I am considering building a winter commuter (possibly Pompetamine). Ideally from a cost perspective I’d single speed it. Ideally my legs would prefer an Alfine hub – as I am fat and unfit.

Commute is 17 miles, mostly quite flat but there are some short hills. Most days I would commute one way only (and go by train the other way).

Google seems to suggest that 32:18 is a common starting point for single speed. My road bike’s smallest chainring is 39, so would running 39:21 give a reasonable idea of the pain I will suffer on this? Something tells me crank length and tire diameter have an impact too – but are they small enough to ignore?

I don’t want to buy this and discover I can’t ride it for its main purpose.

32:18 will be for offroad. Onroad, 39:18 would be about right for an averagely hilly commute imo.

32:18 would drive you mad on the road, way too spinny.
My road tyred mtb runs 42:16 and is fine for me but I have been SSing for a few years so go with warpcows suggestion of 39:18 and see how you get on.

What they said, 39-18 would peobably hurt just enough on the climbs without being too spinny on the flats.

On road in a fairly hilly Edinburgh city centre – 16:40. 26″ mtb

I had a pompino for several years, 48×18 and that was spot on I reckon, 18mph cruising speed, bit of a push away from the lights or if it was windy and I was tired. I managed to drag it up some big seriously steep hills on just for fun rides too. You could try lower gearing but unless you’re really out of shape or have a very hilly commute I would go much lower.

Recently had a cxer running 42×18, offroad on well surfaced tracks was fine but too hard for rough uphill tracks and a bit spinny on the tarmac.

I have a Pompetamine that is used for commuting, heavy duty shopping and child transport. I have 42/18 on it, which spins out when unladen but means I can still get up decent hills with heavy loads on it.

39/12 here but no massive hills on my commute. It’s ok to *swoon*

39/18 would be my starting point, till your up to speed.

OK 39/18… so can I test the survivability of that by simply riding that combination (or the closest I have) on my road bike?

worth remembering that you will be able to push a bigger gear, when the only option is to get off/walk

its also worth getting your head into gear inches, to easily compare combinations.

Try fixed gear, its fun.

For fixed, I think about 65 to 75 gear inches would be a good start. Most hills would be manageable, and not too spinny. Maybe a bit higher gear if you are somewhere flat.
So something like 39×16 or 39×15.

I commute to work on single speed mtb i run 36 x 16. With two longish hills and i find it ok.

Am I odd in thinking a lot of the suggestions are very short gears?
If your commute is mostly flat, you’re really going to be spinning your legs very fast indeed at 39:18… This is road we’re talking about isn’t it?
I would consider something like 42:16 to 46:16 personally….!

DrP

I run 45:18. On my pompetamine. Use it on winter club runs and can cruise at 18mph keep up with the group as long as they don’t go much above 30 and climb hills. I’ve got up a 20% one but I had to use all the road by the end.

Alfine chainsets are cheap on planet x with a discount today. It is much easier and cheaper to change the rear sprocket than the chainring. Pick your chainring size to give you options.

Jonba, I was about to say exactly that. I was tempted to get a slightly smaller sprocket when I recently had to replace it, but concluded that I still need the lower gear for a laden bike on the hills.