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[Ming the Merciless]

I said that in one line... 👅 It will work one end, not the other....

Except you didn’t you said “It won’t work” 🤪

 

[fossyant]

Except you didn’t you said “It won’t work” 🤪

Stick it up your 42 👅👅👅👅

 

[Ming the Merciless]

Stick it up your 42 👅👅👅👅

Won’t fit 😘

 

[fossyant]

Won’t fit 😘

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

 

[ColinJ]

I'd be walking up at 3mph........

You'd be doing VERY well to walk a bike up a 25% ramp at 3 mph!

That's a comfortable walking speed on the flat. I usually do more like 4 mph, but not up steep hills...

 

[silva]

Would be a fun gear on a fixie downhill.

 

[ColinJ]

Anyway, do you remember I fitted and rode one of these 42T rear sprockets a good few years back, but with a 34T front chainring.

I do because I bought one soon afterwards! I haven't fitted it yet because the bike it is going onto needs a new fork.

I currently have a 34/36 bottom gear on that bike but I will be changing to a new chainset*** which has a 39 inner ring and I want to keep my lowest gear for those pesky 20-25% ramps.



*** Donated Powercranks, otherwise I wouldn't bother

 

[Ajax Bay]

there must be an off the shelf rear derailleur option

No there isn't: because this is a ludicrous combination.
@GuyBoden - just have a chain long enough to go large/large. Avoid the smaller sprockets on the middle and larger rings. If you find yourself going faster than 18mph, don't shift to a smaller sprocket: freewheel or pedal faster. The chain interference will be audible once the RD's ability to wrap is exhausted. If you do shift (to a smaller sprocket), as well as the chain-chain-chain friction, the return section of the chain will sag and there's a high risk the chain will unship. All this probably best done in daylight.

 

[Jenkins]

Can SRAM MTB derailleurs run on friction shift? The SX or GX 12 speeds can cope with a 32T front and a 50T rear so might handle the 24/42 combination, but you may lose out at the other end IF you could find a way of making it work with a whatever you've got (guide plates too narrow for 9 or 10 speed chains, etc)

 

[Tigerbiten]

An easy way to get this to work would be .....
Start with a rear derailleur that will work on the 11-42 cassette.
Get new sideplates laser cut with a greater distance between the jockey wheels as this will give you more chain uptake.
Rebuild derailleur.

Easy ......

 

[T4tomo]

The Hope Eagle has a 30T front and a 52T rear using SRAM, that's a 15 inch gear, the same as 24T - 42T.

That is a single chainring set up. Your proposal is a triple adding a further 20T diff into the mix, which is why is won't work.

Yes SRAM and Shimano shifters / RDs cannot be mixed and matched. Cassettes can be interchanged.

 

[fossyant]

As has been said, it's not likely to work on a triple. If you can't get up a hill on a normal triple, without a massive cassette, then it's time for an ebike, or walk.

 

[numbnuts]

Get one of these T 20

View: https://youtu.be/tPbru0dAdmU

 

[fossyant]

You are probably correct, but as you know MTB's have used 26t front with 10-50 rear. Yes, a triple exceeds the capacity, so restricts ring combinations.

I've cycled up a few short 20% climbs last week, so I don't need an ebike at the moment, but, yes in the future I will.

Read this, if you don't believe me about 26t front with 10-50 rear on a MTB, but I know, you know this.
https://www.mtbr.com/threads/running-a-26t-front-with-10-50-rear.1085725/

I know all about 1 x 12T and massive rear cassettes, but it won't work with the bigger triple. No mech will cope with the chain length. You'll be able to use a fair few of the gears, but come the day you forget, and it all jams up.

 

[Ajax Bay]

I've got a 24T-34T-44T front triple and I have a 42T rear sprocket I would like to use.
down tube friction shifters.
But, what rear derailleur would work?

if you don't believe me about 26t front with 10-50 rear on a MTB, but I know, you know this.

You asked what RD would work (triple + 11-42): the answer is 'none'. Who's saying we "don't believe" you about a 1x combined with a huge range cassette? This seems to be your strawman.
Not what you asked to start with, nor relevant (you are not proposing to go 1x).
You say "that's why I'm asking for advice" but not sure you want to hear what we offer.