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 expense tube.
Here is a simple frame design that is becoming more common for wide tire use and is surprisingly easy to construct. The picture above 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 ˝ 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.