Transmission Wear

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axel_knutt
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Transmission Wear

Postby axel_knutt » 7 Nov 2012, 4:02pm

When a chain is brand new it has a pitch of exactly half an inch, and it will sit snugly at the base of the teeth, but as a chain wears it gets longer, and the chain has to ride higher on the teeth in order that the tooth pitch always matches the ever increasing pitch of the chain. Eventually, the chain gets replaced before it becomes worn enough to slip over the tops of the teeth. That’s the familiar cycle of chain wear, but let’s consider it in more detail starting with a couple of hypothetical chains: one an everlasting chain that never wears out and always remains exactly half an inch pitch, and the other a poor quality chain that wears out in no time (a few miles, say).

Consider the everlasting chain first. If it never wears out then it will always remain snugly seated at the base of the teeth, so the chainring wear all occurs at the base of the teeth too, and the familiar hooked tooth wear pattern develops. Eventually the teeth become sufficiently undermined that the chain can’t separate, gets drawn up the back of the chainring, and the ring is scrapped.

Now compare this with the poor quality chain that wears rapidly. As we’ve seen above, the chain rises up the tooth as it wears so although the teeth still wear, the chain would be wearing at a much faster rate, and never remain in the same place on the tooth for very long. So the fast wearing chain would wear a thin skim off the whole height of the tooth before being replaced by one chain after another, each of which would do the same. Many chains later, the tooth would have accumulated the same amount of wear as with the everlasting chain, but instead of all the wear being concentrated at the base it would be evenly distributed up the whole height of the tooth. A pattern of wear like this preserves the profile of the tooth, so although it may become very worn there is no hook to snag the chain and prevent it from separating from the ring. Indeed, because the wear is spread over a larger area it will take many more miles to erode the tooth to the same depth. It’s a win-win situation, on one hand there is less depth of wear for any given mileage, and on the other hand this pattern of wear will tolerate a greater depth worn away before the ring fails.

Look at the rings below, the one at the bottom looks in much worse condition, and indeed it has done many more miles, but it is the one at the top that is unusable because the teeth are hooked and drag the chain up the back of the ring!

Wear_Pattern.jpg


The moral of this tale is that the wear pattern depends on the relative rates of wear of the ring and the chain. If your chain is hard wearing compared with the ring you will get a hooked wear pattern and a short chainring life, if you have a chain that wears quickly relative to the ring the more even wear pattern will prolong the ring life. Short lived chains are not bad for your chainring life, quite the contrary.

Now consider some chains from the real world, with an ordinary lifespan. Let’s compare what happens if you wear out one chain at a time, with what happens if you buy a set of three and rotate them regularly. The diagram below shows how chain wear accumulates with mileage:

Chain_Wear.jpg


The green line shows the wear of a single chain, the red shows the wear for the continually rotated set of three chains, and the orange is a smoothed out average of the red line. As you can see from the graph, the three chains collectively behave in much the same way as a single chain with three times the lifespan, but we’ve just seen above that long lasting chains create a tooth profile that’s more hooked and force you to scrap the ring prematurely. Hence the second moral of this tale is if you rotate your chains, it will reduce the life of your chainring not increase it.

So, to sum up it is the rate that the chain wears relative to the chainring that determines the wear pattern, and a hooked pattern is less tolerant of wear because it prevents the chain from separating. Continually fitting new chains, buying hardwearing chains, and rotating a set of three chains all reduce the chain wear rate relative to that of the chainring, and thus shorten the life of your rings by concentrating all the wear at the base of the tooth rather than distributing it more evenly.
Last edited by axel_knutt on 19 Nov 2012, 4:31pm, edited 1 time in total.
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531colin
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Re: Tranmission Wear

Postby 531colin » 7 Nov 2012, 4:32pm

By the time the chain is worn enough to ride up the chainwheel teeth, won't it have worn the cassette teeth so that a new chain will skip?

My chainwheel teeth eventually hook, but I do sometimes throw away particularly manky-looking chains before the wear gauge tells me to.

Brucey
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Re: Tranmission Wear

Postby Brucey » 7 Nov 2012, 4:39pm

that is all very interesting but I don't think that hooked chainrings are such a big problem;

- firstly when the chain pitch matches the tooth pitch exactly the load is spread over several teeth at once and this makes for very much slower wear, even if it is perhaps producing a hooked pattern eventually.

-Second the supposedly 'useable' tooth profile will suffer accelerated wear with any chain for these reasons; a) the roller load must be concentrated because there is no seat on the tooth for each roller with a matching curve and b) the roller contact on any neighbouring tooth is not tangential to the tooth surface. These two arise from the same thing in this case which is that the tooth is not the correct shape any more. The consequence is that the teeth are likely to be individually and locally loaded again and this once more makes for faster wear. With luck the chainring will quickly develop new wear pockets with a new chain that to some extent restore the tooth geometry local to the roller contact.

-third, a hooked chainring can easily be remedied with a little filing if necessary. This is (IME) only necessary with smaller chainrings; larger ones seem to better allow the chain to be slack on the chainring at the point where the chain disengages.

-fourth, I normally get at least four cassette's worth of chains etc to one set of chainrings. I spend a lot more on sprockets and chains than on chainrings, and so do many others, I am sure.

cheers
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BigG
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Re: Tranmission Wear

Postby BigG » 7 Nov 2012, 5:53pm

Brucey wrote:- firstly when the chain pitch matches the tooth pitch exactly the load is spread over several teeth at once and this makes for very much slower wear, even if it is perhaps producing a hooked pattern eventually.
cheers

The chain pitch surely always exactly equals the tooth pitch. The only issue is how far out on the teeth this happens.

Brucey
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Re: Tranmission Wear

Postby Brucey » 7 Nov 2012, 6:10pm

BigG wrote:
Brucey wrote:- firstly when the chain pitch matches the tooth pitch exactly the load is spread over several teeth at once and this makes for very much slower wear, even if it is perhaps producing a hooked pattern eventually.
cheers

The chain pitch surely always exactly equals the tooth pitch. The only issue is how far out on the teeth this happens.


yes, but surely there are only two conditions which allow the roller to to sit on a properly curved seat on the tooth form and thus not produce a high contact stress and therefore accelerated wear; 1) with a new chain on a new chainwheel and 2) with a hooked chainwheel.

However in the latter case you often get tooth-at-a-time loading anyway.

IME hooking only starts with a worn chain and once it starts chain wear will usually continue unabated. Also hooked teeth rapidly turn into the ones with a triangular form if use with a stretching chain is continued; the overhangs of the hooked tooth form wear off during chain engagement (which happens under load). Better IMV if a new chain is fitted as soon as it starts to produce hooking; it will surely be stretched by this point and will be destroying the sprockets at high speed.

Likewise if a new chain is used with a hooked chainwheel then it wears the hook pocket on a smaller diameter, again during engagement. This process can be noisy but after a hundred miles or so (of this kill or cure treatment) it normally settles down provided it wasn't too bad to start with.

cheers
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Mick F
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Re: Tranmission Wear

Postby Mick F » 7 Nov 2012, 6:58pm

axel_knutt wrote:When a chain is brand new it has a pitch of exactly half an inch .....
Sorry for picking you up on the very first phrase.

Completely incorrect! :evil:

I won't bring up the thread again, but I bought a Sram cheapo - bottom of the range - and it was "worn" or "stretched" straight out of the box. It failed the chain-checker, and it failed the ruler.
Mick F. Cornwall

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meic
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Re: Tranmission Wear

Postby meic » 7 Nov 2012, 7:17pm

It is quite a convincing argument and I was beginning to think that it explains why I get a pretty poor life out of my chainrings.

However there are too many facts against it.

I have a zicral 39 t ring that has done about 11,000 miles using 5 chains all of which were never allowed to the wear limit. The teeth are not and never have been at all hooked, they have worn down to slim daggers, possibly the chain removes the hooking as it comes off rather than get caught on it.

Another cyclist on the other hand, who rode with me most of the time, ran one chain and a steel 39t ring for 9,000 miles at which point his chain was jumping and eventually snapped. The chainring was in much worse condition than mine is. Though of course he is 4 chains richer.

My big ring is not perceptibly worn at all, his big ring was scrapped with the middle ring.

The way I see it the chain ring will not last as long with just one chain, especially a short lived one as the worn chain does destroy the ring rapidly. I have only got a few more thousand miles out of my chainring and it cost me four more chains to do it. Yet I would say as far as the chainring is concerned that running only chains with little extension has got full money's worth out of the chainring.

If I felt like I could get a few more thousand miles out of the chainring but it will start breaking teeth soon.
In my case the teeth have not worn into hooks, it does not seem to actually happen in the way that this theory is based on.

I rotate chains on most of my bikes and replace them "early" and the only teeth that hook are on the small steel rings and that is deformation not wear.

Of course the fact that I keep cleaning my chains may have more to do with it than the rotation and replacement.
Yma o Hyd

BigG
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Re: Tranmission Wear

Postby BigG » 7 Nov 2012, 8:12pm

Brucey wrote:yes, but surely there are only two conditions which allow the roller to to sit on a properly curved seat on the tooth form and thus not produce a high contact stress and therefore accelerated wear; 1) with a new chain on a new chainwheel and 2) with a hooked chainwheel.

However in the latter case you often get tooth-at-a-time loading anyway.

cheers

I may be wrong, but I think that all chain drives are designed so that the roller contacts a nearly straight part of the tooth (actually a volute). This allows it to roll out along the tooth while remaining in contact as the tooth drops away prior to losing contact entirely. They are like gear wheels in this respect. As for the "one tooth" loading, I believe that the chain will always ride out so that the load is distributed (declining exponentially) working back from the first point of contact until the feed-on point is reached. In the case of derailleur gears, the nuetral point is where the tension provided by the jockey wheels equals the tension provided by the main drive. This is likely to be close to the feed-on point in normal use.

Brucey
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Re: Tranmission Wear

Postby Brucey » 7 Nov 2012, 9:05pm

as one sprocket tooth goes 'over the top' the direction the roller is pulled in changes. If the tooth geometry (and/or the chain pitch) is wrong, the roller moves on the tooth whilst under load which is one way in which rapid wear can arise.

Where the chain is fed on to a sprocket it sees a reverse tension and the chain rollers can sit in the 'wrong place' until they see the full chain tension. In the worst case the chain roller moves once (under load) as it starts to bear the full load and again as it disengages. Unless the tooth happens to wear as a perfect volute (or w.h.y.) then some movement of some kind will likely occur with a worn tooth.

It is possible that the type of movement (and thus wear) that is seen is dependant on the speed and the balance of the chain tension on the slack run and the tension run.

The idea that the chain rollers all sit at an equal radius on the side of the tooth is probably closer to the truth when the chain is operating at speed; here the chain sees an additional inertial force that may help it engage more evenly.

I compared the wear on a well-used hub gear sprocket with a new one a while back. The sprocket was badly hooked, so as expected there was significant wear on the loaded side of the tooth. However I was a little surprised to discover that the root of each tooth was also significantly worn; I wasn't expecting this, and it shouldn't be expected if the chain always rides up the tooth as it elongates.

Bottom line (for me, anyway) is that bicycle chains operate in a peculiar way and may well manifest atypical behaviour when compared with other chains.

cheers
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