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I would like to change my bike setup. I have a 8 speed freewheel and a 3 chainrings crankset and I would like to add a single speed crankset in front (around 50 teeth) because I am not using my front derailleur anymore. Is it a good match? If so, what kind of chain do I need? Do I need a new bottom bracket?

Thank you

  • You mean an 8-spd cassette, right? – BSO rider Jun 30 '15 at 21:28
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You probably don't need to purchase a new crankset.

You can remove the the rings you don't need and simply add a new ring. You'll need to know the distance of the bolt holes of your current chainrings/crank. This is known as the BCD (bolt circle diameter) and you can usually find it stamped on the chainring. Common sizes include 130mm, 110mm, 94mm, etc.

Ideally, you'd want a chainring designed for single speed use. Rings designed for multi chainring drivetrains have pins, ramps and cut away teeth to help the chain smoothly move from one ring to another. Since the chain isn't moving, you don't need them.

You will also need single speed chainring bolts which are shorter than double ring bolts.

You may or may not need a new bottom bracket. In a perfect world you want the chain to be in a straight line to the middle cog in back. If you mount your single ring in the middle position of a triple ring crankset you'll probably be good to go. If you decide to go with a dedicated single ring crankset you'll likely need a new BB with a shorter spindle length.

Chain needs are more a function of the rear cogs/derailleur than the front. Any chain made for a 7,8 or 9 speed drivetrain will work fine.

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    Thanks for the quick answer. I forgot to mention that I need to change my crankset because the biggest chainring only has 42T which does not give me enough speed. – poroy Jun 30 '15 at 18:46
  • @poroy: Are your chainrings riveted to your crank, or can you remove them? Good quality cranksets you can remove the chainrings, and you can replace the 42 with a 50. – whatsisname Jun 30 '15 at 19:00
  • @whatsisname: I don't know by heart if my chainrings are removeable, but I know for sure my crankset is a crappy one hehe (a friend gave it to me). – poroy Jun 30 '15 at 19:06
  • You'll probably need new cranks. Look for some that are compatible with you current bottom bracket (probably a square taper?). Look for a chainring that has a Wide-Narrow tooth profile, that will prevent any chain droppage. – Chris Lee Jun 30 '15 at 19:13
  • A chainring designed to be used in a single setup has teeth specifically designed to reduce chain droppage. Also, 42 tends to be the largest chainring with 4 holes to attach to the crank, bigger rings tend to have 5 bolt holes. So geting a new single ring crankset may be quicker than fiddling around with a cheap one. – Jahaziel Jun 30 '15 at 20:17
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This is probably more complicated than you think it is.

This depends on your bike. A lot of bikes which are spec'd with say 42t front chainrings for the big ring can't clear a 50t big ring in the front without the chain interfering with something. You'll also have to worry about chain line and possibly installing something to prevent the chain dropping. But, assuming its possible for your bike, if you have a cheap crankset, you'll likely need to buy a new one which is compatible with your BB, or possibly new rings depending on what crankset you're running.

If you feel your gearing isn't high enough, you're usually better off tweaking the cassette/freewheel (i.e. the gears in the back), or more likely increasing your cadence (with a 42 x 11 at an 80 rpm cadence, which is reasonable, you should be doing around 25 mph. 42 x 14 at the same cadence would be ~20 mph). It is almost surely the case that your cadence is too low.

If you are indeed hitting a good cadence, you probably should replace the whole bike so you get something which puts you in a more aerodynamic position and has better gearing (something like a road race bike).

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