# Thread: Gear Inches - Why use wheel Diameter? It's almost useless...

1. ## Gear Inches - Why use wheel Diameter? It's almost useless...

Old man muttering...

"Gear Inches" = D (Wheel Diameter in Inches) * (Crank Teeth/Gear Teeth)

Example using easy numbers:
• 27" * (30/15) = 54 Gear Inches
• For every full crank rotation, my 27" diameter wheel makes two revolutions
• But I don't traverse 54", I traverse 2*(27*3.14) = 170" (rounded)
• Although "Gear Feet" would be more practical (14.1 in this example)

So "Gear Inches" is only useful as a relative reference point when you are talking to other cyclists. They know that road bikes range from about 30 to 127 (excepting granny gear folk like me). My range, using 700x25:
• 53/11 = 127.1 Gear Inches
• 30/34 (Yeah, I've got L-O-W gears!) = 23.3 Gear Inches
• So I should remember for those rare occasions where I am talking to a gearhead, my Gear Inches range from 23 to 127 - keep repeating until it sinks in, 23-127, 23-127...

In reality, MPH @ 100 RPM is much more useful:
• 53/11 = 37.8 mph @ 100 rpm
• 30/34 = 6.9 mph @ 100 rpm

Using this, since 20% is easy to add or subtract, my cadence is seldom under 80 and my absolute max so far is 120, I can calculate:
• My absolute max speed (45* @ 120 rpm on the steepest downhill I've found).
• My slowest climb at 5.5 mph (20% under 6.9)

There have been instances where I was going slower, but I expect to banish that this coming year thanks to my new hip, new knee, and improved attitude!

* FWIW, that's the highest speed I've ever observed. Unfortunately my CF Roubaix did not feel solid - it verged on a shimmy.

2. Well best I know - at competitive levels - no one does. It is roll out distance and applies to those below age 19.

3. Roll-out or development maybe, ratios perhaps, but expressing the front rear combination suffices.

4. Yes, once the wheel size is understood the gear ratio front/rear is the most useful number. But gear inches is a historical measure thst relates modern bicycles to penny-farthings. It is hard to displace for that reason.

5. As a trackie, I and my brethren speak gear-inches. It's like latin in the old Catholic church

6. The point of a standard is that we may all speak together. Standards are not perfect or immutable - for instance the kilogram in SI is a strange case you can read about. It might not be the best thing if you were inventing it from scratch today. You might like it to be in meters or include crank length. But it's what we've been using and it conveys what we're speaking of in a way we recognize. That's hard to supplant. For your proposal, speed at 100 rpm cadence is useless to me, I can't flail around like that.

7. Originally Posted by tony2v
As a trackie, I and my brethren speak gear-inches. It's like latin in the old Catholic church
I'm lost without my gear-inches chart and was a week ago.

8. if everyone that pondered how far they travel with one turn of the crank (and that indirectly includes everybody that rides a bike, whether they know it or not), and they used the same tire and rim size then, yes, it wouldn't make any sense to consider wheel diameter. it would be a constant and could be disregarded in the formula.

and if everyone used the same ratio(s) between their chainring(s) tooth count(s) and the cog'(s) tooth count(s) it/they could be disregarded too. and we could just use the rear cog tooth count to compare. like on occasion, when two roadies with the same crank boast about sprinting uphill in their "12". the constants are assumed, and know they are not talking about their shoe size.

9. Originally Posted by Darth Lefty
The point of a standard is that we may all speak together. Standards are not perfect or immutable - for instance the kilogram in SI is a strange case you can read about. It might not be the best thing if you were inventing it from scratch today. You might like it to be in meters or include crank length. But it's what we've been using and it conveys what we're speaking of in a way we recognize. That's hard to supplant. For your proposal, speed at 100 rpm cadence is useless to me, I can't flail around like that.
I doubt any new definition of the kilogram will involve crank length.

10. I most often see gear inches referenced w/r/t single speeds/fixed gear, not so much road bikes.

11. Because "42x16=69" is a true statement understood by cyclists but baffles non-cyclists.

12. Originally Posted by hobkirk
• So I should remember for those rare occasions where I am talking to a gearhead, my Gear Inches range from 23 to 127 - keep repeating until it sinks in, 23-127, 23-127...
Don't worry about it. Just knowing your chainrings and cassette is enough. The only people that I know that talk about gear inches are track racers and SS/FG riders.

13. Gear-inch speak has diminished among roadies, but plenty of tourers and C&V-minded enthusiasts still speak it. I find it very useful when comparing between bikes of different wheel sizes.

Perhaps I'd be a big proponent of meters-development if I'd grown up in an SI country, but as it is, there's much to like about a figure that is two or three digits and is independent of the wheel size and the rider's cadence.

14. Originally Posted by caloso
Because "42x16=69" is a true statement understood by cyclists but baffles non-cyclists.

15. Originally Posted by WhyFi
I most often see gear inches referenced w/r/t single speeds/fixed gear, not so much road bikes.
+1. Generally only brought up by old farts and fixed gear riders. Probably used to be more relevant when you only had 5 gears on the rear cassette and had to spend more time selecting appropriate gears for particular courses. Not really relevant these days.

16. Since I started riding back in the dark ages, I've never had a need to know gear inches. I just think in terms of chainrings & cogs.

17. Originally Posted by hairnet
Don't worry about it. Just knowing your chainrings and cassette is enough. The only people that I know that talk about gear inches are track racers and SS/FG riders.
Only true if tires in question have the same OD/circumference. On mtbs, moving from 26 to 29" tires makes a notable difference in gearing.

18. Originally Posted by Looigi
Only true if tires in question have the same OD/circumference. On mtbs, moving from 26 to 29" tires makes a notable difference in gearing.
Sure does. I figured it was a given (staying in road bike talk) but I shouldn't assume anything.

19. Originally Posted by tony2v
As a trackie, I and my brethren speak gear-inches. It's like latin in the old Catholic church
No way this could fail to make me smile...

Originally Posted by caloso
Because "42x16=69" is a true statement understood by cyclists but baffles non-cyclists.
Ditto...
Plus I'm a big fan of arcania...
...so why am I ranting?

20. Originally Posted by hobkirk
No way this could fail to make me smile...

Ditto...
Plus I'm a big fan of arcania...
...so why am I ranting?
You aren't ranting. You are blathering. Totally different animal.

21. Originally Posted by tony2v
As a trackie, I and my brethren speak gear-inches. It's like latin in the old Catholic church
Indeed, GI is the Universal Gearing Language at the track and useful on the road.

One goes to the Velodrome with an assortment of chainrings and cogs: GI accurately describes what the pile of hardware can produce for gearing in each combination.
When one measures timed results in a Kilo that falls below expectations one does not say: "That Pretty Darn Hard gear was too much for me, let's fit a Pretty Hard Gear and try again."

Technical language has evolved in most sports, GI is one of cycling's.

-Bandera

22. It's all GI on the track.

23. Originally Posted by Bandera
Indeed, GI is the Universal Gearing Language at the track and useful on the road.

One goes to the Velodrome with an assortment of chainrings and cogs: GI accurately describes what the pile of hardware can produce for gearing in each combination.
When one measures timed results in a Kilo that falls below expectations one does not say: "That Pretty Darn Hard gear was too much for me, let's fit a Pretty Hard Gear and try again."

Technical language has evolved in most sports, GI is one of cycling's.

-Bandera
All true, but it still doesn't make any sense. The gear ratio of front teeth over back teeth makes a whole lot more sense, or even better the total roll out for which you multiply GI by pi. GI is a stupid, in-between measure that is not in any way intuitive. Saying that everybody uses it doesn't change that.

24. Originally Posted by rpenmanparker
GI is a stupid, in-between measure that is not in any way intuitive.
Latin with it's five declensions of nouns and four conjugations of verbs is not in any way intuitive but the Curia can still tell knock-knock jokes in it after a couple of thousand years or so of use.

We
all know what a 88GI vs. 90GI at >90RPM for 1,000 meters on 165 cranks is. (Ouch)

Do you measure in Metric when doing a kitchen remodel in the US?

Technical languages have a life of their own in their specialist communities......

-Bandera

25. As long as bicycles continue to be made in a wide variety of wheel sizes, gear inches or the European "development" (rollout) are the only ways to make direct comparisons between 2 bicycles with different wheels. Both these tell you the whole story, and 80" is 80" for 20" folding bike as well as a 29r mtn or hybrid.

Other than making bike to bike comparisons, it's all an artificial construct, and we could speak in any other measure we want, ie the gear ratio, crank turns per mile or what have you.

I commute on a 26" mrn bike platform, and ride 700c on the road. Gear inches tell me what I'll need to maintain certain speeds or climb various hills.

Page 1 of 2 12 Last

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•