Step thru and mixte frames for men

Have a friend who has had several spinal surgeries and a very inflexible back and has a hard time getting a leg over the saddle to mount up. I found an old Raleigh step thru chromoly frame and made a Frankenbike of spare parts and wheels for him to try. I rode it a few times in test drive mode and it felt reasonably firm flex wise. I put him on it and he can actually get on and off way better than with a diamond frame. He is a pretty big fella (up over 2 bills) I would guess.
He is hoping to ride it a lot with some up/down mountain passes and gravel grinding on forest service roads. He needs a way better ride than the Frankenbike he is on now.

Any thoughts from the builders on the way to go i.e. mixte vs step thru vs step thru with extra stays for this sort of rider? Have heard all the stories how whippy and flexible the design is unless you build it like a tank, but not too sure how true they are. I have ideas on a lot of bike designs but know about zero on step thru frames.
Thanks in advance

Moke, I have built mixte frames for men, not a lot of them, but I learned a few things. They are stiffer than a step-through, which is a good thing. I prefer the twin diagonal tube design, with a reinforcing sleeve where the twin tubes anchor to the seat tube. If your rider is any kind of an animal, use straight gauge .9mm (.035") for the seat tube.

I like 5/8" for the diagonal tubes, .028" or .035" depending on the rider.

Good luck!

jn

"Thursday"