I'm running 23" bars on all three of my XC bikes.
While I can see the advantages of wider bars (and have tried one, and liked it) the trees where I live seem to grow about 24" apart...

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Blog http://andrewhowett.blogspot.com/p/1.html

I run 600mm KCNC SC Bone w/ Syntace F99 on my 2007 26" Custom Lynskey Ti Hardtail. For my 2011 Rocky Mountain Element Team RSL, I run Syntace 600mm Duraflite Carbon w/ Syntace F109 stem.

I find 600mm give a or a bit works good for 26" bikes.

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http://www.hackracer.com/

635mm on my XC bike.

Gone from 580mm to 650mm on the Scale, and now with the Element FS as well. I still find that I move my hands farther out to the edge of the grips. Not sure if I went with even wider bars if I'd center my hands on the grips of if I'd just grip the outsides like now??? For reference, I run 420mm on my road bike and 440mm on my cross bike.

AFAIK the bar size also depends on your body measures, so the question seems to be a bit inappropriate.

Mine is 580 on the 26".

I decided to go for 4'' wider bar on this season and after 100km rided on my brother bike, I think that i will never back to 'narrow' handlebars.

I use Easton Ec70 monkeylite low rise 660mm, you cant go wrong with this one

I went from 660mm bars to 730mm bars (Thomson) and I love them. Within 20 minutes I realised why wide bars exisit some how it the bike feels more stable.

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www.thecycleclinic.co.uk

710mm Cromag Fubars on my steel hardtail with old Marzocchi Z1

560mm. And no need for a wider one for the trails i normally do.

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Happy Trails !!!
MTB / Giant Xtc '05 / 8.7Kg
ROAD / La Perla Negra - FM015 / 7.0Kg