Gear Inches
 

I ride an older touring bike and I'm working towards riding when I'm able to. I understand gearing based on how many teeth the chainrings have and how many teeth the cogs have, so for me a small ring of 22 is good and the lower the cog on the back the better for climbing.

I hear talk of gear inches and so far I'm confused by it. How do you figure out how many gear inches your bike has in a given combination? Is there an easy rule or do you have to crunch some numbers?

Easy. chainring teeth/cog teeth X 27. "27" because that's the trad road bike wheel diameter. If it's an MTB, use 26. Gear-inches is thus a function of advance distance per pedal stroke. I use a spreadsheet to make the calculations simpler and more adjustable.

A simple explanation: Gear Inches Explained @ Bike Gremlin.

Basically: (teeth on front chainring / teeth on rear cog) * diameter of wheel = gear inches. Or, roughly how far you travel for one revolution of the crank.

So, for example, on a 3x8spd 1990s MTB:

• High gear = (42 / 11) * 26 = 99 gear inches.
• Low gear = (22 / 28) * 26 = 20 gear inches.
• Still lower gear (ie, with a larger cassette) = (22 / 34) * 26 = 16 gear inches.

Here is a useful tool, where you can plug in the specific chainring and cassette components your bike has, and you'll get the numbers calculated for you: Bicycle Gear Calculator @ Gear-Calculator.com.

You take the number of front chain ring teeth, divide it by the number of teeth on the rear cog/ sprocket and then multiply that by the wheel diameter. This will give you gear inches. Now to calculate how far you actually travel which each pedal revolution you have to multiply the gear inches by 3.14.

You can use Sheldon Brown's calculator. You can choose between gear ratios, and gear inches, just choose which one you want with the Gear Units drop down menu.

https://www.sheldonbrown.com/gear-calc.html

Of course, 'gear inches', like Grinches, are largely imaginary. The alert, or even not very alert, reader will note that the diameter of the wheel is not how far you travel for a revolution, that would be the circumference. Circumference equals pi times the diameter, pi being 3.14 etc. So one really has a choice of ratio, gear inches, or development the distance travelled for of revolution of the pedals. A further complication comes from considering the real, functional radius of the rear wheel, depending on the wheel size, tire size, and rider size. Bicycle designers and builders seem to take pains to make sure their work is as arcane as possible. Where would the fun be if every thing was logical and standardized?

Others have explained how to calculate, so I'll add a note...

Gear inches are the equivalent diameter of direct a drive wheel. In other words, imagine a scenario where each rotation of your pedals resulted in exactly one rotation of the that wheel...

A 24 gear inch would be equivalent of a 24 inch diameter wheel (relatively small)
A 96 gear inch would be equivalent of an eight foot high wheel (rather big even by high wheeler standards).

One rotation of that eight-foot tall wheel will move you four times as far as one rotation of that two-foot tall wheel.

That's is why 'gear inches' works so well....it takes tire size into consideration. Imaginary number or not it offers a way to normalize that data so you can make comparisons between setups.

That’s a very good article.

First, 42 tooth chainrings were rare in the 90s:rolleyes:. They are more of a 2000 thing.

But that said the calculator you linked to is one of the best ones around, in my opinion. Using a more normal 44/34/22 mountain crank, the calculator gives this chart. As can be seen in the screen capture below, the calculator allows for the gear inches to be placed above the cogs. I find this to be helpful but I’ve been using gear inches for 40 years.

https://cimg1.ibsrv.net/gimg/bikefor...ca061ce357.png

The drop down box that has “Gear inches” can be changed to speed


https://cimg9.ibsrv.net/gimg/bikefor...db5d417b77.png

Or “Development”. Development is the number of meters traveled with one pedal stroke in a give gear combination.

https://cimg1.ibsrv.net/gimg/bikefor...9260bfe70f.png

Development is in meters but multiplying by 3 (3.3 if you want to be strictly correct) will give you the number of feet. Both of these might be easier to understand than gear inches.

Another thing to point out about the gear calculator is that it has a box marked “Compare”. That feature allows you to compare your current gearing to a new gearing. I find that feature to be extremely useful.

Another visual for gear inches. Picture the old highwheeler or "ordinary". Those tall, almost one wheeled bicycles that preceded the "safety" bicycle that we ride now. 1880s.

Now, that front wheel gives a gear inch of its diameter. 6' wheel" 72 gear inches. (Roughly, the normal everyday gear of a modern single speed; 44-16.) You have a 22-32 (I'm guessing) for a low. 22/32 X 27 = 18.5". A small child's tricycle. High gear of (say) 50-12. 113" No man ever could straddle that highwheeler and ride it. (Now you know why the safety bicycle took off. Not only could anyone ride it but racers could go a lot faster! And short people could compete. Before, all racers were tall. And nobody had a high enough gear.)

I was raised on gear inches. For about 20 years they were totally out of style. Many in the bike world wouldn't have a clue what I was talking about. Those who knew looked at me like i just brought up a tyrannosaurus rex. GI is back in style. Thank you! (The one gear notation that includes wheel diameter.)

whenever I explain it to folks, I figure its not necessary to know the details of the term, but just to think of it as a black and white number that shows you a number to compare to diff setups, like diff cassettes and chainrings, or to compare bikes.
The thing is to have a reference to understand.

example, so back when I bought that Kuwahara, I set off on a fully loaded trip for the first time, four panniers, tent on top of rear rack, handlebar bag. Had too much crap, hadnt trained with all the said crap on bike, and I went somewhere with bugger hills that were a total killer. Had to walk sometimes which was a drag, and ended up with a wonky knee and had to take days off cuz I couldnt ride cuz of buggered knee, get to a local doctor to get anti inflammatories, biking to said office with one leg.....bad experience basically.
Got back and asked my friendly bike shop guys what could I do to lower the gearing on my bike, as it wasnt enough , as was abundantly clear to me.
They suggested changing out the small front chainring, or granny gear as we call it, from the stock 28, to a 24t. Was an easy peasy fix, and someone showed me a chart where I could get the exact "gear inch" number for my bike, pre and post change. Went from a low of 25 g.i. to about 21, and it made all the difference in going up steep hills.
So even if I didnt know the details about g.i, who cares, I now had a reference for my bike with X amount of crap on it going up Y type hills.

Go ahead through the years, and now I know that for certain types of touring, I want and need about a 20g.i. low.
From simple observing, I knew that for other types of touring that I was dreaming of doing, ie Latin America, where I knew the gradients are steeper than in N America and inFrance where I'd been on other trips, plus knowing that I'd be carrying more stuff, that I would want lower than the 19 g.i. on one of my bikes that I did ride in Latin America on a moderately loaded trip.
Hence me getting a bike set up with a 16.7 g.i. low, and as expected, it worked perfectly ---- so bottom line, having a reference and the charts that easily calculate your gearing, you can get a number that you can relate to your real world experience.

voila

as mentioned, the sheldon brown gear calculator is a good one,
and the "html gear calculator" shown my cycco above is also a great one.
with both, you put in the right info, check off "gear inches" and you get a value for your bike.

its very handy for tourers

Clyde1820...Thanks for linking the gear calculator. I'm not much of a math in my head kind of person so this is going to be something I will save to my bookmarks on my browser. It helps when you can see the numbers and where they come from.

using the sheldon brown one, I save screenshots of the chart that gets made from putting in a specific bike info, so I have a reference for a bike.
Here is my Kuw after I changed the granny to a 24, so 50/40/24 with a 13-30 7 spd cassette and 700x28 wheels and tires
and then my more recent heavy touring bike, 44/32/22, with a 11-34 9 speed cassette and 26x2in wheels and tires (for some reason, couldnt attach this bikes chart...no matter, you get the picture)

the % numbers you see on the charts is the % jump between each gear, which is nice to know and useful if you start to be attentive to this stuff and figure out how sometimes on older bikes, the % jumps were kinda big--which in real life is where its hard to find in some given conditions one gear is too low and the next one is too high....more modern bkes with more speeds tend to have smaller jumps than back in the day, or at least manageable jumps but with a much much wider range of gears, ie cassettes of 11-42 for example.
https://cimg4.ibsrv.net/gimg/bikefor...cc89e46991.png

While that works, I never found it to be that helpful. It’s too hard to see which gear would be the “next” gear to use. For example, the 50/26 combination is 52 inches. The only combination that is close is the 40/20 combination. But to get to it would require a downshift on the front and a double upshift on the rear. Alternatively, 2 up shifts on the back could be done first, followed by a downshift in the front. The former is too difficult...since you usually need an easier gear...and the other is too much of a hassle.

But there’s a better way to use the gearing...with and without the gear chart. Notice that a shift down on the back in the 50 tooth gear is about the same a downshift in the front. It’s actually best with this crank and cassette in the 20 tooth cog. The 50/20 is 67”. The 50/23 is 58”. The 40/20 is 53”. It a bit bigger jump to shift off the outer ring to the middle ring but not huge. I’d ride this gearing as a “crossover” (not to be confused with crosschaining). In a “crossover”, you cross over from the outer chainwheels to the middle chainwheels and then proceed with downshifting on the rear. Going up in gear you cross over to the outer chainwheels at the same point.

If you use the graphic gear calculator linked to above, the speed difference (at 90rpm) shows how the crossover idea works in this same drivetrain


https://cimg2.ibsrv.net/gimg/bikefor...a06150e041.png

Using the middle of the cassette is a bit arbitrary. It does give some latitude when it comes to downshifts or up shifts.

If you fiddle with the chainring slider, you can see that a 42 tooth ring results in a closer ratio that would be an even better “crossover’

Cycc, all good points, but I just stuck up this chart because the fellow asking the question recently bought the same bike i bought in 90 or 91.

but as always, this gear chart is the best one for being flexible, shows lots of info, just wish it showed clearly the % jumps between shifts.
but great for showing visually gearing.

hey chris, give us the details of your bike, wheel and tire size (look at sidewall of tire, it will be marked) cassette teeth at the back, and the teeth numbers of the three rings at the front (chainrings)
Chainrings usually have a little stamped number somewhere on them, or you can count.
for cassette, just count the teeth on the smallest cog, prob a 13, and the largest, maybe a 28 or 30.

and then we can play with gearing charts and come up with any real life suggestions of changes to do if needed.

djb...Ok. I'll go downstairs to the parking garage tomorrow to look over the details on the bike. I had input some of the numbers I think I have but I didn't check for certain. Tomorrow I will know for sure.

I find myself using this calculator pretty often.

https://www.bikecalc.com/gear_ratios

Another thing I like about Sheldon Brown's gear calculator is that if you click on CALCULATE a second time it'll take you to another page with the gear chart that you can print out and tape or glue to your handlebar or stem.

Scrrenshot.

https://cimg9.ibsrv.net/gimg/bikefor...18e8231f29.jpg
Cheers

djb...just found out what my gearing is on the bike...it's a surprise for me! I have, on the front, 30-40-50 for the chainrings and I have a 12 to 28 7speed cogset on the back. My wheels are 700*32 Panaracer Gravel Kings and my crank arms are 175mm. The cogset on the back hasn't been adjusted so it's a standard setup; I'm trying to find out what the cogs in between the high and the low are but not having much luck so far. I though my small chainring was a 26; honestly I don't think I've heard of a 30 tooth small chainring.

Road triple:

https://www.thebikelist.co.uk/choose...omplete-guide/

its possible that the original owner changed the crankset from the oval shaped Biopace stuff to this at some point, but not knowing the age of your bike, who knows. There still is a very good chance, almost guarenteed, that the bolt pattern (called BCD) on this crankset is such that you can change the 30t granny to a smaller one.
As mentioned, this is a standard road triple. At some point the vast majority of road triples became 50/39/30, the bike I bought about ten years ago to replace the Kuwa came with one, and I was able to change the 30 to a 26t to get the low gear lower, as I knew from experience that I needed it for touring with panniers.

go to the sheldon brown gear thing and you'll see that this gives that bike a low gear of 29 gear inches, 28.9
and this is why back in the day, even if young, it would be hard on the knees going up hills....

I dont know the specs on that rear derailleur, but a google search of that rd with a model number stamped on it somewhere might get you an answer of its max cassette size, probably 30, but who knows, I havent looked into specifics of whats available in 7 speed cassettes for a while. Last time was replacing a freewheel, and even places like MEC have sunrace 7 speed freewheels 11-28 for 12 bucks or something---but you need to read up on cassettes vs freewheels and then figure out what your bike has...
You can always find cheap used rd's that can handle a wider cassette also
and a new chain, but 7 spd are maybe 12 bucks or so also

in other words, you have options to get lower gearing inexpensively, especially if you do it yourself. if you have to pay a store to do all this , it will add up.....but thats bike stuff, and bike stores have to pay their rent and equipment and mechanics....

but then you've been wondering about getting 36spoke wheels also...but if you get a good bike shop to check out the existing wheels, tension spokes properly, if they are in good shape you might be ok, but you weigh 70lbs more than I do, so who knows....I'd ride the bike as is and see how it is.

good luck reading up on gearing and the specifics of this bike
and then maybe a granny gear change
so you could get this bike geared lower

Here is my take on that topic...

Back in days of 6-speed cassettes, to cover a wide range, the gap between adjacent gears was larger than today to cover same gap with 9 or 10 speeds. This and an associated concept of half-step gearing meant that it was helpful to see how your gears lined up and how much duplication there was in adjacent combinations. So I can see it being relatively more important to understand placement of those combination. With more gears overall today coming from cassettes, those overlaps are less of an issue.

On the other hand, knowing the range from highest gear to lowest gear (particularly for bicycle tourists) is still an important aspect to know how your setup works at the extremes.

Thanks for all your input djb...

I was expecting to see a 26 tooth chainring on my crank...I used to ride 90s mountain bikes and was used to the triples on there. The 30 tooth is a little bit high for my tastes and getting a 24 tooth seems easy enough to do. They are cheap at the moment as I think most people nowadays are into 1x systems. I think my maiden voyage will be happening with a full load of items as there are a lot of things I have to bring. It will be a short ride to the campsite as my legs aren't used to cycling, even a short distance with a full load will test me. A 24 tooth chainring is definitely in my future.

I have a place I like to go for most bicycle things when I'm not able to do the work myself; it's closer to my home and its good work done at a reasonable rate. I'm not sure if this mechanic who works out of his garage has much experience building wheels but I might ask him; I'm also hoping to purchase some spokes and figure I can do that at the same time. I just ordered the rim, I have the hub and tire already so the spokes are the last piece of the puzzle. When the funds come to me they will go towards the wheel build.

I find this forum to be great...not only is it a good way to spend idle time but it's an amazing source of information. To all of you who have taken the time to write I thank you.

no prob
re building a wheel. Ive never done it, but I have replaced a rim that had cracks appearing at the drive side holes--I was able to get a rim that was the exact dimensions as the original rim, so the original spokes were the right length for the new rim.
So just be aware of the importance of figuring out how to get the spoke length for a given rim and hub, if not and you screw up you could end up with useless spokes because they would be cut too short....

and figure out if that crank can take a 24. My Kuwa crank could, it was a standard mtb triple 110/70 or whatever the bcd was, and 24 was the smallest the inner bolts could take.
again, its easy to ask google how to measure and calculate the bcd if not sure.