Motorcycle frames are fabricated from tube steel or aluminium alloy tubing, and either type is vulnerable to bending as a result of impact. Frame shops use metal jigs when straightening frames, and each shop has its own method of straightening. In the event your motorcycle frame is seriously bent in more than one place or at the steering headset, a frame shop is the best solution. However, you can correct minor frame damage in your workshop using basic shop tools. Straightening a motorcycle frame is done with the engine, gas tank, wheels, suspension and all accessories removed from the bare frame.
Secure the motorcycle frame in an upright position on a sheet of 3/4-inch plywood using pipe brackets. Put a bracket over each of the lowest frame tubes and fasten the brackets to the plywood with 3/4-inch wood screws. Drive the screws using a cordless drill with a screw tip attachment. In the event a lower frame tube needs straightening, secure the frame on its side.
Inspect the welds at each end of the bent section of the frame. A cracked or broken weld must be repaired before the section can be straightened. Repair any damaged welds before proceeding.
Attach a length of nylon string line at each end of the section of the frame tube that needs to be straightened. Tape one end of the line with duct tape at one end of the bent section, then string the line to the opposite end of the section. Pull the line so it is straight and secure the other end with duct tape.
Measure the distance from the string line to the centre-most portion of the bend. In most cases, a bend that does not exceed 1/4-inch from the string line can be straightened without the services of a frame shop.
Remove the screws that secure the pipe brackets to the plywood using the cordless drill and screw tip attachment. Set the pipe brackets and screws aside.
Lay the motorcycle frame on the plywood with the bent section of the frame atop the plywood. Secure the motorcycle frame to the plywood using the saved pipe brackets and wood screws. Place a strip of duct tape along the upper side of the bent frame tube to prevent scarring or scratching the metal during the straightening.
Measure a length of 1 1/4-inch half-round steel pipe long enough to span the bent section and afford you a handhold at one end. Cut the piece to length with a hacksaw.
Wear safety glasses and gloves. Position the length of half-round steel pipe over the bend. Stabilise one end of the half-round pipe with one hand. Straighten the bend by striking the half-round steel pipe with a steel mallet. Inspect the straightening as you go by checking the gap between the bend and the plywood.
Remove the pipe brackets and stand the motorcycle frame on the plywood. Attach a length of string line alongside the straightened frame tube using duct tape.
Repeat the straightening procedure until the distance between the string line and the centre-most portion of the bend is no more than 1/32 of an inch.
Enlist a frame shop to evaluate the alignment and truing of the frame.
Do not weld enclosed or sealed sections of the frame tubes. Drill hot-air relief holes, if necessary, before welding enclosed or sealed sections.