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I recently bought an urban bike that comes with a single gear crankset with a 42 tooth sprocket and an 8 speed rear with 12-36 teeth. I would like to swap out the front sprocket to a larger one to gain more top end speed. I'm thinking 48-53 teeth depending on what I find. I wonder how I would go about doing this. Can I just screw off the 42 and attach a 48 to the existing crankset and then lengthen the chain a bit? Or would I need to purchase a new crankset? Would I need to adjust the rear derailleur if I run a larger front sprocket?

  • "Can I just screw off the 42 and attach a 48 to the existing crankset and then lengthen the chain a bit? Or would I need to purchase a new crankset?" - Depends on the crankset. Can you post a picture of it? – renesis Aug 7 '15 at 14:37
  • I am yet to actually receive the bike so I can't take a picture. I tried googling the specified crank but can't seem to find the exact model. It a prowheel pioneer 42t. This is a picture of the bike itself where you can sorta see the crank. orbea.com/ie-en/bicycles/carpe-30 . thank you in advance. – user20955 Aug 7 '15 at 15:14
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    You'll want to make sure there's enough clearance between the new chainring and the chain stay. You should be able to increase your chainring size, but I wouldn't be surprised if a 50+ tooth chainring ended up interfering with the chain stay. Also, you mention you have $500 to upgrade, you might have been better off just putting a couple hundred extra into the initial bike, as you usually get a much better deal on parts when they come already on the bike. – Kibbee Aug 7 '15 at 16:51
  • Yeah man I feel you its just that this is pretty much the only rigid road bike I can find in my country. I'm better off shipping components than an entire bike. Would there be anything I could do to avoid interfering with the chain stay? – user20955 Aug 7 '15 at 17:19
  • Look at that picture, just to the left of the chainring. There is a sort of dimple in the chain stay to keep the ring from hitting it. Depending on how much clearance there is there you may or may not be able to use a larger ring (and note that the bike you get may not exactly match the picture in this regard). – Daniel R Hicks Aug 7 '15 at 19:37
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You'll have to wait until you get the bike & measure the distance between the holes, using this method. Then you can order the appropriate size chainring. Such as those here.

Rather than trying to lengthen the chain, I would recommend you buy a new one as you will be much less likely to have a tight or bent link this way.

You should not need to make any adjustments to the derailleur if you are only changing out the front chainring.

  • So I can put pretty much any size chainring on the existing crank as long as I match the measurements right? How would I know what size chain I should purchase? Would you suggest changing the crank to something more upper scale or does it not really matter? Thanks a lot and sorry if I'm asking dumb questions. – user20955 Aug 7 '15 at 15:43
  • You can order the same 8spd chain as is listed in the bike listing, or go into a bike shop & ask them for an 8 speed chain.Really the only thing you would gain by changing the crank arms / spider would be a tiny bit of weight savings at a high cost. – renesis Aug 7 '15 at 16:03
  • So durability and performance doesn't really differ between cranksets? – user20955 Aug 7 '15 at 16:12
  • Well, when racers speak of performance, small reductions in weight @ high monetary costs do become valid. That said, some of the lighter weight high end stuff may actually be less durable. I would stick with the cranks that are on there. – renesis Aug 7 '15 at 16:16
  • @user20955 different rings will be designed for use with different speed systems, some will be narrower, some wider. Many will have shifting ramps machined into the teeth, some may even be missing a tooth to facilitate shifting. Just so long as you don't get something too wide, like a bmx sprocket, you should be alright. I might also try to avoid the 10-11 speed rings. – renesis Aug 7 '15 at 16:22
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This is a fairly standard looking crank, chainrings should be readily available in a variety of sizes.

When I searched on the specs the pictures and listings didn't match, so you'll have to measure to be sure what you have in front of you.

However, you might want to actually try riding the bike as it is, turning your legs at a fairly standard 90 rpm is around 40kmh with the top existing gearing.

  • Aren't 104mm generally 4 hole? This looks like 5 holes in the listing. Maybe I'm totally wrong though... – renesis Aug 7 '15 at 16:05
  • I'm real strong and from what I read this means I can benefit from a larger chainring? So basically its just a matter of measuring the crank and screwing on a chainring and putting on a new chain? Thanks. – user20955 Aug 7 '15 at 16:11
  • @renesis, Yeah, you're right, didn't see that, could only find 4 arm 104s in google. – alex Aug 7 '15 at 16:11
  • @user20955, Maybe, you're going to want to try. 40kmh is quite a speed to hold for too long. – alex Aug 7 '15 at 16:14
  • While I agree with what alex is saying about top speed & maybe trying the bike first, chainrings are inexpensive & the OP will then have two to switch between in so desired. So I say go for it! – renesis Aug 7 '15 at 16:23
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The general answer is, "yes, you can change chainrings" – chainring is bicycle talk for the front sprocket. The more specific answer will depend on the actual crankset the bike is built with. There are some that don't make it easy to remove a chainring or where replacement rings are hard to find. To know for sure you'd want to have the bike in hand and check – unless you know the make and model of the crankset that comes with it.

All of that said, I think it would be a good thing to hold off for a while on changing anything. In general (again) pedaling at a higher speed (meaning higher RPMs which cyclists call cadence) is easier on your knees and overall endurance. So I'd encourage you to experiment with the existing setup first. At 70 RPM, which many would consider to be on the low side, your top gear (42x12) will give you about 32 km/h (20 mph) and up at 90 RPM you'd be doing 40 km/h (25 mph). Those are respectable speeds on the flat. Downhill you'll be able to go faster (if you want to), but at some point you'll "spin out" and find that you can't pedal fast enough to push the bike any faster.

If you do decide that you want more speed (or more speed at a lower cadence) then you have two options:

  1. A larger chainring in the front (which will make all of your gears higher), or

  2. One or more smaller cogs in the rear (which will make the individual gears higher). The easy way to make this change is to buy a new cassette (the set of rear gears). You could go to a cassette with 11 teeth on the small cog which would take you up to 35 km/h at 70 RPM.

The advantage of changing cassettes is that the interface is pretty well defined (unlike chainrings where there are lots of little gotchas) and the cost is lower (in general). Over time you'll wear out cassettes, so you'll be changing your's from time to time anyway.

If you want to play with possible gearing scenarios the HTML5 Gear Calculator makes it quite easy to see what the impact will be.

  • Changing the cassette would imply finding another 8 speed or switching out the derailuer as well right? This bike runs a shimano altus 310 so maybe I should switch it to a higher spec 10 or 9 speed? – user20955 Aug 7 '15 at 16:49
  • You could go either way. I'd lean towards staying with 8-speed, but you could also go 9- or 10-speed. Either of those would mean a new chain and a new shifter (or going to friction shifting), but would not require a new derailer. Shimano is pretty consistent about the actuation ratio of their derailers. – dlu Aug 7 '15 at 17:07
  • So if I were to go full out I would. 1. Put on a 50 2. New chain 3. New cassette with an 11 tooth 4. New shifters 5. A higher spec derailer. anything else you guys would suggest I should upgrade? Basically trying to make a really reliable but fast commuter. Thanks again. – user20955 Aug 7 '15 at 17:16

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