Page contents

Reason for replacing chains
Chain correct length
How to tell if a chain needs replacing
What if the chain skips
Good makes of chains
What is Chainsuck
Chainline explained

Why you should replace your chain.

Chains, especially on mountain bikes, have a very hard life as they are constantly being sprayed with dirt, grit, mud and water. This stuff mixes with our chosen lubricant to form a wonderfully efficient grinding paste which wears our chain and gears from day one.

As the chain wears it becomes longer thus increasing the distance between each link. This larger dimension of each link of the chain creates wear on sprockets and chainrings thus demanding their replacement too.

It has been proven that if chains are replaced before they become too worn then you will get much longer life out of your sprockets and rings. Mike usually goes through about three chains before the rear sprockets are too far gone. If the sprockets are too worn when a new chain is installed then the new chain will ride up on the teeth of the old sprockets and skip a tooth as torque is applied to the drivetrain.

How you know what length your new chain should be.

New chains come in one length and need to be re-sized for your bike and its drivetrain.
The chain MUST be long enough to be able to be in the largest sprocket and chainring on the bike without doing damage to the derailer.  Do this:   Place the chain around the big sprocket/big ring combo WITHOUT going through the derailer. Match up the chain to its shortest length and add one inch (TWO LINKS) (or one link and a PowerLink) - which is a set of innie plates and a set of outie plates. Break the chain at this length with a chain tool.  Make sure you have two "male" ends if you are using a mechanical link (like a PowerLink).   If you are going to re-rivet the chain (not as strong or as easy as a PowerLink) then you will need a set of male and female ends to join.

Full Suspension bikes - some bikes due to the suspension configuration make the chain extend as the suspension is compressed.  Please disconnect the rear suspension shock and see if passing the rear triangle through its full range of motion extends the chain.  If it does you are going to have to add extra chain to compensate for this.
There is a misconception that somehow the cage length of the derailer (you can buy them in small-medium-long cage versions depending on make and model).   The cage length has nothing to do with the necessary maximum chain length.  The cage length only comes into play when there is slack chain (off the big/big combo) and it decides how much loose chain is wrapped up.  It will determine how much slack you have in the combos approaching the small/small combos.  

BTW - the big/big combo is not to be considered a usable gear but if there is not enough chain to span these two gears if they are shifted into by accident then drivetrain damage will result.

Do not be concerned if the chain droops down when you're in the small/small combo. If you remove links to correct this then it obviously will be too short for the bigger sprockets. As with the big/big being an unusable gear, the small/small is not usable either. If you doubt me, just place the chain on either of these two extreme gears, squat down behind the bike and look at the horrible angle that the chain is running at. Can you say "drive-train wear"?

How you know when your chain needs replacing.

We do this simply measuring the chain with a ruler or tape measure. The distance between any two pins on a new chain is 1/2". As the chain wears this distance increases by a few thousandths of an inch. Over the distance of many links, this increase can be readily measured.

We take the measurement over the distance of 24 links and as the measurement of each one is 1/2" then 24 of them should measure 12". Measure the chain while it is on the bike as the derailer tension stretches the chain slightly. You may also hang the chain from a nail to give the necessary stretch.

So - measure 24 pins from center to center (or edge to edge) and the distance should be 12" on a new chain and no more than 12 1/16" for a chain in use. Measure the chain often - like once monthly - and discard when the measurement reaches that extra 1/16". If you allow the wear to increase to +1/8" you will probably have to replace the sprockets too. Anything above 12 1/8" will have done serious damage to the front chainrings also.

*****  My CHAINS page shows how to measure chain wear, how to clean your chain and how to re-lube. ****

What to do if your chain skips when you apply pressure to the pedals -

There can be three reasons for this. Let's take the easy one first. Did you just replace the chain? Yes? Then let's check for a tight link which can be caused from your chain riveting process. Back-pedal slowly by hand while watching the rear derailer pulleys like a hawk. If the pulleys take a little "hop" while you're back pedaling then that's the sign of a tight link going through them. Relieve the tightness with the special "tight link" spot on your chain tool or grip the chain on each side of the link by hand and forcefully flex the chain sideways to spread the side plates slightly. Re-check the link by back-pedaling again.

The second reason is that you didn't replace the chain soon enough and now you have old, hooked sprockets and a new chain. Sorry - replace sprockets too!

Gulp - the third reason is serious. You didn't replace your chain waaaaaaay past the point that it was worn out and it not only toasted the rear sprockets but the chainrings too. If you compare the chainring teeth to known good ones you will see that they are worn and hooked. Sorry - be more attentive to the needs of your bike in future.

What is chainsuck.

Chainsuck is what happens when you shift down from the middle ring to the granny ring and the chain does not release from the bottom of the ring and gets dragged up between the ring and the chainstay thus jamming the chain. You then fall off into the weeds.

Here are a few things that seem to contribute to chainsuck. If you’re aware of them and attempt to prevent them then this goes a long way to preventing the dreaded affliction.

The things that seem to contribute to chainsuck are -

  • Downshifting under load.
  • Dirty or dry chain.
  • Worn chain. (see FAQ on measuring chains for wear)
  • Worn, hooked chainrings (replace).
  • Burrs on chainrings (file them off).

Those points are in no particular order and your bike may have a number of them or maybe none of them. Some bikes just seem to suck the chain "because" and there is no reason that can be found.

Keeping the drivetrain in top condition goes a long way to preventing the problem but this is not guaranteed. My son had a bike two weeks old that sucked the chain like crazy.

There are "anti-chainsuck plates" on the market - both built into frames like Bontrager and aftermarket ones (like Ringle) that bolt onto the chainstays. I have had experience with both types and if they are not adjusted 100% then they make the problem far worse as the chain gets jammed in there worse than if there was no plate present. The Bontrager plate worked perfectly.  The aftermarket bolt-on ones seem to get knocked out of alignment with the first hit and then the troubles really start. I junked mine real fast.

Check Jonathan Levi's site  for more chainsuck info than you will ever need.

BTW - some people think that chainsuck is when the chain derails off the granny ring while downshifting and gets wedged between the crank and the BB shell. Sorry, but this is not the acceptable definition of chainsuck. I guess it’s just "jammed, derailed chain".

What is Chainline.

On most mountainbikes you have three front chainrings and anywhere from six to nine sprockets on the rear cassette. With the drive chain linking the two, obviously there has to be some relationship between the positioning of these drive parts  relative to each other for acceptable wear and shifting performance.

If we just sit and think of the setup we can visualize the three rings up front plus, let us say for simplicities sake, seven sprockets at the rear. Would you agree with me if I said the middle ring up front should line up with the middle sprocket at the back? You would? Good, then we are on the same wavelength.

As a quick chainline check, shift to the middle ring and the middle sprocket. Lean the bike against a wall and squat down behind the bike. Look real carefully along the chain from the front chainring to the rear sprocket. The chain should run straight back without angling off to the left or the right. This is pretty subtle, so check carefully.

If you have eight gears at the back then there is no center sprocket and the chain is going to be angled no matter which one you're on. Put it on the fourth from the smallest and it should angle a wee bit to the right and when you put it on the fifth from the smallest it should angle a wee bit to the left.

If the chainline is not correct and it angles off too much one way then your bottom bracket spindle is of the wrong length and should be changed. See the topic below of "measuring chainline".

How to measure chainline -

To find this out we have to get into some fairly accurate measuring. To measure the chainline follow these steps.

The rear chainline (at the rear hub) is not variable and is specific to your frame and components so we will measure it and adjust the front chainline to match.

The measurement we need to take at the rear is from the center of the rear axle to the center of the cassette. As the rear hub is offset to accommodate the cassette then the center of the rear hub is NOT the center of the rear axle! Just take a look and you’ll see what I mean.

We need to measure accurately here. Get yourself a steel tape that measures in millimeters. Turn the bike upside down on the floor. Measure and note the width of the rear dropouts. On most modern mtb frames this is 135mm. Measure the width of the cassette and the gap between the cassette and the right dropout - to do this just hook the tape over the large sprocket and eyeball down to the inside face of the dropout. It should be close to a reading of 40mm. Take the smaller number away from the larger (135-40) and divide by two (95 div by 2) which will give you the distance from the axle mid point to the dropout face or…… rear chainline. In our example (from my bike) we got 47.5mm.

Shimano says the rear chainline should be between 47.5 and 50mm so we are in the ballpark!

Now we have to check the front chainline.

With the bike still upside down on the floor, measure the bottom bracket shell width with a ruler and put a pencil mark on the shell at its middle point. Measure from here to the center of the middle ring. You may have to pass the steel tape between the chainrings and eyeball this from above but you'll get it close with some degree of accuracy. I always measure from the 10mm mark as it's easier than measuring from the end of the tape - just deduct 10mm from your answer. Write this figure down and compare with the previous figure from the rear.

As we found before, our rear chainline was 47.5mm and the acceptable variation was 47.5 to 50 mm. If your front figure is more or less than this acceptable range then you're going to have to change the bottom bracket spindle (or unit) to compensate. Some BB units are adjustable for chainline so just move it over by the amount it's out. Just remember - if your front chainline needs to be 5mm narrower then an axle 5mm shorter will only change the chainline 2.5mm!!

Taaaa-daaaaa - we just did the "complicated" job of measuring front and rear chainlines in two minutes!

Back to menu