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Motorcycle Factors: Vehicle Modifications Table of ContentsHomenhtsa

Introduction to Motorcycles

Motorcycle Design


Vehicle Modifications


Lane Use


A motorcycle’s relatively simple design and availability of replacement or accessory components make it easy, inexpensive and popular to modify with unknown safety consequences.

Virtually every part of a motorcycle can be modified, and many modifications can affect the safety of the vehicle. Modifications aimed at improving or changing the way the machine works include those directed at engine performance, comfort, handling, braking, or cargo capacity. Some changes are made to personalize and customize the appearance. Even seemingly simple, routine changes—such as fitting new tires—can change a motorcycle’s handling.

Some changes (such as upgrading suspension components, tires, or brake components) can be purely beneficial, while others can be mostly detrimental from a functional standpoint. Many involve trade-offs. For example, a motorcycle that is lowered to give the rider a more surefooted stance at a stop gives up some cornering ground clearance and suspension travel. Some changes, such as major frame modifications or use of an aftermarket frame, can change the entire character of the motorcycle. Installation of a sidecar or a three-wheeled “trike” kit creates an entirely different class of vehicle that no longer handles or responds like a two-wheeled motorcycle.

Although trailers towed behind motorcycles have become more popular, we know of no data that indicate their effect on motorcycle behavior. Most motorcycle manufacturers warn against their use, however.

Users may install aftermarket components or make modifications that their motorcycles were not designed for or tested with. They may combine modifications that were not designed to be together and when combined have unforeseen effects on the performance of the vehicle. Riders may also fail to understand all the consequences of a change. Some changes also lend themselves to misuse. Adding a cargo compartment or a luggage rack at the rear of the motorcycle, for example, may allow the user to place too much weight there despite labels warning against it. A change in weight distribution can significantly alter how a motorcycle handles.

The Hurt Report showed that modified vehicles were over-represented in crashes. However, the types of vehicles created by the modifi-cations specified in that study—known then as semi-choppers—now constitute the two largest subcategories of original equipment street motorcycle: sportbikes and cruisers.

Because a motorcycle created by an aftermarket or user-created modification is much different than one built by a manufacturer, the current situation has changed too much for that aspect of the study to be relevant. The Hurt Report also found street motorcycles with modified exhaust systems were over-represented in crashes.

As with original equipment, the quality and safety of most aftermarket components have steadily improved, although seemingly they haven’t reached the levels of original equipment components yet. Users have access to more information of such products from manufacturers than in the past, and the user is likely to be better informed of the possible drawbacks to the modification. The liability climate has also made suppliers and installers more cautious about modifying motorcycles.

Some of the most questionable modifications that were popular during the era the Hurt Report was conducted, such as removing the front brake, have fallen out of style. It is not clear, therefore, if modifications are still a significant factor in motorcycle crashes.

The modifications favored by motorcyclists change with technology, fashion, and other factors, which makes most specific regulations unfeasible. Some countries, such as Germany, require that prior to any sale of a motorcycle, any of its modifications must be tested and certified. Although this may prevent some crashes, it may also cause some by limiting the riders’ access to superior tires, brakes, suspension, and other components.


The current role of vehicle modifications in motorcycle crashes should be better understood.

All aftermarket vendors should make safety a priority in the development of motorcycle accessories.


Any future studies of crash causation should certainly examine the role of modifications to motorcycles, particularly major changes such as chassis modifications, sidecars, and trailers. Since some alterations may be under-represented in motorcycle crashes, that issue should also be addressed. Education of riders may be a better approach to dealing with modification-related problems than regulations.


• Study the role of modifications in current motorcycle crashes.

• Educate users about how modifications and loads can change the operating characteristics of their motorcycles.

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