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Can't Stop Posting!(8 posts)

Can't Stop Posting!Mr Nick
Aug 27, 2003 8:46 PM
Since I am laid up right now and can't ride I figure I'd keep bugging everyone on RBR. Well here is my next question. I have a 12-25 Shimano rear cassette with a double in the front. I was thinking about getting a 12-27 rear cassette to give me a little more gear for the hills. The only two rings that would change would be the last two. I would get a 24 instead of a 23 and a 27 instead of a 25. I am not really up on how gear inches work so I don't know how this will actually affect my ride. Is it worth it?

Also maybe somebody has a website I could read about gear inches and how they are calculated while I'm icing my foot.

Thanks
re: Can't Stop Posting!rwbadley
Aug 27, 2003 8:58 PM
I think a 12-25 with a 39 front is quite versatile. I don't know your hills or condition, but I would just keep the 12-25 until it was worn out, then I could decide which way to go on gearing.

RW
re: Can't Stop Posting!MichaelM
Aug 28, 2003 12:32 AM
I have the same set up as you, and was also thinking of changing to a 12 - 27. But, like you say there's a 24 instead of 23, and 27 replaces 25, which doesn't really give me extra gears, what I'm used to riding in 23 I'd have to use 21 or 24, and if I need 25, then I need 25, not 24, but would have to go to 27, which sort of deafeats the object of having a 27 as a bail out just in case.

If that makes sense!

When it's worn out, I'm thinking of going for the 13 - 25 instead,as I'm used to the bottom gears, but could do with something between the 15 and 17.

Michael
all you need to know about gear inches...Frith
Aug 28, 2003 4:15 AM
...is how to type this address.
http://www.sheldonbrown.com/gears/ plug in the numbers and away you go. I'm thinking of getting a 27 too. Others make good arguments as to why not to do it but I don't know if I'm weak or more of a spinner but 25 has me mashing at a ridiculously low cadence going up some hills. I can do it easily enough but I'd rather be spinning a little faster.
If you wait until your cogs are wrecked and then make the decision you may have to end up buying two sets. If you do it now you could try it and still have your 25 in case you hate it...or even swap them out for different occasions.
a better comparison...C-40
Aug 28, 2003 4:53 AM
If you want a more meaningful comparison of two cogs or chainrings, simply divide the old by the new. For instance, 25/27 = .93. That means that the lower cog will provide a 7% increase in torque (or decrease in speed at the same cadence).

The percentage of change between 1 cog shifts is at it's greatest at the 11-12 shift, where it's just over 8%. The percentage gradually decreases to 6% at the 16/17 shift (which many 9 speed user don't have). The percentage of change takes a jump to 11% at the 17-19 shift and then decreases again to 8% at the 25-27 shift (which campy 10 users can't get).

To answer your question more directly, the 21-24-27 progression makes 11-12% changes at each shift compared to 8% for the 21-23-25 progression. The larger percentage of change generally makes sense in the lower gears. If you need help getting up a grade, the wider jumps are not as noticeable and a 3-4 rpm change in cadence will make up the difference due to the larger jump.

I could definitely use a 12-27 10 speed in the mountains. Campy only offers a 13-29, which is great on the low end, but losing the 12 can be a hindrance on a mountain descent.
Okay, why is everyone stuck on gear inches?NatC
Aug 28, 2003 5:55 AM
I hope it's to make me bang my head in frustration, because I am!

You only need to calculate gear inches if you're changing wheel diameter and/or crank length. If the only thing you're comparing is cog/ring changes, then the other measurements stay the same and you can factor them out. Just divide the # of front teeth by the # of rear teeth and you have your ratio.

39/23=1.69
39/24=1.62

39/25=1.56
39/27=1.44

5th grade math!

What happened to your footie?
Do you spin or do you Hammer up Hills?bimini
Aug 28, 2003 5:56 AM
I am a spinner on hills. In my shoes I would go for 27 for races with steep hills.

If you are good out of the saddle or if you don't have any mountains to climb, I would stick with the 25.

(You may want to train with the 25 (or even lower) in the hills to improve power)
re: Can't Stop Posting!MShaw
Aug 28, 2003 9:54 AM
The thing I've noticed about changing cassettes is that I have the same feel going uphill with my 12/23, 12/25, 12/26 cassettes. The ones with the bigger gears just mean I go slower.

YMMV

If you want a gear-inch chart, go look at www.fixedgearfever.com under gear info on the left side of the page. There's a gear progression chart and a gear inch chart for you.

Mike