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How to shorten a fork
Because forks for 20 inch or 16 inch wheels are hard to come by, and often 26 inch or 700C forks are cheap or *free*, it is generally better to shorten forks than to buy them.

Tools required:
  • torch
  • vise
  • hacksaw
  • drill
  • measuring devices
  • hammer
  • files or angle grinder

6/21012 - Warren Beauchamp

You will start off with a fork like this Steel 26" MTB fork. Make sure that the steerer tube is the right one for your frame, as they come in 1 inch or 1 1/8 inch ODs. Also check to be sure the steerer tube is the correct length and that the threads are in the right place.

You can get a die to cut more threads into the steerer tube but they are expensive and often steerer tube are hard steel, so it is difficult.

Alternatively you may be able to use a threadless headset.

The forks will need to be modified three ways. They will need to be shortened, widened, and also you may want to adjust the rake.

The first step is to figure out where you will cut off the fork blades. Using your wheel with tire installed as a guide, mark the forks where the axles meet the fork blades. Ensure the wheel is properly centered, has proper tire clearance, and that it lines up properly with your chosen brakes.

 

Next you will remove the dropouts for re-use. You can use the hacksaw or angle grinder to do this. Note that they extend into the fork leg.

Alternatively you can purchase new dropouts from frame building suppliers.

Here are the dropouts all cleaned up and ready to use.

Remove the paint from the dropouts and the entire fork. I used a wire wheel on my angle grinder, which removes paint quickly.

On your dropouts, measure the distance from the axle to the point where the dropout enters the frame. Measure up that distance from the axle mark you put on the fork and add new marks. This is where you will cut off the forks.

You will want your fork inside diameter to be the same as the dropout width at the cut mark.

Front dropout width - 100mm
Rear road dropout width - 130mm
MTB dropout width - 135mm

Do not shorten the fork blades yet!. You will need that extra length to help in bending the fork.

To ensure the bending on each fork is equal, you will want to clamp a small tube or 3/8" rod to the steerer tube, that extends down between the fork legs.
 You can measure from the center of this centering rod to the inside of each fork at the cutoff point to ensure each leg is bent equally.

110mm = 4.33", so the distance from the center to the inside of each fork blade should be 55mm or about 2.1". 

To widen the fork you will heat about 2" on the inside of each blade to orange hot where they bend below the fork crown, then bend the blade outward by hand. Heating them will make them easy to bend.

Normal width for front wheel dropouts are 100mm. A road rear wheel is 130mm, and a MTB rear wheel is 135mm.

Here's the fork after stripping the paint, heating the fork blades,  and widening the fork.
Next you will adjust the rake if needed. This is done in a similar manner to the widening. Note that it is best to heat the area of the tube that will be stretched by the bending to avoid wrinkled tubing. You will need to use vice grips or a tube placed over the end of the fork blade to get enough torque to be able to bend the blade and prevent burning your hand. Eyeball the fork several way to ensure it is straight.
Now it's time to cut off the fork legs with the hacksaw. Once that is done, mount the dropouts in the wheel and do a test fitting. You will see where the fork will need to be slotted to accept the dropouts. Also check that the brakes will line up properly with this fork length and that you have proper tire clearance to the fork crown.
Once you have ensured the fork length is correct you will slot the end of the forks to accept the dropout. The first cut should be parallel with the steerer tube, and the same depth that the dropout was inserted previously. The cut should start as near to the inside of the fork blade as possible. Make the second cut to widen the slot to the same width as the dropout.
Do another test fit to ensure the wheel is properly centered, has proper tire clearance, and that it still lines up properly with brakes. Cut / file as necessary.
Brakes line up!
Once the dropout is properly fitted, you will notice there is an ugly bit of tube hanging off the outside. We can fix that. Make a cut from the bottom outside of the fork, up to the top of the slot.
Use your hammer to beat this side of the tubing so that the slot is shaped properly again and rounded.
Re-cut the slot for the dropout so that it fits properly
Check the dropout to ensure it is straight by mounting the wheel and eyeballing it. Wheel should be centered in the dropouts. Eyeball the fork from the top with the wheel mounted to ensure the fork is not twisted.
Braze the dropout to the one side of the fork just on the inside. Mount the wheel and eyeball it again. Ensure the other dropout is positioned properly. Braze just the inside of that dropout. Mount the wheel and eyeball it. Finish the brazing.

Clean up with the file or angle grinder. Add some paint.
Finished fork.

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