Already answered in your question in the Road section. http://www.bikeforums.net/showthread...-aero-seatpost

But may help others here more, so copying into this topic. First section is replying to another posters suggestion how to carry enough drink for 5 1/2 hours...

You don't need to carry enough liquid for the whole bike leg!

There will be numerous aid stations, so usually you first lob the empty bidon at a signed spot before the aid station. Then approaching the the aid station call out what you want (water or event sponsors sport drink) and grab the a full bidon from one of the vounteers. You do this still on the move - just slow down a little to make the grab easier...

In regards to the sponsors sports drink, trial it if you plan on taking this to make sure that your stomach can handle it. If not you need to work out how to carry your own preferred brand. Also there is usually a special needs bag option somewhere around mid ride where you leave with the organisers the day prior to the race and can pick up other nutrition/drinks...

Back to bottle location. An old study of bottles on a semi aero frame - http://www.slowtwitch.com/mainheadin...erbottles.html

Behind the seat - The Profile Design holder linked will NOT work on your bike as it attaches to a round seat post. XLAB amongst other companies make single and double bidon holders that attach to the saddle rails. If your seat isn't too far forward this may work.

Pros - Takes regular bidons so can swap at aid stations. Can use one for liquid and the other cage for spares (I use to use a bidon with cut off top to hold a spare tubular and CO2 canisters and CO2 pump)
Cons - unless the bottles are angled back quite a way - not that aero. Takes practice to reach behind and replace bottles. Out of sight, often out of mind so not drinking enough.

Downtube - standard cage

Pros - Takes regular bidons so can swap at aid stations.
Cons - not that aero. Need to move out of the aero position to reach down for a drink.

Downtube - aero bottle and cage

Pros - more aero especially if fitted well.
Cons - Some are prone to release bottles over bumps. Cannot refill on the fly or swap out at aid stations. Need to move out of the aero position to reach down for a drink.

Downtube - Speedfill. Haven't looked into these as weren't available when I did IM's, the last being nearly a decade ago!

Pros - reasonably aero. Can be refilled on the fly with a filling port. Straw in front so don't need to move out of the aero position to drink.
Cons - can't be removed from the bike whilst riding. Depending how serious you are- being able to pour a spare bidon of water on your crotch after peeing on the go is a really good thing.

Handlebars - Jetstream / Torhan's

Pros - Can be refilled on the fly. Straw in your face so drink more often. Don't need to move off the aerobars to drink.
Cons - less aero with the bottle and straw. Cannot be removed whilst on the go. For the same reason as the Speedfill

Handlebars - horizontal bottle cage

Pros - quite aero. Takes regular bidons so can swap at aid stations.
Cons - Need to move out of the aero position to drink.

I ended up using a combination of the options above. Jetstream on the front with my preferred sports drink. XLAB with one regular bidon for water and second cage for spares.