Well strange agent
, I noticed something unusual about the dimensions of my mid-80s Peugeot P-18c mixte bike. It is the only Mixte or drop tube bike I have ever owned, so I selected the size as I might if choosing a Men's road bike.
My seat tube measures 56.5 cm (center to top). The top of the head tube is considerably higher then the top of the seat tube. From the floor to the top of the head tube measures 86 cm. The seat tube from the floor to the top measures 81 cm (yes, 2 inches lower).
If I were to superimpose an imaginary level horizontal top tube, from the center of the head tube to the center of the raised seatpost, my effective top tube length would measure 60.5 cm! This would be a VERY long top tube for a bike with a 56.5 seat tube (that's measured center to top, so figure 55 cm center-to-"effective"-center [if there were a top tube]).
The bike was all original and came with CTA [French] alloy drop bars just as shown in a 1985 catalogue for this model. The effect this geometry has when setting up the bike for my own use, and with saddle and stem top essentially level (as I might do if it were a Mens road bike) is very unusual. The length from the saddle is dramatically stretched out. It was so extreme (and uncomfortable) that I quickly replaced the original bars with narrow moustache bars - which helped me out considerably.
I believe that Mixte framesets in general were really designed for relatively flat or even back-turned bars (such as French "Porteur" bars or raised "Promenade" bars). The idea of offering drop bars on such a bike seems odd. And, I would not be surprised if this was simply done in later years to appeal to an active young woman seeking a more "sporty" bike... but still not wanting the high top tube. However, in this case at least, Peugeot had made no alterations to the frameset to compensate for the additional reach of drop bars.
On all examples of Mixte bikes I've seen, the seat tubes are placed considerably lower than the head tube. I assume this was always with the intention of riding such bikes with bars placed considerably higher than the saddle. This would make the obvious saddle choice something with springs and intended for a more upright rider position.
So, if there is any "logic" to the bikes, my recommendation would be to consider my observations:
If planning to use drop bars, and a typical racing style saddle (either vintage leather or anything modern) anticipate a longer reach than the actual seat tube on such a bike might imply. Because there is no horizontal top tube to measure, it would be difficult to ask a seller to measure the actual length without a lot of confusing detailed explanations. So, perhaps just view it as you might a compact frameset, and maybe just buy a smaller size than you would with a traditional conventional road bike.
Or, if intending to ride it with flat or "touring" bars, just buy it based on the seat tube length, and plan to ride it with a more comfortable cushioned saddle and in an upright position. Hope this makes sense...
Here is a photo of my own bike as currently configured. With the moustache bars I still feel fairly stretched out (notice the forward placement of the saddle)- but more as if I were simply riding on the tops of my brake hoods on a typical road bike, and I can always back off a bit by grabbing the sides of the bars. However, because I really ride this bike only in the city, I'm honestly considering changing the handlebars (again) and adding a sprung saddle - perhaps something like a Brooks Flyer?
I believe it is time to review some of the many examples shown in the extensive Mixte thread in this Forum. Yes, it is always interesting fussing with unfamiliar bike styles...