By Tom Jow
The solstice has just passed and Summer is officially here. Down in the valley the temperatures are on the rise. It's time to get up into the high country as the mountain passes and trails are now free of snow. Sometimes though, getting up those mountain roads and trails is not so easy. Maybe you are relatively new to cycling and don't have the specific fitness or strength to keep up with the more experienced riders. Or worse yet, maybe time constraints and increasing age are conspiring to make it more challenging to get up the hills. Fortunately it may be possible to change the gear ratios on your bike to get up the hills easier. This month I will explain how to use a gear chart to determine the total gear range on the bike and compare individual gear sizes. In addition, we will learn how to select a shifting pattern based on our available gear sizes in order to use those gears efficiently.
The first step in determining our gear range is to find out the size of our front chain-rings and each of the rear cogs. Sometimes the size is stamped on and easy to find. Sometimes it is necessary to count each tooth. With this information we can build our gear chart. A bicycle gear chart displays the size of each gear in a value called gear inches. The formula used to find the gear sizes is:
Chain-ring / Cog x wheel diameter (in inches) = gear inches.
Build your chart with the cog sizes on top, smallest on the left. Place the chain-ring sizes vertically on the left, largest on top. Using a calculator or spreadsheet, enter the values into the formula to find the size for each gear. Enter the value into columns where the two numbers intersect.
The first thing to consider in order to customize your gears is the high and low range. The example in table 1 shows the maximum recommended cassette for the standard double crankset produced by Shimano. The high gear size is 121.46 inches and the low gear is 39.72. These days, 39 inches is a pretty big gear for the recreational road rider. This low gear combination of 39-27 is the lowest gear available for this combination of crankset, cassette and rear derailleur. How then, is it possible to get a lower gear? There are several choices: a triple crankest, a compact crankest, a mountain bike cassette and rear derailleur or a combination of the two. The gear chart makes it very easy to compare all the options.
The triple crankest gear chart would look like this:
The compact double 50/34 with a 12-27 cassette chart looks like this:
The mountain bike rear derailleur and cassette gear chart that looks like this:
Now let's compare these three charts. The first option (table 2) has the lowest climbing gear and the highest two-thirds of the gears stay the same. Option number two (table 3) has both a lower climbing gear and top speed gear. Option number three (table 4) has a low gear that is lower than the current gear and the high gear stays the same. What if we change option two to an 11-32 cassette (table 5)?
With this option, not only is the high and low range equal to the triple crank option, but the three lowest gears are lower than the original. Also, many of the gears are the same in the large and small ring which makes for a simple and efficient shifting pattern.
Another reason for using a gear chart is to compare each individual gear size and how they progress from one to the next. In table 5 for example, the difference between the low gears is four inches, then five, seven, six, nine and so on. Note that there are four gears that overlap; 49, 55,62, 72. Combined with the cross-chain gears (not shown) this reduces our actual non-overlapping gears to only thirteen gears. Thirteen gears out of twenty? Yes, but when you understand how the gear sizes progress, all seventeen are useful.
Let's say we were to shift these gears as we would drive a car. We start in the lowest gear and work our way up to the highest. We could ride in the small ring up to the 72 inch gear and then make a double shift up to the big ring progressing to the 80 inch gear and beyond. An alternative would be to make the double shift in any of the four gears that overlap. On a perfect road this shifting pattern works just fine. Mountain roads, however, go up and down. This is why knowing the gear progression is important. Imagine riding up a hill in the lowest gear, and the road begins to flatten out. As the terrain gets easier shift up to increase the speed and keep the cadence the same. When the chain gets down to the middle of the cassette it's decision time. Shift to the big ring or stay in the small one? Look up the road. Does it go uphill again? If so, use the small ring as long as possible before having to get back into the lowest gears. Flat for a while or downhill, go ahead and shift to the big ring. Pretty simple, huh?
Even so, there is one more factor involved. This final consideration is the difference in size between the gears progression. Referring back to table 5, starting at the lowest gear, look at the difference in the size of the gears. Only the difference between the first 4 gears are 5 inches or less. Now go to table 2. remember the triple chain-ring setup? Build a shift pattern starting from the lowest gear. Examine the differences between the gears and compare them to the chart in table 5. With the triple chain-ring setup, it takes 9 shifts to get to the 72 inch gear. Of those, 7 of them are 5 inches or less. The compact drive 12-32 setup requires just 7 shifts to get to that same 72 inch gear, but only 3 of those are 5 inches or less. What does this all mean? This means that each shift to a new gear on the double chain-ring setup requires a little more power to keep the same cadence. Said another way, each shift (or step) with the triple chain-ring set is a little smaller, a little easier.
Getting up and over the mountains by bicycle, on road or on trail will never be easy. With modern bikes that have 20, 22 or even 30 gears, having the right gear ratio has never been easier. It is finding that correct gear, that may be a challenge sometimes. That's where the gear chart comes in handy. Learning to use a gear chart will first help us customize our total gear range to suit our riding needs. Then it will help us create an efficient shifting pattern. The most direct shifting pattern follows a simple path from low to high in the most equal divisions. The most efficient shifting pattern is one that changes chainrings as little as possible, and that can only be determined by the rider and his/her knowledge of each individual gear.
Got a bike question? Email Tom at [email protected]