Your Motorcycle Frame Keeps it all Together

Skeleton in the fiberglass

What's the sole structure supporting your bike's hulking powerplant, elaborate fenders, weighty fuel pod and your own considerable, middle-aged heft? Yes, Mr. Trebek, it's but a simple metal frame. That mysterious backbone of twisted tubing also affects the performance of your motorcycle, and virtually all the essential mechanical systems are attached to it. It's also a big reason motorcycles are such interesting vehicles—the variety of frame designs is much wider in the motorcycle world than in the four-wheeled field, where frames remain hidden and high costs make unitized auto bodies excruciatingly similar.

Motorcycle frame, the skeleton in the fiberglass
Your motorcycle's frame is what is supporting all of the components of your bike.Illustration by Jim Hatch

Geometry Revisited The steel skeleton isn't just holding up your butt—a basic function of any frame is to provide a stiff, strong, non-flexing mount for the engine, suspension and wheels. The frame also holds all auxiliaries such as the fuel tank, indicators and batteries. Geo­metrically, it must allow for the appropriate steering head angle, desired stiffness, distribution of weight and fixed wheelbase too. To a large extent, frame geometry determines a motorcycle's handling characteristics.

Motorcycle frames are generally composed of a steering head tube with a link to the rear-fork pivot point. From the pivot area, the frame splits out to the sides in order to hold the rear fork/swingarm, while the lower cradle passes back under the engine to link the head tube with the swingarm.

Rock the Cradle

  • Every type of bike has a corresponding frame structure. The type of design may favor functionality, visual appeal, engine type, weight distribution or the cost of its particular model—it depends on the purpose the vehicle is intended for. To accommodate their massive powerplants, cruiser frames tend to be heavier than sportbike frames, which lean toward weight savings and performance gain. There are three basic modern frame types.

  • The most common type is called the cradle or tube frame. This simple design derives its strength from the triangulation of support tubing at major stress points. A center tube starts from the steering head and passes over the engine, where it splits in two around the swingarm pivot, then joins up with the cradle and wraps around the lower engine to link up with the head tube. The cradle consists of down tubes that extend under the engine and reattach to the frame near the swingarm pivot. This frame is relatively lightweight, extremely strong and easy to fabricate. The perimeter frame shown is a variation on this design. Its direct path between two load points makes the frame stiff and light and allows room for engine intake and exhaust systems.

  • In a backbone or spine configuration, the engine hangs from the top of the frame and acts as a structural member. This design requires the frame to be relatively heavy in order to provide adequate strength and support, but since the frame tubes don't enclose the engine, service is simple.

  • A pressed or stamped frame is two pieces of stamped sheet metal welded together, with strength determined by the shape of the stamping. This type of frame is the most cost-effective to produce and is used on inexpensive, small-displacement bikes. A variation is the monocoque frame, in which the body has a structural function. Instead of individual tubes, the monocoque is one large, welded, box structure. The "box" has the steering head welded to the front, and the design arches over the engine to the swingarm pivot. The airbox, cleaner and battery box can be incorporated into the main frame box, thus saving space. Scooters are classic examples.

  • Motorcycle frames are constantly being updated, so it's not unusual to find combinations of these designs.

The Material World The materials used in construction are also crucial in achieving a frame's characteristics. Frames are traditionally made of steel, aluminum alloy or chrome alloy. Steel is coveted for its great strength, malleability and low cost; all these qualities have made it ubiquitous in frame fabrication. Aluminum is also becoming a more popular choice for frames, thanks to its light weight and rigidity. It's one-third the weight of steel, but it also has one-third the strength and is more expensive in its raw form.

Next time you sink your haunches into your cruiser's overstuffed saddle and thumb the starter, consider what's holding you over the asphalt. It'll give you new respect for your old high school geometry teacher.