Gear inches
 

Ok, so gear inch equations confuse me, so I'll just tell u good folks what I have on my folder now, and what I'm thinking of doing if feasible. My front ring is a 48. Largest rear gear is 34, 8 speed cassette. I have a ten speed 11-25 I'm itching to put on, as I have a 10 speed shifter also. I know I have to change the chain. What size front ring would I need to put on to get about the same feel/ ratio as the 48-34 combo. I have a 44 teeth but I'm thinking it probably needs to be even smaller since the largest rear is only 25. Any ideas ? Thanks

Gear inches are an imaginary scale that supposes you have a highwheeler bike, and that number is how large your wheel would be.

So go online and find a gear inch chart, and input the cogs and chain rings you have and are considering. Combinations that produce similar gear inch numbers are equivalent gears - like 50×11 = 53×12.

You'll quickly get used to using the tool.

Have you tried to follow Sheldon Brown's website description of bicycle gears? Sheldon Brown's Bicycle Glossary G

The largest rear cogs are the easiest to pedal, as in up steep/long hills. It's the smallest cogs that are for down hill and high speed use.

But to answer your ring teeth question. 48/34 is about 1.3. So a 25 large cog would need a 32.5 tooth ring to mimic the 48/34. Andy

Forget gear inches (which is stupid anyway). This is simple math.

48 / 34 = 1.412

1.412 x 25 = 35.29

A 35T ring will get you closest, but I don't think I've ever seen a 35. A 36T ring will give you a bit larger low gear. A 34T ring will give you a bit lower low gear.

The equation for gear inches = (Front ring/rear cog)x wheel size. Wheel size is in inches. Do a bit of algebra and solve for Front ring= (gear inch/wheel size) x rear cog. Assuming a 700C wheel, your existing gear, g.i. = (48/34) x 27 = 38 gear inches. For the near low gear of 25 teeth, the ring needed would be front ring = (38 gi/27”) x 25= 35 teeth. If you don’t want to do the math, you can use Gear-Calculator site. Here’s the same calculation I did in graphic form. You can see that dropping to a 35 tooth gear, your high gear is going to suffer significantly. You are going to spin out fairly soon and spend a lot of time coasting.

If you have to have the same gear inches as you do in the low 48F/34R, and your bike uses 700C wheels, then you'll need a tiny 35 or 36 tooth chain ring to ever get that low.

While some like the gear inches, I'm happy with just gear ratios. Assuming I'm comparing similar bikes it's easier to deal with.

Right now, your 48 x 34 low gear works out to 26.?". Pretty standard for a folder. My folder has a 53 x 32 low which is higher than usual. The 11 - 25 x anything is a bad idea though IMO. With a 44T up front your low combination, 44 x 25, becomes 32" which is about where mine is. But your top gear will be 'only' 73". Mine is just over 80". It takes a 36T to get your low to where it is now and then your top gear drops to 60". I wouldn't do it.

My math mistake, miss hit the calculator buttons. Eric F is right on the math of ratios. I won't say inches of development is stupid. Any system that serves as a basis for understanding and can be used to describe to others and be discussed is not wrong. Andy

just use this https://www.bikecalc.com/gear_ratios

48/34 (1.41)is close to 36/25 (1.44)

Gear inches are the chainring divided by the rear cog times the wheel diameter in inches.
So your current low is 48 / 34 X your wheel size (I'll call it 24" for this example) = 34"

Now, you want a 34" low with a 25 tooth cog. So your new chainring is going to be chainring = gear inches X cog / wheel diameter
Chainring = 34 X 25 / 24 = 35 teeth.

I don't know what your current small cog is but we can work backwards to find the now equivalent to your future 11 tooth cog.
Gear inches = 35 / 11 X 24 = 76"
So, with a 48 tooth chainring: cog required = chainring / gear inches X wheel diameter = 48 / 76 X 24 = 15 teeth.

If you keep the same low, you will be losing high end unless your current small cog is a 15 or bigger. (Unlikely.) To get the same high gear as say a 12 tooth cog, you would need a 35 / 96" X 24 = 8.75 ~= 9 teeth.

Math. The curse our legs have to live with. :) And edit: the advantage of math is that you can do it in your head on those long rides with time to kill.

Second edit: Gear inches are easy to visualize, whole numbers between the teens and a little over 100 and completely transferable between bikes and wheels. Three numbers you need to compute but less to carry around in your brain. Yes, less accuracy but hey, it's below the threshold our legs can detect. (Mine anyway.)

Just a quick question to make sure you're getting the info you need.

I gather you have a 1x8 (going 1x10) system. If not, and you have 2 up front, then you should focus on the large/small and the small/large combos.

No response needed, unless it's to say what the other chainring is.

My arbitrary gear comparison scale is better than your arbitrary gear comparison scale! :troll:

Centimeters are stupid! I only use millimeters!

I always just take a simple ratio of back:front gearing, in your current case 48/35 = 1.4 and then just round to an even number of teeth to get a similar gear. For your new 25t sprocket, you'd need about a 35t front ring.

I never bother with the gear inch charts unless I'm making a meaningful change of tire/wheel size or crank. But even then, the simple ratio gets me what information I actually need or at least close enough for actual riding. When I'm looking at a new drive train, I look at the ratios for the highest and lowest gears and then decide if I want to go lower (I never have wanted to go higher!), or stay about the same.

edit Eric F above has better detail than me

jdog: it is all amazingly simple math as cyccommute (Stuart) pointed out.

HERE is the Simple Way that SCHWINN and other bike manufacturers calculated comparative GEAR CHARTS back in the sixties and seventies.
Yes, it is so simple that Jethro Bodine could cypher the calculation.

EXAMPLE:

----The FRONT SPROCKET is your Numerator
----The REAR SPROCKET is your Denominator

This example you've got a bicycle with 700C wheels that has a 45 tooth front chainring and has 15 teeth on the rear wheel's sprocket.

So, 45 DIVIDED BY 15 = "Result"

45 divided by 15 = 3


NOW YOU TAKE THE "Result" AND YOU MULTIPLY IT BY the wheel diameter


**** USE 27 as the wheel diameter FOR ALL BICYCLES WITH 700C (622mm) wheels and FOR ALL BICYCLES WITH 27 inch (630mm) wheels.

****USE 26 as the wheel diameter for ALL Bicycles WITH 650, 650a, 650b, 650c wheels , WITH 597mm, 590mm, 584mm, 571mm, or WITH 559mm 26" Wheels


SINCE THE BICYCLE IN OUR EXAMPLE HAS 700C WHEELS, We Will Use 27 as the wheel diameter for this GEAR calculation.

Okay to recap:
45 front teeth DIVIDED BY 15 rear teeth = "Result" of 3
TAKE THAT "Result" AND MULTIPLY IT BY THE WHEEL DIAMETER
"Result" of 3 MULTIPLIED BY 27 (since we use 27 for any bike with 700C wheels..)
So 45 DIVIDED BY 15 = 3
So 3 TIMES 27 = 81 GEAR
3 X 27 = 81 GEAR

YOU GOT THAT !!! IT IS EASY. IT IS AN EXTREMELY SIMPLE AND CRUDE CALCULATION BUT IT IS A VERY MEANINGFUL, VALID, COMPARATIVE Gear Number THAT IS ENTIRELY USEFUL FOR COMPARISON BETWEEN SIMILAR BICYCLES.
obviously you may have some possible significant differences depending upon if you were comparing two bikes with certain extremely different twenty six inch wheelsets for example. *** The comparison is still valid, useful, and nearly perfect When Comparing Bicycles With The Same Exact Wheel & tire combination.***


Just do that GEAR Number calculation again and again for EACH of Your freewheel cog OR cassette cog gears......or for your prospective possible choices for freewheel or cassette and for potential combinations using different prospective front chainwheel configurations.
YEAH, THIS IS ALL DUMBED DOWN AND Jethro Bodine SIMPLE, BUT IT Works Great and It Is A Meaningful Comparative GEAR Number between similar bikes!!
Just do the simple calculations and compile a GEAR CHART like the factory often did in the old days for use in catalogs/print ads/owners manuals/promotional materials.
...................You can even get your 10 year old kids, or your 10 year old grandchildren to calculate the GEAR numbers and make the GEAR CHART compilation if you are too busy or too lazy to do it!!!

I don't know how gear inches become gear inches but I know the number I like for a low gear is between 23 to 26 inches. My Dahon with 7-speed IGH and 34t front ring has a low of 25.1, high 61.3. Based on your information with 11-25 cassette and 36t ring I came up with a low of 26.9, high 61.1.

I'm a geezer like you who just wants to get this thing up hills, I don't care about flats/downhill speed, I have a different bike for that. 26.9 would be a bit high for me, if you drop down to a 34t front ring you'll have 25.4-57.7. Yeah you'll spin out quick with that top gear.

Here's mine and the two configurations I estimated for your bike.

https://cimg0.ibsrv.net/gimg/bikefor...9aae499a3a.jpg
https://cimg2.ibsrv.net/gimg/bikefor...5dfc97f425.jpg
https://cimg7.ibsrv.net/gimg/bikefor...c66f64b0d6.jpg

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Inches of development (travel along the ground for each pedal revolution) is meaningful and easily understood. However, “gear inches” is the equivalent diameter (not circumference) of the front wheel on a penny-farthing bike. It’s archaic and meaningless, except for this one specific context. I’m sticking with my previous characterization - it’s stupid.

But pi makes the in your head math harder than the simple 27" (or 26 or 24; all numbers that often simplify with the cog teeth nicely).+

Thank you for coming back and saying this. You're welcome! (And yes, tightly spaced cog jumps rule!)

Why do so many of you favor doing math on every combination when gear charts show you every solution simultaneously?

I don't carry gear charts on my rides. Years and years of Excel make the math easy for the big clusters. Easy to save what I like. Print out what I like in stem friendly formats.

It does not. It is a folding bike with 20" wheels. All that effort was for nothing. I'm not sure why so many made this into a TED Talk opportunity.

I'm new to the Bicycle Mechanics section and have quickly learned every thread is a TED Talk opportunity.

How far I travel across the ground (gear development) with each pedal revolution is something relatable to the activity of cycling. How the gear ratios and wheel size of a multi-geared bicycle compare to the diameter of the front wheel of a penny-farthing bicycle (gear inches) is archaic.

What?

How does it relate? When do you use inches traveled per revolution?