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Being relatively new to all this I have some questions:

1. How much does wheel diameter affect gearing choices?

2. How much does crank length affect gearing choices? (I run 170's, which I'm learning is not the norm for SS racers; but I'm not terribly big and neither is my bike.)

3. How often do the rest of you change cogs (depending on terrain, mood, et al)? Example: right now I race on a 26" wheeled Monocog and for short-track season I'm running a 32 x 19 (it's a course with a lot of moguls and bumps). For 'cross season I usually swap to a 32 x 18 for the flatter, drier course; and later in the fall when it gets good and muddy I will swap to a 32 x 20. Since I am using Surly cassette cogs it is quicker and easier for me to just swap the rear cog, and the length of the track slots gives me plenty of room to adjust chain tension without worrying about how many links I have/need.

I'd like to know what other riders use and if their choice of wheel size affects their gearing choice very much. Thanks --bh

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Replies to This Discussion

Wheel size does affect gearing. I've heard it said that the difference between a 29er and a 26" bike is two teeth in back. That's obviously not completely correct (since we're ultimately talking ratios), but isn't a bad rule of thumb for the 32ish chainrings we're talking about for mountain bikes.

I'm less sure of the math surrounding crank length. My intuition says that longer cranks make it possible to push a bigger gear, but conversely make it harder to spin a high RPM, so you'll get less high end out of whatever gear you're running. I tend to see crank length as overrated in general, though, cause I switch back and forth pretty easily between 170s and 175s (and I'm short!).

For cross, I will rarely change cogs (actually, I'm running freewheels, but the same principle applies). For super flat courses I'll drop a tooth in back from my standard, and when I was first riding single speed I would go up a tooth in back for hilly courses, but I don't do that anymore. On the mountain bike, I pretty much always run 32x20 on my 29er, although I've been meaning to pick up a 19.

On the cross bike I run a much higher gear: generally a 39x17 or 39x16. 170 cranks on the mountain bike, 175s on the cross bike.

You can play with the numbers and see how they compare with what you're used to on Sheldon Brown's gear calculator.
I use a flip flop hub & adjust my chain tensioner accordingly. However % have found if I need the easier gears for a hillier course_ if the downhilss are longer vs steaper_ I just spin out. If there's sustained climbing_ with short steep hills, then I can get awayw/ an easier gear.

I think the longer crank is only beneficial if you are taller can put out more power to push a bigger crank. Iwould thin having a longer crank would effect your ability to spin efficieently & effectively_ if you are a novice. I used to ride a ss mtn bike a few yrs ago. It's hard. Most people I know that race them well have been racing a long time or just good mtn bikers... Most don't start out racing ss and race well. Cross is a little different. The lighter bike (mine is 17#'s) is a huge advantage in a muddy course. Easier to shoulder_ less to go wrong. If the couse is flat_ go with the harder gear_ work on power_ & momentum. Remember it's 50% bike & 50% rider. I finally learned if I want to get better, I have to train smart_ work on your weaknesses.... Mine is powere & spinning fast to go fast..... Good lungs & good fitness helps. Good luck this season. Remember to have fun & relax, & just use each race to learn something new.
I currently run a pretty big gear, 41-18. But.. I own a 36 and a 39 so I can change prior to the race. Of course this takes some specific tools, and the skills to shorten the chain and reset the brakes.. But it is all worth it once you have a solid feel for how your body can handle a given course. Did I mention.. "pre-ride" ?
The first SS race this year in New Mexico, was a long course with some long climbs. I used a 34 x 16. I figured I would struggle on the climbs, but smoke people on the downhills. This kind of worked till I got really tired and my back started to hurt on the climbs. There was a long twisty grass section where I lost lots of ground to a guy with a very small gear. I would have been better off with a smaller gear.

I have a chain tensioner, so with a few tools I can quickly change cogs, but I typically use smaller gears, like 34 x 20 and don't change that often. I will probably stay with the 34 x 20 for most of the season.
For short-track I raced 32 x 19 and it mostly worked pretty well (though a couple of very tall berms on the motocross part of the course were so steep I had to get off and run).
For the first Cross Crusade race at Alpenrose I swapped in a 20t cog and it seemed almost perfect for the faster, dry conditions.
This week I'm swapping in either a 21 or 22 for PIR because it's a long course with some steep sections, and because the forecast calls for plenty of RAIN, which will make for plenty of MUD. It will probably feel a little too easy to pedal on the downhills, but just about right for the short, steep climbs I know I'll encounter.
Nice part is having a singlespeed cassette rear hub that makes changing cogs easy. I keep an assortment in a woodn recipe file box in my shed and swap them in and out as needed.
hey beth - of course it depends on pedaling style, power, leg length, etc. but for the longest time I've mostly ridden a 39x17, my seat height is about 76cm, and I'm mid-pack B racer. i used to be a masher, but now post kneecap fracture am more of a spinner, but still that gear is pretty good for me. i do change the cog by one tooth once in a while.

sasha's rule of 2 teeth between 26" and 29" is a pretty good estimate. there' about a 10% wheel diameter difference between the two, 10% is about about 2 teeth based on a 20t cog...just a gross estimate ASSUMING you're running a narrow cx tire on the 26" wheel. If you're running mtb tires it's a lot closer to a 700c wheel.

crank length - we used to see most going really long, but now folks are getting custom short cranks for mtb ss. so no real rule of thumb there.

good luck!
Thanks. The shorter crank thing has always made sense to me, especially for riders under 5' 6".
I scored what is apparently the last set of Truvativ Stylo 1.1's in 170mm length for my singlespeed bike; SRAM says there are no plans to offer that length as an aftermarket item. (sigh)

For this Sunday's race (PIR, super-muddy with lots of short, steep climbs and off-camber action) I'm gearing 32x20 and hoping I can handle it. The only thing I've done to my bike to prepare is to clean everything up (seems futile, with all that mud), and swap in some new salmon brake pads.
I run Conti Cross Country 1.5's in the mud and they are just about the Best Tire That Ever Stopped Being Made. (What IS it with good stuff going extinct?)


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