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Discussion Starter · #1 ·
It is commonly-accepted that aluminum cogs and chainrings wear faster than steel ones, but that judicious application in bigger rings/cogs (with forces distributed over a greater number of teeth) arrives at a good balance point between drivetrain durability and overall weight. However, has anybody considered that having any aluminum wear surfaces might be contributing to accelerated wear even on the steel-to-steel interfaces (chain plates/pins/rollers, steel rings/cogs) of a bike drivetrain?

In another thread, another poster noted that a significant source of drivetrain wear is the oxides of aluminum powder/shavings that gets trapped in the chain lubricant, be that the chain lube overall and/or the assembly grease under the chain rollers. While his theory goes toward an argument for ultrasonic chain cleaning... I was thinking that maybe his theory might be a better argument for a drivetrain with no aluminum wear surfaces whatsoever. With the advent of 1X drivetrains and the existence of steel rings and all-steel-cogged cassettes (Shimano Deore CS-M6100, SRAM NX PG-1230)... this is quite feasible.

Looking up the Vickers hardness:

Aluminas: 10.5-12.7 GPa

Iron oxides: 3.5-5.5 GPa

Which works out to aluminas being 2-4 times harder, and therefore that much more abrasive.

I am not a mechanical engineer, so I do have to ask: am I interpreting this right?

My inorganic chemistry is also quite rusty (heh), so I do also have to ask: being that the wear surfaces are not elemental aluminum and iron, but actually aluminum and iron alloys, would this interpretation still hold?

Silicas are almost as hard as aluminas, and I would assume that a mountain bike's drivetrain gets a good dose of this from the dirt we play on. So would this theory only really be applicable to roadies?
 

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It is commonly-accepted that aluminum cogs and chainrings wear faster than steel ones, but that judicious application in bigger rings/cogs (with forces distributed over a greater number of teeth) arrives at a good balance point between drivetrain durability and overall weight. However, has anybody considered that having any aluminum wear surfaces might be contributing to accelerated wear even on the steel-to-steel interfaces (chain plates/pins/rollers, steel rings/cogs) of a bike drivetrain?

In another thread, another poster noted that a significant source of drivetrain wear is the oxides of aluminum powder/shavings that gets trapped in the chain lubricant, be that the chain lube overall and/or the assembly grease under the chain rollers. While his theory goes toward an argument for ultrasonic chain cleaning... I was thinking that maybe his theory might be a better argument for a drivetrain with no aluminum wear surfaces whatsoever. With the advent of 1X drivetrains and the existence of steel rings and all-steel-cogged cassettes (Shimano Deore CS-M6100, SRAM NX PG-1230)... this is quite feasible.

Looking up the Vickers hardness:

Aluminas: 10.5-12.7 GPa

Iron oxides: 3.5-5.5 GPa

Which works out to aluminas being 2-4 times harder, and therefore that much more abrasive.

I am not a mechanical engineer, so I do have to ask: am I interpreting this right?

My inorganic chemistry is also quite rusty (heh), so I do also have to ask: being that the wear surfaces are not elemental aluminum and iron, but actually aluminum and iron alloys, would this interpretation still hold?

Silicas are almost as hard as aluminas, and I would assume that a mountain bike's drivetrain gets a good dose of this from the dirt we play on. So would this theory only really be applicable to roadies?
With how cheap 10/11spd stuff is compared to 12spd stuff, it seems like this really hasn't been a huge issue until now? Might be we see more aluminum cogs on cassettes now because 12spd stuff is heavier, taking full advantage of the lighter aluminum at the cost of longevity.

Seems like more of a reason to start waxing chains
 

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Discussion Starter · #3 ·
With how cheap 10/11spd stuff is compared to 12spd stuff, it seems like this really hasn't been a huge issue until now?
This issue remains the same regardless of how many speeds your drivetrain has. It's a material and chemistry issue, not a design issue. It's just that eschewing aluminum chainrings in a multi-chainring drivetrain might be too difficult to justify given the weight penalty.

Might be we see more aluminum cogs on cassettes now because 12spd stuff is heavier, taking full advantage of the lighter aluminum at the cost of longevity.
We are seeing aluminum cogs on cassettes because the giant cogs of the wide-range cassettes are distributing the drive load over as many teeth (and more!) than that of chainrings, now and in the past multi-ring eras. If there were 10/11 speed ~500% cassettes, the biggest cogs on high-end cassettes would be made out of aluminum too.
 

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The only issue we had back in the day was small aluminum chainrings, granny gears. Those would wear out pretty fast, but an all-steel drivetrain wasn't magic, it still wore out cassettes and rings and you had to change your chain at regular intervals. Today's modern drivetrains with aluminum rings and a large cog or two last way longer for me.
 

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Both my mountain bike drivetrains are all steel.
  • Shimano 11-speed 11-51t CS-5100 cassette
  • RaceFace 28t steel n/w chainring
  • KMC or SRAM chain, whichever's in the parts bin
Wears like steel. Shifts great. No complaints. I don't weigh stuff.
=sParty
 

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This issue remains the same regardless of how many speeds your drivetrain has. It's a material and chemistry issue, not a design issue. It's just that eschewing aluminum chainrings in a multi-chainring drivetrain might be too difficult to justify given the weight penalty.

We are seeing aluminum cogs on cassettes because the giant cogs of the wide-range cassettes are distributing the drive load over as many teeth (and more!) than that of chainrings, now and in the past multi-ring eras. If there were 10/11 speed ~500% cassettes, the biggest cogs on high-end cassettes would be made out of aluminum too.
Email the drivetrain manufacturers and let us know what they think.
 

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Discussion Starter · #12 · (Edited)
The only issue we had back in the day was small aluminum chainrings, granny gears. Those would wear out pretty fast, but an all-steel drivetrain wasn't magic, it still wore out cassettes and rings and you had to change your chain at regular intervals. Today's modern drivetrains with aluminum rings and a large cog or two last way longer for me.
In my experience, as long as you take care of your chain (and replace when needed) aluminum cogs and chainrings don't wear unusually fast.
I'm pretty sure that's been my experience too.

But what I'm talking about is the wear everywhere else that is steel on a drivetrain that involves some aluminum wear surfaces.

The idea is that the little bit of aluminum that does wear off of the aluminum wear surfaces, by virtue of oxidation, becomes a much worse abrasive contaminant (aluminas) than what wears off from a drivetrain with all-steel wear surfaces (steels, then iron oxides).

Sure, you're supposed to flush contaminants from the drivetrain ASAP, but in the absence of measures such as ultrasonic cleaning, it's possible that an all-steel-wear-surface drivetrain, which of course will be more durable than otherwise... might actually be disproportionately durable because of the elimination of alumina contaminants.

The way to test this would be a controlled wear test between a normal high-end drivetrain (which these days always involve aluminum wear surfaces), vs. an all-steel-wear-surface drivetrain. And the relevant wear rates to be measured wouldn't be on the aluminum cogs, but rather the steel ones.
 

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Discussion Starter · #13 · (Edited)
Both my mountain bike drivetrains are all steel.
  • Shimano 11-speed 11-51t CS-5100 cassette
  • RaceFace 28t steel n/w chainring
  • KMC or SRAM chain, whichever's in the parts bin
Wears like steel. Shifts great. No complaints. I don't weigh stuff.
=sParty
I have a similar all-steel drivetrain on a '05 Heckler, SRAM GX Eagle with a 34T NX ring and 11-50T NX cassette.

The seed of thought for this are my eMTBs, for which I've stockpiled a couple CS-M7100 SLX cassettes. Considering if I might be better off eliminating aluminum wear surfaces with some CS-M6100-12 cassettes. What people attest to dying first with Shimano 12-speed cassettes in an eMTB context are the smallest (and steel) cogs... which I suspect derive from low-cadence, high-assist riding in high gears. But it still gets you thinking.
 

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The big factor you're leaving out of the aluminum/steel/silica abrasive argument, is how each of those particles wear. Aluminum and steel (particles) both wear down and end up with smoother edges. While material like silica fractures, breaking off instead of wearing, leaving sharp edges. Those sharp edges cause more damage. On the grand scheme of things, I don't think it matters all that much of the life of a bicycle drivetrain.
 

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Certainly aluminum dust can make for an abrasive slurry -- worse if there's a lot of dust than a little. Personally I believe that typical wear of aluminum components in a bicycle drivetrain results in a comparatively insignificant amount of aluminum residue/dust. Living in the PNW as I do, I've come to fear water (and whatever the environment adds to it -- typically sand, mud & forest detritus) as the worst metal degrading & chain contaminating agent.

If I lived in a dry climate, I'd probably be more concerned about flushing metallic dust out of the chain/system. As it is, I lube & check chain length frequently; change to a new chain as necessary. YMMV
=sParty
 

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Discussion Starter · #16 · (Edited)
The big factor you're leaving out of the aluminum/steel/silica abrasive argument, is how each of those particles wear. Aluminum and steel (particles) both wear down and end up with smoother edges. While material like silica fractures, breaking off instead of wearing, leaving sharp edges. Those sharp edges cause more damage. On the grand scheme of things, I don't think it matters all that much of the life of a bicycle drivetrain.
Certainly aluminum dust can make for an abrasive slurry -- worse if there's a lot of dust than a little. Personally I believe that typical wear of aluminum components in a bicycle drivetrain results in a comparatively insignificant amount of aluminum residue/dust. Living in the PNW as I do, I've come to fear water (and whatever the environment adds to it -- typically sand, mud & forest detritus) as the worst metal degrading & chain contaminating agent.

If I lived in a dry climate, I'd probably be more concerned about flushing metallic dust out of the chain/system. As it is, I lube & check chain length frequently; change to a new chain as necessary. YMMV
=sParty
As mentioned before, I think for those of us on dusty trails, our most-significant and inescapable contaminant is indeed the silica that we get tossed from the front tire. Hence this is an academic discussion. The consideration of what state the contaminants arrive (silica vs. aluminum/steel shavings that are ground down then oxidized) was particularly interesting and something I failed to consider.

But maybe the roadies and commuters might get some use out of this.

Thanks for playing, guys.
 
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