How to Build a Chopper Frame
Rolling Chopper Chassis
Building a Bike Is a Rewarding Experience
Building a motorcycle in your garage or in a small shop with minimal tools is no small feat. But it can be done and hobby bike builders are doing it successfully, even on their first attempt.
You can build a chopper frame on your own. The satisfaction you will get from building the frame alone is hard to put into words.
In the article I am going to cover a lot of subjects that cover in detail what is required to build a quality chopper frame. I will cover:
- the fabrication processes,
- welding, and much more!
If you want to build a motorcycle from the ground up you will start with the frame (or chassis).
Some first time bike builders like to start with a pre-made from or rolling chassis and then piece the bike bit by bit from there. And that is a good idea because it will give you the experience you need when building from scratch.
But we want to build a chopper frame, so lets begin there...
This Bobber Built By Hand:
Chopper Frame Jig
Assembly and Welding Jigs
Although it's possible to build a frame without a jig it's not a good idea. A jig is a massive help in the frame building process because it holds your frame together as you do your mock up and welding. Without a jig it's very difficult, time consuming, and it's much more likely you'll get a bent frame.
Commercial chopper frame jigs are very expensive. The beginning price could be anywhere from $5,000 upwards for an adjustable jig. The jig would have to be tailored to the bike design, or be infinitely variable. In either case, the cost would be prohibitive to the small shop or the individual builder.
To build a jig yourself, and I mean all the machining and other work yourself, you will have to invest anywhere from $200 to $700 in materials alone. If you tackle this job yourself, remember, it will always remain a "work-in-progress" as you fabricate your frame. To the above right is an example of a builders Jig.
Notice the adjustments that are available for the steering neck, rear axle plates and seat post. There are four leveling adjustments on the feet extending out from the parallel rails. As long as the jig base (parallel rails) and the vertical assemblies are perpendicular to the parallel base rails, accuracy will be achieved in the frame. Many builders use actual major components such as engine, transmission, rear wheel and front forks to ensure accuracy of alignment.
Just because you have a jig to setup the tubing for welding, does not ensure that you will have a perfect frame. The tubing should be mitered and fitted such that the junction fits easily together. If you have to force it, you have done something wrong.
These tools come in all shapes, sizes and prices. One supplier for tube bending and tube notchers is Pro-Tools from Tampa, Florida. The link to their site is located in the RESOURCE section of this document. Another supplier of tube benders is Williams Lowbuck Tools, Inc. in Norco, California. You will also find several other suppliers in the RESOURCE section.
A common question is whether or not a pipe bender can be used to bend tube. The answer is...not if the inside diameter (I.D.), and tubing is measured by the outside diameter (O.D.). Another difference is the wall thickness.
For our example projects, we are using tubing with a 1.25" diameter, and a 0.125" wall thickness. Some tube bending machine suppliers emphatically state that you absolutely cannot use a pipe-bending machine for tubing. Don't be fooled into thinking that you can use a pipe or conduit bender for bending tubing. Tubing is tubing, and pipe is pipe, and never the twain shall meet!
Yet there is a noted frame builder who states that if you are in near desperation and still need to have your frame tubing bent, you can use a 1" thick-walled conduit bender to do the job. The rigid electrical conduit is nearly the same as the outside diameter of ERW tubing. But don't plan showing the bike at some of the better shows. So there you have it. If you have a choice, your preference should be to use the right tool for the job at hand.
Other tools used for measuring include a decent tape measure, carpenter’s bubble level, a steel rule, a machinist’s square, and a nine-inch caliper.
ERW/DOM Tubing Comparison
Motorcycle Frame Tube Material
There are 3 types of tube used to build frames. They are Chromemoly, DOM, and ERW.
- ERW = Electric Resistance Welded. This type of tubing is created with mild steel. The tube is created from a sheet of mild steel, and then it's rolled up.
- DOM = Drawn Over Mandrel. This type of tubing is basically the same as the Electric Resistance Welded tubing except that it has been put through an extra process to take out the imperfections which are supposed to give it more stability (strength).
- Chromemoly is a type of alloy that has both Chromium & Molybdenum and has more strength than the other two types above. But it's a bit lighter.
The most common type of tubing used for chopper frames is DOM. But ERW is nothing to scoff at.
Chopper Frame Poll
What Do You Prefer?
Design & Fabrication Processes
By definition, design integrity is the quality, or state of being complete or undivided. This includes a singleness of purpose in the fabrication of a motorcycle frame. If the frame fails, EVERYTHING fails! Everyone involved in the process must have the same goal as the designer. The materials must be of good, uniform quality.
The miter joints must be done properly and the welds must meet established minimum standards. Any compromise in frame integrity could result in premature failure and personal injury to the rider, or even death.
The designer should define the standards to which the frame is to be made, and demand that those standards be adhered to. Standards would include definitions of all materials used plus the definition of an acceptable weld and overall workmanship. In addition, all material standards that will be acceptable, and expectations of the overall design should be clearly outlined.
Before any successful project fabrication starts, the designer must have a goal in mind, as well as product expectations or there could be chaos. Imagine a motorcycle cruising down the highway at 70 mph, and the frame suddenly comes apart because of poor design or quality of workmanship or materials! The point to be made is simple! Make absolutely sure that every aspect of the project has been carefully considered in detail. You cannot allow for any compromise.
The standards do not necessarily have to be detailed data sheets, but should at least be considered and noted in writing. Hopefully you wouldn't build a house without a set of plans and specifications. The same should be true in most other projects undertaken.
Now comes the time to pull together the first union. Before you jump headlong into the project, you need to have a plan, or at least an idea of your finished product. A sketch will work if you dimension the drawing together with tubing angles.
Without at least this amount of detail, you will most likely have a misaligned and warped mess that you may as well schedule for the dumpster. Using your drawing as a reference, select the seat post where it joins the backbone.
Cut the seat post to length according to your drawing. Leave a little extra space just in case you need it for final fit. If the final piece is too short, you just start the seat post over again. Clamp the seat post tubing and make the rough cut with a notcher.
To finish the miter, use a half-round file to form the miter for a good fit to the adjoining tube. This is one of the reasons for leaving extra length whenever possible. Notice that the mouth does not come to a knife-edge. This would produce a poor penetration of the weld, and therefore a weak joint.
Seat Tube and Backbone
This image presents the near perfect fit of the seat tube to the backbone. The mouth fit is flattened or beveled to permit weld penetration into both tubes.
In this view, you can see the flat or bevel edge of the miter. All miter junctions are not so straightforward. For these, we use a paper template. There are many software programs out there that can help you with this.
Frame Bend Video
Frame Tube Bending
Now comes the fun part. Bending tubing! This guide is not intended for step-by-step procedural instructions on how to bend tubing. That generally is supplied in the manual that comes with your bender. If there is not a manual there, contact your vendor. You will find, however, the pitfalls you need to watch out for in tube bending.
The biggest problem is inaccurate measurement, or selection of incorrectly sized tools for your bender. This could result in several problems such as overall length error, bend angle error due to failure to consider springback, bend deduction distance error for a given O.D. of the tubing, etc. Another problem could be the measurement of the bend angle, or rotation of the frame tube in compound bends. Any of these issues could hamper the assembly of the frame.
Bending tube comes with a unique set of problems that could cost a good sum of money in scrap material. If the bend measurements are grossly incorrect, resulting in overall dimensional error, you have just created a piece of scrap. Now if the angles are correct and the final length is too long, you are probably okay, so just trim the excess.
Welded Up Bobber Frame
Flat and Tube Welding
Material should be thoroughly cleaned to remove all scale, grease or oil residue. Parts to be welded should be properly fitted to each other, aligned and maintained in position during the welding process. To maintain alignment and position, clamps, bars, tack welds or other means may be used during the welding process.
Flat welding is just as the name implies. Welding of horizontal or vertical planes can generally be termed as "flat welding". Preparation is the key to any welding, but more so in flat welding. Butt-welding requires that the two pieces of metal be prepared with a recess or angle to permit better penetration of both ends being joined. Butt-welding generally refers to flat stock or pipe, but can also refer to tubing as well. However, butt-welding of tubing for aerospace and medical applications is usually accomplished by use of computer-controlled
orbital welding machines.
Since the focus of this report is the fabrication of Chopper Frames, we will concentrate mostly on manual tube welding as opposed to orbital welding. In our process, we can choose to use MIG, TIG or Stick welding. Some people insist that the "wire feed" arc welders are better than the conventional "stick" method.
TIG, Stick, or MIG Welding?
The argument for and against TIG, Stick, and MIG welding is almost as old as the first motorcycles ever built.
The preferred method is TIG because it is a very smooth and strong weld. But then again, you can get similar results from MIG welding. And Stick welding is nothing to scoff at! I know welders in their sixties who have fabricated some works of art with an old 'buzz box' welder (an old stick welder).
In truth, there is no difference in characteristics or quality of weld. The "wire feed" systems are faster than "stick" welding, but when using one of the "wire feed" methods for welding tube, the advantage of the speed is lost. So, which one is really better? The opinions out there are as many and varied as the welders that use them. Whichever system fits you and your application, then that's the one for you.
So if you were to twist my arm I would say TIG weld your frame. But TIG welding requires skill and practice. It's not easy, and TIG welders are quite expensive.
MIG welding and Stick welding is easy. MIG being is the easiest of the three.
So if you are not experienced or your funds are limited, go with a good stick welder. You can buy excellent ones for under $400, and good ones under $200. Buy a name brand like Lincoln or Hobart. Norther Tools and Eastwood make pretty decent ones as well.
Tig Welded Tubing
Welding Takes Practice
Welding is an acquired skill that borders on being an "art form". It takes practice to produce a good weld. Don't think that a "pretty" weld is necessarily a good weld! You see only the surface and not the depth of penetration.
Don't be afraid to take a few pieces of scrap and weld them together.
After the weld has cooled down, use a band saw or suitable type of saw to cut across the weld. You can then see the degree of penetration (or lack of), which will let you know real soon just how your welding is! Below is an example of what I am talking about.
The picture here is a TIG weld. Notice in the first photograph how the bead blends well into the metal without a lot of buildup of metal. But you can't tell about the penetration between the tubes without an x-ray of the junction or cut the junction in two.
Look close at the photo on the right, and you can see the penetration between the lower tube and the upper tube. Generally you would not cut your frame in half to see how good your weld is! This is a practice piece.
Wide Tire Chopper
Chopper Frame Assembly Procedures - Jig Versus Non-Jig Assembly
Jig vs. Non-Jig Assembly
To be clear...it is possible to build a motorcycle frame without a jig. Various tools, crutches, and other "insider" methods can be used, but unless the issue is one of cost, it would be very difficult to imagine anyone wanting to build a bike "free-handed" when a jig makes the process so much easier. However, if you are intent on NOT using a jig or have no ther choice, lets get at it!
As mentioned earlier, you need to have a plan, or at least an idea of your finished product before getting to started. A sketch will work if you dimension the drawing, together with tubing angles. Without at least this amount of
detail, you will most likely have a misaligned and warped mess that you may as well schedule for the dumpster.
There are several good sources for drawings if you don't already have a set. The main one that comes to mind is Custom-Choppers-Guide.com. They also carry plans created by BCC Orlando and offer extra stuff for ordering them.
Motorcycle Frame Building Video:
On to the Motorcycle Frame Assembly Process
Okay, now that you have drawings in hand, lets work on getting the necessary tubing and other miscellaneous steel parts gathered together
The tubing we're going to use is 1.125" x 0.120" ERW, unless otherwise specified in your drawings. If you don't have a source for tubing, you can find several under the topic of RESOURCES in this document. Next you miter the various components of the frame and check for good fit. After you finish with this process, you can begin piecing the frame parts together as depicted in the example below.
This section is somewhat akin to a jigsaw puzzle. After you have bent and mitered the tubing, you need some way of temporarily holding things together. Lets assume that you have perfect measurements, angles and orientation. Since we're not using a jig, we need to get to as level a spot as possible in order to start the assembly process. The next figure show one method of holding the pieces together for tack welding.
To align the frame during the assembly process, use the motor (engine), rear wheel and front forks to verify fit and alignment. As you can see from all of this, you need four arms or two people to work together. You can imagine how the frustration can potentially build up.
Place major assemblies into the frame to verify alignment. Of course if they fail to line up properly, you have some adjustments to do. This is why the frame components are only tack welded. You still have a chance to salvage the hard work you have done on the frame.
After you are satisfied with the parts placement, you can begin welding the frame together. Of course there are many other ways to assemble your frame without a welding jig. This is only one way to accomplish the task at hand. You can see from the examples that there has to be a better way to get your frame together correctly and that is why we recommend a welding jig. If you want to build your own jig, there are numerous sources for the plans, including Custom-Choppers-Guide.com.
You probably already noticed that these are the same sources for frame plans. Buy them both at the same time and you just might get a discount.
Mock Up and Tack Welding
Using a Jig
The welding jig must be mounted with all four corners level. The leveling platform should be elevated to knee height.
This could be a series of blocks or a welded frame upon which you place the leveling platform. Any suitable arrangement will work, just as long as the jig is level. After the platform is level, you can then place and mount the verticals to the platform.
Picture 1: Frame with engine and transmission mockup is located in the jig for alignment adjustments and tack welding.
After alignments are satisfied, the frame can be welded together.
Picture 2: This frame has already been tack welded together, using the jig.
This is by no means an exhaustive guide on the building of Chopper frames. It does however give you a frame of reference for what must be done to build a Chopper frame.
The skill level required is alluded to throughout this article. Familiarity with basic hand tools is a definite requirement, as is the use of welders and tube bending tools.
It's up to you now to take the first steps. Get your equipment, plans, and start building!
If you need any more help, resources, or encouragement please comment below :-)
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.