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I'm building a SSCX bike up for commuting and general mucking about on.
Last week I was experimenting with my regular cx bike...46/21 seemed to be a good compromise for my commute (1000ft climbing mixed paths, roads and trails).
I'm looking at the shimano grx single chainset. It's got a 40t ring...so would I need a 18 or 19 cog to get closest to the 46/21 I've been experimenting with?
46/21 is 61.3 gear inches.
40/19 is 58.9 gear inches and 40/18 is 62.2 gear inches.
40/18 is closest, but a tiny bit harder.l
40/46*21
Back when all roads were gravel and hills hadn't been flattened for cars, your great grandad probably rode a bike with 65" gearing, and your great granny one with 60".
So pick a ratio somewhere in that range and you won't go wrong.
They were probably on fixed wheel with open pedals too.
(I'm building something similar right now and I'm aiming for somewhere between 60-65")
I have 40/16 on my Peregrine and works well for me, can get up most things and can spin at 17-18mph on the flat.
I think you said 1000feet climbing, is that long drag or ups and downs?
38:16 with 43c tyres = 66.2in here for easy commuting in Midlands
39:16 fixed or 39:18 free.
The difference is more to do with keeping the fixed gear manageable downhill. 39:18 is just about spinnable on the flats.
Unlike MTB though it's a much bigger compromise. MTB you winch up to the top in a sub 52" gear and in general you can go as low as you like with very little impact on average speeds unless you live somewhere incredibly flat. Because you're climbing as fast as you can almost regardless of gear and descending is always mostly freewheeling. On the road or paths, you spend a lot longer spinnign out which is why fixed works quite well for keeping you motivated.
If you felt 61 gear inches was about right you should be okay with an inch or two either side. Just go for whether you would ideally want it a tiny bit lower or a tiny bit higher but either should work well.
I run a very low fixed gear of 56 (makes it fun spinning at 200rpm downhill!) but can ride a 63 on same loops just as easily but I just prefer the lower gear.
Thanks guys.