9-Speed Ultegra (6500) with Mountainbike Derailleur

Question

I want to go bikepacking with my old road bike. The road bike has a standard crankset (53/39, double not triple), and an 11-28 cassette. The groupset is Shimano Ultegra 6500.
Because I want to do some large climbs in the Alps, I would like a lower gear. I considered changing the crankset, but that seems a bit expensive.
I also have two old city bikes that have mountainbike derailleurs that I could remove: One is a Shimano Deore LX 9 speed, the other is a Sram s500 RD 9 speed.
Can I use one of these derailleurs to fit a 12-36 cassette to my bike, or will that exceed the total capacity of the drivetrain with the non-compact chainset?
I mention both the Shimano and the Sram derailleur because I think the Shimano is more likely to work, but in slightly worse condition than the sram - so if the Sram works too, I will try that.

Nathan Knutson · Accepted Answer

All nine speed Shimano shifters index with all pre-ten-speed Shimano MTB derailleurs.
All nine speed long cage MTB derailleurs with long (SGS) cages will have total capacity sufficient to run any cassette they can clear with a double in front.
36t large cog cassettes came out late in 9-speed mountain's life. Only a very few Shimano 9 MTB derailleurs are rated to clear a 36t for their largest cog spec. Most do 34 and a few only 32.
The SRAM you have is a 1:1 and has no compatibility with the shifter.

Argenti Apparatus · Answer

Yes, you can use the Shimano 9 speed MTB derailleur with the 9 speed Ultegra 6500 shifters. This is because on older MTB groups up to 9 speed and road groups up to 10 speed Shimano used the same actuation ratio (ratio of length of cable pulled to cage movement).
The total capacity of the derailleur is a measure of how much chain slack it can handle and should be greater or equal to (difference in largest/small sprocket teeth) + (difference in chainring teeth). I.e., the size of the chainrings does not matter, the difference between them does. The 53/39 rings only have 14 teeth difference (compact 52/36 or 50/34 have 16) which give you a little bit more latitude with the cassette size.

9-Speed Ultegra (6500) with Mountainbike Derailleur

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