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spin... spin.. spin
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Discussion Starter · #1 ·
“Ask the French girl, she’ll know.” I hear one of the younger guys in the shop say. It is a compliment. The shop has two service techs and my colleagues coming to me for the harder questions is really something that I don’t expect but find to be happening a lot. This time they are wrong, I don’t know the answer but at least I know how to figure it out. To be fair I know right away what to do but what to do wasn’t the question they chose ask of me.

As the industry races to catch up with demand there is still a huge parts shortage. Today the guys are scratching their heads over a mountain bike with SRAM 11 speed group set. The chain and cassette are well beyond needing replacement. The issue is there are no 11 speed mountain bike cassettes left in stock. And 11 speed road cassettes don’t for the most part fit on HG mountain bike freehubs. Not only are these all out of stock in the shop but also in the supply chains. The soonest they will be available looks to be February. So what’s being discussed is compatibility.

“Rachel, will a 10 speed cassette work with a 11 speed shifter and derailleur if we just tighten up the low limit screw so the eleventh shift can’t accrue?” A question that makes me want to respond with, ‘eleven speed shifters only shift ten times.’ No one likes a smart ass so I refrain.

The answer of course is not a simple yes or no. Well, it is that simple but I don’t want to give away just yet why I knew I could make it work. It’s a complex matter of maybe. Or better yet the real answer is, depends. To truly answer with a definitive yes or no one has to understand how shifting a bike works and then delve into compatibility. Of course we could throw it on the wheel and check but then we’ve sized a ten speed chain that can’t be un-shortened. Proof of this is un-shortened is in fact not even a word. Also, it seems to me in the very least to be unprofessional to just guess. Also, if not shifting smooth is it compatibility or installation we need to question? Nope, it’s just better to know it will work before turning wrenches.

“Guys,” I begin. “Compatibility isn’t simple but can be made more so. To do this we need first understand that shifters have an amount of cable they pull and derailleurs have a movement ratio or actuation based upon how much cable a shifter pulls.”

“What??? I thought derailleurs all move the same amount and it’s just shifters pull different amounts of cable.” Says a slender mechanic with greying hair.

“I wish it were that simple. SRAM did that with 1:1 on their 7, 8 and 9 speed kit. But times have changed and the truth is, shifters have a length of cable they pull, then derailleurs have an amount of movement that occurs when cable is pulled. For sake of making this slightly understandable. Let’s make it super complex by looking at why Campy doesn’t play well with others. A ten speed Campagnolo shifter pulls 2.5 mm of cable five times, 3 mm of cable twice and 3.5 mm twice. Nine shifts to cover the ten cogs on the cassette. Why the amount of cable pulled changes is simply why Campagnolo shifts smoother in all gears than other brands. A Record ten speed rear mech has an actuation or ratio of 1.5. Meaning it moves 1.5 mm of each mm pulled by the shifter. So yes your bottom six cogs on a Campy cassette are in fact closer together than the last four. Mix the spacers about on a Campagnolo cassette and it will shift like crap. Use a Shimano cassette with equal cog pitch through out and it won’t have the Campy precision and will shift more like Shimano, well sorta. No one else does this different pull as the chain moves up the cassette besides Campag. Which we will be grateful for when trying to sort out if any or which ten speed cassette has a cog spacing that will work with SRAM eleven speed shifter and derailleur.” I explain to two coworkers who have tuned me out and nod but clearly don’t follow what I’m saying or even want to.

“Sounds good. Let us know what will work then when you have it all sorted.” Says the younger of the guys. They walk away leaving me with a fun adventure. That again I already know how to solve. The final solution so simple it will irritate everyone who never thought of doing it.

It takes me a second to calm down. I love math and am beyond excited at the chance to delve into SRAM X Actuation and what is going on with it. For years SRAM used a 1:1 rear derailleur ratio. Meaning if your lever pulled 3 mm of cable the mech moved that exact space. Then when they came out with ten speed they went to Direct Actuation and finally mtb X Actuation came out with eleven speed. But what is the actual ratio of X Actuation? I was excited to find out.

According to SRAM via the service tech portal eleven speed mountain shifters pull 3.5 mm of cable with all ten pushes. For each of those pulls the X Actuation derailleur moves exactly 1.12 times the 3.5 mm of cable pulled. Taking 3.5•1.12 gives us the exact distance a chain will move each shift and thus the distance the individual cogs have to be apart on the rear cassette. Give or take a small discrepancy. In minutes from the start I know I need a rear cassette that has a spacing of 3.92 mm between the gears. I also know I could technically take any cassette and use the 3.90 mm spacers from any SRAM 11 speed mountain cassette, as I’ve known since they asked me the initial question. But that is no fun. I know the spacers are this thickness because my callipers tell me this when I measure them and SRAM specs claim their own 11 speed mountain cassettes have a spacing of 3.90. Thus .02 must not matter for it to work. Campy would disagree but enough about how much more precise they insist things be. A quick look tells me a SRAM road cassette won’t work. The spacing between the cogs is a much wider 4.0 mm. Obviously it would make the shifts but not smoothly and without some unwanted clatter.

I happen to know the cog pitch for Shimano 11 speed mtb cassettes is also 3.90 thus Shimano and SRAM 11 speed cassettes work with each other’s shifter derailleur combos. Again I know because I measured the spacers of an old cassette we had laying around. But the shifter derailleur must both be either SRAM or Shimano, no mixing matching allowed. Because Shimano mountain eleven shifters pull 3.6 mm of cable and the derailleur ratio is 1.1. 3.6•1.1 is 3.96 so this tells me .06 mm clearly is not enough to cause shifting issues as their cogs are 3.90 mm apart both measured and according to S-Tech (Shimano's service portal).

With no eleven speed cassettes available the search begins for a cassette with a cog pitch of 3.90. If I don’t find one obviously I have the spacers from used 11 speed cassettes to build my own.

A while later I discover Shimano ten speed road and mountain cassettes have a cog pitch of 3.95 as does all SRAM ten speed cassettes. Since I know the shifter derailleur combo moves a chain 3.92 mm and this works on a cassette with a cog pitch of 3.90 my calculations tell me in fact a SRAM 11 speed mountain bike shifter and derailleur pull nearly as close to the pitch on a 10 speed cassette as they do to the one they were designed for.

I spare the lads the headache of hearing me ramble on about how I know and grab the bike throw a ten speed cassette on it. Set limit screws so the final shift can’t be made and cycle it shifting perfectly on a test ride. Then because SRAM specs say it should be 3.90 pitch I swap out the spacers and reassembled the bike shifts flawlessly but not noticeably different than with the 3.95 mm spacers. In the end doing what I had all along planned to do. Only the spacing matters and if one uses the spacers found between 11 speed cassette cogs obviously the number of cogs has nothing to do with how smoothly it will shift. Of course the chain and cassette have to be paired. Well kinda. But that’s for a different discussion.

Handwriting Writing implement Pen Office supplies Font


Not recommending running miss matched kit. But in this case it gave a customer his bike back working. It is his only mode of transportation and he was preferable to this very affordable solution over waiting months for the correct part which is substantially more expensive. And it was the best part of my day figuring it out.
 

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Deranged Touring Cyclist
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5,585 Posts
Sprocket, you are amazing. Thank you for sharing this. Mixing kit in this way is easy to look down upon, but the supply situation is what it is. Better mismatched kit than a bike which can't safely be ridden. So awesome that you could figure a solid workaround and get someone's steed back on the road. Way to go!
 

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Two skinny J's
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21,749 Posts
Schooled :D
 

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Spin Spin Spin
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4,187 Posts
The whole red bike I cobbled together is a mismatch festival of road and mountain parts.......minus the math and with extra see what happens after turning the wrenches. Way less scientific uglier luck based success in this case, lol.

dang, demz is spensive cassettes! why are 12's so much cheaper?
 

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spin... spin.. spin
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1,759 Posts
Discussion Starter · #7 ·
Sprocket, you are amazing. Thank you for sharing this. Mixing kit in this way is easy to look down upon, but the supply situation is what it is. Better mismatched kit than a bike which can't safely be ridden. So awesome that you could figure a solid workaround and get someone's steed back on the road. Way to go!
Totally said it. Not ideal but times are what they are.
 

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spin... spin.. spin
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1,759 Posts
Discussion Starter · #8 ·
The whole red bike I cobbled together is a mismatch festival of road and mountain parts.......minus the math and with extra see what happens after turning the wrenches. Way less scientific uglier luck based success in this case, lol.

dang, demz is spensive cassettes! why are 12's so much cheaper?
It’s the same thing. Really on my own bike I would have just tried things until something worked. Then well. I may have wanted to know why anyways because I’m slightly off my bean. Really I only did the maths to make sure because it’s a customers bike.
 

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Spin Spin Spin
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4,187 Posts
oh, I have to know how things work......even if it won't go back together afterwards......then youtube, lol....
 

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tall old member
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1,478 Posts
Some of that explaining the math lost me but I read it clear through to the end because I love listening to your French accent.
The only cassette I mixed was about 16 years ago I had a nine speed Trek road bike and it wasn’t geared low enough for western Pennsylvania hills. I had a hilly century coming up in two weeks so I found a SRAM nine speed cassette with a thirty tooth big cog. I ordered it assuming it would work. It did sort of. The short cage derailleur was really stretched in that big cog and made a clicking sound. I ordered a long cage derailleur with two day shipping and it came in the day before my century and it worked perfectly. Dumb luck I guess because I didn’t know what I was doing and figured I would learn.
 

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Spin Spin Spin
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4,187 Posts
Yeah, that noise is the cassette cog and der pulley too close when der can't handle that many toofessess.......
 

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spin... spin.. spin
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1,759 Posts
Discussion Starter · #13 ·
Oh I forgot to mention the Pro’s Closet where I bought my 11-40 cassette for my Lynskey had 11 speed mountain bike cassettes in lots of sizes when I bought mine.
Thanks for suffering through my maths. Pro Closet does have many it seems! Thanks for that tip.

Always great when we just try things on our bikes and sort out fun stuff that works. Playing with the bike mechanically can be as fun as riding it.
 
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