The Small and Medium Ortlieb saddlebags arrived, and I have settled on the Medium, which actually packs smaller than the Small, due to less interior crowding and therefore better vertical compression when rolled shut and secured. Bottom line: You really have to look at them in person or send for a couple to have any real idea of size and clearance. Despite warnings by some online sites, they both nicely fit the rails on my B.17, and they both cleared my thighs because they taper in width.
There's few accurate measurements for these bags 'cos they are multidimensional in shape, vary in capacity and compressibility depending on how much they are rolled, and they vary in both interior stiffener length and basic reach from mounting plate to seatpost -- and that reach is adjustable in 4 steps via a movable docking bracket. All sizes end in a half-round plastic cradle that mates with the seatpost and is secured with a velcro strap to stabilize the bag. Proper mounting means moving the bag forward on its bracket until this cradle contacts the seatpost. For my Sherpa with a Brooks B.17 saddle and a long-layback seatpost, the Small *just* fit with some stretching, and the Medium made it with no issues, thanks to a more rearward mounting plate and a longer "nose" to the saddle bag. All sizes have a little plastic guide to allow for a compression strap (to minimize rattles and vertical size) or for nylon webbing adapter straps to mount the bags to sprung saddles. Only the Medium and Large have an additional plastic guide to allow mounting an LED blinky taillight, though I can't imagine how the light would end up vertical.
The Ortlieb instructions say to roll the end shut 2-3 times to achieve secure closure and waterproofness. After squinting at the instructions and making several attempts, I concluded "rolling" equates to "flipping" and thus refers to half-turns (180 degrees) and not full ones (360).
It also means the effective minimum capacity is limited by the "roll" bottoming out at the edge of the interior bag stiffener (3 flips); maximum capacity is 2 flips, which means the bag is larger but less secure and less waterproof, so I'd recommend sizing a bag to allow three flips over the desired contents. This makes for a secure closure and the rear reflective patch is then vertical for maximum visibility. As a bonus, 3 flips/turns makes the "handle" formed by the rolled ends and buckles snuggle close to the bag and adds minimal overall length for a neater appearance and less interference with a rack-top load.
Stuffing the Small size to capacity, I got the following measured dimensions:
Minimum Size (= 3 flip-turns):
Vertically (looking down from on high): 75mm wide at front, tapering to a compartment width of 150mm at the rear, over a distance of 130mm front-to-rear. The rolled material is added to the rear, making an overall length (F-R) of 170mm.
Horizontally (in profile, or looking from the side): The length remains the same, but the height of the storage compartment tapers from 75mm tall at the front to 85mm at the rear, exclusive of the mounting plate and bracketry. Clearance for the mounted bag will vary depending on the angle and length of your saddle rails, and whether it is mounted on the horizontal portion or where the rails turn up.
Maximum size (= 2 flip-turns which I don't feel is nearly as secure):
Vertically: 75mm wide at the front, tapering to a compartment width of 150mm at the rear (10mm wider than when it is flipped 3x). The rolled material is added to the rear, making an overall length of 200mm, since the looser roll makes the bag ens into a handle of sorts that extends rearward instead of snugging against the bag as when rolled 3x.
Horizontally in profile: 75mm at the front to 95mm at the rear (10mm taller than when it is flipped 3x).
Ortlieb's dimensions as listed on the Small product tag are (with little hand-drawn arrows): 9cm high, 7/11cm F-R, and 12cm diagonally, which I guess is length overall, none of which matched my measurements. The Medium packs smaller and weighs nearly the same as the Small, has no crowding or security issues, and snuggles against the seatpost with no strain, so the choice for me was easy.
I won't measure the Medium 'cos I've got it happily packed with stuff and mounted. Pics show the Medium in place with the Sherpa bare and with a full touring load, which it just clears perfectly. Maybe this will help someone else trying to determine which size to buy.
Best,
Dan.
