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Ace of Spades 11-19-2008 02:47 PM

Frame Build

Building a wide tire frame can be quite a challenge to the new builder. The bottom rail of a typical wide tire frame involves not only correct angles but is also compounded by radial rotation during the bending process. A single mistake scraps a considerable length of expensive tube.

Here is a simple frame design that is becoming more common for wide tire use and is surprisingly easy to construct. The picture below doesn't really show the rear width very well…that 140 tire is lost between 14” of axle plate width. (Ignore the water pipe front end and rake adjustable trees…that is another story).

Here is picture of the basic frame parts. Note that there is only one bend in any particular piece of tube. Having single bends reduces errors and increases flexibility of the frame. Tube ends are slightly over length to allow trimming. In the pile (left to right) are the two down tubes/lower rails, seat wishbones, steering neck, rear wishbones, two short connectors for the rear wishbones, rectangular tube seat post crossbar, backbone, seat post and axle plates.

Construction starts with measuring the rear wishbones to get the proper space between the axle plates for the tire being used. This can be anything from stock width to a very wide tire. A short piece of tubing fits snuggly inside the rear wishbones. A slight gap is left so a weld fuses the splice and both tubes into one solid unit.

Now slots for the axle plates can be laid out. Using a piece of 1-1/4” square tube makes it easy to lay a square flat on the tube ends. Find the center of the tube, and subtract ½ of the axle plate width (in this case, axle plates are 3/8” so the square is set 3/16” short of the tube center).

Start slots by cutting inside the marked lines with a hacksaw or abrasive cut off wheel in a die grinder. It is important that the slot is not cut too wide. It is better to leave a little extra material and file the slot to a final size.

Grab the cut slot with a pair of small vice grips, bend up once and pull down and the slot will break out. This leaves a rough end that cleans up quickly with a large file. The slot is cleaned out with a file so the axle plate is a tight fit.

Next will be construction of the seat wishbones to the backbone and upper rear wishbone assembly.

This part is going to cover the build of the backbone, seat wishbones and rear wishbones. The seat wishbone to backbone cut is difficult due to the steep angle involved. The tube in the picture to the left has been marked for the angle (silver line). Note how the cut starts from the front I.D. of the tube (if started from the OD of the tube, a sharp edge would result and have to be sanded of flat again). The cut was started in a drill press using a hole saw and tube-notcher. The hole saw bottoms out before the tube can be cut through. Since the tube is round and the cut is also round, the cut does not follow the marked angle. This excess material forward of the angle line wraps and conformed to the backbones.

This picture below shows how the excess material has been rough cut with a hacksaw (lower piece of tube). The upper tube has been cleaned up by sanding a nice radius from the front seat wishbone ID point to where the hole saw stopped cutting. Laying the tube next to the backbone (center tube in picture) shows how the fit is progressing. A 10” half round file works well to final fit the cope.

After the seat wishbones are trimmed to fit the backbone, the rear ends are trimmed and coped to fit the rear wishbones assembled back on page one. The parts can be assembled flat as shown to the left for a typical straight back chopper frame. Or, the connections can be done at angles to create a drop seat frame or dropped backbone for a drag style bike. By just changing the fitting of the copes, a number of frame designs can be made from this simple set of bends. In any case, it is a good idea to build a simple jig from wood to hold everything in place for tack welding. Blocks of wood screwed to a sheet of MDF or Plywood makes an adequate jig to hold everything in place.

The backbone assembly represents the major part of the frame build so far. The next critical joint is the down legs to the steering neck. After the steering neck height is determined the top of the down tubes are rough trimmed to length. By this point, you need a frame jig or at least a platform where the bottom rails can be spaced and anchored. The down tubes need to have a bit of the center section removed so they move inward toward the centerline of the frame. In this case, ½” was removed. This was done by plumbing up one down tube, measuring in ½” at the top and using a square with a built in level to draw a vertical line. The same process is done for the opposite side. Drawing a flat file between the rough-cut tubes can produce a nice tight fit. The cope was rough-cut with a die grinder and carbide double cut bur. As a few trial fits are done with the neck, the cope is further refined until all gaps are eliminated. Note how the tube ends are squared off and not sharp knife-edges. The square edge along with a bit of a chamfer allows for a full thickness weld between the tube and steering neck.

The next picture shows how the down tube to neck joint looks. A little more chamfer needs to be added for weld penetration and the dark coat from the tubing will be sanded off to keep the weld clean. Although it is possible to build a frame without a jig, it makes fabrication of a frame much easier. Here you can see one of the neck centering cones holding the neck in place. If the centering cone rod is replaced with a shorter rod that has been center drilled on one end, a 2” hole saw can be used to cut the down tube cope for the neck.

Here is the connection at the bottom rail to the bottom rear wishbone. This is going to be a spot requiring an exceptional weld. Adding a gusset for reinforcement is also a good idea. On a typical frame build, the tube that acts as a crossbar just forward of the rear fender under the seat tends to pull the axle plates together making it impossible to get the wheel and spacers installed until the width is corrected. This weld will have an opposite effect and tend to widen the axle plates. The jig or a threaded rod keeps the dimension while this weld is done, reducing the problem. Short welds can help reduce shrinkage on various parts of the frame.

Here are the rest of the parts that need to go into the frame for a HD type engine. (Left to right) Steering neck, front engine mount with quarter round shaped gussets and bosses for forward controls, a rear engine mount, front transmission mount, seat post crossbar, 2 pieces to fabricate a rear transmission mount and the axle plates. Fitting an tacking in these parts will take about as long as setting up the basic frame. Cutting these parts out also takes considerable time since they are cut from ¼” to ½” thick plate depending on the part.

The placement of the remaining parts is done after the frame is completely welded. Obviously some sort of plans will be required to ensure part fitment. Skip sections of weld in stragic locations to minimize distortion by playing welds against each other until all are completed. Install driveline mounts last. It is always best to use some sort of rigid fixture to maintain mount alignment. There will also be a number of other miscellanous parts to add to the frame like kickstand, light, tank, fender and electrics mounts.

Building frames is a path to total control of a project. It is not a path to a cheap motorcycle. Although anyone can learn the skills of metal working and welding, these are skills to bring to the project and not things to learn with the project. Your safety and the safety of anyone within sight of your creation rely on your skill and knowledge. There are a number of fabrication shops that cater to the bike building public at the fabrication level. You can purchase the parts you can't make. You can outsource the operations you can't handle. Some things can be learned on the way.

There is little that has the satisfaction or personality of a project to which you collectively applied your own skill and knowledge.

Steve Spencer

Tim B 03-09-2009 05:24 AM

great article i hope to be able to use it some day thank you

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