Kind of about my performance in 10s l
between 290-310 watts on the same course with a week between them wind conditions changed some ignoring a couple of outliers like the day it rained
26 min roadbike 30mm rims
25 mins roadbike + aerobars
24.30 roadbike + aerobars +aerohelmet
24.10 roadbike + aerobars +aerohelmet+skinsuit
I can add the following experience to this thread.
Road bike: Boardman AiR with clip-on bars 22:59
TT bike: Specialized Shiv TT 22:46 (first race of the year and first on bike in bad conditions ā organiser said that competitors were 45-60s slower than their times last year)
Next week Iāll be racing the same course that I did my previous road bike PB on, variable weather aside itāll be interesting to see how much faster a TT bike is. If I can get in to the 21s Iāll be well happy.
Better get trainingā¦
Cool, report back to let us know how you get on 
Not strictly a side-by-side comparison, but to potentially give a good idea hereās a comparison of two similar efforts. Same wheels. Same kit (non-skinsuit). Road helmet. Only difference is TT frameset and tyres (gp4000s > S-works cotton). Ridden around a favourite of mine, Richmond Park in the sunshine.
2018 (Aero Road bike with TT bars): 3laps ā 52:12 (17:26, 17:22, 17:24)
2019 (TT bike): 3laps ā 49:48 (16:16, 17:18, 16:23) ā second lap thwarted by some traffic
A fairly astounding 1min+ per lap (6.7miles)
Far more than I expected, admittedly my fitness may be slightly higher this year but I wouldnāt say significantly so.
Do you know what your w/kg was this year and the previous?
This year, yes (309w ā 3.8w/kg), last year no. Only started riding w/power in November sadly. Not that it really is a direct comparison to power, my avg. heart rate was 166 this year and 158 last year. So potentially not as big an effort made.
Having ridden both set-ups, TT bike just gets you in a far more aggressive, faster position. Which rewards the effort you put in with speed.
This is an old thread, but what the hey.
When youāre talking tri and iron distance you need to be talking about the bike + run split. Your bike split alone is useless info.
Consequently, the decision over the position you race in, power and speed is about how well you will be able to run off the road bike with bars vs off the tri bike.
Time trials are a different story. Of course you can ride a given TT faster on a TT bike. Thatās not important.
There are many factors to consider in this decision:
How many long distance triathlons have you done and how many are likely in your future?
How good is your position on each bike, and how well can you run after 90km and 180km?
How long will it take you to cover the 180km with enough to run a marathon afterwards?
How long can you hold aero position on each bike and still run a marathon?
When it comes to specific races, and IM Wales being well known as a climbing course, there may be only a few minutes gained over a 10-16hr result. If that is in a position that is a struggle to hold, and/or cripples you for the run (which happened to me), itās a bad choice.
Well I have to completely disagree, bike leg in any tri is the most significant in terms of time elapsed, so itās far from useless to isolate. If anything, itās the most significant to isolate and optimise as much as possible, particularly over 112miles. If you can travel 4-5% (potentially up to 7%) quicker on a TT bike for a relative effort, it means that a) youāre spending the same energy to be faster or b) use less energy to go the same speed and therefore have more energy for the run. That increase at 20mph would jump to 21.25mph, which equates to 20mins of free time over a 112miles. Thatās a significant chunk of sliding time for the run.
Iāve never done an Ironman, but I imagine the bike to run transition is less important over longer distances, than say, Olympic distance where the run is shorter. I can say that I always used to struggle for about 2miles off the bike, but I never trained that transition much and the bike was always my best leg so put more effort in to capitalise which punished my legs. But surely common sense suggests the faster you can go on any given leg, the faster your time will be. Itās that simple. If I can ride ~1-2mins faster on a sprint bike leg due to bike choice Iād take it. Thatās the difference between running a 16 and 18 minute 5k (which requires a helluva lot of training to achieve)
If you canāt hold the position on a TT bike, you either have to train to do so, or reevaluate your position so you can hold it for 100+ miles.
Ultimately, look at Pro-ironman, how many athletes ride road bikesā¦ TT/tri-specific bike are faster.
Bike data is significant, but isolating it is precisely the mistake you need to avoid. 
Nobody performs their best by over-biking, which is what most cyclists do when they move to triathlon. Not such a problem at sprint level, but a sure fire method to ensure the run leg in an Ironman is a death march marathon.
There are no trophies for the best bike split! 
Only if, for that faster time in one sport, you can hold your times the same for the others. Comparing bike fit between triathlon and time trial for comparable bike course distances (sprint tri with 10-15mi TTs, Olympic vs 40k, Ironman and 100 mile, etc.), comfort is more important and run preparation a distinct requirement of bike fit. If you compromise your comfort to have a more aero position, it may save a couple of minutes on the bike, but cost you 10x that on the run if it leads to cramping or having to walk.
Iāve raced Wales with both clip ons & a TT bike, and will do it again this year on a TT.
A TT will always be faster as thereās enough flat road to take advantage of it (especially the small loop to angle) but itās not a massive difference. As others have stated, thereās other advantages to TT over speed/aero/efficeny.
As long as you have the right gearing - Iād go min 30 on the back its a manageable course. Just leave something in the tank for that run.
Letās just hope the wind stays away.