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After assembling a train or its companion wagons, I'm always impressed at how smoothly the wheels glide over the track.

When putting them together, it's just a metal axle with two wheels on each end inserted into the slot of the wheel holder.

Is there a special coating on the axle to limit friction? What makes it so smooth?

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I recall in my school CDT classes we'd make bearings by drilling a hole through solid nylon and pass a fairly tight fitting metal axle through that which would tend to spin quite freely. I also have some old trains axle bricks that are clearly made of at least 3 parts - a black outer shell, a clear base, and then red "tubes" inside that the axles locked into. It's possible that the tubes are made from a slightly different plastic with a lower CoF than standard bricks (which use friction to lock). –  Zhaph - Ben Duguid Jan 24 '12 at 21:52
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Physicist Richard Feynman explains how train wheels work on this Youtube video. It's not necessary to coat the wheels, applying a generous amount of coating may decrease, traction which will effect the trains performance.

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I think the question is referring to the axle friction, not the wheel on rail smoothness :-) –  Rory Alsop Jan 23 '12 at 23:39
    
@RoryAlsop: I'm not quite sure what you mean by axle friction. Which particular piece are you referring to? –  Ambo100 Jan 24 '12 at 19:21
    
See @LarsTech's final sentence. It refers to the axle, not the wheel. –  Rory Alsop Jan 24 '12 at 19:29
    
@RoryAlsop: I don't understand, It might help if I knew the piece or set in question. –  Ambo100 Jan 25 '12 at 19:15
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@RoryAlsop I don't know if the friction of the axle versus the friction of the wheel to the track are mutually exclusive. This is actually a helpful answer, because even though I was focused on the axle rods, it could be that I was focusing on the wrong thing. When you place a Lego wagon on a track, I'm amazed (easily apparently) by how well that wagon will glide over the track, especially on straight track. I've always attributed it to the axles, but maybe there's more going on there. I might have to consider rewording my question. –  LarsTech Jan 27 '12 at 13:49
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