How to Choose Sprockets for Gravel Bikes

What changes when sprockets are replaced?

When a sprocket is replaced, the major change is the gear ratio.

Gear ratio is a measure of “gear weight” calculated by “front gear / rear gear” and is said to be around 2.3 for city bikes.

For gravel bikes, it usually covers about 1.3 to 4.3. However, replacing the rear sprocket will change this coverage and the gear ratio difference between gears.

Replacing the sprocket also changes the weight. Generally, wide-ratio sprockets with larger gears are heavier and cross-ratio sprockets are lighter, but even with the same gear configuration, the higher grade sprockets such as DURA-ACE and ULTEGRA are lighter.

Just by changing sprockets, the weight can change by as much as 50g, which contributes to weight reduction.

“Wide ratio” with a wide range of coverage

If you switch to a “wide ratio” sprocket with a large difference between the minimum and maximum gears, such as 11-32T, you will have a wider range of gear ratio coverage, but the difference in weight when shifting gears will be larger. When shifting gears, it is as if the bike becomes “gunkier and heavier”.

Let’s take a 2x8 speed with 50/34T front and 11-32T rear as an example.

front	rear	gear ratio	gear ratio difference	speed at 80 cadence
50	11	4.55	0.70	45.6
50	13	3.85	0.51	38.6
50	15	3.33	0.56	33.4
50	18	2.78	0.40	27.9
50	21	2.38	0.30	23.9
50	24	2.08	0.30	20.9
50	28	1.79	0.22	17.9
50	32	1.56	-	15.7
34	11	3.09	0.48	31.0
34	13	2.62	0.35	26.2
34	15	2.27	0.38	22.7
34	18	1.89	0.27	18.9
34	21	1.62	0.20	16.2
34	24	1.42	0.20	14.2
34	28	1.21	0.15	12.2
34	32	1.06	-	10.7

As can be seen in the maximum speed, the speed range covered at a cadence of 80 is wider, from 10 km/h to 45 km/h. Because of this, the gear ratio difference is generally larger, from 0.3 to 0.5, and there is a gear ratio difference of as much as 0.7 when shifting from outer 7th gear to 8th gear.

“Cross ratio” with less weight change during shifting

When a “cross ratio” sprocket such as 12-23T is used, the difference in weight when shifting gears becomes smaller.

The image is that "you can ride so smoothly in the next gear that you don’t have to be aware of the gear shift. However, there is a demerit of narrower gear ratio coverage.

Let’s look at a 2x8 speed with 50/34T front and 13-26T rear as an example.

front	rear	gear ratio	gear ratio difference	speed at 80 cadence
50	13	3.85	0.27	38.6
50	14	3.57	0.24	35.8
50	15	3.33	0.39	33.4
50	17	2.94	0.31	29.5
50	19	2.63	0.25	26.4
50	21	2.38	0.21	23.9
50	23	2.17	0.25	21.8
50	26	1.92	-	19.3
34	13	2.62	0.19	26.2
34	14	2.43	0.16	24.4
34	15	2.27	0.27	22.7
34	17	2.00	0.21	20.1
34	19	1.79	0.17	18.0
34	21	1.62	0.14	16.2
34	23	1.48	0.17	14.8
34	26	1.31	-	13.1

Unlike the wide ratio sprocket mentioned earlier, the maximum speed will be slower at 38.6 km/h and the lightest gear will be about 1.31, about 1 step heavier, but the gear ratio difference is generally within 0.2, so you will not suddenly feel heavier after shifting.

Optimal gear ratio for gravel bikes

Gravel bikes differ from road bikes in that they are ridden on rough terrain, but recently road bikes have begun to select sprockets with gear ratios in the neighborhood of 1.0 for hills exceeding 10%, so the concept of gear ratios is basically the same as that of road bikes.

However, if a gravel bike is to be used on trails, such as MTB trails rather than gravel, it is safer to have a gear ratio of less than 1.0 for running performance.

Since gravel bikes are designed to balance the two contradictory goals of “emphasizing high speed on paved roads and driving performance on unpaved roads,” the trick is to have a wide range of gear ratios to cover rather than a small difference in gear ratios.

However, if the gravel bike is used for commuting to work or school or for city riding, the range of gear ratio coverage does not need to be that wide, so a cross ratio is a good choice.

How to choose a sprocket

Front Single

The “front single” with a single front gear is being increasingly adopted on gravel bikes.

In the case of a front single, the number of shifting steps is small in the rear only, and the range of gear ratio coverage is stiff and narrow, so the trick is to widen the rear sprocket to widen the gear ratio coverage.

Specifically, you want to include a sprocket with a maximum rear tooth count of 32T to 36T so that the minimum gear ratio is about 1.0. If the maximum rear tooth count is too small, you will not be able to climb hills.

Some front single users have even combined MTB components to make a magical modification to a super wide such as 11-51T. “How wide a ratio you can make” is very important for a front single gravel bike.

In the case of a front double

In the case of a front double, it is easier to use a cross ratio sprocket because the gear ratio can be greatly increased or decreased in the front gear, such as 52/36T or 50/34T.

The advantage of a front double is that the number of gear ratios can be increased even when the number of shifting gears is small, such as 8 or 9 speeds, so the gear ratios that can be covered are wider, even if slightly closer to cross ratio.

However, even with a 50-34T compact crankshaft, the gear ratio of “front inner (34T) x rear 34T” is finally 1.0, so a wide-ratio sprocket will inevitably be used if you try to include a gear ratio of 1.

Pushing to climb is also an option.

When riding on MTB trails, you will inevitably encounter routes that cannot be overcome by riding. It is normal to “push” your way up such routes on MTB.

In this sense, rather than forcing the rider to use a halfway wide-ratio configuration, one option is to use a cross gear ratio and “push to climb where the route cannot be climbed”.