I need to replace my chainwheel and cassette, and it made me think. I know that the chain has to be a specific length relative to the number of teeth on the gears.

I was thinking about changing the cassette for one which a bigger 1st gear. Suppose I could get one with whatever number of teeth I wanted.

The bigger the better, right? If better means less effort to climb hills. But I have never seen a really big gear, like almost the circumference of the wheel.

I assume that's for a practical reason, not just because it would be very lazy if it worked and look ridonkulous :)

  • 5
    You can (within reason) make the front sprocket smaller rather than making the rear sprocket larger. It's not that hard to get a combo where you can't pedal fast enough to maintain enough speed to keep the bike upright. Jun 8 '16 at 22:25
  • Google suggests they exist: google.com.au/search?q=bicycle+with+giant+chainring
    – Móż
    Jun 8 '16 at 22:40
  • 3
    There is a maximum cog size a rear derailleur can go to. Latest 1x12 MTB setup can get to 50, 1x11 is common with a 42 and 46 is possible.
    – mattnz
    Jun 9 '16 at 0:35
  • You never see weightlifters lifting just an empty bar. Not even beginners. Reason is, its very easy. So when someone close to me 1000 reps with an empty bar, she strained her tendons and couldn't lift for a week. So "Overspinning" might be the word for a stupidly low (large rear and small front)
    – Criggie
    Jun 9 '16 at 1:53
  • @DanielRHicks hit the nail on the head. Much less than 1:1 will be hard to ride unless your balance is excellent, when it will be slower than walking. Slightly less than 1:1 is quite nice on steep hills. A mass-market 3x8 setup can easily do 28:32 or 28:34 - and has been able to for years. There are more modern ways to achieve the same or wider range.
    – Chris H
    Jun 9 '16 at 13:25

In the diagram below (from Park Tool) one can notice that if the innermost (lowest) cog grows much larger it will interfere with the top pulley on the rear derailleur. This is the primary limiting factor for the size of cogs.

rear derailleur adjustment--low end
(source: parktool.com)

For chainwheels, I suspect that the ground would be the physical limit (for a typical diamond frame, anyway). Of course you'd better eat a good breakfast and have a friend with a shed on his car if you plan to turn a ring that big.


One limitation is your rear derailleur cage length. Typical are short (road), medium and long (mtb). If the cage isn't long enough it can't handle a cassette with wide range. For example a short cage might be able to handle 12-26 cassette, but for 11-30 you'll need a long one.

It also depends on range of chainring. Wether it is the range in chainrings, cassette or both you need a right size rear derailleur.

Chain also needs to be right length like you already mentioned.

In small sprocets the diameter of freewheel body puts the limit to 11 teeth. Some special hubs might have even smaller ones.

  • -1: Too much is technically wrong for this to be an acceptable answer in 2016 (5 years ago it would have been OK). SRAM do an 10 tooth on the XD Freewheel body - which is common enough I would not call it 'special' anymore (high end, expensive yes, special, yes - but not in the way you mean.). 9 tooth is available from some third parties. 1x setups can use medium cage with capacity of 37 tooth, meaning a medium cage will easily do a 10-42 cassette.
    – mattnz
    Jun 9 '16 at 4:17

There are a number of factors to consider. For another question, I looked up the specs. on a specific Shimano derailleur. It listed min and max values for both the largest and smallest rear cog as well as a total range. Of course, you may be able to make it work outside that range. The bolt circle on your crank will limit he range of gears you can use. On a full size crank with 130 mm BCD, it is hard to go below 39 teeth. Compacts with a 110 mm BCD usually come with 50/34. There are subcompacts that run 46/30. Some front derailleurs are mounted using a clamp on the downtube, but mine mounts to holes on the downtube. Although there are adjustment slots, I do not know if it would go low enough for a subcompact. If you need more chain, your local shop probably has bits left over from other bikes.

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