Personally, I'd build up one bike that fits well for all events between 5K and 40+K. You're not going to want to sacrifice any additional power for those very short time trials attempting to go too low and tight and you don't really want to sacrifice a ton of aerodynamics in a 40K TT seeking long distance comfort and realistically 40K TTs are not all that long. Sure if you were positioning for a 5+ hour Ironman leg and then running off the bike some comfort tradeoff might be in order but it doesn't sound like that's what you're talking about here.
If anything I'd set up one bike for training and keep one set up with pristine components for race day but keep the positions pretty much identical so you can just jump on and ride fully adapted to that position on race day. That's another reason not to set up multiple positions, you'll adapt and produce your best power and develop your best head tuck in the position you train in the most so don't train in a high position and then race low or vice versa.
You could also use the two bikes to experiment with a traditional low and tight position or a long and high 'faux superman' or 'deep diver' position to see which is faster or which is faster in differing wind conditions but that's a pretty decadent use of two really nice bikes.
If you get your position for 40K really dialed and you spend time riding in that position it will very likely be your best position for short time trials as well. You might be able to push a tad lower for short events but if you pay too big a power penalty trying to deal with a sub-optimal position from a functional standpoint then it won't likely be faster in the short events anyway.
In terms of crank length, if you're challenged by minimum hip angle, that is the included angle between your torso and femur at the very top of the pedal stroke then shorter cranks can help a lot. That's a limit for a lot of folks in an aggressive TT position but not everyone. If your overall position already keeps that angle fairly open (as in 45 degrees or more for folks without a ton of flexibility or at least 40 degrees for the flexible folks) then going shorter in the cranks isn't likely to buy you much if anything. That's definitely one of my limiters and after trying a lot of other approaches I've gone to 165mm cranks at 5'10" tall and my power and comfort are much better even though I run longer cranks on my other bikes.
If you currently have a road bike fit that feels aggressive but very powerful and functional then I'd highly recommend working into your position from your best road position as follows:
- Set up your road bike on a trainer and shoot good side shot video of you hammering at race power in your best hammering road bike position. IOW, your go-to position for closing a gap or driving a chase. Maybe that's in the drops or maybe up on the brake hoods with forearms relatively flat but ride the bike hard in your favorite go-hard position.
- Run the video through the free Kinovea motion analysis software and pull the key angles which are:
- Max knee angle at bottom of the pedal stroke (ideally in the range of 140 to 150 degrees)
- Min knee angle at the top of the pedal stroke (ideally more than 70 degrees)
- Max hip angle from shoulder to greater trochanter to pedal spindle at bottom of stroke (ideally 90-100 degrees)
- Min hip angle from shoulder to greater trochanter to center of knee joint at top of stroke (ideally 40 to 50 degrees)
- Work to set up your TT bike to preserve these same key angles but to do so by rotating the entire position forward around the bottom bracket. You'll do that by sliding your seat forward and raising it to hold that same max knee angle and then dropping the front end. Try not to close up that max hip angle or min hip angle more than necessary as that's where you'll lose power. If you're racing under strict UCI rules you'll likely have to keep your saddle nose at least 5 cm behind the BB center and take the morph exception for 80mm of total bar reach to the shift axles of your aero extensions at your height. If you're not running under strict UCI rules you may be able to push the saddle a bit further forward but only do so as much as necessary in order to rotate that functional road position around trying to flatten out the torso angle and lower the head till it's in line with the back but not higher than the back if possible.
Good visuals of the key angles here: http://bikedynamics.co.uk/guidelines.htm
and here for TT positions: http://bikedynamics.co.uk/FitGuideTT.htm
and a good discussion of how shorter cranks may help in a TT position here: http://bikedynamics.co.uk/FitGuidecranks.htm
Free motion analysis software for measuring angles from videos (don't pose in still shots, your position will change) here: http://www.kinovea.org/
If you really get your position right in terms of a good balance between functional power and aerodynamics I doubt you'll really be able to set up a second much more aggressive position for the short races that doesn't cost you too much in terms of power.
Good luck,
-Dave
