Gearing used to be simple, “back in the day” (don’t you just love cliches?) it was 44-16, 43-16 or NOWT. These days we have a whole load more to choose from and yet there is very little information available to help you choose the right set-up. The following is intended to help guide you through the options but fairly obviously this is going to make it very very dull. There isn’t really a lot I can do to soften the tedium I am afraid, so I can only suggest that you stop reading right now and save this bit for your next bout of insomnia. If you really need to know what the deal is right now then by all means read on, but don’t say I didn’t warn you. Throughout this deluge of tedious odure I will make use of imperial measurements, feet and inches. You may think that we are metricated, but go for a quick ride on your 20” with its ½” (half inch) pitch chain and 2.1” wide tyres for a few miles and then come back and shut up.So, lets dive straight in with the very very basic principals so we all start from the same place.
If you look at your bike you will probably notice that the sprocket on the cranks is considerably bigger than the sprocket on the back wheel. There is a very good reason for this, we don’t want to look like dick-head mountain bikers. IF the two were the same size, one turn of the cranks would cause one turn of the back wheel, so for each pedal stroke you would turn the wheel half a turn and you would move about 31” or two and a half feet. This would make you pedal like the clappers to go very slowly up hills, soon you would feel the need to wear lycra (when really you don’t have the lack of beer belly for it) and shave your legs, sound familiar? So we have a big ass sprocket on the front and a tidgey little freewheel on the back, the old school 44-16 for example. This gives you a chance to look less of a tit, one full turn of the cranks “feeds” 44 teeth worth of chain, 44 teeth worth of chain passes over the 16 teeth of the freewheel and so causes the wheel to turn (44 divided by 16) 2.75 times. So now one pedal stroke turns the wheel 1.375 times moving the bike forward 86” or about 7 feet. So now you have a top speed of say 25mph instead of 8mph. But there is a trade off, as the gearing increases so does the top speed but it gets harder to pedal, so it takes longer to accelerate, if you are riding street with a restricted run-up to some wall ride, a lower gear can make it easier to get the speed you need. To summarise; your gear ratio is the teeth on the front sprocket divided by the teeth on the freewheel. This gives you a number like 2.75, if it’s a bigger number you will be able to go faster but it will be “harder” to accelerate quickly. If it’s a smaller number it will be easier to accelerate but you may find it harder to get the speed you need for a jump-box.
OK so the basics are sorted, what happens when you try to set your bike up. First you have to choose a set-up, you might want to stick with the conventional 44-16 or you might want to try a modern small drive system. By running a 14 tooth freewheel on the back wheel (for which you will need a different hub thread to a 16tooth) you can run a smaller sprocket up front. A 39 will give a ratio of 2.786 (so you gain some extra speed too) and the sprockets will be smaller (so lighter) and more out of the way of grinds. If you run a cassette hub you can get the rear sprocket down to 13 teeth which would let you run a 36 for an even more compact set-up. Fantastic I hear you cry… Until you try to set it up…. Lets assume that for the last 3 years you have run a 44/16 you switch over to a 39/14 and take two whole links out of your chain, suddenly your wheel wont go round! You have a look and see that the chain is too short now and the wheel wont go round because the tyre is rubbing on the chainstays. So you put one of the links back in and the wheel moves back from where you always had it and maybe your chain tensioner has no room or maybe you just don’t like the feel of it. So how do we work that out in advance?
Well the easiest way is to ADD the teeth on the sprocket to the teeth on the freewheel, 44+16 is 60 teeth. Then 4 teeth represent one full link (see: “what is a link” sidebar) so if you had a 41/15, 41+15=56 you could take one link out and it would leave the back wheel in almost exactly the same place. But; 39/14 gives you 39+14=53 so if you take two links out (52 is the nearest) your wheel will have to move forward to make up for the extra tooth and if you only take one away you will have three teeth worth of chain slack to take up by moving the wheel back.
The following simple rules are all you need to remember.
1 full link is equivalent to 4 teeth. 4 teeth represent ½ (half) an inch on the chainstay length.
Or you can create a list of chainstay lengths and gearing options.
CS: 13½(half)”, 14”, 14½(half)”, 15”
Total Teeth: 60, 56, 52, 48, 44, 40, 36
Possible Combos: 44/16, 41/15, 38/14, 34/14, 35/13, 32/12, 33/11, 30/10, 26/10
CS: 13,5/8”, 14,1/8”, 14,5/8”, 15,1/8”
Total Teeth: 59, 55, 51, 47, 43, 39, 35
Possible Combos: 43/16, 40/15, 41/14, 37/14, 35/12, 32/11, 29/10, 25/10, 26/9
CS: 13¾(three-quarter)”, 14¼(quarter) ”, 14¾(three-quarter)”, 15¼(quarter)”
Total Teeth: 58, 54. 50. 46, 42, 38
Possible Combos: 42/16, 40/14, 37/13, 36/14, 33/13, 34/12, 31/11, 28/10
CS: 13,7/8”, 14,3/8”, 14,7/8”
Total Teeth: 61, 57, 53, 49, 45, 41, 37
Possible Combos: 45/16, 41/16, 39/14, 36/13, 37/12, 33/12, 30/11, 27/10
So how are we doing so far? Anyone still reading?
Lets suppose that you have decided to switch from a 43/16 to a 39/14, you are ready for the ¼(quarter) inch change in length and reckon there is enough room to shorten the chainstays from 145/8” to 143/8” without the tyre hitting the brake. BUT will the chain rub the brake bosses? The simple answer is to look at golden nugget of wisdom number 3.
Golden Nugget of Wisdom Number 3
For every tooth lost from the front sprocket the top run of chain will move down closer to the chainstays (or brake boss) by 0.08” ie.3 teeth causes a drop of about ¼(quarter)”.
Lets assume that you work out that this isnt going to be a problem and you go ahead with the change. But maybe after a few riding sessions you decide that the bike just feels wrong and you want the chainstays back at their old length. Well there is a way you can do it. You can buy a thing called a half link. This is a “cranked link” the sideplates are bent so that at one end they go on the outside like normal outer plates and at the other end they are the inner plates with the roller and bush. A half link allows you to take out “one full link” which is two ½(half)” “pitches” long and replace it with the cranked link which is only one ½(half)” pitch long. By using this little device you can get ¼(quarter)” increments in your chainstay length. Useful.
After a while you might get into sprocket tricks, after a few stalls and grinds the chain breaks on you (in the way that they do) chucking you onto your bollocks and stem. Finding this a trifle annoying you might decide to buy a FAT chain. Built for motorbikes these fat chains have taller sideplates, use thicker gauge steel and weigh a lot more, the advantage is that they will take a LOT more hammer than an ordinary chain. So you buy one of these big heavy chains and try to put it on your bike. Unfortunately it wont fit. The tall side plates need tall teeth and little 14 tooth freewheels don’t have them. There are also similar problems with some cassette hubs when you run the smallest sprockets so check before you buy.
Anyone get this far? If you waded through all this crap then now you know everything you need to plan any gear changes. Sorry it was so dull but there you go, I did warn you…..
What is a link of chain. Chain is made up of links (well durr!). Bicycle chain has a ½(half) ” pitch the pins are ½(half)” apart with alternating inner and outer plates. When I talk of a “full link” I am refering to the smallest unit of chain that you can take out at one time which will comprise a pair of inner plates and a pair of inner plates and two pins and two rollers. The whole cabboodle will be 1” long ( 2 times ½(half)”). If you take one “full link of chain out then you need to move your wheel forward ½(half)” , its as if you had taken one half out of the top run of chain and one out of the bottom run.