I would be slightly concerned about the extra leverage this would allow to be put on the seat and therefore the extra strain it would put on the seat tube. It would be interesting to know if Thorn would 'warranty" their frames using this product.
That's an interesting thought, Andy, and one that crossed my mind as well.
*If* one used the adapter to place a short-rail saddle (say, a Brooks B.17) so the clamp hit the midpoint of the rails -- say, 1cm or so further back than the original post -- then the actual layback might be no more than it would be on a Thorn/Zoom large-layback seatpost (
http://www.sjscycles.co.uk/thorn-micro-adjust-alloy-seat-post-with-large-lay-back-400mm-x-272mm-prod23725/ ); the difference is that one has a forged head integral with the post, rather than lagging it off the original head. I sure wouldn't want to hang the saddle waaaaaay back on the rails on one of these. I have seen a variation that was done a bit differently so it sat in the seatpost's cradle; the original clamp assembly was then moved rearward on the adapter. I like that one better.
If you're going to go the long-layback route for a custom purpose as I did on my Folder's telescoping midpost (to accommodate a very tight fold), then it is best to do it from scratch, milling the aluminum 'post stock as needed and brazing the stressed bits from cro-moly. I milled the alu 'post after fitting it with an internal reinforcing slug (this is a prototype in the photo, hence the tool marks), then fitted it with a fillet-brazed chro-moly extension and a milled-from-forging alu clamp with interlocking connection; the end holds an LED blinky between the saddle rails. I'm fussy about my joints, so everything is mitered light-tight with full contact, then I flame-chase my fillets with minimum heat as I lay them rather than grinding or sanding them to profile afterwards. The fillets you see here have just had the flux soaked off and a quick dash with some 000 steel wool to check for any voids. Clean as a whistle. It's not too different from my adjustable, double-locking tandem stoker stems, also designed and built to handle a lot of stress.
I can get away with such a long-layback in this case 'cos the Folder's seat tube angle is pretty steep and the net layback is only about 5mm more than the Thorn/Zoom post's. The net stresses on the 35mm seat lug are nearly identical to what they would be with conventional frame geometry.
Still, to mock up and size for a new frame using an old one, this VK adapter looks like it might be fun to play with.
Best,
Dan. (...who thinks the world looks very pretty through brazing goggles and mask)
