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In trying to achieve a long lasting, low maintenance drivetrain, I've read statements about aluminium oxide being the source of the black colour of chain dirt and also being highly abrasive:

Aluminum oxide from chainrings makes chain dirt black - and it is very hard and abrasive. Grinding wheels are made of aluminum oxide!

John Allen on sheldonbrown.com

Others state that pretty much all the abrasives (including the aluminium oxide) come from the road:

I believe almost all the grit comes from the road. Aluminum is very common in most soils, in the form of aluminum oxide, which is a pretty effective abrasive.

Frank Krygowski in rec.bicycles.tech

My question is this:

How much of the dirt/abrasive on a chain comes from the chainrings, and how much does it really affect chain wear?

My feeling is that the aluminium oxide particles ground off the chainrings would be of very low diameter, and thus wouldn't have a significant abrasive effect compared to the larger particles of sand and dirt thrown up from the road/path itself. Obviously this brings up questions as to the lubricant used (as a sticky lube will attract dirt) and riding conditions, but for the sake of this discussion, I'd like to consider a bicycle which is regularly maintained and ridden in all weather, primarily on made roads.

To bring this into the realm of the practical, my reason for asking is that if the cog material truly does affect chain longevity, I would consider changing to steel cogs (not currently considering the other advantages/disadvantages of steel, which have been covered elsewhere).

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    If the aluminium oxide comes from the chain rings, that means the chain rings are wearing away. But we don't see that in practice, chainrings last a long time, much longer than the chain or cassette. That alone suggests the answer to your question is "no". – Móż Nov 27 '15 at 1:10
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    How common are aluminium chain rings and casettes? The dirt on every single bike chain I've ever had has been black, including the bike I rode as a five-year-old kid and the dirt-cheap mountain bikes I rode as a student 20 years ago. Would those bikes have even had aluminium chain rings or casettes? If not, that's another strike for the theory. – David Richerby Nov 27 '15 at 9:24
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    @David I was wondering that too - surely vintage bikes have steel cogs, and they definitely have black chains. – Simon MᶜKenzie Nov 27 '15 at 9:46
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    To anyone out there with experience with indoor cycling, do the chains on bikes in trainers (or even track bikes in indoor velodromes) end up black? – Simon MᶜKenzie Nov 27 '15 at 9:56
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    'Long lasting' and 'low maintenance' do not go well together. If you cut maintenance of the chain to a minimum you can be sure that you'll soon replace chain, cassette and chainrings. – Carel Nov 27 '15 at 16:09
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I suspect that the abrasion from road/chainring/cogs is negligible, especially when comparing with other causes.

The silica grains (SiO2, present almost everywhere) would be more of a worrying issue than the aluminium oxide, regarding chain wear. The grains are often visible (in sub millimetre size) and easily stick on the chain after a ride on any road surface. These grains create grindings (between chain and cogs/chainrings) and accelerate chain wear.

The applied force also plays important roles in chain wear. An incorrect gear would create a large force between chain pins and cog/chainring. A worn chain would create a large (localized) force between (certain) chain pins and cog/chainring. A chain would wear much faster if the applied force to its pins increase.

In short, aluminium+stainless combination is almost identical to stainless+stainless combination in gearing (i.e. creating the same amount of debris) . I used to make electric motor with high RPM and the best combination is nylon+metal. The clearance/applied force does have great importance over many other causes of wear.

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    As a secondary question, do you believe the statement that "aluminum oxide from chainrings makes chain dirt black" is accurate? I didn't think of checking this before asking, but I see that both SiO₂ and Al₂O₃ are both white - does that immediately disprove this claim? – Simon MᶜKenzie Nov 27 '15 at 1:10
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    In short, no. I think the claim is not vigorous. Al oxide and Si oxide are white in powder form. When lubricant is added, they are still relatively white/grey/dark grey, depending on the lubricant. However, this is an ideal laboratory set up. If you have notice, a ride on a rainy day will make your rim covered in black (thin) film of dirt. The dirt contains organic matter, which makes it colour black. Your chain will eventually pick up enough dirt. – Nhân Lê Nov 27 '15 at 1:31
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    aluminium is silver in colour, and can be black in powder form. However, aluminium oxide is white in colour, and is relatively white in powder form. – Nhân Lê Nov 27 '15 at 1:42
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    Yes, i think it is a great theory. I however cannot verify this claim. It could be more complicated than just simple Al powder coated by oil. For example, oil may decompose into different compound and gives black color. – Nhân Lê Nov 27 '15 at 3:04
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    @SimonMᶜKenzie I'm skeptical. My feeling is that you'd need an awful lot of powder to colour a silver chain uniformly black: I'd probably start with a 50/50 mix of oil and powder if I wanted to "fake" that on a clean chain. It seems that components would have to wear extremely quickly to produce that kind of effect. Also, if it is aluminium powder, aluminium is much further down Moh's scale than steel (2.5 vs about 5), so shouldn't cause much abrasion. – David Richerby Nov 27 '15 at 9:22
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Interesting question. Real world conditions are messy with multiple factors impacting any analysis, as such my answer will be speculative, but based on a number of sensible working assumptions.

Chainring degradation as a source

First lets consider how much material comes off a chainring. Generally speaking, if you replace your chain regularly before it stretches, it should be quite easy to get 10,000km+ out of a set of chain rings before needing to replace them. This indicates a slow rate of wear, and therefore a small amount of contaminating/abrasive material. Furthermore the chain ring touches the rollers of the chain, not the whole chain width. As such, the small amount of material that does get deposited gets deposited on the roller. It still needs to make its way to the edge of the roller and then down the side of the roller and into the contact area between the roller and the "bushing-inner plate" before it will start contributing to chain wear. Given the location of the abrasive material and the limited and slow movement of the rollers (this of course depends on your gearing - e.g., Big-big will have slower roller movement than Big-small), I suspect this is a very slow and low volume process.

Road debris as a source

Now contrast this to road debris. Even when riding on pristine roads your frame will often get covered in a fine layer of dust. This dust is being kicked up primarily from the front wheel. Next look at the proximity of the chainring to the front wheel. It is unfortunately located in very close proximity to the dust plume kicked up by the front wheel. This will cover the chain with a lot more debris. It will also force debris into the gap between the roller and the inner plate, which will make it easier for the debris to make its way down into the deep recesses of the chain roller / bushing-inner plate assembly. Given the volume of debris this will likely be the predominant source of abrasive material and therefore wear.

Lubes

I noticed you mentioned choice of lube

Obviously this brings up questions as to the lubricant used (as a sticky lube will attract dirt) and riding conditions

This is the common wisdom, but I personally question it. Over the last 3 years I have been carefully recording chain longevity under different lubes and conditions. Under the same conditions (brand/model of chain and season) a thicker wet lube resulted in longer chain life. Sometimes the difference relative to a light or dry lube was startlingly (e.g., 1000 km more life on an Ultegra chain). I suspect the thicker/stickier lube remains in place better (some sort of hydrodyamic property that I can't remember the technical term). As for debris, it does have an effect, but I suspect it is somewhat over-rated as I only see a small drop (e.g., ~300-500 km) during the winter under the same lube chain combo and in some pretty nasty muddy conditions as I keep off the main roads and ride on more gravel during the winter.

In the end I suspect choice and frequency and quality of lubing may actually play the biggest role!

  • It's not "thick" lube that's the problem so much as "wet" lube. A "wetter" lube is stickier. Wax, eg, is quite thick but not very sticky, and is known to produce longer chain life. – Daniel R Hicks Nov 27 '15 at 14:29
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    @DanielRHicks - the best chain longevity I have attained is with a "thick" and "wet"lube, so I personally have some doubts over this the conventional wisdom of "wet" == bad. Did you have a source for your thick wax claim? If so I will try it next. I have found all the wax lubes in the past I have tried didn't work as well as claimed. – Rider_X Nov 27 '15 at 15:07
  • The only decent (true) wax lube is the factory-applied stuff. Otherwise it's too difficult to apply correctly. And of course a lot depends on how often you clean, plus weather conditions. You want dry lube in dry dusty conditions or if you are not real good at cleaning your chain on schedule. Wet lube is better in wet conditions or if you're a bit obsessive about cleaning your chain. – Daniel R Hicks Nov 27 '15 at 15:19
  • Thanks for the detailed answer - regarding lubes, did you find a positive correlation between stickiness and longevity - i.e. did the stickiest lube produce the highest longevity, and the driest the least, or was it more of a general trend? Like many (most?) cyclists, I've gone through a lot of different lubes too, but I haven't been very scientific about measuring performance. What are your lubes of choice, and what are your main selection criteria? – Simon MᶜKenzie Nov 29 '15 at 21:56
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    @SimonMᶜKenzie - Right now my interest is in chain longevity under average maintenance. I have found the best results with thicker lubes and worst with thinner lubes. Ideally the lube needs to remain in place in between the roller and inner bushing plate when under pressure. I haven't had enough time try all types, but so far I have had the best luck with finish line cross country lube - year round, dry or wet. – Rider_X Dec 1 '15 at 7:22
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Aluminum oxide from chainrings makes chain dirt black

This specific claim is nonsense, since aluminium oxide is white.

enter image description here
"Oxid hlinitý" by Ondřej Mangl - Own work. Licensed under Public Domain via Commons - Wikimedia

  • I absolutely agree - do you think you'd be able to add anything to the discussion below Nhân Lê's answer? – Simon MᶜKenzie Nov 27 '15 at 8:22
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Al oxide sandpaper seems to match the colour seen building up on the chain with aluminium chainrings. Otherwise it is metal itself perhaps, or mixture. Using steel rings seems to cut down on this buildup (I have changed out to steel on previous bikes). I have used wax for a few years now but I add tungsten disulphide to the wax so this is now blacker than black :-) but works well. Perhaps doesn’t last as long or is not as easy to replenish as wet lube but has bonus of dry clean chain - ie won’t mark clothing / legs, after the excess wax is removed from outside surfaces of the chain at least. Just got my on-one stainless steel single front ring, new sram chain is there, just awaiting my sun race 10sp wide range cassette, and will then change all over to 10x1 setup.

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