If you're going next level on your build, and plan to build or modify a chassis, you'll need a jig. In this article, I'll explain the features of my jig. If you want something similar, you can pick up a set of plans from my store.
TL;DR show me the video:
This is not my design, but its similar to that shown by Ron Covell in his video Building a Chopper Chassis.
I have a copy of this old film on DVD, and I essentially built one from the images and Ron's descriptions in the footage. I bought a bit of tubing but the rest was out of stock I had lying around.
Anyway, the plans I have created are for a replica of my jig.
Lets get into the details.
Dimensions and units of measure
Since I'm in Australia and all our steel stock is metric, the plans are shown in metric dimensions with metric hardware.
Depending on whether you are bilingual with units, this might be a show-stopper for my American friends.
None of the dimensions in the jig are critical, so you can substitute other sizes if you have different material on hand, or want to build the chassis jig using inch-sized stock.
If you can build a frame or jig, I figure you can alter the dimension to suit your build.
Hows it work?
The general approach for this style of jig is a rod passing through the steering neck, clamped with a pair of cones. This secures the front of the frame.
At the rear, a post can move fore and aft to suit the frame wheelbase. From this post, a bracket hold rods that are inserted into the rear axle bores.
This style of chassis jig suits rigid chopper frames, where the rear axle plates need to be clamped accurately in relation to the steering neck, but with some imagination, you could adapt the rear post to clamp the swinging arm pivot or some other feature on the frame. If you can fabricate a frame or this jig, I'm sure you can work out this detail.
As you can see in my frame, I used some heavy steel C-sections for the rails. This is simply because I had picked them off a scrap heap for free. The plans show rectangular tube which is a more likely material for builders.
In the plans, you'll see some "crush tubes" welded into the rails. When you bolt the rails to the legs, the bolts will easily crush and deform the tubing in the rails. The crush tubes strengthen the area around the bolted joint.
You could make the rails as long as you like, 2400mm (or 8") should be a good start.
I mounted my chassis on casters to move it around easily. You can do the same or leave them off and bolt the jig down to the floor if that suit you better.
The use of a 50mm wide section for the legs sets a 50mm gap between the rails. This is useful as we'll see shortly.
I welded crush tubes into my legs as well.
At the front of the jig is, well, the front support. This is a vertical post that can be clamped to the rails. On the front face is a plate with a long slot down the centre.
The slot allows the neck bracket to be adjusted up and down to suit the frame you're building or modifying.
The neck bracket has a pivot point allowing adjustment of the neck rod angle to suit the rake of the frame on the jig.
A pair of machined aluminium neck cones can be screwed into the steering neck to clamp it in place. You'll need a lathe to make these, or buy them from my store:
At the rear is a similar structure. Since the axle or swinging arm pivot is lower than the neck, the rear vertical post can be shorter than the front one.
The rear support can slide fore and aft on the rails adjusting for wheelbase before being clamped down.
It uses a similar slotted front face plate to allow the axle bracket height to be adjusted to suit.
Since I've seen axles in both 3/4" and 5/8" (~19mm and ~16mm), I built my jig so I can fit axle rods of both sizes to suit different frames.
The 50mm wide center channel allows a variety of brackets to be fabricated to support the bottom of the frame.
Ideally, you'd make this bracket specific to the bike frame to clamp existing features, such as engine mounting points. This holds the frame securely and accurately for welding.
The plans contain 36 pages of drawings for every part in the jig. These are proper engineering drawings, fully dimensioned with welding callouts. Here are some samples and images of the model which was the basis for the drawings:
The drawings are supplied in a PDF file, prepared on A4 paper. For my USA friends, resize the PDF to suit letter-sized paper when you send it to a printer.
Purchasers of the plans will be added to a dedicated email mailing list, so any drawing updates or changes will be supplied free-of-charge.
Email support for customers is available through the Contact page on my website.