It seems that lately I have been seeing a lot of questions about what length top tube you need to get to run drop bars off road. This is a question that has some unspoken myth and serious consequences for "real" off road uses. Let me explain.....
First off, it is imperative that you know how to position your drops for off roading. The whole reasoning for even using drops might be useful here, but I'll just say that one of the benefits is that your hand grip can be more relaxed in the rough if you are in the drops. So, it is a generally understood thing amongst drop bar aficionados that you ride in the drops all the time.
If this is understood, then your drop bar positioning becomes clear. The bars drop section must be as close to the same height from the ground and distance from your saddle as your current flat bar/riser bar set up. You cannot achieve this by using a shorter top tubed frame. Why? Well, a shorter top tubed frame is, in reality, a bike one size too small for you! You wouldn't buy a frame that size for your "normal" bars, so why would you do this for a drop bar bike?
The reason that this myth got started was because the function of fitting a drop bar to a properly sized frame is the function of the proper stem. Usually a short reach, high rise affair that for years was nearly impossible to get easily. To compensate for this lack of stem choice, some made the assumption that using a shorter top tube, (ie: a bike a size too small) would solve the stem issue. Not a good idea for proper off roading.
Now stem choices in high rise/short reach are becoming more common, (IRD, Salsa, and a few others com to mind) and custom builders are available to do this sort of work all across the nation. (Check out the inner-web-o-sphere for more) There is no excuse now for not using the proper stem.
Just keep in mind that you will need to accommodate the reach factor of the bar into your stem calculations along with the bars drop measurement. Yes.......you will have a goofy looking stem! There isn't anything that you can do about that unless you go custom. If you end up loving drops for off road, you won't care about the stems looks anyway. Heck, you might even grow to love that look.
So, next time you see someone talking about getting a "shorter top tube" for their drop bar set up, set them straight. It is a bad mistake waiting to happen!