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[ColinJ]

How about a compromise... Turn it into a double by removing the big ring, possibly with a slightly bigger second ring - a 36?

A 36/11 top gear should be good for 40 kph/25 mph?

 

[Tigerbiten]

Just checked the specs of the Shimano RD-M8000-SGS 11 speed rear derailleur.
The published limits are .....
Max capacity:- 47t.
Max front difference:- 18t.
Largest sprocket:- 40t(3x11-speed), 42t(2x11-speed), 46t(1x11-speed).

You can get around the first limit fairly easily by just not using some of smallest sprockets when on the small chainring. I've done that in the past.
As for the other two limits, it's a case of trying it and seeing how hard they are.
You want 20t in front and the limit is 18t and you also want 42t(3x11-speed) vs 40t(3x11-speed) max.
You may well find that you can exceed one limit or the other but not both together.
So I'd be prepared to switch to a sub-compact double if the limits are to hard and a triple doesn't work.
And then see want the limits are for a sub-compact double.

Luck ............

​

 

[figbat]

Sorry, I'm having trouble understanding that if a 24T front and 42T rear will work on a MTB, then why can't it work on a triple.

I understand that some ring/sprocket combinations will prohibited using a triple, but that's no big deal.

I'm going to fit the parts and see if it works. If it doesn't, no harm done.

Because the derailleur and chain length have to account for the minimum and maximum possible combinations. On a 1x setup, the derailleur will be fairly close to fully-relaxed on the 11T and pretty well extended on the 42T. If the derailleur can just about cope with this situation, you can't then expect the chain to go all the way around a bigger chainring as well - the derailleur's job is principally to shift speeds on the cassette but it is also a chain tensioner, taking up the slack as different chain lengths are needed. The longer the cage, the more scope it has to cover big differences, but there is still a limit.

 

[T4tomo]

I'm going to fit the parts and see if it works. If it doesn't, no harm done.

except to all the skin you lose when the rear end jams and throws you off.

reread the advice you've had on this thread, google rear derailleur capacity, read sheldon brown.

streuth some people are hard to save from themselves.

 

[Once a Wheeler]

I admire your puritan insistence that cycling at any speed is preferable to getting off and pushing. Hats off!

 

[Tigerbiten]

Because the derailleur and chain length have to account for the minimum and maximum possible combinations. On a 1x setup, the derailleur will be fairly close to fully-relaxed on the 11T and pretty well extended on the 42T. If the derailleur can just about cope with this situation, you can't then expect the chain to go all the way around a bigger chainring as well - the derailleur's job is principally to shift speeds on the cassette but it is also a chain tensioner, taking up the slack as different chain lengths are needed. The longer the cage, the more scope it has to cover big differences, but there is still a limit.

The 11-42 cassette uses a 31 teeth capacity.
The Shimano RD-M8000-SGS 11 speed rear derailleur has a total capacity of 47 teeth.
That leaves 16 teeth for the chainrings.
It's possible to go over the 47 total tooth capacity as long as you don't use the smallest 2-3 sprockets when on the smallest chainring.
I've run an X9 setup like that on my first bent trike.
It just gets noisy with the chain rubbing against itself if you forget and shift to small-small.
But as long as you can safely get into the big-big combo then nothing will jam.

What may stop the OP is the top jockey wheel on the RD-M8000-SGS is not at cage rotation axis.
So the greater the range between the chainwheels then the closer it gets to the big sprocket.
So with a 20t difference in front the top jockey wheel may well touch the biggest sprocket when running big-big with the chain at it's minimum length.
You may well want to go 2 links longer with the chain and play around with the B screw to see if you can still keep clearance there.

Luck .............

 

[C R]

I've installed a SRAM X5, the gears work fine using just the 38T, 40T and 42T largest rear sprockets with the 24T front chainring.

The middle chainring, 34T, functions fine with all of the rear sprockets.

The compromise has been that the three largest rear sprockets obviously can't be used with the largest, 44T, front chainring, the other rear sprockets are ok.

The pull ratio for the gear lever is a lot longer, but the downtube shifter copes as usual.

View attachment 638104

View attachment 638105

What cadence do you need in the lowest gear to stay upright?

 

[cougie uk]

You should laugh... dealing with a 39 x 26T max on my road bikes and I'm getting old..

Aaaah that is what I had on my bike when I first did the Alpe.
Mate has just bought a gravel bike with a 1x set up and he has a 42x42 bottom gear. I can't see why you'd need lower than that - I'd be falling off on hills !

 

[ColinJ]

Mate has just bought a gravel bike with a 1x set up and he has a 42x42 bottom gear. I can't see why you'd need lower than that - I'd be falling off on hills !

I have a 22/32 bottom gear on my mountain bike. I didn't use it very often but there were some very tough rough-surfaced ramps at around 25-30% that I definitely DID need it on.

 

[Ajax Bay]

I've installed a SRAM X5

Neat. Shortening the section of outer would increase the radius of that cable and result in less friction. Just a bit, keeping the QR axis clear.

 

[Ming the Merciless]

I have a 22/32 bottom gear on my mountain bike. I didn't use it very often but there were some very tough rough-surfaced ramps at around 25-30% that I definitely DID need it on.

Yep and I have my old mtn chain set on my recumbent, bottom gear 24 / 34 and definitely not fall off territory. Those who do must have very low cadence.

 

[ColinJ]

What does it look like in the big ring and smallest sprocket?

 

[Ajax Bay]

What does it look like in the big ring and smallest sprocket?

Suspect the chain looks as if it's on the large ring (of three) and the smallest sprocket. This combination is not on the operating perimeter.

 

[ColinJ]

What does it look like in the big ring and smallest sprocket?

Suspect the chain looks as if it's on the large ring (of three) and the smallest sprocket.

Show us what it looks like with the chain on the big ring and smallest sprocket!

This combination is not on the operating perimeter.

I, er, flipping heck...

 

[Ajax Bay]

Colin, what I'm suggesting is that an image of large/large and small/small (the two edge cases) would offer some illumination, whereas large/small would offer none.