Rear derailleur capacity – calculation

Recently, I have faced the problem of the rear derailleur capacity in my bike. It’s important if we want make changes with cassette or chainset. Lucky, this issue isn’t complicated and can be described in few simple words and only one pattern to calulcate everyting important.

Most of bikes have two or three cranks in the front and 8-11 in rear. The relationship is very simple – the more teeth on the crankset at the front, the higher speed we can get (but it will require more strength, there is nothing for free). And the less, the lighter pedaling will be. On the rear, the dependency is reversed – more teeth, more the target fits in to the hills and terrain, the less likely it is to ride fast on a flat surface and roads.

I have currently a compact cranket on the front (50 teeth on a large crank and 34 on a small one), on the back a cassette that has 10 laces – from 12 to 25 teeth. My last trip to the “mountains” made me realize, that such a set is a bit too small even for my Polish conditions – it was hard sometime. Considering that I want to fight again in the mountains many times, it is necessary to change quickly. Here comes the problem, the rear derailleur capacitylimit. It does not have unlimited ability to handle laces and cranksets. Manufacturers happily give us a derailleur capacity that corresponds to the pattern:

Capacity = (number of teeth on the largest front crank – number of teeth on the smallest crank disk) + (number of teeth on the largest lace at the back – number of teeth on the smallest lace at the back)

And simple example with Shimano 105 5701 rear derailleur.
Capacity: 33T.

Crankset: 50/34T, so: 50 – 34 = 16.
Cassete: 12/25T, so: 25 – 12 = 13
33 – 16 – 13 = 4
We can still make changes (on crankset or cassette) with four teeth. To clarify: rear derailleur can be also limited to max. teeth on largest lace. We should check this before cassette changes.

Knowing this pattern and currently owned hardware, we can quickly calculate what cassette will be able to operate with our derailleur. In my case, it will be cassette 11-28: smallest lace will be useful for pulling, three additional teeth will allow you to go much lower uphill. If current capacity does not satisfy us and does not allow for scheduled replacement, it is necessary to replace the rear derailleur – not necessarily a completely different model. Everything depends on the length of the trolley, because the market will be longer and shorter, both of which are characterized by other capacities.

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