I bought a new Trek Fuel EX8 29er. The bike came with a shimano M552 3x10 crank, SLX direct mount F derailer, 11-36 cassete, and XT shadow+ R derailer. I wanted to change to a 2x10 set up. I bought a SLX crank with 38/24 gearing. The LBS installed the crank and made adjustments to the Front derailer to reasonably accommodate the 2x10 crank. I am using SLX shifters that have a switch on the bottom that allows 2x or 3x. The shifters are set to 3x. Anyway I occasionally drop my chain when shifting from the 38t to the 24t front sprocket. This seems to only happen when I am in the two largest gears on the rear cassette. I have since ordered the proper 2x front derailer to hopefully fix the problem. But I have heard the rear derailer could cause a chain drop if you don't have the right length cage (med or long cage). I am using the same gearing as what comes stock on the Fuel EX9 29er but not sure if that bike has the same length cage as mine. Thoughts?