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  1. #1
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    Installing my 20 T chain ring

    On a normal 64mm bolt circle diameter 22 T is normally the smallest ring you can use. But on my 29er, there isn't a good option for a cassette with a 36 tooth cog, so to get the extra range, I installed a 20T ring on the front instead (this is a lighter weight than going to a 12-36 cassette also).

    I got the ring off Ebay for about $30, but now I see you can get the same thing for $15?
    CHAIN RING SPROCKET MOUNTAIN 20T TOOTH 64 BCD STAINLESS | eBay

    This shows the size of the 20 T compared to the normal 22T.
    Attached Thumbnails Attached Thumbnails Installing my 20 T chain ring-p1020730.jpg  


  2. #2
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    If you install the ring without modifying the crank, it almost looks like it fits, but it doesn't. The chain is resting on the crank post where the bolt inserts. The posts have to be filed down a bit to give the chain clearance. In this picture the chain looks like it's seated ok, but it will not hold and skip if you try to ride it like this.
    Note that the chainring tooth is not sticking up above the chain at the bolt location like the teeth are on both sides of the bolt. The chain is not seated down far enough because its resting on the crank post behind the ring.
    Attached Thumbnails Attached Thumbnails Installing my 20 T chain ring-p1020733.jpg  

    Last edited by smilinsteve; 10-22-2011 at 02:32 PM.

  3. #3
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    Once I filed down the post, the chain seemed to rest on the bolt head. This is not good either, so I had to file down the bolt heads as well. To file the bolt heads, I tightened them down and then colored the section that needed filing with a Sharpee. Then I held them this way, which was better than trying to hold the threads with pliers.
    Attached Thumbnails Attached Thumbnails Installing my 20 T chain ring-p1020742.jpg  


  4. #4
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    This is what the posts look like after filing.
    Attached Thumbnails Attached Thumbnails Installing my 20 T chain ring-p1020744.jpg  


  5. #5
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    After filing you can see there is clearance between the chain and bolt head. Note how the bolt head is no longer round.

    It seems to work fine now, but need a real uphill challenge to test it out. Shifting from small to big ring is slower than it used to be, but I think I just need to adjust the front derailleur a bit more.
    Attached Thumbnails Attached Thumbnails Installing my 20 T chain ring-p1020741.jpg  


  6. #6
    There's always next year.
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    Yup-- brings back memories when I had to do all that to fit an action tec 20t on my XT crank a few years ago. Good pictorial!

  7. #7
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    Quote Originally Posted by padrefan1982 View Post
    Yup-- brings back memories when I had to do all that to fit an action tec 20t on my XT crank a few years ago. Good pictorial!
    How did the 20T work out for you? Were you able to get good front shifting?

  8. #8
    There's always next year.
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    Quote Originally Posted by smilinsteve View Post
    How did the 20T work out for you? Were you able to get good front shifting?
    Similar to your experience. It was a tiny bit slower going from the 20 to the 32 middle (I was running 20-32-42), but nothing that was horrid.
    I have since taken it off, however, because when I tried a 2x9 set up, with 24-36 chainrings, I found out for my fitness, in my part of the world, and my bike, I was faster up everything with the 24 granny, and no change in in rear cassette.

    I have kept that 20t ring though, just in case I ever move to a area where climbs require it, or a different bike needs it. I like having some options.

  9. #9
    Te mortuo heres tibi sim?
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    Curious why you aren't able to do the 36t cassette?
    Florence Nightingale's Stormtrooper

  10. #10
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    Quote Originally Posted by scrublover View Post
    Curious why you aren't able to do the 36t cassette?
    For 9 speed, as far as I know, the only option is the Shimano HG 61 which is really heavy, or exotic (very high priced) Ti cassettes like Actiontec or Lucky Nino.
    It seems both Shimano and SRAM decided you don't deserve a nice lightweight cassette with 36T big cog unless you switch to 10 speed.

    So, my first attempt at lower gearing was to buy a 36 T cog from MTBTools on Ebay. You slide the 36T on your free hub, add a spacer, then use 8 of the 9 cogs of your normal cassette (remove the 11T). Well, adding the 36T steel cog made my set up weigh about the same as buying the HG 61, and it didn't shift that well, and I bent it.

    So, I figured I could just keep my light 11-34 (SRAM PG 990 305 g), and lower the gearing up front instead. This option is lighter as follows:

    PG 990 = 305
    PG 990 + 36T from MTB Tools= 440g
    Shimano HG 61 = 420g

    There's not much weight difference switching from a 22 to a 20 up front, but still, overall using the 20 up front with my 11-34 in the back saves me a quarter pound over those other two options.

    Did a serious ride today for my fist test of the new set up, with over 4000 ft climbing. It worked great. No skipping, no chain suck. I rarely used the low low combo, but it was nice to have.

  11. #11
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    Hey smilinsteve, this is a great post you made. I know this is a little late, but thanks. Question: your thoughts on aluminum vs steel freehub bodies for the large cog when you used it?

  12. #12
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    Quote Originally Posted by skim500 View Post
    Hey smilinsteve, this is a great post you made. I know this is a little late, but thanks. Question: your thoughts on aluminum vs steel freehub bodies for the large cog when you used it?
    My hubs are Bontrager race light and I don't even know if they are steel or Al.
    I never had a problem with freehub body damage from using the 36 T cog, but I didn't use it for very long.

    But I could see the concern. All the torque on the cog is applied to the small area of contact between the one cog and the FH body. It's not distributed as if it were attached to the other cogs or a carrier. So with an AL body, it might dig into it. There's probably someone on this forum who has tried it.

  13. #13
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    Thank you!

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    Well done. Just like how I did it, except I had access to a metal lathe to reduce the diameter of the bolt heads small enough to clear the chain. I'm now running an ebay 20T granny and Anderson 30T and 40T rings. Now my 29er has about the same gearing as my old 26" did. (And my old legs need this gearing to ride in the santa monica mtns.)

  15. #15
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    Quote Originally Posted by roadrider1 View Post
    Well done. Just like how I did it, except I had access to a metal lathe to reduce the diameter of the bolt heads small enough to clear the chain. I'm now running an ebay 20T granny and Anderson 30T and 40T rings. Now my 29er has about the same gearing as my old 26" did. (And my old legs need this gearing to ride in the santa monica mtns.)
    I imagine you have to go through the same filing process to make the 30T middle work?

  16. #16
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    Quote Originally Posted by smilinsteve View Post
    I imagine you have to go through the same filing process to make the 30T middle work?
    Thats right. The Anderson Machine web site shows just filing a bevel on the mounting tabs, but that isn't enough. You need to file in a parallel groove similar to the above pix to allow adaquate clearance for the chain. But once you do, it works great.

    Product Info Andersen Machine's Blog

  17. #17
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    Hey Smilinsteve,

    Got a question for you. I got my Action Tec 20t (which took forever), and filed the crank posts, it went great. I looked at the bolts and they "looked" okay (XT cranks). Should I file them anyway, and how do you know what part to file when they're not screwed down?

    Thanks. skim

  18. #18
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    I took the bolt heads down the same amount as I took off the crank posts:


    - DSCF1256

    That was about 1mm off each. Since I had access to a lathe, I just turned the heads of the bolts from ~12mm to ~10mm, but I imagine you could mark on the bolt where it lines up when tightened then remove it and grind of file a flat on the outer edge.

    I may have taken off a bit more than minimally required judging by the chain link clearance:


    But I find that chain lube (I use a dray wax lube) does tend to build up in those areas after a bit of riding, so a little extra room helps.

  19. #19
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    Thanks for this detailed info. Great pictures too!

  20. #20
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    Quote Originally Posted by skim500 View Post
    Hey Smilinsteve,

    Got a question for you. I got my Action Tec 20t (which took forever), and filed the crank posts, it went great. I looked at the bolts and they "looked" okay (XT cranks). Should I file them anyway,
    You can try using them without filing. You will know it right away if there is interference.

    and how do you know what part to file when they're not screwed down?

    Thanks. skim
    I screwed them all the way down, and used a sharpy to color the top part that needed to be removed. Then I took the bolt out and filed away the part I colored with the sharpy.

  21. #21
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    On a side note you can shape round parts very easy by inserting the part on a drill and then use a grinder/file/dremmel


    Ps: thanks for the great write up..

  22. #22
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    Great tip. Great thread.

  23. #23
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    Good pics of the modification. I tell everybody about this solution to get better gearing for steep hills, but I find most people here are afraid to work on their own bikes. I am a native Californian too, so it must be something else!
    The guy who makes those sprockets on Ebay is right here in Simi Valley. Nice guy, and makes a good product.

  24. #24
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    Interesting thread! I thought I was going to have this problem when I was planning the build of my 29er. I checked out the gearing,
    ratios, saw the 24t granny and thought to myelf:

    "The wheels are bigger, the granny ring has 2 more teeth, therefore I'm gonna have to get custom chainrings to achieve lower gears".

    In practice I've found that I get on just fine with the XT 42-32-24 setup. So much so that my beautiful Turner Flux sits in the garage whilst I ride my Salsa El-Mariachi all the time. I'd say if you are just about to swap from 26" 9-speed to 29" 10-speed don't panic. Give the standard setup a try, get used to the different pedalling cadence, and you may find (like me) that you prefer it.

  25. #25
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    Quote Originally Posted by Tea@Dimbola View Post
    Interesting thread! I thought I was going to have this problem when I was planning the build of my 29er. I checked out the gearing,
    ratios, saw the 24t granny and thought to myelf:

    "The wheels are bigger, the granny ring has 2 more teeth, therefore I'm gonna have to get custom chainrings to achieve lower gears".

    In practice I've found that I get on just fine with the XT 42-32-24 setup. So much so that my beautiful Turner Flux sits in the garage whilst I ride my Salsa El-Mariachi all the time. I'd say if you are just about to swap from 26" 9-speed to 29" 10-speed don't panic. Give the standard setup a try, get used to the different pedalling cadence, and you may find (like me) that you prefer it.
    It depends on the terrain you ride of course, but going from 26 to 29 and adding 2 teeth to your small chain ring would not be something I could just adapt to.

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