Wheel deflection

On Wed, 29 Oct 2008 00:42:34 -0700, Chalo wrote:

Carl Fogel wrote:

Chalo wrote:

jim beam wrote:

Jobst Brandt wrote:

Time of loading has no effect on metals

untrue. Â*many materials react differently depending on loading rate.
Â*for example, this is why you have nail guns - they succeed at a
high rate on driving nails into concrete whereas trying to do the
same job at a lower rate always fails.

Concrete is the issue. Â*Whack a nail into wood, or push it in with an
arbor press, and it's about the same (although the whacked nail might
hold better due to local heating of the wood resins).

[snip]

DearChalo,

Usually I follow you because your posts are clear to even the meanest
intelligence.

But this is a concrete example (sorry, couldn't resist it) of how even
you can overestimate the dregs of your readership.

Can you explain concrete versus wood a little more from the nail gun's
point of view?

Concrete is very strong in compression, but subject to being
disintegrated by shock. The shock created by a powder-fired fastener
accomplishes what a similarly large but steady axial load on the same
fastener can't, pulverizing a small zone around the nail and allowing it
to penetrate. Without the shock, a nail pushed sufficiently hard would
bend or break before penetrating the concrete enough to fasten to it.

indeed.




Wood is a resilient material and does not need to be shocked to allow a
fastener to penetrate. Thus a nail can be shot, pounded, or simply
shoved into place with comparable results in whichever case.

not always the case. long thin nails can be driven without buckling when
done at high velocity in wood. not so at low velocity.

On Oct 28, 9:04*am, jim beam wrote:
On Mon, 27 Oct 2008 21:08:19 -0700, steve wrote:
Does anybody know if a rim would react differently to a sudden load
versus a gradual load? What I mean is if you were to apply a gradual
load on a wheel would that rim be able to handle a higher load before
permanantly deforming than if you were to apply the same load very
quickly?

for the same load magnitude, strictly speaking, it depends on the rate. *
in typical riding situations, you'll see no difference, only at very high
loading rates.

why?

I have been doing lateral and radial testing on multiple wheels for
some time now using a load that increases in incriments. The load is
applied without the tire since the tire would greatly complicate
things for the simple minded such as myself. I have been wondering for
some time know if my testing is invalid since the load is not be
applied quickly like while you are riding and hit a bump. I understand
that things get very complicated when you throw a bike tire on the rim
but I was assuming that the bike tire would just allow the rim to
handle higher loads before plasticly deforming since the tire is able
to distribute the force, allowing it to be applied to a larger part to
the rim. Thus my data would be similiar to a "worse case senario"
while riding. I also found that most of the wheels I tested, with the
exception of carbon rims, could handle close to a 1000lbs or radial
load without permanantly deforming. Since I can't do the math I don't
know if this would be hard to exceed in a real world senario. But It
seems that it should be harder than it obviously is based on the
number of rims you see that have flat spots in them from excessive
radial load, especially since the rim has a tire on it to help
distribute and absorb some of the force.

Steve

On Oct 28, 4:16*pm, Frank Krygowski wrote:
On Oct 28, 1:25*pm, wrote:

Frank Krygowski wrote:
Does anybody know if a rim would react differently to a sudden load
versus a gradual load? *What I mean is if you were to apply a
gradual load on a wheel would that rim be able to handle a higher
load before permanantly deforming than if you were to apply the
same load very quickly?
With suddenly applied loads, there's an inertia effect that
generally makes things worse.

Let's not leave it at such a vague description. *The question is load,
regardless of its origin, inertial or dead weight. *The rate of stress
application has no effect on metals, at least up to acoustic
frequencies.

That depends on exactly what Steve meant by "load."

The classic example is gently placing a load on a spring, vs. suddenly
releasing the same load onto the same spring. *Even if the load is
(barely) touching the spring just before it's released, the spring
momentarily deflects twice as far as with the gently placed load, and
the peak force on the spring is twice as great.

Those are two different loads. *The question was about load
application and possibly duration causing plastic deformation.

If Steve could tell us more about exactly what *he means, we could
determine whether the sudden application of an object onto a spring is
properly analogous. *I believe it's likely, but he's given no details.

If the load were applied by a mechanism that didn't involve mass and
inertia, I don't believe that effect would be present. *But bike wheel
loads do involve mass and inertia.

Bicycle wheels do not involve mass AND inertia. *Loads are mainly
rider and bicycle mass and their inertia. *The mass of the wheel and
tire is insignificant in that respect. *An example of that would be to
toss a wheel, with inflated tire, into the air and watch it bounce
undamaged on pavement. *To damage a wheel in that manner would take a
hefty toss, especially with a fat (2+ tire) inch.

Bicycle wheels support mass. *That mass has inertia. *It makes a
difference.

Example: *A 200 pound bike+rider sitting still on level ground causes
certain stresses in the various wheel components. *That same bike
+rider dropping one foot down onto level ground causes much more
stress. *If Steve was envisioning anything similar, the fact that
wheel masses are low has no bearing.

So, Steve, what do you have in mind?

- Frank Krygowski

I was envisioning a force being applied to a rim with tire in the
radial direction from a bump while riding versus, a load being applied
by a bottle jack in gradually increasing incriments on a bare rim.
Obviously the tire complicates the issue so if we were to take the
tire out to the equation would the wheel be able to handle the same
amount of force whether it was applied gradually from a bottle jack or
very quickly like when you are riding and hit a curb?

Steve

On 2008-10-29, steve wrote:
[...]
I was envisioning a force being applied to a rim with tire in the
radial direction from a bump while riding versus, a load being applied
by a bottle jack in gradually increasing incriments on a bare rim.
Obviously the tire complicates the issue so if we were to take the
tire out to the equation would the wheel be able to handle the same
amount of force whether it was applied gradually from a bottle jack or
very quickly like when you are riding and hit a curb?

Yes (or very close to the same amount). It's just not very easy to
estimate what the force on the rim is when you hit a kerb.

The tyre means less force on the rim when you hit the kerb, and probably
as you suggested it also spreads the force out a bit meaning less stress
on any part of the rim.

By the way, how do you know how much force the bottle jack is applying?
People usually hang weights off things for this kind of experiment.

On Oct 29, 1:30*pm, Ben C wrote:
On 2008-10-29, steve wrote:
[...]

I was envisioning a force being applied to a rim with tire in the
radial direction from a bump while riding versus, a load being applied
by a bottle jack in gradually increasing incriments on a bare rim.
Obviously the tire complicates the issue so if we were to take the
tire out to the equation would the wheel be able to handle the same
amount of force whether it was applied gradually from a bottle jack or
very quickly like when you are riding and hit a curb?

Yes (or very close to the same amount). It's just not very easy to
estimate what the force on the rim is when you hit a kerb.

The tyre means less force on the rim when you hit the kerb, and probably
as you suggested it also spreads the force out a bit meaning less stress
on any part of the rim.

By the way, how do you know how much force the bottle jack is applying?
People usually hang weights off things for this kind of experiment.

On Oct 29, 1:30*pm, Ben C wrote:
On 2008-10-29, steve wrote:
[...]

I was envisioning a force being applied to a rim with tire in the
radial direction from a bump while riding versus, a load being applied
by a bottle jack in gradually increasing incriments on a bare rim.
Obviously the tire complicates the issue so if we were to take the
tire out to the equation would the wheel be able to handle the same
amount of force whether it was applied gradually from a bottle jack or
very quickly like when you are riding and hit a curb?

Yes (or very close to the same amount). It's just not very easy to
estimate what the force on the rim is when you hit a kerb.

The tyre means less force on the rim when you hit the kerb, and probably
as you suggested it also spreads the force out a bit meaning less stress
on any part of the rim.

By the way, how do you know how much force the bottle jack is applying?
People usually hang weights off things for this kind of experiment.

I bought an "s"type load cell which is placed between the bottle jack
and wheel. The wheel is held in place at the axle by two 2" dia
screws which are attached to 6"square tubing. This allows me to put
high loads on the wheel without a significant amount of flex. At the
moment I can easily do lateral and radial loading but I am trying to
figure out a reliable way to measure wind-up or tortional load of a
rear wheel from pedaling.
By the way, does anybody have an idea of the max amount of force in
lbf or Nm that a human can produce at the center of the hub?

Steve

Steve

On 2008-10-29, steve wrote:
On Oct 29, 1:30*pm, Ben C wrote:
On 2008-10-29, steve wrote:
[...]

I was envisioning a force being applied to a rim with tire in the
radial direction from a bump while riding versus, a load being applied
by a bottle jack in gradually increasing incriments on a bare rim.
Obviously the tire complicates the issue so if we were to take the
tire out to the equation would the wheel be able to handle the same
amount of force whether it was applied gradually from a bottle jack or
very quickly like when you are riding and hit a curb?

Yes (or very close to the same amount). It's just not very easy to
estimate what the force on the rim is when you hit a kerb.

The tyre means less force on the rim when you hit the kerb, and probably
as you suggested it also spreads the force out a bit meaning less stress
on any part of the rim.

By the way, how do you know how much force the bottle jack is applying?
People usually hang weights off things for this kind of experiment.

I bought an "s"type load cell which is placed between the bottle jack
and wheel. The wheel is held in place at the axle by two 2" dia
screws which are attached to 6"square tubing. This allows me to put
high loads on the wheel without a significant amount of flex. At the
moment I can easily do lateral and radial loading but I am trying to
figure out a reliable way to measure wind-up or tortional load of a
rear wheel from pedaling.
By the way, does anybody have an idea of the max amount of force in
lbf or Nm that a human can produce at the center of the hub?

Do you mean torque? lbf is a unit of force, but Nm is a unit of torque,
and you were just talking about wind-up and torsional loads. And you
might have meant lbft or something (foot-pounds, also a unit of torque).

It's reasonable to say that the most force the rider can put on the
pedal is his whole weight. You can get a little bit more by pulling up
on the handlebars, but it's not going to be that much more.

So, an 80kg rider pushes the pedal with a maximum of 800N. The crank is
0.17m long, so that's a torque of 800*0.17 = 136Nm at the bottom
bracket.

In a low gear on an MTB, the sprocket has the same number of teeth as
the chainring, so that would also be 136Nm at the back wheel. On a
typical road bike 39x23 bottom gear, it would be 136 * (23/39) = 80Nm.

"Ben C" wrote in message
...

In a low gear on an MTB, the sprocket has the same number of teeth as
the chainring

22/32 or 22/34? :-)

On 2008-10-29, Clive George wrote:
"Ben C" wrote in message
...

In a low gear on an MTB, the sprocket has the same number of teeth as
the chainring

22/32 or 22/34? :-)

Well I said, "a" low gear. Mine was 6-speed originally and went to
24/24. I know they go lower than that these days, but didn't know how
low.

Do they really go to 22/34?

In article
,
pm wrote:

On Oct 28, 1:58*pm, wrote:
Frank Krygowski wrote:
[content-free definitional posturing]
[more content-free definitional posturing]

I wonder why is is that an insistence on rigid adherence to
(arbitrarily decided and often inconsistent) terminology so often
correlates with the same person's complete failure to communicate,
whether in the role of producer or receiver of information.

The typical failure insists that accurate diction is essential to
accurate communication; yet when the process of communication has
progressed so that any outside observer can see what information is in
play, the failure goes on long after the fact to insist that words be
used *his* way -- though by any measure the communication of
information is long complete. One is forced to conclude from the fact
of his actions, in contradiction to his stated beliefs, the failure
has no pragmatic interest in the transmission of accurate information.

Physics, the theory of elasticity, and mechanical engineering
have a common language in which terms are defined precisely.
Definitions are vital in every calling. Use of precisely
defined terms by those not skilled in the art as if the
speaker is skilled in the art is troublesome because the
unskilled speaker is using words in an everyday sense,
but in a situation where the technical meaning is called
for or expected, leading to miscommunication, confusion,
imputation of misstating the facts, and rancor.

--
Michael Press