This post compares the advantages and disadvantages of 11-25 and 11-28 cassettes.
The Advantages of 11-28 Cassettes
- Easier Climbing
The first obvious difference between 11-28 and 11-25 cassettes is the largest sprocket (28T vs. 25T).
The larger 28T cog makes it easier to climb. To understand why one has to be familiar with the term gear ratio.
The gear ratio reveals how many times the rear cog and consequently the rear wheel rotates per one revolution of the cranks.
To calculate the gear ratio, it’s necessary to know the number of teeth on the chainring and the cog.
For example, if the chainring has 42 teeth, a 28T cog results in a 1.5:1 gear ratio. Or in other words, the rear cog will make 1.5 revolutions per one rotation of the cranks.
Meanwhile, a 42T/25T gear has a 1.68 gear ratio. Thus, the rear wheel will spin 1.68 times for every rotation of cranks.
The smaller the gear ratio, the easier it is to pedal in that gear because the wheel has to move less for every rotation of the cranks. The 0.18 difference that we have in this case doesn’t sound like much, but it matters when trying to cover hard hills.
An 11-28 cassette is undeniably better than an 11-25 model for people who do a lot of climbing.
- Lower Gearing Without Changing The Chainring
By switching from an 11-25 to an 11-28 cassette one will instantly acquire a slightly lower gearing without having to swap chainrings.
Meanwhile, if you want the low gear that an 11-28 cassette offers while running an 11-25 cassette, the only option will be to switch to a smaller chainring. The change could give you an even lower gear, but it will come at the expense of the bike’s top speed.
For example, if the bike has a 42T chainring and thus a 1.5:1 gear ratio when running an 11-28 cassette, a 37-38T chainring will be needed to get the same gear ratio with an 11-25 cassette.
The 37T chainring will provide the necessary low gear, but it will also lower the potential top speed of the bike.
The highest gear will become 11/37 which has a gear ratio of 1:3.36.
To calculate the potential top speed of the bicycle in that gear, it’s necessary to know two more parameters:
- The circumference of the wheels
- The RPM of the rider (the number of crank rotations per 60s)
For illustration purposes, I will apply the formula to a bike with 700x25c wheels with an approximate 2111mm circumference. The RPM of the rider will be 90.
Step 1: Determine The Gear Ratio
In the 11/37 gear, the gear ratio is 3.36. Thus, the rear wheel makes 3.36 rotations for every spin of the cranks.
Step 2: Determine The Total Number of Rear Wheel Rotations
The rider is pedaling at 90 RPM. Therefore, the rear wheel makes 90×3.36 rotations = 302.4 rotations.
Step 3: Distance
To know the traveled distance, one has to multiply the number of rear wheel rotations by the circumference of the wheel.
In this case, the number is 302.4 x 2111mm = 638366.4mm = 638.36m = 0.39miles = 0.638km.
Step 4: Calculate Speed
To calculate the speed of the bike, one needs to divide the traveled distance by the time needed to cover it.
In this case, the speed is: 10.63m/s; 38.28kph; 23.78mph
However, if the bike keeps its original 42T chainring, the potential top speed will be 12m/s; 43.5kph; 27mph.
Note: The reduction of top speed could be avoided if the bike has multiple chainrings. In that case, the smaller chainring can be downsized while keeping the large one as big as it is to preserve top speed.
The Disadvantages of 11-28 Cassettes
- Large Jumps
A typical 11-speed 11-28 cassette such as CS-R8000 has the following progression:
The jump between the last two cogs (25T to 28T) consists of 3 teeth and is therefore classified as large.
The downside of larger gear transition is the increased difficulty to maintain fluid cadence (RPM).
That said, the effect, in this case, isn’t massive because the largest 28T cog is used only when riding uphill. The jump would be a lot more annoying if it was in the middle of the cassette because those are the most frequently used cogs.
The Advantage of 11-25 Cassettes
A typical 11-speed 11-25 cassette has the following gradation:
Meanwhile, some 11-28 cassettes have the following gearing:
The mid-gearing of some 11-28 cassettes is missing a 16T and an 18T cog. Also, the jump from 25T to 28T is large. Consequently, some riders feel that 11-28 cassettes do not offer the necessary gearing for maintaining smooth cadence.
The Downside of 11-25 Cassettes
- Higher Gearing
As already explained, 11-25 cassettes have a higher gearing making it more difficult to climb.
Comparison of 11/12-25 and 11/12-28 Cassettes Among Brands
The gradation of cassettes differs among brands. The tables below point out the differences:
11-25 and 11-28 Cassettes
- The 10-speed 11-25 cassettes of SRAM, Shimano and Campagnolo offer identical gearing and miss an 18T cog.
- The 10-speed 11-28 cassettes of SRAM and Shimano differ by the last three gears. In the case of Shimano, the final jump is from 24 to 28 whereas that of SRAM’s cassette is from 25 to 28.
- The 11-speed 11-28 cassette of Shimano offers a smoother transition between the final gears.
12-25 and 12-28 Cassettes
- Shimano’s, SRAM’s and Campagnolo’s 11-25 10-speed cassettes offer the same progression.
- Shimano’s 10-speed 12-28 cassette comes with a smoother transition between the last few gears.
- Shimano’s 11-speed 12-25 cassette offers one of the smoothest gradations with minimal jumps between the gears.
- SRAM isn’t producing 12-25 and 12-28 11-speed cassettes.
The Most Important Factor When Choosing Between 11-25 and 11-28 Cassettes
If you have to cover excessive hills or simply want an easier gear to reduce joint stress, a cassette with a 28T cog will help. That said, an 11-28T cassette does not help climbing as much as cassettes with 30+ teeth.
Or in other words, a 28T cog is still pretty close to 25T and may fail to make the dramatic difference that the rider might expect.
If you’re going to ride primarily on flat terrain, then the 28T cog does not provide as much value. In that scenario, the actual gradation of the cassette is more important.
Why Not a 12-25/28 Cassette?
Truth be told, the vast majority of people do not need the speed that an 11T cog provides. A 12T cog offers more than enough speed for most people’s needs.
This represents a direct incentive to look into 12-25/28 cassettes. The main benefit of 12T cassettes is that they offer a smoother transition between the mid-gears which are the most frequently used anyway. (read more)