Motorcycle Frame Modifying Using Jigs and Fixtures

of 01

Motorcycle Frame Modifying Using Jigs and Fixtures

A fabricator is manufacturing a jig for this Ducati frame before making any changes to it. Note: The headstock is the next item to be secured on the jig. John H Glimmerveen Licensed to

Motorcycle manufacturers spend considerable amounts of resources on designing and developing their motorcycle frames. In many areas they are forced to compromise to be competitive on price in the marketplace, so a design that works for everyone is rare. Modifying a frame is often done to personalize a motorcycle--building a café racer, for instance. But these modifications must be done carefully so as not to compromise the integrity of the machine.

When the manufacturers produce a frame, they do so with the help of jigs and fixtures. These jigs not only align the various parts, they are also used to clamp items during the welding process. If tubes, etc., are not clamped during welding, they will pull as the weld cools, causing misalignment.


Strategic alignment on a motorcycle frame primarily involves the headstock, engine and swing-arm. As these items are some distance away from each other, any misalignment will be greatly exaggerated. For example, if the headstock is just a few degrees out of line, by the time the misalignment has reached the tire to road interface, the wheel could be offset from the center line by a considerable amount.

When modifying a frame, (for example, by removing the stock rear fender loop), it is imperative to hold or brace the frame before any cutting. A steel brace between the two outer frame rails will greatly reduce the amount of twist or pull as any tubes are removed.However, as in frame  design in general, triangulation is advisable to ensure a tube stays in its correct position. 

Adding or removing small brackets is unlikely to affect the geometry of a motorcycle frame, but welding must be kept to a minimum so as not to distort it. In addition, the mechanic or fabricator must be extremely careful the hes does not cut into the main tube--for example a frame downtube. Any small cuts into frames can lead to sudden failure under load. If the frame does get damaged in this way, the mechanic must get the offending cut welded to ensure the tubes integrity. 

Major Modifications

Making major modifications to a frame must be done by an experienced fabricator/welder. These major modifications must be done with the aid of a jig (as seen in the photograph), the manufacturing of which is a highly skilled endeavor.

As an example, the Ducati frame seen in the photograph is to be extensively modified. To successfully accomplish these modifications, the owner has made a substantial jig or fixture to hold the frame’s critical geometry in place. The fabricator has located the swing-arm pivot points, the lower engine/gearbox mountings and the rear shock’s top mount. These items were located first to ensure the various points were correct to the jig’s mounting face. The headstock location will be the final part to add to the jig.

Although using a jig in the manner described will greatly reduce the risk of misalignment, the finished frame must be checked for accuracy. Although a certain amount of misalignment is acceptable (it will vary depending the use/type of frame in question), the owner must keep it to a minimum.

Occasionally, an owner will contemplate "smoothing out" the welds on a frame for appearance purposes. This must be resisted as the weld strength will often be reduced and can lead to failure of the construction. 

It is highly desirable to seek the advice of an experienced fabricator before commencing any modifications. 

Further Reading:

Motorcycle Frame Alignment

Welding on Classic Motorcycles