Tri Bike vs Road Bike with aero add ons

Performance is a hard thing to measure. 

You can measure difference is weight, you can measure differences in drag, you can measure differences in bike geometries etc, but it is hard to put those into a formula to calculate how much fresher you will feel of the bike or how much faster you are going to be able to run. 

My understanding is that a Tri Bike basically takes your position form a road bike and when looking at your from the left profile rotates it clockwise.  So...all your angles are the same on a tri bike but you have less drag.  My understanding is that for a time trial you start to reduce the hip angle, etc by getting into a lower more aggressive position so you are reducing drag by changing your fit rather than reducing drag by changing the bike geometry.    

If you are taking a road bike and putting aero bars on it you are not going to get the full-body clockwise rotation so I am guessing that means that you are going to be reducing the hip angle to get lower into the aero position.    

I think I heard a rule of thumb of something like 20% reduction in drag (but drag is not the only factor in performance) in the aero position vs an upright position.  I am sure that you aren't going to lose 20% of you power by going aero so lots of gains can be made with aero bars on a road bike.  I don't have a road bike to compare with but on the bike trainer with virtual power my in-ride instruction will ofter say if I can't maintain the target power to come out of the aero position.  In, or out, I don't see much difference in the power I can generation but I would say it might be as much as 1-2% more power out of aero than in aero.  If you have a road bike with aero bars and you are opening up the hip angle when you come out of the aero position that might be more like a 2-3% increase in power generation (I am totally making up numbers here).  So if my made-up numbers are anywhere close you might see a 1-2% increase with a Tribike over an equal level street bike. 

However, if you have a high-end street bike that you can equip with aero gear you might be faster on it than going for an entre level tribike.  I did my first Triathlon on a high-end street bike that I borrow from my brother's friend and averaged 21.5 MPH without any bike training at all.   When I got into triathlons 8 years later I got a $300 used Triabike with an aluminum frame.  I trained on it for 4 months before my debut race and only averaging about 20 MPH on it for similar rides to the race I did 8 years earlier (but am happy to say that after 5 years training and some bike up-grades am doing 24 MPH at similar races).  

So my guess (again shooting from the hip on these numbers) is that you could see a 1-2% improvement from a road bike to Tribike if the two bikes are equal level bikes, but that if you have to choose between a high-end road bike with aero gear and an entry-level Tribike that your high-end road bike may put you 2-3% ahead of the entry-level tribike.   Since these numbers are all made-up anyways we will assume that they factor in the difference in run times too.

Just to add to the very thorough post above...

I think if finances are tight you can get a HUGE jump going to aerobars and I'd recommend it to anyone.      They are so cheap in comparison to a new bike.  Is it perfect?  Heck no, but with minimal investment and effort you're in a much better position.

That said, in addition to the difference in geometry you also get bar end shifters on a tribike.  That is a big time and aero savings if you don't have to constantly move from your position.  If you're in hilly area, requiring lots of shifting, it matters more.  

Rhetorically...

What are your tri-goals and where are you relative to those goals?

What are your life goals relative to your tri goals?

IMO, what you “give up” to ride a road bike with clip-ons only matters if you’re near the pointy end and want to get further towards the point  

when I started tri’s, I had a mountain bike.  I got an old road bike, added clip-on bars, exchanged it for an old tri-bike, and eventually got a new road bike and added clip-ons.  My total investment is comparable to an entry level tri-bike And I’ve spent more $$ on race fees than bikes.  I can tell you, the choice of bikes hasn’t been limiting my performance!  

The aero setup is good IF you can remain in it.  If the course is hilly or you’re not disciplined, the advantages evaporate quickly.

Is there some “excitement “ to get a $5K tri bike?  For sure, but then there’s also “excitement “ to have that $5K in the bank for whatever other goals you have.