A couple of thoughts:
- Lower is not always better when it comes to TT positions from either a sustainable power standpoint or even an aerodynamics standpoint. It sounds like you're attempting to ride in a very low position if your knees are brushing your chest. A good rule of thumb is to drop your shoulders (acromium process or flat hard bony feeling part right above your shoulder joint on the outside of each shoulder) so that it's level with the point in your mid to lower back where your lats (the large triangular wing shaped muscles on either side) originate (IOW, the lower pointy end of your lats). Getting lower than this might make your chest seem really low, flat or even downsloping but it generally starts exposing your upper back to the wind so you start trading wind drag off the chest for wind drag off the back which doesn't generally buy you anything from an aerodynamics standpoint but costs you with discomfort, issues like you seem to having with your knees, and often loss of sustainable power from an overly tight hip angle.
- There are three basic contact point adjustments that raise your torso angle but also change other things so you've got to think big picture when making adjustments. Basically it comes down to:
- Aero bar pad height which you adjust either by changing stem angle or stem spacers or using pad risers to lift your pads up higher. It's usually best to think of this in terms of pad drop or the vertical distance between the top of the saddle and the top of the aero bar pads.
- Aero bar reach. This isn't as obvious, but the more you stretch out forward the lower you get for the same pad drop. When your upper arms are at nearly a right angle to your back (conventional aero fitting for skeletal support and long distance comfort) you'll be higher for a given pad drop than if you've got an open elbow angle (arms stretched forward from the shoulders in a semi or 'faux superman' position). So if you're stretched pretty long and your clip on bars allow you to slide the pads back a bit to shorten the cockpit it will also bring your shoulders up which increases your torso angle and could resolve the problem with knees brushing your chest.
- Seat fore aft position and height. Seat changes should start with a focus on proper seat height for good max and min knee angles while pedaling in the aero position and secondly for obtaining a large enough hip angle (typically 90-96 degrees for aggressive aero positions) but the fore aft position of the saddle also impacts your overall reach and with it how open your elbow angle is and that impacts your torso angle just like moving your pads fore or aft. Remember that your effective seat height decreases as you slide your saddle forward and increases as you slide it back so seat angle changes require seat height changes which changes pad drop so this stuff is pretty interconnected and changes should be thought of as part of the whole system.
Check out some of the info on this site regarding TT fitting: http://bikedynamics.co.uk/FitGuideTT.htm
It can also be useful to look at TT photos and see how low serious riders are or are not going on their TT bikes.
FWIW, I spent several seasons and a couple of fittings from experienced TT fitters working on getting as low as I could. I did a lot of training in those positions and got my power pretty close to what I can manage in my road position but raced an awful lot of mediocre time trials where my power was good but my times were middle of the field at best. I could ride with up to 20 cm of pad drop (I'm 5'10" tall) pretty comfortably, sustain decent power in that position and could see reasonably well up the road while holding a deep turtled head tuck but my times were still pretty dismal. This season I've gone the other way entirely. I'm riding a mere 8cm of pad drop but stretched out right up to the UCI morph limit (80cm between BB center and aero bar ends) with my seat all the way back to match my road bike angles. My power is the same, it feels way too comfortable riding so high but my times are much better this year with a win and a couple of podium places in TTs this season so far.
So lower is not necessarily better. It might be for you with your good lower back flexibility but I'd get some fitting photos and start by getting your shoulders even with the origin of your lats and see what that does for your knee-chest issues and see how that works for you on the road. A power meter makes aero field testing easier but even without one you can do roll down testing to compare positions and you can do TT training in various positions to see what actually helps you go faster.
BTW, don't ignore the value of a deeply 'turtled' head tuck. Regardless of the rest of your position this can really buy you speed while time trialing. A lot of otherwise fast looking positions are ridden by folks that hold their heads way up in the air which isn't helping them.
Here's side shots that I posted a while back comparing the higher position I'm riding now (top) to the lower position I was previously riding. To my eyes that low position in the bottom photo looks a lot faster but field testing and race results tell me otherwise.
[IMG]cyclingforums.com500Open ↗[/IMG]
-Dave