"brevettata" tandem?

wrote:
wrote:
Is this a matter of rim caliper brakes being less effective on small
wheels? Or what?

With a 20" wheel you can skid a drum, can't with a 'full size' wheel

Dear Andrew,

Any idea why?

The torque exerted on the wheel by the road surface is proportional to
the radius of the wheel. The opposing torque exerted by the brake is
proportional to the radius where the pads are located.

Therefore a given size of drum (or disk) brake will be more effective
when used with a smaller rim compared to a larger one. OTOH, rim
caliper brakes have almost the same effective radius for both the
brake surface and the tire/road surface and the short-term braking
power isn't affected much by wheel size. But a smaller rim is less
able to dissipate heat, so small-wheel bikes with rim brakes are more
subject to problems related to rim heating.

A Muzi wrote:

wrote:
Can you skid a 20" rim caliper? (Fogel Labs does not stock the wheel
or courage needed to test what I presume are front wheel skids.)
I'm not arguing, just curious whether all brakes are more effective on
smaller wheels or if reducing the wheel size favors drums over rim
calipers or what.

No mystery, I think you can _see_ it. The ratio of the drum diameter to
rim diameter is more favorable. Ask any mechanic who ever built a
Schwinn Crate new out out of the box. On a tile floor.

Another way to look at it is that the drum brake surfaces are moving
past each other about 35% faster with a 20" wheel (since the wheel
will be turning faster at a given speed). So... to the drum brake,
the forces stopping the 20" wheeled bike from 20mph will be equivalent
to the forces stopping a 27" wheeled bike from only about 15mph.

OTOH, the heat ultimately generated will be the same either way.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame

On Wed, 07 Mar 2007 18:01:52 -0700, Mark Hickey
wrote:

A Muzi wrote:

wrote:
Can you skid a 20" rim caliper? (Fogel Labs does not stock the wheel
or courage needed to test what I presume are front wheel skids.)
I'm not arguing, just curious whether all brakes are more effective on
smaller wheels or if reducing the wheel size favors drums over rim
calipers or what.

No mystery, I think you can _see_ it. The ratio of the drum diameter to
rim diameter is more favorable. Ask any mechanic who ever built a
Schwinn Crate new out out of the box. On a tile floor.

Another way to look at it is that the drum brake surfaces are moving
past each other about 35% faster with a 20" wheel (since the wheel
will be turning faster at a given speed). So... to the drum brake,
the forces stopping the 20" wheeled bike from 20mph will be equivalent
to the forces stopping a 27" wheeled bike from only about 15mph.

OTOH, the heat ultimately generated will be the same either way.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame

Dear Mark,

I'd wondered about the ratios that you and other posters have
mentioned, but I'm still wondering about the skidding and whether it's
a small front tire on clean, dry pavement.

I can see that the drum's leverage improves with a smaller wheel, but
it seems as if the rim caliper would still have an advantage.

In any case, I don't see yet how a smaller front wheel would allow
skidding on clean, dry pavement when a larger front wheel won't skid.
Maybe there's some geometry change? Or maybe I've misunderstood which
wheel?

Meanwhile, this is as good a spot as any to announce that I saw what
may be the first turkey vultures of spring, circling in the late
afternoon sun and being harried by what turned out to be a pair of
crows.

Whenever they turned, the almost-level sunlight would light up the
white trailing undersides of their black wings in a gorgeous fashion.

Alas, by the time I got a camera out, I could barely see just one
vulture, lurking in the shadows of its customary 70-foot blue spruce:

http://i19.tinypic.com/2nvtizn.jpg

It's a wretched picture, taken through a fork in a tall elm tree, but
that's why they like their tall spruce--plenty of privacy, with easy
take-offs and landings.

Cheers,

Carl Fogel

In article . com,
"Scott Gordo" wrote:

On Mar 7, 2:07 am, Ryan Cousineau wrote:
In article ,
A Muzi wrote:





Ryan Cousineau wrote:
I'm trying to chase down the provenance of a bike I haven't seen yet.
At a guess, I google-translated the name, and (as many of you know)
it
means "patented" in the land of Giros and Fiats.
Any ideas what make of tandem would likely be misidentified this way?
Or
is it a real marque?
I hope to take a look at it fairly shortly.

A Muzi wrote:
Can't help with your specific problem but it happens all the time.
He has no idea what it is. He looked at a part marked -
Brevettata= Patented, Italy
I'm going to see the bike tonight (the price is in my range, and I
just want a "around the park" tandem for me and the missus). I don't
want a bike that will fall apart in a mile, but looks are probably
more important than how many gears it has.

Any general remarks on tandems of this type? Is there likely to be
some freaky Italy-specific issues I might not expect versus, say, an
English or American tandem of the same era?

Send photos to me or some other doddering old bikie for a general
assessment. Or, better, post some photos for RBT review! There aren't
simple guidelines but we can help you avoid known or obvious troubles.

Well, you can't help me avoid trouble now that I've bought the bike!

It turns out it was...a 20"-wheeled folding(ish) tandem!

http://flickr.com/photos/rcousine/41...n/photostream/

I've had a couple of those gimmicks in the non-tandem version. One
said that it was made by Bianchi, and made in Italy. I've had another
one with identical construction that bore some Frenchy name I can't
recall.

All I can say is that what they had in style they lacked in any sort
of torsional rigidity. They're commically flexy, from the handlebars
on down, so doubling up on one, ESPECIALLY as a single speed...if you
see anything that looks like a hill, head in the other direction.

Keep an eye on them quick releases, especially the one that holds the
stem in place. The ones on my bikes were an insult to Tullio.

The good news is there are _no_ quick releases on this bike. The places
where you would expect them, there are these inch-tall, heavily-built
collars held together with 8mm hex bolts.

Even the frame hinge is held together that way.

If you look upthread, this frame has a lot of the tropes of Andrew's
pink Bianchi, most notably the bulged head and seat tubes.

With all that said, it'll be a funny little ride and will def raise
some smiles.

Undoubtedly. It's wife-approved!

ps: Oh yeah, I don't believe that ANY of those parts followed any kind
of standard, so don't leave it out in the snow

Andrew: wanna sell your Bianchi?

--
Ryan Cousineau http://www.wiredcola.com/
"I don't want kids who are thinking about going into mathematics
to think that they have to take drugs to succeed." -Paul Erdos

Ryan Cousineau wrote:
Andrew: wanna sell your Bianchi?

Heck I just bought it in 1975 and it's only been welded and painted 3
times - I'm just getting used to it!

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

On 2007-03-08, wrote:
[...]
I'd wondered about the ratios that you and other posters have
mentioned, but I'm still wondering about the skidding and whether it's
a small front tire on clean, dry pavement.

I can see that the drum's leverage improves with a smaller wheel, but
it seems as if the rim caliper would still have an advantage.

It does, a rim caliper always has the best leverage. With a drum or disk
brake, you have to compensate for the reduced leverage by adding more
leverage somewhere else in the mechanism.

I haven't looked at drum brakes, but I imagine the cable pulls a lever
which operates the shoes. How long that lever is is quite important-- it
might be much longer than the equivalent lever on a rim caliper. And/or
there may be some mechanical advantage inside the drum in the way the
actual shoes are operated.

This is why I'm surprised that drum brakes should be force-limited--
you'd think that it wouldn't be too hard to design in an equivalent
mechanical advantage to offset the fact that the brake is acting at a
smaller radius, and allow any rider with averagely strong hands to apply
enough force to lock the wheel.

Force is one thing, the rate at which a brake can dissipate energy is
another, and that seems generally to be the harder part of brake design.
But that's more significant for long descents and prolonged braking,
usually less so for a quick stop or sudden brake application intended to
provoke a skid.

It may be that drum brakes aren't specifically designed for a size of
wheel, so when you put a drum brake all geared up for a large wheel on a
small-wheeled bike the brakes are sharper than you expect, so it becomes
easier to lock the wheel and skid inadvertently.

wrote:

Mark Hickey wrote:

Another way to look at it is that the drum brake surfaces are moving
past each other about 35% faster with a 20" wheel (since the wheel
will be turning faster at a given speed). So... to the drum brake,
the forces stopping the 20" wheeled bike from 20mph will be equivalent
to the forces stopping a 27" wheeled bike from only about 15mph.

OTOH, the heat ultimately generated will be the same either way.

snip

In any case, I don't see yet how a smaller front wheel would allow
skidding on clean, dry pavement when a larger front wheel won't skid.
Maybe there's some geometry change? Or maybe I've misunderstood which
wheel?

Consider that, on the 20" wheel'd bike, the brake surfaces are moving
past each other 35% faster, thereby producing 35% more friction at a
given brake lever force. I think that explains it (though much
simplified, and I'm sure minus a number of fascinating details that a
brake expert would add).

Meanwhile, this is as good a spot as any to announce that I saw what
may be the first turkey vultures of spring, circling in the late
afternoon sun and being harried by what turned out to be a pair of
crows.

Niiiice. I was recently visiting my son and his family in East Texas,
and was out for a morning ride on my daughter-in-law's MTB through a
local preserve by a lake. As I came around a bend in the trail, a
pair of bald eagles swooped out of a tree, down the trail in front of
me. What amazed me most was the SIZE of the larger of the two, and
the noise generated by their (huge) wings. Sadly, no camera (and even
if I had one, I'm sure I would have still missed the shot due to an
overdose of awe).

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame

On 2007-03-08, Mark Hickey wrote:
wrote:

Mark Hickey wrote:

Another way to look at it is that the drum brake surfaces are moving
past each other about 35% faster with a 20" wheel (since the wheel
will be turning faster at a given speed). So... to the drum brake,
the forces stopping the 20" wheeled bike from 20mph will be equivalent
to the forces stopping a 27" wheeled bike from only about 15mph.

OTOH, the heat ultimately generated will be the same either way.

snip

In any case, I don't see yet how a smaller front wheel would allow
skidding on clean, dry pavement when a larger front wheel won't skid.
Maybe there's some geometry change? Or maybe I've misunderstood which
wheel?

Consider that, on the 20" wheel'd bike, the brake surfaces are moving
past each other 35% faster, thereby producing 35% more friction at a
given brake lever force.

If you're going at 20mph, the rim is rushing past the brake shoes at
20mph. This is the same for any rim diameter.

On Thu, 08 Mar 2007 07:56:08 -0600, Ben C wrote:

On 2007-03-08, Mark Hickey wrote:
wrote:

Mark Hickey wrote:

Another way to look at it is that the drum brake surfaces are moving
past each other about 35% faster with a 20" wheel (since the wheel
will be turning faster at a given speed). So... to the drum brake,
the forces stopping the 20" wheeled bike from 20mph will be equivalent
to the forces stopping a 27" wheeled bike from only about 15mph.

OTOH, the heat ultimately generated will be the same either way.

snip

In any case, I don't see yet how a smaller front wheel would allow
skidding on clean, dry pavement when a larger front wheel won't skid.
Maybe there's some geometry change? Or maybe I've misunderstood which
wheel?

Consider that, on the 20" wheel'd bike, the brake surfaces are moving
past each other 35% faster, thereby producing 35% more friction at a
given brake lever force.

If you're going at 20mph, the rim is rushing past the brake shoes at
20mph. This is the same for any rim diameter.

Perhaps you missed the part of the thread wherein it was mentioned
that drum brakes were under discussion...

On 2007-03-08, wrote:
On Thu, 08 Mar 2007 07:56:08 -0600, Ben C wrote:

On 2007-03-08, Mark Hickey wrote:
wrote:

Mark Hickey wrote:

Another way to look at it is that the drum brake surfaces are moving
past each other about 35% faster with a 20" wheel (since the wheel
will be turning faster at a given speed). So... to the drum brake,
the forces stopping the 20" wheeled bike from 20mph will be equivalent
to the forces stopping a 27" wheeled bike from only about 15mph.

OTOH, the heat ultimately generated will be the same either way.

snip

In any case, I don't see yet how a smaller front wheel would allow
skidding on clean, dry pavement when a larger front wheel won't skid.
Maybe there's some geometry change? Or maybe I've misunderstood which
wheel?

Consider that, on the 20" wheel'd bike, the brake surfaces are moving
past each other 35% faster, thereby producing 35% more friction at a
given brake lever force.

If you're going at 20mph, the rim is rushing past the brake shoes at
20mph. This is the same for any rim diameter.

Perhaps you missed the part of the thread wherein it was mentioned
that drum brakes were under discussion...

Ah, now I get it. The smaller wheel spins faster (rpm) than the bigger
one for a given road speed. So assuming the drums are the same radius,
then the brake surfaces will be rushing past each other faster on the
smaller wheel.