From: jobst.brandt@stanfordalumni.org
Subject: Re: brake pads for wet weather?
Newsgroups: rec.bicycles.tech
Message-ID: <rXAza.16501$JX2.1012672@typhoon.sonic.net>
Date: Sat, 24 May 2003 02:39:19 GMT

Chris Zacho writes:

> A kinda-sorta semi off topic question, does anybody know if the
> device known as the "Rim Rake" is still made?

> For those who don't know, it's a small device that you clamp on with
> the brake pad. it rides behind them and contacts the rim slightly
> before the pads, scraping mud and water off before it gets under
> them.

This is one of those devices invented by people unclear on the
concept.  If the leading edge of a brake pad, pressed firmly against
the rim, does not wipe the rim clean and dry, than a light weight wipe
won't do it either.  In fact, it takes more than a sharp edge and
pressure to clean water off surfaces... something that should be
apparent to anyone who has operated a razor blade.

On the other hand, people marvel how garden snails can glide over the
edge of a razor, even though they do the same while shaving daily.
This makes more apparent that tribology is foreign science, even to
many mechanical engineers and physicists.

That water cannot be entirely wiped off a rim becomes evident to those
who have ridden with snow piled on the inner circumference of the
wheel.  When braking begins, ever so ineffectively, it generates heat
that melts the snow.  The brakes do not start working significantly
until ALL the snow is gone.  This exercise makes the concepts of wet
braking strikingly clear as does the spattering of a little water on
the rim, as it causes sticky braking and squeals as the droplets enter
the pad to rim interface.

Jobst Brandt
jobst.brandt@stanfordalumni.org
Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: brake pads for wet weather?
Newsgroups: rec.bicycles.tech
Message-ID: <Z8sBa.17931$JX2.1109780@typhoon.sonic.net>
Date: Thu, 29 May 2003 18:17:29 GMT

George Daniels writes:

> Do you have personal experience with the device?  Seems that a
> sweeper contacting the rim just ahead of the pad would accelerate
> the pad's demoisturizing-unwetting of the rim.  A rectangle of poly
> from a h20 bottle held by an arm extending from the usual brake
> mechanism, like a snowplow!

I guess this should be in the FAQ but isn't.  Water adheres to
surfaces much more strongly than apparent.  Shaving is an example of a
water film remaining between skin and blade as a lubricant.  A clearer
example may be wet window glass.  Wiping a squeegee over a wet window
seems to remove the water but careful observation will show that water
remains behind to dry off after the wiper has passed.  This can be
demonstrated by wiping the squeegee dry before making a second pass
over the same path.  The squeegee will not glide on the glass that is
now dry.  This demonstrates the presence of water passing under the
lip on the first wipe.  More pressure on the wiper will not alter the
outcome.  It takes heat to dislodge water layers from surfaces.

> But when thinking of these things you gotta wonder-"well, SunTour
> didn't think of making it so there must be a good reason for the
> thing not getting on the market."  Maybe the plow jams under the pad
> and sends the rider under a bus and his 27 family members to the
> local lawyer.  Like chain ring amputations when removing pedals.

I'm not familiar with that event but the reason we don't have wipers
on rims is that they don't work.  You can be sure that many who do not
have the technical understanding attempted such inventions.  I see
them all the time at bicycle trade shows.  You will note that there
are many brake pads with snowplow wedges and voids in their faces to
accomplish what you suggest.  None of these do any good and are
evidence of the lack of technical expertise in the industry.

Jobst Brandt
jobst.brandt@stanfordalumni.org
Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Disc brakes squeal due to loose pads?
Newsgroups: rec.bicycles.tech
Message-ID: <Ml%Bc.18248$Fo4.238266@typhoon.sonic.net>
Date: Tue, 22 Jun 2004 19:01:00 GMT

David Damerell writes:

> That is not true. I am not disputing that your brakes do not perform
> well; I am merely pointing out that, since rim brakes can work
> perfectly well in the wet, that is because yours are inferior, not
> because of an inherent limitation of the design. Obviously one can
> equally well obtain an inferior disc brake; that would not imply
> that no disc brake works well.

I disagree on your assessment of rim brakes on wet rims.  I have
performed tests on wet braking that conclusively show that it doesn't
work the way most riders visualize.  I have had the opportunity to
brake in water deep enough to submerge the rim and found that there
was practically no brake effect during the immersion.  Subsequently I
rode in snow so there was a snow pack on the inner periphery of the
rim.  On braking, this snow melts supplying a continuous flow of water
to the braking surface.  Braking came back to typical wet weather
effect as the snow was gone and shortly after that full effectiveness
returned.

This is a good test to perform if you ride in snow.  I did it because
I ride in snow on occasion and had the distinct feeling of no brakes
in such circumstances.  It's not riding through snow but getting it on
the rim that does the mean trick and exposes water lubrication for
what it is.  That's also why chains work well until the rain stops,
and then they squeak.  RR trains have skidding problems on wet track.

Braking in rain, rims are not as wet as one might assume judging from
these experiments.  The effect is mainly a partially wet and warm rim
which is not what occurs on a disc brake that is rapidly heated above
boiling temperatures in use.

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: Disc brakes squeal due to loose pads?
Newsgroups: rec.bicycles.tech
Message-ID: <B95Cc.18345$Fo4.238809@typhoon.sonic.net>
Date: Wed, 23 Jun 2004 01:37:37 GMT

Michael Press writes:

>> I have had the opportunity to brake in water deep enough to
>> submerge the rim and found that there was practically no brake
>> effect during the immersion.

> By "No brake effect during immersion" you mean that braking was
> completely ineffective during immersion?

Yes, there was practically no brake effect as long as the rim was in
the water although there was a small and unacceptably low retardation
that fits the description of complete brake fade.

>> Subsequently I rode in snow so there was a snow pack on the inner
>> periphery of the rim.  On braking, this snow melts supplying a
>> continuous flow of water to the braking surface.  Braking came back
>> to typical wet weather effect as the snow was gone and shortly
>> after that full effectiveness returned.

> So by "Braking came back to typical wet weather effect" you're stating
> that when the melting snow supplied a flow of water on the rim, the
> effect was the same as during immersion, that is little or no braking
> ability?

Yes.  Try it, you'll like it... as the old saying goes.

Jobst Brandt
jobst.brandt@stanfordalumni.org