Good general rules of thumb- Tribars, skinsuit and aero helmet are all good investments.
- For the helmet, get a proper one, not one of these 'aero road helmets' - they're no better than a backwards cycling cap and for the likelihood you'll actually fall off in a TT, you might as well go bareheaded. The Bell Javelin and Giro Selector can be picked up pretty cheap these days, but are good solid all-round bets.
- Set the tribars up so your hands are basically touching in the middle.
-If you're worried about the length of the headtube, get tribars that place the bars under or in front of the handlebars rather than on top, so the arm pads can lie directly on the bars.
These Ritchey ones would do the job, as would some similar ones made by 3T or the low mount ones made by Zipp.
- To get your bars even lower, fit a steeper stem. Planet X tend too be a good source of these.
- Ride around in your TT position a lot to adapt yourself to it. If you set up a super-aggressive position straight away and race in nit without practice, you'll be down on power.
- Don't worry about your laid back position. Massively forward tri positions have gone out of favour in the TT world: the thinking these days is that getting your saddle low is really important aerodynamically, and pushing it back will allow you to do that without losing too much power - you'll have to adapt to the tighter hip angle but to an extent that's trainable. Also, excessively high and forward saddles are bad for bike handling.
TestingSince you've got a power meter, a
Chung test is the best way to estimate your drag coefficient. Basically you ride round in circles, the algorithm shunts all the unknown variables into 'virtual elevation' and then you alter your CdA until VE = Zero. There's free software available that allows you to do this: see
Golden Cheetah.
Ideally you want somewhere with zero wind and traffic, and a constant surface so your rolling resistance stays the same - i.e. a velodrome. Outside, it can be quite hard to control those so you won't be able to fine tune, only to get a rough idea, but the circular nature of the course means that you can gauge the accuracy of the test by how well all the VE values line up at the end of each lap.
The quick way to do this is to pay someone to do this for you in a velodrome, but if you don't have a TT bike yet, don't spend £500 on doing that, put it towards any of the things above which are all low hanging fruit.
[SD beat me to it on the Chung testing link]