Hi Richie!
Some quick answers for you...
Is the gearing you mention Dan the lowest you can go and stay in warranty?
Yes, at present, thanks to Rohloff's revised standards.
Whats the thinking behind a Stainless ring.
Extremely tough and long-wearing. Stainless is harder than aluminum. Teeth are tougher, too, if you manage to high-center on something. Added advantage: If you're contemplating a Hebie Chainglider, the thinner Surly stainless 'ring will fit with minimal rubbing, unlike the very high quality but thicker Thorn 'ring. However, Hebie don't (yet) make a 36T front-piece for the Chainglider; 38T is their current minimum.
If you go with a Surly stainless chainring, do note Surly's caution that in some sizes/BCDs, the 'ring is a bit thick for 9-sp chains and will bind slightly for a bit till the chain adapts:
Our 94bcd,104bcd and 110bcd x 34/35/36t chainrings are made with a 2.2-2.3mm thick stock plate and work best with 5-8 speed chains. Many folks are using these on 9 speed drivetrains but this can often results in a "tight fit" right out of the box (This will loosen up and shifting should improve after the chain breaks in). The 110bcd 38-50t and all 130bcd chainrings are machine finished to have a 2.1mm tooth thickness and thus work fine with 9spd chains.
If I change should I change the chain and /or the rear cog at the same time?
Depends entirely on the state of wear for the cog and chain, though of course you'll have smoothest meshing if you change the lot and start fresh. The cog is stainless as well, so the most likely point of wear if things "look good" is the chain. If possible, give it a measure, or keep in mind whether you've had to tension it, as stretch (actually wear at the bushless ends/pins is the key determinant).
Hope this helps!
All the best,
Dan.
