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You MUST have a firm grasp on what you want your bike to do. Different types of usage demand different designs. For example, if you want your bike to be a Saturday night stoplight-to-stoplight racer, you will design the bike differently than if you want a dependable street bike.

Even within the "Saturday Night Special" category there are sub-categories. Let's look at some possibilities.

      Street bike
         Dependable
         Saturday Night Special
            Straight line, stoplight to stoplight
            Twisties and turnies
         Touring
      Race bike
         Drag race
         Oval race
         Road race
   

Now, within each of these broad categories, there are sub-categories and sub-sub-categories. The point being that you must know how the bike is to be used BEFORE you start building it. Once you have determined that, you can sit down and start listing components and beginning the design in conjuction with any restraints.

As an example of that last statement, is there a cubic inch or cubic centimeter limit? This will apply mainly to race bikes but if you're going to have a dual purpose machine, you have to take into consideration what class you're going to run in. And will the engine for that class be usable on the street?

A general rule of thumb is that the more uses the bike will see, the more compromises you will have to make in it's design.

Let's take drag racing for example. If you're going to build a dual- use bike, street and an occasional trip to the strip, you will need to compromise cam and carb choice to insure the bike can be used for both purposes. If you were building a drag-race only bike, you would not have to compromise so much. You would be building for higher RPM usage and not worried about low and mid-range RPM.

For the drag-only bike, the next step to consider is engine size. Are you going to be running in a class that has a size limit? Are there body or frame limitations? Or are you going to be running either bracket or in a class that is wide open.

The best place to start is:

    What type of bike do I have or do I want to build?
    How is the bike going to be used?
    What type of engine do I want to use?
    Are there any, and if so what type of constraints on the
        construction of the bike are there?
    HOW MUCH MONEY DO I HAVE!!!!! - (perhaps THE most important question)

So, at this point you have picked a usage for the bike, size for the engine and type of usage. Next is to pick the RPM range the bike will run in. If you are building a 4 cylinder crotch rocket, you may have a peak RPM of 12,000 to 14,000 RPM. If you are building an early vertical twin Triumph, you are limited to somewhere around 6500 to 7500 RPM.

As another example, if you are building a Harley, you may be limited to around 6000 to 7000 RPM. With a Sportster, maybe around 7000 to 7500 RPM tops. Of course with a Harley, there are other considerations. Due to the numerous options available, connecting rod length must be considered at this point. With a Japanese 4 or a Triumph, there are not enough alternatives to be concerned about. The point here is that you must know what is available for your chosen bike.