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How do you set proper chain length, small cog to small ring, big to big, or small to big?

post #1 of 9
Thread Starter 

I've seen conflicting methods for determining chain length, for example:

 

 

Small cog to small ring:

 

 

 

 

Small cog to big ring:

 

 

 

 

Big cog to big ring:

 

 

 

 

On SheldonBrown.com he promotes the big to big technique:

 

"The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that one complete link (one inch, -- one inner and one outer half-link) overlaps. In almost all cases, this will give the optimum length."

 

http://sheldonbrown.com/derailer-adjustment.html#chain

 

So....

 

Which is correct, or better, and why?

 

Thank you. :)

post #2 of 9

Well..it all depends..  :-)

 

There are multiple ways but not all work for all bikes..  The primary goal is to have enough chain to prevent breaking the rear derailleur when using the big-big combo and having max length of the chainstay included if there is rear suspension.  This requires the correct rear der cage and ring/cog combo as well.

 

If you have too much chain slack after making sure you don't break the rear der on big/big then you don't have enough chain wrap in the rear der, which requires some modification to the drivetrain.

 

The Shimano short-cage dual-front small range rear cassette system was generally big front/small rear and make the der cage perpendicular to the ground.

 

Sheldon's method is one I commonly used for triple fronts.

 

There is some leeway depending on the drivetrain which allows you to build-in some slack for less resistance (if you believe in that sort of thing) or you can tighten it up if you want less chain slap.

 

Again though, the overriding factor is don't break the cage when you go to big-big.

post #3 of 9
Thread Starter 
Quote:
Originally Posted by Scott43 View Post
 

Well..it all depends..  :-)

 

There are multiple ways but not all work for all bikes..  The primary goal is to have enough chain to prevent breaking the rear derailleur when using the big-big combo and having max length of the chainstay included if there is rear suspension.  This requires the correct rear der cage and ring/cog combo as well.

 

If you have too much chain slack after making sure you don't break the rear der on big/big then you don't have enough chain wrap in the rear der, which requires some modification to the drivetrain.

 

The Shimano short-cage dual-front small range rear cassette system was generally big front/small rear and make the der cage perpendicular to the ground.

 

Sheldon's method is one I commonly used for triple fronts.

 

There is some leeway depending on the drivetrain which allows you to build-in some slack for less resistance (if you believe in that sort of thing) or you can tighten it up if you want less chain slap.

 

Again though, the overriding factor is don't break the cage when you go to big-big.

 

I should've pointed out which drivetrain I'm using and what kind of bike, doh!

Road bike, double front chainrings, Dura Ace short cage rear, 11-25 cassette.

 

That said, what would you recommend and why?

 

Surprised me that there are 3 methods being promoted out there. One should be correct, unless they all result in the same number of links. :)

post #4 of 9

Well, the problem is the system is not perfect.  It would be nice if they all worked the same, but they don't.  Some people build drivetrains with crazy ranges.  Or they use a borderline cage length for the wrap they need.  And suspension thew a curveball because the length of the chainstay can change with suspension movement which changes chain slack needs.  So there really can't be one way of doing it, other than the way that doesn't break the der cage!  :-) 

 

Shimano's standard instruction for your setup is big front, small back and make the the line drawn between the pulley wheels on the derailleur perpendicular to the ground.  That's what I would do and verify that you have enough tension in small/small to prevent grinding and enough slack for big/big without over-tensioning the cage. That's what I would do.

post #5 of 9

You didn't note if you're installing your mid-compact or still working with your 53/39.  I assume you're running a short cage rear der (derailleur)?

 

Either way (standard or mid compact) the large/large method is your best choice for establishing length. ( i.e. wrap and add two links.)  As noted, this protects your system if you shift into the large/large combination (cross-chaining), which isn't a good idea because of chain angle and heavy wear.  

 

The small/small method is more of a check to determine if your der cage has enough "wrap capacity" to take up slack for extreme drop ratios.  Small/small is again not a desirable combination in-use because of stress and wear on the driveline.  However, new school doesn't seem to be as concerned about cross-chaining - "use all the gears you paid for and replace when things wear out".    

 

A short cage Shimano der with 11-25 and standard or mid-compact rings should have no problem taking up slack.  If it was me, I'd set it up using the large/large method, then assemble. Run through all the gears and see if you have excessive sag (or rubbing) in the small/small combination.  If it does rub or have excessive slack and you plan to ride small/small, then you will want to tune out the rub in some way.  You can adjust the B-screw to add some tension into the system, but that can have a negative impact on shifting response (not as quick/precise) across your cassette.  Scott43's suggestion to follow Shimano protocol is a better idea if you're going to ride small/small.

 

If you run your chain a little "short" for the gear combos you have, you can tune the system for slightly more crisp shifting.  Depending on how short, you may need to be very careful you don't shift to large/large.  If the chain is too short for your large/large combo, best case is the system will bind and not turn.  Worst case - something will break/bend ... most likely the rear der.  

 

Depending on what you optimize for, you can probably swap in/out a 2-3 tooth cassette difference without resizing your chain.  (i.e swap an 11-23 for an 11-25)   On rare occasion I swap in a wheel with an 11-28 in place of an 11-23 without resizing my chain for a very hilly ride.  I just know I cannot shift large/large - or even large/2nd large - without undesirable consequences.  Driveline set up is a bit like choosing skis - what terrain are you going to ski most and optimize for that.   

 

There's also a "calculation method" for chain length.  Probably on Sheldon's site somewhere.  Probably more appropriate for a high-travel mountain bike, where you need to account for many more variables than a fixed rear-triangle road bike.  I'd keep life simple and use the large/large method and be done with it.  


Edited by medmarkco - 4/7/14 at 1:16pm
post #6 of 9
Thread Starter 
Quote:
Originally Posted by Scott43 View Post
 

Well, the problem is the system is not perfect.  It would be nice if they all worked the same, but they don't.  Some people build drivetrains with crazy ranges.  Or they use a borderline cage length for the wrap they need.  And suspension thew a curveball because the length of the chainstay can change with suspension movement which changes chain slack needs.  So there really can't be one way of doing it, other than the way that doesn't break the der cage!  :-) 

 

Shimano's standard instruction for your setup is big front, small back and make the the line drawn between the pulley wheels on the derailleur perpendicular to the ground.  That's what I would do and verify that you have enough tension in small/small to prevent grinding and enough slack for big/big without over-tensioning the cage. That's what I would do.

 

Thank you, will spend some good time w/the bike on the stand shifting and observing before I get it out on the road. 

Ordered a new chain tool over the weekend. I have a feeling I'm going to be experimenting a bit, and the tiny chain tool in my saddle pack would be a pain to work with.


Edited by Super D - 4/7/14 at 2:01pm
post #7 of 9
Thread Starter 
Quote:
Originally Posted by medmarkco View Post
 

You didn't note if you're installing your mid-compact or still working with your 53/39.  I assume you're running a short cage rear der (derailleur)?

 

Either way (standard or mid compact) the large/large method is your best choice for establishing length. ( i.e. wrap and add two links.)  As noted, this protects your system if you shift into the large/large combination (cross-chaining), which isn't a good idea because of chain angle and heavy wear.  

 

The small/small method is more of a check to determine if your der cage has enough "wrap capacity" to take up slack for extreme drop ratios.  Small/small is again not a desirable combination in-use because of stress and wear on the driveline.  However, new school doesn't seem to be as concerned about cross-chaining - "use all the gears you paid for and replace when things wear out".    

 

A short cage Shimano der with 11-25 and standard or mid-compact rings should have no problem taking up slack.  If it was me, I'd set it up using the large/large method, then assemble. Run through all the gears and see if you have excessive sag (or rubbing) in the small/small combination.  If it does rub or have excessive slack and you plan to ride small/small, then you will want to tune out the rub in some way.  You can adjust the B-screw to add some tension into the system, but that can have a negative impact on shifting response (not as quick/precise) across your cassette.  Scott43's suggestion to follow Shimano protocol is a better idea if you're going to ride small/small.

 

If you run your chain a little "short" for the gear combos you have, you can tune the system for slightly more crisp shifting.  Depending on how short, you may need to be very careful you don't shift to large/large.  If the chain is too short for your large/large combo, best case is the system will bind and not turn.  Worst case - something will break/bend ... most likely the rear der.  

 

Depending on what you optimize for, you can probably swap in/out a 2-3 tooth cassette difference without resizing your chain.  (i.e swap an 11-23 for an 11-25)   On rare occasion I swap in a wheel with an 11-28 in place of an 11-23 without resizing my chain for a very hilly ride.  I just know I cannot shift large/large - or even large/2nd large - without undesirable consequences.  Driveline set up is a bit like choosing skis - what terrain are you going to ski most and optimize for that.   

 

There's also a "calculation method" for chain length.  Probably on Sheldon's site somewhere.  Probably more appropriate for a high-travel mountain bike, where you need to account for many more variables than a fixed rear-triangle road bike.  I'd keep life simple and use the large/large method and be done with it.  

 

Correct-a-mundo! I forgot to mention which chainrings! :)

 

Okay, here's where it gets tricky (well....just a little). I bought both the Mid Compact 52-36 (which I'm installing first), and also a Standard 53-39 (which I'll install after the Summer, when I'll hopefully be a manly man from continuous quality training rides)?...Okay, well, let's just say I'm looking forward to not being in the slower section of the pack. At least that's my plan. Which could change. Depending on whether I can successfully shed another 15 lbs and get my pace waaaaaay up. Which remains to be seen. Can't you just feel the bristling bravado coming from my end? :D

 

It's humbling, I gapped my friend (who rides 9k mi/yr) on Saturday half a dozen times by 1-3 minutes and had to wait, PR'd up the wazoo on Strava, but I'm still several MPH's off the pace from the fast guys in the area. Are they drafting UPS trucks? Are they built like 13-yr-olds? Are they eating a lot of beans? What gives?!! ;-)

 

I'll be running 11-25 in the back, most of the time. Occasionally, will switch to 11-27 or -28 (haven't bought my mountain cassette yet, but it'll be one of those two configs).

 

Btw, thanks for the detailed post, very insightful once again.

post #8 of 9
Quote:
Originally Posted by Super D View Post
 

 

It's humbling, I gapped my friend (who rides 9k mi/yr) on Saturday half a dozen times by 1-3 minutes and had to wait, PR'd up the wazoo on Strava, but I'm still several MPH's off the pace from the fast guys in the area. Are they drafting UPS trucks? Are they built like 13-yr-olds? Are they eating a lot of beans? What gives?!! ;-)

 

I'll be running 11-25 in the back, most of the time. Occasionally, will switch to 11-27 or -28 (haven't bought my mountain cassette yet, but it'll be one of those two configs).

 

Btw, thanks for the detailed post, very insightful once again.

 

I was passed by more than a couple of 13 year olds while grinding up the local climb in my annual MTB race a year or two ago.  The announcer tried to console us old fellas.."Yeah, but they weigh 75lbs, haven't killed any brain cells with alcohol and don't have any cool scars!"  I just thanked my lucky charms that they had chocolate milk at the finish line to keep me going...  While not happy with my "form", I am comfortable with my ranking in the standings!!!  :-D  Not last!

post #9 of 9
Thread Starter 
Quote:
Originally Posted by Scott43 View Post
 

 

I was passed by more than a couple of 13 year olds while grinding up the local climb in my annual MTB race a year or two ago.  The announcer tried to console us old fellas.."Yeah, but they weigh 75lbs, haven't killed any brain cells with alcohol and don't have any cool scars!"  I just thanked my lucky charms that they had chocolate milk at the finish line to keep me going...  While not happy with my "form", I am comfortable with my ranking in the standings!!!  :-D  Not last!

 

Hehe, love it! That's what I think it should say on the event t-shirts. "Hey, at least I'm not last!" 

 

And then, the guy who DOES finish last gets a special one. :D

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