History of Motorcycle Frames

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History of Motorcycle Frames

Each week RideApart looks back at key milestones in motorcycle history from technical innovations to significant model introductions to racing successes and, of course, some of the disastrous things we’d rather forget. This week we look at the origins of the motorcycle frame and some of the developments it has undergone in more than 100 years.

It’s a fact that in the early 1900s, motorcycle manufacturers were in effect nothing more than bicycle makers who found a way to strap an engine to a bicycle frame. This was a bit of a hit and miss affair, as riders ran the risk of the engine often shaking itself loose and falling out of the frame on the move. However, providing you didn’t go faster than 20 mph, which was probably very unlikely as most engines at that time (both cars and bikes) were not that powerful or that fast, you were for all intents and purposes riding the first motorized bicycle that we know today as the motorcycle.

The bicycle frame design that was used back then is more or less the same set up we have today on bicycles with tubes connected to three critical points – the head stock where the steering is mounted, a crank bearing case where the pedals are located and a tube carrying the saddle.

To make a bicycle into a motorcycle, many of the early manufacturers simply bolted or clipped an engine onto the frame. There was no clutch or gearbox and the rider continued to use the existing bicycle pedals to both start and add additional momentum once they were on the move.

Despite the fact you now had an engine mounted on your bicycle frame, the brakes remained the same too and were notorious for being next to useless even on a conventional bicycle let alone on a bicycle fitted with a motor.

As engines became more powerful, motorcycle manufacturers began to see some limitations to using a conventional bicycle frame. Most notable was the fact that the tubes at the critical weld points suffered stress cracks causing the frames to break and collapse if the engines didn’t fall out.

History of Motorcycle Frames

Prior to the First World War, motorcycle development and engineering moved at a rapid pace and manufacturers started to use purpose built frames that could handle bigger capacity, more powerful engines.

To keep this simple, as there have been multiple versions of motorcycle frames over the years, all motorcycle frames are made from welded aluminum, steel or alloy and in some cases carbon fiber.

The sole purpose of a motorcycle frame is to act as base, which all of the components (gas tank, engine, suspension, handlebars etc) can be bolted to. Usually the engine sits inside the frame, the swingarm is attached using a pivot bolt (allowing the suspension to move) and the front forks are attached to the front of the frame via the steering head.

Buell XB12S Lighting
Buell XB12S Lighting

As always there are exceptions to the rule – Buell for example used the frame on its XB12S Lighting as a fuel tank, but in general the following are the types of frame you can expect to find on most motorcycles.

The most conventional and widely used set up, which was used very early on in motorcycles and is still found on bikes today, is the cradle frame. Effectively it’s a loop design, either with single or double tubes, which fixed to the steering head and wraps in, or loops, both the engine and gearbox. If a cradle frame on a motorcycle doubles at the exhaust it is sometimes referred to as a split single cradle frame.

A double loop frame is known as a duplex cradle and is derived from a single cradle frame and is often found on custom bikes as well as regular road bikes as it offers a good compromise between rigidity, strength and lightness.

History of Motorcycle Frames

However, in recent years the perimeter frame has started to surpass the cradle, as it is both stronger and lighter. Derived from racing motorcycles, where a rigid set up is required, the swing head and the rear swingarm are joined in the shortest way possible.

Essentially this done by using two beams that descend from the steering head in the most direct way possible to the swingarm, passing around the engine. Early perimeter frames were made from steel, but in order to improve rigidity to weight ratios it led to most manufacturers adopting aluminum perimeter frames.

Since the 1920s, manufacturers have also used what is called a triangulated frame or trellis frame. Also derived from racing and used on many European motorcycles, the trellis is easy to manufacture, offers exceptional strength and uses the same principles as the perimeter frame as it directly connects the steering head and the rear swingarm. But the frame is made up of a large number of short steel or aluminum tubes welded together to form a trellis.

There is also the underrated backbone frame. This typically utilizes a large diameter top tube, which runs under a motorcycle’s gas tank, and which in some cases have also doubled up as an oil tank on some bikes. This top tube is the dominant load carrying part of the frame, fabricated from heavy gauge steel, but with thin tubing walls to keep the bike’s overall weight down.

Some motorcycles have and continue to utilize the engine and in some cases the gearbox as well, as a stressed member of a motorcycle frame. Typically the steering head is mounted to the upper section of a bike’s engine, while a secondary frame carrying the rider and rear suspension is bolted to the rear.

Monocoque frame
Monocoque frame

Finally there is the monocoque frame – monocoque is a French word that means single shell. On a car or motorcycle, there is normally a frame or chassis that is covered in body panels. With a monocoque the external skin of the vehicle supports most of the load.

On bikes it’s almost exclusively used on racing motorcycles rather than street bikes. A motorcycle monocoque is a single piece unit that incorporates the seat mounting, tank and tail section of the bike. It’s immensely strong and very rigid but expensive to mass-produce.

What motorcycle frames have you experienced and which set-up do you prefer?

  • William Connor

    I have ridden all of these except the monocoque and the original motorized bicycle. Honestly, they have all worked very well. There have been more differences in suspension, engine, and breaks that were weak points or strong points on bikes that vast;y overshadowed what type of frame it was.

    • di0genes

      If you have ever ridden a honda dream or cub, allstate puch or similar 50′s and 60′s small motorcycles you have ridden a monocoque framed mc too.

  • ThinkingInImages

    Duplex cradle, although I like the look of a trellis frame. I thought Buell had a great idea using the frame as the fuel tank. With FI there’s little reason to have a gravity feed tank design.

  • tbowdre

    I hate to nit pick but that is an XB9S

    • TP

      Boom roasted. Also the fuel in frame is used in all XB9s and 12s, the 1125′s and the newer 1190′s as well.

      Great article though.

      • Guest

        Derp. He grabbed the image from Wikipedia and still go it wrong. http://en.wikipedia.org/wiki/File:Buell_Lightning_XB9SX_CityX.jpg

        • TP

          Yeah, but in his defense though they are 99% the same. They have different crankshafts and throttle bodies and that’s pretty much it, isn’t it?

        • Miguel

          In my earlier comment I mentioned the black art of motorcycle design. Eric Buell is the voodoo priest of motorcycle frame design because on paper the XB motorcycles aught to be completely different from the way he made them work and feel. I’m so impressed by what he was able to do with an Harley engine, which should never be on a sport bike, that I want my next motorcycle to be an XB. I would love to chat with him about motorcycle design.

  • akaaccount

    One significant thing that people often overlook about the Buell XB frame is that it also serves as the cooling system for the engine. One of the reasons that Buell could get away with taking a Harley lump that isn’t particularly reliable making 50 HP and make it reliable with twice that is that the frame acts as a tunnel to not only increase flow across the heads, but regulate it. The cooling fan blocks air when off and forces it through when on. The biggest problem with air cooled engines isn’t overheating, it’s over-cooling.

    • Harve Mil

      Overcooling? Is that why my XB12 sounds like a shop vac with the oil cooler cooling fans coming on after 15 minutes of in town riding? No exaggeration, people yell over to me fairly regularly, “Hey buddy, what’s wrong with your bike?” I just tell them that’s just the turbo and drive off before they can followup.

      • akaaccount

        Yes, air cooled engines NORMALLY over-cool. Without regulation, (like a normal air cooled motorcycle engine) the engine cooling fins have to be designed to discharge the maximum amount of heat generated (heavy load) under the worst conditions (no wind.) The XB heads and frame are designed to discharge less heat, so they require the fan under high heat generating conditions. All at the expense of sounding like a drying fan on a grain silo. I have an XB and I’m quite familiar with people’s comments. Do you think that your bike is overheating whenever the fan is on or something?

        • Harve Mil

          I’m sure the bike isn’t overheating. The only thing that overheats is my temper when the drying fan on the grain silo makes people stop and stare. I understand the perilous nature of air cooled engines, and the unenviable task that Erik had making performance bikes out of the Harley parts bin. That was more of a joke on the Buell (which I love) due to how often those cooling fans kick on. Can you think of any other bike that has a more annoying fan?

          • akaaccount

            Ha absolutely not. There’s a Buell commercial on Youtube where a guy and girl are eating at an outdoor cafe when a guy pulls up and parks in front of them on a lightning. The girl’s all turned on, but come on. She’d really be annoyed by the giant blow dryer that was just parked beside her.

      • Carter

        I tell them its the turbine spinning down on my Ulysses and Firebolt. And the Ulysses has a “Loud Fans Save Lives” decal on the hard bags…

  • IAmAConservativeICannotBeWrong

    the zx-12R and 14R have monocoque frames

    and the Panigale is a half-monocoque

  • Harve Mil

    The Norton Featherbed gets no love?

    http://en.wikipedia.org/wiki/Featherbed_frame

    For more I’d recommend Top Dead Center 2. Chassis and Suspension, 1-4. p85-127.

    http://www.amazon.com/gp/search?index=books&linkCode=qs&keywords=0760336083

  • labradog

    I don’t care if you disappear my comment, this article is still shallow bullshit.

    • KeithB

      I will admit that I suggested an article on this topic. Some of us are interested.
      No different that one on brakes or any other part of a bike, but I will agree that better illustrations would have been nice.
      The great thing about posted articles is you have the choice to read them…or not!

  • Mitchel Durnell

    ‘the trellis is easy to manufacture’

    Well, actually no, as a trellis in almost all cases requires a human to hand weld the section together, adding time and cost as well as introducing variance in how rigid each frame is. Suzuki had an interesting solution to this for their SV; the second generation SV uses a stamped/cast truss in place of a trellis.

  • Campisi

    When I hear monocoque, I think early Vespa (or CT70).

  • Miguel

    One aspect of frame design that is unique to motorcycles and worth mentioning is built in frame flex. Designing the right amount of frame flexing is the real ‘black art’ of motorcycle frame design. Too much flex and you get bad braking and handling traits like chatter that can lead to nasty things like tank slappers. Too little flex and the motorcycle is nervous during cornering. Car frame designers can solely focus on reducing frame flex while motorcycle builders need to find just the right amount of flexing.