Drivetrain wear explained

We explain the technicalities of drivetrain / chain wear to cyclists on a weekly basis. Here we’ve broken it down to the basics. Understanding this issue will save you some time, money and keep you safer on the road.
THE BOTTOM LINE (Too long, didn’t read!):
If nothing else just understanding this will make your life a little easier: Bicycle chains wear with use and need to be replaced regularly to prevent unnecessary damage to the rest of your drivetrain. Check your chain for wear frequently or take it to a professional and ask them to check it for you.
As the chain wears it starts to cause subtle wear to your cogs. At a certain point in time this wear will become “damage” and the chain won’t connect with the cogs correctly, causing a sudden jerk or crunch in the pedals when pushing hard. This can cause the chain to snap, your feet to slip off the pedals, or you to lose balance - all of which can be dangerous especially when cycling in traffic. The first time this happens is always a surprise, but it’s usually when pulling away at a traffic light, or roundabout.
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Sadly at this point it’s too late, in fact your chain and cassette may be >0.75% worn with ZERO symptoms.
It’s basically impossible to put a timescale or mileage on when you need to replace the chain as it depends on too many different factors. Luckily there's a tool which does away with guesswork and tells you when the chain has begun to wear - before it's even visible. With practice you’ll get an idea of the mileage you should expect on YOUR bike.
In our experience once the chain is over 0.75% worn it’s highly unlikely your existing cogs will take a new chain without slipping under pressure (even if it wasn’t slipping already!) For this reason we recommend replacing the chain BEFORE it gets to 0.75% worn.
If a chain is over 0.75% worn we’ll recommend replacing both chain and cassette for two reasons:
Even if the old cogs take a new chain without slipping, they can still SPEED UP the wear to the new chain. We can't know if a new cassette is needed until test riding with the new chain (andany other necessary repairs), if you wait until that point to opt for a new cassette there may be a delay if we need to order it in.
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A chain checker is a simple “ruler" about 6 inches long, with a hook shape in one end to fit in the chain’s links. The other end of the tool will not fit into the corresponding link on a new chain, instead the tool only fits in to indicate when your chain goes past two key points in its lifetime:
A: 0.5% worn = the chain is 0.5% "longer" than when it was new (0.0%). At this stage the chain should still fit the cogs fairly neatly, but is getting about 1/2 way through it’s useful life**.
B: 0.75% worn = at this stage the chain may not be fitting well to the cogs - this is often visible as “gaps” between the chain and cogs, (put it on the largest front chainring to see this clearly)
**Drivetrain wear happens exponentially which means as it happens, it happens more quickly! “Halfway” through a chains useful life does not necessarily mean you have “another 6 months” left!
Remember chain wear is inevitable, but there are a few factors that influence how long a bike chain will last before it needs replacing.
#1 Pedal power - use the lowest (easiest) gear you can bear to be in
Cyclists who cycle with a higher cadence (fast feet, light pressure) can sometimes get away with a chain >0.75% worn without replacing the cassette.
If you cycle with “slow feet and hard pressure” then you’re probably using the smaller cogs on the rear cassette, these wear out more quickly because there are fewer teeth sharing the load and the harder you push on the pedals the harder the chain has to work.
#2 A clean chain -
Road grit and dirt will combine with chain oil / lube to create a “grinding paste” which increases abrasion between the metal parts of the drivetrain. This shaves tiny particles of your drivetrain away with each pedal rotation. It’s impossible to “stick” those particles back once theyre gone!
Check out our short video on how to clean a chain quickly and easily.
#3 Lubrication’s what you need (if you wanna be a record breaker)
Once you’ve cleaned and dried your chain it'll need some lubricant to help the metal parts engage with each other without unnecessary friction.
Less is more. A little oil applied frequently is better than drenching it as too much oil can attract lots of dirt and you’re back at issue #2
See our article on which oil to use
#4 Use a chain checker
These tools are very cheap. Once you get the hang of using a chain checker regularly and understand what it's telling you, you'll ride with more confidence in your drivetrain.
#5 If in doubt, switch it out
Regardless of all of the above, simply riding your bike with a worn chain is really the only factor that matters in drivetrain wear. Sadly it’s the reason why people routinely spend more money on a cassette, chainrings, or in many cases even a new crankset - than they actually need to.
Chains are fairly inexpensive compared to cassettes and chainrings. They’re also easy to fit - see our handy guide on how to do this...
They may well be right! From 12 years experience and without even looking at your bike we can tell you that:
If you’ve been riding it at all, and didn’t already know about chain wear, statistically your chain is likely to be over 0.75% worn and therefore your cassette is worn too.
To save time explaining this complex phenomenon and because of the high likelihood you’ve gone past the “replace chain only” window (0.5><0.75%) most shops will advise you need a new chain and cassette EVERY time!
The only way you can know for sure is to ask for a photo of their tool on your chain, (we provide this on request) or by using your own chain checker.
To re-iterate, you may not be experiencing any “symptoms” - but they could appear at any moment, and this is why we recommend replacing these parts
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