Singlespeed Gearing

smurfylicious

Make a note of what gears you're using on the road bike you already have. Then divide the chainring teeth by the sprocket teeth. Do the same for the bike you're thinking of buying and compare. If tyre sizes are different you'll need to multiply by tyre circumference to allow for the difference.

I'm buying a ss/fixie soon. Although I've never had one before I already know what gearing I want because I know what the chainring/sprocket ratio is for the combination I use most on my geared bike. I also know which hills I'll be able to get up using that combination.

One other thing to consider is whether you get a whole number when you calculate the chainring/sprocket ratio (e.g. 48/12 = 4). If you do then the same places on your rear tyre will be worn repeatedly. The Sheldon Brown website has some good information on this.

smurfylicious

Below is the link to "Skid Patches". A way of calculating how many wear spots you will have if you do skid stops. But obviously it also applies to the number of wear spots you will have from "normal" riding.

http://sheldonbrown.com/fixed.html#skid

MrChuck

It's stating the obvious but it depends on the tradeoff you want to make. I have 45x16 and while I labour up steep hills it's not too spinny on the flats. Uphill starts from the lights are a bit of a PITA, especially where you could do with a spurt. Previously I had 42x16 which I found a bit spinny, YMMV though. That's for 700c wheels.

Also that stuff in the previous 2 posts about flat spots would only apply to fixies, to state the obvious again!

DF33

Why not just work out the chain length for each 16 17 18 on the ss. Then do the same for the nearest chainring / sprockets on your normal bike.
Ride into work without changing gear.

How does it feel? etc.

Plenty of chain inch tables on the net.

ex-pat scot

I have a similar commute to you.
20 miles each way.
15 flat(ish) miles and 5 hilly ones – 16% up and down.
I bowl along on the flat around 19-22 mph depending on mood and wind.
I find the 42 x 16 is perfect gearing for me.
I think the 42 x 18 option is pretty close to a 42 x 16 and would be fine for you.
There’s a lot of machismo around pushing big gears, but to me the whole point of fixed wheel riding is to build suppleness and spin comfortably.

gaspode

I ride 48x18 on my fixed gear - used to be 42x16, but I was finding it hard-work on downhills (my old legs couldn't turn fast enough and I was spinning out) - change to 48x18 gives me a couple of extra MPH on the top end, but doesn't make hills too difficult.
If you're intending to stay singlespeed (as opposed to fixed), then it won't be an issue as you can freewheel when you get the speed up...
I'd agree with the suggestion of trying to stick in one gear on your roadbike (use the online calcs to find a combination that is similar to each of the options), but remember, that:
1. your singlespeed should be lighter than a geared bike
2. your singlespeed will be more efficient as you're not losing power through the gear change/mechanism
(so don't expect the singlespeed to exactly match the geared bike with the same gearing......)

mz__jo

i would have gone for the 18 with a 48 ring around Bristol (and even that is a bit big for me) but then I have always spun a bit. If its a 42 ring then 16.

Velonutter

For training on my fixed I ride a 48/16 and I then ride mostly flat.

During the winter If I want to spin then I ride a 48/20 and get up most of the small lumps in my area.

Han2130

I've got a 38 chainring and 16 sprocket on my 29er. Very spinny on the flat but will get up the 1 in 5 hills round here.

Monty Dog

I'd start with no greater than 48x18 as you'll probably find the others too hard - I ride 48x17 fixed and that can be pretty fearsome on a lot of local routes which aren't too dissimilar to the roads around Bristol. Something like 46x17 would be better suited to rolling roads and have the ability to spin easily at 30kph on the flats.

smurfylicious

Just taken delivery of my bike and been for a 2.5 hour ride so I can now answer the question a bit better. The gearing is 42-18 and felt more or less as I'd expected. I didn't have to walk up any hills, and my route included two that were marked as 'single arrow' on the OS map (which means between 1/5 and 1/7). It's a tad 'spinny' on the flat but I was prepared for that as I've been practicing spinning on my older (geared) bike, I don't even ride that fast anyway so no problem. Downhills were fine as I have a freewheel at the moment rather than fixed.