Culley22 wrote:
So...I have 11-32 on my rear gearing. I have no clue what these numbers mean for riding
Alright, gearing.
Imagine that you're riding on a wheel, with cranks attached directly to the axle of the wheel. You might call it a unicycle. Add a second wheel and call it a bicycle if you want, it doesn't matter. The important thing is that the cranks are fixed to the wheel axle, so that one revolution of the cranks corresponds to one revolution of the wheel.
On this machine, every time you turn the cranks forward by one revolution, you move forward by a distance equal to the circumference of the wheel. If your wheel has a circumference of 168 inches, you'll move forward by 168 inches. By using a smaller wheel, you reduce the amount of resistance that you have to push against on the pedals, but you also have to spin faster for a given speed. So, the size of the wheel describes how high the gearing is.
Now, wheel circumference is kind of clunky to measure on the fly. It's easier to measure wheel diameter, so back in the 19th century when bicycles were actually built like this, people would describe the gearing by saying what their wheel diameter was. The gearing that results from a wheel with a diameter of 60 inches is called 60
gear inches.
But we quickly run into a problem: if we want a very high gear, we need to use a huge wheel. Suppose we want gearing equivalent to what Eddy Merckx used for his hour record: we would need to use a wheel with a diameter of about NINETY-EIGHT INCHES, or about two and a half meters.
That's a bicycle for giants.
So, we need a solution. Instead of using a huge wheel, we'll use a reasonably-sized wheel. In fact, we'll use a wheel where the outer diameter of the inflated tire is around 26.4 inches, which is approximately what happens when you mount a 25mm tire on a 700c wheel.
Now, if we attach the cranks directly to this wheel axle, we have a really low gear: a 26.4" gear! That's lower than the lowest gear on most road bikes. So we need to do something other than attach the cranks directly to the wheel axle. We'll use a
chain drive.
So, we stick the cranks somewhere else on the bike, and we'll call this spot the
bottom bracket. And we'll bolt something to the cranks: a toothed sprocket wheel that can engage with the chain. Because this is a ring-shaped object that engages with the chain, we'll call it a
chainring. In our case, let's suppose that our chainring has 52 teeth: if we put a chain onto the chainring, it will pull that chain forward by 52 teeth every time the cranks make a full revolution.
Now, we need a wheel to be driven by the chain, so we stick a second toothed sprocket wheel onto the rear wheel. We'll just call this one the rear
cog or rear
sprocket, because why not. In our case, let's suppose that this cog has 14 teeth: this means that the rear wheel will make one revolution every time the chain gets pulled forward by 14 teeth.
We now have our complete chain drivetrain!
Okay, so what's the gearing?
Every time we turn the cranks through a complete revolution, we pull 52 teeth of chain. Every time we pull 14 links of chain, the wheel turns over. So with every crank revolution, the rear wheel spins 52/14 = 3.71 times.
So, our gearing is 3.71 times higher than what we'd previously expect for a 26.4-inch wheel. And 3.71*26.4 = 98, so our gearing is 98 gear inches. So our gearing is equivalent to that 98-inch wheel we were previously wanting to build, but we don't need to be a giant to ride the bike! Hooray!
But, having only one gear on our bike is kind of annoying. Maybe for our climbs we also want a gear that's only half as high. To accomplish this, we can simply double the size of our rear cog. So we create a cassette consisting of a 14-tooth cog and a 28-tooth cog. A 2-speed 14-28 cassette! Yay!
Errr... actually, this cassette still kind of sucks.
Maybe we're feeling fine in that 98" gear (52-14) when we're doing 30mph, and we're also feeling fine in that 49" gear (52-28) when we're doing 15mph. But what if we're going 20mph? In the 52-14 we're slowly grinding at the pedals with tons of resistance, and in the 52-28 we're having to spin the cranks at a super-fast 137 revolutions per minute.
So we really want some additional gears in between those two extremes. If this in Eddy Merckx's era and our freewheels tend to have five cogs, it's likely that the additional three cogs will be 17-teeth, 20-teeth, and 24-teeth. So this is now a 5-speed 14-28 freewheel, but a more precise description is that it's a 14-17-20-24-28 freewheel. After all, 14-16-18-22-28 would also be a 5-speed 14-28 freewheel, but our intermediate ratios are very different: they're more closely grouped in the higher gears, and more widely-spaced in the lower gears.
If we want to get really creative, we can also add a second chainring. If this is in Eddy Merckx's era and one of our chainrings has 52 teeth, it's likely that our second chainring has 42-teeth. 52-42, the standard classic double! Or at least, until people started calling 53-39 a standard classic double. Perhaps one day, 50-34 will be called a standard classic double. Oh well.
Anyway, the gearing math works out pretty much the same. 26.4*52/14 was 98, but 26.4*42/14 is 79. So if we're in our 42-tooth chainring and our 14-tooth cog, we have 79 gear inches. That's about 19% lower than the 98 gear inches that we get when we're in the 52-14 combination, because the 42-tooth chainring is about 19% smaller than the 52-tooth chainring.
Quote:
but was under the impression the “32” is for really big hills (true?)
I don't know.
What's a "really big hill?" If a road winds upward at a 2% gradient for 100 miles, you'll end up climbing over 10,000 feet. I'd call that a "really big hill", but it's not very steep and I don't think I'd need a super-low gear to manage it!
Also, how high of a gear the 32-tooth cog corresponds to depends on the wheel size and the chainrings. Now, most road and TT/Tri bikes have a wheel in the neighborhood of 26"-27", so there usually aren't big differences there (unless we bring stuff like 650c wheels into the picture...). But chainring choice can have a big impact. Like if you have a 53-39 crankset versus a 50-34 crankset... the 39-32 combination is 15% higher than the 34-32 combination. It's just as big as the difference between a 28-tooth cog and a 32-tooth cog!
Or, consider my gravel bike:
The lowest gears on this bike are much lower than the lowest gears on my road bikes, because the unpaved roads in the foothills can have long sections of steep gradients with loose material.
But, that cassette doesn't look like the big cog is very big! In fact, it looks suspiciously like a 28-tooth cog! And as it happens, it
is a 28-tooth cog.
The subtle detail that's not clearly visible in the photo is that the smallest chainring has only 24 teeth. The lowest gear on this gravel bike is a mere 22 gear inches. Even at only 6mph, I can still be pedaling at ~90rpm if I want to.
So even though the big cog isn't that big, it creates a very low gear when paired with an itsy-bitsy chainring!
BUT ALSO We need to consider the rider. Even if we can agree on what a "really big hill" is, and even if we're accounting for the size of the chainring, we can't say "such-and-such ratio is for such-and-such" riding if we don't know who the rider is. The variation in rider ability that exists in the real world is
HUGE. Elite climbers can dance up hills well over twice as fast as weak cyclists. On a steep mountainous ascent, some average guy might be struggling to turn his pedals over in a 34-32 gear combination, but Giulio Ciccone might be totally at ease on the same hill in a 39-23 combination.
So is the 32-tooth cog for "really big hills"? I don't know. Maybe. Could be. Depends.