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Cable rub


LazyTrailRider
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I swopped the cables on my brother's bike a while ago, and noticed that the brake (hydraulic) housing which had been zip-tied on had rubbed a nice fat groove into the fork crown. Literally almost as deep as the housing itself. Weirdly though, the housing itself is still perfectly fine...

 

Now I'm no metallurgist, but my brain is baffled by how a piece of plastic (the housing doesn't seem like metal to me from inspecting it) can destroy a piece of alloy like that. Or are fork crowns just really really soft and we're naive thinking they're tough?

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No expert on this, but I've seen instances where two metals/alloys of different "hardness" rubs against each other, and the harder of the two comes off second best. Sure the same can happen with a tough material like a cable housing rubbing against a metal.

 

I'm sure JB will have an explanation for this.

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The alloys used are In themselves tough but still is alloy which is a softer form of metal.

 

The housings and hydraulic hoses are plastic coated which cause friction on a microscopic level, it acts like very very fine sandpaper and eventually wears it away.

 

This process is accelerated by dust and mud.

 

That's my theory anyway.

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I swopped the cables on my brother's bike a while ago, and noticed that the brake (hydraulic) housing which had been zip-tied on had rubbed a nice fat groove into the fork crown. Literally almost as deep as the housing itself. Weirdly though, the housing itself is still perfectly fine...

 

Now I'm no metallurgist, but my brain is baffled by how a piece of plastic (the housing doesn't seem like metal to me from inspecting it) can destroy a piece of alloy like that. Or are fork crowns just really really soft and we're naive thinking they're tough?

 

This is very common and a big problem in industry. Conversely, this phenomena is used to our advantage in certain circumstances.

 

What's happening here is not that the plastic itself has abraded the aluminium* even though it looks like that. What happened is that the hard silica particles in soil got between the housing and fork. Since the plastic is the softest, the grit embeds in the plastic and now abrades the aluminium. It cannot move against the soft plastic but it does move against the aluminium. Hence the groove without any apparent damage to the plastic.

 

It is a big problem with Rox motion control dampers.

 

Engineers and metallurgists use this phenomena to create, for instance, hard aluminium. Some cars have camshafts and crankshafts that run in aluminium journal bearings (bushes). At first glance it looks stupid, yet these bearings last forever because the soft metal has some hard metal particles inside. The hard steel part actually only runs against the hard crystals in the metal matrix.

 

 

Alloy is a term used for a mixture of metals. Even though it is a mixture, we generally call it whatever the majority metal in the mix is. In other words, if it is an aluminium alloy, we just say aluminium. There are some exceptions. - steel and brass for instance. The former is an iron alloy and the latter a copper alloy. Fork sliders are made from magnesium.

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The alloys used are In themselves tough but still is alloy which is a softer form of metal.

 

 

Alloys can be harder or softer than the base metal. Alloy per se is not a softer form of metal but a mix of two or more metals. In the case of steel, the other "metal" is often carbon, which is not a metal. The nomenclature here is strange.

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What happened is that the hard silica particles in soil got between the housing and fork. Since the plastic is the softest, the grit embeds in the plastic and now abrades the aluminium. It cannot move against the soft plastic but it does move against the aluminium. Hence the groove without any apparent damage to the plastic.

 

Ah, that makes perfect sense now that you've explained it. I noticed that the King headset on my new bike was also starting to get its anodizing rubbed off (nevermind the black fork crown also getting silver lines on it) and foolishly thought that putting those little rubber sleeves on the housing would stop it.

 

Is there any worth in these "carbon" rub stickers people put on stuff? Hmmm, are they even "carbon" to start off with? Is there any way to stop my expensive headset (and frame) getting slowly nano-vapourised?

 

BTW Johan, my father (Henri Hattingh) suspects he knows you and I thought I'd ask... I mentioned your name the other day and he's convinced he went to Technikon with you. He said if it's you, you might remember Pine Pienaar as well?

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Yes, I'm a firm believer in those frame stickers. Whether they have carbon weave print on them or not. Being the cheapskate that I am, I get offcut pieces of clear vinyl sticker from my local sign shop and put that on the frame. You almost can't see it. I refuse to pay R90 to R140 for a few patches of that stuff nicely packaged as bike stuff.

 

Henri???? No not me. I only went to reformatory and was expelled even there. Besides, in every class in every Afrikaans school there is a Pine Pienaar.

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