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I am replacing my smallest cassette cog for the 3rd time.

I have been using Harris Cyclery's Aluminum Cog

Crupi's Chromoly Cog is about 3 times as expensive. Is that still worth it?

Can anyone comment towards how much Chromoly increases the cog durability?

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To any later party viewing this, I am trying Crupi's Cog, but having some issues. 1. The cog's spacing is too narrow for Shimano chains below a 10-speed. 2. I'm still seeing the skipping behavior I had replaced the cog and chain to fix. @nickg has pointed out in Can the Tensioner Spring go Bad on a Rear Derailleur? that the Crupi Cog may have different tooth spacing which could now be causing the skipping. –  Jonathan Mee Aug 16 '13 at 12:04
    
I am disappointed to report that the Crupi Cog will always skip on a multispeed bike. The reason for this is that the teeth are tapered. So the derailleur's spring is not strong enough to keep the chain from climbing out of the valley when I stand on the pedals. In contrast the Harris Cog's teeth have straight drop offs so the derailleur does hold the chain in the valley. This will be my last comment on this issue so I'd like to give a shout out against Crupi, I'm really disappointed in their service as far as the road biking community goes. –  Jonathan Mee Sep 6 '13 at 12:44
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1 Answer 1

up vote 6 down vote accepted

The key property is hardness. For uniform materials (like cogs), hardness directly affects wear resistance. The harder the metal the longer it will last.

Some digging around wikipedia suggests that typical Brinell hardness values are:

It seems likely that most aluminium cogs will be 6061 or closely related alloys, so you're looking at the steel being about twice as hard as the aluminium ones. So it should last about twice as long.

Since people generally replace the entire cassette because the smallest cog is worn, by using an aftermarket one you're prolonging the life of the rest of the cassette by only changing it when the second smallest cog is worn.

Sorry for all the links, I couldn't find a single page with all the info on it.

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I wonder how true the assumption "twice as hard equals twice as long" is? –  PeteH Aug 1 '13 at 18:18
    
There's defintely a point where something can be too hard, and thus become brittle. Using the Brinnel scale, glass rates at 1500 HB, which is much higher than other materials mentioned, but glass is obviously a poor material for making cassettes. On the other hand, hardened tool steel is rated at 1500-1900 HB, and would probably be well suited to such a task. So, while hardness is a good indicator, it's probably not the only think you should be using to determine which material is better than another for this purpose. –  Kibbee Aug 1 '13 at 18:32
    
@Kibbee -- Actually, ceramic might be a good choice (though perhaps a bit too apt to shatter when some hotshot jumps on the pedal). And my gut feel is that the steel cog would last about 3x as long as the Al one. –  Daniel R Hicks Aug 1 '13 at 19:53
    
Another thing to consider is friction -- I have a vague suspicion that the Al cog would result in slightly lower friction, though how significant is hard to even guess. –  Daniel R Hicks Aug 1 '13 at 19:55
    
@DanielRHicks I agree that steel would outlast aluminum, but I'm just saying that hardess is not the only thin you need to look at in cassette materials. I also think that ceramic might be good at not wearing down in the usual way, but I think that they'd be really easy to shatter even when your trying to be careful. If they chain gets caught between cogs, the thing would be toast. –  Kibbee Aug 1 '13 at 20:59
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