What is the correct terminology for bike speeds?

Factual Questions
1

I recently bought a 27-speed mountain bike – 3 sprockets in the front and 9 in the rear. What I want to know is how does one describe the gear they are in? I have always kind of assumed the relationship was multiplicative. So you would number your sprockets from 1 to 3 in the front (1 being the smallest, i.e. easiest) and 1 to 9 in the rear (with 1 being the largest, i.e. easiest). Thus 1 in the front combined with 4 in the rear would give you 4th gear, 3 in the front with 7 in the rear, 21st gear, and so on. The larger the number, the harder it is to pedal, with greater resultant velocity. Of course I know that there are combinations and overlaps, so each gear is not unique.

However, I have spoken to other non-velophiles, and they use no such system.

So is there is correct system?

(another question: is sprocket the correct term for what I am referring?)

2

You’ve got it right. There are other ways of measuring gears. The most common one is to divide the mumber of teeth on the front chain ring by the number of teeth on the rear sprocket and then multiply the result by your wheel size. This gives you how far you go forward* with each pedal stroke.

  • Cyclists are notoriously bad at math.
3

I just refer to them by their combined number as a two digit group. So 2 at the front and 6 at the back would be 26. Of course it makes no numerical sense at all, and bares no relationship to the actual number of gears available. I only use it for my own reference and it means that there is a distinct number for every gear combination.

4

I usually say somthing like “3-6”, then explain what I mean. Cyclists seem to go by the number of teeth on the gears: “54/39” or something. Don’t know how they remember that.

5

The front “gears” are called chain rings, with a standard setup being two chain rings with 52 and 43 teeth. The rear are called cogs, and a 7-“speed” gear cluster will have odd- or even-numbered cogs, for instance, 11-13-15-17-21-23-25. That’s real basic, there are any number of variations on top end bikes, including the addition of a third chain ring that’s starting to pop up on a lot of bikes. So in bike vernacular, you can say “I was hammerin’ in 52-13 when I bonked so bad I had to park on the side of the road and feed the carp”. (Going flat out in big chain ring, second to last cog, dehydrated/fuel spent, blew what’s left of lunch/breakfast)

6

Note that you should avoid the extremes (far left in front, far right in rear)

Multiplying the gear ratio by the wheel size gives you the “wheel inches” which means the bike would act the same if it was one big wheel (like the old fashioned bikes) of that size.

Here is the table for my bike (Trek 520)



		Front	
Rear	30	42	52
32	25.8	36.2	44.8
28	29.5	41.3	51.2
24	34.4	48.2	59.7
21	39.4	55.1	68.2
18	45.9	64.3	79.6
16	51.7	72.3	89.6
14	59.1	82.7	102.4
12	68.9	96.5	119.4
11	75.2	105.2	130.3

(using 700mm ~= 27.56 inches for wheel size)

Frankly, I just use the middle front chain ring unless I’m going up or down a significant hill

Brian

7

Forgot to mention, my previous post is referring to road bikes, most mtb’s have three chain rings. The third or smallest chain ring is usually called the granny gear.

8

I think you mean 53/42. But that would be for a really fit racer. Most people would have a set-up like 53/39 for the front.

9

Thanks everyone. Somehow I knew this would be complicated, but I actually think I understand the jist of it. I think one of my dreams is to be part of the cycling subculture! :smiley:

10
11

Doh!! I stand corrected. :smack:

12

Gear sizes for cyclists are an absolutely important aspect of racing.

On mainland Europe they use the most logical method of calculating gear ratios, the distance traveled forward in one revolution of the cranks.

In the UK, and former British colonies the process is far more arcane.

All gear ratios in such places are calculated by converting the gear ratio to a wheel size.
This relates to the earlier days of cycling, when those who rode cycles had penny farthings, or ‘ordinary’ machines.

During those days, there was only one gear, and that was determined by the size of the front wheel, the larger the wheel, the larger the gear.
This of course meant that your height determined just how large a front wheel you could have.
This one-to-one crank to wheel rotation ratio actually meant that just about every rider was undergeared, they simply could not pedal fast enough and this limited top speed.
Not good when you are rolling downhill and you have neither a freewheel device, nor good brakes.

When chains became economic to produce(shafts were tried but were just too heavy and inefficient) another type of machine came into being, this had two equal size wheels and the rider was far closer to the ground.
It was much easier to ride and easier to learn, and far less likely to lead to a crash, so they were marketed as ‘safety’ bicycles, and the large wheeler, which was at first very much more common, became the ‘ordinary’.

One huge advantage of chain drive, even without a freewheel, is that by changing the size of the sprockets, you can easily change the gear ratio and this made it possible to exploit more of the torque that the rider can produce.

The calculations on the gear ratios were aimed at comparing the ordinary with the safety, and gearing meant, effectively, that riders of safety machines could turn a much larger diameter front wheel than the ordinary rider could.

The result was that on a safety you could ride faster and further, what is more, by having a wheel with a number of sprockets on it, you could change the gear to the one most suitable for the terrain upon which you were riding.
Since this was in the days before freewheels were invented it meant you had to dismount, unfasten the back wheel and move the chain by hand and move the wheel to maintain chain tension, and fasten the wheel up again.

To calculate gear ratios the UK way,

divide number of chainring teeth(Ct) by the number of sprocket teeth(St), and then multiply by the radius of the wheel(Wr).

(Ct/St)*Wr
If you look in any specialist cycle shop you will find gear charts where this has already been done for you.

Down one side to the chart will be the chainring sizes in teeth, and across the top will be the sprocket sizes.

You can do some fairly interesting maths here to illustarte the use of powers, iterative calculations and if you really want you can get into logarithmic curves, and some integration too, true you have only a limited number of points to plot since the number of teeth can only ever be whole numbers but its not a bad little study to do.

Depending upon the kind of rider you are and what sort of riding you aim to achieve, I can give you ots of useful advice on how you should assess your gearing needs, and since everyone is differant, customised gearing should be tailored to suit your needs rather than stick with the standardised shop stuff that the casual rider uses.

13

Thanks casdave. I think I am pretty basic at this point, so don’t think I’ll be needing anything customised quite yet.

But a dumb question: I am using my mountain bike with lugged tires to bike to work on paved roads. I have noticed a whirring sound coming from the front. Is it pretty safe to assume that it is coming from the friction of the lugs on the pavement? I haven’t had a chance to look very closely, but it doesn’t feel like anything is rubbing up there.

14
      • Get “city” tires–1.5-inch wide slicks for MTB’s. There’s expensive ones at bike shops, but even cheap $10 ones at Wal-Mart will work better. Inflate them to 50-65 PSI and you will like them much better on pavement, they accellerate and roll a lot easier than big fat knobbies. {And do note-you will need 1.5-inch-compatible innertubes as well!}
        ~
15

Yes, I had the option to switch the knobbies for city tires when I bought the bike, but chose not to. First of all, I do like to go offroad occasionally, and couldn’t be bothered with changing tires at that time. But secondly, my commute is only 7 km, and I want to get as much exercise out of it as possible. So a little extra drag doesn’t bother me.

I do wonder about the humming noise though, in case something is rubbing and will eventually wear through…

16

casdave – Maybe I missed something, but how, exactly, is your ‘UK’ system for describing gear ratios different from the ‘continental’ system?

The 'UK" system you outlined basically expressed the gear set up as the size of a imaginary wheel you’d need to get the same distance moved per crank turn if the imaginary wheel was being driven 1:1 (or as an imaginary ‘ordinary’ wheel. Same thing). So a gear of ‘72 inches’ or whatever means you’re putting forth the same effort (and moving the same distance per crank turn – 72 inches) as if you had a 72 inch ordinary, or a 72 inch rear wheel with equal sized front and rear sprockets.

So how would the ‘continental’ system be different (other than using metric units)?

17

That sound IS the knobbies pounding the pavement. Nothing to worry about. If the tire spins freely when you lift the front wheel off the ground and spin it with your hand then you can be assured that the brakes aren’t binding.

18

What you are describing is usually called “gear inches.”

I believe what casdave meant by “continental” system is “development,” i.e. how far the bike moves forward for every crank rotation. It’s gear inches multiplied by pi.

You can calculate either number on the Sheldon Brown’s gear calculator page.

19

Quercus

The UK system does not actually measure the the actual distance you would travel forward upon one revolution of the cranks, it actually shows the size of a wheel diameter, thus in UK terms a 72" gear is meant to refer to a theoretical wheel size.

This is obviously a completely impratical wheel for a bicycle but the system was devised to compare ,what was at the time the less common, ‘safety’ cycle to the much more common ‘ordinary’ cycle.

The ‘Continental’ system does not do this, it simply records how far a machine wil travel upon one turn of the cranks, which is a real world measurement rather than the theoretical UK method.

In the end, either method is used to compare gear ratios in a chart, and both give relative values to gears, so the need for a real world measurement compared to a theoretical one is just as valid, so long as you compare like with like.

20

Cool post, casdave. I’d seen the term safety bicycle in writings of the period and wondered in passing where the term came from. Now I know. I vaguely figured it was the closer to the ground part – hurts less when you fall over.

DD