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I'm willing to buy a 2015 Shimano 105 groupset and i have got 2 options in choosing the cassette, the first is 11-32 and the other is 12-25. I have no idea what these numbers indicate and which one would be more suitable for me knowing that i'am still a beginner.

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    Welcome to Bicycles @Ramy. Good question - I am amazed that it has not been asked here before! – andy256 Jan 10 '15 at 23:33
  • The gears (cluster or front chain rings) can be easily replaced - gears and chain are a maintenance item anyway. Focus on setting up for what you need now, and upgrade (sideways grade?) as and when your cycling improves. – mattnz Jan 12 '15 at 7:28
  • @mattnz, that doesn't help if s/he doesn't know what s/he needs now. – Holloway Jan 12 '15 at 12:27
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    @Trengot: I got the impression from the question the OP might think its like buying a car and choosing a 1600 or 2000 engine, where you are stuck with that choice for life. Its important the OP understands its not only possible, but quick and cheap to change gearing if you want, hence a comment not an answer. – mattnz Jan 12 '15 at 20:32
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If you imagine a cassette, will have a bunch of sprockets on it. The current 105 range (which came out last year) will have 11 sprockets, the earlier 105 had 10 sprockets.

The notation you've noticed simply means that for one of these cassettes, the smallest sprocket has 11 teeth, the largest has 32 teeth. And the second cassette has smallest sprocket 12 teeth, largest sprocket 25 teeth.

So these numbers are basically the "range" of gears covered by a cassette.

You might imagine that a large range (e.g. 11-32) is good, as it would give you lots of teeth to climb steep hills, but also a small cog which you can use to power downhill. And this is correct, but there is also a flip-side. A larger range of sprockets means that there can sometimes be a jump of a few teeth between each sprocket, which can be less smooth when changing gear. Hence, if you don't actually need the large range, a smaller range would give you a smoother ride.

As regards which one you should choose, it's not really down to whether you're a beginner or not, more to do with how good a climber you are. If you feel you might struggle going uphill, a 32-tooth sprocket is probably going to be a good choice for you.

The other thing to remember is that, when you're thinking about gears, the cassette is only half of the story, the other half being the cranks. The number of chainrings, and the number of teeth on each chainring, will also affect the gear range and smoothness. But that's probably a different question...

  • ok that's for the cassette, but even the crankset available in two options 50X34 and 52/36 is it the same as the the cassette ? – Ramy Jan 10 '15 at 12:49
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    @Ramy exactly the same principle for the cranks (except at the front, a low number represents an easier gear) – PeteH Jan 10 '15 at 16:03
  • Sorry, but i can't understand at the front aren't all the Cranks at the front ? – Ramy Jan 11 '15 at 10:54
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    My basic advice would be: there will come a time when you have sufficient experience that you will know exactly what combination of cranks & cassette you want, and this question becomes irrelevant. But obviously you're not there yet. Given that, I would tend to pick the combination which gives a lot of flexibility, sacrificing smoothness if necessary. 50/34 and 12-25 is therefore a reasonable combination. So too is 53/39 and 11-28 (or even 11-32). Bear in mind that if you get the choice really wrong and want to change (unlikely), a new cassette is a lot cheaper than a new crankset. – PeteH Jan 11 '15 at 12:42
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    Thinking about the number of "I need a higher gear" questions we see here that turn out the correct answer is "you need to spin faster" - I would suggest no bigger than 50 on and no smaller than 12 on the back for the highest gear- along the lines of youth gearing. It will help a beginner to have a good selection of very low gears if they ride any hills - encouraging spinning on climbs, so I would probably suggest 34/28 low gear (I know its very low) if hes got any hills. – mattnz Jan 12 '15 at 7:24
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Numbers denote number of teeth on smallest cog i.e. 11-32 has smallest cog with 11 teeth, and largest with 32 teeth.

11-32 gives you larger range of transmission ratios while 12-25 gives you smoother steps between the speeds. When calculating which one to choose also take into account number of teeth you have in front, and the kind of terrain you would like to drive. Also make sure your rear derailleur has capacity to consume chain slack for your given combination

  • even the crankset is available in 52/36 and 50/34, i think the one which gives smoother steps between the speeds would be better, right ? – Ramy Jan 10 '15 at 12:27
  • It depends on the terrain an on your fitness. If you have a hillier ground, go for 50/34 and 11-32. If the ground is flatter 52/36 and 12-25 will be better. Your LBS could fit an 11-25 cassette as well. Because there are more than the two you indicated to chose from. And cassettes are easy to swap. – Carel Jan 10 '15 at 16:38
  • I use a 12-27 cassette which could be an intermediate option between the two originally listed. – Holloway Jan 12 '15 at 12:33
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Smoother steps between the gears is a nice to have - but if the gearing is not suitable to you - it becomes irrelevant.

It's mentioned in the answer above "rear derailleur has capacity". This refers to the the length of the cage on the rear derrailleur. They come in three varieties (with Shimano anyway) - short, medium and long. Each length has a specified teeth capacity.

The teeth capacity required is calculated as the total difference between smallest and largest sprockets on both chainset and cassette. ie. 50/34 with a 28/11 is 33t - check with the manufacturer but iirc this is a medium cage.

(to complicate things - you can always squeeze a few extra teeth beyond the manufacturer's suggestion).

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