i'm signed up for an ironman in sept '06. currently, i have a fairly entry-level, older GT Strike road bike. i'm not terribly attached to it and am thinking about purchasing a tri bike. i'm wondering how different the feel will be and if training on the road bike will be nothing like the tri bike due to the seat post angle differences. everyone i've talked with about triathlons say that tri bikes make a significant difference, although fit is of primary importance. i don't think i'm fit very well at the moment either, so that's a moot point. any advice/comments would be appreciated.
You could do the ironman on either type of bike. Depends on a number of factors. I have a road bike, and a steeper - angled tri bike. The road bike, I find climbs better, and is more nimble when riding on a tight course or in a group. The tri bike for me feels more comfortable in aero position for longer periods of time, and is a little easier to run off of. It's awesome and fast on a long course that doesn't have a lot of tight turns. If your ironman has a very hilly course, you might consider a road geometry. If you like doing group rides a lot, and are going to use the bike for other than triathlons, you might consider a road geometry. Maybe try a couple of bikes out at a bike shop, and get a sense of what the difference is. In the end, you will probably get a lot of different opinions from people you talk to, but comfort and fit - especially on iron distance is most important.
I asked the same question last year when I signed up for my first Tri. (Carpinteria Olympic)
The course was curvy and hilly and I guessed that the benefits of a Tri-Bike would be diminished by the amount of time climbing hills and steering down tight steep 2 lane roads.
I brought my road bike (Look La-Vie-Claire) and it was perfect for the course. I averaged 21 MPH on my roadie.
I probably could have done about as well on a Tri bike, but I would have not been as comfortable and definitely more fatigued going into T2. (I felt great at T2)
Tri-Bikes are definitely the way to go if the course is flat and fast and not twisty and hilly. (I am bringing my roadbike to Wildflower this year for sure)
I agree with Argonaut66. I live in central Maine, where hills and curves are par for the course, and flat sections of road are rare. I train and race on a Giant road bike and wouldn't trade it for a tri bike unless I moved to a part of the country where the topography is considerably different than it is here!
AFTER DOING A TON OF RESERCH ABOUT AERODYNAMICS VS WEIGHT, AND HAVING RACED A TRI BIKE IN BOTH CALIFORNIA AND TEXAS, I WOULD HAVE TO SAY TRI BIKE... THIS BEING JUST MY OPINION. I DO ALL MY TRAINING AND RACING ON THE SAME BIKE, A '05 CERVELO DUAL. IT IS MY OPINION, BASED ON RESEARCH AND EXPERIENCE, THAT A TRI BIKE IS THE WAY TO GO. OF COURSE, IT COMES DOWN TO PERSONAL TASTE AND COMFORT. I HAVE RIDDEN A LOT OF HILLY, CURVY ROADS, AND I THINK IT IS ALL A MATTER OF FAMILIARITY WITH THE BIKE AND ROAD THAT MAKES THE DIFFERENCE. IF YOU ARE GOING TO DO AN IRON MAN ON ROAD BIKE, AT LEAST HAVE SOME CLIP ON AERO BARS FOR THE FLAT STRETCHES. IF YOU HAVE QUESTIONS ABOUT AERODYNAMICS, TRY DOING A GOOGLE SEARCH ABOUT IT. AS FOR CLUB RIDING, I HAVEN'T HAD ANY PROBLEMS RIDING IN GROUPS. AGAIN, I BELIEVE IT COMES DOWN TO FAMILIARITY WITH YOUR BIKE. THE MORE RACES YOU DO, YOU WILL ALSO SEE A LACK OF ROAD BIKES, AND A TENDENCY TOWARDS TRIBIKES. THERE COULD BE A REASON FOR IT...I LOVE MY TRI BIKE AND WOULDN'T TRADE IT FOR ANYTHING! GOOD LUCK!
Weight of the bike will become a more significant factor than aerodynamics if it is a hilly course, like say the St Croix half ironman. I have a cervelo p3 tri bike and a trek 5200 carbon road bike, and the trek would be better for the hilly half ironman. It is of course modified to move the seat position forward, and lower the forward body with angled seatpost and cowhorn/aero bars/ bar-end shifters.
You can get a lot of the tri bike feel and features on your road bike by using a profile design type fast forward angled seatpost which will put your body position forward 2 to 3 inches. A longer stem might be in order also to achieve the most aero and best hip angles possible. Making these changes to your road bike can give you a better idea of how you are going to feel on a tri or time trial bike.
Lastly, don't forget that it is the training to ride aero that is more important than equipment. Take the spacers out from under your stem and get the stem agle pointed downward as much as you can. Train yourself into holding aero as long as possible, which will be difficult and hard on your low back at first..
I signed up for the Coeu d'Alene Ironman this year. It will probably be my first and last Ironman distance. My wife is being very supportive but, unless a miracle happens and I qualify for Kona, one will have to do. I just had to try it. I have a Softride Tri bike and a good old road bike with a steel chromolly frame and add on aero bars. On an Olympic distance I cut about 5-7 minutes off my bike time. Average speed went from 20.78 to 22.93 mph. Some of that might have been training. The bike definitely made the biggest difference. My Softride is not the lightest at 21 lbs my steel bike is 26 lbs. My own size 6?2? and weight at 188, is the biggest factor for climbing. By the time Ironman comes around I should be at a trim 176. Haven?t been there since High School, WOW.
I like the road bike much better for handling and down hill, however I did get going 56 mph on the Softride. I probably could have hit 60 on the steel fram and felt comfortable. There was no way I was going to move from aero position to the breaks at that speed 56 mph. My Softride has 650?s, I would definitely suggest 700?s for bigger riders. But I got the Softride for a good price, actually a screaming deal, so I?ll ride it in a few more races before letting it go to a smaller rider.
If you are looking at new bikes to buy, now is a good time to pick up last year?s model at a discount. You can learn a lot about bike design and stuff on the internet and in Tri books. Just remember that everything they do to Tri bikes is to make them fast and aerodynamic. The wheels, the length, the weight the bars the angle, actually that has a lot to do with saving you for the run. I have done everything I can to make my road bike match my tri bike and I?m still a few inches longer.
Good luck in making your choice. Any new tri bike is going to be better than what you have. 
IM MOO 06 will be my first ironman-distance race as well, and I will be riding a tri bike there. Having looked at a lot of pictures, I would say that the vast majority of riders at previous Madison races have ridden tri bikes, but there were plenty of road geometry bikes as well; if you chose your road bike you would not be alone.
There are definitely advantages to both types of bikes on a course like this, so I say go with what you are most comfortable with. My tri bike is the only good bike I own so that's what I'll be on. Regardless of which way you go, IM WI is less than eight months away, so you need to make a decision soon. Pick a bike, get properly fitted, and stick with it.
Something else you might want to consider is the age of your bike. You mentioned that it was older...how old? Buying a new bike might not only get you a new frame, but also all new components. You don't want to go through all the training and sacrifice of preparing for your race, only to have something to crap out on you at mile 80. Having all new componentry is an insurance policy against a day-ending mechanical.
And, more than anything any particular bike can do for you, YOU need to be prepared for a long and grueling day on the bike. Practice hills, practice in the heat and wind, practice going long. People suffered on that course last year...over 200 of them didn't make the bike cutoff. Commit right now to not be one of them in September 2006.
An Ironman represents a huge investment of your money and your time. Do what it takes to get there and enjoy your day.
Best of luck! I'm off to go ride my trainer.
I can't believe I even still see this debate. If you look at the times from Kona (I know it's a little flatter there), the average person on a tri bike rode 10 minutes faster than road geo, and RAN 5 MINUTES FASTER! It's not just a bike race it's a triathlon.