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Thinking Around the Box
Screw the box. We're cutting it up for templates!
By: Shaun "Scrapyard" Kelly
See other issues of the Naked Newsletter Streetfighter Motorcycle Frame

 

In the wonderful world of Fighterdom, you are only limited by your imagination. In the grand scheme of things, we are working with 3 main elements: the wheels, the engine and the seat. The way in which these elements are connected, to work as a whole, are practically limitless. Chassis design, suspension components, and body design are all key factors of what makes a motorcycle unique. Each part that is assembled with the machine not only adds in its function but also to it's style.

In coming installments I'm going to focus on ideas and theories that hopefully inspire some of you to go out to the garage and build some unique and interesting stuff. This is not going to be a blow by blow tech section of how to install parts or make things work. That's for you all to figure out. Although I will throw in some tech handy info of how things work when I can. In large my job is to drop little ideas in your brain and get the wheels turning. It's time for all of us to get away from store bought parts and services and get back to getting in there and getting greasy.

One thing that has always intrigued me was frame design. With out a frame we basically have a pile of random parts, but there's more to it then just a place to bolt it all together. This is the beginning and end of how well your bike is going to perform. You can put the best suspension, biggest brakes, and lightest wheels you can afford on your bike, but if the frame is weak and twitchy all those killer parts are for nothing. In the day and age we live in, it's hard to beat the quality and construction of the latest and greatest factory superbikes. Let's face it. They have the time and money to put all the R&D and design into them they want until you have a super light, super rigid frame. Now that doesn't mean we have to swear off the idea of designing our own. This is kind of the crème de la crème of building a special. A truly one off frame separates the men from the boys so to speak. For those of us with older bikes this can greatly help overall performance and handling by making the bike much more sturdy and responsive. But also on a styling note this is where you can really start laying out the look of a bike. The chopper crowd has been stretching and raking frames for decades. This was for a mater of style. But when you start building your own frame you can make it all work together. You can have the style and performance in one package.

 

Different Streetfighter Frame Designs :

Trellis Frames:

For those of you that have been around the fighter scene, you no doubt have seen the beautiful examples built by Spondon, Harris, RAU and some of the later builders such as Martek, Steelheart and PEST. The most common design of these frames is a trellis format, similar to Ducati and MV Agusta. Built from either large diameter aluminum tube or chromoly steel, they offer a very light and rigid construction, but also give the builder a range of adjustability in component placement and over all layout. Mounting points can be made to fit many different engine configurations and also allow for a range of suspension locations.

 

Backbone or Cradle-Style Frame:

The next form of frame configuration is the backbone style or cradle frame. Most commonly found in older bikes and traditionally styled cruisers. RAU has adapted a version of this to act as a stressed member frame. Instead of a full cradle frame that has mounting points for the engine to sit into, they use the engine to mount the frame. This style does away with the lower frame rails. It incorporates a large diameter tube for the "backbone" and a section of down tubes to connect to the engine mounts. Honda has also adapted this style frame on their Hornet line. This setup is not only very rigid but also allows to showcase the engine. The down side is the frame has to go over the engine witch can make the bike rather tall.

 

Twin-Spar or Twin-Beam Frame:

The twin spar or twin beam frame is one of the most commonly used frames among sportbikes. If you've owned a GSXR, YZF, CBR or ZX of the last 10 years you've ridden a twin spar. They are very rigid, and strong frames. They can be heavy and bulky compared to other examples but in recent years the manufacturers have come up with new technologies to help cut the weight while still maintaining the strength. Arguably not the prettiest of designs but can be made to work as eye candy if done well. Another advantage of this design is the ability to hold or hide some of your components or fluids. Case in point the Buell XB series with its fuel in frame design.

 

 

Monocoque Frame:

One of the most unique designs is the monocoque. This can be done a number of ways. A number of panels that bolt together, tubes that mount in triangulated shapes, even just bolting everything to the engine. It's a complex structural design that pulls the machine together in a very unorthodox way.

As for materials these frames can be made from, the range is also large. Steel and aluminum being the most common, although there are examples that have been made of carbon fiber, stainless steel and titanium. All these materials require a certain amount of skill to work with. Steel is probably the easiest and cheapest although is the heaviest of materials. The up side is it is the most resilient to flexing forces and if designed right can weigh in very close to an Aluminum frame. Aluminum would be next up the scale but takes more skill to work with. Welding aluminum can be tricky. It melts faster then steel and you have to have a good working knowledge of this material for it to weld right. It is very rigid and strong though and also half the weight of steel to gain the same strength. Titanium is very expensive and hard to get in large quantities but is the lightest and strongest of the more common metals. Carbon fiber is also getting harder to get and requires a totally different set of skills to work with. It also requires quite a bit to gain the same strength as the aforementioned metals. It is brittle though and has been known to shatter under the right forces.

So how do we lay all this out and make it work? Well, like I said earlier, that's what you have to figure out. I will say this. Start drawing lines from the steering head to the swingarm pivot and go from there. Look at some new bikes and how they are laid out. Specifically, look at there mounting points; steering, engine and rear end. Break out your imaginary pen, A.K.A. your finger, and start drawing lines from one point to another. I'm interested in what you'll come up with. There's a lot more to this as well, such as component positioning, center of gravity, rake, trail ride height etc., but we will attack these issues another time!

 

Here are some interesting sites to check out that'll delve into this issue more.

http://www.eurospares.com/other.htm - a great site with many articles and photos dedicated to building sport oriented bikes.

http://www.spondonengineering.co.uk/ - The official home page of Spondon Frames.

http://www.steelheart-engineering.com/ - An up and coming frame builder.

http://www.dbbp.com/ - Some awesome examples of motorcycle design. HD based primarily but check out the CAD Bikes section for some neat ideas.

-Shaun "Scrapyard" Kelly

Page 3 - Naked Newsletter - Streetfighter Motorcycle News - June 2007

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