# Rear sprocket size?



## oxford_guy (8 Feb 2009)

Hi - am converting my Ridgeback Neutron hybrid to single/fixed speed hub, as after much abuse the hub gears are FUBAR and I'm only going to use that bike as a winter commuter hack/pub bike, so is not worth replacing the hub gear unit (would be around £200...). Anyway, my local bike shop is offering a decent Surly OneXOne flip-flop hub with the correct OLN size (135mm) at half price, so will be fitting this, but was wondering what would be a good rear sproket size to fit? My chainset ring has somewhere around 44-46 teeth (was not 100% sure when counting the teeth), and I will be mostly using the bike on fairly flat roads. Thanks!


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## MajorMantra (8 Feb 2009)

It's a matter of taste, and whether you're a spinner or a grinder, but for fixed I'd suggest starting with a gear in the 70-something inches range for flattish roads. You can fiddle with ratios here:

http://www.sheldonbrown.com/gears/

For freewheel I'd think you'd want a slightly easier gear but if you really don't have many hills you may not feel the need.

I run a 74" gear (48/17) in hilly Edinburgh which seems to work for me. It's a little on the high side for some of the hills but it has the advantage that my top speed is higher. If I spin at 100rpm I do 22mph, 150rpm gets me 33mph, and 200rpm (which I likely haven't hit yet, I only just fitted a speedo) would get me 44mph.

Matthew

EDIT: Doh! I actually counted the teeth on my cog and realised that I've been running an 18t for all this time, so I'm actually only at 70GI. The shame...


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## oxford_guy (8 Feb 2009)

MajorMantra said:


> It's a matter of taste, and whether you're a spinner or a grinder, but for fixed I'd suggest starting with a gear in the 70-something inches range for flattish roads. You can fiddle with ratios here:
> 
> http://www.sheldonbrown.com/gears/
> 
> ...



Thanks for the link Matthew, am not sure what my crank length is, where on the crank do I measure this to/from? Or should I just estimate based on common crank sizes (though I don't know what is usual...) for a 700c hybrid? Thanks

Assuming I had 170mm crank (is that a reasonable assumption?) 45x16 would give me 75.3 inches - is that going to be too high? Am not sure what my cadence is.


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## MajorMantra (8 Feb 2009)

It's likely you have a standard size like 170mm, but anyway the crank length doesn't affect gear inch calculations, it's in the calculator for doing gain ratios.

You might well be happy with 75.3 inches, you won't know til you try. My fixed came with the similar 48/17 setup and since it seems to work pretty well I've never bothered to change it. New cogs aren't especially expensive so if you decide you don't like it you can always swap it out.

Matthew


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## oxford_guy (8 Feb 2009)

MajorMantra said:


> It's likely you have a standard size like 170mm, but anyway the crank length doesn't affect gear inch calculations, it's in the calculator for doing gain ratios.
> 
> You might well be happy with 75.3 inches, you won't know til you try. My fixed came with the similar 48/17 setup and since it seems to work pretty well I've never bothered to change it. New cogs aren't especially expensive so if you decide you don't like it you can always swap it out.



Okay, cool. I'll have a chat with the guys at the bike shop too and see what they think. BTW will be using both single and fixed at different times, I think, would the same ratio be okay for both? This stuff is all new to me, but am learning alot, thanks to all the advice on here, what a great resource this place it


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## MajorMantra (9 Feb 2009)

oxford_guy said:


> Okay, cool. I'll have a chat with the guys at the bike shop too and see what they think. BTW will be using both single and fixed at different times, I think, would the same ratio be okay for both? This stuff is all new to me, but am learning alot, thanks to all the advice on here, what a great resource this place it



With fixed you get a 'helping hand' in that your forward momentum keeps the cranks turning through top-dead-centre so you don't have to push through it. This means that with a given gear ratio, it's easier to go uphill with fixed than with a freewheel. 

Because of this, I think it's likely that you'd want a slightly easier ratio with the freewheel. That said, once you try fixed you may never want to go back - it's quite addictive and enormously enjoyable. The first time I rode fixed I couldn't stop grinning like an idiot because it's such a neat feeling. It's also great for strength. I don't cover all that many miles but I'm noticeably stronger since I went fixed.

Matthew


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## Oddjob62 (9 Feb 2009)

oxford_guy said:


> Okay, cool. I'll have a chat with the guys at the bike shop too and see what they think. BTW will be using both single and fixed at different times, I think, would the same ratio be okay for both?



Often people will stick a slightly lower gear on the ss side. This is so if they need to do a slightly hillier route, or are just plain knackered, they can have an "easier" gear option. Obviously the main issue with this is how much adjustment room do you have on your dropouts


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## oxford_guy (9 Feb 2009)

Oddjob62 said:


> Often people will stick a slightly lower gear on the ss side. This is so if they need to do a slightly hillier route, or are just plain knackered, they can have an "easier" gear option. Obviously the main issue with this is how much adjustment room do you have on your dropouts



Not a bad idea, I could go:

45/16 = 75.3 on the fixed side
45/17 = 70.8 on the single speed side

What do people think? The horizontal dropouts should allow a reasonable amount of adjustment


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## Origamist (9 Feb 2009)

45/18 would be my choice ss, with 45/16 fixed.


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## tyred (9 Feb 2009)

When the bike had gears, which ratio did you use most often? That should give you a good clue. You could probably go slightly higher than that as the bike both lighter and more efficient without a derailleur.


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## oxford_guy (9 Feb 2009)

tyred said:


> When the bike had gears, which ratio did you use most often? That should give you a good clue. You could probably go slightly higher than that as the bike both lighter and more efficient without a derailleur.



Am not sure, as it had hubs gears, not a derailleur before...


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## tyred (9 Feb 2009)

oxford_guy said:


> Am not sure, as it had hubs gears, not a derailleur before...



Sorry. Didn't read the OP properly

Still, which gear did you use most? You should be able to find info on the hub type and using your rear sprocket size, work out what the gear ratio was.


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## Oddjob62 (9 Feb 2009)

Could you not borrow someone's bike so you can have a rough idea?


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