I read in a Mountain bike book that supposedly you should avoid "crossing the chain" when shifting.

According to this book, "crossing the chain" occurs either when you are on the top-most chain ring in front, and the top-two gear sprockets in the back, or on the bottom-most chain ring in front, and the bottom-most gear sprocket in the back.

I know for a fact I have done this.

Is it harmfull to the shifting system?

I do notice when I am on the middle or lowest front chain ring and the bottom-most gear sprocket, I hear a little bit of noise from my chain - I thought the chain just needed to be tightened.

What is the theory behind this?

Thanks!

The issue is that a chain has a lot of strenght in tension, but due to the thickness the chain is not strong laterally (from the side). Cross chaining puts a lot of lateral tension (sideways flex) and will cause the chain to wear out prematurely. Also, being in the big-big combination causes the rear derailleur to be pulled tight, and imposes a lot of force on it. I've known people myself included that have ripped rear derailleurs off their bikes just while pedalling. i.e., climbing up that grueling hill in too high a gear and muscling it!

Plus, with all the gear combinations and possible ratio's, you should be able to find a similar feeling gear using the middle chainring and one of the middle cogs. Without the lateral flex imposed on the chain.

L8R

Thanks!

One question...

How do these combinations impose lateral flex on the chain?

It imposes lateral flex since when you go big big or small small combination they are the farthest away from a straight chainline you can get. So the chain is not in a straight line going to the rear cassette from the front chain ring so causes more stress..

Let me try to creat a picture for you: Picture looking down from above:

Perfect Chain-Line:

(cogs) (chain line) (chainrings)

--------------
------------ -------------
----------
-------- -- -- -- -- -- -- -- --------------------
------
---- ---------------------------
--

Cross Chaining: (Big/Big)

(cogs) (chain line) (chainrings)

-------------- --
------------ -- -------------
---------- --
-------- -- --------------------
------ --
---- -- ---------------------------
--




Cross Chaining: (Small/Small)

(cogs) (chain line) (chainrings)

--------------
------------ -- -------------
---------- --
-------- -- --------------------
------ --
---- -- ---------------------------
--

L8R

Okay.....

I'm a newbie so bear with me.

Wouldn't the chain be more diagonal if it where on the top front chainring and the lowest back gear spocket, or lowest front chain ring and top gear sprocket?

Let me try again.


Arrrrgggghhhh can't get the pic to work.


Anyways, no, the chain becomes diagonal in the two extremes, putting lateral flex on the chain.


In the Big Chain ring/Small cog the chain is still on the outside half of the drivetrain and not as extreme.


L8R

from the Park tools website: www.parktool.comRepair Help


Shifting issues related to chainline
If either the front or rear sprockets are either inward or outward relative to the other, there may be certain shifting problems. However, if a bike chainline measures off, yet the bike has no problems shifting, the bike should be considered acceptable. No "fixing" of the chainline is then required. The following are typical problems that may be caused by chainline issues.
Chain jumping off large chainring when front derailleur is correctly adjusted.
Chain riding off lower derailleur pulley when derailleur or hanger is not bent
Chain rattling on inner faces of front chainrings.
Chain derailling off inner chainring when front derailleur correctly adjusted.
Front derailleur cannot be adjusted to stop over shifts while still allowing good shifting.
On certain bikes, the chain may tend rub and rattle against the front rings while riding in certain gear combinations. This is common on many bike when riding in the so-called "cross-chaining" combination of the smallest front ring, and the smallest rear cog. An example of a gear combination that is likely to rub is shown below.



Generally, the shorter distance from the bottom bracket to the rear hub, the more likely a rattle from gear combinations will occur. There may in fact be several gears that are unuseable on any given bike. It is possible to minimize or eliminate this problem by moving either the front or rear cogs. Even if the bike is has so-called perfect chainline, it may help the riding and shifting to create an "error" to solve shifting existing issues.

cross gearing is the other way around from what you said at first. It is when you are in the biggest in front, smallest in back or vice versa. As in the diagram in A2psy's post.

I get it now!

Hence the rattling I've been hearing when on the middle or lowest front chain ring and the lowest back gear cog.

Thanks for the info!

the good thing is that whatever gear ration you're in when you're crossed, there's an eqivalent in the next ring somewhere.

Crack'n'fail -

Actually - unless I'm dyslexic, the picture that A2psy sent is exactly what I was talking about - small/small and big/big gear ratios.

My terminology is probably different than yours...

now that I read it again, I see you did have it written correctly, based on your questions i thought you were thinking it backwards. sorry.

No big deal -

Just glad to have an answer!

Crossing the chain also makes the drivetrain work at cross purposes, if you see what I mean. Using the smallest chainring with the smallest cog is like saying "go as slow as you can, as fast as possible" and the big-ring, big-cog combo says "go as fast as you can (big ring) as slowly as possible (big cog)." It's like driving a 4x4 in low range but 5th gear... why not switch to Normal range and second or third?

That said, I'm guilty of the cross-chain crime several times a day. If I'm commuting in traffic, I don't usually drop out of my big ring. Even at a stoplight, I just punch down to the second-to-biggest rear cog. Why...? Because I just want to drag off the line and punch through the rear shifts, not make that ponderous, acceleration-robbing, chain-might-derail-or-break shift from the middle ring to the big ring under hard acceleration. I've accepted that chains and cassettes will wear out eventually anyway, and I'll trade a little lifespan for a practical benefit