Joined: Feb 2012
Posts: 19,354
Likes: 5,472
From: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
Shimano front der cable pull ratios are different between road and mountain bike, but you know that. Not sure how far back in time this difference is valid. Also i believe that the cranks have slightly different ring C-C spacings, road and MtB. The thumb levers work well because the front is a friction unit.
Options- First would be Ergo with a JTech adptr for the rear. Front is not indexed and read with the Shiftmate works pretty well. Second would be bar end levers. Third might be trying a road front der with STI levers, being only a double crank the differences in stroke might not be enough to be a problem.
One issue is the 42 front big ring. Any road der will not have a good cage curve match to this small a ring. This aspect falls off around a 46 ring.
In the end you might just have to try and see with what ever combo you come up with. Andy.