Frame Geometry

Here we discuss frame geometry; an important area to be familiar with.

In all Cruisers, the foot pegs and shifter are located a long way forward so your legs can stretch out. On a well designed bike, this is a comfortable riding position - Cruisers make good touring bikes because of this feature combined with their stability at high speeds. A badly designed Cruiser drops all your weight onto your Coccyx (tailbone) and is a far from comfortable ride. The placement and shape of the handlebars stretch the arms out and add to the laid back style. Choppers are an extreme, stripped down variation of the Cruiser, with larger front fork rake and banana seats. Choppers carry a V-twin engine, usually a big one.

The frame geometry is how this look and style is achieved. Here's the frame diagram again:



chopper frame drawing

Different chopper looks are achieved by changing the length of parts of a standard frame. It's what's known as stretching. Extend the downtube for example and you can get a more obtuse angle for the forks and a longer overall bike (the front wheel moves further away from the frame). It's done for looks, for a better fit for a particular rider, or to change the way the bike handles. It isn't done by just welding an extra few inches into the frame where you want to stretch it, but is done at the time of building the frame, with all the angles and lengths carefully worked out. To understand this frame geometry section easily, I suggest getting some paper and a pen and doing some drawing of your own. By the end of this section, you'll be able to design the chopper of a stickman's dreams!

Stretching takes place in these three main areas - in the rear by extending the wishbones and bottom rails, up by the neck by extending the top tube and in the downtubes to shift the angle of the neck. This frame geometry diagram shows where I mean:



extending wishbone

This is what happens when you make a rear stretch. The axle has effectively moved up, lowering the bike:



rear stretch

Next in our motorcycle frame geometry section is a stretch in the front end. In this case a combination of two of the stretches I mentioned above. In this case the effect has been to lengthen the bike without altering the rake. If just the bottom tube had been extended then the rake would have grown (see below for the full explanation of rake!):



front end stretch

Frame Geometry Conclusion:

With your pen and paper you can draw entire bikes to see what happens to the frame and the rest of angle of the ride and forks, the height of the ride from the ground and so on. To understand the change in handling characteristics that result from stretching the frame, we need to look at rake and trail.

Free Frame Building Mini Course:

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