Designing the Frame

Designing of the steel tubular space frame, where to start? What is the mission of frame in track motorcycle? Engine, wheel axles, swing arm to frame axle, seat, shock absorbers and fuel tank are in certain places. All these affect to the lap time. Frame combines these all together. It has to be stiff, so that chassis will work and light so that overall weight doesn't be any more than it has to be. Balancing once again in the world of different kind of terms.

Designing is understanding, you have to understand matter widely as possible while you keep our eye on the goal. By understanding and often over time with designing software you can get insight, which will take you closer to goal or in worse situation further away of it.

I will tell about designing of our track motorcycles frame, one part at the time. I will start from the attachment of the front shock absorbers till I come to the rear wheels axle attachment.

Front shock absorbers attachment:

1. Front wheel axle

2. Lower attachment bearing of front shock absorbers

3. Upper attachment bearing of front shock absorbers

Designing front shock absorbers attachment main boundary conditions are force generated by maximal braking situation and front rake angle defined by chassis design (a). Force generated by maximum braking situation comes from friction of tire and asphalt. Rubber type and condition of asphalt affect to this considerably. Force moves via wheel axle to shock absorbers, from those to attachment bearings and from there to frame. Force is descriped in picture by yellow arrow at front wheel axle point (F). Measument A descripes the distance from lower attachment bearing to front wheel axle. This distance creates moment to force F. Longer the distance is, more force is going to frame. This distance is affected by wheel size and shock absorbers maximum stroke. Forces generated by maximum braking is descriped at the points of bearings by green arrows. Distance B act as so-called counter moment, longer distance is, less have to frame support at point 1, but this lengthens shock absorbers and some tubes in frame. As good as possible middle course in this once again.

We decided distance between bearings to be 170 mm, this is possible by the parts that we got from Suzuki and this is justifiable by FEM calculations. So purpose of frame is to endure forces that are descriped by green arrows at bearing points. This is possible best if the tubes and shock absorbers are in 90 degrees angle (perpendicular) in other words direction of maximum forces at bearing points (see picture below). Tubes 4 and 3 are not excatly in this angle because it was better to take them directly to the engine attachment points. Because engine and air filter box has to have room frame circles them, this provide also needed torsion stiffnes to frame. Tubes 5, 6, 7, 8 and 14 are there to provide the proper triangulation. This is what I explained briefly at my last post (Frame Type and Material). Tubes 9 and 10 carry the loads forward. Ideal situation is, example in braking situation is that all the tubes in frame take forces, then frame can be very light. With excellent triangulation this is nearly possible.

In the next post I will tell more about, example why tubes 9, 10, 11 and 13 go to that node and why is the node where it is. These things are hard to clarify clearly so if you have something to comment please do so.