Finite element models... Rim section modulus... Gavin

Ron Ruff said:

I'm building some FE models of bicycle wheels. Last night I was
playing around with the rim sections and trying to verify that they
were correct, and I took a look a Gavin's paper:
www.duke.edu/~hpgavin/papers/HPGavin-Wheel-Paper.pdf

Early on, he did some experiments on a rim to determine Izz and Irr,
and the numbers he got seem impossibly low. Izz= 795 mm^4, Irr= 1200
mm^4, J= 1139 mm^4. He doesn't say what the dimensions were for the
rim he used, but if it was a shallow box rim ~20mm wide then Irr seems
about right, but Izz should be much higher... and J as well, unless
the rim is an open channel.

Jobst used an Irr in his 2D model of 1124 N/mm^4, so that one seems
about right. Also, here:
http://www.astounding.org.uk/ian/wheel/
Ian used an Irr of 1187 mm^4, and an Izz of 6847 mm^4 for his 25mm
wide box rim. I'm looking at a 30mm tall, 19mm wide rim that weighs
460g and the calculated values are Irr= 8,000 N/mm^4, and Izz= 4,000.

I know that some of you here have studied this paper, and I'm
wondering if these values from Gavin are incorrect, typos, or... there
is some reason why *calculating* the section modulii doesn't work.
Gavin tested the rim to get his numbers, which makes me wonder...
since this is one of the easier things to just calculate.

Dear Ron,

Professor Gavin's a nice fellow, so you might try emailing him and
asking:

[email hidden]

Cheers,

Carl Fogel

Ron Ruff said:

I'm building some FE models of bicycle wheels. Last night I was
playing around with the rim sections and trying to verify that they
were correct, and I took a look a Gavin's paper:www.duke.edu/~hpgavin/papers/HPGavin-Wheel-Paper.pdf

Early on, he did some experiments on a rim to determine Izz and Irr,
and the numbers he got seem impossibly low. Izz= 795 mm^4, Irr= 1200
mm^4, J= 1139 mm^4. He doesn't say what the dimensions were for the
rim he used, but if it was a shallow box rim ~20mm wide then Irr seems
about right, but Izz should be much higher... and J as well, unless
the rim is an open channel.

Jobst used an Irr in his 2D model of 1124 N/mm^4, so that one seems
about right. Also, here:http://www.astounding.org.uk/ian/wheel/
Ian used an Irr of 1187 mm^4, and an Izz of 6847 mm^4 for his 25mm
wide box rim. I'm looking at a 30mm tall, 19mm wide rim that weighs
460g and the calculated values are Irr= 8,000 N/mm^4, and Izz= 4,000.

I know that some of you here have studied this paper, and I'm
wondering if these values from Gavin are incorrect, typos, or... there
is some reason why *calculating* the section modulii doesn't work.
Gavin tested the rim to get his numbers, which makes me wonder...
since this is one of the easier things to just calculate.

The box section rims that used to be near-standard were
rather shallower than they were wide. For example a GP4 or
G40 or MA40 or MA2 type rim is probably about 18-19mm
wide and 10mm tall, neglecting the bead hooks (which of
course didn't even exist on tubular rims). Since Irr is the
moment in the radial, 10mm dimension, it's not surprising
it is smaller.

If you approximate the rim as a rectangular beam 19mm
wide, 10mm tall, and 1mm wall thickness: the moment
of a solid beam of base b and height h is I=bh^3/12; of a
hollow beam, using superposition, it's
I(hollow) = I(solid) - I(missing inner part). I get
that Izz = 2441 mm^4 and Irr=858 mm^4. Obviously
rims aren't perfect rectangle sections and Gavin and Jobst
may have used different size rims and assumptions.

Ben

Quoted message said:

Professor Gavin's a nice fellow, so you might try emailing him and
asking:

Thanks Carl! I'll give that a shot...

BTW... finally got out on the road today... calm and in the mid 60s!
If hadn't needed to walk through snow-melt-mud for half a mile to get
to the road, it would have been perfect...

Quoted message said:

If you approximate the rim as a rectangular beam 19mm
wide, 10mm tall, and 1mm wall thickness: the moment
of a solid beam of base b and height h is I=bh^3/12; of a
hollow beam, using superposition, it's
I(hollow) = I(solid) - I(missing inner part). I get
that Izz = 2441 mm^4 and Irr=858 mm^4. Obviously
rims aren't perfect rectangle sections and Gavin and Jobst
may have used different size rims and assumptions.

For your configuration that sounds about right for the assumptions you
made (that would be a <300g rim), but Gavin had an Izz of only 795 N/
mm^4... which is less than 1/3rd of yours! That's what I don't
understand. His Irr looks about right but Izz and J seem way too
small. I can't think of any rim shape that would give the numbers he
found. An open rim would be poor in torsion, but should still have a
high Izz, since it is essentially a "C" channel in that direction.

Ron Ruff said:

Gavin tested the rim to get his numbers, which makes me wonder...
since this is one of the easier things to just calculate.

And, as all scientist know, when calculations vary from reality,
reality is wrong.

An FEA that is not validated by experiment is just a guess.

On Feb 5, 3:50 pm, "[email hidden]" <[email hidden]>

Quoted message said:

The box section rims that used to be near-standard were
rather shallower than they were wide. For example a GP4 or
G40 or MA40 or MA2 type rim is probably about 18-19mm
wide and 10mm tall, neglecting the bead hooks (which of
course didn't even exist on tubular rims).

Are you familiar with the M4 or MA4 tubular? Can't seem to find any
info on Google. It is starting to make some sense though, if this is a
light rim and has 38 large holes and 36 small ones, plus the sockets
would probably not add much to the stiffness. I haven't had a double-
eyeleted rim in awhile so I'm not even sure what they look like, or
how much the eyelets weigh.

In article <[email hidden]>,

Quoted message said:

And, as all scientist know, when calculations vary from reality,
reality is wrong.

No, that's supply-side economists.

Quoted message said:

Ron Ruff said:

Gavin tested the rim to get his numbers, which makes me wonder...
since this is one of the easier things to just calculate.

And, as all scientist know, when calculations vary from reality,
reality is wrong.

An FEA that is not validated by experiment is just a guess.

When calculations and experiment (which is not the same
thing as "reality"😉 disagree, one of them has some problem
with the assumptions. This is when we learn something.
Or at least, Ron and I are learning something.

One reason for validating an FEA by experiment is to
then generalize the FEA model to something you haven't
experimented on. You don't then say, "Well that new model
was not validated by experiment, so it's useless." There's
a difference between being careful and being stubborn.

Ben

Ron Ruff said:

On Feb 5, 3:50 pm, "[email hidden]" <[email hidden]>

Quoted message said:

The box section rims that used to be near-standard were
rather shallower than they were wide. For example a GP4 or
G40 or MA40 or MA2 type rim is probably about 18-19mm
wide and 10mm tall, neglecting the bead hooks (which of
course didn't even exist on tubular rims).

Are you familiar with the M4 or MA4 tubular? Can't seem to find any
info on Google. It is starting to make some sense though, if this is a
light rim and has 38 large holes and 36 small ones, plus the sockets
would probably not add much to the stiffness. I haven't had a double-
eyeleted rim in awhile so I'm not even sure what they look like, or
how much the eyelets weigh.

No, I don't know much about M4 tubular rims. One of the experienced
shop owners on here might (Andrew Muzi?) I would guess that they
are not too different from the GP4, which is also a tubular, but not
superlight (more like a tubular G40 than a light GEL280). I might
have an old GP4 around and if I do I will try to measure it and
take a picture.

Ben

On Feb 6, 4:21 pm, "[email hidden]" <[email hidden]>

Quoted message said:

Quoted message said:

On Feb 5, 12:12 pm, "Ron Ruff" <[email hidden]> wrote:

Quoted message said:

Quoted message said:

Gavin tested the rim to get his numbers, which makes me wonder...
since this is one of the easier things to just calculate.

Quoted message said:

And, as all scientist know, when calculations vary from reality,
reality is wrong.

Quoted message said:

An FEA that is not validated by experiment is just a guess.

When calculations and experiment (which is not the same
thing as "reality"😉 disagree, one of them has some problem
with the assumptions.

An experiment tries to test a hypothesis, which is what I would call
an FEA, against reality.

But I agree, it is possible to make incorrect assumptions in an
experiment- you think you are measuring something, but you're not.

Quoted message said:

On Feb 5, 3:50 pm, "[email hidden]" <[email hidden]>

Quoted message said:

The box section rims that used to be near-standard were
rather shallower than they were wide. For example a GP4 or
G40 or MA40 or MA2 type rim is probably about 18-19mm
wide and 10mm tall, neglecting the bead hooks (which of
course didn't even exist on tubular rims).

Ron Ruff said:

Are you familiar with the M4 or MA4 tubular? Can't seem to find any
info on Google. It is starting to make some sense though, if this is a
light rim and has 38 large holes and 36 small ones, plus the sockets
would probably not add much to the stiffness. I haven't had a double-
eyeleted rim in awhile so I'm not even sure what they look like, or
how much the eyelets weigh.

A classic tubular steel eyelet weighs 1.42g ( ten are 14.2g)

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

Quoted message said:

In article <[email hidden]>,

Quoted message said:

And, as all scientist know, when calculations vary from reality,
reality is wrong.

Tim McNamara said:

No, that's supply-side economists.

Right! CBO said tax receipts would tank after tax cuts. In theory.
They are _way_ up since 2003. In actual cash revenue, not theory.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

Quoted message said:

Quoted message said:

Quoted post said:

The box section rims that used to be near-standard were
rather shallower than they were wide. For example a GP4 or
G40 or MA40 or MA2 type rim is probably about 18-19mm
wide and 10mm tall, neglecting the bead hooks (which of
course didn't even exist on tubular rims).

Quoted message said:

Ron Ruff said:

Are you familiar with the M4 or MA4 tubular? Can't seem to find any
info on Google. It is starting to make some sense though, if this is a
light rim and has 38 large holes and 36 small ones, plus the sockets
would probably not add much to the stiffness. I haven't had a double-
eyeleted rim in awhile so I'm not even sure what they look like, or
how much the eyelets weigh.

Quoted message said:

No, I don't know much about M4 tubular rims. One of the experienced
shop owners on here might (Andrew Muzi?) I would guess that they
are not too different from the GP4, which is also a tubular, but not
superlight (more like a tubular G40 than a light GEL280). I might
have an old GP4 around and if I do I will try to measure it and
take a picture.

sorry, don't have one

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

A Muzi said:

Quoted message said:

In article <[email hidden]>,

Quoted message said:

And, as all scientist know, when calculations vary from reality,
reality is wrong.

Quoted message said:

Tim McNamara said:

No, that's supply-side economists.

Quoted message said:

Right! CBO said tax receipts would tank after tax cuts. In theory.
They are _way_ up since 2003. In actual cash revenue, not theory.

Ahem. The stock market _STOPPED_ going down in 2003, didn't it?? I
think its been reported (New York Tikmes) that the vast majority of
revenue increases since 2003 are from securities capital gains
(remember, they cut taxes on capital gains and dividend to encourage
people to flip securities), NOT from more people paying income tax on
earned income ...

It's the classic example of presto-chango with my left hand, while my
right hand is furiously doing things to fake up the intended result
....

- Don Gillies
San Diego, CA

A Muzi said:

Right! CBO said tax receipts would tank after tax cuts. In theory.
They are _way_ up since 2003. In actual cash revenue, not theory.

Of course, CBO was right twenty years earlier!

Pat

Ron Ruff said:

On Feb 5, 3:50 pm, "[email hidden]" <[email hidden]>

Quoted message said:

The box section rims that used to be near-standard were
rather shallower than they were wide. For example a GP4 or
G40 or MA40 or MA2 type rim is probably about 18-19mm
wide and 10mm tall, neglecting the bead hooks (which of
course didn't even exist on tubular rims).

Are you familiar with the M4 or MA4 tubular? Can't seem to find any
info on Google. It is starting to make some sense though, if this is a
light rim and has 38 large holes and 36 small ones, plus the sockets
would probably not add much to the stiffness. I haven't had a double-
eyeleted rim in awhile so I'm not even sure what they look like, or
how much the eyelets weigh.

Well, I found a GP4 tubular rim. For those of you who like
looking at grotty old Mavic rims:
http://www.ucolick.org/~bjw/misc/rbt/img_2970_33.jpg
http://www.ucolick.org/~bjw/misc/rbt/img_2967_33.jpg
The rims are a GP4 tubular, G40 clincher, and MA40 clincher
in order front-back 1st pic and left-right 2nd pic. The
crusty old tubular glue hides the spoke sockets (but it is
a component of the terroir, which is critical for French rims).

The rims all have more or less the same shape, slightly
convex-inward spoke bed. I measured the width, total
height (including the bead hooks for the clinchers) and
depth of the concave tire bed from the rim high point
(again including the bead hooks):
GP4 19.4 12.0 4.2 mm
G40 19.2 13.0 7.5
MA40 20.4 14.1 7.5

So the main box of the rim is maybe 19-20mm wide
by 6-8mm high.

Ben

Quoted message said:

A Muzi said:

Right! CBO said tax receipts would tank after tax cuts. In theory.
They are _way_ up since 2003. In actual cash revenue, not theory.

Pat Lamb said:

Of course, CBO was right twenty years earlier!

Yes, they famously predicted ten of the last two recessions.

We have broader home and stock ownership than ever, healthy growing
economy, low unemployment and we're absorbing millions (like it or not,
we are) of immigrants into a growing workforce. Low taxes grow the
entire system effectively.

All that just galls the [censored] out of some people.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

A Muzi said:

Quoted message said:

A Muzi said:

Right! CBO said tax receipts would tank after tax cuts. In theory.
They are _way_ up since 2003. In actual cash revenue, not theory.

Pat Lamb said:

Of course, CBO was right twenty years earlier!

Yes, they famously predicted ten of the last two recessions.

HarHarHar! Wow, anyone clever enough to repeat that joke must have
studied economics at the graduate level!

Quoted message said:

We have broader home and stock ownership than ever, healthy growing
economy, low unemployment and we're absorbing millions (like it or not,
we are) of immigrants into a growing workforce. Low taxes grow the
entire system effectively.

All that just galls the [censored] out of some people.

What galls the [censored] out of me is the reality behind this nonsense. It
is fundamentally false manipulation of data to leave the impression
that, for example, rising tax receipts SINCE the 2001 TAXCUTS, that
have finally reached a point where tax revenue was higher than 2001,
means that tax receipts are higher than they would have been without
the taxcuts. It is just utter [censored]. The government, the country,
the people are in debt like never before, and the number of high-risk
mortgages made for amounts beyond houses' present worth iis in crisis
territory. China literally holds enough US Treasury paper to destroy
our economy if they suddenly decided to sell it. And don't get me
started on 30 years of stagnant wage growth for the middle/working
classes while the top 1% has seen their income jump in the same
period, or the wonderful and rewarding new jobs being created every
day at the Wal-Mart, or the corporate profits that have not shown up
AT ALL in the wages of the workforces of those corporations. Yes, it
sure does gall the [censored] our of me, yep it does.