gear inches- do they tell the whole story- Mtbr.com

# Thread: gear inches- do they tell the whole story

1. ## gear inches- do they tell the whole story

I am trying to decide what gearing to use in the Butte 100 and am trying to compare the gear I'm running on my 29er to what riding buddies use on their 26ers.

question: is it really just a mathematical equation? Does a 29 er using 32/19 giving you 48.8 gear inches really ride just like ( or very similarly) to a 26er using 32/17 which equals 48.9 gear inches?

Or is there something else at play when comparing between the 2 sizes of bikes?

2. Nope, that's it. Other than the fact that the 29er wheel rolls across stuff better so it's more efficient, and it weighs more so it's less efficient.

--Sparty

3. What Sparty said. For every positive of a given wheel size, there is the negative.

Which ones do you prefer?

4. Originally Posted by Sparticus
Nope, that's it. Other than the fact that the 29er wheel rolls across stuff better so it's more efficient, and it weighs more so it's less efficient.

--Sparty
That's the quote of the day right there. And I'm a 29"er guy through and through.

5. Originally Posted by snack64
I am trying to decide what gearing to use in the Butte 100 and am trying to compare the gear I'm running on my 29er to what riding buddies use on their 26ers.

question: is it really just a mathematical equation? Does a 29 er using 32/19 giving you 48.8 gear inches really ride just like ( or very similarly) to a 26er using 32/17 which equals 48.9 gear inches?

Or is there something else at play when comparing between the 2 sizes of bikes?
You have to take crank length into account as well. (though it's not that relevant since most SS mtbs have cranks between 175 and 180 mm)

Sheldon on gain ratio

6. OK, not an SSer, but an interesting question, so thought I'd add my \$0.02

As I said I don't ride SS, although you may ride the same gear inches, but different ring/cog combos pedal easier/harder than others, is this purely a chain cross thing because of the gears or does it make an easier gear having almost identical cog/ring sizes as opposed to bigger ring/smaller cog or small ring/big cog?

7. hmm...i'd say maybe not

on a wide open rolling hill trail a 29er should be able to maintain higher momentum\speed which would allow a higher gear to be ran than on a 26er

but on a tight trail with steep climbs the 29er would need a lower gear than the 26er

so prolly depends on the trail...horses for courses

8. Pick a low gear. The last 50 miles has 12,000' of gain.

9. Originally Posted by LyNx
OK, not an SSer, but an interesting question, so thought I'd add my \$0.02

As I said I don't ride SS, although you may ride the same gear inches, but different ring/cog combos pedal easier/harder than others, is this purely a chain cross thing because of the gears or does it make an easier gear having almost identical cog/ring sizes as opposed to bigger ring/smaller cog or small ring/big cog?
Identical gear inches being a given, the larger the polygons, the more efficient the drivetrain.

A 34x17 will be more efficient than a 22x11 even though the gear inches are identical.

This has nothing to do with the OP's 26" vs. 29" gear inch question and frankly I'm not even sure it has anything to do with yours, LyNx. Perhaps I'm just babbling again.

--Sparty

10. Originally Posted by snack64
I am trying to decide what gearing to use in the Butte 100...
I've been stuggling with exactly the same issue especially since the info about the Butte-100 course is so inconsistent...I mean how many different elevation gain statements has there been made...12k, 12-15k, 16k and most recently 18k - what is it? So at this point it's just a guess...do I run 32x19 or 32x20 or...

11. Checkout the calculator in my sig if you want to crunch numbers. I find it helpful in comparing wheel sizes to gain ratio and gear inches.

asphaltdude plugged Sheldon's explanation, always a great place to start.

12. ## gearing and elevation for butte 100

Originally Posted by Super E
I've been stuggling with exactly the same issue especially since the info about the Butte-100 course is so inconsistent...I mean how many different elevation gain statements has there been made...12k, 12-15k, 16k and most recently 18k - what is it? So at this point it's just a guess...do I run 32x19 or 32x20 or...
yeah, i hear ya. I live here and have been training on a 32 x 19 but am leaning towards running 32x20 on race day. I have done 80-90% of the second half of the hundred twice and although not the most accurate in the world my watch altimeter has given me ~ 8000' of climbing. I have not been on the first half at all as everybody who has been on it has gotten lost.

All that being said, it would be nice if we had a good solid and reliable profile to go by.

13. This race makes it really tough to pick a gear. Lots of flats and lots of climbing.
What's more frustrating for you....Spinning out or walking?

Originally Posted by snack64
I am trying to decide what gearing to use in the Butte 100 and am trying to compare the gear I'm running on my 29er to what riding buddies use on their 26ers.

question: is it really just a mathematical equation? Does a 29 er using 32/19 giving you 48.8 gear inches really ride just like ( or very similarly) to a 26er using 32/17 which equals 48.9 gear inches?

Or is there something else at play when comparing between the 2 sizes of bikes?

14. With that much elevation you are either climbing or cruising downhill. I'd be rolling 32x32.

15. Originally Posted by ionsmuse
Pick a low gear. The last 50 miles has 12,000' of gain.
Are you positive about that figure? I've also heard mixed statements about the actual elevation gain (snippet from the site)

Elevation Gain
Submitted by Anonymous on Sat, 07/17/2010 - 23:00.
What is the expected total elevation gain for the 100 mile course?

Elevation gain
Submitted by Anonymous on Tue, 07/20/2010 - 07:49.
Sorry about the late response to your question! The 100 mile is approx. 18,000'

Bob's elevation profile
Submitted by Anonymous on Tue, 07/20/2010 - 23:40.
shows 14,792 feet of gain. Bob, is that accurate for this year?

16. ## elevation gain butte 100

Originally Posted by p nut
Are you positive about that figure? I've also heard mixed statements about the actual elevation gain (snippet from the site)

Elevation Gain
Submitted by Anonymous on Sat, 07/17/2010 - 23:00.
What is the expected total elevation gain for the 100 mile course?

Elevation gain
Submitted by Anonymous on Tue, 07/20/2010 - 07:49.
Sorry about the late response to your question! The 100 mile is approx. 18,000'

Bob's elevation profile
Submitted by Anonymous on Tue, 07/20/2010 - 23:40.
shows 14,792 feet of gain. Bob, is that accurate for this year?

Yeah, like I said when I rode almost the whole 50 mile section (minus 700' climbing) I got a little over 7000'. While my altimeter watch may have been off a little it sure didn't feel like 12k of climbing to me. While I have heard from several sources that the first half is easier than the second half, none of those sources has ridden the first part.

So, rest assured it'll be a lot of climbing, some flats, some harsh 2 track, some sweet singletrack and some hike-a-biking. No matter how the day unfolds, you'll enjoy your post-ride beer. Alot.

17. Originally Posted by Sparticus
Identical gear inches being a given, the larger the polygons, the more efficient the drivetrain.

A 34x17 will be more efficient than a 22x11 even though the gear inches are identical.

This has nothing to do with the OP's 26" vs. 29" gear inch question and frankly I'm not even sure it has anything to do with yours, LyNx. Perhaps I'm just babbling again.

--Sparty
Ah...I have never seen that stated that way, but I have noticed that over the years...bigger CRs and bigger cogs feel smoother and seem to give me more per pedal stroke compared to a smaller combo of equivalent gear inches. I figured there was some science somewhere. I went from a 32/20 to a 34/21 and I like it better.

18. To directly answer the question, no, in my opinion gear-inches do not tell the whole story.

Someone mentioned it above, but (again, in my opinion) Sheldon Brown's "Gain Ratio" is the best way to simultaneously take into account gear ratios, wheel sizes, and crank lengths.

Read over his explanation of Gain Ratio carefully, and sketch it out and do the math - it makes sense. I was impressed with how simple yet clever it was after I understood what was he was doing.

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

EX: a 29er with 175mm cranks and a 32/18 gearing:

29" = 736mm dia = (368mm radius / 175mm) = 2.1, meaning the circumference of the wheel is a factor of 2.1 times the circumference of the orbit traced by the crank arms.

Apply gearing:
2.1 * (32/18) = 2.1 * 1.73 = 3.73 Gain Ratio

Basically, for every inch the crank arm travels in it's orbit, the wheel rolls forward 3.73 inches. Higher Gain Ratios mean the bike travels farther for a given crank movement, at the expense of requiring more applied torque, and vice-versa.

Increasing the number of chain ring teeth or wheel diameter increases GR, while increasing the number cog teeth or crank arm length decreases GR.

Cool huh? I hope I explained it right...

19. i'd pay attention to tires too.

pressure, traction, rolling resistance, etc. will affect effort on a long ride...which can make a particular gear-inch feel like another.

20. Originally Posted by Cygnus
i'd pay attention to tires too.

pressure, traction, rolling resistance, etc. will affect effort on a long ride...which can make a particular gear-inch feel like another.
Hmmm good point... there is definitely a difference in diameter between a 2.1 and a 2.3. I suppose you could measure your overall wheel & tire diameter and use that radius in the calculations.

It would be interesting to see how much it affects the numbers. We can do a quick and dirty estimate:

a 2.1" tire = 53.34mm
2.3" = 58.42mm
Radius of a 29er = 368.3mm

Assuming 32/18 and 175mm cranks, and adding the tire width to the wheel radius yields:
Gain Ratio (2.1" tire) = 4.283
Gain Ratio (2.3" tire) = 4.335

So, going from a 2.1 to a 2.3 only changes GR by about 1.2%. I'm not sure I would notice a difference that small.

But, I think it would be hard to factor in tire pressure, rolling resistance, traction, ride length, etc. Just too many variables, and difficult to quantify numerically. Not that they aren't factors, but at some point you just gotta fish or cut bait...

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