Bicycle of Theseus: Continued

[Note: more bicycle geekery. Previous geekery here.]

I just finished replacing the last of the drivetrain components on my bike: after replacing the freewheel and chain, there was still a grinding/clicking noise while pedaling hard. I have never replaced the chainrings (front gears at the crank) over the life of this cycle, and looking closer, they were pretty "sharkfinned" (i.e., teeth with an assymetric shape, due to directional wear). I've had problems that are symptomatic of chainring wear: when standing to climb a hill, the chain has slipped, resulting in a violent kneefirst drop into the handlebars and a metal-rending sound.



As a side note, these are the original chainrings: Shimano Biopace--a kinda neat elliptical chainring technology that is unfortunately no longer available.

I completed the replacement and went out for a test spin: the bike feels great now; I'm very happy. The ride is extremely quiet (you can hear the tires running against the ground, instead of [crunch] [tic] [crunch] [tic] [crunch] [tic], and it feels like power is being transmitted more efficiently. So if any of you have been experiencing high mileage bicycle problems along these lines, I'd suggest replacing various parts of your drivetrain (probably chain, freewheel, and chainrings, in that order, which is also cost order).



Anyway, my original chainrings were steel, and I thought I might be able to get a set of replacements that were the same. I really didn't need anything fancy--just something that didn't crunch, tick, and skip all the time. However, I went to the bike store, and everything they had available (even by special order) was aluminum. I ended up buying $120 CAD of chainrings (~$30-45 per ring), by Blackspire (hey... made in Canada, eh?--out of BC). It was a special order, being an 'economy chainring'--I just wanted to get my bicycle back up and running correctly, not add performance parts.

First, it surprised me that they were this expensive--I got a freewheel for $60, and it seemed like there was more engineering in that, as opposed to flat machined metal plate stock.

But it also got me curious why steel chainrings were not available: speaking as a former material scientist, it seems like this would be a good application for a very strong, hard/wear resistant material, especially for non-weight-obsessed, non-performance cyclists like me. I started Googling ("chainring steel aluminum difference"), and came up with some links. A review of Surly Stainless Steel Chainrings provided good background:

These days, most chainrings are cut from some sort of aluminum alloy. It's a pretty good material choice, combining low cost and light weight, and it offers decent service life provided that you clean your chain regularly and replace it as it wears.

Those last points are key. Aluminum is a relatively soft metal. If neglected, aluminum rings can 'pit' and 'shark fin', thus compromising the chain interface.

...

Steel is the material of choice for smaller rings or cogs with lower levels of chainwrap, resulting in a workload that is spread across fewer teeth, and thus increased friction and wear. It has been estimated that, in such applications, the lifespan of a steel part can be as much as five times longer than that of an aluminum equivalent!

A comment in parentheses, "We won't even talk about the non-stainless cheapo stamped chainrings on the inexpensive bikes you see at big box retailers. Apples and oranges, indeed." So that makes sense--my factory chainrings were steel, and I can imagine that they were pretty low cost.

A board discussion on aluminum, steel, and titanium chainrings raised a point of view similar to mine: I'm not looking for the lightest weight components. It's nonsense to pay premiums for lightweight materials when you're going to load up a touring bike with 40+ lbs of gear (clothes, camping & cooking, etc...). I'd be willing to use steel chainrings if they were made properly and compatible with the ultegra triple crankset that I already have. They ended up concluding that not many steel chainrings were available, and the jury was out on whether titanium chainrings were worth it. Overall, they concluded that you should clean and lubricate your chain regularly, and just suck it up and replace the drive components as they wear out.

I wonder if the aluminum chainring-steel chain combination might be a good thing, though, due to tribology (i.e., the field of friction/lubrication/wear)--I remember hearing (during my undergrad days) that a good wear combination is a softer and harder material (as opposed to two equally hard materials). If I remember correctly, two equally hard materials "grind" the grit between them, causing increased wear. In contrast, a hard/soft combination makes the latter conform to the former, and 'traps' the grit by pushing it into the softer material. Any of you hard-core mech E types want to confirm/debunk this line of reasoning?