compact crank sprocket sizes



fredf

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Apr 8, 2004
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Am getting FSA Energy compact crank with new Trek 5200. I'm going for compact crank instead of Ultegra triple...I still want a granny gear of sorts..would like gear inches range of approx 30" to 100".
Chainrings are 50/34.
Please suggest specific rear cogs (9 speed) (eg: for 12-25 or 26 cogs...or should I go 13 to 26?)
 
fredf said:
Am getting FSA Energy compact crank with new Trek 5200. I'm going for compact crank instead of Ultegra triple...I still want a granny gear of sorts..would like gear inches range of approx 30" to 100".
Chainrings are 50/34.
Please suggest specific rear cogs (9 speed) (eg: for 12-25 or 26 cogs...or should I go 13 to 26?)
Sheldon Brown has a gear inch chart at www.harriscyclery.com Hint you cannot get 30 GI with a 34 ring and a off the shelf road casette which only to to a max large cog of 29(campy). A shimano 27 cog and 34 ring only gets you 34 gi, and a 34x 29 is 32 gi.
 
[A shimano 27 cog and 34 ring only gets you 34 gi, and a 34x 29 is 32 gi.[/QUOTE]
I want to be able to do hills in my neck of the woods...not mountains but I am concerned that I be able to get up reasonable sized hills.
Shimano's Ultegra triple can go to 31.5 on a 30/25 combination. RSA compact can go to 35.7 on a 34/25 combo.
Is there a 'significant' difference between the two? I want to know if it is worth going to a triple to get the 31.5 gear inch?
thanks
 
fredf said:
[A shimano 27 cog and 34 ring only gets you 34 gi, and a 34x 29 is 32 gi.
I want to be able to do hills in my neck of the woods...not mountains but I am concerned that I be able to get up reasonable sized hills.
Shimano's Ultegra triple can go to 31.5 on a 30/25 combination. RSA compact can go to 35.7 on a 34/25 combo.
Is there a 'significant' difference between the two? I want to know if it is worth going to a triple to get the 31.5 gear inch?
thanks[/QUOTE]Why are you limiting yourself to a 25 cog? How could anyone else know what you need. You go with your prior experience. No experience? Then the triple is the safe option and sure bet.
 
What's your lowest gear now? You can use this http://www.panix.com/~jbarrm/cycal/cycal.30f.html to compare what you're riding now to what you would be settting up. That'll probably have more meaning than just gear inches. Depending on what you're riding you might have a 42 or even 44 small ring and if you have a 23 or even a 21 in the rear that's a big gear for some hills, but if you just need one more gear there's no need to go get a 34/25 combo.
 
boudreaux:
can you just answer one question:
how close to 31.5 is 35.7? Is it a direct mathematical relationship...ie: it is 14% easier? Is this a 'significant' difference, understanding, of course, that only I can tell for sure.
 
I just went from a triple with a 52/42/30 with a 12-25 cassette to a 50/34 compact with a 12/27 cassette. The gear inches went from about 31.5 to 33.1. I do a climb each week that is 3200 ft. elevation in 10.8 miles. I used the compact for the first time this week. The difference is noticable, but still in the ball park. However, I don't think I would be happy with a 12/25.
 
[QUOTE=I just went from a triple with a 52/42/30 with a 12-25 cassette to a 50/34 compact with a 12/27 cassette.

What sprocket sizes do you have? How can you go from a 12 to a 27 in 9 cogs? or do you have 10 cogs.
I am switching from an Ultegra to FSA, so I assume I have to stay with 9 cogs and so I'm wondering what sizes you went with to maximize efficiency without large % changes in gearing?
Shimano only has 11,12,13,14,15,17,19,21,23,25,27 in their lineup and my salesman says that the teeth don't line up properly if I don't stick to a set...is this true? I get to choose 9 of these...which doesn't allow me to go 12-17...how did you do it?
FSA only does: 12-25.

thanks
 
fredf said:
boudreaux:
can you just answer one question:
how close to 31.5 is 35.7? Is it a direct mathematical relationship...ie: it is 14% easier? Is this a 'significant' difference, understanding, of course, that only I can tell for sure.

Yes, there is direct relationship of gear ratio to cadence. Suggest you look at cadence and climbing speed for your answer. The 34/25 will give you 88% of your cadence in a 30/25. If you can maintain say a 90 cadence now on the steep hills without killing your legs, you'll be down to 79 rpm. The force required from your legs will go up 13% to maintain the same speed of course....which may not be too bad depending on how strong your legs are.

However, if you're struggling now to maintain 60 rpm with your triple, (around 5-6 mph) the compact will reduce that to 53 rpm. For me, that low cadence means constant out of the saddle work, not something I want to do on long rides with several mountains to climb.

Or, you can just go out and try the climb with your triple, using the 30/23 and 30/21 combinations....those bracket the 34/25 ratio. Find out the difference for yourself.
 
fredf said:
Yes, I went to a 10 speed Dura Ace. But Shimano also lists a 12/27 cassette 9 speed: http://bike.shimano.com/product_images/CS/ev_images/CS_6500_EV.pdf

I also agree in looking at what the gear sizes will do to your cadance. I lost about 4 rpm at my climbing speed. I also wanted to be sure I would be able to stay seated on the long climbs. Here is a spreadsheet I used to look at the impact: http://www.midcalracing.com/gears.xls
 
fredf said:
Shimano only has 11,12,13,14,15,17,19,21,23,25,27 in their lineup and my salesman says that the teeth don't line up properly if I don't stick to a set...is this true? I get to choose 9 of these...which doesn't allow me to go 12-17...how did you do it?
FSA only does: 12-25.

thanks
shimano has a 12x27. Even if there was a possible cog sapping senario,and the shift ramps on the cogs did not line up,you would likely not know the difference. The salesman was blowing hooey. Sheldon Brown sells custom casettes with misaligned teeth,and noone gripes. Others have swapped cog for years.
 
rjcamsr said:
Yes, I went to a 10 speed Dura Ace. But Shimano also lists a 12/27 cassette 9 speed: http://bike.shimano.com/product_images/CS/ev_images/CS_6500_EV.pdf

I also agree in looking at what the gear sizes will do to your cadance. I lost about 4 rpm at my climbing speed. I also wanted to be sure I would be able to stay seated on the long climbs. Here is a spreadsheet I used to look at the impact: http://www.midcalracing.com/gears.xls
RJCAMSR:
thanks, this is exactly the information I am looking for!!
 
fredf said:
RJCAMSR:
thanks, this is exactly the information I am looking for!!

I haven't done the math on the ratios but I swapped out a 105 double for an energy crank. I have a 12-25 cassette and have found that once my conditioning started to improve I don't need the 25 anymore for my area and I have found myself spinning out the 12. i keep a high cadence while climbing seated I don't like to drop below 80.

I'm ready to go to 11/23 and am going to change out the 12/25 for an 11/23 Sram cassette. I can always put the 12/256 back on if I think I will need to for riding steeper climbs.

Acutally I'm hoping to get to the level where I can put the 105 double back on with the 12/25.

My ride starts out with a hard climb of about a quarter mile before I get to any level ground.

You may find this chart interesting

http://www.fullspeedahead.com/downloads/Compact Gear Ratio comparison.pdf
 
shadco said:
I haven't done the math on the ratios but I swapped out a 105 double for an energy crank. I have a 12-25 cassette and have found that once my conditioning started to improve I don't need the 25 anymore for my area and I have found myself spinning out the 12. i keep a high cadence while climbing seated I don't like to drop below 80.

I'm ready to go to 11/23 and am going to change out the 12/25 for an 11/23 Sram cassette. I can always put the 12/256 back on if I think I will need to for riding steeper climbs.

Acutally I'm hoping to get to the level where I can put the 105 double back on with the 12/25.

My ride starts out with a hard climb of about a quarter mile before I get to any level ground.

You may find this chart interesting

http://www.fullspeedahead.com/downloads/Compact Gear Ratio comparison.pdf

that charts worthless its so obviously selective for fsas sake, why not a 11-23 wotj a tripple for eg