In this video, I talk about how to work out your gear ratio and the difference gearing between compact and semi compact cranks.

Video Transcript

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Hey, it’s David Heatley here, and I just wanted to talk to you about gear ratios. Now, I had this question asked of me, “How do I work out my gear ratios? Why are some of my gears similar to other gears?” So to get into it, what I want to do is first show you the formula, and it’s a gear inch formula. I’m just going to share my screen.

All right. So the gear ratio formula is your front chain ring divided by your rear sprocket times 27. And the reason why we use 27 is that it’s the standard that we’ve used for quite a few number of years, and obviously, we ride on 700c nowadays. But the issue is that when you’re talking to gear ratios with people that ride bikes, especially when we’re down on the track in the velodrome, well, you always talk about gear ratio inches. So that’s the standard that we use.

And what I’ll do is I’ll just go to a gear ratio chart. So this is a chart that I created a few years ago, and you notice it only goes down to 30. And a few years ago, 30 wasn’t as common. People were riding, generally, 52 or 53/39s. That was really the common thing, and with the advent of semi-compact and compact crank sets and bigger cassettes for road bikes, we’re seeing differences in gear ratios.

So let’s go through and explain how this work. So we see 11’s usually the smallest cog that you can get your rear cassette, and this is the chain ring. So we’ve got … You know, it goes up to 54, and it goes down to 24. Now, 53 is here, so that’s 53, and with all those gear ratios. So a 53 11 will give you a 103 inch gear, okay? And we’ll see, actually, when people talk about running out of gears on a compact crank set … we’ll just go to a compact crank set and look at what the gear ratio is on a compact crank set. So the largest chain ring on a compact crank set is 50, which gives you 122 inch as opposed to an 130 inch gear.

Now, the interesting thing is that if you have a standard crank set, 53/39 … there’s 39 there. All right. And you compare it to a compact crank set, which is a 50/34, you’ll see that the top gear for, say, a 53 x 12 is 119 inches.

Now, if you go to a compact crank set and put an 11 on your bike, which is a 50 x 11, it’s 122. So you’ll see that a 50 x 11 is a bigger gear than a 52 x 12, all right? So if you’re running a 53 … sorry, 53 x 12 and you go to a compact crank set and you put on a 50 x 11, you’ll actually have a bigger gear on your bike as your top gear, all right?

Now, let’s look at the bottom range of the gearing. So if we go to a 50 x 30 … what is it? 39 for a compact crank set … where’s 39? Here it is, here. And we look at, say, 28, which is … 28 down through here, and here it is. So that’s a lower gear, so that’s 37.6 inches. Or well, we usually round it out, and that would be 37-38, because we’d round it up, because it’s .6. So you round to 38 inches.

Now, if we look at a compact crank set that gives you a 34, you’ll see that the number here is 32.8, so 33 inches. Now, you’ll notice that there’s not much of a difference between those two gears, but enough to make a difference. And what happens with these gears, you’ll find that towards the upper end, the changes between one gear and another is quite big, the change between one tooth and another. If we look at 53, we’ll see 119 and 130 here, all right? So big difference in the number of teeth. But when we go down this range here, we see the difference between one tooth and another tooth is a lot less, alright? So just understand that, that as you get lower on your cassette, you need more teeth to make a substantial difference in your gear inch, all right?

So now, the other thing that I wanted to cover off in this is that sometimes, you get overlaps in your gears. So if we look at, say, a standard crank set, 53, and we look at some of the gears down here, if we’ve got a cassette that goes from 28 through to 11 … obviously, we don’t have all of these gears here. But you will see that if we’re on the big chain ring, we’ll see that the big on big, big chain ring, big sprocket at the back if we’re running a 28, the lowest gear we’ve got is 51.
If we look at 39, here’s 39 here. And we look at the gear ratios on that chain ring that we get, we start off with 37. And we go up to on the 11th sprocket, 95 inches. So you’ll see that there’s actually quite a bit of overlap in those gears, all right? So you’ll see that when you’re on the big chain ring, on the big sprocket on a 28, you’re at a 51 inch gear. And you’ll see that that’s equivalent … here’s 52. It’s equivalent to sitting in your low chain ring on a 52 x 20.

So that overlap is important, because what it means is that some people feel a bit ripped off. They’ve got a bike that’s got 11 speeds, so they’ve got 22 gears, and some of the gears are similar on the lower chain ring as they are on the big chain ring. But the great thing about that is that when you’re on the big chain ring, when you’re riding fast along a flat and riding downhill, well, then you’re on the big chain ring. And you’ve got this selection of gears that you can ride along, all right?

Now, when you’re on a small chain ring, again, you’ve got … You know, you usually use the small chain ring for riding up hills, riding into headwinds. So when you get into the small chain ring, you’ve got this range of gears. And it means that you don’t have to switch from the big chain ring to the little chain ring or the little chain ring to the big chain ring to select gears in sequence, like a car.

So it means that you’re not having to change from little chain ring to big chain ring all the time when you’re just basically riding in this range of gears for a particular type of terrain, so uphill or into headwinds. And then, you’re riding on this range on flat roads and in tailwinds and down hills.

So I just wanted to explain that to you. Look, let me know if you’ve got any questions. It’s been David Heatley here from Cycling-Inform.