Comparison of Auminium, Steel and Carbon forks?

[jim beam]

Mike wrote:
> In article <[email protected]>, [email protected] says...
>> [email protected] wrote:
>>> On Apr 19, 10:18 am, Mike Rocket J Squirrel
>>> Right. Three forks of the same make and model should be exactly the
>>> same (although variance is likely to go up substantially at the bottom
>>> end of the market), but you can't just test one carbon fork next to
>>> one aluminum fork and say that the difference is the material.
>> but previously you were saying there would be "vastly different results"
>> - which of these conflicting statements would you have us believe?
>>
> This could be a result of wishful thinking of the form: " I spent vast amounts of hard-earned cash on these new forks,
> so I had better be able to detect a vastly improved ride"...
>

so what happens when attenuation is still apparent with cheapo sub $100
carbon forks vs. expensiveo custom steel costing more than twice that
much then? would that be wishful thinking too? given that one of the
material properties of cfrp is that it does indeed attenuate vibration...

 

[Ben C]

On 2008-04-21, [email protected] <[email protected]> wrote:
[...]
> False dichotomy, jim. One doesn't have to personally run tests and
> publish results to recognize the ******** component of the advertising
> hype that pops up in bike magazine ads and articles - things like
> "rigid, yet compliant," "superfoods that increase healing power,"
> "sealing gaps at the molecular level reduces friction at racing
> speeds." Yes, and "incomparable, magic ride quality."

Occasionally the ad copy does hint at some genuine innovation or
improvement, although it still makes it sound like BS, causing one's
bogosity detectors to register a false positive.

Basically products either improve or they don't and you always get some
new advertising BS regardless. The two are only very loosely coupled.

Often the clever stuff isn't even mentioned in the advertising BS at
all.

 

[blackhead]

On 17 Apr, 23:23, blackhead <[email protected]> wrote:
> Are there any impartial tests that have been done on Auminium, Steel
> and Carbon forks? Some people say carbon absorbs vibration better than
> steel and Aluminium, others say it makes little difference... etc etc

For the fork to attenuate vibration, it has to do what a shock
absorber in a car shock does; store the energy when flexed similar to
a spring and dissipate it via some dampening mechanism, otherwise
you'll get the vibration released through the whole frame as a
resonance. It certainly stores energy when flexed, but I don't see how
the energy is dissipated. Realistically, I think the tyres will have
the greatest influence on how smooth a ride you will have because the
fork suspension system is too primitive and if carbon does have an
advantage over steel and aluminium, it may have more to do with its
weight for a given required strength.

 

[jim beam]

blackhead wrote:
> On 17 Apr, 23:23, blackhead <[email protected]> wrote:
>> Are there any impartial tests that have been done on Auminium, Steel
>> and Carbon forks? Some people say carbon absorbs vibration better than
>> steel and Aluminium, others say it makes little difference... etc etc
>
> For the fork to attenuate vibration, it has to do what a shock
> absorber in a car shock does; store the energy when flexed similar to
> a spring and dissipate it via some dampening mechanism, otherwise
> you'll get the vibration released through the whole frame as a
> resonance. It certainly stores energy when flexed, but I don't see how
> the energy is dissipated.

word of the day: "hysteresis". all polymers have hysteresis, hence they
absorb/dissipate energy.


> Realistically, I think the tyres will have
> the greatest influence on how smooth a ride you will have because the
> fork suspension system is too primitive and if carbon does have an
> advantage over steel and aluminium, it may have more to do with its
> weight for a given required strength.

 

[Ben C]

On 2008-04-21, blackhead <[email protected]> wrote:
> On 17 Apr, 23:23, blackhead <[email protected]> wrote:
>> Are there any impartial tests that have been done on Auminium, Steel
>> and Carbon forks? Some people say carbon absorbs vibration better than
>> steel and Aluminium, others say it makes little difference... etc etc
>
> For the fork to attenuate vibration, it has to do what a shock
> absorber in a car shock does; store the energy when flexed similar to
> a spring and dissipate it via some dampening mechanism, otherwise
> you'll get the vibration released through the whole frame as a
> resonance. It certainly stores energy when flexed, but I don't see how
> the energy is dissipated.

You would expect steel and al to give most of the energy back as you say,
but CF might actually absorb a bit (your forks would warm up a little).

If the forks are a bit springy but non-damping that still could have an
effect on ride and perceived vibration. If they were very springy, not a
very good effect.

> Realistically, I think the tyres will have the greatest influence on
> how smooth a ride you will have because the fork suspension system is
> too primitive and if carbon does have an advantage over steel and
> aluminium, it may have more to do with its weight for a given required
> strength.

I think you're right that's the main reason for using them. Any
vibration damping is a happy side-effect.

If you're making a cheap bike so can't budget CF for the whole lot you
usually start with a CF fork followed by CF seatstays. There must be a
reason for doing it in that order-- why not CF for the three main tubes
and al for the fork and rear triangle? The previous generation of
slightly budget steel bikes did that: 531 for the three main tubes,
something cheaper for the fork and rear triangle.

 

[[email protected]]

On Apr 21, 3:33 am, Ben C <[email protected]> wrote:
> On 2008-04-21, [email protected] <[email protected]> wrote:
> [...]
>
> > False dichotomy, jim. One doesn't have to personally run tests and
> > publish results to recognize the ******** component of the advertising
> > hype that pops up in bike magazine ads and articles - things like
> > "rigid, yet compliant," "superfoods that increase healing power,"
> > "sealing gaps at the molecular level reduces friction at racing
> > speeds." Yes, and "incomparable, magic ride quality."
>
> Occasionally the ad copy does hint at some genuine innovation or
> improvement, although it still makes it sound like BS, causing one's
> bogosity detectors to register a false positive.
>
> Basically products either improve or they don't and you always get some
> new advertising BS regardless. The two are only very loosely coupled.

Ben, the same phenomenon exists when marketing everything: window
cleaners, bottled water, spark plugs, makeup, toothpaste and
beyond.

You can judge for yourself whether brushing with Ultra-Brite really
makes women 35% more attracted to you. I won't comment on that. But
when the ads' claims involve mechanical concepts that I've spent a
lifetime studying, using and teaching,
I'm willing to trust my judgment. Some items might be genuine,
significant advances; some might deserve testing; but most are either
blatant hype, or else subtle shifting in the balance that always
exists between benefits and detriments.

Of course, you don't have to trust me. You're free to spend all you
want on magic forks that absorb road shocks without deflecting, wheels
whose molecular surface produce less air resistance, titanium bolts
that are 30% lighter than steel ones, etc. Many people do, hoping
those things will somehow transform their cycling.

My friends who have tried such gimmicks never have managed to ride
away from me.

- Frank Krygowski

 

[Mike]

In article <[email protected]>, [email protected] says...
> Mike wrote:
> > In article <[email protected]>, [email protected] says...
> >> [email protected] wrote:
> >>> On Apr 19, 10:18 am, Mike Rocket J Squirrel
> >>> Right. Three forks of the same make and model should be exactly the
> >>> same (although variance is likely to go up substantially at the bottom
> >>> end of the market), but you can't just test one carbon fork next to
> >>> one aluminum fork and say that the difference is the material.
> >> but previously you were saying there would be "vastly different results"
> >> - which of these conflicting statements would you have us believe?
> >>
> > This could be a result of wishful thinking of the form: " I spent vast amounts of hard-earned cash on these new forks,
> > so I had better be able to detect a vastly improved ride"...
> >
>
> so what happens when attenuation is still apparent with cheapo sub $100
> carbon forks vs. expensiveo custom steel costing more than twice that
> much then? would that be wishful thinking too? given that one of the
> material properties of cfrp is that it does indeed attenuate vibration...
>
No - that would not be wishful thinking - it would suggerst that _those_ specific el cheapo cf forks were more
vibration-absorbant than those _specific_ expensivo custom steel forks. Doesn't tell you much else though. Sure, cf can
attenuate vibrations, but so can steel bedstead springs and cheese souflee - you wouldn't build forks from either of
these I think though.

The point of arguement is to determine if carbonfibre or aluminium or steel is inherently better for vibration
absorption. Arguements along the line of "these forks are more absorbant than those.." are obviously irrelevant unless
it can be shown that with significant statistical relevance - "these" are almost alawys cf (for example) and "those"
are always steel. Arguements along the lines of "this material will make more vibration absorbant forks because it has
property x" are also irrelevant unless it can be demonstrated that
a) the material really does have property x
b) property x really does increase vibration absorbance in forks madefrom it
c) the vibration-reduction would be significant enough to be observed by riders in a blind study
d) the material does not have other properties (y and x) that might reduce its ability to absorb vibration.
e) I am sure you can think of more...

Property d) is of significant importance. If, for example, aluminium uses tubing of greater diameter than steel to
provide the necessary strength, one would have to check if steel forks with greater diameter also improved vibration
absorbance. If, for example, carbon forks require different fork geometry and aluminium inserts at the ends where where
the wheel attaches, then one would bneed toi check if it was those design canges rather than the base material that
improved vibration absorption over aluminium or steel.

Demonstrate a combination of blind studies and/or laboratory testing and/or significant popular opinion and you will
have a valid arguement, but neither of your 'arguements' above are convincing.

Mike

 

[Mike]

In article <[email protected]>, [email protected] says...
> blackhead wrote:
> > On 17 Apr, 23:23, blackhead <[email protected]> wrote:
> >> Are there any impartial tests that have been done on Auminium, Steel
> >> and Carbon forks? Some people say carbon absorbs vibration better than
> >> steel and Aluminium, others say it makes little difference... etc etc
> >
> > For the fork to attenuate vibration, it has to do what a shock
> > absorber in a car shock does; store the energy when flexed similar to
> > a spring and dissipate it via some dampening mechanism, otherwise
> > you'll get the vibration released through the whole frame as a
> > resonance. It certainly stores energy when flexed, but I don't see how
> > the energy is dissipated.
>
> word of the day: "hysteresis". all polymers have hysteresis, hence they
> absorb/dissipate energy.
>
Yep, and steel and aluminium forks absorb and dissipate energy too - otherwise I could wander out into the corridor
(where I store my bike during the day) and observe the forks still 'humming' from the morning's ride. What you need to
do is demonstrate that vibration absorption in cf is:
a) significantly greater than in steel or aluminium
b) significantly greater at vibrational frequencies and amplitudes that are detectable to riders
c) significantly greater in structures with dimensions and geometry typical in forks

Come on Jim - where is your evidence - these are bike forks, not tuning forks and are all significantly damped. None of
them will retain any significant vibrational motion after the first half-wave produced by the shock transmitted through
the wheel to the fork, and the acceleration and amplitude at the top end of a rigid fork (i.e at the handle bars) for
any given road surface is dependant firstly on tyres, secondly on fork geometry, and only perhaps thirdly (and a
distant third) on fork material.

Mike
>
> > Realistically, I think the tyres will have
> > the greatest influence on how smooth a ride you will have because the
> > fork suspension system is too primitive and if carbon does have an
> > advantage over steel and aluminium, it may have more to do with its
> > weight for a given required strength.
>

 

[jim beam]

Mike wrote:
> In article <[email protected]>, [email protected] says...
>> blackhead wrote:
>>> On 17 Apr, 23:23, blackhead <[email protected]> wrote:
>>>> Are there any impartial tests that have been done on Auminium, Steel
>>>> and Carbon forks? Some people say carbon absorbs vibration better than
>>>> steel and Aluminium, others say it makes little difference... etc etc
>>> For the fork to attenuate vibration, it has to do what a shock
>>> absorber in a car shock does; store the energy when flexed similar to
>>> a spring and dissipate it via some dampening mechanism, otherwise
>>> you'll get the vibration released through the whole frame as a
>>> resonance. It certainly stores energy when flexed, but I don't see how
>>> the energy is dissipated.
>> word of the day: "hysteresis". all polymers have hysteresis, hence they
>> absorb/dissipate energy.
>>
> Yep, and steel and aluminium forks absorb and dissipate energy too - otherwise I could wander out into the corridor
> (where I store my bike during the day) and observe the forks still 'humming' from the morning's ride.

er, no. in that misconceived example, energy is dissipated into the
environment, not in some bizarre internal absorption mechanism.


> What you need to
> do is demonstrate that vibration absorption in cf is:
> a) significantly greater than in steel or aluminium
> b) significantly greater at vibrational frequencies and amplitudes that are detectable to riders
> c) significantly greater in structures with dimensions and geometry typical in forks
>
> Come on Jim - where is your evidence - these are bike forks, not tuning forks and are all significantly damped. None of
> them will retain any significant vibrational motion after the first half-wave produced by the shock transmitted through
> the wheel to the fork, and the acceleration and amplitude at the top end of a rigid fork (i.e at the handle bars) for
> any given road surface is dependant firstly on tyres, secondly on fork geometry, and only perhaps thirdly (and a
> distant third) on fork material.

you're grasping completely the wrong end of the stick. [and after you'd
been given a clue with the word of the day too.]

hysteresis is a physical property of polymer deformation. it's a
property of that class of materials that you cannot avoid. given that
fact, the question then simply becomes one of "how much" it happens.

now, if /you/ want to muck about on a wild goose chase, /you/ go ahead
and research how much different fiber density, lay-up, fiber length,
orientation [etc] affect how much it can be made to /not/ happen, be my
guest. but don't come in here with a bunch of underinformed presumptive
nonsense again, there's a good boy.





>
> Mike
>>> Realistically, I think the tyres will have
>>> the greatest influence on how smooth a ride you will have because the
>>> fork suspension system is too primitive and if carbon does have an
>>> advantage over steel and aluminium, it may have more to do with its
>>> weight for a given required strength.

 

[jim beam]

Mike wrote:
> In article <[email protected]>, [email protected] says...
>> Mike wrote:
>>> In article <[email protected]>, [email protected] says...
>>>> [email protected] wrote:
>>>>> On Apr 19, 10:18 am, Mike Rocket J Squirrel
>>>>> Right. Three forks of the same make and model should be exactly the
>>>>> same (although variance is likely to go up substantially at the bottom
>>>>> end of the market), but you can't just test one carbon fork next to
>>>>> one aluminum fork and say that the difference is the material.
>>>> but previously you were saying there would be "vastly different results"
>>>> - which of these conflicting statements would you have us believe?
>>>>
>>> This could be a result of wishful thinking of the form: " I spent vast amounts of hard-earned cash on these new forks,
>>> so I had better be able to detect a vastly improved ride"...
>>>
>> so what happens when attenuation is still apparent with cheapo sub $100
>> carbon forks vs. expensiveo custom steel costing more than twice that
>> much then? would that be wishful thinking too? given that one of the
>> material properties of cfrp is that it does indeed attenuate vibration...
>>
> No - that would not be wishful thinking - it would suggerst that _those_ specific el cheapo cf forks were more
> vibration-absorbant than those _specific_ expensivo custom steel forks. Doesn't tell you much else though. Sure, cf can
> attenuate vibrations, but so can steel bedstead springs and cheese souflee - you wouldn't build forks from either of
> these I think though.
>
> The point of arguement is to determine if carbonfibre or aluminium or steel is inherently better for vibration
> absorption. Arguements along the line of "these forks are more absorbant than those.." are obviously irrelevant unless
> it can be shown that with significant statistical relevance - "these" are almost alawys cf (for example) and "those"
> are always steel. Arguements along the lines of "this material will make more vibration absorbant forks because it has
> property x" are also irrelevant unless it can be demonstrated that
> a) the material really does have property x
> b) property x really does increase vibration absorbance in forks madefrom it
> c) the vibration-reduction would be significant enough to be observed by riders in a blind study
> d) the material does not have other properties (y and x) that might reduce its ability to absorb vibration.
> e) I am sure you can think of more...
>
> Property d) is of significant importance. If, for example, aluminium uses tubing of greater diameter than steel to
> provide the necessary strength, one would have to check if steel forks with greater diameter also improved vibration
> absorbance. If, for example, carbon forks require different fork geometry and aluminium inserts at the ends where where
> the wheel attaches, then one would bneed toi check if it was those design canges rather than the base material that
> improved vibration absorption over aluminium or steel.
>
> Demonstrate a combination of blind studies and/or laboratory testing and/or significant popular opinion and you will
> have a valid arguement, but neither of your 'arguements' above are convincing.
>
> Mike


wrong end of the stick mike. it's a property of the material. you go
ahead and show how much hysteresis and therefore absorption /doesn't/
happen. [using a spell check wouldn't hurt you either.]

 

[jim beam]

[email protected] wrote:
> On Apr 21, 3:33 am, Ben C <[email protected]> wrote:
>> On 2008-04-21, [email protected] <[email protected]> wrote:
>> [...]
>>
>>> False dichotomy, jim. One doesn't have to personally run tests and
>>> publish results to recognize the ******** component of the advertising
>>> hype that pops up in bike magazine ads and articles - things like
>>> "rigid, yet compliant," "superfoods that increase healing power,"
>>> "sealing gaps at the molecular level reduces friction at racing
>>> speeds." Yes, and "incomparable, magic ride quality."
>> Occasionally the ad copy does hint at some genuine innovation or
>> improvement, although it still makes it sound like BS, causing one's
>> bogosity detectors to register a false positive.
>>
>> Basically products either improve or they don't and you always get some
>> new advertising BS regardless. The two are only very loosely coupled.
>
> Ben, the same phenomenon exists when marketing everything: window
> cleaners, bottled water, spark plugs, makeup, toothpaste and
> beyond.
>
> You can judge for yourself whether brushing with Ultra-Brite really
> makes women 35% more attracted to you. I won't comment on that. But
> when the ads' claims involve mechanical concepts that I've spent a
> lifetime studying, using and teaching,
> I'm willing to trust my judgment. Some items might be genuine,
> significant advances; some might deserve testing; but most are either
> blatant hype, or else subtle shifting in the balance that always
> exists between benefits and detriments.
>
> Of course, you don't have to trust me. You're free to spend all you
> want on magic forks that absorb road shocks without deflecting, wheels
> whose molecular surface produce less air resistance, titanium bolts
> that are 30% lighter than steel ones, etc. Many people do, hoping
> those things will somehow transform their cycling.
>
> My friends who have tried such gimmicks never have managed to ride
> away from me.
>

hearsay and waffle. you're an idiot krygowski.

 

[Tom Sherman]

"jim beam" wrote:
> [...]
> wrong end of the stick mike. it's a property of the material. you go
> ahead and show how much hysteresis and therefore absorption /doesn't/
> happen. [using a spell check wouldn't hurt you either.]

Funny that someone who never uses capital letters where they are
required should complain about the spelling of others.

--
Tom Sherman - Holstein-Friesland Bovinia
The weather is here, wish you were beautiful

 

[[email protected]]

On Apr 20, 11:33 am, Ben C <[email protected]> wrote:
> On 2008-04-20, [email protected] <[email protected]> wrote:
> [...]
>
> > Right. Three forks of the same make and model should be exactly the
> > same (although variance is likely to go up substantially at the bottom
> > end of the market), but you can't just test one carbon fork next to
> > one aluminum fork and say that the difference is the material.
> > Whatever vibration parameter you're measuring is going to have a big
> > enough range across different models of carbon forks that it's going
> > to overlap with the range across different models of aluminum forks.
>
> Sometimes different materials dictate a different shape.
>
> For example, I think I'm right in saying that if you make an aluminium
> fork as flexy as you can make a steel one the aluminium one will fatigue
> badly. So you have to make it a bit thicker and/or fatter and stiffer.
> Sure you _could_ make a fork just as stiff out of steel, but you don't
> have to and might not.
>

Material dictates shape, but there's still a working range. If you
have a steel tube of a given flexural strength, you can match that
strength with aluminum by using an appropriate ratio of wall thickness
to tube diameter. The aluminum tubes are going to range from big with
thin walls (stiffer than the steel tube) to small with very thick
walls (less stiff than the steel). Where the stiffness of the steel
tube falls in the working range for aluminum will depend on how big
and how stiff it was to begin with.

>
> So it's better to compare complete forks as sold and then say something
> like "out of the 100 forks tested, the CF ones mostly absorbed vibration
> better than the Al ones".

Exactly. I would even plot the distribution curves to see how much
overlap there was.

 

[[email protected]]

On Apr 20, 10:28 pm, [email protected] wrote:
> On Apr 20, 3:10 pm, jim beam <[email protected]> wrote:
>
>
>
> > Ben C wrote:
>
> > <snip for clarity>
>
> > > I would expect there to be much more likely to be non-negligible
> > > differences between forks.
>
> > of course. krygowski, being an intelligent, informed engineering
> > professor with access to the correct instrumentation, already the owner
> > of a cross-section of different forks, a sound knowledge of the
> > principles, and with an open, inquiring mind, has already tested this
> > position and is simply waiting for an opportunity to publish his
> > results. or he's simply an idiot voicing underinformed opinion as fact
> > and who has no inclination to actually test any damned thing that could
> > possibly upset his comfy luddite little world.
>
> False dichotomy, jim. One doesn't have to personally run tests and
> publish results to recognize the ******** component of the advertising
> hype that pops up in bike magazine ads and articles - things like
> "rigid, yet compliant," "superfoods that increase healing power,"
> "sealing gaps at the molecular level reduces friction at racing
> speeds." Yes, and "incomparable, magic ride quality."
>
> And a careful reader will note that I was simply giving my speculation
> on what a fork comparison test would show. While I'd be willing to
> bet with my friends on the issue, I wouldn't testify in court unless
> I'd performed the proper test, or seen results I judged worthwhile.
>
> Again, the worthwhile test would be a blind, on-road comparison test
> using multiple riders, where the other factors were held constant.
> It's the only way to filter out the placebo effect.
>
> - Frank Krygowski

An even easier test is to take a carbon fork fresh out of the box,
grab it by the steerer, and whack one of the dropouts against
something. It's going to hum like a tuning fork for several seconds.
It may seem clear to some that the polymer matrix in the composite is
soaking up significant amounts of energy, but there are two things
going against that theory. 1) Polymers dissipate energy proportional
to their volume and the amount of strain they're under. A carbon fork
does not contain enough epoxy under enough deformation to dissipate a
significant amount of energy. 2) Manufacturers are constantly pushing
to use less and less epoxy in their layups. It's dead weight, and
doesn't contribute to the strength and stiffness of the fork. In all
of their advertising, they somehow forget to explain what their latest
revolutionary process (it was nanotubes last I heard) to increase
fiber volume ratio for increased strength to weight was doing to the
fork's imaginary damping properties.

 

[Ben C]

On 2008-04-22, [email protected] <[email protected]> wrote:
> On Apr 20, 11:33 am, Ben C <[email protected]> wrote:
>> On 2008-04-20, [email protected] <[email protected]> wrote:
>> [...]
>>
>> > Right. Three forks of the same make and model should be exactly the
>> > same (although variance is likely to go up substantially at the bottom
>> > end of the market), but you can't just test one carbon fork next to
>> > one aluminum fork and say that the difference is the material.
>> > Whatever vibration parameter you're measuring is going to have a big
>> > enough range across different models of carbon forks that it's going
>> > to overlap with the range across different models of aluminum forks.
>>
>> Sometimes different materials dictate a different shape.
>>
>> For example, I think I'm right in saying that if you make an aluminium
>> fork as flexy as you can make a steel one the aluminium one will fatigue
>> badly. So you have to make it a bit thicker and/or fatter and stiffer.
>> Sure you _could_ make a fork just as stiff out of steel, but you don't
>> have to and might not.
>>
>
> Material dictates shape, but there's still a working range. If you
> have a steel tube of a given flexural strength, you can match that
> strength with aluminum by using an appropriate ratio of wall thickness
> to tube diameter. The aluminum tubes are going to range from big with
> thin walls (stiffer than the steel tube) to small with very thick
> walls (less stiff than the steel). Where the stiffness of the steel
> tube falls in the working range for aluminum will depend on how big
> and how stiff it was to begin with.

OK I have two questions.

Strength is one thing, fatigue life is another. Any component has to
have enough of both.

For fatigue life we have the S-N graph of mean stress against number of
cycles. You need to reduce the stress (by increasing the cross-section
for the sake of argument) to the point where the component will survive
plenty of cycles (I don't know, 10^6 or something).

The component also needs to be strong enough so as not to yield when you
hit a reasonably normal bump in the road and so forth.

An ideal component engineered for the lightest weight has just enough of
each of strength and fatigue life and no more.

So I'm making a tube of a given length out of a given volume of metal. I
have two parameters: wall thickness and tube diameter. If I'm just
pulling on the tube (loading it axially) all that matters I think is
cross-section, so it's moot whether I go for almost a solid rod or a
thin-walled "oversize" tube.

So let's say I'm bending it. It might be the top tube on a bike (that
might be simpler than the fork). Suppose it's strong enough, but its
fatigue life is too short. Would it help to make the tube fatter but
thinner-walled, or vice versa, or not?

My second question is: if I make two tubes that are both just right for
both yield strength and fatigue life for a particular application, one
out of steel and one out of aluminium, is it inevitable that one tube
will be stiffer than the other, or is there enough working range that
after satisfying the strength and fatigue requirements the designer can
also make them the same stiffness? (Of course they will have different
diameters and thicknesses).

 

[Mike Rocket J Squirrel]

On 4/22/2008 9:13 AM [email protected] wrote:

> On Apr 20, 11:33 am, Ben C <[email protected]> wrote:

>> So it's better to compare complete forks as sold and then say something
>> like "out of the 100 forks tested, the CF ones mostly absorbed vibration
>> better than the Al ones".
>
> Exactly. I would even plot the distribution curves to see how much
> overlap there was.

Whoa. Too useful.

--
Mike "Rocket J Squirrel"

 

[Peter Cole]

Ben C wrote:

> So let's say I'm bending it. It might be the top tube on a bike (that
> might be simpler than the fork). Suppose it's strong enough, but its
> fatigue life is too short. Would it help to make the tube fatter but
> thinner-walled, or vice versa, or not?

Yes, the larger cross section will have lower stress for the same load.

>
> My second question is: if I make two tubes that are both just right for
> both yield strength and fatigue life for a particular application, one
> out of steel and one out of aluminium, is it inevitable that one tube
> will be stiffer than the other, or is there enough working range that
> after satisfying the strength and fatigue requirements the designer can
> also make them the same stiffness? (Of course they will have different
> diameters and thicknesses).

The two materials are different, so that makes direct comparison
problematic. Generally, to get comparable fatigue life, the aluminum
will use a bit more material than needed for comparable strength, so you
have a choice of similar fatigue, but aluminum stronger, or similar
strength with aluminum less durable. The limit for steel tubing diameter
is dent/crumple resistance. This has been all pretty well worked out
empirically in bike frames over the last couple of decades.

 

[[email protected]]

Peter Cole wrote:

>> So let's say I'm bending it. It might be the top tube on a bike
>> (that might be simpler than the fork). Suppose it's strong enough,
>> but its fatigue life is too short. Would it help to make the tube
>> fatter but thinner-walled, or vice versa, or not?

> Yes, the larger cross section will have lower stress for the same
> load.

Another aspect of this is that for the same deflection, stress is
higher in the larger cross section element. For this reason, I have
seen more wrinkled aluminum than steel tubes from bending. Before the
advent of aluminum frames, steel frames wrinkled their downtubes at
the transition from thick walled tube ends to thin wall. Forks in
contrast did not wrinkle, having no localized wall thickness
transition nor excess "beam height" as did aluminum forks.

>> My second question is: if I make two tubes that are both just
>> right for both yield strength and fatigue life for a particular
>> application, one out of steel and one out of aluminium, is it
>> inevitable that one tube will be stiffer than the other, or is
>> there enough working range that after satisfying the strength and
>> fatigue requirements the designer can also make them the same
>> stiffness? (Of course they will have different diameters and
>> thicknesses).

> The two materials are different, so that makes direct comparison
> problematic. Generally, to get comparable fatigue life, the
> aluminum will use a bit more material than needed for comparable
> strength, so you have a choice of similar fatigue, but aluminum
> stronger, or similar strength with aluminum less durable. The limit
> for steel tubing diameter is dent/crumple resistance. This has been
> all pretty well worked out empirically in bike frames over the last
> couple of decades.

To avoid the stiffness problem (that increases with the third power of
"beam height"), that causes failures elsewhere, some aluminum forks
were made to external steel dimensions but with greater wall
thickness. That was a dud and you probably cannot find such a frame
still in regular use. Weight weenies have done little good for the
durability and repairability of bicycles yet that is what sells.

Jobst Brandt

 

[Ben C]

On 2008-04-25, Peter Cole <[email protected]> wrote:
> Ben C wrote:
>
>> So let's say I'm bending it. It might be the top tube on a bike (that
>> might be simpler than the fork). Suppose it's strong enough, but its
>> fatigue life is too short. Would it help to make the tube fatter but
>> thinner-walled, or vice versa, or not?
>
> Yes, the larger cross section will have lower stress for the same load.
>
>>
>> My second question is: if I make two tubes that are both just right for
>> both yield strength and fatigue life for a particular application, one
>> out of steel and one out of aluminium, is it inevitable that one tube
>> will be stiffer than the other, or is there enough working range that
>> after satisfying the strength and fatigue requirements the designer can
>> also make them the same stiffness? (Of course they will have different
>> diameters and thicknesses).
>
> The two materials are different, so that makes direct comparison
> problematic. Generally, to get comparable fatigue life, the aluminum
> will use a bit more material than needed for comparable strength, so you
> have a choice of similar fatigue, but aluminum stronger, or similar
> strength with aluminum less durable. The limit for steel tubing diameter
> is dent/crumple resistance. This has been all pretty well worked out
> empirically in bike frames over the last couple of decades.

Thanks for the answer. I'm thinking then that it is quite likely that in
practice aluminium frames and forks _will_ be stiffer than steel ones
(when comparing similar styles of bike) as a consequence of designing
them for sufficient fatigue life?

But I still don't know really-- the aluminium frame may be stronger, but
aluminium has a lower modulus than steel, so it's not obvious that it's
going to be stiffer.

 

[Ben C]

On 2008-04-25, [email protected] <[email protected]> wrote:
> Peter Cole wrote:
[...]
>>> My second question is: if I make two tubes that are both just
>>> right for both yield strength and fatigue life for a particular
>>> application, one out of steel and one out of aluminium, is it
>>> inevitable that one tube will be stiffer than the other, or is
>>> there enough working range that after satisfying the strength and
>>> fatigue requirements the designer can also make them the same
>>> stiffness? (Of course they will have different diameters and
>>> thicknesses).
>
>> The two materials are different, so that makes direct comparison
>> problematic. Generally, to get comparable fatigue life, the
>> aluminum will use a bit more material than needed for comparable
>> strength, so you have a choice of similar fatigue, but aluminum
>> stronger, or similar strength with aluminum less durable. The limit
>> for steel tubing diameter is dent/crumple resistance. This has been
>> all pretty well worked out empirically in bike frames over the last
>> couple of decades.
>
> To avoid the stiffness problem (that increases with the third power of
> "beam height"), that causes failures elsewhere, some aluminum forks
> were made to external steel dimensions but with greater wall
> thickness.

This sounds interesting but I'm not sure I understand exactly what
you're saying. What's "beam height"-- is it tube diameter in this
context-- and what were the failures elsewhere?

> That was a dud and you probably cannot find such a frame still in
> regular use.

What was wrong with it?

> Weight weenies have done little good for the durability and
> repairability of bicycles yet that is what sells.

Well they are weight weenies not durability or repairability weenies.