I'm not an engineer* so this may all be impractical speculation, but...
Has there ever been an attempt to design a two-speed freehub, or something similar?
What I mean by that, is a sprocket-thing that slides onto a standard freehub, so the chain can be set up as it would for a single speed, but the sprocket-thing has a simple two-speed gear system inside it. Looks like it should be feasible space-wise with a single planetary gear and perhaps 20t or larger cog**.
Another option that crossed my mind is having two full-length chains around two pairs of chainrings and sprockets, with some simple system that ensures only one at a time engages with the rear wheel.
Basically I'm thinking the simplicity of single-speed but with a bail out gear, without having to buy an expensive two-speed crankset like Schlumpf drive or an internally geared hub.
*at least not a practical one, I somehow have two degrees from an engineering department
**reminder, not an engineer
I thought about this, with a chain on both sides, for a 'double speed'. I tried to think of loads of mechanical ways of making it work, but I think the best option is to do it electronically at the push of a button. So it looks from the outside like a SS with a chain on both sides, but inside the hub there's a dog clutch that engages the freewheel on one side and disengages the other.
A planetary gear is no good because it's lossy, and no SSer will go for that. The idea behind two gears is that you are essentially still SSing, but you have a long gear when you need to hit the road to and from trails, which seems to be the big downside that even die-hard SSers acknowledge.
There is a very old design for a bike that is two speed that you pedel forwards for first gear and backwards for second.
https://en.wikipedia.org/wiki/Retro-direct
The issue with sliding onto a freehub is that the entire mechanism will be rotating, so you won't be able to attach a cable. You could implement some sort of wireless electronic shifting, I guess, but that's getting a bit silly 🙂 I assume we're not talking about investing the sort of R&D that it'd take to reinvent a Rohloff.
You could have a tensioner and derailleur, of course (my Brompton is two speed), but then you might as well add a fistful more gears.
I'd say the viable options are a hub gear or a standard get-off-and-do-it-by-hand dinglespeed.
A slightly leftfield approach is continuously variable transmission (CVT), which I think was tried briefly on bicycles, but which would be pretty unsuited to off-road use.
On the retro-direct front, it's probably worth reading the linked Kent Peterson articles (the first two two linked from this one) on getting one up and running:
http://kentsbike.blogspot.com/2008/12/retro-direction-perfection.html
What you are describing is essentially a hub gear.
There is no need to constrain yourself to fitting the gearing inside a conventional freehub as you are not mounting a cassette on it, just a single sprocket. Much better to make the hub shell bigger and use a clutch...like a hub gear.
Sturmey Archer do a 2 speed hub gear
link
@tjagain have you ever tried Retro-direct? My mate did it to an old 50's road bike, my god it's weird! I suppose you'd get used to it, but that little back pedal to level out for corners, gives you a little boost of higher geared acceleration just when you really don't need it! As I said you'd probably get used to it but old habits die hard!
with a bail out gear, without having to buy an expensive two-speed crankset like Schlumpf drive or an internally geared hub.
As soon as you start having to machine gears, it will start to get expensive. You might as well just go for a proper hub gear.
you could run gears by using 2 chains from 2 chainrings to 2 sprockets without any of the faff with cables
run an inside ring of (say) 42t at the front, and an outside ring of 32t
then at the back, run a 21t and a 16t. because the ratios are the same, no need to switch between them - 2 gears that are both engaged all the time.
How's that going to work?
@5lab ??? Whats the point in that? Both gears are the same ratio, extra chainring, cog and chain for nothing.
I thought about this, with a chain on both sides, for a ‘double speed’. I tried to think of loads of mechanical ways of making it work, but I think the best option is to do it electronically at the push of a button. So it looks from the outside like a SS with a chain on both sides, but inside the hub there’s a dog clutch that engages the freewheel on one side and disengages the other.
I've got something in my head that I think would work. I'll try to sketch it out, but in words...
It would need a pair of single speed sprockets on the freehub, but both float on bearings rather than being splined. On the inside of each sprocket (i.e. the sides facing each other), there's something like a DT Swiss ratchet interface.
In between the sprockets, you have a splined spacer on the freehub, which holds the bearings the sprockets are mounted on apart and engages the freehub. This has another layer of splines around it (no need for these to be Shimano or anything), so you can mount another bit of metal on top (call it bit X). Bit X is slightly narrower than the spacer underneath, and it has the same ratchet interface on both sides needed to engage the sprockets.
Essentially, bit X always engages with the freehub via the spacer underneath, but it can float laterally so it only engages with one sprocket at a time. A spring could be buried in there so there is a default engagement direction. The trick is to get bit X to float the other way with a shifter. I think you could do that by having a bearing ring around it that would be stationary even when pedalling, so could have a cable attached to it to pull against the spring tension and engage the other cog.
Edit: if that makes sense it'll be a miracle
If you're using a sprung tensioner, you could just leave the granny ring on a triple chainset and kick the chain down when faced with a steep climb? Could probably do the same if you were prepared to stop if you had sliding dropouts, or even have two chainring / sprocket combos of about the same tooth count 32/16 + 24/24 for example. If that's too low-tech, what are you trying to achieve that's not reinventing the Sturmey-Archer hub?
Pedersen made 2 and 3 speed gear hubs (rather than epicyclic). They had a pear shaped hub shell / flanges, so required 12 different lengths of spoke which I think would explode my head trying to lace up.
@legometeorology I get that part. I'm thinking you need two chains and sprockets, effectively just shifting between which geartrain is actually driving the wheel? I think that "could" work. Getting the clutch ring to shift could be done with a rotating ring, similar to the SA rotary hub, though I think you'd need a double cable desmo actuator.
You could fashion a mechanism to move the sprockets across the cassette if you use fewer sprockets (and can keep them moving freely). This was common back in the middle of last century - I remember my dad talking of a 4 speed version, I think it was rod operated but I could be confusing it with the front derailleurs of the day.
You could then get maybe half the gears but maintain a perfect chainline - with tensioner of course.
Alternatively, dinglespeed works if you want to avoid chain tensioners but are prepared to stop to change gear. It's pretty much suitable to put it on one gear for your commute/ ride out to the trails, then run the lower gear for MTBing - have seen it done but not done it myself. You just have to make sure the chainring plus sprocket teeth adds up to the same total in both gears.
@Legometeorology. Sounds complicated, what is wrong with a derailleur? If it's just two cogs an old short cage mech and a friction thumbshifter is all you'd need, cheap and about as simple as you'll get. Adding extra bearings, sliding elements and clutches defeats the simplicity no?
Setting the actual mechanism of that "two sprockets on a freehub" idea aside for one moment…
If you run two drivetrains this way, they need the same effective chainstay length (otherwise you need a tensioner, in which case you might as well use a derailleur). For that to be the case, the pairs of sprockets have to both differ by the same multiple of four (eg 32:16 and 36:20).
Let's say you want one gear to be about 50" or so on a 27.5" wheel. You want the smallest sprocket you can (otherwise the next bit gets even worse), so you're probably looking at 22x12.
Now you want a bail-out gear of, what, maybe 30"?
This is where your problem starts, because even if you fit a 58x48 you're still not even down to 33". A more workable 46x36 would get you about 37", to be fair (equivalent to 32x24, so lowish middle ring on an old triple setup), but is that low enough to be worth the faff?
You just have to make sure the chainring plus sprocket teeth adds up to the same total in both gears.
The ideal in terms of keeping the same effective chainstay length is paired multiples of four; same-total gearing generally works absolutely fine but the length differs slightly (chain tugs will need adjusting and disc calipers may need checking); other combinations may be possible depending on how much leeway you have on your dropouts/tensioner/brakes, and the fine details differ slightly depending on what type of dropouts, brakes and brake mounts you have.
Another possibility would be 2 concentric freewheels, both with sprockets attached, one of which (the higher gear) could be disengaged to allow you to change gear. You then run 2 chains, chainrings and sprockets, and confuse people.
The ideal in terms of keeping the same effective chainstay length is paired multiples of four; same-total gearing generally works absolutely fine but the length differs slightly
Are you sure? Can you explain why?
I can, but it means drawing some triangles and doing some basic trig, and I can't be arsed to upload pictures 🙂
Essentially it's because (a) if the angle of the straight run of chain relative to the effective chainstay differs then you won't fit the same positions of BB/hub axle without some slack; and (b) chains need to be built in pairs of links.
If you use a half-link chain then it's multiples of two, not four.
How is this different to a Sturmey Archer 2 speed hub? You can even get a kick shift one, no cables necessary!
https://road.cc/content/review/33373-sturmey-archer-s2-kickshift-hub
The SA hub:
The shift motion is a lot more subtle than you might expect and requires a little bit of finesse; you can change just by shifting the pedals back just a short way, so if you stop at the lights and rock the pedals back to starting position there's every chance you'll change ratios.
Sounds like it'd be a right ball-ache for MTBing. And I can't seem to find out if it runs on planetary gears or not.
I looked at this a while back, but for a 3 speed.
I came to the conclusion that the trick would be to replace the freehub itself, because these days their width is sufficient to fit a 3 speed planetary in there.
However then the question is why not just build a 3 speed wheel then you get a stronger wheel because with equally spaced hub flanges there's no uneven tension.
For a 2 speed, I'd go for the S-A S2K - a disk brake 2 speed that changes without the need for cable, just back pedal slightly.
It's not just the number of chain links that is important its chainstay length to. So as the PCD goes up at the primary gear the chainstay length has to decrease as more of the chain is being wrapped around the chain ring.
I'd try inverting the rings ideally so 50 34 compact road at the front then 34 50 at the back, That would ensure the same chain length and chainstay length so tensioners and brakes shouldn't need adjusting. Would still need a tensioner to switch the chain between the two without moving the wheel in the drop out though.
2 lots of 50:34 and two lots of chain to wrap around it probably weighs a damn sight more than a cassette and derailleur........
Oh yea, i'm more than happy with my mech's and dont see the point personally but as an engineering challenge thats where i'd start.
Only one chain though, i'm not getting all this double chain talk.
Jesus, I thought this thread would die off immediately but now I've got too much to read...
Four quick replies:
I currently run a dingle speed 29er with a 37:21 and 39:19 set up and horizontal dropouts. It's nice, but even with a 10mm axle that allows the wheel to drop straight out it takes long enough to change that I only bother once in a 2hr ride.
I've ran a similar set up with a tensioner before and 2 or 3 chainrings. Much quicker to change gear but a lot less smooth than the above. May as well run a rear mech IMO.
Internally geared hub: probably the sensible answer, but it's be nice to get something simpler and compatible with any freehub. I thinking running two chains, sprockets and chainrings could do it.
That retrodirect thing is genius, but no way I'm going to back pedal my bike offroad at high speed!
How do you engage and disengage the 2 chains? mixed gear ratios on the same shafts just locks it all up.
I rented a Felt cruiser on holiday with a back pedal 3 speed, may even have been coaster to. I didn't ride to far on it but it just felt clunky and vague.
How do you engage and disengage the 2 chains? mixed gear ratios on the same shafts just locks it all up.
2 independent freewheels, so when both drivetrains rotate the lower gear freewheels a bit. You then "just" need to disengage the higher ratio to drop down to the lower gear.
SRAM do a 3speed hub IIRC if you wanted something off the shelf. I remember someone trying to flog an 81 speed SRM Switchback on ebay for ages a long time ago that used one (with 3x9).
Or Truvative HammerShmidt did the same for the chainset. Was neither light or strong though, heavier than the equivalent gearing and didn't like jumps.
You then “just” need to disengage the higher ratio to drop down to the lower gear.
Some sort of electromagnet arrangement for the pawls? 🙂
If you run two drivetrains this way, they need the same effective chainstay length (otherwise you need a tensioner, in which case you might as well use a derailleur). For that to be the case, the pairs of sprockets have to both differ by the same multiple of four (eg 32:16 and 36:20).
When I say bail out gear, I don't necessarily mean they have to be massively different, but this is a problem for sure. Even with my current 37:21 & 39/19 dingle speed, I give the chain-tug bolts one turn when manually switching gear to get the tension right.
Obviously the ideal is a X:Y & Y:X set up like coppice says, but I think the rings have have to be huge to work.
Something like a 33:16 & 30:19 may just about work I would think.
How do you engage and disengage the 2 chains? mixed gear ratios on the same shafts just locks it all up.
The long explanation I typed above was an attempt to describe the idea I have in my head, which would work sans electricity I think. But Bez's replies have highlighted the biggest issue may be effective chain length.
To be clear, the only reason I thought this could be a good idea is if it was genuinely as smooth and low maintenance as a dedicated single speed
I'd guess a front mech with 2 rings and a single rear with a chain tensioner is as good as it's going to get.
I’d guess a front mech with 2 rings and a single rear with a chain tensioner is as good as it’s going to get.
At the moment, yes...
Ive done that before and currently getting bits together to do it again. The type of tensioner that works best is one that apes a derailleur cage, like the Paul Melvin, Acor or Shimano (Nexus, I think).
Mrs ran tringlespeed like that on her commuter for quite a while using up old bits (triple chainset, old road rear mech and a Surly screw on wheel we had lying around. When I worked out the ratios it was remarkably close to a Sturmey.
Having had a good local ride today with 10 gears I realise I'd be very reluctant to lose them all.
That 2-speed Sturmey Archer looks interesting. I like the idea of no shifter and backpeddling instead and it's not that heavy, but it seems like accidental shifting is going to be pretty common if using on more techy offroad stuff, if it's even up to that?
I used to have the Sturmey Archer 2-speed with coaster brake.
Bloody nightmare - I was always in the wrong gear in traffic.
Sold it to LeeLovesBikes on this forum who was delighted with it.
900g rear hub for 1 extra gear!
I always get to either
a) it defeats the object of a singlespeed or
b) You might as well have a derailleur and cassette
Would a very simple pinion type gearbox be a go-er?
