shift a little as the bamboo compresses between the dropouts and the carbon layup, but more likely they would split. To address this, Jame thought of putting a washer underneath so that the force would be translated directly upwards. As I didn’t have the right size washer on hand, Jame cut a fairlead in half, removed the plastic, and voila! We 5-minute epoxed it all together and used electrical tape to hold it in place until the epoxy set. The nice thing about electrical tape is that epoxy does not bond to it. Do not use masking tape with epoxy!

Finally we had to tack on the top tube, down tube and head tube (which form the front triangle). This part is really tricky because you have to make sure that the whole front triangle is set in line with the rear triangle, while at the same time ensuring that the head tube is directly vertical. Starting with very well mitred joints really helps here because everything more or less lines itself up, you just have to fine tune to get it bang on.

We first installed the headset and fork so that when the frame is leaning forward on the front dropouts, we would know that the head tube is vertical. Then we used a clever little trick I am calling the “bungee method”. It is a modification of Sheldon Brown’s “string method” for ensuring that a steel frame is aligned after spreading the rear triangle to accept a wider rear axel. By using a bungee rather than a string, the bungee serves the dual purpose of ensuring alignment, and holding the parts together while the epoxy sets. I ran a piece of bungee through both dropouts and around the head tube, then tied it so that there was a decent amount of tension. Then we epoxied each joint, and rested the frame on the bottom bracket shell and the front dropouts. We measured and adjusted the alignment continuously until the epoxy set. As you can see in the second picture below, the gap between the right (from a rider’s perspective) bungee and the top tube is larger than the gap between the left bungee and the top tube. That indicates that the alignment needs adjustment.

With the 5-minute epoxy set, the frame was assembled, tacked, and ready for the wet layup. I should have been more excited in this picture, except that it was starting to get late and we still had a lot of work to do.