How to Choose a Cassette: 11-30 vs. 11-34, Teeth for Hill Climbs and Chain Length

How to read "12-speed 11-34"

A cassette marking gives the number of speeds and the teeth on the smallest and largest sprockets. A 12-speed 11-34 stacks 12 sprockets from 11T to 34T. The smallest (11T) is the top gear and the heaviest; the largest (34T) is the low gear and the lightest.

• Speeds: choose the same number as your derailleur and shifters (12-speed won't fit an 11-speed bike)
• Top (smallest teeth): the smaller, the better at high speed. 11T is common, and those starting at 10T need a compatible freehub body
• Low (largest teeth): the larger, the easier the climbs. 28–34T is mainstream, and gravel cassettes go to 36T or more
• Sequence in between: even with the same 11-34, the middle teeth differ by speeds and maker, and the sequence affects how it feels to use

Tooth sequences and jumps of the main cassettes

How much the weight changes when shifting to the next sprocket (the jump) depends on the ratio of teeth. 11T→12T is 9.1%, 17T→19T is 11.8%, and 30T→34T is 13.3%. With a jump over 10%, riding at a cadence of 90 rpm and changing one gear drops you to around 81 rpm, so you tend to notice your leg rhythm changing at each shift.

An 11-speed 11-34 skips by 2T from the smallest sprockets, as in 11-13-15, so the jumps at the top end used on the flat are a large 15–18%. A 12-speed 11-34 goes up 1T at a time from 11T to 15T, so it has the advantage of fine steps at the top end despite its wide range. Even with the same "11-34," 11-speed and 12-speed feel quite different.

Close-ratio versus wide-ratio

A cassette with little difference between largest and smallest and small jumps between neighbors is called close-ratio, and one with a big difference that reaches light gears is called wide-ratio. Neither is better; each suits different places to ride.

A close-ratio cassette like 12-25 runs 1T at a time from 12T to 19T, with jumps of only around 6%. It's a satisfying setup for holding around 30 km/h precisely on the flat, but even with 50-34 the lightest gear is 1.36, so grades above 8% get hard.

11-30 vs. 11-34: the lightest gear is about 12% lighter

The 12-speed 11-30 and 11-34 share the five sprockets from 11T to 15T, and differ from 16T on. The 11-30 includes 16T for finer steps in the middle, while the 11-34 drops 16T and adds 34T.

• Lightest gear: 34×30 is 1.13 and 34×34 is 1.00, so 11-34 is about 12% lighter (34×30 is about 13% heavier than 34×34)
• Cadence climbing at 10 km/h (700×28C): about 69 rpm on 11-30, about 78 rpm on 11-34
• Flat gears: going from 15T to 17T, the jump is 6.7% + 6.3% on 11-30 (via 16T), and a large 13.3% on 11-34
• Gear range (with 50-34): 401% on 11-30, 455% on 11-34

11-30 suits fine adjustment around 30 km/h mostly on the flat, and 11-34 suits courses with mountain passes and steep climbs. Many people choose 11-34 as insurance, feeling that "I may not use the 34T, but it's reassuring to have," and it also helps when your legs are spent late in a long ride.

Teeth for hill climbs: cadence by gradient and power

The lightness you need on a hill depends on the gradient and the power you can sustain. With a combined rider and bike weight of 78 kg and 700×28C tires, these are climbing speeds by gradient and the cadence on a 34T front at that speed. They're estimates that include rolling and air resistance, and vary with wind and road surface.

150 W is a guide to the power a rider used to road bikes can sustain on a climb for 30 minutes to an hour. At this power, 34×30 spins at 71 rpm on a 6% grade, but at 8% even 34×34 drops to 63 rpm. For passes with long sections of 8–10%, 11-34 is still heavy, and 46-30 or 48-32 at the front becomes an option.

• Mostly hills up to about 5%: 11-28 to 11-30 is often enough
• Climbing passes of 6–8%: 11-32 to 11-34 makes it easier to hold around 70 rpm
• Steep climbs of 10% or more: if 11-34 isn't enough, consider 46-30 or 48-32 at the front, or a wide cassette such as 10-36

The relationship between gradient percent and angle is explained in Converting Slope Percent to Degrees. Linking the gradient shown in map apps with how steep it feels makes it easier to prepare your gears.

Before changing: rear derailleur maximum sprocket and capacity

When fitting a larger cassette, first check what your rear derailleur supports. Manuals and spec sheets list the maximum low sprocket (largest low-side teeth) and total capacity. Exceed either and shifting can fail, the chain can go slack, or at worst the derailleur can get caught in the spokes.

1. 1

   Check the maximum low sprocket

   Confirm that the largest sprocket on the cassette you want to fit (34T, say) doesn't exceed your rear derailleur's maximum low sprocket.

2. 2

   Calculate the total capacity

   (Largest front − smallest front) + (largest rear − smallest rear). For 50-34 × 11-34, (50 − 34) + (34 − 11) = 39T.

3. 3

   Compare with the spec sheet

   It's usable if the calculated value doesn't exceed your rear derailleur's total capacity. Short-cage and long-cage values differ, so check the model number right to the end.

Chain links after changing the cassette

A larger low sprocket also needs a longer chain. A common formula for geared bikes is 2 × chainstay length (inches) + largest front teeth ÷ 4 + largest rear teeth ÷ 4 + 1, rounded up to whole inches and doubled to get the number of links (1 inch = 2 links).

Each extra tooth on the largest rear sprocket needs about half a link more chain. Going from 11-30 to 11-34 adds 4T to the low gear, so depending on the chainstay you may need 2 more links. On the actual bike, you can also check by wrapping the chain around the big ring and largest sprocket without passing through the derailleur, rounding the length that just joins up to an even number, and adding 2 links.

FAQ

Which is better, 11-30 or 11-34?

For mostly flat riding, 11-30 with its finer middle gears. For mountain passes and steep climbs, 11-34. With 50-34, the lightest gear is 1.13 for 34×30 and 1.00 for 34×34, so 11-34 is about 12% lighter.

How many teeth do I need for hill climbs?

At a combined 78 kg and 150 W, a 6% grade gives about 71 rpm on 34×30, and an 8% grade about 63 rpm even on 34×34. For passes of 6–8%, use 11-32 to 11-34; if there are steep climbs of 10% or more, also consider smaller front gears.

How many chain links after changing to 11-34?

With 410 mm chainstays and 50T at the front, the formula gives 110 links. Each extra tooth on the low gear needs about half a link more, so going from 11-30 (108 links) to 11-34 adds 2 links.

How do I calculate total capacity?

(Largest front teeth − smallest front teeth) + (largest rear teeth − smallest rear teeth). For 50-34 × 11-34, that's 16 + 23 = 39T. Confirm it doesn't exceed the total capacity in your rear derailleur's specs.

What's the difference between close-ratio and wide-ratio?

Close-ratio cassettes have small jumps of 6–10% between neighbors, making it easy to match your pace finely on the flat. Wide-ratio cassettes have larger jumps of 12–18% but a lighter lowest gear, making hills easier.