Chain inch formula
08-08-17, 05:39 PM
#26
Senior Member
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 39,069
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 141 Post(s)
Tagged: 1 Thread(s)
Quoted: 5981 Post(s)
Liked 2,902 Times
in
1,610 Posts
half a senior moment, namely the association with chain.
Gear inches refer to the effective diameter of the rear wheel as if it were a directly driven wheel on Penny Farthing.
The formula is front sprocket divided by rear sprocket (the drive ratio) times the actual diameter of the rear wheel. In Europe they use a similar formula based on "development" or "rollout" which is the distance the bicycle travels in one revolution of the pedals. It's equal to the gear inches times Pi, to convert rear wheel diameter to circumference.
Gear inches refer to the effective diameter of the rear wheel as if it were a directly driven wheel on Penny Farthing.
The formula is front sprocket divided by rear sprocket (the drive ratio) times the actual diameter of the rear wheel. In Europe they use a similar formula based on "development" or "rollout" which is the distance the bicycle travels in one revolution of the pedals. It's equal to the gear inches times Pi, to convert rear wheel diameter to circumference.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
08-08-17, 05:44 PM
#27
Senior Member
Gear-inches is quite esoteric, but it does give people a handle on what certain bikes will do for them. For example, we have Bike Fridays with 20" wheels. The chainrings are road triples, and there are 9sp cogsets on the rear. I can tell by using gear inches that a Bike Friday in my stable has a lower granny gear than just about all my other bikes, but runs out of possible speed at the top end with, let's say, a 108 gear-inch total... much less than my other road bikes.
Shame about the direction the thread is suddenly taking, though.
Shame about the direction the thread is suddenly taking, though.
08-08-17, 05:47 PM
#28
Senior Member
Sequential shifting is not something the average cyclist concerns themselves about. Especially as few of them ever shift from one chainring or the other.
If someone wants to be "pure" about shifting through all the available gears in sequence, have at it. Trust me, it gets pretty old very quickly.
If someone wants to be "pure" about shifting through all the available gears in sequence, have at it. Trust me, it gets pretty old very quickly.
08-08-17, 05:58 PM
#29
Senior Member
Join Date: Oct 2014
Location: Portland, OR
Posts: 13,015
Bikes: (2) ti TiCycles, 2007 w/ triple and 2011 fixed, 1979 Peter Mooney, ~1983 Trek 420 now fixed and ~1973 Raleigh Carlton Competition gravel grinder
Mentioned: 131 Post(s)
Tagged: 0 Thread(s)
Quoted: 4887 Post(s)
Liked 4,052 Times
in
2,627 Posts
Sequential shifting is OK up to a point, but it can get messy if you have a triple crankset.
Doing up a gear-inch chart will probably show you that there are repeated or very close-together results.
Hence the simplification for triples --
Small chainring for steep climbing, and use half the cogs from biggest down;
Middle chainring for most riding, using the full range of cogs on the rear*; and
Big chainring for fast speed using half the cogs from the middle down to the smallest.
* On triples, the chainline should be set up for the middle ring and the middle cog on the middle. It also is the chainring likely to get the most wear, and therefore is the one that likely needs to be changed out more often than the other two.
On doubles, a similar explanation could be used -- small ring for general use and climbing; big chainring for speed. Either chainring on a double should be able to run through all the rear cogs. There might be repeated ratios when you work out the gear-inch chart.
The "purest" form is single chainring to a cogset on the rear. All the gear-inch ratios will be unique.
In practice, the repeated ratios on triples and doubles don't really cause much of an issue, and especially if you are skilled at shifting the front and rear gears at the same time.
Doing up a gear-inch chart will probably show you that there are repeated or very close-together results.
Hence the simplification for triples --
Small chainring for steep climbing, and use half the cogs from biggest down;
Middle chainring for most riding, using the full range of cogs on the rear*; and
Big chainring for fast speed using half the cogs from the middle down to the smallest.
* On triples, the chainline should be set up for the middle ring and the middle cog on the middle. It also is the chainring likely to get the most wear, and therefore is the one that likely needs to be changed out more often than the other two.
On doubles, a similar explanation could be used -- small ring for general use and climbing; big chainring for speed. Either chainring on a double should be able to run through all the rear cogs. There might be repeated ratios when you work out the gear-inch chart.
The "purest" form is single chainring to a cogset on the rear. All the gear-inch ratios will be unique.
In practice, the repeated ratios on triples and doubles don't really cause much of an issue, and especially if you are skilled at shifting the front and rear gears at the same time.
I would rewrite 'The "purest" form is single chainring to a cogset on the rear. All the gear-inch ratios will be unique." to ' The purest form is single chainring to a single cog on the rear. The gear-inch ratio will be unique.'
As for marching through my gears by gear inches? Nah! Life's too short. I keep my FW/cassettes as narrow as possible and shift like a road racer. Big changes in front, fine tune in back. Doubleshift ahead of time to be ready for what's coming so it can be done all on the rear derailleur.
Ben
08-08-17, 06:22 PM
#30
It's MY mountain
Join Date: Sep 2006
Location: Mt.Diablo
Posts: 10,010
Bikes: Klein, Merckx, Trek
Mentioned: 70 Post(s)
Tagged: 0 Thread(s)
Quoted: 4381 Post(s)
Liked 3,052 Times
in
1,653 Posts
No rage here chief but i feel a little hostility from you . Try it . work out your GI chart . If you have more than one chain ring it is not going to be the same . You will then see that as you go up in chain inches you will have to make some double shifts to stay in sequence . Some incremental shifts will require that you have to go up 1 or 2 cogs when you go up a chain ring . Or maybe down a cog and down a chain ring to get to the next sequential gear in terms of Gear inches . Some combinations may be close to repititous . If you take the time to work out your chart you will see what I mean . Some people just can't admit they are wrong though and will not take the chance of finding the truth about something they are mistaken about . Go in peace grasshopper .
The ratio is the front teeth divided by the back teeth.
Ratio times diameter = gear inches.
A chart of gear ratios will give exactly the same shift progression as a chart of gear inches for any given bike.
08-08-17, 06:24 PM
#31
Senior Member
Thread Starter
Join Date: Jul 2017
Location: lake martin alabama
Posts: 92
Bikes: caad 8 c'dale , trek madone 5.2
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 36 Post(s)
Likes: 0
Liked 1 Time
in
1 Post
half a senior moment, namely the association with chain.
Gear inches refer to the effective diameter of the rear wheel as if it were a directly driven wheel on Penny Farthing.
The formula is front sprocket divided by rear sprocket (the drive ratio) times the actual diameter of the rear wheel. In Europe they use a similar formula based on "development" or "rollout" which is the distance the bicycle travels in one revolution of the pedals. It's equal to the gear inches times Pi, to convert rear wheel diameter to circumference.
Gear inches refer to the effective diameter of the rear wheel as if it were a directly driven wheel on Penny Farthing.
The formula is front sprocket divided by rear sprocket (the drive ratio) times the actual diameter of the rear wheel. In Europe they use a similar formula based on "development" or "rollout" which is the distance the bicycle travels in one revolution of the pedals. It's equal to the gear inches times Pi, to convert rear wheel diameter to circumference.
08-08-17, 08:13 PM
#32
Senior Member
Most geaer-inch or ratio charts will also show that out of, let's say a 27sp bike (3x9), probably only 18 or so of those are useful in the sense that they are not repeated and/or are further enough apart to make a shift worthwhile. It irked a lot of people new to this because they figured that they had paid for a 27sp bike but only got 18 speeds to use.
I've never done the calcs on a 33sp bike (3 x 11), but I cannot imagine it being much different.
I've never done the calcs on a 33sp bike (3 x 11), but I cannot imagine it being much different.
08-08-17, 09:44 PM
#34
Senior Member
Join Date: Feb 2006
Location: Sin City, Nevada
Posts: 2,897
Bikes: Catrike 700, Greenspeed GTO trike, , Linear LWB recumbent, Haluzak Horizon SWB recumbent, Balance 450 MTB, Cannondale SM800 Beast of the East
Mentioned: 19 Post(s)
Tagged: 0 Thread(s)
Quoted: 523 Post(s)
Likes: 0
Liked 231 Times
in
183 Posts
The last time I used the gear calculator to figure out gear inches was when I last bought a new recumbent trike. It happens to have 30 gears and I know some of them overlap. I just wanted to know the upper and lower limit (21.9-124.2 GI) so I had a low enough gear to get up a steep hill and to be able to pedal effectively downhill. It doesn't make any difference what each gear is because I shift gears when there is a change in the terrain and I am either pedaling too fast or two slow for what I consider a comfortable cadence. It happens that for me, the middle chainring has a range wide enough for nearly all of my needs. I almost never use the granny ring and rarely the large chainring. Don't obsess on figuring out which gear to go to next but pick them to get the right cadence and let it go.
I guess in your mind, I paid for 30 gears and only got ten.
I guess in your mind, I paid for 30 gears and only got ten.
08-09-17, 06:28 PM
#35
Senior Member
Join Date: Feb 2004
Location: St Peters, Missouri
Posts: 30,225
Bikes: Catrike 559 I own some others but they don't get ridden very much.
Mentioned: 16 Post(s)
Tagged: 0 Thread(s)
Quoted: 1572 Post(s)
Liked 644 Times
in
365 Posts
Why do you think that you still have to do that?
Back in the olden days "10-speed" meant 2 on the front and 5 on the back. To keep the spaces reasonable between gear ratios, we had to squeeze every possible ratio out of that combination. My old Raleigh had alpine gearing, sometimes called a 1 1/2 step. That meant that a front shift was equal to 1 1/2 rear shifts. I'd go back and forth shifting both derailleurs, fishing for the gearing "sweet spot". It was easy to get confused about how to access the next easier (or harder) gear.
Today we have an abundance of gears. My trike has 10 cogs on the back, a whole bicycle's worth by my thinking, and 3 on the front. I don't need to continually shift chainrings back and forth anymore. I mostly just leave it in the middle ring and only use the granny ring for steep hills and the big ring for riding either downhill or down wind. The 10 cogs on the back are spaced close enough to suit me.
I don't need to tape a "cheat sheet" to my handlebar anymore.
Back in the olden days "10-speed" meant 2 on the front and 5 on the back. To keep the spaces reasonable between gear ratios, we had to squeeze every possible ratio out of that combination. My old Raleigh had alpine gearing, sometimes called a 1 1/2 step. That meant that a front shift was equal to 1 1/2 rear shifts. I'd go back and forth shifting both derailleurs, fishing for the gearing "sweet spot". It was easy to get confused about how to access the next easier (or harder) gear.
Today we have an abundance of gears. My trike has 10 cogs on the back, a whole bicycle's worth by my thinking, and 3 on the front. I don't need to continually shift chainrings back and forth anymore. I mostly just leave it in the middle ring and only use the granny ring for steep hills and the big ring for riding either downhill or down wind. The 10 cogs on the back are spaced close enough to suit me.
I don't need to tape a "cheat sheet" to my handlebar anymore.
__________________
My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.
My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.
08-09-17, 07:07 PM
#36
~>~
There is no down side to having the GI progression for every bike I own (except the FG
) taped to the stem. I can handle the weight of a scrap of paper w/ actionable data.
Old School methodology still in use in the Fifty Plus (50+) sub forum? Go figure.....
-Bandera
Last edited by Bandera; 08-10-17 at 07:44 AM.
08-10-17, 02:25 PM
#37
What??? Only 2 wheels?
Join Date: Apr 2010
Location: Boston-ish, MA
Posts: 13,438
Bikes: 72 Peugeot UO-8, 82 Peugeot TH8, 87 Bianchi Brava, 76? Masi Grand Criterium, 74 Motobecane Champion Team, 86 & 77 Gazelle champion mondial, 81? Grandis, 82? Tommasini, 83 Peugeot PF10
Mentioned: 189 Post(s)
Tagged: 0 Thread(s)
Quoted: 1225 Post(s)
Liked 657 Times
in
235 Posts
Wow. Now let's dive into a discussion of half-step crossover vs. 1 1/2 step.
Ultimately it comes down to "Do I need to shift now?" If the answer is yes, then what's the best choice, front or rear or both? And if the terrain is going to change again in 100 yards (or 100 meters) should I bother? It almost doesn't matter what the actual ratios are, expressed as either gear-inches or development or whatever. If you need to shift, do it.
Ultimately it comes down to "Do I need to shift now?" If the answer is yes, then what's the best choice, front or rear or both? And if the terrain is going to change again in 100 yards (or 100 meters) should I bother? It almost doesn't matter what the actual ratios are, expressed as either gear-inches or development or whatever. If you need to shift, do it.
__________________
Real cyclists use toe clips.
With great bikes comes great responsibility.
jimmuller
Real cyclists use toe clips.
With great bikes comes great responsibility.
jimmuller
Thread
Thread Starter
Forum
Replies
Last Post
Fangowolf
Clydesdales/Athenas (200+ lb / 91+ kg)
11
02-11-13 09:36 AM
