I got inspired to make an overall by several things. One is my strong interest in space suits: most of the early ones that were used in Gemini, Mercury and Apollo programs were mostly soft, sewn garments with one main zipper opening. That fascinates me, a sewn garment, something I could potentially make with my hands, can protect a human being from some of the most some extreme conditions in the universe.
The second reason to make an overall was that it gives more freedom of movement. Its lower part does not have to rest on the waist/hip area, I see it as an advantage over a conventional pants: the waist/hip area of the body does not keep its shape and dimensions constant when body moves (think of a pants which fit fine when you stand, but cut in the waist and expose your lower back when you simply sit down), hence by removing the need of the lower part of the coverall from resting on the waist the comfort is improved.
One thing that most of the people would see as a disadvantage of an overall is a need to take off whole upper part, when visiting bathroom, and that is one of the issues that I tried to tackle.
I installed a zipper on the rear lower part of the coverall that allows to use bathroom, it has certain limitations, but in general it works really fine.
Big issue of this zipper placement is its curved shape, one side of zipper tape have to be stretched and another compressed to lie flat with the garment, it is impossible to stretch zipper tape, so the result is less than perfect. I wonder why there is no (as far as I know) zipper where zipper tape is flexible, I can conceive of a zipper where only very narrow strip next to the coil itself would have a regular weave and the rest of the tape would be woven on bias, giving a great ability to lay zipper flat, yet curved.
I faced this issue with zippers related to other experimental feature of this piece: ventilation openings. There is two armpit vents, one on each side, and an asymmetric one on the back. Overall traps air inside and it's hotter than a separate pant+top combination, since the only openings allowing convection are the cuffs and neck. Besides, I was thinking about vents in rain coats and jackets for a long time, analyzing best position and shape and here I made a first trial of some ideas. For example, let's take an armpit vent, most of the jackets will have it in the side seam of bodice with some part in the seam of the sleeve. When it is located that way there is no forces that pull the edges of the opening apart, so even when the zipper is open I think it does not provide much of ventilation, surely it lets hot air to escape, but it does not make a use of any wind that could improve convection. My first idea was to put the vent on the front of the arm, so that wind can enter, second idea was to make it curved, like an upward looking crest, so that when open it does not create just a straight line opening, but instead an 'eye' shaped opening, allowing more air circulation.
And indeed the ventilation is great through such an opening. However curvature of a zipper is an issue, as well as chafing of the open vent on the bare arm, when wearing a backpack.
Back vent took its shape because of arm reach restrains. I approximated position and angle that would be more or less comfortable for a human hand to reach behind the back and open/close the zipper. That shape worked for me, however once again zipper curvature posed a problem.
The most successful vent, and the simplest one, is the one on the chest: it runs along main zipper and has no curvature, so works just fine.
Main zipper is positioned diagonally because I wanted to reduce bunching up of the zipper when seated, for example, zipper that runs vertically in the center of the bodice is subject to more compression than a diagonal one, since it runs along the plane of the body that bends forward. So zipper installed diagonally to that plain is not bending over itself.
Last part that I experimented with was a gusset. Since I am a woman and I ride bicycle I don't want to have any seams between me and a saddle. I am pretty happy with a gusset I made on this coverall.
By making one piece with a lot of experimental solutions I was able to learn a lot, so next one would be improved in all areas.