I started to admire my new rear derailleur …… good news …… it is an eTap wireless …. Very cool …… even comes with ceramic bearings in the derailleur cage …. Cool …… BUT
(stock photo)
I was looking at the basic overall Rube Goldberg design of bicycle rear derailleurs …. They suck …… Large aero profile and exposed to damage.
Rear bicycle derailleurs look like something the alien Klingons foisted upon us … as punishment …
How about a rear derailleur that is mounted on the inside of the rear seat stay and works like the front chain ring derailleur (no tension cage) with tension pulley(s) mid chain attached to the chain stay?
Way more aero …… simpler ……and location protected from damage …… and … would obsolete all existing rear derailleurs …
The RD serves two functions: moving the chain from cog to cog, and taking up the chain slack. It’s the latter that leads to a bulky un-aero design.
Yeah - it’s hard to come up with a design for something that can take up the range of slack dictated by the extremes in cog ratio that ends up being more elegant than the existing design.
But once you’re given an existing chain tensioner design, the parallelogram addition to move the chain from cog to cog starts to look ingenious.
Edit: I welcome someone throwing out the existing design and trying to come up with something better, though…just don’t think it’s going to be easy.
back in the mid-90s i did a lot of riding on an (already) old Raleigh 3-speed. internal hub, so it’s just a direct loop. the whole process of pedaling just felt so much more direct and efficient.
apparently, the numbers don’t back this up. (something about internal friction in the hub being less efficient than the bends of a rear derailleur). i still loved it, and i’m still waiting for someone to really get the internal gear hub right.
Personally I think the future for TT bikes is a gearing system composed of expanding and contracting chainrings: one at the crank and one at the rear hub. They would both be operated by worm drive servos and actuated wirelessly. As the front expanded the rear would contract and vice versa. This would automatically control chain slack and it would also be designed so that chain tension would be as low as possible and, obviously, chain line would be perfect. It would also provide for “infinite” ratios within its gear range and the shifting would be controlled by something like the Baron Biosystems automatic shifting: just program in the power you want to hold and go
I’ve never worked out what the maximum gearing range would be though… that could possibly be a problem.
It was the aero ugliness of the rear derailleur that got me thinking about moving the derailleur inboard of the seat stays and chain stays … probably mounted inboard and low on the chain stay with nothing hanging outside of the chain stay … and a stepping motor … or worm drive … that fed the chain across the the lower teeth of the cassette … but then if that was available … I would really prefer your single expanding/contracting sprocket (front and rear) and get rid of the multi cog cassette for a single adjustable sprocket which would provide a much narrower and faster rear triangle … and as you say … and always straight chain line.
Of the two approaches … moving the rear derailleur inboard should have the same efficiency as the old system … the expanding chain ring system would need to exhibit the same or better efficiency to be useful for racing … I have seen more efficient looking designs (less moving parts) for expanding chain rings than the one I posted above where the expansion and contraction takes place and locks in place with no extra spinning jockey wheels.
Well, in that patent sketch of chainring-gearbox-thingy I see a chain tensioner, which I imagine is still needed, and looks like what you didn’t want to have.
Further, in your op you imagined a seat stay derailleur similar to front. However, won’t that position be where the chain is exiting the cogs? And thus won’t shift the chain? You have to de-rail as chain is entering cog (or chainring). Maybe I misunderstood. Also any tension pulley under the chain stay will have to stick down even further than rd. Geometry is optimized with dr as far back as possible. (Assuming you could even account for side to side movement of chain.) And of course tensioner can’t go on top.
with wireless technology here, I wonder how much of the existing RD design can be (re)moved. I am not sure if the next “solution” (aerodynamically) can be a separate purchased item, in my mind you move the necessary mechanics into the chainstay and have not much more than the actual cage/pulleys exposed. Something like a modern version of that S1.
We built a Zipp Super9 Disc wheel with SRAM i-motion 9 or 10 speed hub shortly after Zipp was bought by SRAM. It was beautiful, but unfortunately the internal friction of even the best internal gear hubs is a killer at this level. It’s been too long for me to remember the details, but the efficiencies inside internal gear hubs depend on the gear and range from ~90-95% of a derailleur setup…so even if you tweaked, coated, and ceramic the heck out we were looking at best case 2-3% friction losses compared to derailleur…which is far more power lost than could be gained back through aerodynamics.
I did, however, completely love the look of a track bike with 11 speed rear. That thing is floating around the Zipp archive somewhere I’m sure as I haven’t seen it in years, but would be fun to dig it out and have a go!
Having sat for long hours with the development group at SRAM that handles this part, I can tell you that the biggest challenge is handling the wide ranges of cassettes and chainring use that customers want. If everybody was still riding 42/52 front and straight block rear…we could probably make the things significantly smaller and possibly simpler…but once you start introducing large chainring steps, and multi-tooth rear gaps, plus the need for the derailleur to takeup and handle pretty large frame misalignments (of which modern carbon bikes tend to be way worse than old metal bikes…) you find that the derailleur is doing a whole lot more than just changing rear gearing.
Google “SunTour S1 derailleur”
How do you remove the rear wheel in that setup? Don’t see how you could pull the wheel far enough forward to clear the jockey wheels.
Really, that’s the other thing that the conventional rear derailleur offers versus internal hub designs, ease of rear wheel removal.
big hollow axle (thru axle ish) with all the intelligent parts, with the end having the pulley/cage, doubling as skewer and derailleur. Not sure how you get it to move up the cassette in a small package though.
