I'm in the slow process of building up a Pompetamine commuter bike which I want to build up with an 8spd Alfine hub. My commute is 8 miles long with a some lengthy flat road stretches but also with a couple of shortish but solid road climbs, the main one being a couple of mins in granny ring (30t) & 2nd or 3rd biggest cog of a 12-26 cassette on my Spesh Sirrus.
I'm hoping the 8spd Alfine will have enough range to get a decent speed up on the flat sections but low enough to get me up the main hill without too much trouble but guess it depends upon the chainring I spec? 11spd Alfine may be an option if the price comes down a bit more upon release, as I know it has more gear range...but it's still pricey for my budget!
Any advice on the best way to determine what size chainring I need?
Find out what gearing is in the alfine (eg: what the equivalent 8 speed cassette would give you). Then spec a chainring to suit.
WHS + you need a 30-23 and poss a 52-12, that's a range of 332% Alfine is 300% IIRC? Sounds like it will work, you'll have bigger jumps though.
Some kind chap on here put all the ratios on a spreadsheet. E mail me for a copy or IIRC Sheldon brown has one.
With a 18 t sprocket on the hub it effectivly acts as a 11/34 casette if that helps you.
45 looks about right for the same bottom gear as you use now and thats a top of 45 / 11 equivalent.
Thanks for the advice guys, that's really helpful.
Just had a go on Sheldon Brown's site, and looks like as you say a 45/18 combo would be about right. I notice most of the Pompetamine complete bikes come with a 48/18 set up which would take a bit more effort on the steep climb so will keep an eye out for a 45t chainring in the sales!
Make sure the chainline of your Alfine matches your front ring.