Iron distance (70.3 and full) road bike vs tri bike time savings

The difference will almost entirely be due to whether the position on the tri bike is substantially more aerodynamic than on the road bike.  That is, it's not really the bike.  It's how the bike allows you to position your body.  Most will be faster on a tri bike, some will not.  Some will be substantially faster on a tri bike (1+ mph), most will not. 

Agree with the previous post 100%, but I have noticed the main difference in how my legs feel going into the run.  My legs are much more fresh on the tri bike.  I have been told a road position uses more hamstrings and the tri position uses more quads.  Therefore your hamstrings are more fresh for the run.

When I moved from a road bike with aero-wheels and clip-on aero-bar to a tri-bike I picked up almost a half MPH at my 18-19 MPH typical ride pace.  The difference for me was that I could stay in a more aggressive position for much longer on the tri-bike.  The more forward design gave me more shoulder comfort and I could stay down longer.  In all honesty, I don't feel any advantage for the run and the bike speed difference wasn't significant either.  But, as my kids say "Wow, that's a hecka cool looking bike!"

There is some placebo value in that if you got the extra bucks to spend.  If money is tight, adding aero wheels to the roadie would probably make more of a difference.

 

No.  There have not been "a lot" of studies on this.  In fact, any evidence to this effect is quite thin at best.  One study, that is questionable, showed some relationship and has been repeated often by others as 'evidence'.

I noticed a bit of a improvement when I switched from a road to tri bike ...but ... I honestly believe that it was due more to the actual bike quality.. I had an older below entry model road bike and just switching to any decent bike would have most likely got me the same gains,,

Of course I credit it to the tri-bike usually ...  

Hate to say this, it all depends.... On a hilly course, I am faster on my Road bike, on a flatter course, I am faster on my Tri bike. I'm doing Lake Stevens 70.3 next month and plan on riding my road bike with clip ons. Last time I did that race, I was on my Tri bike. I'm running 50mm deep rims on my road bike, and back then on my Tri bike 50mm front and Tri Spoke rear. So the Tri bike "should" be a little more areo.

I don't find running off either bike any easier. I do bricks off of both bikes so maybe that helps. What I find makes a nice difference is for the last few miles on the bike match your pedal cadence to your run cadence (if they are different). For what ever reason this seems to make my legs feel fresher right as I start to run.

x2

I went from a heavier entry level road bike with clip ons to a light weight tri bike (that I was properly fitted for) and I was able to increase my average speed by 1-2 MPH. I'm not sure that would have been the case if I had a higher end and lighter weight road bike.

http://www.slowtwitch.com/mainheadings/techctr/geometry.html

Read this entire article.  Its very informative.  Most people who have tri bikes have an improper fitting.
http://www.slowtwitch.com/mainheadings/techctr/bikefit.html


A properly adjusted and fitted tri bike will allow you to be very comfortable in the aero position, allowing you be more aerodynamic throughout the race.  An aero position on a tri bike IS more aerodynamic than an aero position on a road race bike.

The time you gain on a tri bike depends on wind speeds and direction.  The ability to run better afterwards depends on how aware you are of your body.  Some will see it, most won't.

And for the record, Road bikes have geometries of 76 and below seat post angles.  Tri bikes are 76-83 or so degrees.  Many companies market their road frame geometries as tri bikes.  Please be very aware when buying a tri bike.

My javelin tri bike is 76 deg.  My next bike will have a 78 deg seat post angle or more.

After completing my first HIM on a road bike with clip ons, I wanted to upgrade.  After much research and realizing I wanted the bike for long triathlons, the choice was obvious and I love my tribike.  I am faster on my tribike because it keeps me more aero and fresher for the run.  That being said, I think it is an individual choice.  Just depends on how you want to use your new bike.  Good luck and enjoy your new bike!