When to replace the chain on a bicycle?

Updated: 28/10/2020.

This post explains when to replace a bicycle chain. For a detailed explanation of what exactly happens when a chain gets used and worn, look at this post: Bicycle chain wear (elongation)

Contents:
1. Chain design
2. Chain wear – elongation
3. When is the time to replace the chain on a bicycle?
4. How to measure chain wear?
…4.1. Method A – ruler
…4.2. Method B – pulling the chain off the front chainring
…4.3. Method C – using chain checker tool

1. Chain design

Bicycle chain transfers pedalling force to the rear wheel. Compared to alternative systems for drive force transfer (like universal joing (cardan), belt etc.) it has very small mechanical losses – it is efficient. This is an intersection of a chain with bushings, predecessor of the modern chain:

Chain plates are connected with pins. Over the pins there are rollers – used to make sliding over chainring teeth easier. Previous version of chain had bushings over pins – between pins and rollers. Modern chains however don’t have these bushings – they are bushingless chains.

Advantages of modern chain compared to old bushing chain design:

• Cheaper to produce (this should also mean cheaper for the buyer, but is not always the case)
• Less likely to brake if changing gears under load
• Provides better lubricant flow inside the rollers
• It has more lateral flexibility, which enables more cross chaining on bicycles with lots of sprockets

Disadvantages of modern chain compared to old bushing chain design:

• Slightly worse mechanical efficiency (for a few percent)
• Lubricant is quickly washed out of the area between the rollers and pins. So while they are more quickly lubricated, they loose lubricant (and allow the dirt to enter the area between rollers and pins) just as fast. This openness of rollers and pins is Achilles heel of modern chains. Unless rigorously cleaned and re-lubed, they wear out rather quickly.

2. Chain wear – elongation

Chain gets worn through use. Wear is mostly affected by dirt and lack of lubrication. Grinding pastes are made with combination of sand and oil – this is also present on a dirty chain. Perfectly clean and lubricated chain inside a car engine often lasts for tens of thousands of kilometres, taking a load of over 100 horse power often. So to have the chain last long – it has to be clean and lubricated. Still, even with good maintenance, chain gets worn sooner or later.

How to tell that the chain is worn?

Chain wear is determined by it’s elongation, often, technically inaccurately called “stretch”. Chain elongation does not occur because of material stretch, but because of pin wear.

Length of one link of a new chain is exactly one inch (2.54 cm). One chain link includes one pair of inner and oune pair of outer plates. In the picture above, that is marked as “2 x PITCH”. As the pins wear, the distance between adjacent pins gets bigger – making the chain effectively longer.

Since chainring teeth are optimized for chain with 1/2″ (1.27 cm) step, longer chain will not mash with the chainrings properly. Because of a longer step between pins, a worn chain will not put similar load on all the chainring teeth, and will tend to “climb” on the teeth. This further increases chain wear and it also increases chainring wear significantly.

Skipping chain: when the chain gets too worn, it can climb too high and when pedalling hard it can slip off the chainring teeth. This results in pedal suddenly dropping, like the rear wheel has slipped suddenly.

Rear sprockets wear: if a bicycle is ridden with an elongated chain, the rear sprockets will quickly get worn so the chain skipping  will occur with a new chain – due to worn rear sprockets. Rear sprocket wear (except an extreme one) is impossible to determine by looking. Even a fraction of a millimetre wear is enough to cause skipping of a new chain. One of the reasons I’d never advise a purchase of a second hand cassette.

Front chainring wear: if the bike is ridden longer with a very worn chain, even front chainrings will get worn.

Here a chain slipping is the least likely to occur, but since the sprocket teeth get worn, the chain will have noisy disconnection at the bottom of chainring, or it will get stuck (this is called “chain suck”).

Chain suck effect shown on video:

3. When is the time to replace the chain on a bicycle?

1) When the chain gets elongated for about 0.5% of it’s length, it is the time for a new chain.

If the chain is changed at this mark, the life of chainrings and sprockets will be longer. Rear sprockets will last from 2 to 4 chain changes this way, before they too get worn and need a change. Sprockets need to be changed only when they start skipping with a new chain. Front chainrings will last many times longer than the rear ones with regular chain replacing.

2) If the chain is not changed at 0.5% elongation, but stays used until it reaches 1% elongation, then rear sprockets will need to be replaced along with the chain – they will definitely be worn. Gear changing on bicycles with a rear derailleur will be slower and it will worsen as the chain wears over the 0.5% elongation point.

3) If the chain is not changed past the 1% elongation, then even front chainrings will relatively quickly get worn. So when the chain is changed eventually, both the rear and the front chainrings will also need replacing. Changing gears on bikes with derailleurs will be noticeably worse and there is also a higher risk of a too worn chain snapping.

4. How to measure chain wear?

4.1. Method A – ruler

With a ruler, simply. The chain is measured when it is straight and tensioned, best directly on the bicycle.

Inch ruler: since one link is exactly 1″, measure 12 links. It is easiest to measure form the middle, to the middle of the pins.

If the 12th link pin aligns with about 12″ an 1/16″, then the chain is about 0.5% elongated.

If it aligns with about 12″ and 1/8″, then the chain is elongated for about 1%.

Centimetre and millimetre ruler: 10 links are measured. Zero is aligned with the middle of the first pin and it is then checked where the 10th link pin is aligned. If it aligns with 25.4 cm, the chain is new.

If the middle of the 10th link aligns with just over 25.5 cm (more precisely 25,53 cm), the chain is about 0.5% elongated.

If it aligns with almost 25.7 cm (more precisely 25.65 cm), then the chain is about 1% elongated.

4.2. Method B – pulling the chain off the front chainring

At the front chainring, towards the front part of the bike (at “3 o’clock”), one should try to lift the chain off the chainring.

This method is not too precise, but is good for a quick check.

4.3. Method C – using chain checker tool

There are simple tools for checking chain wear. They are just leaned on the top of the chain. If they drop in, the chain is worn and it’s time to change.

There are labels on the left side of the tool 0.75 and 1.0. The 0.75 measures approximately 0.5% of chain elongation, while the 1.0 is a 1% elongation mark.

Measuring using such tools is quick and easy. The flaw of this tool is that it often gives a “false alarm”. It adds roller wear into results – not just the pin wear that affects stretch. This means that often when it shows a time for a new chain, measuring with a ruler shows less than 0.5% elongation. This is often the case with chains that have a bit softer rollers, or a bit smaller rollers (sometimes even new chains show wear with this tool). I’ve made an experiment to show that roller wear does not affect chain to chainring engagement.

That is why such tools are mostly good as a quick check when to start measuring with the ruler – to determine a change time.

The exception are Shimano tools (links point to product pictures and prices at Amazon.com – as an Amazon Associate I earn from qualifying purchases.) TL-CN40, TL-CN41 and TL-CN42. The picture shows difference of ordinary and Shimano tool:

The problem of ordinary chain checkers can be partially eliminated by moving the right hand side roller to the left manually with a screwdriver, but this usually requires a “third hand”. 🙂 However, even using this technique, the “ordinary” tool still measures distance between the two roller sides facing each other. So roller thickness still affects the measurement results.

It is an interesting fact that a worn chain is much more flexible to the side than a new one. No way it is a precise method to measure for wear, but it can be informative when looking at a 2nd hand chain quickly (buying which I wouldn’t recommend):

Related post – Chain length sizing for bicycles with derailleurs:

A simple, accurate and precise tool for chain elongation measurement (link to Amazon.com with the tool pictures and prices – as an Amazon Associate I earn from qualifying purchases.): Park Tool SBC-1.