# Compact with SRAM Force - How big cassette?



## AJ88V (Nov 14, 2012)

Want to buy a new cassette for my training wheels so I don't need to switch over. Current setup is (stock) SRAM compact front (50/34) with a Force rear derailleur on a 12-25.

I was thinking it might be nice for the spare cassette to be useful for bigger climbs, say 27 or 28 rear. The Shimano Tiagra 10-speed cassette is way cheaper than anything else and barely heavier than my SRAM. Available ratios are 11-25, 12-28, and 12-30. The 28 is 11% bigger and the 30 is 20% than my current setup.

1) Will either the 12-28 or 12-30 work with the current derailleur (Force only comes with short cage)?

2) Is there any reason why the bargain Tiagra 10-speed cassette is a particularly poor choice?

Many thanks.


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## aclinjury (Sep 12, 2011)

if it were me, I'd go for the 12-28 if I expect to climb anything longer than 2 miles at 7%+. 

and 12-30 if expecting 7%-10%+ for anything longer than 5 miles.

Makes very little sense to use a compact with a 12-25 cassette unless all you do is 2%-3% rollers all day long and nothing else.


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## tom_h (May 6, 2008)

You need to lookup your rear derailleur's "chain wrap" spec at SRAM website.

Chain wrap = (difference in # chainring teeth) + (_max _difference in # cog teeth).

Eg, for the 12-30 cassette, the total "wrap" required would be
= (50-34) + (30-12)
= 34

A 12-30 is going to have some huge gaps in gearing, probably fine for touring and rec riding, but would be very annoying for me.

I'd probably install a new longer chain if you went to a 30t cassette.

Unable to answer your Tiagra question.


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## AJ88V (Nov 14, 2012)

Thanks, guys. I was definitely thinking of the 12-28, not the 30. The 11-25 hasn't been a problem for most of my riding, although my fat @ss would have appreciated a little help on a few local hills.

And thanks for making me look this up myself. I found this 2010 guide
http://cdn.sram.com/cdn/farfuture/SBapqB1KCQ3-0NgWUaIzVaqNXbofxsy1YhKU5AjHm3k/mtime:1301686365/sites/default/files/techdocs/my10-sram-tech-manual-rev-a.pdf
If I'm reading page 6 correctly, the max rear sprocket is a 28T and the max chain wrap is 33T, so the chain wrap (50-34) + (28-12) = 32T which is less than the 33T spec'd. Or I could even go 11-28 (SRAM has that combo).

So 28 it is. Now I gotta decide if I want to spring the extra bucks for a SRAM or Shimano 105 or Ultegra for at least twice the cost of the cheapest Tiagra price ($35). Opinions welcome


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## AJ88V (Nov 14, 2012)

For future reference, here's drive train compatibility chart from SRAM
http://cdn.sram.com/cdn/farfuture/tGNvDAuQGUZkAF5hLerl7FIEdqAosDkn76qcIF6UVrw/mtime:1336500655/sites/default/files/techdocs/gen.0000000004185_rev_a_compatibility_map_my13_road_components.pdf


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## ziscwg (Apr 19, 2010)

You might be able to get that Force RD to work right with a 30t rear cog. 

They do make the Force RD in a midcage now which gets you up to 32 in back. Since it's your training rig, an Apex or Rival midcage works. They are $60-90 iirc. You are then free to choose whatever cassette you want to run that day.

I run an 11-32 on the back with an X.9 10 spd RD. There are a lot of long climbs around here. For flatter ride in the valley, I do put the 11-27 on. I don't do that too often because I like my DH..


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## cxwrench (Nov 9, 2004)

tom_h said:


> You need to lookup your rear derailleur's "chain wrap" spec at SRAM website.
> 
> Chain wrap = (difference in # chainring teeth) + (_max _difference in # cog teeth).
> 
> ...


chain wrap capacity is one half of what he needs to know. the other half is 'max cog size'. don't forget that...on a short cage derailleur the 30 _might_ work. SRAM specs the derailleur for 28t max, which means a xx-30 cassette might (as in a big maybe) work but you'd have to try it on your bike to see. it all depends on the length of the derailleur hanger.


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## skinewmexico (Apr 19, 2010)

12-30 works with short cage SRAM, as long as you don't go big-big.


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## SystemShock (Jun 14, 2008)

Personally, I don't really like either the 12-28 or the 12-30 Tiagra cassettes. With them you lose the 16t cog, which some ppl find to be pretty useful. 

A 12-28 SRAM cassette wouldn't any better, 'cuz you keep the 16t but then have that lame jump from 19t to 22t. Derp. 

If it was me, I'd move up to a 105 cassette (price difference is like 25-30 bucks over Tiagra) and go with a 12-27, if possible. It's the same jumps as your current 12-25 'sette throughout most of the range, just a slightly wider spread on the last couple of climbing gears.

Of course, if you don't think 34/27 is low enough as a bottom gear, then it's a no-go. But I think you'd be hard-pressed to tell the difference between 34/27 and 34/28, if you think the latter is low enough.

SRAM also makes a 12-27, but I forget if they make a cheap one.


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## jrduquemin (Oct 4, 2009)

I'm running an 11-28 10 speed cassette on my bike with the compact cranks. Not had any problems with it....


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## AJ88V (Nov 14, 2012)

SystemShock said:


> Personally, I don't really like either the 12-28 or the 12-30 Tiagra cassettes. With them you lose the 16t cog, which some ppl find to be pretty useful.
> 
> A 12-28 SRAM cassette wouldn't any better, 'cuz you keep the 16t but then have that lame jump from 19t to 22t. Derp.
> 
> ...


This is very helpful. I hadn't really considered the ratio steps all the way through the range. Should make a little spreadsheet and post the ratios.

Also agree about 27 vs. 28 max. Truth is, I almost never even use my smaller chainring. My only real reason for trying something else is that I don't want to switch cassettes to switch wheels, and if a cassette must be bought, then it might be nice to add a little capability. Thx.


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## looigi (Nov 24, 2010)

AJ88V said:


> For future reference, here's drive train compatibility chart from SRAM
> http://cdn.sram.com/cdn/farfuture/tGNvDAuQGUZkAF5hLerl7FIEdqAosDkn76qcIF6UVrw/mtime:1336500655/sites/default/files/techdocs/gen.0000000004185_rev_a_compatibility_map_my13_road_components.pdf


Whoa. Figuring out that chart is a challenge, especially the shifter/derailleur compatibility. Only by already knowing/understanding what's compatible was I able to figure out how the chart works...


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## cxwrench (Nov 9, 2004)

skinewmexico said:


> 12-30 works with short cage SRAM, as long as you don't go big-big.


^ another example of crap advice ^ 
there is NO way you can be sure this will work on every bike, AND not being able to go big/big means it doesn't work. sorry, but you're doing it wrong.
guess i'm back to my old self...


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## kbwh (May 28, 2010)

aclinjury said:


> Makes very little sense to use a compact with a 12-25 cassette unless all you do is 2%-3% rollers all day long and nothing else.


I use an 11-23 for the +/-6% stuff. 50x12 is too short...


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## skinewmexico (Apr 19, 2010)

cxwrench said:


> ^ another example of crap advice ^
> there is NO way you can be sure this will work on every bike, AND not being able to go big/big means it doesn't work. sorry, but you're doing it wrong.
> guess i'm back to my old self...


How about this, your Highness? 12-30 works on a Cervelo RS with a short cage SRAM. With a longer chain, it might make less noise in the big-big. I found it to be an acceptable weekend alternative when I had a ride with some big hills. Better? I don't want to be the person who makes you screw up your new found reputation as a nice guy.


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## ziscwg (Apr 19, 2010)

cxwrench said:


> ^ another example of crap advice ^
> there is NO way you can be sure this will work on every bike, AND not being able to go big/big means it doesn't work. sorry, but you're doing it wrong.
> *guess i'm back to my old self...*


We're glad your are back. 

I almost never go big/big because I hate the chain noise. It' odd as I like things as quiet as possible on my road bike. However, on my mtb, I like the loud "angry bee" buzz of my Chris King hub. I like the loud grip shift click too. 

I do agree with you that it should be able to shift big/big without rippping the RD hanger off. Sure, one may be like me and rarely use it. If you can shift into it, it should be there.


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