Direct driving a Modern Cycle
Just because modern cycles are geared, does not mean you can’t experience the same “thrill” as the old, direct-drive cycles. To do that, you need a gearing configuration that produces one front wheel turn for one pedal turn.
Of course, it would not be quite the same because the gearing does not “drive” the front wheel. It is a slave to whatever the rear wheel does so your gearing needs to turn the rear wheel at the same rpm as the pedals.
The answer is the front and rear chain wheels would need to have the same number of teeth. The actual number would not be important. As long as you have the same on both, one pedal cycle would translate into one turn of the rear wheel.
Direct Drive Speeds
How could I use this information to determine my speed for a 27 inch tire? The question to ask is how far do I travel for one turn of my front tire? The answer is one tire circumference of the tire, or 84.857 inches or 7.07 feet.
Next, how can we get speed information from this? The key is cadence which tells us the time it takes to turn the front chainwheel once, and therefore the rear wheel as well. At 60 rpm, we would be turning the rear wheel once every second, producing a speed of 7.07 feet/second, or equivalently 4.82 mph.
Why direct drive does not make the grade
The chart demonstrates why rear-wheel direct drive does not work any better than front direct drive. If you consider that a non-Elite cyclist has cadences between 60 – 80 rpm, that means for 27 inch tires, their speeds would be between 4.82 – 6.43. At best, elite cyclists might achieve 120 rpm, but only resulting in 9.64 mph. Let’s look next at why gearing solves this problem.
Next Topic: Chainwheels and Derailleurs