Motorcycle Design for Handling (Continued)
by:shift1313

There isnt really a whole lot to talk about on frame construction. There are many different types of frames out there and typically you will see a perimeter frame or twin spar frame on most bikes now. Older bikes typically slant the motor forwards and stick the motor partially in the frame using the under tank area as an airbox. Bikes like the r1 take it a step further and rotate the cylinders more to keep the motor out of the frame. This allows the frame to be much thinner, but the basics of the frames are still the same.

When designing a frame you really have to think about the forces. The weakest point of most frames is the steering head. This is a single point(unlike the rear) that the frame terminates at. Most bikes will come off of the steering head and go as wide as possible. Older bikes that ran a "spine" from the steering head tend to flex more in turns and under braking loads. It is easy to see that a perimeter frame would offer more stiffness at the steering head over the old design. The rear of the frame where the swingarm attaches also needs to be very strong. Most frames, old or new, will have a large solid section here. Either cast or machined. Older bikes will have thicker plating with bends in the edges around this area and have a solid section for the swingarm pivot. This is because under turning the swingarm can introduce a tremendous twist in the frame.

Connecting these two points(steering head and swingarm pivot) can really be done anyway you want. A trellis style frame(ducati) is very common because it is fairly easy to build with minimal weight and very good strength. Here is a pic of a ducati frame at the steering stem.

 

Here is an sv650 frame

you can see how they tried to weld their tubes as far apart as possible to spread the load out on the steering head. It is also important to note that most frames use the motor as part of the structure(stressed). Espeically the hondas which use the motor as the swingarm pivot as well as part of the frame.

When designing the rear end of your frame you should always make your swingarm pivot as close to your CS sprocket as possible. This is the best location for it for several reasons. The most important other then the rear end stuff already discussed is problems with chain clearance. if your pivot is off line or far away from your sprocket then your chain will do funny things during your suspension travel.

When designing a frame a good background in mechanics is helpful. Knowing what structures offer what stiffness is important. The most used geometric shape, for good reason, is the triangle. Triangles are really good for framing because of the way they distribute the load applied to them.

Another important thing to mention is joints. If you are using round tubing it is very important to notch your tubing properly. By no means do you want to weld two tubes together that arent touching. A gap means you are soley relying on your filler material to hold. You always want to use welding rod that is rated higher then your material but by no means would you ever want to trust a weld bead alone.

At all your critical joints you want to make sure to use gussets. If you are using square tube for your frame you want to make sure your gussets are on the corners of the tubing. This is actually the same thing you want to do with round tubing as well but you will bring your gusset half way down the side of your tubing. I will try to provide drawings later. Again this is to spread the load out and place it on the strongest part of your tubing. You want to gusset any point that will see several loads. For instance your shock mounting point, motor mounts and your steering stem to name a few.

When constructing the frame you want to avoid any unneccessary bends in your tubing of choice. Bends will introduce weakness that you dont want. Any bend, especially without the proper bender, will stretch the material out on the outside of the bend leaving you less structure. If you need to bend 90 degrees try to bend two 45 degree segments if you can. This isnt always the case but as a general rule you should try to minimize this. Especially if you are using square material as it will buckle the inside of the bend on your tubing greatly reducing the strength.

I will mention welding here but if you are tackling a frame you should already know a good bit about welding. You should know how heat affects the metal. Too much heat, cooling too quickly or creating a cold weld should all be common knowledge. There is alot at stake when you build a motorcycle frame. Your frame jig should also be very stout to help prevent distortion when welded but you should also try not to make large weld beads, but rather do small sections at a time.

Continued on next page