Retro Grouch 

Chairing teeth divided by rear cog teeth times 27.

MacG

not quite. [chainring] / [cog] * [rear tire effective diameter measured in inches]

27 will only work if you have a rear wheel that rolls as an exact 27 inch circle. A 27" wheel/tire does not have a 27 inch diameter, either. 27x1-1/4 is about 27.1 inches, 700x23 is about 26.3 inches, and 26x1.5 is about 24.9 inches.

Better yet, just use this thing I wrote a while back. It's designed mainly for singlespeed/fixie math, but it's pretty handy overall.
https://www.basementfreaks.com/members/karl/gearing/

Al1943

Actually Retro's simple formula is plenty good enough because gear inches are normally used to compare various gear combinations so any constant will work. The actual diameter would be important only if you were calculating development, how far the bike travels with each revolution, and then you need to include pi in the formula. By using the generic 27 you can quickly calculate gear inches by hand or on a simple calculator.

Al

'nother

OK, if you're extremely anal about it, but:

(39/25) * 27.0 = 42.12 (52/11) * 27.0 = 127.63
(39/25) * 27.1 = 42.27 (52/11) * 27.1 = 128.11
(39/25) * 26.3 = 41.02 (52/11) * 26.3 = 124.33
(39/25) * 24.9 = 38.80 (52/11) * 24.9 = 117.71

With the possible exception of the 26 x 1.5, I'd say RG's formula is quite reasonable for a quick estimate.

OLDYELLR

Well, 27" has always been taken to be the rolling diameter of a 27" tire and even for 700c. For road bikes, 27" is good enough, even if it's out by 1/2", because you'll never feel the difference.

And speaking of gearing, anybody here feel at all comfortable with the European concept of "development"? I think they tried to introduce it in Canada some years back, but nobody was having any of it.

Retro Grouch 

Yup. Retro doesn't do the roll out thing to find tire circumference when he's setting up a computer either. Sometimes close enough is close enough.

HillRider

Worse yet, the Euros express it in Meters so we are even less familiar with what it means in terms of gear ratio and peddling difficulty/bike speed. Gear-inches may be archaic but it has an old and honorable history and we all know how to interpret the numbers. That's really all we need.

I also agree that 27" (or 26" for MTB calculations) is plenty close enough for what we are trying to determine. You don't use a micrometer when a yardstick is adequate.

Al1943

Gear inches is convenient because it's easy to remember that 100 gear inches is a high gear, most road bikes have something higher. And 35 gear inches is pretty low, but a triple will have lower.

AL

Gonzo Bob

Actually, I use development. I don't use a cyclo-computer so I'll often estimate my speed by knowing what gear I am in and estimating my cadence. If my cadence is 100rpm (pretty typical for me), then my speed in km/h is gear*6 - i.e. if I'm riding a 5.5m gear and pedaling 100rpm, I'm going 33km/h. The same calculation from gear-inches to mph is possible but harder to do in your head.

CdCf

I wouldn'ty consider that even close to close enough. My computer has an error of less than 0.5% at the moment. The setting was found by riding a long stretch of road where the distance was known to within 10 m. That's probably close enough for me, but it's still almost a kilometer off after a 200 km ride.

AnthonyG

Using an accurate wheel diameter might be a mute point for most of you but for me I have/had bikes with 3 different wheel sizes so in order to compare gears between the bikes being accurate is important.

700x23c is 26.3", 650x23c is 24.3" and I had a 24" wheeled bike that was 23.6".

Regards, Anthony

HillRider

Well, you are using three significanty different rim sizes so the difference should be significant. What we are saying that agonizing over the "true" diameter of a 700x23 vs a 700x25 is not worth the time or effort. Using 27 for all 27" and 700c wheels, 26" for MTBs and 24" for 650c/24" wheels is really accurate enough.

Wogster

Technically the diameter of a 700x23c tire at 8 bar, and at 9 bar is different, maybe only .5mm but still different.

HillRider

That's exactly what I meant by agonizing over minor differences. I realize you are humorously making a point but there are those who would take this kind of thing seriously.

Al f R

[QUOTE=HillRider]Worse yet, the Euros express it in Meters so we are even less familiar with what it means in terms of gear ratio and peddling difficulty/bike speed. Gear-inches may be archaic but it has an old and honorable history and we all know how to interpret the numbers. That's really all we need.
QUOTE]
Hi HR & Co
Just a perspective from a relative newcomer to biking, 4 years in fact after a 30 year "rest". Gear inches may be of use to old hands [or more accurately, those that have been around bikes a long time & not metrically orientated] but from my POV the use of "development" is more appropriate. I can easily equate to something that is real i.e. 50/12 X 2115 [tyre rollout of my 25mm] = 8.81 Metres being the top gear rollout of my current bike per crank revolution, compared to 48/11 X 2140 = 9.33 Metres top gear rollout of the old hybrid bike.These would respectfully be 112.5 & 117.8 gear inches. I have my own spreadsheets for the complete gear rollouts of all the common gear ratios for me to compare.
OK, so these gear inch examples will tell me which is the taller ratio, but my metric gear rollout [which is what I call it] is a real measurement compared to just a comparison between figures. BTW I grew up with imperial measurements, so inches themselves are not foreign to me, but after living with metrification for 30 odd years [Australia took the plunge & did it] it is by far a simpler system.
I also like accuracy in the wheel circumference/computer settings scenario, which equates to lots of white paint dots on my driveway, so pi not required Al1943 that's just the way I am, pedantic maybe but I count my air swings in golf as well.
Al