Condensed Answer: Cassette size depends on the terrain and the shape of the rider. If you live in a flat city, then a cassette with higher gearing but smoother transitions (e.g., 11-25) would work fine. If you have to cover a hilly area, then going for a larger cassette such as 11-32/34 is a logical choice.
The relationship between chainrings and cassette sprockets is described by a term known as gear ratio.
The gear ratio determines how many revolutions are made by the rear cog in use and respectively the rear wheel per 1 spin of the cranks/chainring.
To find out the gear ratio, one needs to know the number of teeth on the chainring and the cog. For example, if the chainring has 52 teeth and the rear cog has 12, the gear ratio is 52:12 or 4.3:1.
In other words, the rear wheel makes 4.3 rotations per 1 full spin of the chainring.
The larger the gear ratio, the faster the gear combination can be.
More revolutions of the rear wheel per 1 spin of the cranks equal a greater travel distance and consequently speed. However, a larger gear ratio demands greater input from the rider.
Cadence and Gear Transitions
The term cadence refers to the rotations of the cranks per 1 minute. Higher cadence such as 90 RPM is known to offer a combination of efficiency and higher average speeds.
It’s easier to maintain a higher cadence when the jumps between the gears are minimal. That way the rider can pedal as smoothly as possible.
In different, massive transitions disturb fluid pedaling by drastically increasing or lowering the effort needed to pedal.
The table below compares the gear jumps of 11-speed cassettes:
When the number of gears is the same, cassettes with a lower first gear come with larger jumps between each sprocket.
The difference isn’t dramatic, however. The average jump on an 11-25 cassette is 8.5% whereas that on a 11-32 cassette is 11.2% (3.3% difference).
Truth be told, most recreational riders rarely obsess over their cadence and probably wouldn’t feel the larger jumps. That said, the absence of very low gear will quickly become apparent during a climb.
The next table compares the weight of 11-speed cassettes:
Conclusion: As expected, when all parameters are equal (brand, model, material, design…etc.) larger cassettes weigh slightly more due to the bigger sprockets. However, the extra weight is about 50 grams at best and doesn’t change much unless one is trying to build the lightest possible bicycle.
Gear Ratio Comparison
The next table compares the lowest and the highest gear ratios of the cassettes above:
|Cassette||Top Ratio||Low Ratio|
The top speed offered by the cassettes is identical.
The main difference comes from the low gear. In the case of 11-25, the rear wheel makes 1.44 turns per 1 revolution of the cranks when riding in the smaller ring and the 25T cog.
In the case of the 11-32 cassette, the lowest gear gives us a 1:1.2 ratio. Or in other words, the rear wheel rotates 0.24 times less per 1 spin of the cranks. The difference is 20% or 1/5.
Over the course of a long ride, that 20% will quickly become significant.
Gear Inches Comparison
Gear inch is a term that indicates how much a bicycle moves per 1 revolution of the cranks.
The value depends on the gear ratio and the diameter of the wheel (tire included).
Note: Gear inches do not actually provide the traveled distance in a standard unit of measure (e.g., inches). To find that value, it’s necessary to multiply the gear inch by 3.14.
The table below compares the gear inches of the cassettes above when the bike has 700×25 wheels:
|Cassette||High||Traveled Distance||Low||Traveled Distance|
|11-25||125.14||392 inches||38.10||119.6 inches|
|11-28||125.14||392 inches||34.13||107 inches|
|11-30||125.14||392 inches||31.75||99.7 inches|
|11-32||125.14||392 inches||29.9||93.88 inches|
In general, gear inches under 30 indicate easy gearing designed for hills. Only 11-32 cassettes satisfy these criteria. The lowest gear inch of 11-25 cassettes is 38.10.
What Size To Choose?
If you’re a recreational rider or simply commute on your road bike, then the most logical choice is to get a cassette with low gearing (at least a 32T large cog). The same applies to touring cyclists who have to carry a lot of gear over diverse terrain for long periods.
Unless you’re in a racing situation, then the fluidity that a small cassette offers is inconsequential.
If you intend to use the bike primarily in “sports mode” so to speak, and you don’t live in a particularly hilly area, then an 11-28 cassette is an acceptable choice. It still has a notably lower gearing than 11-25 while offering acceptable transitions.
If you want to use an 11-25, then it would be wiser to go for a compact crankset (50/34) instead as a method to lower the gearing.
Personally, I have a retro road bike with 52/46 cranks and a 7-speed 13-26 freewheel. The gearing is quite high, but I manage because I don’t have to cover extreme hills. That said, I wouldn’t use the bike for long touring, and if I lived in a hilly area, I would switch to lower gearing.