Chainrings and Cogs - How and Why?
 

This was posted in a different thread:

"You should use only the big chainring with the smallest cog."

Can anyone explain the above quote to me? I'm only starting to ride often, will be beginning a 10 mile commute soon, and don't want to screw my bike by riding incorrectly.

Thanks in advance.

That's incorrect. You should always avoid the big-big and small-small combinations. But other than those combinations, feel free to mix and match chainring and cog combinations (assuming your derailleurs are properly adjusted and your chain is the correct length).

It's all about keeping as straight a chainline as possible. Lets say you've got a spiffy brand new 27 speed MTB. There's a lot of side to side play that you can introduce by running your granny ring up front with the smallest cog, or the big ring up front with your biggest cog. It puts undue stress on the chain and other components of the drivetrain. You'll wear out chains, rings, and cassettes pretty quick.

Thanks for the info, that's how I've been riding already, mixing and matching. When I read that quote I figured I was doing something wrong. What's the reason for avoiding the big-big and small-small? I can't say I've ever tried to ride in those combinations, seems like they'd be difficult, but is that the only reason? Or can you screw something up that way?

EDIT: Nevermind, just read CliftonGK1's post, and I think that explains it.

well, put it in the small/small and look at what your rear derailleur is doing.

Notice how in the front it goes from small to big? And in the back it goes from big to small? So if you put the chain on the small-small, it goes at a really steep left-to-right angle. You're almost guaranteed to have the chain rub on the front derailleur. Same but opposite on the big-big. Plus, those combinations put a real strain on the rear derailleur. Finally, if your chain or cassette is old and worn, you may have slipping.

So, it's not a good idea.

what if you only have a 5 speed cassette, does it still matter?

My guess is that the more cogs and chainrings you have, the more it matters. So if you had a double up front and a 5 speed cassette, the big/big or small/small would be a lot less extreme than if you did the same on a 10 speed triple.

Err...I'm kind of slow...

Can someone translate into slow/fast terms?

Let's say the front gear is:
1=slow (big)
2=med
3=fast (small)

And the rear gears are 1 (slow) to (6) fast

Which combos do I avoid?

Yeah I don't think it matters at all w/ a 5 spd cassette and a double. I'm trying to figure out why my triple / 9 spd road-y bike is so much pickier than my triple / 8 spd mtn bike. Both have 22t gap at the front, and the mtn bike has a wider gap at the back, yet it's much more tolerant. I *cannot* use the 3 most extreme gears w/ either big or little on the road-y (leaving 21 net from the 27 combinations), but I can use them all, if I wanted to, on the mtn bike.

The extreme combinations, at either end of the slow/fast spectrum. On my triple/9 speed, for example, I avoid using the last 2 or 3 small cogs in the rear when I'm in the small chainring up front; when I'm cruising along in the big chainring, I avoid the last 2 or 3 larger cogs in the rear.

One way to think this through is to check out a gear ratio chart for your particular gearing. The resulting numbers will show you where your specific gear combinations overlap and you can then think about a gearing strategy, so to speak, that avoid cross-chaining. Here is Sheldon Brown's calculator: http://www.sheldonbrown.com/gears/index.html.

Short vs. Long cage derailleurs, maybe? A long cage on the MTB will make it more tolerant to taking up and letting out slack on the chain to reach all the gears (even the "bad" combos you shouldn't use.) A short cage roadie derailleur won't allow as much play in your chain slack.

One thing to remember is that many of the gear combinations are duplicates anyway. For example, using my touring bike gearing, with a 46/34/20 crank and a 11-13-15-17-20-23-26-30-34 cassette the gear ratios (in gear inches) are:

... 46 34 20
11 114 84 50
13 97 72 42
15 84 62 36
17 74 55 32
20 63 46 27
23 55 40 24
26 48 36 21
30 42 31 18
34 37 27 16

If we start in the lower right corner and number that gear as 1 and the bottom of the middle column is 10 and the bottom of the left hand column as 19, I can describe a shift pattern. Notice, however, that there are lots of duplicate gears or near duplicates. In the middle and large chain ring, I count 7 duplications.

Just for the sake of demonstration, I start at the top of a large hill in gear 10 (middle ring, largest cog) after having come up the other side. (We'll leave the granny stuff for later.) As I start down, I shift to 11, then 12, then 13 (34/23 combination). At this point you should notice that there is a 55" gear at the 15th gear (34/17) but there is also a 55" gear at the 22nd gear (46/23). Here I have to decide where I want to go. If I make the jump to the large chainring at this point, a 15" jump or do I shift one more up (14th gear, 34/20) and make a 17" jump at the next gear? Since I'm on a downhill, I'll probably make the shift to the 14th gear.

Another decision: Do I make the shift to the 15th gear (34/17) or do I shift up to the big ring on the front? I shift to the big ring and go to gear 23 (46/20) combination. The reason is that I've got a lot of speed now and will probably run through the gear quickly, so I want to be in the high range. Also the step from 15th gear (34/17) to 24th gear (46/17) is large and I'd rather have a smoother transition.

One thing I would never do, is go further up the gears in the middle ring from about 15th gear (34/17). The steps from middle to high range get larger and larger which means the transition from middle to high range is very abrupt. I absolutely would never shift all the way to the 34/11 gear and then shift up in the front. That's a 30" step which is absolutely huge!

For the granny, I start a the bottom of a hill in my tallest gear (27) and go 27, 26, 25, 24, (dump to middle range) 15, 14 and then, because I'm losing momentum fast, I dump the low range (20 tooth granny) and then shift 5, 4, 3, 2, 1 and then walk if that isn't enough:D The transition from the middle range to the granny is going to be very abrupt but at that point I don't really care. I need a low gear fast or I'm not going to get to the top of the hill.

Because of the range on my gearing, would also probably never go higher, in low range, than 5. My chain starts to get too slack at that point.

Hope this long winded post helps. Go do your own calculations and look at where you have duplicates (we all have them) and then figure out your own shift pattern. You want the smallest jumps from gear to gear you can get.

Edit: Sorry about the poor quality of the chart. If you take the gears I gave above and run them through Sheldon's gear calculator, you can see what I'm describing better.

Numbers are my hated enemy. Should I just give up on figuring out what you're talking about, or is it really valuable information? I really appreciate the info but I just completely suck at numbers. I don't even know how to find the numbers you're talking about. I checked out Sheldon's page, but got lost there as well.

Thanks.

Don't worry about the numbers. You really only need to remember a couple of things: Assume that your cadence (the rate at which your feet revolve) stays constant. As your front chainrings get bigger, the pedalling gets harder, but you can go faster -- just like being in a higher gear in a car with a manual transmission. As your rear cogs get bigger, the pedalling becomes easier but you go slower -- just like being in a lower gear in a car.

The rear cogs are arranged so that the largest is furthest to the left, and the smallest furthest to the right. The front chainrings are arranged so that the largest is furthest to the right and the smallest is furthest to the left.

Use whatever combination of front chainring and rear cog allows you to maintain your preferred cadence comfortably, except:

Keeping the chain on the largest rear cog (i.e., the cog furthest to the left) and the largest front chainring (i.e., the cog furthest to the right) will mean that the chain will run diagonally. This will cause wear to the chain and derailleur. Similarly, avoid having the chain on the smallest rear cog (the cog furthest to the right) and the the smallest front chainring (the chainring furthest to the left) for the same reason.

All you really are doing is avoiding having the chain running at an extreme diagonal. This is an picture of what to avoid:

http://www.parktool.com/images_inc/r...help/CLG03.gif

The numbers refer to the number of teeth on the chainrings and cogs, so 46, 34, 20 are the no. of teeth on the largest, middle and smallest chainrings and the 11, etc., refer to the teeth on the cogs, so the 11 tooth cog will be the smallest and so on, up to 34 or whatever.

Your highest gear (the hardest and the one in which you can ride fastest - prbably downhill) will be:

(Big chainring) 46 x 11 (smallest, right hand sprocket)

Your lowest gear (for grinding up hills) will be 20 (chainring) x 34 (sprocket). The smallest chainring is commonly referred to as the granny ring, presumably on the grounds that even your granny could ride along in that.

Armed with this info, count the number of teeth on each of your chainrings and sprockets, return to the Sheldon Brown site, print off the info on gears and read thro' carefully. If you take your time and try to ignore your "number block", you should be able to understand it and apply it to your own bike.

Good luck

What if you are using a single chainring up front?

I am building a commuter/utility and to simplify it will be single chain up front (probably 39) and 8 spd 11-32 in back. I plan on using a standard double crankset, but just one chain ring.

thoughts?

Don't get hung up on the numbers. Numbers are just the same as words really. The calculator does all the work for you so all you have to enter is what gears you have on the front and back. You have to count them all and write them down, then enter them into the calculator. It will spit back the ratios. Now I work in gear inches (because I'm old and that's the way I was taught) but it doesn't matter what ratios you use, if you do the calculations you will see that there are all kinds of the same numbers in different places on the chart. For the most part (assuming a road bike or bike ridden on the road), unless you have some old funky half step gearing, if you start in the middle chainring and the largest cog on the back, you should shift up 3 or 4 cassette cogs and then 'crossover' to the big chainring in the front and then shift upwards as you go faster. On a downshift, 'crossover' to the middle chainring at the same spot. This will give you the smoothest and smallest transitions. Drop to the granny gear (if you have one) when things get steep.

The charts that Sheldon Brown's calculator generates just lets you see where the duplicated gears occur. That's what that poorly generated chart that I made tells you. Look for the patterns...you can do it! It's one of the things we little monkeys are good at.;)

Thank you for the diagram, Canonet. A picture is truly worth a thousand words (or a million numbers).

I've been working sporadically on a page explaining gears to beginners. No tooth numbering and gear inches, just theory and shifting practices. Would you mind if I incorporated your diagram, with credit? I will post a link to the page here when I have it uploaded to my site.

I actually lifted it from the Park Tools web site. :rolleyes:
My guess is that there are other similar ones on-line.

Okay, what do you all think of this page (especially OP)?

Thanks, I thought the page was helpful.

I use the big-to-big combo routinely for stoplight launches. If the drivetrain wears a little faster, I really don't give a darn; I want to accelerate to cruising speed rapidly without having to upshift in front. In fact, I usually jump two cogs at a time on the first couple upshifts anyway. Not quite 0-45mph in two seconds like my avatar, but I do the best I can ;)

Yeah, don't worry too much, your bike will complain if you try it. The chain will chatter as it tries to assume such an unnatural gearing.

Yeah, you won't instantly destroy the chain riding like that, it will just put undue stress on the chain and cause it to wear out faster. If you like a particular gear combination and don't mind the extra noise and having to fix the chain sooner then just ride that way and don't worry about it. :)

I use a single chainring up front and 8 cogs in the rear on my Mongoose. The big and little cogs definitely put a strain on the chain, but I've only had it come off once and that was when I hit a very bad bump and them immediately changed gears afterward. I have noticed that I have to spend more time getting the rear derailleur just right for it to operate smoothly and quietly, but it works. Preferably I'd like to get some sort of chain guard that mounts like an outer chainring and rig up some sort of clamp with an arm from the seat tube to keep the chain in place, but I'm still trying to figure that out. In the mean time I'll just keep an eye on it and try to be careful. :D

P.S. And try to avoid the gear combination I've done, which is a 42 in the front and a 12-23 in the rear. :o I built the bike as cheaply as possible, so I just used what I had lying around. Needless to say my legs are getting stronger (and more sore!)

Great work, John!

That sounds just about perfect for my own topography (but not low enough for trailering, but that's why I have another bike).