tomtomtom123

I am thinking about the K3 Plus 16". What is the longest rear derailleur cage, practical for 16"?

I currently have a 20" Dahon bike with 12-34T cassette and 44/24T chainrings which gives me 519% gear range, and would like to maintain this or higher range.

For my 20" rim with 2.15" (55mm) tire, I have a RD 4700 GS rear derailleur, and with 34T cog, the lower pulley on the cage can swing to around the outer diameter of the rim, meaning that it could potentially swing to around 45mm above the ground when the tire is fully inflated.

For a 16" rim with Big Apple tires, 2.00" (50mm), the ground would be perhaps 55mm closer to the lower pully, meaning that it could potentially hit the ground with the same GS length cage and 34T cog.

would this mean that 16" rim would need an SS length cage?

With the limited chain wrap capacity of SS cages, I was thinking that a K3 Plus would need the Sturmey Archer CS-RK3 hub with 177.7% gear range. Combined with 11-34T cassette would result in 549% gear range. I thought about taking the last 3 cogs on the spider of the CS-HG700-11 11 speed cassette (27-30-34) and mixing it with other 11 speed road cogs to get 11-12-13-14-15-16-17-19-27-30-34. I'm not sure if the 8 teeth jump is problematic between 19-27.

The shimano specs don't show any SS cages that can go to 34T max cog. Or is there one that would work?

I was thinking of taking a Shimano Zee 10 speed RD and using an 11 speed MTB shifter with a cable reduction of 90-94% to get the correct cable pull.
There were other posts here with people using the Zee RD on 16" bikes, although I have not found any threads about someone using the Zee RD on 11 speed cassette. I guess the main concern is whether the Zee RD can move the extra 2 to 4mm for 11 speed MTB or Road cassette vs 10 speed.

Otherwise, if not using the Sturmey Archer hub, I would consider a triple chainring with 54/38/26, and 11-28T cassette to maintain 529% range with the minimum wrap capacity of 45, but requires at least a GS length cage.

base2 

I think you are on the right track with the Sturmey Archer CS‐RK3. They also make a non-disc brake version, the CS-RF3 if you have no intention of using a disc rotor.

I don't know about the newer Dahons but, mine has 130mm drop out spacing & no disc tabs. (My solution for that bike was a Schlumpf Speed-drive with a 34 tooth ring, a Nexus 7 with a 17 tooth cog to be with in Shimano's ratings, & a Surley Singleator to provide chain tension.)

The Sturmey archer hubs are 135mm over locknut dimension. This could provide complications. Especially if yours is not steel. Namely cold-setting such short chain stays & hanger realignment.

As far as derailleur length:
Sturmey put an arbitrary 34 tooth max cog rating on the hub...That's not how gearing works.

I think as they are well aware the market the hub will be used in is Adult road bike conversions & similarly, cruisers, single speeds, etc...The smallest road cranksets readily available & popular are usually around 50-34. All other uses, such as folding bikes have bigger chain rings & small wheeled childrens bikes are unlikely to be driven very hard. So they simply put a 34 tooth arbitrary max cog rating to limit the drive ratio to 1:1 to avoid improper use in mountain bikes where this hub would be tempting. But specifically to avoid the further torque mulitplication of super low 24, 26, 28 tooth small rings of mountain cranksets when combined with the ridiculously large 42, 46, 50 tooth cassettes (meaning sub 1:1 ratios) readily available on the market these days.

This is a totally reasonable course of action by Sturmey. 1:1 would be one of the lowest input ratios of any internal gear hub on the market. IMO, it's a testament to the strength & durability of the hub that they would feel confident in such a holistic approach to their ratings..

I'd wager any cog set with a low/low ratio of 1:1 or higher would work fine & still be well within the hubs capabilities.

All this is to say, if the biggest, big cog your current derailleur or wheel radius from the ground, will accomodate is a 34, then fine: Run a 34 tooth (or larger) chain ring instead. That'll be your lowest low safely achievable.

EDIT: Now that I re-read your post, you're right. There are no Shimano SS cages that will accomodate a 34. A 28 tooth cassette is the best you can do. A 28-42 crankset with the Sturmey & an 11-28 cassette would still get you 12.5 to 84.5 gear inches. Honestly, wouldn't that be enough? I'd just find any 1x chainring larger than 28 (probably closer to 42 or so) & call it good enough.

I am currently building up a 700c wheel with a CS-RK3 with a 11-42 cassette to be driven by an Absolute Black 42 tooth 1x chain ring. 680% range. If I'm wrong, then...I guess we'll be wrong together. 😉

Jipe

On a 16" ETRTO349 Bompton size wheel with 35mm wide tires (Continental Contact Urban 35x349) I use a Shimano 11s Ultegra RD-6800-GS with a 9-32 cassette that doesn't hit the ground (something like 25-30mm ground clearance worst case).

What 16" wheel size do you mean ? ETRTO305 I guess (since there are no Big Apple 50mm wide in ETRTO349 but well a 50x305 Big Apple) ?

A 50x305 wheel is about 14mm smaller in diameter than my 35x349, so I guess that the same setup won't hit the ground on your 50x305 wheel.

tomtomtom123

I couldn't find published specs for k3 plus so I assume when they say 16" that they mean 305. A 34t will be 4mm lower to the ground compared to a 32t. So comparing 349 to 305, if you get 25mm clearance on your 349 and 32t, then a 305 and 34t may touch the ground. You also have to consider that a big apple will be at lower pressure and can compress closer to the ground when hitting a bump.

K3 plus is disc brake with quick release so I assume 135mm OLD, which is why I mentioned the cs-rk3 instead of the cs-rf3. I would not want both a planetary gear hub and multiple chainrings, as it makes the bike heavier and shifting more complicated.

I'm kind of wondering if the SS cages that state 30t max cog could do 36t with the b screw all the way in. But all the SS cages don't have Shadow+ with clutch. I'm thinking of using an oval chainring for the first time, and the zee RD has a clutch to retain the chain, but I also wonder if the clutch has enough backlash to not apply too much force and drop efficiency during the small oscillation of the RD cage when the oval makes small changes the chain length as it revolves. I still don't know if the zee RD has enough movement to cover 11 speed cassettes.

base2 

Ok then, you are golden. The CS-RK3 is your best bet. Still, grab a ruler & measure the drop out width before ordering. A 1x chainring around abouts 42 gets you ~18-85 gear inches (452%) with a standard 11-28 cassette. You'd need the cassette to be one that fits on a 8,9,10 speed free hub body, regardless of how many individual cogs are on it. I think there is 8, 9, 10 speed versions from most brands.

Then all you'd need is a cheap matching 3x mountain shifter set from Microshift...and you'd be set.

Fentuz

did you look at the altus derailleur M-310 ? it is fitting on many kids bike; it gives better clearance on my kid bike than the sora short cage on my jetstream.

Jipe

There are 14mm difference on the diameter, so only 7mm lower to the ground = 18mm ground clearance. It should be OK.

The 34t cassette isn't an option as the Ultegra RD-6800-GS doesn't allow a 34t cog, 32t is the maximum (same for the Shimano 105 RD-5800-GS, the current newest Shimano Ultegra and 105 rear derailleur are very different, it is difficult to predict their ground clearance),.

tomtomtom123

The m310 RD is for 8 speed cassette. I don't know if it has enough movement to go across an 11 speed cassette which is 3 to 4mm wider, even if an adapter gets the cable pull right.

In your example with 6800, The difference between 349 and 305 is 44mm. Divide by 2 equals 22. Add 4 for the difference between 34t and 32t. Subtract maybe 10 for the difference between 50 and 35 tire.
22+4-10 = 16mm closer to the ground. If your setup only had 25mm clearance with 349 with 35 tire and 32t, then the 305 with 50 tire and 34t will have only 25-16= 9mm clearance to the ground. 9mm is extremely small so it can hit rocks and the branches. When hitting bumps, the tire will compress and goes lower. If there is a sudden flat tire, the RD could smash into the ground.

I measured approximately 45mm clearance with 406 and 55 tire,34t and 4700gs RD. The difference between 305 and 406 is 101/2= around 50.

So 305 with 50 tire,34t and 4700gs will already hit the ground.
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Jipe

The difference between 50mm and 35mm tire is 15mm. The overall diameter of the ETRTO305 with 50mm tire is about 405mm while the one for the ETRO349 with 35mm tire is about 419mm, diameter difference is about 14mm.

The pressure in an ETRTO305 50mm Big Apple cannot be as low as for a bigger wheel Big Apple tire, so the compression of the ETRTO305 Big Apple isn't very big, not much bigger than with a 35-349 tire.

As said, the 34t is irrelevant with RD-6800-GS or RD-5800-GS because both are limited to 32t.

But they accept a 9-32t cassette which has a wider overall ratio than a 11-34 cassette and fits better with a small wheel folder.

Note that the derailleur cage is the closest to the ground on an intermediate sprocket when it is more or less vertical, not on the largest sprocket where it is tilted to the front !

tomtomtom123

Oh, yes your math is more accurate. I was dividing the difference in single tire height by 2, when I shouldn't have, since there are 2 sections of tires through a cross section of the wheel.

But still, with a 50mm tire, if there is a flat, there is a chance of the cage hitting the ground or some object. I know that the cage at the max extension will not be pointing straight down, but angled forwards, however if you go over some tall object like a curb, the point of contact with the curb could be at the location where the cage is closest to the perimeter of the tire. Also, if you drop the chain off the chainring while on the largest cog, the cage will swing back and potentially strike the ground or some tall object you're passing by.

Also, I pump 50mm tires to 3 bar, while 40-47 to 4 bar. The effective diameter (sinking) of the tire changes a lot at lower pressure while loading the bike. I don't remember the exact measurement, but when I dropped the 55mm tire pressure from 3 bar to 2.5 bar and sat on the bike, the bike had a difference in height somewhere around 3mm to 5mm. A 35mm tire at higher pressure would probably have lesser changes. Also, change in tire width doesn't equal the same change in tire diameter. I assume the change in height is less, because the cross section of the tire isn't a perfect circle. Where the wire beads sit, the circle is cropped off. So the difference in diameter between a 50mm and 35mm tire isn't 15mm, but actually less, I would guess 10mm.

splithub

When i converted a bike that small and lightweight into IGH drive, i was disappointed by the weight impact, it lost a big part of it's easy handling, balance. On my Spin5.1 i drive a Sora3000 SS (specced for 32T max), handling a 36T without problems, might vary from bike to bike. On the german folding bike forum is a guy riding a Zee converted to 11-speed. I can't see 50-305 tires to fit on the K3+ front. Have a look at older SRAM RR derailleurs, they might give you some clearance, i like it on my 14" Spin2:

14\" Spin2 SRAM Rival

Jipe

With a 50mm wide tire and 135mm rear spacing, the derailleur cage is very close to the side of the tire.

The chance that an object can hit the cage is therefore very small because the object must be so close to the side of the tire but not under the tire.

Now, there is indeed a risk.

Note that on most folding bikes an IGH require a chain tensionner that can also hit the ground.

I agree with Splithub: an IGH on a small wheel folder reduce the reactivity of the bike which is part of the pleasure to ride a mall wheel bike (unless you choose the expensive Rohloff hub).

Fentuz

I run it on a 9 speed 11-36 cassette and it works well. I believe it would work up to 10 in 11-34.