Gears

[Oaklandhills]

I know there have been plenty of threads on gears, but I need to educate myself more on the gearing ratios. That said, I have visited Sheldon Browns website and would like some clarification on the calculations. If I use a 172.5 crank, 9 speed, 52-39, 12-25, what are the numbers next to the cassete size. For example, 12 - 8.6, 13-8.0, 14-7.4, etc...?

I am looking to jump from a 12-23 to a 12-27 and am trying to understand the difference.

Thanks.

[Needs Help]

...what are the numbers next to the cassete size. For example, 12 - 8.6, 13-8.0, 14-7.4, etc...?

Those are the gear inches given the chainring and cog combination. Two combinations of chainring and cog that have the same gear inches will mean they will feel the same when you are pedaling.

whoa...Sheldon Brown uses what he calls "Gain Ratios" which, unlike gear inches, measures the "true" leverage you have in any one gear by taking into account wheel size and crank length. You can still use gain ratios like gear inches to compare different combinations to see which gears are nearly identical. Sheldon Brown has a glossary explaining what both "gear inches" and "gain ratios" are"

http://www.sheldonbrown.com/gloss_g.html

[JohnS]

Sheldon Brown explains that those are the gain-inches. That is how many inches forward the bike will move with each inch movement of the pedal.
Basically, in going from the 12-25 to a 12-27, you gain a 27 and lose a 14 tooth cog. In the real world, you will have a slightly easier gear for hillclimbing.

[Oaklandhills]

Thanks guys, could you clarify a bit more. I know it will be easier, but I do not fully understand the movement issue. When you say the wheel will go 4.1 inches on a 52-25 for each pedal movement , what do you mean by movement?

[jeffg]

It looks like the numbers you quote reflect Sheldon's proposed gain ratio standard. It includes crank length in its calculations, if I recall. Anyway, the lower the number, the lower the gear. You should be able to see that a 39X27 is equal to a 34X23, etc. I use gear inches since I am used to that and it makes sense to me, but either standard is just fine as long as you compare apples to apples.

best,

Jeff

[JohnS]

OaklandHills- It means that for each inch of movement the crankarm makes in the invisible circle as you pedal, the bike will roll forward that many inches. It's all explained on his site.

[Brian]

Here’s another way to look at it… a 39-25 provides 42.1 gear-inches. That is, for each complete revolution of the pedals, your bike will move forward 42.1 inches. A 39-27 provides 39.0 gear inches. All things being equal, that’s a decrease of a little over 7%. In a perfect world that would mean it will be 7% easier to pedal in the 27 on identical grades. How do you quantify 7% to your legs and lungs? Beats me. But it will feel easier.

[mavic1010]

Brian:

That's great info...and then by calculating rpm with the gearing, I'll be able to determine at what cadence I need to be spinning in the easier gear to go at the same speed in the harder gear....

[Oaklandhills]

Brian - thanks. That description is working the best for me. How exactly did you calculate the 42.1 and the 39.0? I understand the decrease as a %.

Thanks again.

[Climb01742]

one of the things i dig about this phorum--it's like sesame street for cyclists. damn educational. seriously. i finally understand gear inches. cool.

[JohnS]

Oaklandhills- It's on the Sheldon Brown website! He merely left the default tire size (27") and put in a 39/53 crankset and a 9sp 12-27 cassette.

[Ozz]

This is a pretty cool calculator for checking out different gear combinations:

http://www.jbarrm.com/cycal/cycal.html

[BigMac]

Brian's analogy of the 7% gain differential is correct however his assertion that a 42.1 gear-inch correlates to 42.1 inches forward travel per single crank revolution is incorrect. Gear-inch really has no correlation relative to modern drivetrains, it harkens back to the era of direct drive large wheeled bikes, the ones where a crank was connected to the front hub and gear choice was dictated by front wheel diameter. Gear-inch is simply the gear ration multiplied by wheel diameter.

There is in fact a metric gear range measuring standard referred to as "Gear Development" that expresses forward travel in meters per pedal revolution. Of course this requires measurement of actual wheel circumfrence.

As Brian has pointed out, gear-inch ratios allow the user to compare actual gear ratio gains and gear ratio spacing which is all that really matters in developing an appropriate gear selection choice.

Ride on!

[Climb01742]

in one of the above examples, someone mentioned, for example, gaining a 27 while losing a 14. are there any rules of thumb about gears you should usually have, versus those you could lose without really noticing? for example, in the example i cited, is a 14 all that useful? i can understand how useful a 25 or 27 might be for climbing, but between say an 11 and a 17, what's normally useful? sorry if this is a dumb question. i am, afterall, dumb.

[JohnS]

I think it's dependent on cadence, etc. Just occasionally look down and see which gears you use a lot. Just remember that a two gear jump at the lower end will be a bigger perceived difference than one on the high end. Ex. : going from a 14 to a 12 is more than a 25 to 27.

[BigMac]

Climb:

You're the most qualified person to answer this as you presumably know best your preferred riding technique and terrain. If you're primarily a flatlands rider who prefers pushing big gears, a tightly spaced, tall geared cluster is your obvious choice. Lots of steep, steeps preferring a high cadence spin style you may consider shorter gear ratios with maybe a 12 at the tall end for bombing down those steeps once you've spun your way to summit. I would suggest you break out the calculator or surf Google for an online gear-inch chart. Following Brian's suggestion, simply calculate all 16, 18 or 20 gear ratios your prospective setup offers. Mark the crossover ratios in red such that if these are duplicate ratios it will be of little consequence in that you'll hopefully be avoiding crossover situations anyway. If all ratios are relatively evenly spaced with plenty of range at high and low ends, you've found your cluster. If not, mark the areas of duplicity or overlaps and consider evaluating alternative OEM cassette ratios or perhaps consider a simple swapping of 1 or 2 cogs to create a custom cluster that suits your needs.

From personal experience, the Campy10 12-25 generally nets relatively wide gear range with even spacing and very few duplicates. If you need lower gearing, the 13-29 is decent but if you jetison the 16T and add a 12T you'll net better spacing and better range, particularly if you're using an alpine double such as 50/36 up front. Gear-inch charts are very easy to calculate and an invaluable tool for evaluate proper gear choices. Good luck.

Ride on!

[Brian]

Big Mac,

You’re absolutely correct! My formula only used the wheel diameter (27”), not the circumference. You would have to multiply my answers by pi to get the actual distance traveled with a single crank rotation. Therefore the distance traveled would be:

39-25: 132” (39/25*27”*3.14)
39-27: 122” (39/27*27”*3.14)

As you also stated, it still results in the same 7% difference. Nice catch!