Can I run a SRAM 2x10 with 3x9 derailleurs?

or, what is the difference between ESP and Exact Actuation?

I've been dreaming of both converting my main ride to a 1x9 and being strong enough to rock the Front Range climbs without a granny years before SRAM debuted its 2x10 system. I've been fairly successful at avoiding shifting out of the middle, but that granny does occasionally pay its own way.

I went to a SRAM demo day a couple of months ago at Golden Bike Shop where they were demoing a bunch of brands of bikes all set up with XX and X.0 2x10 kit.

red=mt., blue = road?

I didn't feel much difference in the rear shifting, but absolutely loved how the front just slammed left and right into the two rings without blinking, under almost any load. I loved the decrease in ratio overlap and the loss of forbidden gear combinations. However, I just don't have the budget right now to pick up a new drivetrain.

I asked the SRAM rep what the minimum equipment change would be to go 2x10 from a 3x9 system that was a handful of years old. He told me that I needed it all: Both derailleurs, shifters, crank, chain, and cassette. Maybe it's my mistrust of authority, or maybe salespeople, but somehow I just don't think that the path the derailleurs travel can be that different. I want to believe that I can get away with keeping my old derailleurs and saving some money. (It's hard to even call it "save" if you don't have it to begin with!)

Years ago, I tried making my own stick shifter (not successfully, but I'm still working on it) for a cruiser. I took apart a Shimano shifter and was surprised that the teeth on the shifter weren't the same distance apart. I was excited to see that SRAM used a 1:1 shifting ratio. I'm still not sure exactly what that means, but assume that for every unit of cable pulled, the rear derailleur moves that same unit distance away from the cassette (perpendicular to the wheel axle).

SRAM image of Exact Actuation's 1:1 ratio

My 3x9 kit seems to be called E.S.P. by SRAM and claims a 1:1 ratio. The new 2x10 kit claims the same ratio, but seems to have dropped the E.S.P. and is now called Exact Actuation. Assuming the cable pull ratio has stayed the same, I should be able to use my old derailleurs to build up a new 2x10 bike, right? I just can't find anything on the internet about the difference between E.S.P. and Exact Actuation.

If both have the same cable pull to movement ratio, the only difference can be that the path the parallelogram travels, right? And that's assuming there even is a difference. Does anyone know? Can anyone help me figure this out?

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I'm not getting any answers here, so I'm still looking. I found this interesting answer from 2008:

ESP means 1:1 ratio. For sram, anything that has a number (X.0, X.9, X.7) is 1:1. Anything with a name (Rocket, Attack) is Shimano compatible.

This is what I suspected. Buuut, I still would like to know what the difference is between ESP and Exact Actuation, and, ultimately, whether I can run X.0 9-speed derailleurs with the rest of the drivetrain being X.0 10-speed.

From what I understand, ESP was introduced on the mountain side (when?) and Exact Actuation on the road side in 2006. That made the two systems not intercompatible. For example, you couldn't run a force rear derailleur with X.0 shifters if you were putting flat bars on a cyclocross bike. Starting with the new 10-speed mountain kit, all (? maybe there's some Shimano compatible stuff left?) ESP is gone, and all SRAM stuff is Exact Actuation.

In 2006, Leanord Zinn reviewed the SRAM road kit when double tap was introduced in Velo News:

Both derailleurs get “SRAM Exact Actuation,” which is not quite the one-to-one actuation ratio of SRAM mountain derailleurs, so Force and Rival shifters are not compatible with SRAM X.0 rear derailleurs.

Now, this contradicts SRAMs information about ESP, which they state is 1:1, and Exact Actuation, which they also claim is 1:1.

In their 2008 Technical Manual, page 25, SRAM states this about mountain derailleur compatibility:

X.0,9,etc. Shifter Compatibility : SRAM 1:1 Actuation Ratio 9 / 8 speed shifters

ESP Shifter Compatibility : SRAM 1:1 Actuation Ratio 8 / 7 speed shifter

( Which would lead me to believe that they dropped the ESP moniker when the updated to 9-speed.)

And this regarding Red/Force/Rival derailleur compatibility:

RD Shifter Compatibility : SRAM Double tap shifters (Red / Force / Rival

Which makes no reference to actuation ratio and there is no reference to "Exact Actuation" in the entire manual.

And this regarding X.0/X.9 trigger shifters:

Front Trigger Shifter Compatibility : SRAM & Shimano

Rear Trigger Shifter Compatibility : SRAM 1:1 Actuation Ratio

To summarize:

  • SRAM's tech manuals say that X.0,X.9 9-speed is 1:1
  • SRAM says Exact Actuation is 1:1 [source]
  • Zinn says Exact Actuation is not exactly 1:1 [source]
  • There is a general LBS/forum consensus that 9-spd. mountain and road are not compatible
  • SRAM's tech manuals indicate a lack of compatibility between 9-spd. mountain and road
  • SRAM doesn't seem to be internally consistent with use of its ESP moniker
  • The term Exact Actuation wasn't used for the first several(?) years that the 10-speed road stuff was out.

rad accurately summarizes the surprising lack of compatibility data from SRAM. I think that 1:1 is only a design philosophy, not a mark of compatibility. Older SRAM marketing describes the advantage of 1:1 being that adjustment errors are not amplified through a 2:1 derailleur ratio. So 1:1 is just part of a bigger the equation:

  • 1:1 only describes the ratio of cable travel to derailleur travel.
  • Cog pitch = cable travel * derailleur ratio
  • Shifter indexing determines the distance the cable travels between clicks, which determines the distance the derailleur travels.
  • It's possible for different 1:1 models to use a different cable travel distance.
  • Some SRAM product summaries describe the new WiFLi 10-speed gear uses precise 3mm steps over the entire range.
  • Since 10-speed setups use a thinner chain and tighter cog spacing than 9-speed, then a 9-speed 1:1 shifter should pull a little more cable per click.
  • Some users claim to have measured average cable step distance of a 9-speed SRAM shifter at 3.1mm per click. I don't see any contradiction there.

So even if your derailleur and shifter are 1:1, you still have to match shifter pull-per-index to the distance between cog centers.