Triathlon - 75 or 78 degree angle

Hi everyone,
Im new to this forum and new to triathlons. I plan on competing in my first triathlon this summer. I have been cycling, running, and swimming for years just never competitively or immediately following each other. I have a question about the bike though. I was curious what the benefit of a 78 degree angle over a 75 degree angle and which one is better for a beginner triathlete. Thank you for the help guys. I am planning to build a bike and was unsure which angle to go for. I know the new Louis Garneau is a 75 but with a specific forward post it would be 78. I also know most bikes tend to be 78 and was curious why.... Thanks again
JP

in short (though i am everything but a bike-tech-expert):

in triathlon (longdistance tris in particular) you want to stay in the sleek aero-position. 78° gives you more comfort and space for breathing while in that crouched position.

that's why many triathletes who do use a 75° frame at least use a forward seatpost to simulate the 78° angle...

What is the advantage/benifit of a 78 degree vs. a 75 (or less)? Here is my take on it...
It comes down to a matter of choice (and what you believe comfort to be) and it's intended application (TT or Triathlon? - What distance?)
A 75 degree would be more comfortable as it is closer to a road bike geometry. If you're a beginner you will most likly be doing either sprint or Olympic distance events - "Aero" is not that big of factor for shorter races. A road bike with clip on aero bars is all you really need, and the upside to this is you have a "Road Bike". A 78 degree geometry is designed for aero bars and is really TT or Tri specific. The geometry allows you to be in a more aggressive forward position (a combination of "aero and power" output).
Having a strong body core is also important if you are going to ride in an "Aero Position" on a more aggressive position, as it will be needed in T2 and the miles ahead on the run.

I have 2- Tri bikes (used only for Tri and TT events), and both are very differnt bikes.
1. Cannondale Ironman 5000 with a 78 degree seat tube angle (my Olympic up to 1/2 IM distances)
2. A Trek-TT with a 73.6 Degree seat tube angle (my Ironman distance bike)

It comes down to a combination of multiple things as to which is better.
1. The fit applied to the bike regardless of seat tube angle.
2. Optimal power output (with respect to the race distance it will be applied to)
3. Optimal Aero positon (with respect to the race distance it will be applied to)
4. The materials and component selection (or bike set-up) again - with respect to the race distance it will be applied to. (seat height and positon, Bar height and position, amount of reach which is a compination of bars and stem choices, and frame geometry).
5. Is the "combined geometry" right for your body? IT IS NOT just the "seat tube angle" you should be thinking about here.

The Cannondale has a much more aggressive position and fit, in that I am more forward and access a larger percentage of my leg muscles that are used for running (not a good thing if I need to run a marathon after riding 112 miles) which is fine for the shorter distance events since I'm a strong runner. The Cannnondale was designed to be a Triathlon bike and at the same time - has been used at Ironman with great success. But - the way I have it set-up - it is a much better bike for shorter stuff like Olympic up to 1/2 IM distances. For me the Cannondale (being Aluminum) it very stiff, and the bars I have on it allow me to climb better and offer better bike handling capability. I also use this bike for short TT only events.

The Trek geometry is much more close to a typical road bike, and it was designed for Lance for just that very reason. Pro riders want a TT bike to NOT BE that much different than their daily road race machines. Also, there are different rules that cover races like the Tour de France type races vs. an Ironman competition. So the Trek with it's factory fork is a UCI legal-TT bike (such as seat position in relation to bottom bracket, how "aero" the fork is, etc..). But then again - it has also been used to win at Ironman with Tim Deboom riding it. The Trek is Carbon, has a softer more forgiving ride, and with it's fit is something that will leave me in better shape when it comes time to run a marathon. The bars (HED one piece carbon) are more "aero" and better for the long rollers at Kona.

Because I have specific race goals I own multiple bikes for a specific applications. But, if I had to choose only 1-bike to do it all - it would be a good road bike (with a typical road geometry) that I would also use for Tris as well. If in the future Tri's become a real passion where you race several events/year, and you are after qualifing for IM Hawaii or trying to place within your age group - THEN look for the Tri specific bike. You can have the same amount of "Fun and enjoyment" with a road bike in a Tri. I spend much more time on my 2-other "Road specific bikes" and my traininer than I ever do on my Tri-specific bikes.

Instead of thinking seat angles, thing about the rotation of the rider about the bottom bracket. You can ride in any rotation, from way back (think recumbent) to way forward.
A std road bike puts the rider rotation to distribute the weight between saddle and bars, with a hip flex angle that riders find comfortable.
Most tri riders are not natural born cyclists, they are runners or swimmers. Having more rotation enables them to achieve an aerodynamically flat back with less hip flex, and the extra weight at the front is supported by the aerobars.