Chassis Stresses..


April 12, 2000

Hi again...I've been goofing off the last few months trying to get some bikes done and haven't had time to write much. As chassis builders, we have to understand the loads being applied to the chassis in order to counteract them and build a good handling motorcycle. Obviously the more HP and the bigger the rear tire, the higher the applied loads are going to be.

First lets look at the steering head. On a drag bike there is virtually no load being applied to the steering head. As a matter in fact, On a fuel bike, or any bike that carries the wheel way down tack, you could cut the backbone off at the seat and the front motor mounts and still make a good run until you shut the bike off in the lights and there was no front wheel. There are some loads encountered during braking but these are generally straight back and not side loads. Any good strong backbone will usually do the trick here.

The loads we have to worry about are caused by the pull on the chain as the Horsepower is transferred to the rear wheel. The more H/P and the bigger the tire, the worse it is. The rear sprocket is offset from the chassis center line to clear the tire and as a result of this offset there is a tremendous side pull on the rear of the chassis...see Figure-1...
In testing I have seen the rear pulled over as much as .190 and I suspect in a high H/P bike it may even be more. With proper bracing we have gotten this down to .010 to.020. When you see a bike going down the track and the rear is wiggling side to side, I suspect the rear of the chassis is being pulled over as far as it can go then is springing back and doing it again and again etc.

There is another chain related load we have to counteract...see Figure-2...
As the H/P is applied the chain is trying to pull the rear sprocket up. I have see it lift up over .200 in testing. It is actually trying to bend the chassis in half between the two sprockets. This twists the rear wheel in the chassis and contributes to the bike feeling like it is rolling under as the front wheel comes down. When you get a combination of the two it can make the run exciting to say the least...Then you add tire shake to this and look out....I'm not going to get into how to fix these problems here, but as you walk around the pits look and see how the various chassis builders addressee these problems. That's it for now, I'm going to go and shovel the damn snow we got today....Grrrrrrrrrr....

Later...
Pup...

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2000