Bicycle Mechanics - Chainline with multi-speed bikes

Recently I've seen some discussions in various forums comparing gearing of "compact" cranks and "standard" cranks, blah, blah, blah. Anyway, that's not what my question is about. Some of the posts in these threads state that because of "bad" chainline, you shouldn't use the largest three cogs on the big ring or the smallest three cogs on the small ring (9-speed cassette).

I've always gone with an "all-but-one" rule. For example, I run a 39x53 with a 12-23 cassette and I figure there's just as much chain deflection in the 53/12 as there is in the 53/21 (assuming the chainrings are centered on the cassette). Similar deal with the 39/23 and 39/13. Am I missing something here? Why do folks limit the "cross-chaining" when it has no more chain deflection than using the lowest and highest gears?

2 reasons why cross-chaining should be avoided:

1) the angle is the most severe that you can put on a chain-this will drastically increase lateral wear (chain and cogs), and will significantly increase the chance of something breaking.

2) This also taxes the limits of the capacity of the rear derailleur. In the big-big combo, the derailleur is all the way forward, putting increased tension on the chain-not good, especially when the lateral forces are at the maximum. Also, sometimes a big bump could let the jockey wheel hit the cogs...

In the small-small combo, the chain is as slack as it will ever get, and it's bouncing around all over the place. In some bikes, the chain can even rub against itself, causing even more damage.

A good rider never cross-chains with any 'newer' bike. I occasionaly cross-chain with an old road bike that has a double ring and a fairly close-ratio 6-speed freewheel, but I've checked that the derailleur has plenty of capacity.

2 reasons why cross-chaining should be avoided:

1) the angle is the most severe that you can put on a chain-this will drastically increase lateral wear (chain and cogs), and will significantly increase the chance of something breaking

I agree. But the angle is equally severe in the lowest and highest gears. I guess I'm not supposed to use those, either. :)


2) This also taxes the limits of the capacity of the rear derailleur. In the big-big combo, the derailleur is all the way forward, putting increased tension on the chain-not good, especially when the lateral forces are at the maximum. Also, sometimes a big bump could let the jockey wheel hit the cogs...

In the small-small combo, the chain is as slack as it will ever get, and it's bouncing around all over the place. In some bikes, the chain can even rub against itself, causing even more damage.

But my rear derailer is not even operating near it's maximum capacity. Going from 53/21 to 39/13 requires a wrap-up capacity of at least 22T and I believe my rear derailer is rated at 28T or 29T. As long as chain length is "correct", there are no problems with chain bounce or too much tension.

But the angle is equally severe in the lowest and highest gears

No, it isn't. You forgot to account for the extra distance of the chainrings. In "top gear", you have a lesser angle than you would in "small-small" gear.

No, it isn't. You forgot to account for the extra distance of the chainrings. In "top gear", you have a lesser angle than you would in "small-small" gear.

I'm not talking about using small-small or big-big. I'm trying to understand if there is a legit reason to not use the next two smallest cogs with the small ring or the next two biggest cogs on the big ring. The chainline for these combinations is no worse than the chainline for the lowest and highest gears. But some people say they shouldn't be used.

Gonzo,

I don't think your initial premise is correct. For the chain line to be the same angle at the largest cog as the smallest, the entire cassette would have to be centered on one of the chain rings.

This would only be true if you're running a triple crank. In that case, then the lateral angle, anyway, would be the same, and you should only avoid middle chain ring and smallest few cogs because of derailler capacity and a "soft" chain.

With doubles, however, the center point in the front should be the middle of the two chain rings while in the rear it is always the middle of the cassette. In this case you would have slightly different angles from the front to either the top or bottom cog.

Clear as mud, no?

There is nothing wrong with using the combinations next to the extreme crossovers, but they may indeed cause as much driveline wear as the lowest and highest gear ratios. I gear my bikes such that second gear is my normal low, with first reserved only for the steepest hills, or a long climb at the end of a tough ride.

The ideal is to divide you rear cogs into as many groups as you have front cogs.
Say you had a 9x3 setup.
With the smallest front cog you would only use the largest 3 rear cogs.
With the middle front cog you would only use the middle 3 rear cogs.
With the largest front cog you would only use the smallest 3 rear cogs.
This would keep the chain line as good as possible.

That isn't really pratical and depending on the relative postion of the front rings to the rear might mean you have to move the boundries to get the best chainline.

I don't use the bigest 3 cogs with the big ring and don't use the smallest 3 with the small front ring.

The extra wear from using the crossover gears compared to moving the chain one cog over proberly isn't that much. It's just a trade off of how good a chainline you want against the number of useable gears you want. Don't care about wear then use all the gears, riding in muddy conditions it might be better to keep the chainline as good as possible and not use some gears.

There are also setup issues, somepeople don't use a chain long enough to make the big/big combo and sompeople don't have a long enough cage mech to take up all the slack either.