Largest cassette on a 20" Dahon?
 

It's time for a new derailleur and I figured that while I'm at it, time for an upgrade from an 8 speed to a 10 speed cassette and new derailleur.

That said, what is the largest practical cassette that would work?

46T maybe 50T?

The issue or limiting factor is not the cogset. The limiting factor is ground clearance for the RD. On 10-speed 406, you want something along the lines of Deore M6000 which is rated to 42t. That's what I run on my 406er. Here it is...
https://cimg5.ibsrv.net/gimg/bikefor...0ddb8e6d92.jpg

https://cimg8.ibsrv.net/gimg/bikefor...c69d2fc8d2.jpg

.
The other GS alternative is to jump to 11-speed with this job rated for 46t.
https://cimg7.ibsrv.net/gimg/bikefor...36e731325b.jpg

What gear-inch value are you trying to achieve?

Is that a long or medium cage?

Shimano nomenclature

• SS: short cage
• GS: medium cage
• SGS: long cage

Once again, on 406 wheels you want no longer than GS.

In my locality the Shimano RD-M6000-GS part has become rather scarce. That's the one you want for 10-speed and 42t max. Grab one if you can. Were I building a 406er, I'd use that or move up to 11-speed with the Shimano RD-M7000-GS part, good for 46t. You wanna blow more coin, there's also the RD-M8000-GS part, good for 46t and 11-speed. Are you seeing the pattern?

I run up a 11-46T nine speed cassette on one of our Terns, using Microshift’s medium length Advent rear derailleur. With a 52T chain ring that gives me a 88”-21” range which works for my hilly city, but needs either additional links adding to a standard chain or an extra long one.

A straight-on image with the RD at its lowest position would be great.

https://cimg6.ibsrv.net/gimg/bikefor...587b2b558.jpeg
As requested, clearance appears similar to that of a Shimano GS rear mech.

Yeah...thanks...I think...🤔 That's not a good image, even worse than mine. Certainly insufficient to make any determination. Come on, Aotearoa!
​​

Give me a day or so and I’ll take a better one ;)

Great. Images like these. Close up and straight on two dimensions.
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Shimano RD-M6000-GS, 53-406 tires
https://cimg1.ibsrv.net/gimg/bikefor...07348fd306.jpg
https://cimg2.ibsrv.net/gimg/bikefor...70937dffab.jpg
https://cimg3.ibsrv.net/gimg/bikefor...20ddb03db6.jpg

Personnaly, if going off road, i would keep a short cage derailleur. I tried medium and the bottom pulley kept getting caught. With shimano current short cage (equivalent to 10yrs old medium cage), i run 36T without problem and 40T with the better derailleur.

if you are just using the bike on the road, medium is “ok” although in the British winter it is not great as the pulley get covered with mud, salts etc

As requested…
https://cimg1.ibsrv.net/gimg/bikefor...ee8e1b695.jpeg
https://cimg3.ibsrv.net/gimg/bikefor...750e1b085.jpeg
https://cimg5.ibsrv.net/gimg/bikefor...8892fce06.jpeg
For reference, tyres are 50-406 Schwalbe Big Apples. I agree with Fentuz, I wouldn’t go heavily off-road with this setup - for that, I would probably replace it with a 11-38T cassette, the Microshift Advent super short rear mech and a 46T chainring and forfeit anything higher than a 75” top end.

Here are pictures of the Microshift Super short 8 speed on 20 inch wheels.
https://cimg1.ibsrv.net/gimg/bikefor...bf2f62a88.jpeg
https://cimg2.ibsrv.net/gimg/bikefor...54494f175.jpeg
https://cimg5.ibsrv.net/gimg/bikefor...c23efa270.jpeg

i’m worse than that :D I run 11-40 with a 44 :P

Now that you mention it, there’s a 11-42T cassette with a 44T chainring on our other Tern Link, also with Microshift Advent - I installed that combo on my wife’s bike back when 11-42T cassettes were the largest Microshift were making…

So, the answer is 50T

https://cimg9.ibsrv.net/gimg/bikefor...bf00477b97.jpg

Man, my Origami Wasp was rocking a 58t + 11t-28t freewheel w/16" wheels, and I never had an issue. Enjoy that baby!!!

I updated my response to say "58T",... My bad! :p

NOTE TO ALL: If a couple of you want to be snippy and rude, please do it via Private Message. Thanks.

(Off topic posts removed)

RD's like those make the case for IGH's
I have several 20"s and lost an RD to a pothole and have bent a few over the years.

I recently installed a 40t cassette on my bike with a Shimano 9050 Di2 derailleur system. Ground clearance isn’t great, but the gearing range is enough to get up the steepest climbs I can find in Tokyo.

Saved.

Temper tantrum noted.

Cry some more.

I'm very impressed with some of the pieplate lows I'm seeing on 20"s above. Bigger than I thought possible. There are several factors:
- (obvious) Clearance of the derailleur to ground. This can be mitigated by using the shortest chain possible, so the cage swings forward, away from the ground, when on the lows.
- Lateral clearance between inside of derailler and the tire, as well as same for chain, with even the mildest of wider tires, such as 1.75", even more difficult approaching 2", and this is less mitigated by a shorter chain as noted above.

With my double crank (50/34), I don't need as much of a low, though I'd still like lower than what I have. But the info above may help me with my other Dahons that have not yet been converted to doubles.

I agree that the GS (mid) length derailleur cage is best choice for 20" with 1X crank and hyperwide cassette.

I run a GS with 50/34 11-30, and I don't use the shortest length chain possible (big-on-big, plus 2 links), but the longest chain possible (small-on-small, and derailleur still taking up slack, the cage horizontal aft with still a small gap above, more cage windup with the cage slightly up would take up no more chain), and I still have plenty of cage travel forward if I want to go to a 32 or 34 low cog.

I think some of the latest derailleurs for 1X, may have the cage pivot between the two pulleys, as this may give more clearance. EDIT: CONFIRMED IN PHOTOS ABOVE, MUCH BETTER CLEARANCE, AT LEAST WITH A SHORT CAGE, I THINK THAT IS THE WAY TO GO, BUT I WOULD STILL NEED AT LEAST A GS LENGTH. The question now is, with a 50/34 double, how big a cog can I go in back with an 11 high and a GS cage? Gear calc tells me that 42 low cog would get me down to 15.1 gear inches, from current of 21, I think that would be sufficient. Now to figure RD capacity... full cog difference or just half? Sheldon Brown says full 360 degrees. So... 42-11=31, 50-34=16, so I need 47 tooth capacity... looking now, the guide I see is old, both GS and SGS say max cog of 34, so with 1X RDs, I know they can use a bigger cog, but capacity should not change as much, and I don't think I can use a GS if I want a 42 low AND keep my double crank in front; GS says 33 cap and SGS says 45 cap. I think a long cage may be possible with one of those newer RDs with the pivot between the pulleys. EDIT: My cheapo GS Tourney TX RD has 43 tooth total capacity, and I've heard those ratings are conservative, it might do 47. But I think the pantograph angle is such that it might not be able to handle a 42T low. The derailleur itself is not indexed, so might actually work on 10 speed with a 10 shifter. But I think the pulleys may not work with a 10 chain.

Another note about RD cage lengths: I'm noticing that on some of those GS rated for very large lows, the link above the cage is extra-long, to space the jockey (top) pulley low enough for the large low. Meaning, that a GS may be longer than expected and still hang too low to the ground.

Due to the oversize seat tube on a Dahon, and the front derailleur adaptor, I can't work a triple crank without an extra-wide bottom bracket to space the crank way out. I'm instead using a wide double, 50/34, and I LOVE the hollowtech II style cranks. I can't fit a smaller ring on a 5 on 110 BCD crank. So I'd love to fit a wider range cassette to go lower than 21 gear inches without sacrificing my 85 inch high. I'd need a new crank or rings to go above 9 speed cassette, might have to do. I love the low cost and durability of 7/8/9. But still preferrable to me over an IGH.