Wednesday, April 13, 2011

Chopping up a Bicycle Frame - DIY tutorial from

Chopping up a Bicycle Frame

Recycle and reuse are the name of this game, especially when you are creating a rolling work of art from the ground up. Practically every single part of a standard bicycle frame can be recycled, and many of the parts are key components in any frame such as the head tube and bottom bracket. Every scrap frame you can find will have some good reusable parts, even one that has been run over by a bulldozer, so never turn down any free bicycle junk! Let's cut up an old frame into its individual tubing components and see what we get.

Figure 1 - The sacrificial lamb

Every steel bicycle frame can be butchered up into reusable tubing for your projects. Aluminum and carbon fibre frames are not very useful to the garage hacker, so don't bother trying to recycle them for your projects. Carbon fibre is obvious by visual inspection, but aluminum can be tricky to distinguish from steel when it is painted. A quick test is to place a magnet on the frame. A magnet will not stick to aluminum, so that will tell you right away if the tubing will be of any use. Also, aluminum will not throw sparks when using a grinder, but at that point, it may be too late!

The typical upright bicycle frame shown in Figure 1 was salvaged from the local dump and is awaiting a new life as some radical new recumbent bike or trike.

Lugged frames should be avoided
Figure 2 -Lugged frames should be avoided

In addition to an aluminum or carbon fibre frame, there is one other type that should be avoided if you have enough scrap to choose from - a lugged frame. A lugged frame will often show up in the form of an old "granny bike" or 1970s single speed cruiser. Although these frames are steel and can be welded, the bottom bracket and head tube will not be reusable due to the lugged construction of the frame. Lugged frames have inserts for the frame tubing as can be seen in Figure 2 of the bottom bracket. If you cut the bottom bracket or head tube from a lugged frame, there will be massive holes in the parts, making them very difficult to reuse. Any frame that has the tubing welded directly to the bottom bracket and head tube will be fine, but try to avoid lugged frames if you can.

Types of grinder discs
Figure 3 - Types of grinder discs

Although you can do most of your frame butchering using only a hacksaw and a hand file, an angle grinder will certainly make the job much easier. The three different grinder discs shown in Figure 3 from left to right are: flap disc (sanding disc), zip disc (cut-off disc) and a standard grinding disc. The zip disc will rip though a bicycle tube like a hot knife through butter, and a single disc will probably be able to chop up 10 or more frames. The larger grinder disc is good for removing the leftover tubing on the bottom bracket and head tube as well as cleaning or fish-mouthing up the ends of tubes for welding. The flap disc is like a finishing disc, able to clean up paint and small amounts of leftover weld metal, bringing the surface to a like-new condition. If you plan to dig deep into this hobby, it's a good idea to have a good collection of each of the three disc types in your toolkit.

Making the first cut
Figure 4 - Making the first cut

The first cut you make into a triangular frame will be the one that will cause you the most problems. Because of the stresses put on the frame during the welding process, it will tend to collapse onto itself as the first cut is made through the tube. When using a zip disc, you have to be aware of this and make sure you have a really good grip on the grinder, or stop short of the end and simply make the last bit of the cut with a hacksaw. When cutting frames, I either have them in a vice as shown in Figure 4, or place them on the floor and stand on the frame to keep it from moving.

It does not matter where you start cutting, and it really depends on how you plan to use the frame. Just remember that any triangular shape is going to want to grab your zip disc at the end of the cut, so be prepared!

Avoiding a grinder disc snag
Figure 5 - Avoiding a grinder disc snag

Figure 5 shows how the joint can collapse on itself after the first cut has been made. The top of the joint has a gap made by the zip disc, but the bottom of the joint has closed up, causing the grinder disc to jam. Usually, this is not a problem as long as you are holding on, but I usually complete the first cut with a hacksaw just in case. A jammed disc could fracture, especially the very thin zip discs.

The front triangle removed
Figure 6 - The front triangle removed

Figure 6 shows the front triangle cut from the frame. The top tube and down tube are now held in place by the head tube, and since the triangular structure has been opened, the head tube joint can be easily cut without any grinder disc jamming issues.