SRAM Conversion

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SRAM Conversion

Postby surfin' addiction » Thu May 24, 2012 11:48 am

Now, before anyone flames me for bringing up a topic which has been done to death (Sram Vs. Shimano), I have done a search but everything I have found has been to do with higher end components. Unfortunately I'm not at that standard so I am looking for lower end advice.

I haven't been MTBing for very long now, I just started recently as an extension of my road cycling as a way to get out and see some different scenery and try something new.I have a 2007 Norco Sasquatch but I recently damaged the rear derailleur.

My question is, I was thinking of converting the bike over to SRAM X7 from the Shimano Deore. I am wondering how easy this will be and how much other stuff I will need to change other than the derailleur and shifter. I currently have as I said, a Deore rear derailleur and front shifter and the read hub is also Deore. To run SRAM X7 which is 9 speed instead of 8, will I need to replace the rear hub to accommodate a 9 speed cassette? Also, the current Deore derailleur is a long cage. What is the difference between running, long, medium or short cages?

If you are able to answer my questions and advise on a parts list that would be great. Most work I can do myself except re-lacing the rear wheel if the hub requires a change.
-Josh-

Whips: Merida Racelite 905, Apollo Record Track, Norco Sasquatch.
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by BNA » Thu May 24, 2012 1:04 pm

BNA
 

Re: SRAM Conversion

Postby WestcoastPete » Thu May 24, 2012 1:04 pm

Gudday mate.

You can swap out the cassette to a 9sp easily; you won't need a new hub. SRAM or Shimano cassettes will work.

You'll also need a 9sp chain, RD and shifter. That's all.

Long/Medium/Short cage is dependent on the cassette range and chainring size.

Here's a great explanation from a thread on mtbr.com:

Speedub.Nate wrote:
Quick answer: The medium cage will work, but you'll drop your chain if you accidentally shift to the small-small combo. Suspension *could* be a factor, depending on how much "chainstay growth" your frame experiences as your suspension cycles.

Long answer:

Derailleurs have a rated capacity. This is their ability to take up excess chain. After all, you need just about all of your chain to run in the big-big combo, whereas you have a bunch of extra links doing nothing when you run in your small-small combo.

Not that either of those cross-chain combos are normal to run in, but let me get to that in a minute.

Manufacturer stated derailleur capacities are as follows:
Shimano long = 45T; medium = 33T
SRAM long = 43T; medium = 37T; short = 30T

Speaking from experience, Shimano is a bit conservative in their capacity rating. I can only assume the same is true of SRAM (I'll get to that, too).

The easy capacity formula is to add your big ring & cog sizes, then subtract your small ring and cog sizes. It looks like this:

cap req'd (T) = (BIG ring - small ring) + (BIG cog - small cog)

...so for a typical 44-32-22 mountain crank & 11-34 cassette...

T = (44T - 22T) + (34T - 11T)
.. = (22T) + (23T)
.. = 45T

Using this simple forumla, you would need a derailleur with a 45T rated capacity to absorb all the possible extra links of a typical 27-speed drivetrain.

(I make the assumption SRAM stated capacity is conservative, since they list 43T as the long cage capacity -- 2T short of what is required by this forumla).

Where do shorter cage lengths come into play? Right here!

Even though the long cage will, in theory, take you down to the 22x11 gear combo and hold adequate chain tension, let's be logical: 22x11 is a combo you don't use!

Rather than use the generic formula, let's map out the capacity for each gear combination (based off of a Shimano cog pattern; SRAM will be slightly different):



44x34 starts off at zero because in that combo, all of the chain is being used up by the ring and cog, and the derailleur needs to take up none of it. As you shift through the cassette range (moving down the column), the amount of free chain increases as the cog size decreases.

Take a look at the useable gears, which I've outlined in green and yellow. Those fall near the stated capacity of the medium cage derailleurs. (I mentioned that Shimano's stated capacity is conservative, and in practice, I find their medium cage to be closer to 39T.)

For instance, in the middle ring (32) and the small cog (11), the table shows you've got to absorb 35T. This is near the stated capacity of either of the medium cage derailleurs. This gear combo remains useable, but you'd be better off shifting to your big ring for better chain tension.

You can also see that to use a SRAM short cage derailleur (30T capacity) on this drivetrain would leave you with two or three unusable gears while in the middle ring, and only about three useable gears from your granny ring. (Any number greater than 30T on the table would be near the limits of the short cage derailleur.)

Oops! Accidentally shifted into the unusable "red zone"? Nothing major: the derailleur cage folds back on itself, the chain droops, and you maybe drop the chain if you don't catch it in time.

In my opinion, it'd be stupid to size a chain any smaller than what is required to shift into big-big. If you accidentally force a shift into that combo, which is certainly possible when you're tired or "in the moment", you don't want to break anything. So chain length will be the same no matter what derailleur you choose.



Benefits of a shorter cage length?
- snappier shifts
- better chain tension
- less chain slap / greatly decreased drivetrain noise (!)
- better obstruction clearance / improved spoke clearance.
- slight weight loss -- but you gotta be a real weight weenie to appreciate this one.
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Re: SRAM Conversion

Postby surfin' addiction » Thu May 24, 2012 2:42 pm

Wow! Thanks WestcoastPete, that was quite a bit of reading. I feel better now than I did when I spoke top me my LBS and they were like it'll coast about $550 do a conversion, when really it's probably only $200+ if you shop around for parts. Knowing I don't need to change a hub is a help too.

I am still a little stumped though as to which length cage I could use. I am currently running a 22T/32T chain ring with a bash guard but was going to drop one and have say the 32T and a chain guide and would be maybe looking at a 11-32 or maybe even an 11-26 9 speed cassette. So I am guessing either way I look at it I'll need a long cage derailleur unless I used a different set of gear ratios.

I may need to hunt around a bit, see what's available and then buy a kit which is all compatible.
-Josh-

Whips: Merida Racelite 905, Apollo Record Track, Norco Sasquatch.
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Re: SRAM Conversion

Postby WestcoastPete » Thu May 24, 2012 11:35 pm

You should be fine with a medium cage RD.

(32-22) + (32-11) = 31. SRAM medium cage RD capacity is 37.

That said, if you ever wanted to put a big ring back on, you'll need a long cage, and your current setup will work fine with a long cage, so it might be worth just using one of them.
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