Motorcycle frames have everything to do with performance and feel of the motorcycle. The motorcycle frame, suspension, and engine work in tandem to deliver an overall riding feel of the motorcycle. Modern day motorcycles are built on a variety of frames, taking into consideration things such as weight distribution, frame strength, and geometry.
Note: Information presented on this page is for educational purposes only. Always rely on your motorcycle owner’s manual and the manufacturer’s information for full user handling, care, instructions, etc.
Introduction to Motorcycle Frames
In the early days, little engines were fitted to bikes and declared motorcycles. Luckily we have come a long way and frames have changed. The first pure mc frames were built from thin steel, and in later years the steel was pressed to give it more strength. The major battle among Japanese manufacturers to build more powerful engines left many frames too weak resulting in wobbly / weave riding characteristics.
As more strength was required from the frame, steel tube designs were introduced with the Norton Featherbed leading the way. Steel tubes were triangulated to form a strong complex web in which the engine would rest. Later development saw the engine become a member of the frame as the steel tubes were linked with the stability of the engine.
Generally a cruiser or custom motorcycle will have a longer frame and the weight ratio more towards the back. This gives the motorcycle a slower reaction time, bigger moving space and overall easier going riding position. Sports motorcycles tend to have a shorter frame and the weight more forward. This makes the motorcycle steer sharper, react quicker and a more edgy riding style.
Motorcycle Frame Components
Before you purchase a motorcycle frame, or even design one yourself, you first need to figure out the amount of rake, stretch, tubing thickness, neck, width, offset, and other factors.
A big degree rake is what you are after if you want a radical show bike and don’t care much about handling on the road. However, for real road stability and ease of handling, you’ll want a lower degree. Around 30 to 35 degrees is widely considered to be the best for handling. Remember, the higher the rake degrees, the less cornering capability the motorcycle will have. A longer front end may look really cool, but the bike will be hard to handle on the road.
Custom motorcycle frames are described in terms of “stretch”. The amount of stretch in the downtubes refers to how much higher the custom frame is than the stock frame. Stretch in the backbone refers to how much longer the custom frame is than the stock model frame. Stretching the backbone and raising it up creates different ride configurations. The trick is to get the grouping just right. Too much stretch will make the bike flex a lot on the road. Raising it too high will make it uncomfortable to ride for anyone but a very tall person.
Neck style is another element to think about in a custom frame. An hourglass shaped neck uses old style fork cups that come with the races already installed. With a straight neck, you have to install bearing races.
You’ll want to think about the weight of the motorcycle frame. It makes a big difference in how smooth a ride your motorcycle will have; generally the heavier the frame, the smoother the ride. Of course, if you know you’ll be drag racing, you may want to go with a lighter frame.
Sportster frames are designed for the Sportster engine and come in a variety of styles. Due to the added weight and rumble of the big Sportster engine, these custom frames need to be extra strong. There are also Sportster frames designed for oversized tires. The Hardtail Chassis is a good choice for larger tires.
While Sportster frames have no stretch, rolling chassis frames are any type of rigid, swingarm, or softail motorcycle frame with front and rear wheel assembly and a stretched backbone or front downtubes.
If you prefer a long, low look to your motorcycle and you aren’t so concerned about comfort, you’re going to want a rigid motorcycle frame. It gives the feeling of sitting down low on your bike with the front end stretching out in front of you.
The softail frame, popularized by Harley in the early 80’s, was in such high demand that the company couldn’t keep them in stock. The hardtail look is still the most popular frame design for custom motorcycle builders today, but the design now incorporates horizontally mounted rear shocks underneath.
Twin-spar motorcycle frames are mainly used by sport bikes because they maintain their strength against the incredible power output of high-performance sports bikes. Racing bikes occasionally use titanium, magnesium and carbon-fiber.
The trellis motorcycle frame, which is lightweight and very firm, also offers greater strength than a twin-spar frame, but is more complex and difficult to build.
FXR is widely regarded as one of the best motorcycle frame designs ever created. It is great for both handling and long distance riding. If you’re building a racing bike, dirt bike or are going to be riding on rough terrain and need durability and extra suspension, FRX motorcycle frames are your best option.
Motorcycle Frame Definitions
- Single cradle frame – a main backbone above the engine with one (or two) down tubes in front of the engine. Additionally the frame surrounds the engine with a sub frame going to the back.
- Stressed engine frame – the engine becomes part of the frame (saving weight and space) additionals are bolted on to the frame, which creates a stiff chassis.
- Backbone frame – the engine sits below a top frame bar (tube). This kind of frame is mostly for smaller engine motorcycles
- Diamond frame – a single tube hangs down from the backbone on which the engine is bolted, only then does it form a complete chassis for the motorcycle. Mostly used for smaller engine motorcycles.
- Frame Rake – refers to the steering head angle which often is the same as the angle of the fork tube angle.
- Frame Trail – refers to the travel distance of the wheels / frame. Said simply – the shorter the trail the more direct a motorcycle steers.
- Wheel Base – the wheel base of a frame refers to the distance between the wheel centers. (overall length of a bike is the distance between wheel ends)
Motorcycle Frame Tips
- Frames are made to be stiff, if a frame flexes the wheels aren’t in line anymore and the steering goes bad on you.
- Frames get most of their strength from the material used, form of the tubes, the structure of the tubes and reinforcements.
- Periodically check all bolts on the frame (frames don’t need much maintenance) but do check all bolts often, you’ll be surprised at what comes loose over time.
- Over time the motorcycle (frame) might feel a little unstable this is often caused by worn and loose bearings (e.g. a clunk in the steering, drifting in a straight line, vibration in the bars, etc)
- Anytime you are dealing with a motorcycle from a crash the frame will need to be checked by an expert.
- Frame maintenance should be a regular part of your service schedule. For example, check swing arm bearings and linkage, lubricate swing arm and bearings, check steering bearings, check nuts and bolts.