via Cycling Tips
Beyond the big ring: understanding gear ratios and why they matter
by Matt Wikstrom – August 11, 2014
Over the past decade, the number of gears on road bikes has steadily increased. Current groupsets now provide up to 22 gear combinations produced by two front chainrings and 11 rear cogs. That should be enough to contend with just about any terrain, but there is no prescription for perfect gearing. Rather, the individual must determine which gear ratios suit his or her riding style and needs.
Before the advent of the chain-drive, cyclists determined that the size of the drive-wheel had a profound impact on the speeds that could be achieved. Penny-farthings were not designed with a huge front wheel for aesthetic reasons — the massive circumference allowed higher speeds provided the rider was strong enough to turn the gear.
The introduction of the chain-drive improved the efficiency of the bike because gears could be used. By combining a large cog on the cranks with a small one on the wheel, a single turn of the cranks produced multiple revolutions of the rear wheel, so it could operate just like the massive drive-wheel of a Penny-farthing.
Calculating the number of wheel revolutions produced by a bike’s gearing is simply a matter of determining the ratio of the chainring to the rear sprocket. For example, when a 53T chainring is paired with a 12T cog, it has a ratio 53:12 or 4.42 — that is, the rear wheel rotates 4.42 times for every rotation of the crank. In contrast, a 39x25T selection produces a gear ratio of 1.56.
COMPARING GEAR RATIOS
A road groupset can offer a variety of gear ratios ranging from 1.21 to 4.81 in increments of 0.15-0.40. Rather than trying to understand the significance of gear ratios directly, they can be transformed into more meaningful values in one of two ways.
The first method is to relate the gear ratio to wheel size by multiplying the gear ratio by the diameter of the wheel (Figure 1A). In the case of a road wheel, 27 inches can be used for simplicity although the true diameter of a 700c rim fitted with a 23mm tyre is more like 26.3 inches. The resulting value, gear inches, is the equivalent diameter for a direct-drive wheel (like the front wheel of a Penny-farthing).
Put another way, gear inches provides the diameter of a wheel that has a circumference equivalent to the distance a geared bike will travel with one turn of the cranks and the chosen gear ratio. Thus, riding a high gear ratio such as 53x12T is equivalent to pushing a wheel with a diameter of 119 inches. In contrast, a low gear ratio like 39x25T is equivalent to a 42-inch wheel.
The second method, meters of development, is calculated by multiplying the gear ratio by the circumference of the wheel (measured in meters, Figure 1B). This value represents the distance the bike will travel with one crank revolution. Thus 53x12T yields 9.28 meters of development compared to 3.28 meters for 39x25T.
Gear inches (or meters of development) are typically presented in a gearing chart or graphed so that various combinations can be compared (Figure 2). My introduction to gear charts came with BMX racing where they proved invaluable for fine-tuning gear selection to suit the track and weather conditions. Track riders will do the same, swapping the front chainrings and/or the rear sprocket to increase or decrease the gear ratio in increments as required.
The important thing to note is that even minor differences in a ratio (e.g. 1 gear inch) influence how easily the bike can be accelerated or the maximum speed that can be attained.
OUT ON THE ROAD
In practice, road riders typically don’t pay much attention to gear ratios due to the abundance of choices available to them. However, there is still considerable value in fine-tuning the range of gear ratios at your disposal to maximise their utility.
I typically see riders deciding on their gearing on the basis of the largest or smallest gear ratios. Some will insist on an 11T cog, however the only time they ever use it is on a steep descent. Such thinking also fuels the debate on the merits of standard (53/39T) versus compact (50/34T) cranksets but the two offer comparable ratios. Indeed, pairing an 11-23T cassette with a compact crankset offers near-identical ratios to a standard crankset coupled with a 12-25T cassette (Figure 3).
There is a subtle difference between standard and compact cranks though. As shown in Figure 4 below, the rate at which the ratios increase is greater for standard rings than compact rings. For example, the ratios for the 11-15T cogs increase at an average rate of 7.0 gear inches/tooth for a 53T chainring compared to 6.6 gear inches/T for a 50T chainring. This difference defines the true distinction between the two cranksets by generating a different feel to the gearing at the high end.
Deciding on one over the other is difficult to do without some experience with each, but in general, the lower rate of development offered by compact cranks will suit novices and enthusiasts while racers will prefer the extra grunt offered by standard rings.
There is another consideration. Every crank and cassette combination suffers from some redundancy that reduces the number of discrete ratios on offer. A standard crankset paired with an 11-speed 11-25T cassette offers 15 discrete ratios (Figure 5A); by contrast, compact cranks paired with the same cassette afford 16 discrete ratios (Figure 5B). The difference is dictated by the combination rather than the size of the chainrings since 16 discrete ratios can be achieved by pairing an 11-28T cassette with a standard crankset (Figure 5C).
PERFECT GEAR RATIOS
So what makes for perfect gearing? Ideally, you want a combination where every upshift and downshift delivers a change in gearing that perfectly suits your legs. Riders that like to spin are likely to prefer small steps while those with more strength and a preference for a slower cadence will want bigger steps.
Thus, the former will enjoy compact rings and/or a straight block (which provides cogs in 1T increments) while bigger chainrings and/or larger increments in the cassette are likely to suit the latter. Bear in mind though, the suitability of the gearing is subject to your form and the terrain.
FINAL THOUGHTS AND SUMMARY
Ultimately, gearing is a personal choice and every rider should have the freedom to decide the matter for him/herself, but it will involve some experimentation. Fortunately, groupset manufacturers offer plenty of choices with 11-speed transmissions and compact/semi-compact/standard chainring combinations. The latter is further helped by new crank designs that allow compact, semi-compact, and standard chainring combinations to be interchanged.
It is futile judging the value of a particular chainring and cassette pairing on the basis of a single ratio. A 53T chainring will always offer a higher gear ratio than a compact 50T chainring but the two offer different rates of development, which ultimately dictates how the gearing evolves over the range of the cassette. At the same time, devoting some consideration to the overlap in gear ratios between the big and small chainrings may extend the versatility of the gearing.
It won’t be long before bike computers are paired with electronic groupsets to collect data on how much time is spent using each gear ratio. At the same time, there is also the prospect of sequential shifting for road groupsets. Perhaps these and other innovations will usher in a new era of intelligent gearing?
What sort of gearing do you prefer? Compact or standard crank? Or perhaps a mid-compact? Why have you got the gearing setup you’ve currently got? Is it simply about the range of gears for you, or something more?