Chassis Design

James DiTullio
September 23, 1999

Chassis: Motor Placement

Puppet speaks:

Hi again...I get asked a lot about where to mount the engine in a chassis someone is building. There is no Magic number...It all depends on a thousand things, but most important is the amount of H/P available. The more H/P you have, the more traction we can generate...It's all a matter of leverage...

I have drawn three pictures to help explain my theories...Everyone thinks that moving the engine back puts more weight on the rear tire, well it does when the bike is sitting there, however things are different when it's going down the track. In figure 1, we have the engine up against the rear tire and a short set of wheelie bars. If you hit the throttle on this, it would jump up, hit the wheelie bars and flip over backwards smashing you into the pavement and give you one hell of a headache. This would not be a desirable situation. I believe that on a drag bike, the wheelie bars are the pivot point. On acceleration the bike is trying to rotate itself around the wheelie bar wheels.

In figure 2.. we have lengthened the wheelie bars but left the engine in the same place. When we hit the throttle on this model, the wheelie bar wheels will hit the ground and prevent the bike from flipping over. Gravity is pulling down on the engine putting weight on the rear tire. As long as the wheelie bars are on the ground we can look at the wheelie bar wheels as a fixed point and the bars and frame as long lever with a wheel in the middle of it. With the engine this close to the tire we can only apply slightly more weight than the engine weighs to the tire. The acceleration forces will actually lift the chassis up and unload the tire. Bikes like this have a tendency to hop as the tire hooks, then lifts up ( unloads ) then drops down, hooks again etc. These bikes also hit the bar real hard which upsets the chassis adding to the problem. Longer wheelie bars will help this problem somewhat.

In figure 3.. We have moved the motor way forward. The trick here is to have enough H/P to hold the front end up 1100 to 1200 feet. When the engine is in the right spot the chassis will drop at the hit of the throttle and plant the tire, then pull itself up. There will not be a lot of pressure on the wheelie bar if the engine is in the right spot. As you can see in the drawing, the engine is hanging way out increasing the leverage on the rear tire as long the front wheel in the air. If the front tire drops to early, there is to much weight on the front and not enough on the rear resulting in wheelspin... Most of the acceleration is over at the 1/8th mile mark so we try to carry the wheel at least that far.

This is just one part of loading a chassis, some of the other things that effect the chassis are, crank height, output shaft height, size of sprockets, angle of top run of the chain, rider height and location and of course where you mount the fuel tank, battery etc. We'll talk about these things at a later date. Next we'll discuss mounting the engine...