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The Chain Wears The Sprocket

Thinking Date - Tuesday, February 19, 2019

I seem to be going against the grain again. It's not my intention to be awkward or argumentative, it is my intention to gain knowledge. As such I am prepared to be proven wrong. I await a wisdom more sage than I, which ought not take too long.

The given wisdom is to replace the sprockets when you replace the chain. The reasoning is the chain has warn to the new pitch of the now warn chain. Eh... What?

What is chain pitch?

Chain pitch is how long each individual link is. If your chain has a 10mm pitch and 1,000 links the chain will be 10mm multiplied by 1,000, that will be 10 metres long. A "520" chain has a pitch of 15.88mm. That, dear reader, will be 5/8ths of an inch, leading to the "5" in "520". If your chain has 112 links then it will be 1778.56mm long, or 1.778 and a bit metres long.

Pitch is measured pin centre to pin centre.

Of course this is if your chain is perfect and with zero tolerances. The world is not perfect so don't expect your new chain to be precisely this length. Knowing what it should be is handy though because you can compare what it should be to what it is, thus knowing how much it has "stretched".


Save for the ridiculously minuscule molecular level it's safe to say metal chains under normal use don't stretch. They wear, but you'd have to be a pedant to correct someone talking of chain stretch. As the chain is in use each and every link pivots back of forth. Once tight tolerances become slack. Imagine our 520 by 112 link chain started out life at 1779mm long and each link wears by 0.1mm that is 0.1 times 112 giving 11.2mm of extra length. Our chain is now 1790.2mm long.

This is why motorcycle chains can be adjusted, to take up with slack as the chain wears. But what cannot be adjusted are the sprockets. 

Sprockets are engineered to the pitch of the chain, at new. The tip of each tooth on a 520 chain will be 15.88mm apart... Hang on. Not quite. Sprockets are circular. This causes explanation issues because of radii. Oh look, here's a diagram. 

A diagram to show the difference between the inner and outer radii on a sprocket
The tips of the teeth are, well, more than the "pitch" apart.

So for the purpose of explanation I have created a miraculous "flat" sprocket. Don't like this? Well just imagine it as a section of a 50,000 toothed sprocket ok? Otherwise we're into a whole world of pain and my MS Paint skills aren't that good.

The teeth of a flat sprocket makes drawing the diagram easier
My "Flat" Sprocket. The teeth, troughs and centres are all exactly 15.88mm apart.
The rollers of the chain sit perfectly in the teeth
My perfect new chain sits perfectly in the perfect new teeth of the new sprocket.

So the gaps between each tooth are the same as the pitch of the chain. The chain slots neatly into the matching gaps. With a little lubrication the whole system works perfectly and the world is a happy place. Until the chain wears. The gaps on the sprocket have not changed (yet) but the chain links are now further apart.

The teeth are in the same place but the chain is falling out of sync
Our old chain no longer slots neatly between the teeth of the sprocket.

The teeth of my perfect sprocket are still 15.88mm apart. But the links are say 16.00mm apart. As the roller part of the chain reaches the sprocket teeth instead of the roller slotting neatly into the gap between the tooth it hits the back of the gap aka what is the front of the next tooth. The next tooth has arrived before the next gap in the chain.

As such the latest roller to hit the sprocket takes all the tension. The rest of the chain on the sprocket has loosened off and dropped in a saggy baggy manner into the troughs between the teeth. The chain under tension's pitch is 16.00mm, the chain in the sprocket troughs is resting at 15.88mm give or take. 

This wears the leading face of the tooth, more than it ought to. This action elongates the well, the gap, the trough, the space between the teeth. All the teeth will wear, elongating the troughs. But what it does not do is change the distance BETWEEN the tips of each tooth.   

The worn sprocket starts to get the distinctive saw-tooth look
"Saw-Tooth" Yet the distance between the tips of each tooth are still exactly the same.
A worn sprocket starting to get the saw-tooth look
My sprocket is also "saw tooth", but the "pitch", the gap between the tips, is still the same.

What we end up with is, in my opinion, still a 520 pitch sprocket but with malformed gaps between the teeth. 

Thus and therefore when a new 520 chain is fitted to the sprocket with the malformed gaps it will land quite happily onto the sprocket and sit, as it should, in the elongated gaps. 

A new chain sits in the worn troughs of the worn sprocket just fine
A new chain on the worn sprocket teeth. 

This all comes back to a comment I saw on a forum. "The chain wears the sprocket, the sprocket does not wear the chain".

Be warned though, this is NOT an excuse to ignore the condition of your sprockets. In the final image above you can clearly see the teeth are now thinner due to the wear on one face side. As a young, nieve and poor teenager I was sat at the traffic lights with the clutch out in first gear but going nowhere. The chain was still there, it took me quite a while to realise there were no longer any teeth on my sprocket.

You're welcome to counter Ren's thinking in the comments below. Or if you wish to be more thorough you can create your own explanation as a post with images, diagrams etc. Contact

Reader's Comments

Pocketpete said :-
I understand the logic but surely a chain as well as wearing down will indeed stretch. Steel can be pretty stretchy stuff despite being as hard as well 'steel'

They replaced the actuator rods on the tornado jet from high grade steel to titanium because it didnt stretch as much. I think they were talking about 0.1mm over 5 years of cycling. Hardly seemed much but was important enough to warrant the change.
20/2/2019 12:13:44 PM UTC
CrazyFrog said :-
Pocket Pete - I'm sure you are right, and the chain may stretch, ie the side plates may stretch. However, I would expect the amount they stretch to be completely insignificant compared to the 'stretch' caused by pin wear.

Ren, you've convinced me...
20/2/2019 1:25:13 PM UTC
Ren - The Ed¹ said :-
Yes the metal of the chain may stretch bit I'd think it was so minor as to be negligible. Tornado jets are probably under a lot more strain than bike chains. I wonder if they'd ever let me bodge a tornado jet...?

CrazyFrog - I've convinced you... What, that I'm an idiot?
20/2/2019 4:19:26 PM UTC
Rod said :-
Yes Ren, I have followed your logic and agree to a point.
The only doubt I have is this. If the chain was under constant light load I would totally agree, but motorcycles accelerate and decelerate so the play between the chain and worn sprocket will be moving backwards and forwards in the elongated sprocket gaps. I am thinking this will put more strain on the chain and hence wear it more quickly.
20/2/2019 7:05:18 PM UTC
Snod said :-
My chains will often see at least a couple of rear sprockets and several front sprockets, I think the last chain on my CB250 saw at least 5 fronts in it's 40K mile life! Keep the sprockets in good condition and the chain will last a long time, in my experience. That was a DID VM though, the JT Z3 on my Z250SL does not seem to be faring so well at a paltry 9K..
20/2/2019 10:15:01 PM UTC
Ren - The Ed¹ said :-
What surprises me are the huge variations in experience. From Ian only adjusting a chain once in 20k, Snod preferring to change sprockets more often than chains, Pocketpete killing a chain in 7k and my questionable habits. I've seen 37k out of a chain right down to about 9k.

Is it our individual use scenarios, care regimes, motorcycles and riding styles or is it possible chain quality itself is in fact VERY variable? Even between chains from the same marque and model.
21/2/2019 6:51:04 AM UTC
Pocketpete said :-
Is it just luck how long a chain lasts.

I sprayed Castrol lube and put chain wax on my old chain. Lasted 7k. Thus ones lasted 4 so far and I adjusted the new one once by a tiny amount. The only difference is in using old engine oil and grease to lube it. It certainly looks like it will last longer.

I suspect quality control is poor from Honda. Looking online chain life varies from 6k to 25k. That's a big variation.
21/2/2019 10:38:41 AM UTC
Rod said :-
In my experience the two thing which help chains last are regular lubrication and adjustment, and do not use the bike in bad weather as the rain will wash off the lubricant.
I also use a mixture of grease and oil, I have always used gear oil and grease and it seems to work so I have never tried old engine oil.

I did have one bike where I only lubricated it at the regular service intervals and never adjusted the chain once in all the time I owned it. The chain still did not need adjusting or replacing after 19 years and nearly 60,000 miles!!

This was because it was shaft drive.
21/2/2019 12:21:53 PM UTC
Ian Soady said :-
"What surprises me are the huge variations in experience. "

I suspect that a lot of it (like tyre life) is down to riding style. I've always been what I consider to be mechanically sensitive in my style although my consistent 55 mpg may suggest that I'm not too gentle...

Maybe the shock loadings from dropping the clutch, changing down without matching engine revs etc cause extra wear? I confess I don't know.
21/2/2019 3:29:44 PM UTC
Ted said :-
Way back in my distant youth I could not even afford any sort of proper bike.

So, as a poorly paid apprentice on less than £3.00 week I bought a 5 year old 70cc Capri scooter (sorry guys, yes, a scooter, it was transport) Garelli engine and a fully enclosed chain drive......
Which it was perfectly safe to ignore eh?

Scooter cost me £30.00, all the dosh I had in my "post office book" :-(

Well, "proper" scooters, Like Lambretta had a triple row chain drive in an oil bath.
Vespa (I think) were a permanently engaged gear drive. Loads of nice sticky oil stuff to keep it working.

My old bicycle had a chain, how often did that need oil, let alone adjusting.

When the clutch started feeling a bit jerky and there was this little rattle that I had not noticed before.
Time to investigate.

Oppps, I suppose about a 1/4 of the rollers fell on the ground when I took the chain case off.
Rear sprocket was hooked enough to use for fishing.

Lesson learned.

It would seem that we never get experience until we have learned a lesson that has cost us money.
5/3/2019 11:56:20 AM UTC
Ren - The Ed said :-
Cheers Ted. Tell me was it black and white back then and did the Hovis tune play constantly in the background? By 'eck I know tha wuz only an apprentice but 3 quid a week! Mind you I bet a loaf of bread wur only 2 'n 6.

I also bet the parts to fix were more than the £3 a week you were being paid.
5/3/2019 3:55:23 PM UTC
Ted said :-
Hi Ren, I started work at an engineering in Charlton, South East London, September 1963.
I woke up one morning to 12" of snow in the street where I lived.
Scooter wheels were only 12", so the 5 mile journey to work was interesting to say the least.
That was the worst winter since 1947 (so I was told)But in 1947 mum provided my transport ;-)

No "Hovis Hills" to ride down in South London, but just as many bumpy roads.

I do remember when I got my first car at the age of 21, could still but 4 GALLONS of Petrol for a £1.00
get change.

We had two TV channels and even though only in B/W were getting "excited" about the new BBC2

Originally styled BBC2, it was the third British television station to be launched (starting on 21 April 1964),

How easy pleased we were as young people.

WTF turned me into a Grumpy old man ?

Oh yeh....LIFE lol.

6/3/2019 9:08:54 AM UTC
Ren - The Ed said :-
I'm working on being curmudgeonly, regular readers will know it is going very well indeed. I must say I'm thoroughly enjoying being a grumpy old grump. It's a lot easier than pretending to be happy, I'm at my best when I'm complaining.

If thy didny av th'ills fert wayk up n daan int snow tha doesn'y know tha's born lad. It took me 5 hour fert geet up't local th'ill tay me ouse cos it wur froz so solid even't cat cudn't wayk up it.
6/3/2019 4:50:34 PM UTC

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