Does anyone have any experience with the 9" carry-on size that Rick recommends and Lufthansa's requirement which states 40cm or 7.8"?
I do not know the precise sizes but I did experience one full flight to Germany on Lufthansa where the staff was asking people to check largish "carry-on" roller bags. It could also be that those folks had more than one bag as I was not paying that much attention. It was a flight in November. With heavy coats and multiple bags, the overheads were overflowing even after some folks were asked to check bags.
The specified max thickness is 20 cm or 7-7/8", 1-1/8" less than the MAX diminsion of the Rick Steves bag, and Lufthansa does have sizing frames at a lot of their gates. I've never seen them make somebody fit their bag in, but they could. . . . . ¶ But no one says that you have to fill your bag to the max it will hold. The convertible bag is pliable and, if not stuffed, will conform to the max thickness. I've tried my Rick Steve convertible in the frame several times, and it always fit. The rigid frame of the roll-aboard, on the other hand, probably won't fit. . . . ¶ A bigger concern for me is the weight (8 kg, 17.6#), and they have weighed my bag.
We were a little worried last November as our new lugagge slightly exceeded limits (annoyed because this was a line of luggage and they seem to ignore the airlines). Anyway, Lufthansa did not check the size and some folks clearly had oversized carry-on (another pet peeve). However that may be somewhat dependant on the crew at that airport (VYR)