Coaster brake theory

"Adam Rush" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

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?

You could hack all day long with a brass saw on a steel shell and not get that far! The brass is
much softer. And there IS wear. Usually, the brass expanding collar wears out, but that usually
takes quite a few years or some very hard use. Shimano hubs use 2 plates that push into the hub.
Those are not brass but some other softer metal I suspect and they wear out as well. Additionally,
there is lubricant on that interface, although I don't know if it mitigates wear significantly as it
must be pressurized out of the contact zone in some way.

Cheers,

Scott..

(Adam Rush) said:

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?

I think that all coaster brakes use a driver that pushes two large cones together as the rider
backpedals (actually moving one cone towards a stationary cone), which then pushes the brake shoe(s)
against the inside of the hub shell. The hub shell is usually chromed steel and hence has a very
hard surface, while the brake shoes are usually bronze (although I have a pair of Bendix coaster
brake shoes that appear to be steel). The shoes wear much faster than the hub shell.

Adam Rush said:

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?

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.

--
Andrew Muzi www.yellowjersey.org Open every day since 1 April, 1971

Tim McNamara <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

[email hidden] (Adam Rush) writes:

<snip>

Quoted message said:

I think that all coaster brakes use a driver that pushes two large cones together as the rider
backpedals (actually moving one cone towards a stationary cone), which then pushes the brake
shoe(s) against the inside of the hub shell. The hub shell is usually chromed steel and hence has
a very hard surface, while the brake shoes are usually bronze (although I have a pair of Bendix
coaster brake shoes that appear to be steel). The shoes wear much faster than the hub shell.

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).

Jeff

(Jeff Wills) said:

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.

I did a rebuild on a '50s middleweight coaster brake wheel for a friend of mine-- it was in some
ways a completely different creature than a modern wheel. The 28 spokes were deeply butted,
comparable only to today's 14/17ga spokes with their very short thickened sections. The rim was
steel and very flexible, but it sprang back from unbelievable flexure without permanent deformation
(I was pre-truing it using a surface plate). When I went to do the final truing on it, I decided to
rebuild the New Departure hub in order to steady the axle for fine adjustment.

When I opened the hub, I thought, "Oh. One of those." I took note that the "lubricant" had turned
into an inky black tar-like solid. Since the friend for whom I was doing this favor was a diesel
mechanic, I decided it looked more like his department in there, and I closed the hub back up.

Chalo Colina

Quoted message said:

Quoted message said:

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?

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.

Blued chrome?

(Jeff Wills) said:

[email hidden] (Adam Rush) wrote in message
news:<[email hidden]>...

Quoted message said:

Quoted message said:

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

Blued chrome?

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.

See _The Birth of Dirt_ by Frank Berto regarding the history of the Repack race- named for having to
repack the coaster brake hub for exactly the reasons you describe. A Google search ought to turn up
a few things, too, including an interesting report about the Repack Reunion race held in the 90's.
Joe Breeze or Charlie Cunningham or somebody has some of this stuff on a Web site somewhere,
including some pages from the Fat Tire Flyer and such.

[email hidden] (Adam Rush) wrote in message news:<[email hidden]>...

Quoted message said:

Quoted message said:

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

Blued chrome?

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.

Jeff

Quoted post said:

Originally posted by A Muzi
On large wheels with big riders and long descents they get hot - to the point of grease running out
and blued chrome.
[/B]

Not following the significance of the wheel size.

A large wheel size has a slightly higher slightly higher rotational energy with a heavier large rim vs. a lighter small rim (angular velocity times radius are equal)-even at that it's trivial compared the kinetic energy of the bike and rider's speed.

Is there some other overlooked issue on wheel size?