What aspects of a tt frame are really important and what is just for looks? Also how important are they?
rear wheel cutout
aero tubes not just the downtube
rear wheel close to the frame. and how close should it be
your going to get this a lot but fit is by far the biggest, meaning the proper position (achievable on most frames), on a bike which allows it to handle well in that position (not achieveable on a lot of frames).
After fit comes aero tubes, cables inside the frame, light weight.
Styrrell
I’ll second what smtyrell99 says.
This aero data from the Cervelo site is interesting.
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fit
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aero downtube
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aero seatstays
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narrow headtube
I was refering to more of an aero look. I am not looking for a new frame just tring to learn. If you look at some high end bike they have pretty differnet designs. If you tak ethe trek equinox 11, p3, the the felt b2. The b2 doenst even have a rear wheel cutout. the p3 has a huge cutout and the trek has huge aero tubes everywhere. Aero wise is one better than the other?
What’s important and what’s for looks?? This might be an interesting thread…
IMHO
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Some type of cut-out for the rear wheel. It doesn’t have to be huge, just something for the rear wheel to hide behind. How close should the rear wheel be?? I think the closer, the better. However, I’ve heard some ‘theories’ that say there is such a thing as tooooo close. Not sure if there is any research behind this or just a way to justify standard drop-outs (cheaper and easier) vs. adjustable.
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Aero down-tube and seat-stays… top-tube and chain-stays don’t matter.
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Forward seat-tube angle, 75 degrees or better.
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Internal cable routing is important to some degree… although I also think it’s partly cosmetic. A ‘clean’ look is desirable for most people (myself included).
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Weight should be something to consider, but most TT/Tri frames are not light compared to other frames.
In the end, I think bike fit and positioning are the most important things. I see far too many people on really expensive equipment with horrible positions.
JB
The gearheads on this forum will likely miss out on this small detail, but the most important aspect of a TT bike is the engine riding on top. Aside from that, once you pass the $1000 price point, any top pro could win Kona on any of the bikes with the right tweaking of frame size, crank length, cleat position, seat position, stem and handlebar position.
If you guys recall, Tony Rominger set the World Hour record on a double diamond round tube Colnago. Greg Lemond won the 1989 TdF final Versailles to Paris TT on a double diamond round tube bike averaging something outrageous like 53 kph. Its the engine and the position of the engine that is the most important, especially for all the mere mortal posers on this forum.
A bike that matches your dimensions is key. Cables routed internally is good, rear wheel cut out is still being debated, I say round steel tubes could still make you win if you have the motor. Aero top tube is unnecessary and actually looks quite silly.
I would still worry more about wheels, then possibly fork (after the most obvious thing: your position) before the profiles on the frame tubes.
…Bunnyman, I think you got the ordered mixed up. First worry about your position. Then worrry about the other items (You got the order of those correct though) 
I am exhausted. I just hope my blog is a bit more on it.
Please read my blog and comment.
and how it mates to the fork design as well.
Hows your TTing going at the moment gary ?
Its the engine and the position of the engine that is the most important, especially for all the mere mortal posers on this forum.
true but where’s the fun in that 
also, I’m on the downslope - not going to get any faster/stronger between here and eternity, so any time I can buy some extra speed, I’m interested…
Fair enough, but the main point is that there a “free speed” if you can come up with an aero position that you can hold in for the duration of your race in real world circumstances (cross winds, false flats, bumpy roads etc), then you can be faster without spending any cash. Once you are there, go nuts spending on gadgets. Its all fine tuning. Many people doing see the forest from the trees, riding like a sail and then spending cash on “more aero” carbon cranks (you get the picture)
All excellent points here, but I can’t believe that no-one’s mentioned stiffness/power transfer yet! I could beat Lance in a TT if he were given a soft enough frame (you know - like made of jello or something ).
I’ll take stiffness over aero tubes, but fit above all.
"Hows your TTing going at the moment gary ? "
I am slow.
Gary- I don’t want to hear you crying about being slow! I keep seeing your results on the local TT’s and they’re not slow. How did you feel on last weekends Santiago Cyn TT? I hadn’t done that race in 2 years and forgot how hard it is. I think I burnt a little too much on the hills. I finished in 28:51 and noticed that you had the same time. What is your next race?
dp wrote: there a “free speed” if you can come up with an aero position that you can hold in for the duration of your race in real world circumstances (cross winds, false flats, bumpy roads etc),
This is one of my reasons for going to a suspension bike. Not only can I stay in my best aero position over bumps, I have a stronger position because I can rotate my pelvis more forward without pain. My bike frame is plenty stiff, although I can bob up and down if I just pound the pedals…so, in one way, it’s not considered stiff. It has taught me that my old-time habit of pushing against the seat wasn’t as productive as it felt.
To answer the question, I agree with everyone that says fit as being most important (assuming you are dealing with a certain given power source).
Next TT is a little hillclimb in Poway (www.socalttseries.com) just for fun, then some TT’s in Apr/May before states on May 28th in Palmdale.
And yes, I am slow. Slower than I was last year