Try duplicating your road setup on the mtn bike. The relationship between my knees, the seat height/position and the pedal spindle are all basically the same on the two bikes, with small allowances on the mtn bike for getting behind the saddle. Since your ride isn't technical at all, you should be able to get pretty damn close. If you get everything set up the same and you still have trouble, then the Q-factor may be the problem.