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Gear Ratios(12 posts)

Gear RatiosMichaelM
Apr 9, 2003 3:42 AM
I'm running a 12 - 25 cassette, with my inner front ring being a 39, and I'm after a lower gear for climbing some of the steep stuff but without changing to a wide spread cassette.

If I was to change my front inner to a 38 (which I think is the smallest I can go on a 105 double chainset), so that my low gear is now 38 x25, what would this be equivalent to in terms of keeping a 39 front??? (I dont think that 38x25 is the same as 39x26).

Thanks.

Michael
For a relative number, divide front by back...TFerguson
Apr 9, 2003 4:22 AM
The lower the number, the easier the gear.

39x25=1.56
38x25=1.52
39x26=1.50
39x27=1.44

There's not much difference here, even between 1.44 & 1.56. Do the math on your current 39x25 and on the next highest gear that you can feel a significant difference when climing. This will show you the relative difference you are looking for.

TF
here's a spreadsheet that helpsDougSloan
Apr 9, 2003 6:22 AM
http://www.midcalracing.com/gears.xls
http://www.sheldonbrown.com/gears/ (nm)KEN2
Apr 9, 2003 8:00 AM
Related questionanalog1
Apr 9, 2003 1:12 PM
I live in San Francisco=big hills. I am going to be building a bike with campy record 10. I'd like to have a double up front. What cassette should I get so that I can make it up the hills?

This is all so new to me!

Thanks.
how fast (or slow) do you ride?DougSloan
Apr 9, 2003 1:27 PM
What's the slowest you go up the hills?

Use the spreadsheet (link above) and plug in the numbers for your speed and cadence. You may well need a 29 cog for San Fran.

Doug
how fast (or slow) do you ride?analog1
Apr 9, 2003 1:42 PM
Hmm. Not sure. I know that there are some hills I have inched up. Clocking in at ~5mph on the granny gear of my mtn. bike. I do not know what my cadence is. I've never measured/had it measured. I usually prefer to muscle up the hills instead of spinning.

Ideally, I'd like to be able to make it up the hills without having to spin like crazy on the flats.
Here's a stab at itKerry
Apr 9, 2003 4:26 PM
Assuming you can pedal smoothly at 50 rpm, then 5 mph requires a 34 inch low gear. With a 39 CR, you'd need a 30 cog. With a 38, you'd need a 29 cog. Campy offers a 13-29 10s cassette. How much ever faster than 5 mph you can actually go, then you can ratio down the cog (e.g. 6 mph then you could go with a 30*5/6 = 25t cog with a 39 CR). Only you can tell us how fast you can climb these hills, and this defines the gears you need.
Do the steepest hill on your MTB…TFerguson
Apr 9, 2003 4:58 PM
and see what ring and cog you are using. Use the chart to calculate the gear inches using MTB tire diameter. Then use the chart to with road tires and find the cog with a 39 ring. If your crank arms are different lengths, you will have to ratio that difference in at the end.

Post the MTB gears and measure your tire. It would be interesting to see the results.

TF
re: Related questionukiahb
Apr 9, 2003 9:53 PM
I'm from SF, and most people do need something like a 28T for the steepest hills, so the Campagnolo 13-29 cassette would work well. It is also possible to buy 12 and 13 T cogs (Branford Bike sells individual cogs), leave out the 16 T, and make your own 12-29 cassette...though some would not be happy w/o the 16 T....I'm going to try this on my bike...the 13T that comes w/ the 13-29 cassette won't work as a #2 cog as it is designed to work w/ the lockring.
hybrid cassetteDougSloan
Apr 10, 2003 6:57 AM
If you happen to have a 10 speed 12-25 and 13-29 cassette, you can combine the smaller cogs of the 12-25 with the larger ones of the 13-29. I can't recall exactly which ones, but I've done this a number of times.

Doug
yep, thats how I am doing it...ukiahb
Apr 10, 2003 10:58 AM
I have both cassettes, the combination that works is to use 12-15 from the smaller one, and 17-29 from the larger...I'm willing to lose the 16 in order to have the 12 AND the 29 since I live in a very hilly area...