Aero enough?
 

Since adjusting my bike fit, my thighs now bump into my lower ribcage when I get into a lower position, and my knees come up above my forearms. Is this sufficiently aero?

Congratulations on hitting your ribs instead of your gut. Or maybe not. Many pros have their gut lower. Those guys have real small chests.

You can never be aero enough if you can get more aero. You may end up going slower as you put out less power, or crashing as you can't handle the bike, but if there is more aero to get, then you don't have enough.

On the chance this is not a trolling thread you are likely too low and going slower. It depends a bit how fast you were going before. If you ride at 30mph, I take that back, but if you ride under 20mph, the loss in power is more significant than the aero - most likely.

Generally you should fit for max power first, then rotate seat/stem/bars to get most aero while maintaining power and comfort. Exceptions are ITTs and cases where riders are willing to give up power for aero as they are going so fast.

No trolling, honest! My power is still low, as I’ve only recently resumed conditioning/training. I’m hitting just over 20 mph on my speed drills.

Your comments about power should help me evaluate and fine-tune my position.

The unfortunate reality of aero positions is that the stronger you are, the easier it gets to hold an aero position. If you go for too aero of a position while not having the requisite power, you will get numb hands and tired arms.

BUT assuming that you get the requisite power to comfortably hold an aero position - try the following:

1) Move saddle up and forward VERY gradually. This is basically getting you closer to the TT position. More weight on the hands and pedals, less on the saddle. But it will allow for a flatter back. Revert your change if you experience hand discomfort, neck discomfort, or your hour power goes down too much (due to over fatiguing of quads)

2) get a saddle that allows for maximum rotation of the hips. Hopefully you already have this, and aren’t compensating for a lack of hip rotation with spinal flexion. At some point, most people who aren’t genetic freaks will need to resort to spinal flexion to get more aero but you should max out your hip rotation first. Stop rotating your hips once you start feeling impingement of the hip joint.

3) Get a shorter crank. Many studies have proven that the power loss from somewhat shorter cranks is negligible to nonexistent, but the comfort and aerodynamic gains are real. Find someone who you know can very easily hold an extremely aero position with a straight back. Chances are that, relative to their height, they will have short tibias or low kneecaps. Compensate for your longer tibias by going for a shorter crank. If you are 5’10” or under, you can almost certainly get away with 165mm cranks. Under 6’2”, 170. Something around that. Too short is better than too long. Realize that it will take quite a few miles to adapt to the shorter crank before it feels natural. Your RPMs will have to increase in order to maintain the same foot speed.

4) Get narrower bars. If you are riding in a pack or racing, this is a nice little benefit. You’ll reduce the amount of space you need in a pack, but also be marginally faster. Just remember that narrower or wider bars than “shoulder width” will make it harder to reach the bars because of how triangles work. So you may need a shorter stem - this will make it slightly harder to handle the bike, but it shouldn’t be an issue.

5) get taller eye protection. Your head accounts for a massive amount of drag. Craning your neck also hurts and reduces the sustainability of the aero position. Find glasses that fit close to your eyes and allow you to duck your head down all the way and look through your eyebrows without getting wind in your eyes. In combination with an aggressive position, this will make a noticeable difference.

6) work on your upper body/core strength. Having a perfect position is less useful if you can’t sustain it when fatigued or when putting down small amounts of power, like when in a pack.

I’m not an expert on aerodynamics, and I cannot say exactly how much this will save you, but I can guarantee you that it will save you more than the difference between a lightweight bike frame and the most aero bike frame.

This is a non-answer, but...

When your chin is next to the stem with a flat (or rounded) back, your forearms parallel to the ground, your knees brush the toptube and your elbows are just wider than the knees = you are probably close to best aero, assuming the proper skinsuit/helmet. but the 'grasshopper position' with head hiding behind uptilted forearms on tribars was a thing before UCI squashed it. And Grahman Obree's Superman is position is legendary.

....so (as always) = It Depends.

Most aero without 'most powerful' position may not be the Fastest Position.

I don't know your goals, but I assume speed. At ~20 mph power and comfort are likely going to make you go faster than aero. Aero matters, just not that much at that speed.
The balance should be more toward comfort and power rather than aero if speed is the goal.
There are still little things to get more aero.

As in the post above get forearms parallel to the ground and pointing into the wind. With a lot of power, that can be done with bars really low. But if your bars being low mean you have to have straight arms with your forearm length in the wind - your bars are too low (for your power at 20 mph). Don't copy pro position unless you have pro power. Some things are fine to copy, being low and flat is not one of them.

Clothing should be obvious. Watch you elbows sticking out and your arms crossing. You can roll your shoulders in a bit. Shaved legs (and arms) are more aero, but you can save that for higher speeds.