|CyclingSite > Lists & Articles > Gearing 101 Tutorial|
|Gearing 101 Tutorial|
|By Rox Heath|
How to examine your current gearing and decide on what changes (if any) you would like.
This is kind of a long lesson. Split it up and do pieces as your schedule allows.
Before you begin this lesson you need to count the teeth on each gear – both front and rear – on your bike. Write this info down and save it for the lesson.
First let's agree on some terms:
A "gear" is a wheel with teeth that mesh with another gear or a chain.
A "T" stands for teeth. So a 26T gear has 26 teeth.
The gears on the front by the pedals are usually called "chainrings". I have had them range from 20T to 52T although I'm sure they come larger (and maybe smaller).
If you fasten several rear gears together it becomes a "cassette". Some bikes have all of their gears in one cassette, some have just a few "glued" together and then some individual gears fastened to them to form the whole rear cluster. For ease of communication we are going to refer to the whole rear cluster as a cassette when doing our calculations.
Technically, a "cog" or "sprocket" is just the tooth on the gear, but those terms are often used to indicate the whole gear.
The "shifters" are those things (usually on the handlebars) you move to change gears. They can be levers, pushbuttons, or part of the handgrip.
The "front derailleur" is the little gadget down by the chainrings that nudges the chain back and forth from one chainring to another when you shift.
The "rear derailleur" is the gadget in the back that has a couple of small wheels and a long arm that the chain wraps around and runs through. As you shift it moves to the gear you want, lining the chain up to ride up on the new gear.
Getting your gear inch numbers
Now dig out that grubby piece of paper you wrote your gear count numbers on and let's get started...
Go to http://www.sheldonbrown.com/gears/ and start plugging in numbers. Enter your Wheel Size (or close to it) and ignore the Crank Length for this calculation. Under Gear Units choose "Gear inches". Under Chainrings type in the sizes of your chainrings (smallest to largest). If you only have two then leave the last box blank. If you have a Stock Cassette and can find it in his list then go ahead and choose it. Otherwise go to Custom Cassette. Enter ALL of the sizes of your rear gear cassette (smallest to largest). If you run out of gears just leave the right end boxes blank. If you have an Internal Hub choose the proper configuration. Now click on the "Calculate" button. Either print this page or copy it down somewhere. And say a heartfelt "Thanx!" to Sheldon Brown for saving you from having to figure all this out for yourself!
These numbers are the gear inches for each gear combination your bike will do. They are also the numbers that let you compare these combinations.
What is a gear inch number?
Definition of gear inches: Imagine you are on one of those old antique bikes with the big front wheel with the pedals attached directly to it. The gear inch number is the diameter (the distance measured across a wheel from one side to the other and passing through the center) of that big front wheel. Naturally, the bigger the diameter (gear inch number) the farther you will go for each pedal revolution. Similarly, the smaller the diameter (gear inch number) the shorter the distance for each pedal revolution. You can also see that it would be easier to push the pedals on the smaller wheel than on the larger.
For the curious, the distance traveled for each pedal revolution is the gear inch number times pi (3.14). For the mathematically disinclined who can't find a calculator just multiply the gear inches by 3. Close enough! Imagine you are spinning along a nice even stretch of flat ground. Every time your right foot hits the lowest point of a pedal revolution a spot of paint hits the road. This is the distance you would get if you measured between those spots. Now, to reverse this process (again for the "just curious" group)... Think of how far you would like to travel for each pedal revolution, divide by 3 (or 3.14) and you have the gear inch number you would need.
Looking at your gear inch numbers
Now we want to use the gear inch info for our bikes to really look at what we have. I find it much easier to look at this with the format Dick Marr uses in his book, Bicycle Gearing. To convert it over take a piece of paper and make a table. Along the left edge of your paper list the sizes of the rear gears you have starting with the smallest. Across the top list the sizes of your front chainrings, starting with the smallest. Then fill in all of the numbers the website calculated for you. I like this format because I can now get on my bike and stare at the piece of paper and the order of the chainrings on the paper matches the chainrings on the bike. Your table should now look like this:
(The rear cogs are the left-hand vertical column and the front chain rings are across the top.)
Notice I have rounded the numbers off. This table is for a 21 speed bike. Your table will reflect your bike 's gearing .
As you can see from the table this bike has a gear inch range of 19 to 103 (smallest number to largest number). For the average (not hardbody) first timer on CO this is a pretty good range. With the 19 gear inches most people in reasonable CO shape should be able to handle those 5 mile long 5% grade hills although they may need to stop and take a break occasionally. The 103 gear inch combo will let you cruise along at a pretty good rate of speed (unless there's a head wind) on those flats or moderate downhills. On the steep downhills you won't be able to pedal. Cyclists who can spin fast will not need as high of a gear inch number for the top end since they are doing more pedal revolutions than the rest of us. Or they keep the high gear inch combo and just go faster still.
Now as we look at this table the first thing to notice is all of the overlap between the columns. It looks like you are traveling the same distance at the top of the 20T column as you are at the bottom of the 42T column. So why not just throw away the middle gear? In order to ride comfortably and efficiently you need more overlap than just the two combos. Part of the problem is that to get from the top of the 20T column to the corresponding combo on the 42T column you would have to go through a large number of shifts - and each shift causes you to lighten up on your pedaling for a short time. The overall effect is a loss of momentum and an unacceptable slowing.
Now, put the middle chainring back in. This immediately "outlaws" the top of the 20T column and the bottom of the 42T column because they cause the chain to stretch diagonally across the gears. This causes excessive wear on both the chain and the gears. Occasionally, you may just decide to grin and bear it, but you are wearing out your bike faster.
One other point I need to make here - When you are in a small gear and jump to the next you will notice a difference of 3 or 4 teeth. When you are in large gear and jump to the next larger you will only notice a much larger difference in size. The person riding the above gearing will feel like the difference between the 19 and 22 is similar to the difference between the 87 and 103. Similarly, the average person can't tell much difference between 19 and 20, but they can between 19 and 22. They will feel no difference at all between 47 and 49 - so just think of them as "equal".
Now I am going to copy the table here again so we can refer to it easily:
Start noticing the shifting patterns. Make a copy of your table and start marking it up. Your table may have much more overlap and smaller steps between gears than mine does. We will get into that at a later lesson. If I were to shift upward in smooth steps as I accelerated I would use the first three (or four) of the combos on the 20T column and then shift to the 30T column. For instance I might use 19, 22, 26, 30 on the 20T column and then jump over to the 31 on the 30T column. At this point I have actually shifted between "equal" gears which is somewhat pointless. So after a little practice I move both shifters nearly simultaneously. (Don't move them simultaneously or your chain may jump off.) I jump from the 20T chainring to the 30T chainring in front. I also downshift 2 gears in the rear so I end up at 36. I then usually climb up the middle chainring to the top before jumping over to the 87 on the 42T chainring.
Now in unloaded, on-the-flat acceleration you will not want to hit every step. Just like the truck driver with the empty truck you are going to skip gear combos. But it helps to make this clearer to see the full pattern of gear inches on your bike. Now go out there and ride around and notice the patterns of shifting you have established and what gear inch numbers you like for different types of terrain. Try some different shifting routes of gear combos and see if they are smoother or easier. Experiment with this info. Dick Marr suggests making a small copy of your table and fastening it to your handlebars to make it convenient to refer to.
[Time out for some bike riding...]
Back to work...
If you have done your homework you should now be very familiar with your shifting patterns and have a good idea of which gear combos you like for different terrains. Make a note of the gear-inch numbers for your favorite combos because you will need that info later in this lesson.
If it has been a few days go back and read the top of this so that it is familiar to you. You will be using it when looking at the new gearing you are contemplating.
Now let’s compare a couple of different gear set-ups:
Bike B (our old friend from above):
The difference between two adjacent gears is called a "step". Thus, on Bike A, the difference between the top two gears is 103 – 95 or 8 gear inches. On Bike B the difference is 103 – 87 or 16 gear inches. Twice as much! Okay, that’s all the math for now…
An advantage of having those gears close together is that you can shift small amounts smoothly. If you are carrying a big load of gear (touring) you would probably prefer smaller steps, although the larger steps would certainly work once you were used to them. Another reason to have smaller steps is to help you accelerate faster. It takes more muscle to accelerate those bigger steps fast.
Low and High Ends
Looking at these two tables you can see that Bike B has a MUCH lower gear than Bike A. In fact, when Bike A is in its lowest gear the corresponding gear inch number on Bike B is halfway between its third and fourth gear up. This makes a huge difference to the riders on those loonnngggg hills. If you are a hardbody who likes to mash up those hills Bike A would probably be your choice. If you are not a hardbody or carrying a lot of gear you would probably want to go down to 19 gear inches (or similar). Another group of people who want lower gear inches are those that are good at spinning fast.
As far as the high end goes – if you spin fast you don’t need as high of gears to go the same speed. If you are a slow spinner you will want them. Some bikes go MUCH higher than 103 gear inches. If you are racing or one of those who has to get from camp to camp in a hurry then this is important to you. If you are going to comfortably cruise along stopping to gawk at the scenery and getting distracted you may not even get into that 103 gear inch combo except on the downhills (and you don’t even really have to have it there).
The point I am trying to make here is that gear inch ranges are extremely individual. Don’t let your buddy’s set-up influence you unless he or she has a very similar body and riding style. And even then, you will choose gears differently.
So where do you go from here? Try to imagine what you want. If you are going to change to a gear set-up that has a narrower range it is easy. Just imagine that you are missing the gears you are going to delete and ride without them. How do you like it?
If you are going to add more gears to the bottom try to imagine how many more steps you would like – or give it up as hopeless and go for something around 19 gear inches – which will probably work for most CO hills. Usually CO does not have too much in the way of loonnnggg hills at more than 5 or 6% grade. And if you are new at this and run out of low gears you can always walk or stop often on the steep, short hills. The more you are used to all kinds of hills and your gearing the easier the hills will be. One other note – spinning in that 19 gear inch combo I tend to go around 3 to 3 ½ mph. Much slower and I start to really wobble. If you want fewer gear inches you need to either spin faster than I do (not all that difficult) or have better balance.
As far as the high end gears. Being able to still pedal while the bike goes faster is fun. However, it is not necessary unless you are in a hurry. My philosophy as a sight-seer riding a bike has been to be sure I get the low end and then get whatever I can on the high end, but not be very driven about it. Somewhere around 100 gear inches is nice. 113 is nicer still, but not super important to the sight-seer.
Naturally, the larger your gear inch range is, the bigger the steps between the gear combos. I really noticed the change when I went from Bike A (with the narrower steps) to Bike B. At first I didn’t like it, but I really wanted that 19 gear inch combo. After a few hours on the bike it seemed normal and I am sure that if I rode the gearing for Bike A now I would feel like its gears were too close together.
Your Ideal Gear Set-up
From what I have been told the middle chain ring costs more to change – especially on an older bike. This is because it is usually what everything is fastened to. With an older bike this may mean you are replacing the whole assembly including the crank arms. So let’s try to keep that middle chain ring the same if we can.
Now that you have some idea of what you want, try to make up a gear inch table for it. At this point we don’t need to bother with each individual gear combo – just go for the high and low end of each chain ring. Start at the lowest end. Is the low end gear inch number what you want? If not, you can either change the rear cassette or the small chain ring. If your rear cassette has a good, wide spread of steps you may want to lean toward changing the chain ring.
Remember that "mountain bike" rear gear cassettes tend to have a wider range with a higher tooth count gear than "road bike" cassettes. Your bike doesn’t care what the cassette is called and will use either (although the derailleur may need to also change). Common MTB cassette sizes are: 11-32T, 11-34T, 12-32T, and 12-34T. Some people worry about excessive wear on an 11T, but a fair amount of that can be solved by replacing your chain when it starts to wear. Chains wear out relatively frequently and a worn chain eats gears.
Use the website at http://www.sheldonbrown.com/gears/ and plug in the numbers you want for your ideal rear cassette and each chain ring, keeping your current middle chain ring if possible. There should be an obvious overlap between each ring. Remember from our first lesson that you will want each chain ring to overlap at least 3 or 4 gear combos from its next chain ring. Ignore the warnings on the website about drivetrain wear. These numbers are flagged because you have not entered the data for all of the gears you will have.
Transpose the results into our gear inch table format leaving out the middle gears.
Bike B’s chart would now look like this:
Make sure that your favorite cruising gear lands in both the middle and large chain rings. This way you can cruise in the large chain ring and vary both up and down or you can cruise in the middle chain ring and go on down for those steeper rollers. For instance, if I like to cruise at 66 gear inches then I can use both the middle and large chain rings effectively. If I climb most hills at about 22 or 26 gear inches, then I can still have some room to shift down for the steep or long hills.
Now we will look at the derailleurs and figure out what you will need. Sorry there is a little math here, folks. You can always use a calculator! Just hang in there and take it one sentence at a time.
Front derailleurs are rated by three numbers. First they want to know what the maximum number of T are that they have to jump between. This is called "top-low maximum capacity". To get this number subtract your smallest chain ring from your biggest. This is 42 – 20 or 22T in the above example. Write this number down. When you look at front derailleurs your maximum can’t be bigger than the derailleur’s maximum.
The next number is the "top-middle minimum capacity". Subtract the middle chain ring from the large chain ring. In the example this is 42 – 30 or 12T. You can’t pick a derailleur that has a minimum that is less than yours.
The third number the derailleur cares about it how many rear gears you have. Some only work with nine. Some are happy with seven eight, or nine.
If you look at Shimano’s website at http://bike.shimano.com/mtb/index.asp you can see the specs on their XT line of front derailleurs. Look at the table and you will see where the above numbers are listed. Shimano has many other lines of both mountain and road derailleurs listed on their website to look at. They have many, many more not listed on this website. Go to your local bike store and ask. The derailleurs with the large ranges are not listed on the website. Remember that it is okay to mix mountain and road bike gears, derailleurs, and shifters. The bike won’t care and many new bikes come with a mix.
Here is the table reprinted:
Rear Derailleurs have five different ratings you care about. First is the "total capacity". This is basically how many T of extra chain it can wrap around itself and hold onto when you are in your smallest sized front and rear gears. To get this number you want to find out how many extra teeth of chain it gets from the front and then from the back. First subtract the front again (largest minus smallest) or 42 – 20 = 22T for my example. Now subtract the rear (largest minus smallest) or 28 – 11 or 17T for my example. Now add your two numbers together. This is 22T + 17T = 39T for the example. So in order to handle the example’s gearing the rear derailleur must have a total capacity of at least 39T. More is just fine. Write down the number you came up with for your gearing.
The next number you need to know is the "maximum sprocket" size. This is just the largest gear on your rear cassette. (28 in the example.) If your number is smaller or equal to the derailleur’s maximum it’s okay.
Next is "minimum sprocket" size. This is the smallest gear on your rear cassette. (In the example it is 11.) If your number is larger or equal to the derailleur’s minimum it will work.
The derailleur specs also list "front difference". This that "top-low maximum capacity" that you calculated for the front derailleur. It is just the largest chain ring minus the smallest. Don’t exceed this number. In the example it was 42 – 20 = 22T.
The final spec you need is simply the number of gears your rear cassette has. Some derailleurs work with several sizes, some just like one size.
One set of rear derailleur tables to look at is on Shimano’s website at: http://bike.shimano.com/mtb/index.asp Again, remember that this is but a small sampling of what is available.
Your Current Derailleurs
It is generally pretty hard to tell what your current derailleurs can really handle. The easiest way is to take your bike to your local bike store and they can look it up (assuming your bike is fairly new). If they are experienced they may be able to look at an old set-up and guess how much more T of chain the derailleurs could handle. You can also fudge this to a certain extent, but you will lose the ability to get in some gear combos because of chain rubbing, or too much or too little chain available.
If you replace ALL of your gearing you are going to spend a lot of money. Replacing a cassette or a chain ring is not bad at all. Derailleurs hurt a little more. Remember many changes require a new chain. Of course your old chain may be worn anyway…
At this point figure out what you would need to replace for your ideal gearing and if it is too spendy then go back and re-figure it. I usually go through this process several times before I reach a compromise that I am happy with.
7 speed bikes use a wider chain (and gears) than 8 or 9 speed. Be careful to specify the number of speeds you are using when ordering parts. Also, some road and mountain bike derailleurs use different amounts of cable-pull to shift between gears. You should check to see if your shifters and derailleurs are compatible before you shell out the money.
Now you have your bike re-geared and you just jump on it and ride off into the sunset! Right? Wrong!!!
In some ways you have just bought a brand new bike. Take some training rides and get used to stopping and starting and accelerating up to cruising speed. Some sort of cross-town trip usually works great for this. Then find a small hill (overpass size if you are slow on hills). This hill should have very little traffic. Practice those new low gears and where you would get into the lower chain ring from the middle chain ring. Practice shifting up again when you hit the top. Do this over and over until it is smooth. The reason why you want a small hill is because it is easy to just coast to the bottom and start over when you make a mistake – which you will. Get used to stopping and starting on a hill. Get used to starting on a hill when you have stopped in a too-high gear (you may already be familiar with this!)
Now ride a route with a series of rollers. When you are good at that go for a nice, long hill and get used to how often you need to stop and rest your legs now. Often people need to stop spinning and just relax their legs after 10 or 20 minutes of constant spinning. Find your comfortable limit so you can plan on it. Take a quick break off the bike and move and stretch your legs. Scarf up some calories…
For the slow climbers – remember that 3 mph IS a speed (as someone pointed out a couple of years ago) and even at 3 mph you are going to hit the top of that hill eventually! Just get some gearing that you are comfortable with for that length of time (possibly 2 or 3 hours.)
Congratulations! You are a graduate of Gearing 101. If you want a diploma go to your nearest office supply store and choose the border stock of your choice. Now compose an elegant document and print it.
And for the mathematically inclined - the formula for gear inches is G = F x D divided by R. Where F is the number of teeth on the front chainring, D is the diameter of the rear wheel in inches, R is the number of teeth on the rear gear, and G is the gear inch number. For a 700 mm wheel just use 27 inches – or you can pay attention to tire size and get really accurate!
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