Chassis Design

James DiTullio
August 29, 1999




Tony Lang's; by Puppet

Chassis: One Size Doesn't Fit All.

Puppet speaks:

The first thing we have to do in building a bike is to design a chassis. We need to figure the wheelbase, seat height, engine placement, steering head angle, wheelie bar length and more. The easy way would be to make one that handles good and make a bunch of them. It's also the way to make a lot of money. The problem with that is…A chassis is very horse- power sensitive in that the engine placement depends on the amount of horsepower available. There is no…"one size fits all".

I build chassis with aluminum axle blocks in the rear, so you can move the rear wheel up to six inches back as your horsepower increases. If you move just the engine up one inch not much happens, but if you move the rear wheel back one inch you are moving ALL the weight including the rider one inch, and this makes a difference. Of course, if you are building a bike in a wheelbase restricted class like ProStock this won't work, and you have to be exact the first time. When a chassis is working right, at the hit of the throttle, the rear of the bike will drop and plant the tire, then pull itself up. You have to be careful that the wheelie bars are high enough, so that they don't hit before the tire is planted. You also must have enough air in the tire, so it doesn't collapse. A VCR camera is good for spotting these things…unless you have an observer with a photographic memory.

In the higher horsepower bikes we try to carry the front wheel 1100 to 1200 feet before setting it down and going through the lights. The reason for this is that when the front tire comes down you are taking weight off the rear tire and are not getting the full amount of weight transfer that is available. You can do this by moving the engine back, but if you do this you have to much weight on the wheelie bars and are losing transfer that way…

In my IDEAL world...the bike would plant the tire, pull itself up with a minimal amount of hit on the wheelie bar and come off the bar about 400 feet while carrying the front wheel to about 1200 feet without touching the wheelie bars. Elmer Trett's bike used to do this when things were working just right...Sometimes it would carry the wheel through the lights, this meant we needed to move the weight a tad more forward.

Engine placement is important because all of your acceleration is in the first 660 feet. In the first 660 feet a fuel bike will go from zero to 193 mph in just under four seconds. The last half of the track it only gains about forty mph. in a little over two seconds, so...weight transfer (engine placement) is critical...The idea is to make the chassis work and slip the clutch as little as possible...

In the next article I will try to explain how engine placement affects the amount of weight on the rear tire and why moving the engine ahead puts more weight on the rear tire...

later...
pup...





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