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Discussion Starter · #1 ·
I'm running 53/39 front chain rings and 11-25 cassette - all 7800 DA. Can someone please tell me how large I can go on the rear cassette (27, 28??) with the standard rear dreailer and whether it is possible to get a smaller front chain ring (less than 39) without also having to change the large chain ring.

I'm getting old and need some relief on the bigger hills.

Thanks
 

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roger9 said:
I'm running 53/39 front chain rings and 11-25 cassette - all 7800 DA. Can someone please tell me how large I can go on the rear cassette (27, 28??) with the standard rear dreailer and whether it is possible to get a smaller front chain ring (less than 39) without also having to change the large chain ring.

I'm getting old and need some relief on the bigger hills.

Thanks
Assuming you have a short cage RD, the max tooth capacity is 29T and the max cog size is 27T. Between your current (53/39) chainrings and a 12-27 cassette you'd be up to a total tooth capacity of 29.

Although Shimano's specs err on side of caution, your real limiting factor here is that you have to use a cs-7800 cassette IF you're also using a FH-7800 freehub, because it has taller splines requiring the use of the cs-7800 cassette. If you're using a different model freehub, you can probably go with another compatible brand/ model (SRAM, 105, Ultegra) offering up to a 28T cassette, but changing to a smaller inner ring will likely cause a problem because you'd be above a 30T total capacity.
 

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Adding to PJ532's advice...you could go one tooth smaller on the inner ring, but be advised that 38t is the smallest allowed by the bolt circle diameter of your crankset.
 

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Discussion Starter · #5 ·
PJ352 said:
Assuming you have a short cage RD, the max tooth capacity is 29T and the max cog size is 27T. Between your current (53/39) chainrings and a 12-27 cassette you'd be up to a total tooth capacity of 29.

Although Shimano's specs err on side of caution, your real limiting factor here is that you have to use a cs-7800 cassette IF you're also using a FH-7800 freehub, because it has taller splines requiring the use of the cs-7800 cassette. If you're using a different model freehub, you can probably go with another compatible brand/ model (SRAM, 105, Ultegra) offering up to a 28T cassette, but changing to a smaller inner ring will likely cause a problem because you'd be above a 30T total capacity.

Thanks (ditto Silver Star)

I'm actually using a Cannondale Si crankset so I assume it too is limited to 38T for the front due to BCD.

I do have a short cage RD - I'm not sure what you mean by "total tooth capacity" or how it is calculated. Max cog size speaks for itself.

I'm using Mavic Ksyrium SLs - will these take the 28T.


Thanks in advance
 

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roger9 said:
Thanks (ditto Silver Star)

I'm actually using a Cannondale Si crankset so I assume it too is limited to 38T for the front due to BCD.
Is it the alloy crankset which has a removable spider? If so you could get a compact spider and 50/34 rings.

I do have a short cage RD - I'm not sure what you mean by "total tooth capacity" or how it is calculated. Max cog size speaks for itself.
Capacity is calculated by taking the difference between the chainrings (53-39=14) and the difference in the cassette (27-12=15) and adding them together giving 29 total. The compact set up would be (50-34)+(25-11)=28.

I'm using Mavic Ksyrium SLs - will these take the 28T.
No problem.
 

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Just an observation, but depending on how much 'help' you're looking for on the climbs, what you propose (IMO) may not buy you much.

If you are able to either convert your existing crankset to compact or replace it and run either a 12-27 or 11-28, that may be a better option. The latter may be pushing it a little because you'd be at a total tooth capacity of 33, but apparently there are people running this combo without problems. A 12-27 would be another option and would drop tooth capacity to 31.

Here's something that may help you decide:
http://sheldonbrown.com/gears/
 

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roger9 said:
I'm running 53/39 front chain rings and 11-25 cassette - all 7800 DA. Can someone please tell me how large I can go on the rear cassette (27, 28??) with the standard rear dreailer and whether it is possible to get a smaller front chain ring (less than 39) without also having to change the large chain ring.

I'm getting old and need some relief on the bigger hills.

Thanks
I currently run a 30T cog on an Ultegra 10 speed short cage RD; it runs fine... at least on my Giant TCR C (based on the factory RD hanger length; yours may be different). The chain length is the same I had for a 27T cog but I NEVER use the Big/big combo. I have also tried a 32T cog but it rubs on the guide pulley. It just needs about 1 to 2 MM more of space to keep from rubbing.
Things that I have not tried in order to run a 32T cog: (just to mess around to see what the real limits are)
a longer B screw.
a 10T guide pulley (instead of the factory 11T guide pulley)
A longer RD hanger
 

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Inner chainrings are pretty cheap. Cassettes are often cheap... well okay, not at your Dura Ace level, assuming that's the only thing that's compatible w/your 7800. I'd try 38t on the inner ring, and 12-27 on the cassette. That'll give you about a 10% lower low gear.

If your RD has enough capacity (and you can often get away with slightly exceeding capacity), and/or your drivetrain can take non-DA-7800 cassettes, then things like 11-28 and 12-28* become options too, though I think the 15-17 jump in the 11-28 is pretty sucky.

If that isn't enough, then yeah, time to suck it up and spend the money for a compact crank... or even a triple, though that's a last resort, due to the additional expense of new derailleurs and new shifters on top of the crank's cost. :nonod:

/ Some ppl also get low gears by running big-ass mountain bike cassettes with double cranks (and Shimano has come out with new, 10-spd MTB cassettes)... assuming no odd compatibility issues, that could work, but that's a new rear derailleur along with the cassette, and the big jumps between cogs will likely be annoying for road riding. Some ppl also say that such setups tend to shift poorly on short-chainstay bikes.


* 12-28 is allegedly a new SRAM cassette that's coming out in a few weeks. IRD already makes one, but it has the lame 15-17 jump
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Discussion Starter · #10 ·
Thanks to everyone for their input - great to be in a position to make a decision based on solid advice.

I think I'll start with a 12-28 cassette and see how it goes. It's an easy task to swap cassettes when I don't foresee hills and the "big" cassette can sit in the top drawer waiting for service when required.

Ride on !!
 

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roger9 said:
I think I'll start with a 12-28 cassette and see how it goes. It's an easy task to swap cassettes when I don't foresee hills and the "big" cassette can sit in the top drawer waiting for service when required.

Ride on !!
Cool, let us know how it goes. :cool:

Btw, according to SRAM's website, there's two versions of that 12-28 cassette.... one that goes 21-24-28 for the last 3 cogs, and one that goes 21-23-28. The former would be a lot better IMO.

And again, they're not out yet, but allegedly soon.
.
 

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Discussion Starter · #12 ·
SystemShock said:
Cool, let us know how it goes. :cool:

Btw, according to SRAM's website, there's two versions of that 12-28 cassette.... one that goes 21-24-28 for the last 3 cogs, and one that goes 21-23-28. The former would be a lot better IMO.

And again, they're not out yet, but allegedly soon.
.


Yeah, I agree it's probably worth the wait and the 21-24-28 would be better.
 
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