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Discussion Starter · #1 ·
I'm looking at the out of the box SRAM XX chainring combos and I'm thinking I may want to mix and match based on my riding. Offered are a 26/39, 28,42 and I think a 30/45. I was thinking a 30/39 would be ideal for the type of riding I would do. Don't think I need more or less than that and a 30/39 would keep me from switching the front chainring as much.

I could buy the 26/39 crank, then just buy the 30 small chainring separate.

Today the guys at the shop told me that their SRAM rep told them we shouldn't mess with the gearing as they were done that way for a reason. Putting a 30 with a 39 is going to cause me to loose something, maybe shift performance, but we didn't really nail it down. I trust them but just want to understand the reasoning. Anyone from SRAM out there? Or anyone chased this question down? What do you loose and if you know why?

Thx
 

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Discussion Starter · #3 ·
rockyuphill said:
Most likely the shift ramps won't be sync'd for mix and match ring combos.
Starting to make sense to me the more I think about it. I'm just a little slow sometimes. Guessing it based on angles.
 

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If all you need is a 39 for your big, just go with the 26/39. It is much more often that people need more low gear than high. The difference between a 26 and a 30 for your small ring is 1 cog on an 11-34 cassette (26-11 is about the same as 30-13). Wish they came out with a "timed" set of 24/36 as well. I have no need for anything bigger than a 36.


G
 

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If you use your mtn.bike for anything other than riding on the trails then a bigger chainring is a nice option. I have to ride 30 minutes on the road to get to any decent trails. I also have a paved bike path that is a blast to ride on. I can spin out of my 44/11.
We even have some "mtn.bike" races like Paris-Ancaster that have large sections of smooth trails that you can absolutely use a big ring on.
 

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Discussion Starter · #9 ·
I'm still not understanding this though.

Tnanks for the responses but the conversation took a little detour with why would I want 30/39 but and that is not what I was asking. I don't want/need a 26 especially if I have a 36 in back. If I had a 30 on the back I could stay there longer without having to shift the front rings. I don't live in a place where I need a 26/36 combo to crawl up a 5 mile hill. And I don't need a 42 or 44 either as I don't commute on this bike. 30/39 is just my preference if I could have it.

So hoping someone can still help me with the original question of why does the shift performance degrade if I would change the 26 ring of a 26/39 combo to a 30 so I have a 30/39.

I had read the content on the SRAM site
http://www.sram.com/en/XX/engineering/x-glide.php
but it doesn't point out the why.

Here's all it really says "With X-Glide, all four upshift locations are identical and can be picked up by any outer link. " and I'm not sure what that has to do with the size of the small chainring.

From what I can see the front ring has no ramps and is just vanilla. Maybe there is something on the thing??
 

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The ratios between large/small in the XX cranksets is due to the ideal timings between the teeth on the small/large sprocket.

In short - there is a predefined, least effort, angle between two chainwheels that results in the cleanest shift between them. In other cases you will get noisy shift, a skip when a chain loses tension when it mashes to a new cog, or even a misshift.

On the circumference of a chainwheel you can get several possible locations of this "perfect" shift path. The precise amunt of these is determined by lowest common denominator of the chainwheels.
Example: 22/32/44 chainset
22/32 have the lowest common denominator of 2, which means that between those two only 2 perfect shift paths exist. In effect you might need to spin half of a revolution to make a shift.
32/44, these have LCD of 4, thus 4 perfect shift paths can be put.

Obviously, manufacturers want to optimise the perfect path by adding pins/cuts etc to aid the chain in making a shift (esp upshift).

The cool part about sram XX is that its chanring sizes were chosen to get really high number of perfect paths. In the 26/39 crankset the lowest common denominator of the sizes is 13, which means you need to make just 1/13 crank revolution to make a shift, which results in really quick shifting in the front. Sram additionally machined the large chainring to even further speed up the shifts in 4 locations.

On the 28/42 you get 14 perfect paths.

Now imagine you replace granny on 26/39 to 30/39. In the new setup lowest common denominator is just 3, resulting in just three possible perfect paths. And to add instult to injury, these 3 paths might not be even timed, like in the original crankset, thus you get nothing...

TL DR - XX cranksets have their chanrings mated for perfect shifting performance. Replacing them incorrectly ( without paying attention to sizes or orientation ) may, and probably will, severely degerade front shifting performance.
 

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Discussion Starter · #11 ·
uzurpator - good stuff and I'm thinking I'm finally starting to visualize it. Appreciate you taking the time to explain in terms I can understand.

You are saying that a perfect shift involves a specific position on both chainrings? If that is the case then I see the reasoning. This piece of info was key.

So playing back what you just told me.....
For an upshift that position perfect position would be every other tooth on the bottom ring and every third tooth on the big ring. Given the ratio each position would have constant properties of angle distance etc and we have more of those positions. I guess the distance is an exact set of chain links too and if the rings had a different ratio that distance would be hard to maintain constant - or only at 2 or 3 positions. Same applies for the downshift. Every 3rd tooth on the big ring would then be not used for shifting purposes. This is what is shown in the diagram with the A in the middle.

That diagram with XXPONETIAL INOVATION in the middle lead me to believe there were only 4 places where a pickup or release would occur. Based on my understanding now this drawing is incomplete....maybe only 4 were shown for clarity but in this case it led to me not understanding.
 

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The gearing is so teeth align for quicker shifting
26 is 2 x 13, +13 = 39
28 is 2x 14, +14 = 42
By having rings with teeth of equal multiples it allows every other tooth to line up with one on the other ring.
Just had a SRAM XX clinic.
 

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Discussion Starter · #14 ·
Musturd said:
The gearing is so teeth align for quicker shifting
26 is 2 x 13, +13 = 39
28 is 2x 14, +14 = 42
By having rings with teeth of equal multiples it allows every other tooth to line up with one on the other ring.
Just had a SRAM XX clinic.
Yeah - thinking you are saying the same thing that uzurpator said and I finally realized. SRAM could have created a better graphic to show that. I'm convinced now at least in principle. We'll see how it works on the trail.

This graphic still puzzles me though. They talk about 4 ramps but 14 optimal shift locations. 4 != 14

https://sram2x10.com/wp-content/uploads/SRAM_MTB_MY11_10sp_011110_6.jpg

This one does show that you need equal number of teeth on the big/small ring and why the ratio is important but doesn't show how the chain lays over each ring when it shifts.

https://sram2x10.com/wp-content/uploads/SRAM_MTB_MY11_10sp_011110_7.jpg
 

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I have a WAY easier solution (that should work).

You're looking to up your small ring to make it harder to pedal, but want the big ring to stay the same. Instead of a 26 you want a 30. But you'll keep the 39.

Screw the 36-11t cassette and get a smaller one. Won't a 10spd road cassette work? Try 11-32 or a 11-28:eekster:

Ttyl, Fahn
 

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Is there no other option to mix wirth just two or three pickups? I never new i needed more than the two i have now with xtr, let alone 13.

Will 26.42 or 26/28.45 work?

As is, the only way I can see me moving to xx is buy buying two cranks and/or a second spider and swapping cranks like the pros ... Is this maybe the master marketing plan???

Looking at sheldon browns ratio calculator, i could easily go xx on 26/42, and would absolutely fly on 26/45, and would not care as long as at least 1 pickup point
 

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26/42 will not give youany pickup points for clean shifts. You'll lose most of the benefit from XX. As a bonus, since SRAM expects you to have the consecrated chainring combos, the front der is most likely shaped to take that into account.16T difference is evil anyways.

45/26 i 19T difference, so no existing front der will shift that, and if you need that, then, sir, you need a triple ;)
 

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THey are maintaining a size ratio between small and large ring, so the teeth line up in 14 specific locations and give consistent shifts ...



riding29 said:
I'm still not understanding this though.

Tnanks for the responses but the conversation took a little detour with why would I want 30/39 but and that is not what I was asking. I don't want/need a 26 especially if I have a 36 in back. If I had a 30 on the back I could stay there longer without having to shift the front rings. I don't live in a place where I need a 26/36 combo to crawl up a 5 mile hill. And I don't need a 42 or 44 either as I don't commute on this bike. 30/39 is just my preference if I could have it.

So hoping someone can still help me with the original question of why does the shift performance degrade if I would change the 26 ring of a 26/39 combo to a 30 so I have a 30/39.

I had read the content on the SRAM site
https://www.sram.com/en/XX/engineering/x-glide.php
but it doesn't point out the why.

Here's all it really says "With X-Glide, all four upshift locations are identical and can be picked up by any outer link. " and I'm not sure what that has to do with the size of the small chainring.

From what I can see the front ring has no ramps and is just vanilla. Maybe there is something on the thing??
 

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SRAM XX Chainring differences

I have a GXP XX crankset and need a 39 ring and there seem to none to be had anywhere. The BB30 XX 39 ring is available. What is the difference? Can it be used on a GXP XX Crankset? If not, why?

There seems to be some pretty knowledgeable XX people here and I was getting no response on the drivetrain forum.
 
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