Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  


Go Back   > >

Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

User Tag List

Reply
 
Thread Tools Search this Thread
Old 12-17-09, 07:09 PM   #1
trail-rider
Member
Thread Starter
 
Join Date: Sep 2009
Location: SoCal
Bikes: TIME Speeder & Gary Fisher Hoo Koo E Koo
Posts: 38
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Compact Gear Ratios

I'm confused ... comparing compact and standard cranks, is the difference between the small gears (36.7 v. 42.1) bigger or smaller than the difference in the big gears (112.5 v. 119.3)? I know the difference in gear inches for the small chainring is about 5 and for the big about 7 gear inches, but I was wondering if the 5 gear inch difference on the small is more noticeable (maybe a greater percentage of something?) than the 7 gear inch difference on the large. I am thoroughly confused! Thanks!

Gear in chart for a compact and standard with a 12-25 cassette ...
Attached Images
File Type: jpg untitled.JPG (15.9 KB, 11 views)
trail-rider is offline   Reply With Quote
Old 12-17-09, 07:33 PM   #2
AEO
Senior Member
 
AEO's Avatar
 
Join Date: Jun 2007
Location: A Coffin Called Earth. or Toronto, ON
Bikes: Bianchi, Miyata, Dahon, Rossin
Posts: 12,258
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
try it in mph@90rpm
__________________
Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;)
http://sanfrancisco.ibtimes.com/arti...ger-photos.htm
AEO is offline   Reply With Quote
Old 12-17-09, 08:07 PM   #3
davidad
Senior Member
 
Join Date: Feb 2009
Bikes:
Posts: 4,901
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 24 Post(s)
Try this. http://www.panix.com/~jbarrm/cycal/cycal.30f.html
davidad is offline   Reply With Quote
Old 12-17-09, 08:18 PM   #4
Al1943
Senior Member
 
Join Date: Feb 2005
Location: Oklahoma
Bikes: Trek 5500, Colnago C-50
Posts: 9,433
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by trail-rider View Post
I'm confused ... comparing compact and standard cranks, is the difference between the small gears (36.7 v. 42.1) bigger or smaller than the difference in the big gears (112.5 v. 119.3)? I know the difference in gear inches for the small chainring is about 5 and for the big about 7 gear inches, but I was wondering if the 5 gear inch difference on the small is more noticeable (maybe a greater percentage of something?) than the 7 gear inch difference on the large. I am thoroughly confused! Thanks!
Yes, 5 is smaller than 7. The numbers on the chart are gear inches not ratios. Gear inches are linear values that are used for comparative purposes only. They don't tell you anything else. This chart assumes that the compact crankset chainrings are 50 and 34 teeth, and the standard crankset chainrings are 53 and 39 teeth. It also assumes a working wheel diameter of 27 inches (50/12 X 27 = 112.5). If you multiply "gear inches" by pi you get "development" or how far the bike travels for each full crank rotation (112.5 X 3.1416 = 353.4 inches).

Al
Al1943 is offline   Reply With Quote
Old 12-17-09, 08:31 PM   #5
trail-rider
Member
Thread Starter
 
Join Date: Sep 2009
Location: SoCal
Bikes: TIME Speeder & Gary Fisher Hoo Koo E Koo
Posts: 38
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Thanks! The link saved me a lot of time ...

I was in the process of converting each combination to mph. Here's the output for my extreme gears

I guess what I'm really trying to figure out is if the lower gears you get with a compact are worth sacrificing the higher gears you get with a standard crank. But I know there are TONS of posts about this, so no need to respond, I'm just trying to understand the issues by looking at the numbers.
Attached Images
File Type: jpg untitled.jpg (69.8 KB, 6 views)
trail-rider is offline   Reply With Quote
Old 12-17-09, 08:39 PM   #6
trail-rider
Member
Thread Starter
 
Join Date: Sep 2009
Location: SoCal
Bikes: TIME Speeder & Gary Fisher Hoo Koo E Koo
Posts: 38
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by Al1943 View Post
Gear inches are linear values that are used for comparative purposes only. They don't tell you anything else.
Thanks! This is exactly what I was wondering ... gear inches ARE linear values. I think I was getting confused with the proportional nature of the rings on the cassette.
trail-rider is offline   Reply With Quote
Old 12-17-09, 09:28 PM   #7
rm -rf
don't try this at home.
 
rm -rf's Avatar
 
Join Date: Jan 2006
Location: N. KY
Bikes:
Posts: 3,736
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 66 Post(s)
Comparing different large chainring sizes for the same cassette?
Mike Sherman's Gear Calculator can help.

Load your typical cassette settings, then set the chainrings to 34, 50 and 53.
Scroll down to the Speed at X RPM section. You can change the rpms to see how the different chainrings would affect your speed at that cadence. Both the 50 and 53 speeds will show in the chart.

I have a 50-13 as my biggest gear ( a 13-26 cassette) and I spin out at 32-33 mph (about 120 rpm). Any faster on a downhill, and I'll just coast.

Last edited by rm -rf; 12-17-09 at 09:31 PM.
rm -rf is offline   Reply With Quote
Old 12-17-09, 10:20 PM   #8
trail-rider
Member
Thread Starter
 
Join Date: Sep 2009
Location: SoCal
Bikes: TIME Speeder & Gary Fisher Hoo Koo E Koo
Posts: 38
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Thanks for all your help. I'm finally getting a grasp of how bicycle gearing works. Now I just need to go out and ride some different setups to feel the difference.
trail-rider is offline   Reply With Quote
Old 12-18-09, 08:48 AM   #9
DaveSSS
Senior Member
 
Join Date: Sep 2008
Location: Loveland, CO
Bikes: Two LOOK 585s, one KG461
Posts: 4,987
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Think in terms of percentages, not gear inches. At the top end, a shift from an 11T cog to the 12T cog is 8% or 10 gear-inches. At the low end, a shift from the 23 to 25 is the same 8%, but less than 5 gear-inches. Percentages make the most sense, when you consider that an average cadence is in the 100 rpm area. Each shift would create the same change in cadence or the same increase in available torque.

My other favorite formula is what I call "equivalent gears". When comparing different crank setups, a simple formula allows an easy comparison. For example, let's say you have a 53/39 with a 12-27 and want to know what setup with a compact provides about the same low gear. 27/39 x 34 = 23.5 That tells you that a 34/23 is not quite as low, but a 34/24 would be lower. 27/34 x39 = 31 That tells you that a 39/31 would be needed to match the low gear of a 34/27. The same forumla works nicely to compare triple cranks to a compact.
DaveSSS is offline   Reply With Quote
Old 12-18-09, 09:10 AM   #10
joejack951
Senior Member
 
joejack951's Avatar
 
Join Date: May 2004
Location: Wilmington, DE
Bikes: 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i (RIP), 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2014 Islabikes CNOC 14 (son's)
Posts: 10,189
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 189 Post(s)
For most purposes, Dave's "equivalent gear" formula is all you need to know. I use that method all the time for assessing gearing changes or comparing two bikes (keep in mind that this formula ignores wheel/tire size which can be a factor in gearing too). Only when I want to dig into the specifics do I bother with anything else. At that point, I'm looking at mph at 90rpm. Much more useful than gear inches.
joejack951 is offline   Reply With Quote
Old 12-18-09, 11:01 AM   #11
jack002
Senior Member
 
jack002's Avatar
 
Join Date: Feb 2009
Location: Southwest MO
Bikes: (1) 1993 Cannondale R900, red
Posts: 594
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
When I was considering a compact I worked out all the new gear inches and saw that the old-way 1st gear was about the same as the new-way 2nd gear. So for me, it was like adding one lower gear and losing the topmost one. Realising that helped me sort it all out. (BTW, it made a big difference for me, most worth it!)

Some have said the way they plan gearing is to get the lowest gear they need for the terrain they ride and let the top gear fall where it will, and also "I never need more than 90 for the top gear". Both seem like a good idea to me.
jack002 is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -6. The time now is 06:41 PM.