From: jobst.brandt@stanfordalumni.org
Subject: Re: Wooden rims
Newsgroups: rec.bicycles.tech
Message-ID: <YTrQd.6939$m31.86649@typhoon.sonic.net>
Date: Tue, 15 Feb 2005 19:16:08 GMT

Mark Schecter writes:

>> Wood is a singularly poor choice of material for a braking surface.

> Just as a point of interest, the cable cars in San Francisco use
> wooden brake shoes bearing downward against the steel tracks as the
> friction material of choice.  It is cheap and easy to replace, which
> is needed every few days. But the wood is the consumable, not the
> keeper part.

SF Cable Cars are not a good example of braking technology.  SF Cable
Cars are practically devoid of working brakes, relying on the cable to
climb and descend at constant 9.5mph speed.  The wooden blocks are
holding brakes used for boarding passengers while the cable is allowed
to slide through the grip.  Cable Cars also have wheel brakes that are
hardly used because wheels slide easily and cause flat spots.  The
front and rear wheel brakes are operated separately by the gripman and
conductor respectively.  Besides, there is no need for a brake other
than the wood block parking brake, the wheel brakes being ineffective
in stopping the car even on milder grades.

http://www.sfcablecar.com/gripmans.html

On this site you can operate the various brakes and grip, as well as
the bell, which has the correct tone for a real cable car.  The noise
you hear is the cable running over slot pulleys, something residents
along the route appreciate like the sound of surf at the beach.  It's
background.

What the website model does not show, is the third lever that operates
the steel wedge which is the best example of failure in brake design;
the so called emergency brake.  This "brake" is a steel wedge that can
be driven into the cable slot.  It has a contact area of about a
square inch with no heat dissipating surface or cooling means.  It has
never prevented a runaway from a collision at Bay Street (bottom of
the Hyde Street hill) and always shown to do what one should expect,
melt the wedge and weld to the cover plate upon impact with traffic.
That it requires a cutting torch to be removed, proves to the press
and onlookers that it has great stopping power.  What is missed is
that molten steel is a good lubricant and that a brake must dissipate
energy.

The cause of these runaways is also obvious, since they nearly all
occur on the Hyde Street hill where there is a passenger pickup just
before the crest of the hill.  Unless the gripman makes a special
effort, the car will not be up to synchronous speed with the cable
before the crest of the hill where grip must lift the cable from the
crown wheel under the street.  At this point, a sliding cable can
readily peel out of the grip and occurs more so early in the life of a
grip that has not yet worn a pocket from cable slip.

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: Wooden rims
Newsgroups: rec.bicycles.tech
Message-ID: <LXNQd.7096$m31.88627@typhoon.sonic.net>
Date: Wed, 16 Feb 2005 20:22:03 GMT

 Mark Schecter writes:

>> http://www.sfcablecar.com/gripmans.html

>> On this site you can operate the various brakes and grip, as well
>> as the bell, which has the correct tone for a real cable car.  The
>> noise you hear is the cable running over slot pulleys, something
>> residents along the route appreciate like the sound of surf at the
>> beach.  It's background.

> Jobst, thanks for the illumination.  That site is really fun and
> illustrates the mechanism for the layperson.

> I didn't mean to suggest the cable car as an example of good brakes,
> merely as an instance of the use of wood as the friction pad
> material.  Yet, as the wheel brakes are also not very effectual, if
> there were a better material for the track brakes, woodn't they use
> it?  For instance, sintered metallic or ceramic as in auto brake
> shoes?  In a certain sense, it seems that the design is elegant in
> its sufficiency to the purpose, not perfect or ultimate, but pretty
> darn good for the job at hand.

As I said there is no real running brake on these cars and wood does
excellently for the parking brake.  Unfortunately it gets a little
dynamic work as the car rolls to a stop on the level places where
passengers board.  The wheel brakes are supposed to do that job and do
so fairly well but gripmen also know that the wooden pads do a fine
job of stopping the car from 9.5mph on the level.  The lever that
operates the wooden blocks has enough force to lift the wheels off the
track when few passengers are on board.  Having the grip in the slot
prevents the car from getting off track when this occurs.

Jobst Brandt
jobst.brandt@stanfordalumni.org