2 Questions

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I can't quantify the speed difference but it should be significant along with the more efficient energy use on the roadie. 

I don't know of any studies showing what difference you can expect between the two bikes on the same surface.  The two bikes are meant for two different terrains.

In general, road bikes are built for speed and Mtn bikes give you more stability on the trails.  There's a great deal of difference depending on many other things.  For example, my Mtn bike, an older Gary Fisher, is set up with a fairly aggressive tread pattern for trails- There's still a big difference on the road when I switch to tires on that bike to those with a smoother profile and higher pressure.  I've never clocked the difference - I just know its big. 

If you are asking which is better in triathlon, road bikes, designed for speed and efficiency, generally position you much closer to the top tube and the pedals. The hunched-over position is more efficient for getting power from your legs.  They're also much lighter than Mtn bikes and so better for triathlon.

Edited by Gritty 2009-07-04 9:09 PM

The speed difference between mountain and road bikes depends on many factors but, I can give you a data point.  I have ridden my primary training route many times.  On my mountain bike with tires made for the dirt, I average a little over 15 MPH, on my road bike I average 17.8 MPH, on my tri bike I average around 19 MPH.  This is just for me and my bikes, YMMV.  A lot depends on body position, tires, wind conditions, power, and relative flatness of course.

 

thanks for the information. Now to the trek site to find difference between road and tri bikes.

Roger 

I ride the same route all the time and a couple of weeks ago when my road bike was being serviced I rode my MTB in for a change.

On the MTB I rode at 19.18mph

On my road bike in very similar wind/temp conditions I recently rode it at an average of 21.77 (max I've ever managed is 23.37mph).

So I would guess for us non-pros it's about 2-2.5mph

Hope this helps.

For #2, there are wayyy too many factors involved to give a simple answer on that question. Mountain bikes will be slower overall, but they still can work just fine.

What I would recommend is using your mountain bike for some sprint races (lots of people do!).  The one cheap upgrade you can do to it that will undoubtedly help a lot is to get some skinnier, smooother street-specific tires for it.  You can't put roadie tires on it, but there are a lot of commuter-type street tires out there made to be put on mountain bikes - they can be found for cheap too.  Hook those up in place of the knobbies on there now, and that will definitely help a lot.

After you get some race experience under your belt & decide if you want more, then we can look at other option to upgrade the mountain bike (perhaps different gearing setup) or you can consider getting a road or tri bike.

Cheers, Chris