Coaster brake theory

[KNTain]

Okay, I just got my first coaster brake today: a Sachs Torpedo singespeed hub. I take the sucker apart and have a look at how it works. There's a big golden collar which expands against the shell when backpedaled. Okay, mystery solved for today, but it begs the question, why don't these things eat through the shell in mere months if they operate on this principle? Is there something else going on here? I assume that the collar is brass and I know the shell is steel. This hub had been on a bicycle for ~30 years, so it can't be as simple as it seems, right?

[whatisthename]

Yep- take a look at a motorcycle that's been ridden long and hot.
You'll see that the chrome exhaust pipes are discolored near the cylinder head. The chrome plating becomes discolored due to heat.

I've done this- long ago. I put a Shimano coaster brake on Schwinn
Sidewinder and rode down the Mt. Wilson toll road (above Los Angeles, back before they gave rangers radar guns). 2/3 of the way down I stopped and found that grease had melted and run down all of the spokes and the hub shell was turning blue. The hub was literally smoking.

Needless to say, that was the end of my downhill coaster brake experiments.

[GratefulNole]

Yes, that is a brake band. It expands against the hubshell.

On large wheels with big riders and long descents they get hot - to the point of grease running out and blued chrome.

In normal city use they last a good long while.

[whatisthename]

You're describing most current single-speed coaster brakes- Shimano,
Bendix, made-in-Taiwan. (Multi-speed internal hubs with coaster brake, IIRC, use a large diameter bronze band expanded in the barrel of the hub.) Old school mechanics know that the New Departure style brakes used alternating steel and bronze plates that were squeezed together for speed reduction ("braking" is too generous a word). In operation it's similar to a multi-plate clutch as used in motorcycles or some cars.

The bike shop where I got my start had some New Departure spare parts in stock... but I can't recall actually overhauling one. Bendix hubs were far more common by then (late '70's, early '80's).