Gearing, the Fine Points
In the early days of bicycling there were no gears. The "penny-farthing" or "ordinary" bicycles had a big front wheel and a small back one. The pedals were connected directly to the front wheel. The bigger the front wheel, the faster you could go at a given cadence
, or pedal revolutions per minute. Also, the bigger the front wheel, the harder it was to pedal up hills. Of course, if you were short, you had a small front wheel. The size wheel you could ride was limited by the length of your legs.
In 1885, John Starley invented the diamond frame "safety bicycle" that most people today think of as a bicycle. The safety bicycle featured a rear wheel driven by a chain and sprocket. The chain and sprocket allowed the effective size of the rear wheel to be multiplied by the ratio of the number of teeth on the pedaled sprocket and the number of teeth on the sprocket attached to the wheel.
If the pedal sprocket had 40 teeth and the wheel sprocket had 20 teeth, then the wheel would turn two revolutions for every one revolution of the pedals. If the wheel was 26 inches in diameter, it would be equivalent to an ordinary bicycle with a 52 inch wheel. Today, in talking of bicycle gearing, we speak of "gear inches", which is just the effective size of a driven wheel if there were no gears. In other words, what size wheel the equivalent ordinary bicycle would have.
Today, bicycles mostly have multiple gears in the form of front and rear derailleurs or internally geared hubs. The effect is as if you had an ordinary bicycle where you could change the size of the front wheel as you rode. Drop down to a small wheel for easy pedaling on hills and then shift up to a big wheel as you roll fast down the other side.
A surprising number of riders are confused by the gears on their bikes. There's really no need for that, gears are very simple. The idea is to use the gears to match your cadence (how fast your legs are spinning) to the speed of the bike.
Cadences that are too slow don't transfer much power to the rear wheel and can injure your knees (all those single speed and fixie riders out there will keep the knee surgery industry humming along for decades). Cadences that are too fast tire you out quickly. Generally you want a cadence of about 60 to 80 for relaxed cruising and 100 to 140 for bursts of speed or for climbing steep hills.
You should always shift your bike to stay in the optimal cadence range. You don't need to know the exact numbers. Relaxed cruising will feel relaxed. If your knees start to feel strained, shift to a smaller gear inch value. If you're about to climb a hill, shift to a gear combination that will let your legs spin faster.
Note that the front and rear gears on a bike have opposite effects, i. e. shifting to a bigger chainring in front increases gear inches and thus pedaling difficulty and speed. Shifting to a smaller
cog in back has the same effect. Fortunately, the numbers on your handlebar shifters account for that. Higher numbers always increase gear inches, lower numbers decrease it, regardless of whether you're shifting the front or rear gears.
Use the gear calculator to the left to figure out how to shift your bike for a given speed and cadence. Fill in the Bicycle Information part (the default values are common for recumbent bikes), select a desired cadence and speed, and then click Calculate.
It will show you the front and rear gear combination that best matches that cadence to that speed. It will also compute the gear inch value for that gear combination. The rear cog number that is displayed matches the number on your rear (usually right side of the handlebars) shifter.
You will notice that to get the optimum gear inch value, you often have to shift both the front and rear gears. Some riders prefer not to shift the front and use only the rear. That's fine. To force the calculator to do that, pick the front ring you use, set its number of teeth, and set the other two rings to "None".
Adding an Electric Mountain Drive unit replaces your stock crankset with an EcoSpeed freewheeling crankset. This allows the motor to drive the chain without driving the pedals. The maximum size sprocket an EMtnD1 can accept is a 48 tooth. The other sprockets would typically be 38T and 26T or 24T. Front sprocket size is the only choice you need make with the EMtnD. Gearing calculations are otherwise the same.
Click the Installed: Yes button and the calculator will take into account an Electric Mid-Drive unit. The EMD introduces a slight step up in the gearing to compensate for being able to cruise faster and not needing ultra low gears for hill climbing. Click Calculate again and you will notice that now you're in smaller front rings and/or larger rear cogs for a given speed and cadence combination.
Also displayed is the actual motor RPM for that gear and speed combination. Like you, the rider, the motor has an optimal speed range that allows it to supply maximum power at maximum efficiency. Generally, the motor should be between 30% and 90% of its maximum RPM. That's between 1200 and 3600 RPM for both the BMC and Powerpack motors.
Notice the drop down box labeled "Optimize Shifing For". If you select "Power" the calculator will find a gear combination that keeps the motor as close as possible to the RPM where it produces the most power. Select "Cruising" and the calculator picks an efficient cruising RPM. Again, you don't need to know the details while riding. Generally, it will be good enough if you just shift into your big front ring for cruising, the middle ring for maximum power, and the small ring for slow, hard pulling such as climbing a very steep hill or pulling a heavy trailer. Then you just shift the back gears to find a comfortable cadence. With the EMD installed, you don't necessarily have to downshift if you start to feel knee strain. You can just press the throttle and get more power out of the motor.
When you clicked the installed button, a number of option boxes appeared set to default values. The defaults are generally the gearing that the EMD ships with. It is possible to tweak the EMD to optimize gearing for your application. For instance, if you have a very aerodynamic bike, the standard gearing will be too low and your top speed will be lower than it could be. So, you would want taller gearing. That can be done by changing the Motor Speed Reduction ratio or the Mid-Chainring size.
Take note of the Mid-Freewheel drop box. Changing the mid-freewheel size alters the relationship between the rider's cadence and the motor RPM as well as the overall gearing. By playing with mid-freewheel and front chainring sizes you can match a particular motor RPM with a particular cadence. If you are trying for absolute maximum speed, you would figure out what cadence you produce the most power at and then alter the mid-freewheel and front ring sizes so that the motor hits its peak power rpm at the same cadence. You would also alter the overall gearing so that top speed would be achieved at exactly that cadence.
Sound complicated? Don't worry, the standard options get you 90% of the way there and are fine for almost all applications. But, it's nice to know that if you need to tweak performance for a particular need, it's easy to do either before or after purchase. Feel free to play around with the various options in the EMD box to see what effect they have.