Road vs MTB bike geometry

[Dennis]

Here is a request for information. Maybe one of you knows about biomechanics and can explain this to me.
as some of you know, I have been trying to find a good road bike frame for myself. Luckily my LBS has one of those adjustable "fit bikes" so that we can set up the geometry of different bikes to see how it would feel.
anyway, it turns out that I need a road bike with about a 52cm top tube length, which is common in smaller sizes. (with 9 or 10 cm stem). so, being curious, I broke out the K2 mtb measurements and find out that even small mtbs don't have top tube lengths anywhere near that short. even the medium size Oz that I have has a 56.3 top tube, which is extremely uncomfortable for me on a road bike setup.
what gives? I would have thought that since you are down in a more aero position on a road bike that the top tube would need to be longer than on an mtb. obviously, I am not thinking correctly about the geometry somehow. or, unfortuntely, Blue Crush, being a med. Oz frame is waaaay too big. I do have minimal standover on it, but its acceptable. anyway, who can tell me what gives?

[Matno]

Well, there are a LOT of factors that go into sizing a bike frame. What size "fits" you and what size feels good can be totally off. In my experience, it's rare to find a bike shop who actually knows how to use a fitting bike.

However, what they told you doesn't sound way off. Actually, it initially SOUNDED off to me, but then I measured my bikes, and lo and behold, it's right! I would have sworn that my Large 5000 had a shorter top tube than my road bike. Not so. It is 58cm while my road bike is 56. I know that they are both the right size for me, so if your Oz is a medium (56cm) then the right road frame for you might be a 52. That's fairly small, but since I don't have any idea how tall you are, or how you are proportioned, I can't really say what will fit.

Also, the drop bars on a road bike extend your "virtual top tube" length by quite a bit. I imagine your Oz would also feel pretty uncomfortable with drop bars. The mountain bars are MUCH higher - about 5 inches above the headset on my MTB vs 1 inch below on the tops and 4 inches below the headset in the drops on my road bike! That's a potential difference of 9 inches - just counting the vertical difference. Add to that the horizontal difference, which is 3-4 inches longer in the drops or on the hoods of the road bike, and you have a much longer position with the same size frame. Thus, if you were to get a road frame the same size as your Oz, it would indeed feel much longer. If you're new to road biking, I suggest you figure out a way to test one for a couple of weeks. You might be uncomfortable on a 54cm now, but they kind of "grow on you." (Whatever you decide, I'm sure you'll be happy!)

I hope this is helpful!

[Dennis]

Matno,
I have come back to road biking after a long absence, but everything you said made perfect sense. Why didn't I think of the height difference of the handlebars? The size of a road bike vs a mtb just struck me the other day because I had the Oz next to the Trek 1100 I have been using as my entree back into roadiness. The Trek is too big for me (56cm top tube or so) but I got for 100 bucks. Its amazing that after I rode every day or so for a month how comfortable I got on it, except my neck and upper back are uncomfortable when I look up to see where I am going!!!
anyway, the guy with the fit bike seemed ok. We just set the up to mimic the geometry of several bikes, then I got on to see how comfortable it would be. Smaller bikes are definitely better for me!! We're talking 50 cm for the Calfee! The Look KG 386 had too long a top tube no matter what size we tried.
even the 52 cm Aegis was a bit too big.
I think the 51 cm Tommasini carbo light might be the ticket. (see pic in my gallery) its aluminum with a carbon fork and carbon seat and chainstays (all Columbus Muscle carbon with the Ti mesh). its a very classic looking bike besides, although I'm not sure about bonding alu to carbon, but its Italian and I have a 9 spd Record group with Mavic brakes ready to go right on it!!
anyway, thanks for your input. It explained alot that I was curious about.