Are roadbike stems limited to 6 or 8 degrees and are mountainbike stems a steeper angle? I see a stem of 17 degrees offered. Is that a mountainbike stem? Is an 80 degree stem the same as a 10 degree stem?

Road bikes generally have a stem closer to 90 degrees, but there is no hard-and-fast rule. THe only reason is that many people on road bikes prefer a more stretched out position and many mountin bikers prefer to be more upright. Also, the 'look' of the straight-forward stem on a road bike is what most people are used to so they resist putting a more upright stem on for aesthetic reasons.

But most modern bikes (mtb and road) have a 1-1/8 (28.6mm?) steerer tube clamp and a 31.8mm or 25.4mm bar clamp, and so the stems are often interchangeable. There are a few other older size standards of steerer tubes and bars, though, so if you have an older bike it isn't quite so straightforward.

I see Deda selling 35mm bars and stems.

After two back surgeries I prefer a more upright position and a shorter stem. The bike is a size bigger than it should be for me so I want to bring the bars back and up. Any recommendations?

Suggestions:

1. Get a bike that is the correct size
2. Get a shorter higher stem - as long as the steerer clamp and bar clamp are the right size then the stem will work.
3. Get a bar with some flare (Randonneur bars) or shallower drops to give you more useful hand positions
4. Get a bike designed for an upright riding position - a hybrid or 'flat bar road bike' or one of any number or 'relaxed geometry' road-sport bikes available.
5. Swap to flat or riser or trekking/butterfly bars (this is usually not straighforward so this would be the last thing I would try)

17° is the classic "standard" road bike stem angle. It complements a 73° head tube angle to result in a perfectly flat stem.

A 10° stem can also be considered an 80°/100° stem, However if you see a stem listed as 80°, then I would be concerned that it would be a quill stem since just about nobody quotes threadless stems by the big numbers, they're usually quoted by the degrees from perpendicular spec, either 10° or +/- 10°.

I'd also like to correct what LarDasse said about clamp diameters. 25.4mm drop road bars are fairly rare. The most common sub-31.8 road size is 26mm. Those are also getting pretty rare nowadays and soon will be pretty difficult to find new.

35° stems are fairly common and a couple of 45° stems are out there. You just need to figure out how high and back you want your bars. There is a good chance your steering is going to get squirrely if you go way short and tall. With hardly any weight on the front and a short tiller the handling may not be optimum.

Because of the way stems are measured, a 100mm 35° stem has WAY less reach than a 100mm 10° stem, here's a chart to help visualize:

Very helpful, thanks much.