Martin, Ian,
Many thanks to you both for your thoughts.
I figured while I was designing a mount, it would be worthwhile to make it universal to fit my other bikes as well as a variety of bags in case I acquire another in the future. Martin's right about different bags fitting the same mount despite variations in size, and all seem to have identical spacing for the leather strap holes. Similarly, Brooks saddle loop placement is close-enough between saddle models to be considered "standard" within a certain margin.
I have some interesting challenges, because the bikes I will be fitting won't directly accept ready-made solutions.
For example, my Nomad uses a Thudbuster LT (Long Travel) sus-post that declines downward and rearward through its travel. My two randonneur bikes use Thudbuster ST (Short Travel) sus-posts that do the same, but to a much lesser degree. Two of these three bikes sport my adapter mount for Ortlieb underseat bags (see:
http://thorncyclesforum.co.uk/index.php?topic=4523.msg86243#msg86243 ). The third bike has a strap-on underseat bag (for a multi-tool, patch kits, and spare tube) and just has the Brooks saddle loops available. My gravel-grinder/dirt road rando bike has a rigid seatpost and Brooks B.17 with just enough room for the Ortlieb bag mount, which I'd like to leave in place as I will use it more frequently on that bike compared to the Carradice, which will be used for on-the-fly camping and tours made with its limited capacity in mind (limited capacity compared to four panniers).
So.
What I'd like to do is employ a variation on Doneral's excellent earlier suggestion and make a sort of bimini clip to engage the saddle rails, but do so in a way that avoids some of the problems of commercial bimini clips (used in boating to secure canvas tops over bridges and cockpits, etc). Those come in limited sizes and variants are only available in aluminum, which can wear pretty quickly. I'd like stainless.
My first thought was to produce them from stainless sheet, but the bikes have different saddle-to-rack distances (I will use a rack to support the underside of my Carradice Camper Longflap) and this, coupled with different bag dimensions, means I would need to produce clips in a variety of lengths/drops.
I am probably stubborn in trying to accommodate a default mounting of the Ortlieb underseat bags (I have two Mediums and one Large), but they are truly waterproof so my tools don't get soaked/rusty, mount solidly, and get my most needed tools out of the panniers so I don't have to dig for them and they're readily available when I need them so I can be quickly on my way again if minor repairs or adjustments are needed. If I use the Carradice on one of my bikes, then I'll remove the Ortlieb and transfer the contents to one of the Carradice's pockets, probably in a small drysack so the package is still waterproof and the items remain all together.
Unfortunately, the Bagman needs about 5/8in of parallel saddle rails to attach. My Thudbuster-mounted saddle rails don't have that much room available, so that's why I needed to make the brackets so the Ortliebs could attach to the wider flared section of Brooks rails. On the other bikes, the Ortileb mount is in the same place the Bagman would need to go and I don't want to get out tools to make a swap for something that should be a fast operaton. I considered replacing the Bagman bolt with a q/r saddle bolt so one Bagman could be moved toollessly from bike to bike, but there is still the space issue.
Ian's ideas for the Bagman mount and mating plastic bracket are good ones, and the q/r feature is what I'm after. I'll give it some more thought. What I want to do is get the bag on and off each bike quickly if I need to go into a store, or take it into the tent to un/pack if it is raining. The difficulty is getting a mount that will work on any of these four bikes with the restrictions I'm up against.
It would not be difficult for me to make my own version of the Bagman and the plastic mount from aluminum billet, but clearances are the big issue. Of course, it would be faster and easier to buy a ready-made solution and I would prefer that if I can find the right combination.
Thanks for the suggestions; most welcome!
All the best,
Dan.
