Originally Posted by
Kuromori
Sliders/rockers are mostly for the ability to have single speed tensioning ability with vertical dropouts or discs, or to minimize wheelbase. Some people use it to minimize the wheelbase when they swap wheels and tires and tuck the wheel in as far as possible. Fewer use it to "tune" the ride somewhere in the middle and think I really wish the chainstay were 3mm shorter. The ability to "lengthen" the chainstay to get the axle under pannier loads is only in reference to the shorter wheelbase you would theoretically be running with the panniers off. It only really matters if getting a 17mm shorter wheelbase with panniers off is worth the hassle of adjusting the dropouts, the high cost of the dropouts, and the weight penalty of the dropouts.
Longer chainstays can do everything shorter chainstays can except maintain that tight wheelbase, so the real question is how important are short chainstays with panniers off to you, not if you want to sometimes lengthen the chainstays for panniers. I see the chainstay on this design is 405mm, and I don't imagine you want to use up half the rocker to run a 397 chainstay. Even if we add the full 17mm to 405mm, 422mm still isn't a great length for panniers. Of course you can pack smaller panniers to keep the weight forward, or join the lowrider front rack cult, and not have to worry about heel clearance or wheelbase at all.
I concede that if you're playing with bent seat tube chainstay lengths, then you might actually fine tune the chainstay length to be as short as tolerable instead of as short as possible. Some people might actually find a +17mm toggle in chainstay length useful as it's not entirely unnoticeable, but this would basically be people that actually bother to move the wheel forward and backwards in horizontal dropouts to tune the ride. Not very many people. 17mm won't get you the range between a tight sub 410mm chainstay, and a 440mm plus pannier happy chainstay.
This is such great insight - thanks, Kuromori. Also, thanks for explaining how these dropouts impact variable wheelbase. It's now clear to me that these dropouts may be useful to adjust the wheelbase length, particularly to allow for larger or smaller tires on the bike. Tire swap could be great for those riding (for example) the divide tour or similar (anyone agree here?). I see the pannier eyelets are actually fixed to the seatstay on this dropout, and any modification to the axle position within the dropout wouldn't have much of an impact on pannier position relative to the rider (thinking of heal to pannier collision here). In saying that, I would like the ability to run a 30mm or perhaps a 32mm wide tire on this bike. If the space isn't available between the tire and the chainstay or the seat tube - this dropout design could be useful. However, I think there're other ways to address this problem without the need for a rocker dropout (perhaps a pinch in the seatstay or a forward offset on the seattube). Feel free to comment on any of this; all this is pretty new to me.