Motorcycle owners often emulate post-World War II "bobber" motorcycles by cutting off the tail end of their rear fender. Steel motorcycle fenders are also cut and rewelded to change the angle of the fender's diameter, to shorten the fender while retaining the rear section (removing a section from the middle of the fender), to widen the fender by cutting it down the middle lengthwise and welding in a steel strip, or to narrow it by cutting out a lengthwise piece from the middle and welding the two narrower halves together. With careful planning, execution and the proper tools, a home-cut fender will look the same as if a skilled professional had cut it.
Visualise the amount of material you wish to cut from the fender before you remove it from the motorcycle. Measure the amount to be removed and record these measurements. If the fender is painted, remove the paint and any body filler applied to the steel, or have it professionally removed.
Place the fender on a flat surface and hold it securely. Mark your fender with a marking pen, using the measurements you recorded. Place the straight edge of a manila folder along the marks and use it, as a guide, to mark your cut line.
Reinstall the fender loosely on your motorcycle. Make sure that you are satisfied with the proposed fender length as well as the angle of the proposed cut. Make any adjustments to your measurements and, if necessary, mark a new cut line while the fender is still on the motorcycle and check it again.
Remove the fender from the motorcycle. Place it on a surface where you will be able to hold it securely while being able to use both of your hands to make the cut. It is safer to have a helper hold the fender while you cut it.
Install a cutting wheel on the arbor of either your cut-off tool or angle grinder. Tighten the cut-off wheel to manufacturer's specifications. If you are using an air tool, wait until your air compressor is charged, adjust the regulator's PSI setting then attach your tool to the compressor's hose.
Put on your face shield and ear protection. Clear the cutting area of anything flammable. Cut-off tools throw off showers of sparks that are capable of igniting flammable materials. Holding either the cut-off tool or angle grinder as recommended by its manufacturer, slowly cut along your marked line. Do not bear down too hard on your tool. Let the tool do the work for you.
Apply cold water to the freshly cut surface of the fender with a clean rag to cool down the metal. Reinstall the fender loosely on the motorcycle. Stand back and check the length and angle of the cut. If you are dissatisfied, remeasure, mark and cut again. If you are satisfied, remove the fender from the motorcycle.
If your cut involves a curve that is too tight to safely accomplish with your cut-off tool or angle grinder, make smaller straight cuts then grind them into a smooth curve later with a grinding wheel.
Never remove the safety guards installed on cut-off tools or angle grinders. Always install the proper cut-off wheel on an angle grinder. Do not use a grinding wheel to perform cutting operations. Your fender will require finish grinding, sanding, surface preparation and repainting before reinstalling it on your motorcycle. If you are not confident in your skills involving any of these tasks or do not have the proper tools, have a qualified professional perform the work for you.