Motorcycle Design for Handling (Continued)
by:shift1313

 

Ive been thinking about this topic for a few days and im really at a loss with what to write about it. I think the best thing i can do is maybe make a few cad drawings of things like the steering stem, swingarm mount and stuff just to show generally a good way to do things. There are really infinate possibilities on how to connect the steering head to the rear of the frame. One thing I would like to mention is a good way to test your frame once its built. Because i can almost guarantee the weakest point will be the steering stem, you can bolt the rear of your frame to something sturdy like the floor or a metal work bench, possibly with the swingarm and shock linkage. With the rear bolted to something take a long tube and slide it through your steering head and that will let you put some good twisting force on your frame. You should be able to see right away where your frame is giving or if its rigid then hats off to you.

here is a spondon jig

those are just a few. In general you need to have locating points for your swing arm pivot and steering stem. The more points you can locate with your jig the better. Alot of people start with the motor. The jig should be able to hold the motor at ride height. Unless you are dead certain on everything it is to your benefit to make things adjustable. Extruded aluminum framing is available for a high price but it is very common on frame jigs because of its adjustability. You can also use structural I beam and weld plates on to it for mounting. For example weld a flat plate on an I beam with 4 holes in it. Then make a plate that is the same width but longer with sloted holes in it. This plate will have some 4"x4" square tubing going up to your steering stem. This will allow you to move it forward or back if needed. If you are 100% certain the location of your points and you are certain you can weld your jig together(and still be able to remove your frame) then by all means. Any adjustment point will also allow for small error if it moves so keep that in mind as well. Whatever you use for the base, or spine of your jig, it needs to be sturdy. I beam, large box tube etc, these are all fairly cheap compared to buying a jig. If you go down this page link here you can see the turbo 900 project will lots of good pics of mikes frame jig. In the picture you can see the extruded aluminum i mentioned.

This is a great way to do it. You see he used cases and has a "holder" for the crank. Also take a good look at his steering stem area. You can see how he triangulated the bottom tube with the top tubes back about a foot from the frame. Also below the bottom tubes there is a plate with holes in it. This is another way to spread the load out. I can imagine that this bike will be very stiff.

Material Choice

Well first with frame construction there is no perfect frame. Every frame will be suited for its application and wont be optimal for other uses. Your material choice and frame design have to go hand in hand. You cant design a frame without the material in mind before hand. An aluminum frame must be designed different then steel. The only thing that will remain constant between different materials will be the actual construction of the joints. If you are using square tube you can simply cut the tubing at your angle and weld. This does provide some limitation though. Say you have a piece of square tube sitting vertical and you want to join one that is cut at 45degrees but you also wante it rotated 30 degrees. There will be alot of "overlap" of material and is not a good joint to weld together. On the other hand if you run round tubing you can "notch" the tube at this compound angle with out any problem. A simple explaination of notching is simply using a hole saw bit to cut your tubing. Basically you dont want to weld a joint together that has any gap to it. So with round tubing you want your cut to be round as well. There are lots of notching setups out there but if you take your time and measure and setup carefully you can do it with a drill press. Notching setups usually cost between $100+ USD. Well worth it if you are doing alot of notching. Also the choice of square tube vs round tube will require different gussets at the corners and joints. If you are considering building a frame i reccomend you take a look at some factory frames and see how their gusseting is done.

I mentioned round and square tubing but you are not limited to these by any means. You can build your frame out of sheet metal if you want to. You can make wooden forms and hammer form your sheet metal around these forms in two pieces then weld them together making your own "box" section tubing. One popular thing to do is use 1"x1" square tube and weld 3 together making a 3"x1" frame section. This dosent weigh much but it offers "ribs" in the center of the frame section for increased rigidity. Again these are just a few things that are done but you are only limited by your imagination.

Now with type of tubing and frame sections out of the way you have to think about materials. Materials will dictate the thickness of your sections as well as the difficulty in forming and welding your frame together. And with that comes more deforming from heat, possible contamination in welds and weak spots. By far steel is going to be the easiest choice for several reasons. You have several options for welding and forming steel. Steel weld is generally not as suceptable to contamination as aluminum. With steel you can use thinner material then with aluminum and will require less structure. Steel, especially a series with chromium or molybdenum have high tensile strength vs aluminum alloys. But with that comes a higher weight. The breaking strength of say a 4130 chromo tubing is going to be 4 times larger then that of an aluminum alloy but the specific weight is going to be about 3 times larger. This will change depending on the alloy but this is a good general idea. Since steel is so much easier to work with, and more maleable it is usually the choice of custom frame builders. Factory bikes have big advantage with r&d and machining facilites. Factory frames will usually be aluminum. The main sections will be either custom extrusion or formed sheets welded together. The steering head area will usually be cast with extensions to weld to the main sections. And then the rear engine cradle and swingarm area will usually be machined, or cast of a combination of both. Constructing a frame this way allows you to use the needed alloy where its best suited. So the rear cradle and the steering head can be designed to transfer the forces into the main section of the frame which can be designed as light as possible but still rigid enough. This is going to be way out of the range of a home builder but if you have a large enough budget you might be able to outsource some of these parts cast to aid in your build.

You also have composite materials like carbon fiber or fiberglass that "can" be used as framing. I wouldnt attempt this without a serious background in composite work as well as some extensive computer software to annalyze stress and help design appropriate framing. It is very important with composites that the proper core material, thickness and direction of weave be taken into account. If you design a stressed member and your weave isnt in the correct orientation the frame will fail very quickly. You also introduce engine vibration which can and will cause failure. I would only use composites for low stress or bodywork applications only. Also it might be important to note that structural components made from composites have a much shorter life expectancy then other materials. Even aluminum frames have a lower life then steel ones.

With all that being said I cant tell you want material or even what thickness material to choose. You have to either start with that in mind or design your frame and then pick the material to suit your needs. If you dont have a background in materials or fabrication i would first study some physics on forces. Buy a book on motorcycling design or do some research on the internet to figure out what forces a motorcycle frame is subject to. Designing for a 250cc vs 1000cc will be totally different, as well as a standard vs a GP bike. If you have the time and money you can do a trial and error approach. I dont reccoment it but it is one way. You will save yourself alot of time and guess work if you either use drawing software or know someone who does. Software has very powerful stress annalysis built in that can be a big help when designing a frame. Tony Foale has a great website with softare on it that might be useful.

http://www.tonyfoale.com/

There are alot of great photos, tech articles and basic helpful knowledge. Tony has built alot of bikes and designed some crazy things. One useful piece of free software on his site is the tubing profile cutting guide. You can input what diameter tubing and what angle it will join another tube and this will give you a printout to lay around your tubing. Very helpful if you dont have a tubing notcher. There is also a structural sections calculator which might help when designing your frame if you dont have a cad software package with this feature. I havent read through the whole site but if you are serious about building a bike then i highly reccomend atleast checking out the articles section and downloading the freeware.

Good luck!

I was just reading up on the new 2007 r6(because i love the swingarm) and i found two interesting bits of info in their features section. First one was their weight bias:
"MotoGP-style 52.5-percent front-wheel weight bias helps produce razor-sharp handling."

...and second was...

"Light, Controlled Fill swingarm pivots high in the frame, for increased anti-squat effect and mid-corner stability. "

I thought the first was was interesting since thats how i biased my bike, now i know I was right and there is a reason it handles well. Second thing basically leads back to the section on the rear end. Having the right chain pull angle which is directly related to the swingarm angle.

-shift1313