My original bars were 750mm with 70mm stem.

I swapped to a 760mm bar with 50mm stem – it feels better, but im definatly seeing a lot more of my front axle and things might just be a little cramped – thinking of sticking on a 60mm to see if that helps.

I was told that in normal riding conditions your bars should be in line with the axle? Surely this must now be old way of measuring as if I go to an even wider bar I will see even more of my axle?

Is there a general rule of how much shorter your stem should be to how wide your bars are? So if my new bars are 780mm compared to 750mm or the originals, how much shorter stem from the original 70mm?

I also want to try 780mm bars as moved my grips out by 10mm each side and felt pretty good.

No… it depends on what works for you.
Bar width, arms out straight then drop hands from elbows and measure the width, stem length depends on what your reach is like and if you have the right length frame.

I think the seeing the front axle thing is pretty much irrelevant now, if it ever was.

FWIW, I found wide bars changed my upper body posture from an “elbows down” arm shape to an “elbows out” one. Whereas I liked a lot of sweep on narrower bars, I am not so fussed on wider ones (which is not what you might predict). Also, hunching down over the bars is not uncomfortable as my chest is more open and I can still breathe freely. So a (even) shorter stem would not be such an issue cramped position wise. All of which was a bit counter-intuitive for me, the only bit that wasn’t was that I needed a somewhat shorter stem. And I still like the bars about saddle height or not too much higher.

Or to put it another way, you just got to see what works for you

As above- the old road rules of thumb apply less the further mtbs get from classic road geo and riders get from seated spin positions. Wider bars put you in a more commanding position when standing and will help open your chest. A long stem plus wide bars probably amplifies leverage too much plus risks being that push over the edge from well weighted contact spot to nose heavy OTB waiting to happen…

I think I have quite a wide span – when I was trying the press up position with a pair of 760mm bars in front of me, i was definatly in a slightly wider position than the bars themselves.

Have my eye on some Nukeproof 780mm bars so might just go ahead and order! They do a zero degree rise or a 5 degree rise stem so further choices!

I think it also depends onyour height. 760 mm bars on someone who is 5’2″ is very different to someone who is 6’2″

5ft 11, but think my arms a little longer than norm!

Shoulder width over height is more important for me for bar width 780/800 and 5’9

As above you can ignore the “bars in line with hub” guide, it’s affected by head angle, fork rake etc.
If you’re feeling a bit cramped, try 60mm though – if 750mm bars and a 70mm stem felt right, 760mm/50mm will probably feel a bit short. There’s a lot more tolerance in getting an OK position on a mountain bike compared with a road bike though.

Will try 760mm with 60mm before I get a 780mm bar, but thinking if I go 780mm I can go 50mm