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  1. #1
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    2 chainrings 28/36 combination: is it possible?

    I'm currently riding a bike with 2X9 setup.

    I use a 9sp 11-34 SRAM cassette, with 24/36/bash Truvativ GXP chainrings. My derailleurs are SRAM X9.

    I never use the 24t chainrings as it it too small and useless for me. I'd like to change it for a 28. It would give me 28/36. 8 teeth difference.

    Is there any downside with this setup. I search the internet, forums, manufacturers sites, derailleurs manuals from shimano and SRAM and did'nt find ANYTHING about this subject. Seems like I'm the only one on earth who wants to know that.

    Does anyone can help me about that. Or maybe you use that setup and can confirm me that it works. I don't want to buy new chainrings that won't work.

    Thanks for helping me!

  2. #2
    Nouveau Retrogrouch SuperModerator
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    Quote Originally Posted by brucel33
    I'm currently riding a bike with 2X9 setup.

    I use a 9sp 11-34 SRAM cassette, with 24/36/bash Truvativ GXP chainrings. My derailleurs are SRAM X9.

    I never use the 24t chainrings as it it too small and useless for me. I'd like to change it for a 28. It would give me 28/36. 8 teeth difference.

    Is there any downside with this setup. I search the internet, forums, manufacturers sites, derailleurs manuals from shimano and SRAM and did'nt find ANYTHING about this subject. Seems like I'm the only one on earth who wants to know that.

    Does anyone can help me about that. Or maybe you use that setup and can confirm me that it works. I don't want to buy new chainrings that won't work.

    Thanks for helping me!
    If you have the chainstay clearance for the 28T ring it will work fine.
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  3. #3
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    Shiggy pointed to the only possible obstacle.

    Since you can't mount them in the middle and outer positions you'll have to mount the 28T in the granny position, and on some frames there might be a chainstay clearance issue. The 28t will extend roughly 3/8" in radius beyond your existing 24t, so you can eyeball and see if it'll clear before buying.

    Otherwise you might be able to re-space the BB or spindle to buy the needed room, which might be a good idea to improve the chainline based on a double crank using granny and mid chainrings.

    BTW- Depending on your cassette this might work out very nicely, giving you a front shift equal to 1-1/2 cassette steps, and improving the selection of usable mid range gears. I' use a 7t chainring step on my road bike for the same reason.
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  4. #4
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    Yes I'll put the 28T on the granny and the 36T will stay in the middle. The bike is already set with 2 chainrings and is centered for this, so I won't need to re-space the BB. I think there is enough clearance from the chainstay.

    I choosed to put a 28T instead of the 24T, because it's the biggest granny gear that Truvativ makes.

    Can't want to try this setup. Maybe I'll be using the granny again, which I stopped using about 9 years ago...

    Will try to think about giving you my feedback of this setup later. You'll have to wait though, because I live in Canada and I won't ride before may, when the snow will be gone...

    In the meantime, I'd be happy to hear from other people who tried it too!

    Thanks

  5. #5
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    i have a 26 in the granny and a 32 in the middle then a 38 shimano dh ring as a big ring, it shifts great, very quick. 6 tooth gaps even with no ramps or chamferd teeth shift excellent. My front deraileur is not spaced absurdly far away from the the slightly downsized big ring.

  6. #6
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    it will be a little close together though too? I run 28/38 on my 29er and thre is still a ton of overlap...

  7. #7
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    You are going to end up with a BUNCH or redundant gears. The 36/11 and 36/13 are the only additional ratios you gain on that end....and the 28/30 and 28/34 are the only unique gears on the other end. You end up with only 11 unique gears and a fairly narrow range.

  8. #8
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    Quote Originally Posted by davep
    You are going to end up with a BUNCH or redundant gears. The 36/11 and 36/13 are the only additional ratios you gain on that end....and the 28/30 and 28/34 are the only unique gears on the other end. You end up with only 11 unique gears and a fairly narrow range.

    I know that. But as I NEVER EVER use the granny, with that setup I will at least have 2 more lower ratios: as you say: 28/30 and 28/34. Better than not using the granny at all, and I don't really any gear lower than that...

    Also, I considered having just one 36T middle ring. Then I'd have to take off the front derailleur and shifter. Also change the spacing of the BB so that is is centered with the middle ring. And also use a chainguard. That's where I'm not sure. A bike mech told me it weighs as much as the derailleur, plus it causes friction...What do you think?

    I'm a cross-country rider on the edge of all mountain: the terrain here is very very rocky rootsy and technical, with lots of ups and downs, but with no significant mountains. I consider myself a good rider, I'm in good shape and I can spin and keep my momentum on the 36T ring on any climb, technical as can be...I banned the granny 9 years ago, so I don't ever shift on the front, except on my other xc hardtail with 22-32-44, which you guess I use only 32-44...

  9. #9
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    If you can get the range you need using a 1x9 arrangement, i can't see how removing the FD and shifter could cause problems or add weight. You may not need to add an outer chain guard, or can consider leaving the FD on and locking it into place by the limit screws.

    If you prefer to stay with the double front, calculate the cassette steps as percentages and use the pair of rings to create in between steps in the mid-range. For example if the cassette mid-range steps are about 10%, use chainrings that differ by 5 or 15%, to create half steps.

    BTW- if you're looking for a larger granny, one should be available, though not necessarily from Truvative. What's the BCD of your granny position?
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  10. #10
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    Quote Originally Posted by brucel33
    I'm currently riding a bike with 2X9 setup.

    I use a 9sp 11-34 SRAM cassette, with 24/36/bash Truvativ GXP chainrings. My derailleurs are SRAM X9.

    I never use the 24t chainrings as it it too small and useless for me. I'd like to change it for a 28. It would give me 28/36. 8 teeth difference.

    Is there any downside with this setup. I search the internet, forums, manufacturers sites, derailleurs manuals from shimano and SRAM and did'nt find ANYTHING about this subject. Seems like I'm the only one on earth who wants to know that.

    Does anyone can help me about that. Or maybe you use that setup and can confirm me that it works. I don't want to buy new chainrings that won't work.

    Thanks for helping me!
    I agree with the sentiment that having two rings so close together in size seems a little odd. You are only getting about 2 extra gears over a single ring setup. Seems to me you are are a good candidate to go 1x9, or going with a larger big rig, like a 38 or 40.

    OTOH, if it works for you, go for it!

  11. #11
    Nouveau Retrogrouch SuperModerator
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    Quote Originally Posted by brucel33
    I know that. But as I NEVER EVER use the granny, with that setup I will at least have 2 more lower ratios: as you say: 28/30 and 28/34. Better than not using the granny at all, and I don't really any gear lower than that...

    Also, I considered having just one 36T middle ring. Then I'd have to take off the front derailleur and shifter. Also change the spacing of the BB so that is is centered with the middle ring. And also use a chainguard. That's where I'm not sure. A bike mech told me it weighs as much as the derailleur, plus it causes friction...What do you think?

    I'm a cross-country rider on the edge of all mountain: the terrain here is very very rocky rootsy and technical, with lots of ups and downs, but with no significant mountains. I consider myself a good rider, I'm in good shape and I can spin and keep my momentum on the 36T ring on any climb, technical as can be...I banned the granny 9 years ago, so I don't ever shift on the front, except on my other xc hardtail with 22-32-44, which you guess I use only 32-44...
    Since you never use the inner ring now and are happy with it there is no reason at all to mess with the chainline if you went to a single ring. Effectively, you are already doing that.

    You would not "need" a DH style chain guide either. You have the outer guard ring now. You might need a N-Stop or similar on the inside, but probably not.

    I see no reason not to stick with your plan and just mount the 28T inner ring and have the 2 bailout gears, just in case. Cheap, easy and gives you options.
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  12. #12
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    I think the 2.5mm change in chainline is not really worth going through much effort to address. 100% of three ring outboard cranks are 2.5mm 'too wide' when used with a standard 9 speed 135mm rear hub....but it is generally very easy to address and if it is effecting shifting or wear, or you simply want to be 'on spec' then take the 10 minutes to swap the spacers around.

    As for a 'guide', since you do not now use or need any kind of tensioner or bash, I dont see why you would need one if you ran only a single ring (you would end up with less chain tension and chainline variation). You could just lockout your front derailleur, or use something like the e-13 XCX if you needed the extra chain retention.

    Either way, I just wanted to make sure you were aware of the range and overlapping. If the two ring system gives you the spread you need, that is all that really matters.

  13. #13
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    mathematicly the overlap is there but realisticly you shouldn't use the big/big and small/small anyway, since you shouldn't use those gears on whatever range of front gears you have, you can't really count that as part of the overlap. Also "overlap" is not necessarily a bad thing,if you were climbing in the 28 and as the hillcrests you start to shift up on the cassette at some point in the middle of the cassette you shift to the large ring, the jump won't be to big, it'll be a smooth quick shift and if it feels a little to high it won't take much shifting on the cassette to find a comfortable gear. The same is true for a big ring climb,if you decide to bail to the little ring. Don't fear the overlap,if make an effort to use all the gears anyway,your drivetrain may last longer instead of wearing out one chainring and having the other one brand new doing nothing,use mostly the big ring and the top half of the cassette early in the ride and mostly the smaller ring and the bottom half of the cassette for the second half, something like that.

  14. #14
    Nouveau Retrogrouch SuperModerator
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    Quote Originally Posted by herbn
    mathematicly the overlap is there but realisticly you shouldn't use the big/big and small/small anyway, since you shouldn't use those gears on whatever range of front gears you have, you can't really count that as part of the overlap. Also "overlap" is not necessarily a bad thing,if you were climbing in the 28 and as the hillcrests you start to shift up on the cassette at some point in the middle of the cassette you shift to the large ring, the jump won't be to big, it'll be a smooth quick shift and if it feels a little to high it won't take much shifting on the cassette to find a comfortable gear. The same is true for a big ring climb,if you decide to bail to the little ring. Don't fear the overlap,if make an effort to use all the gears anyway,your drivetrain may last longer instead of wearing out one chainring and having the other one brand new doing nothing,use mostly the big ring and the top half of the cassette early in the ride and mostly the smaller ring and the bottom half of the cassette for the second half, something like that.
    Big/big in this case is just fine. It is just a triple crank without the outer ring so the "big" ring is the middle ring which can be used on all cogs.
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