It is possible to build a motorcycle frame without a jig. But, a jig will make the process a lot easier. Note that a jig just holds parts in place for fitting and tack welding. A jig will not hold a frame straight during welding. It is possible to build a “welding fixture” to hold a frame for welding but the light tube in a frame will still be distorted slightly by the heat of welding. Careful thought and welding process will reduce distortion to a livable amount without a heavy fixture or some kind of normalizing process.
Here is a picture of a frame jig with a frame bolted in. The jig is based on two pieces of 5” steel mill channel. The channel forms a backbone for other parts to the jig to locate off of. The jig locates the neck, including height and rake angle. The rear axle height is located. The bottom rails are located and level. An “engine fixture” locates all of the mount holes for the engine and transmission. This jig is for a Harley big V-twin style motor and separate transmission.
Here is a picture of the neck jig. The neck jig post is made of a 2”x3” 1/8” wall steel tubing. A couple of plates clamp to this post allowing vertical adjustment for height. A ¾” rod on a pivot, holds two cone shaped metal parts that center the neck on the jig and hold the neck at the correct rake. The next picture is the axle post. It is made of 2”X4” ¼” wall steel tubing. ¾” “axles” hold the axle plates level in the axle adjustment slots. Spacers and shaft collars lock the axles in place. Spacers hold the width of the axle plates. There are two 1” square 1/8” wall steel tubes to hold the “axles”. The square tubes are welded to a ½” plate that is bolted to the top of the post. Note the small holes at the four corners of the ½” plate. These are set screws that allow the plate to be “jacked” slightly for leveling and small adjustments. Both the neck and axle posts are bolted in place so they can be moved forward or backward, up and down as needed. Spacers can be added under the ½” plate on the axle post for increments between boltholes.
In the middle, the lower rails are setting on 1-1/4” tubing (this frame required shims also). The width of the lower rails are held apart by 1-1/4” spacers. The engine fixture is 1”x4” steel bar. The base is bolted to the channels by a threaded rod that passes through to the bottom of the channels with a piece of angle and nut. In the next picture, the pencil is pointing to another setscrew “jack”. There are four at each corner of the bottom engine plate to adjust the fixture.
Why is there all of the adjusters? Most steel shapes are not very square. In the picture below, a steel rule is laid across the top of both 5” channels. The resulting surface is not flat. However, the lower rail supports rely on the outside edge of this jig so only that edge needs attention. The setscrew “jacks” take care of everything else. Careful measurements and shimming ensures everything is level and plumb.
If a softail frame is to be built, clearance and jigging for the forward shock mount needs to be allowed. A 5/16” plate fastened under the 1” transmission fixture plate locates the side transmission mount.
In this jig, the engine and transmission fixture plates un-bolt from the fixture and can remain with the frame to ensure the mounts stay level and hole spaces stay in place while welding the frame. The next picture shows the engine fixture with the plates removed. One the jig is all squared up, pins are added to the plates to ensure they bolt back into the same place.
Last is a picture of the frame jig with the frame removed and all of the fixtures put back in place. With different spacers and engine fixtures most any kind of frame can be built in this type jig.
A few hints about building jigs: Work as accurately as possible (the final product is a direct result!) By careful measurements, good layout and careful drilling, holes can be located very accurately. The channel backbone forms a 2” wide gap. Everything on the jig is referenced off of this gap. Center everything in reference to the 2” center gap. A wire attached to the sides of the neck and axle post is stretched tight. The centerline of the engine plates is 1” inside of the stretched wire. Level the channel backbone, and then ensure everything added to the backbone is also level. Shim as required making everything level and plumb.