Chain inch formula
 

When I was riding my dawes galaxy touring bike in the 70's and 80's I had a little chart taped to my handlebars that showed the sequence of going through the gears . There was a formula that converted gear ratio to a number for inches of chain . For instance , My 11 : 52 was 108 inches I think , Anybody remember that ? Or am I having a senior moment here ?

Has nothing to do with the 'chain' but the gear inches are calculated as Chain Ring teeth / Cog teeth * Tire Diameter in inches. So in your example it would be 52/11 * 27 = 128 gear inches (108 would be a 52:13 combination). It's equivalent to what an old Penny-Farthing would have as the wheel diameter to get an equal forward movement with each pedal rotation.

If you came up with a 108" top gear, it was likely you were calculating a 52/13 and a 27" wheel.

Here's a link to a gear calculator Mike Sherman's Bicycle Gear Calculator and you can choose gear inch or gain ratio. The Sheldon Brown website has an explanation of both terms Sheldon Brown-Bicycle Technical Information.

Thanks . I guess i go by feel . I think it helps to know the next closest gear when on gradual slope changes . Like if going up on the chain ring , then dropping up two cogs or down on the chain ring and down one cog . I suppose it was more critical when I rode with loaded panniers back in the day . I've gotten about a thousand miles in now and my double shifting is getting more precise . sometimes a too far out of sequence shift can mess with my momentum . I suppose the best thing to do is get another k miles done as quick as possible . no need to over think it I suppose .

Rather than think in inches, I have the number of teeth of each cog in memory.

Gear inches confuse all new riders and a lot of veteran riders.

Even though I can use it, I still find myself thinking backwards to undo the calc.

example:
My fixie has a 70 inch gear.
OK, that's about a 39x15.

Cavendish sprints in a 120 inch gear.
OK, that's about 54x12

OK but knowing how many teeth are on the cogs does not give you the sequential shift pattern . For example , if you are going to your next highest gear from where you are you might have to do it by going up on the larger chain ring and dropping up 2 larger sized cogs to get to that next closest higher gear . other wise you are skipping up 2 or 3 gears at change rather going in closest sequence of change . Of course if you are going up or down a steep grade you can skip a lot of gears with no problem but more subtle changes require staying more closely to the actual sequential change in the gears . Does that make any sense to you ?

I can see what you mean but making sense in practice, not really. For me anyway. I usually don't do anything on the back when I shift the front, at most shift it once. With a compact I usually shift once, sometimes twice. I don't need to remember gear inches for that.

IOW, not sequentially through the gears always to the closest next one. I stay in one ring, shifting the back, until I run out of gears. Or if I'm generally going slow, small ring and if generally fast, big ring.

It's a very important thing to know when riding/racing on the velodrome. It's important to find a gear that not too big or too small, if it's too big then when there's an attack you can't accelerate quick enough, but too small you can accelerate, but are spun out as the speed ramps up (see speed is important ;)) Trackies speak in gear inches (Velodrome Shop Track Cycling Gear Chart). I ride a 50x15 which is a 90 inch gear.

tooth count as a ratio x wheel diameter in inches

Gear inches are what we have wound up with, but we could just as easily have adopted rollout distance which seems like an easier thing to grasp, and which some other countries use.

So instead of a 120-inch gear (a 10-foot diameter wheel on a penny farthing!) you'd say 31-feet travel per crank rev.
Then you could relate your gears in exactly the same way but maybe have a little better feel for what they mean.

Or you could do the inverse and compare them by how many crank revolutions = 100 meters... sort of like in the UK they report gas mileage as "gallons per 100 miles" instead of mpg.

I guess skipping a few gears between shifts because you don't know how to shift sequentially was a bigger deal on my 12 speed 40 years ago than it is on a 20 speed these days . Still I do a lot of double shifting ; meaning I shift the front and the back at the same time . This is also a lot easier these days with modern shifters compared to double shifting with one hand on downtube levers that I started with or even with the Suntour bar cons on my Dawes that made it lot easier than the former .

Scott, I do both but the gear inches are a real blessing when you are riding fixed and using multiple chainrings! My Mooney now sees 46, 44, 42, 38 and 36 tooth chainrings. I have all the cogs from 12 to 24. Thinking inches makes it so much easier!

Also, using gear inches makes it very obvious why the safety bicycle was such an improvement over the high wheeler as a racing machine. Very few men had legs long ewnough to straddle much bigger than a 60" wheel. Racing a 60" gear! That was the first fix gear ride of the winter after most of the snow was cleared!

Ben

Even though my training in physics and electrical engineering makes me a big fan of the metric system, I happen to like gear-inches, because they work like percents of the 52/14 top gear combination that prevailed in the 1970s, making them very intuitive. I know I like to have about 95% of this much pedal force at the high end (hence my 50/14, 46/13, etc.), and about 40-45% for a road bike low (conveniently 42/26, about all some of the older short cage rear derailleurs could handle). For loaded touring or mountain biking, one drops into, say, a 25-inch gear, which has 1/4 the pedal force and 4X the RPM of our default 100-inch top.

Easier to calculate gear ratios, if you're doing it on the fly for some reason. We don't usually change tire or wheel sizes during a ride :lol:

75 3/4 GI on my fixed gear, which is handy if I'm thinking about changing to an 18 tooth cog, but there's not much point to knowing it when just riding. IMO.

The point is that if you figure out the gear inches of each gear combination or ratio then you can determine the sequential shift pattern and learn to use all those gears on your bike more efficiently . If it does not matter to you I don't care . Peace .

I've ridden with gear inches taped to the stem old school style for as long as I can remember, it's how track riders think gearing.
Knowing my terrain, what GI I'm in and what progression is "next" either up or down at the right time to change chainrings (or not) w/o dropping cadence is seen at a glance until it becomes automatic. Not knowing the precise gearing progressions and shift options is just guessing.

Going from bikes equipped 53/39, 50/34 and to a IGH/derail hybrid it takes a glance to find where that ~70GI is on each that I'm used to on the FG. :foo:

-Bandera

Not at all, just trying to be helpful. Gear ratio on your bike is exactly the same as gear inches, but not proportional to the wheel size. Which doesn't matter for your purpose. It gives you the same thing, while being easier to calculate.

No, it does not . Just Knowing the number of cogs on the gears or the ratio ,which is the same thing does ,not give the sequential gear order the way that the gear inch number does . And it seems more like you are trying to be a wise guy than trying to be helpful . But whatever . New information is hard for some to wrap their brains around . Go with what you think you know . I don't care .:lol:

Did I offend you somehow? If so I apologize as such was not my intent, and I promise I won't try to "help" you further.

Promises , promises . :lol:

Of course it does. Ratio is proportional to gear-inches.

Think about this before you rage respond.

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.

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 .