big chainring 

I forgot the formula for this. If you know the gear inches and count your RPM how do you figure your speed?

DrPete 

Here's the formula: https://www.sheldonbrown.com/gears/

Squirrelli

There's also the Rabbit Calculator, but it is more of a SSFG thing.

531Aussie

It's easier to use a roll-out chart showing meters travelled for one rev.
This one is from Analytic Cycling.
https://www.analyticcycling.com/Power...Gear_Disc.html

For example, 90 revs in a 53x11:
From the chart, a 53 x 11 has a one-rev roll-out of ~10.157m.
90rpm x 10.157 meters = 914.13 metres in a minute.
914.13m x 60 = 54.85km/h


Gear Development.

.......39.....42.....52.....53

11 7.474 8.049 9.965 10.157
12 6.851 7.378 9.135 9.310
13 6.324 6.810 8.432 8.594
14 5.872 6.324 7.830 7.980
15 5.481 5.902 7.308 7.448
16 5.138 5.534 6.851 6.983
17 4.836 5.208 6.448 6.572
18 4.567 4.919 6.090 6.207
19 4.327 4.660 5.769 5.880
20 4.111 4.427 5.481 5.586
21 3.915 4.216 5.220 5.320
22 3.737 4.024 4.983 5.078
23 3.574 3.849 4.766 4.858
24 3.426 3.689 4.567 4.655
25 3.288 3.541 4.385 4.469
26 3.162 3.405 4.216 4.297
27 3.045 3.279 4.060 4.138
28 2.936 3.162 3.915 3.990

531Aussie

that Rabbit calculator looks better coz it uses crank length and has more teeth combinations

rollin

I can't get the link to work but just curious, why would crank length matter in all of this? A revolution is a revolution is a revolution right?

531Aussie

Ha! That's what I also would've thought, but it makes it look more comprehensive

big chainring 

I used to calculate this in my head when I was a youngster and cant remember how I did it. You got gear inches, Pi r squared in there, 60 minutes in an hour. 5280 ft in a mile. Throw me a bone here someone.

big chainring 

OK is this it?
Pi x Gear inches X RPM, Divided by 12, X 60, divided by 5280 = MPH.
Theres a shorter version to this i think.

big chainring 

I think I have it.

Simplifying the above. Pi or (3) X gear inches, x RPM divided by 1000 = MPH.

merlinextraligh

I like this calculator. Easy to see change over points with different setups.

https://www.gear-calculator.com/#

bidaci

Rabbit calculated gain ratio too

https://sheldonbrown.com/gain.html

merlinextraligh

You got to be really old to think in terms of Gear inches. Thinking of gearing in Gear inches no longer made sense after the demise of the penny farthing.

(That said I'll still think in gear inches, just because that's the way we did it back in the day.)

Phil_gretz

My answer would be "yes" and "no". To most cyclists, this figure doesn't make practical sense, except for one instance.

As I'm learning to ride a fixed gear bike (for variation, and for exercise benefits), I can look at an approaching hill of, say 200 yards in climb length. Knowing that my gear inches are ~72 (or ~2 yards), I can say to myself "the cranks must go around a hundred times to get over this." It's helpful if the hill is steep, to know when the burning and gasping will end.

Also, in designing a custom build, some thought to gear inches helps to choose cassette/chainring combinations, depending on how I plan to shift.

PG

merlinextraligh

You don't know what " Gear Inches" means.

Gear inch is a calculation that tells you what size of wheel on a direct drive bike (i.e. a penny farthing) would be equivalent to your gear.

So 72 gear inches is like riding a Penny Farthing with a 72" wheel (which would require one hell of an inseam)

And you would go a bit over 6 yards in one crank revolution with a 72 gear inch, not 2 yards.

Carbon Unit

I think inches gear are useful just to see the percentage difference between two different gears. For example, what is the difference between a 53X26 and a 50X29? I would look at inch gears and then the percentage difference. Is too old school?

Carbon Unit

This formula assumes a flat road. If you had in grade, then speed comes down. Some where I saw a calculator that included grade and weight I think.

merlinextraligh

That works, but gain ratio and speed at a given RPM do the same thing, and unless you grew up thinking in Gear Inches, it's difficult to conceptualize for many.

To me in specing gears for a situation, it's more relevant to know that I've to be able to do 9mph to turn over a 39/27 at 80 rpm, than to know it's a 39 gear inch.

I still think in Gear inches, just because that's how I learned, and I know what 39 gear inches feels like. But it's a pretty silly convention to compare things to something that hasn't been in common use for at least 100 years.

AdelaaR

Apparently it's no longer needed to calculate all this stuff.
They've got these neat little devices these days where you can just put in your wheel's circumference and then while you ride it tells you your speed in real time.

merlinextraligh

Unless you're coasting grade wouldn't change anything.

carpediemracing 

I think the magic number is 336. Maybe 360. I don't remember.

Gear inches x cadence / magic number = speed in mph.

If you really calculate gear inches, i.e. based on the actual diameter of the wheel, then it's accurate. Gear inches also goes across wheel sizes, so if you use real dimensions, you can compare a gear on a 650c wheel and a 700c wheel. Or 24". Or whatever.

For general comparison, 700c wheels (and various tires from about 19-25c), gear inches works. So a 50x15 on the track is a 90", so it's kind of like between a 53x15 (95") and 53x16 (I forget what that is).

Or, like when I couldn't put the bike in the big ring, I remembered that a 53x15 = 42x12 = 39x11.

idc

Ok, slight deviation from OP, but why are so many decently nice road bikes sold with anything greater than 11 teeth on the smallest rear cog? Surely when you're going down a hill for awhile you'd like the biggest gearing you can get? It reduces the maximum speed of the bike.

I'm researching bikes and wondering why this is the case. I assume I should just not worry about it because you can always increase your cadence a bit, but there's a limit to that.

Carbon Unit

It wouldn't change speed if you were climbing a 12% grade? Sorry but I am confused.

bidaci

A direct drive without slippage will not change your speed for any given rpm. Your rpm may be lower which would account for your lower speed. If you coast because you speed is greater than your rpm then it would increase.

Did I say that right?

Carbon Unit

OK, I see I get it now. Yes you said it right, thanks.