FORK STEM SHORTENING TUTORIAL
When making your own human powered vehicle, it is often necessary to shorten a head tube and fork for reasons such as under seat steering clearance, making a matching fork set, or simply for aesthetic purposes. Cutting a head tube down in size is as simple as marking and cutting, but to shorten the fork hardware to match requires cutting and welding with careful alignment. Now, let me get this out of the way before we continue: the section above the fork is technically called "the steerer tube", not the "fork stem", but since the term fork stem is more often used, I will use it as well. No point dwelling on grammatical inexactitudes when the time would be better spent on building something!
This tutorial assumes you have already cut your head tube to the desired length or have one you are planning to use that is too short for the fork stem. Install both bearing cups into the head tube and then install the top hardware on the threaded fork stem (including the top bearing) as shown here. While pressing the head tube up into the top hardware, measure the distance from the underside of the lower cup to the widest part of the fork bearing race. This measurement will represent the amount of material that needs to be taken away from your fork stem. Shown here is a measurement of 3 and one eighth of an inch to be removed.
Mark your fork stem using you measurement about an inch above the bearing race so that the welded area will not end up inside the bearing cup where the clearance is minimal. There are two methods that can be used to trim cut the head tube stem: freehand cutting with a hacksaw or zip disc and a tubing cutter. If you don't have a tubing cutter, you can make a cutting guide by wrapping some tape around the fork stem so that it makes a straight line around the circumference of the fork stem tubing.
A tubing cutter is an inexpensive hand tool that will make a perfect cut by running a sharp disc around the tube much like a guided pizza cutter. The benefit to using a tube cutter is that the joint will be perfect when you place the two pieces back together for welding. To use the tubing cutter, place the cutting wheel on the marked line, tighten the clamp wheel, and then turn the tool around the tubing until it there is little friction. Repeat this process until the cutting wheel has made it completely through the tubing. Cutting a fork stem will take about 30 seconds using this tool.
It is important that the fork stem halves are welded back together with good alignment so that the bearings will seat flat into the cups and races. If the parts are not in good alignment, then there will be excessive friction in your steering system. To set the parts up for alignment, a small section of angle iron can be used as a cradle to hold the parts in perfect alignment for tack welding. This process is made easy if you had cut the parts using a tubing cutter since they will butt up together in perfect alignment. With the angle iron guide holding the two halves together, clamp them into a vice as shown here to secure them for tack welding. Before welding, make sure that you have the bearing race installed onto the crown section and place some tape over the race to protect it from welding spatter. You may also want to protect the fork stem threads using the tape as well.
Make a few tack welds around the joint while the assembly is held in alignment by the angle iron guide and vice. Once the parts are securely tack welded, you can safely remove them from the vice and complete the welding around the entire joint. After welding, you may need to use a sanding disc to take down the height of the welded area slightly so that the bearing cups will fit over the joint. Only take as much weld metal away as necessary so you don't weaken the weld. In this photo, the lower weld has been ground slightly to allow the bearing cup to fit over the welded area.
When you have completed the shortening of the fork stem, your hardware will fit back together and show no signs of modifications. Since many of the DIY recumbent bikes shown here have single boom frames, a shorter head tube looks a lot nicer welded to the 1.5 inch square tube frame. For some of the bikes and trikes that have low ground clearance and under seat steering, a minimal head tube is necessary to maintain decent ground clearance under the vehicle.
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