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Discussion Starter · #1 ·
So I am riding a 20" Karate Monkey these days. Nothing special in the build. It is
geared. I was wondering (I am being lazy), since it is built with standard Shimano
XT stuff, what happens when one takes 26" wheel gearing and stick 29" wheels on it?

The mechanical advantage drops and my low end gear is not as low as the same
gearing on a 26" bike with the same gearing? By how much????

I have read people worrying about crank arm length too? I guess I am asking
because I have noticed I am getting pretty deep into the low grears already
on steep pitches (a novice off-road rider, steep pitches, approach 45 degrees?)

As to facts, after much measurement, the folks put me on a 20" KM Frame. I am
6.1 with a 32" inseam. Seems to fit. The person who fitted me also rides a
KM as their daily ride. This is my contribution to the knowledge base, as opposed to
just asking silly questions I could answer with a bit of math :rolleyes

-r
 

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!bike said:
So I am riding a 20" Karate Monkey these days. Nothing special in the build. It is
geared. I was wondering (I am being lazy), since it is built with standard Shimano
XT stuff, what happens when one takes 26" wheel gearing and stick 29" wheels on it?

The mechanical advantage drops and my low end gear is not as low as the same
gearing on a 26" bike with the same gearing? By how much????
-r
There has been discussion about that on this board in the past, and I imagine if you search this forum you will be able to find it.

If I remember right, if you were to run a 20 T small ring up front and a 34 T large cog in the rear cassette - you would closely match at least the granny of a 26" wheeled bike. I would have to search the forum for the "math" that was presented, but perhaps that equals what a 22T/34T rear cog or maybe it was a 32 T rear cog would present on the smaller wheeled bike. I'm not sure about the middle and large chainrings, but imagine it is a couple of teeth or so difference with them as well if you were looking to match the same exact gearing ratios of a small wheeled ATB.

On my Karate Monkey, I run the Race Face Next LP compact chainrings up front (22/32/44) and the SRAM 11 - 34 cassette in the rear with the jump from 28T to 34T between the largest 2 cogs. So far, so good. My Fisher 293 has the XT rings and a SRAM 11 - 34 rear cassette.

Does anyone remember the link to that discussion?

BB
 

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Discussion Starter · #3 ·
BruceBrown said:
There has been discussion about that on this board in the past, and I imagine if you search this forum you will be able to find it.

On my Karate Monkey, I run the Race Face Next LP compact chainrings up front (22/32/44) and the SRAM 11 - 34 cassette in the rear with the jump from 28T to 34T between the largest 2 cogs. So far, so good. My Fisher 293 has the XT rings and a SRAM 11 - 34 rear cassette.

Does anyone remember the link to that discussion?

BB
BruceBrown, I will poke around. Your information is a good starting point when
I get the itch to climb trees ;) I have to admit I am lovin my KM :D

-r
 

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Yep, a 29" tire is almost exactly 10% bigger in diameter than a 26" tire of the same width, so it effectively makes the same chainring-cog combination 10% "higher". To achieve the same effective gear you'd need to reduce the size of the chainring by 10% or incease the size of the rear cog by 10%. Bruce is right, assuming you're used to a 22t small ring and 11-32 cassette on your 26" bike, you can still approach your accustomed granny gear, or even go lower, on your 29"er by using a 20t small ring and 11-34 or 12-34 casette:

22t ring, 32t cog: 0.6875 gear ratio * 26 inch rear wheel = 17.9 gear inches

22t ring, 34t cog: 0.647 gear ratio * 29 inch wheel = 18.8 gear inches (5% higher than what you're used to)

20t ring, 32t cog: 0.625 gear ratio * 29 inch wheel = 18.1 gear inches (indistinguishable from what you're used to)

20t ring, 34t cog: 0.588 gear ratio * 29 inches = 17.05 gear inches (5% lower than what you're used to).

A lot of people report that because their 29"ers climb better than their 26"ers and they can get with the same gearing as before, even though the big wheels basically make for 10% taller gears. I have not found this to be the case personally, but if that works for you a good compromise might be to keep your chainring but get a cassette with a 34t cog: overall this will split the difference between your old bike and the same gearing on your new bike. If you get the 12-34 cassette you'll have almost exactly the same high gear as now, and if you get the 11-34 it'll be about 10% higher than what you have now, perfect for those fast-rolling wheels, right? Remember that the 12-34 will wear significantly longer than the 11-34 and (in my opinion) has much better spacing between gears than the 11-34 or even the traditional 11-32.
 

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!bike said:
I have noticed I am getting pretty deep into the low grears already
on steep pitches
I miss a lower gear than 22/34 on my niner. There's one uphill track section where I live that's about 200 feet high and partly real steep. It's not rocky or difficult in any other sense, just very steep. I have my handlebars low and don't have a wheelie problem, so when it's dry at that climb I simply can't maintain the power needed to keep the wheels turning for the whole climb. And it annoys me every time. 20/34 would reduce the power need, which I think would help me up. Or I just need more exercise... Then I think it was Cloxxki who said something about 20T not being compatible with Fisher frames. Maybe possible with some tweaking?

Also I have found that I only "need" the 44/11 at almost scary speed at some steep downhill asphalt sections. So I think I wouldn't miss that one. Rather, I would appreciate having more usable gears on the big ring. So maybe something like 38T or 40T would be better.

And finally, if I go down in granny ring and big ring, I would probably want to match the middle ring accordingly, to not having to take too big jumps between any two rings. 29T or 30T perhaps.

I have also been thinking about fixing this "problem" by dumping the 11T cog, and getting a 38T rear cog, putting it inside the 34T, and having the rear derailleur work for that. How ever I am going to succeed with that...
 

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Discussion Starter · #7 ·
Thanks everyone (esp GlowBoy)

Just like to say thanks to everyone, and esp glowboy. I studied your comments
and calculations and thanks for doing all the math. Great analysis, I have kept
a copy for later reference if I decide to gear down (I am 22/34 right now.)

-r
 

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Go to Sheldon Brown's site, and use the gear calculator. You can type in your cassette cogs and chainrings for the 26" tires, and then do the same for the 29" tires. Print both outputs, and compare them. That way you'll have all your gear combinations for both setups that you can compare side by side. You can also experiment with new ring and cassette sizes and figure out if it will be worthwhile for you to buy a new ring/cassette or not.
 

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I have also been thinking about fixing this "problem" by dumping the 11T cog, and getting a 38T rear cog, putting it inside the 34T, and having the rear derailleur work for that. How ever I am going to succeed with that...[/QUOTE]

Is this possible or are you just fantasizing?? Can you buy a big ring? I have the same prob and think this would be a great solution.
 

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No 38t cogs that I know of

You could use a Spicer Granny Cog Adapter to mount a 38t chainring on your freehub, except (1) I don't think they make any 38t rings with the bolt patterns the GCA is made in, and (2) the thickness of the GCA+"cog" will mean you'll have to get along with a couple fewer cogs, and will probably leave you with a big enough horizontal gap between the 38t and the next one down that it won't shift right.

Only real option I can think of would be to have Boone (see SS board for a link) fab up one of their Ti cogs for you. At $2.50 per tooth it will be horrendously expensive, though beautiful. You must post a picture if you get one.

- Dan
 

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Tbone said:
Is this possible or are you just fantasizing?? Can you buy a big ring? I have the same prob and think this would be a great solution.
Basically just dreaming. I was figuring taking a front ring and bolt it or something to the 34T cog or to the cassette structure. Probably challenging... Haven't though about what may be needed to do with the derailleur. I more likely will change the front rings, but then it seems I also need to change cranks and bb since I have Shimano today. Wish I knew about their apparent inflexibility on ring sizes when I chose crankset...
 

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29er gearing

Haven't looked at the post in a while, but I've been very happy with my current setup and wrote about it sometime ago. I run a 20/30/42 salsa ring set mounted on a raceface turbine crank. I have a 180mm set up on my Ti race rig and a 175 mm on my GF 293. The front deraileure was a little tricky to set up on the Ti race rig because of the relationship of the derailure to the chainstay and the smallish rings, but didn't have that problem on the 293. The quality of the salsa rings provide beautiful shifting and have never experience any chainsuck or mis shifts. I've found that this set up with a 12/34 rear cassette gives me all the gearing I 've ever needed both on the high end and low end. This was the best set up I've found and I tried a bunch.
 

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Discussion Starter · #13 ·
Maybe a better motor

I was out tonight, and do have to admit, at times I feel a little bit that I could use
something a little lower ... but maybe it is not the gearing but the motor :rolleyes:

But I do appreciate the posting, I am keeping them for later consideration.

-r
 

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how to know for sure

gear inches=#front teeth divided by #rear teeth divided by wheel diameter.
Grab yer calculator and compare yer 26" wheel bike's gears to yer 29" wheeled bike's.
No printouts or websites needed, you can do it in the shop.
 

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Discussion Starter · #17 ·
Don't know, skipped the 26" thing

I don't really know, I went from road to 29" skipping the 26" thing except
for some messing around. Maybe just slow the cadence a bit on long steep
uphills ... or like with skiing ... the hills will wear down faster than I ;)

I have looked for a 20T chainring for Shimano XT, TA seems to build them,
but I can find a US source? Where to find a 20T chain ring???

On the shimano XT changing the inner chainring seems easier than the cassette????

I do believe a 29" is faster than a 26" all things being equal. Would be fun to run
the same course on a 29" and a 26".

-r
 

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29er are fast

I've raced both 26 (two years) and 29 (last season) and once I got the gearing corrected I know I'm faster on the 29er in almost every condition we have here in Texas. Not only is it faster, but I'm physically better at the end of the race. And no 26 can out sprint, outclimb or out technical a 29er given equal riders.

The only place I found rings was through Salsa. When you buy the matched with, correct ramps and pins, it works superb. Had to buy the raceface cranks because of the small ring size. Like I said the only problem was the front derailleure placement relative to the chainstay because of the small rings. Once I worked that out I've had zero problems and some tremendous benefits. Sometimes I think many of the problems some have is mismatching rings.
 
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