I have been riding and commuting on a Focus Mares CX for about a year. It's bloody fast on tarmac and gravel roads, but painfully slow on rutted or rocky trails. So, I'm currently building up a Surely Bridge Club, rigid, sub 10kg rig, for commuting/riding/bike packing. It will have drop bars and can accommodate both 27.5 and 29. The reason I chose the bridge club was largely geometry, I tabulated 7 frames and the Bridge Club was the frame with the best geo 'compromise', I also had a budget. I suggest you do something similar - find out what riding you want to do and identify the most suitable bikes, then tabulate the geo, and go from there. It's not always appropriate to simply buy a MTB and put drop bars on it, most of the time you'll have issues with geo. But, you will find that some modern frames will work, main factors are BB height, reach, wheelbase. The other consideration is drivetrain, keeping in mind that road controls will not always operate with MTB gears, it's a different pull ratio, but you can get an adaptor for this from wolftooth. I'm using ultegra controls with ultegra RX rear mech, and Hayes mechanical discs. My gearing will be 38 or 40t x 10-40, which will provide a good range.
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