Have you tried copying your true crotch shape? It is quite revealing and may even explain some of your pant fitting issues. There is an easy and inexpensive Aluminium Foil method which I also think of as the “tin foil crotch”. There are several methods actually and I’ve tried a few in the past. I wanted to repeat the exercise because I seem a bit befuddled as to how much length I need to cover the undercarriage i.e. the crotch extension length.
For this exercise, I like to use a flexible ruler
and I use small stretchy hair elastics to ‘mark the spot’. Spots (plural) in this case. I tied 1/4″ elastic around my waist. Then slid 5 elastics onto my ruler. (I had to wrap the elastic around the ruler twice to get them to stay in place.) I put one at the zero mark. This is the end that I put on my back at the 1/4″ elastic. I formed the ruler around my crotch up to my front waist and slid the last elastic up to that point. The other 3 elastics were moved to sit at my inseam and at the front and back points where my body curves underneath. I like to double-check everything. So standing in front of the full length mirror, I placed a 12″ ruler between my legs. Initially it was slanted just like I know that I am. My undercarriage is not level. It is tilted towards the back. However, I made the ruler level and moved the 2 elastics marking beginning and end of curve. It is with relief I can tell you, they didn’t move by much. I took the curve off my body. Had to do that a couple of times because it would reform. Finally, I measured how much I needed to bring the marker at the front waist forward so that the ruler could slide down and off without further distortion.
Before starting this exercise, I had cut a square of 18″ wide tissue paper and made an inverted cross (No I’m not a sat*n^st. That’s just the first word that popped in my mind) Now I placed the formed curve on the tissue with inseam on the upright and curve markers on the cross line.
Did I not warn you this would be revealing? Before we go further, I see inaccuracy. I have the curve marks placed about level, when logically the back mark should be lower than the front. The other thing I notice is that I have my front waist lower than my back. Absolutely wrong. Just as my undercarriage dips towards the back, the back waist lowers. But I know these things, so I think I can work with them. If it becomes too much of a visual distortion for me. I would cut a new tissue, make a new cross and trace “My Bowl” in a more accurate angle.
First thing that strikes me is how un-U-like I am. When I open a pants pattern and put front and back crotches together, I usually see a big U, sometimes a big and little Ll. In years passed, some patterns would have a V-shaped crotch. (The V never worked for anyone.) According to the visual above, I am very round. Like a fishbowl. I am relieved to see that the curve as I marked it is much longer than most patterns allow. I am relieved because I have added as much as 3″ to a pattern to give me enough length. However, I am not immediately operating on my fitted pants patterns. They fit. Why mess with a good thing? But I did wonder why they work. And I think “Bias” is the answer. To my knowledge at this point, a pants crotch is cut almost entirely on a bias if not true bias. Bias, no matter whether working with stretch or nonstretch fabrics is amazing in its ability to conform to the object (my body) beneath.
So another exploratory exercise that concludes with me not knowing where this knowledge is taking me, IOW what do I do next. But I am relieved to see the shape and the position of the markers. It confirms my actual sewing experience.