ScottCommutes

I assume the middle front gear on the front of a bike lines up with the middle (lets call it 4th) gear on the back. What do the outer front gears line up with? Do they simply line up with 3 and 5 on the back? Would it make more sense to spread them apart a bit such that they line up with perhaps 2 and 6 on the back? I'm thinking in terms of getting a better chain line in high and low gears. Does this configuration exist? Would it be possible to shift? Do riders prefer the outer front gears tight to the middle so they can cross-chain to more ratios?

Bill Kapaun

Do what works best for YOU and the gears you use while staying withing limits of the FDER travel.

I tend to use the middle ring the most, so I use a slightly longer spindle.

Kontact 

No, it would absolutely be a bad idea to respace your chainrings. The chain will get caught between them or not drop all the way.

Bikes work fine and don't need your re-engineering.

JohnDThompson 

You line up the middle ring with the middle sprocket of the rear cluster to minimize chain deflection in the extreme (large-large and small-small) gear combinations. The inner and outer rings in front don't need to line up with anything; the chain has enough lateral flex to accommodate.

70sSanO

It doesn’t make sense to spread the chainrings apart as reducing the chain angle in one direction increases it in the other. Plus the amount you spread needs to be inconsequential due to chain width.

Since you stated the 4th position cog as the center, I’m guessing it is a 7 speed. With modern chains it is pretty moot.

John

13ollocks

I think the "perfect chainline" is a bit of a myth - yes, there are specific chainring/sprocket combinations that'll give you a perfectly straight chain line, but everything else is a compromise. Paraphrasing Bill Kapaun, as long as your chain isn't rattling off your FD, you're good. Agonizing over frictional losses through chain friction is the preserve of high-level competitors who chase every marginal gain - not for "regular folks"

grumpus

You need to stay near current spacing or the chain will fall in the gap, but you could add extra rings and quadruple chainsets are not unknown. Adding more than one extra ring will likely exceed the reach of most front derailleurs, you'll also be increasing your Q factor. Cross chaining really isn't a significant issue for most riders, even with 12 speed spacing, and the gears we use most are usually somewhere in the middle anyway.

Duragrouch

As long as the middle chainring is centered versus the cassette, you should be able to use all gear combinations. That said, when on the lowest or highest cogs, I tend to use the inner ring on lowest and the outer ring on highest, which is easy to do because the gearing has a lot of overlap (duplicate gearing in different chainrings). When cruising along it makes little difference, but if climbing or hammering on the pedals at higher torque, a straighter chainline puts less stress on the chain; Where the chain bends laterally, even though not much, the chain plates on the outside of the curve will see more pull than on the inside, and hard chain pull puts the chain into a tighter bend there, if cross-chaining. It's just one of the things you do to be nice to your drivetrain, same as not shifting under hard load.

stratman

Mine's a nine speed. It looks like yours is a seven speed.

If your drive train is set up so chainline is the same at the crankset and the cassette when you are in middle ring and middle sprocket, the chainline will be sort-of-straight when it's in the inner ring and sprocket six (counting the smallest as one). Or five. Five and half, if there was one.

Why? Because the sprockets on the cassette are closer together than the chainrings are. The sprockets on seven speed are 5mm apart:
https://www.sheldonbrown.com/cribsheet-spacing.html whereas the chainrings are ~8mm apart. By the way, this is one (rare) occasion where Sheldon gets it wrong. Somewhere he states chain rings are 5mm apart, which is incorrect.

This means that on my setup middle ring to sprocket five is a straight chainline, inner ring to seven is sort-of-straight, and outer ring to 3 is sort-of-straight.

I do try to use the gears so that I'm keeping a straight-ish chainline. But I have read somewhere (I think) that the wear due to out of line chains isn't actually that great. I avoid big-big and small-small, as there's no possible reason to use them, except poor planning or not paying attention. And I expect that bad wear or poor efficiency only happens at those extremes. But generally speaking flattish/gentle up hills is middle ring, downhill/wind behind me is big ring, up hills is inner ring and the corresponding sprocket choice will naturally give me a fairly straight chainline. The most "unstraight" I usually am is 2 sprockets out, which is about 12 mm, I think. Of course it means that the most "stressed" combination - inner ring to biggest sprocket - is non-straight. But I hardly ever use that. It's a 24 teeth chainring and a 34 teeth sprocket, which I think is about 18 gear inches on my bike. That's huffing puffing and cursing up a very steep hill. Efficiency is the last thing on my mind. Not falling off sideways at 3mph is my main concern.

My gearing works perfectly. I can get all 27 gears, even the ones I never use, without any chain rub, and every change up or down is smooth and immediate. So I'm not about to start changing anything.

Any changes you might make could possibly give you unintended consequences. For instance, if you put a longer bottom bracket in to get a wider chainline at the crankset, you can't be sure the front derailleur will still work. Unless you find a way of moving that out, though somebody somewhere does an eccentric FD shim.

And certainly respacing the sprockets sounds like a receipe for disaster. Assuming you've got indexed shifting, isn't the indexing going to be all wrong?

As others have said, obsessing over the "perfect chainline" isn't really relevant for most riders on a multispeed derailleur bike. Getting them all working smoothly is a great deal more important, and then, especially on a triple, sensible use of gears will give you a more-or-less straight chainline anyway.

With hub gear/single speed things are different. There is some point aiming for a dead straight chainline. Both to reduce wear and also because single speed chains don't have the same sideways flexibility that multispeed chains do.

Mr. 66

Sure, space out the chain rings, lol.

ScottCommutes

Thanks for an excellent answer! I have no intention of changing my gear spacing. I just wanted to understand how bikes are engineered so I can make more intelligent choices when riding.

The gears at the back obviously need to be tight together to fit between the wheel and the frame. The gear set at the front, however, is always narrower than the rear, so there are design choices about how wide to space it out. You indicated the gear-to-gear spacing at the rear is 5mm, and the front as approximately 8mm. So now we are getting to the heart of my question - why the slightly wider spacing? Is it due to construction limitations, to help the front derailleur operate better, or for chain line or clearance purposes?

stratman

I personally don't know why the spacing at the front is wider, is the simple answer. But a useful side effect is that the chain will usually be straighter, depending, of course, upon how one uses the gears.

SJX426 

The spacing in the front accommodates the large range of front ring tooth count. An example would be a 52/42/30 ring set with the chain on the 30 and the 2nd larger sprocket from the smallest, nearly cross-chaining. If the rings were closer the chain on the 30 might rub on the 42.
For best performance, you want the a correct chain line. All About Bicycle Chainline (sheldonbrown.com)

Iride01 

What intelligent choices do you need to make that knowing this answer will solve? The makers of the bike expect you to be able to shift to all the gear combinations. They sometimes caution you not to run long in the small/smalls.

slighter wider spacing to what? The difference between the spacing on the front rings compared to the rear sprocket spacing? I'm just guessing, but probably because when the chain is on the small front, when it's on the smaller rear cogs, the angle of the chain to the rear would have it's side plates rubbing the next bigger ring on the front.

ScottCommutes

So the front has bigger jumps between gears than the rear, thus the need for a little extra space there. Makes total sense - a design limitation. Taller people with bigger frames, longer chainstays, and longer chains would have less chance of the rubbing you describe.

ScottCommutes

Let's say that you are rolling down a small hill to the bottom and then climbing out the other side. And let's say that at the bottom you are in your highest gear, but climbing back up you will need your lowest gear.

Because it all happens so fast, you don't use every possible gear. You're going to do every click on the rear shifter and only make the two shifts on the front. The question is when to make them.

Here's the same idea with nomenclature:

My bike has:

11,12,13,14,15,16,17
21,22,23,24,25,26,27
31,32,33,34,35,36,37

Do I go through
37,36,35,25,24,23,13,12,11
or
37,36,26,25,24,23,22,12,11
or
37,27,26,25,24,23,22,21,11
or something else?
I'd like to have this particular highest to lowest shifting pattern pretty well practiced and memorized because it is a fundamental part of riding the bike that I don't want to think about too much in traffic.

Iride01 

So are you thinking all of us should make our chain stays longer to make a more better fix for what they component and bike manufacturers have already accounted for?

ScottCommutes

Obviously not. Just thinking that, all other things being equal, a big bike should shift slightly better than a small one because the angles on the chain aren't as severe.

Iride01 

When in a larger front ring, if I'm only going to be in that lower gear ratio (larger tooth sprocket) on the rear for a very short time, I shift the rear. If I'm going to be there for a while, and I have about the same gear ration on a smaller ring, then I shift the front and back. Though if needing a two or three sprockets lower ratio then some times just a front shift to a smaller ring works.

If you are trying to memorize numbers, then that is insanity. At least for me. Looking at the cogs while on the bike is not allowed either! That makes for a bad habit of looking at them too much and not looking forward or everywhere else one needs to look as often as one should.

When you can't shift any higher or lower on the front or the back, then you know where you are. That's really all you need to know.

ScottCommutes

The numbers (by happy coincidence) roughly correspond to how fast you can go (in mph, but even closer in kilometers) in each gear, at least on my bike.

stratman

To be blunt, imagining that you have to memorise all that is crazy. If you are trying to keep those numbers in your head, and change gear due to some prescribed numerical pattern, I'd say you aren't riding in the safest way.. Your username implies you commute, presumably in traffic. Your attention should be directed at other road users, looking out for those that might kill you. Not thinking about sprocket teeth numbers. I don't even know what mine are, except the biggest and smallest. What I do have is one of those Shimano gear indicators for the cassette. I'm on washing line shifters and I quite like having it there, not sure it's really necessary. I mainly have it to stop me trying to shift past the end of the range, which on the largest sprocket/chainring might strain the shifter/cable.

https://www.ebay.com/itm/19617378884...cAAOSw-KJlmaO-

As far as the front chainring goes, I'd say I'm in the middle chainring 70% of the time, outer chainring 25% of the time, inner chainring 5% of the time. I find that I hardly ever use the lowest gear (inner ring, largest sprocket). But when I do I'm glad it's there. On the other hand I spin out more and more frequently in top. So maybe I should get a bigger outer. But I'm at the limit for my FD, so that would lose me some of the lowest gear.

I read the road ahead (if I don't know it already). If it's flattish or maybe some gentle up hills, I'll be in the middle ring and change the rear cassette to suit. If it's flat or downhill or the wind is behind me or it's safe road and I want to go fast I'll be in the outer ring, again changing rear cassette to suit. Lots of up hills, maybe steep, I'll be in the inner ring. And that indicator thing will be very roughly in a position corresponding to the range of sprockets most appropriate to chain ring (inner to big, middle to middle, outer to small).

In the example you quote, a downhill followed by an uphill, I'll drop gears a little early, so I'm not changing under a lot of force. If there's a stop at the bottom, some sort of junction, I'll change down to a gear I know I can start in. My shifters (Shimano Ultegra) can down (up?) shift 3 gears at once, which occasionally makes things a little smoother. I sometimes wish it would multishift in both directions.

NB - I've never been clear about what "low" and "high" means in terms of bike gears. Or what shifting "up" or "down" means.

stratman

Actually, I've just read that again.

It doesn't make sense. What do those numbers mean? You haven't got a rear cassette with 21 sprockets, have you? Presumably the smallest is 11 teeth, and the largest is 37. With 5 others in between.

70sSanO

Have you actually calculated that angle?

A few years ago I did a rough estimate because of the ongoing cross chaining argument. It is easy math. The angle is quite small at the extremes. The result of changes in cassette is frightfully minute.

I can’t imagine increasing chainstay length while maintaining good design geometry would have any appreciable difference.

Wading around in tenths, more appropriately minutes, of a degree is pretty far into the weeds.

Johh

ScottCommutes

Some examples:
11 means the easiest gear to pedal front and back (the lowest gear possible on the bike)
12 means the easiest gear in front, the second easiest in back
21 means the middle gear in front, the easiest in back
27 means the middle gear in front, the hardest in back
37 means the hardest gear to pedal in front and in back (top speed in this ratio)

The first digit (1, 2, or 3) corresponds to the small, middle or big gear on the front. The second digit (1-7) corresponds to the back gear (largest=1, smallest=7).

I like visualizing the gears this way. For example, I try to start from a stop in "22" and then shift up to "23", then "24". The two digits together make a number that roughly corresponds to how fast I will go in that gear. I think of the front gear in the "10s" place because it makes a bigger difference.

Also, I am a fan of the mountain bike style shift levers that I have. You can literally "feel" with the side of your knuckle what gear you are in without looking or moving your hands. No other style of shifter has this benefit.

KCT1986

He is probably referring to the numbers on the shifter indicator. 3 (large chainring), 2 (mid), 1 (small), and 7 (outermost sprocket), ...1 (innermost sprocket). So, 37 if something like 42T front, 11T rear....