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Recreational Cyclocross and Gravelbiking This has to be the most physically intense sport ever invented. It's high speed bicycle racing on a short off road course or riding the off pavement rides on gravel like :The Dirty Kanza". We also have a dedicated Racing forum for the Cyclocross Hard Core Racers.

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Old 09-28-10, 08:25 PM   #1
sabazel
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Forks: Aluminum = Widowmaker?

I've heard that aluminum forks are "widowmakers." I think that my bike has such a fork, though it may be steel. I don't think it it, though.

My question is: What sort of differences do the different fork materials make in terms of handling/shock absorption? I know it's going to be rigid, but will a non aluminum fork make a huge difference in terms of handling on bumpy conditions?
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Old 09-28-10, 10:48 PM   #2
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I raced for two seasons on an old Haro with a 1" threaded aluminium fork. Never had a problem. My current bike, a Surly with a Bianchi carbon fork, feels much more comfortable, but the geometry and fit are so different that it's hard to say anything about the fork per se.

My gut feeling, however, is this: with 34 mm knobbies at 40 psi, I'd be hard-pressed to identify a noticeable difference in the ride quality between two forks with the same rake. I can barely tell the difference between my super-stiff Ksyrium ES's and my super-flexy Ultegra R600's under those conditions.
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Old 09-29-10, 06:26 AM   #3
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If you want to know if your fork is steel or alu, use a magnet.

As for the rest, the only real advantage of a steel fork is that has a slower failure mode than alu - ie it will tend to gradually bend in extreme circumstances where alu would snap.
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Old 09-29-10, 08:19 AM   #4
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Alu forks use significantly thicker tubes than a frame would. These would be less likely to catastrophically fail.*



*disclaimer - This is a gross generalization and I'm sure there exceptions.
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Old 09-29-10, 08:21 AM   #5
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Alu forks use significantly thicker tubes than a frame would. These would be less likely to catastrophically fail.*



*disclaimer - This is a gross generalization and I'm sure there exceptions.
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Old 09-29-10, 08:50 AM   #6
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The thing is this: Steel has a stress threshold that, if you never exceed it, will result in an infinite fatigue life. Aluminum does not have such a limit. Apply enough cyclic loads to an aluminum part, no matter how small the loads, and eventually the aluminum part will suffer a fatigue failure.

That said, if you design enough margin into an aluminum part, the number of load cycles required to fail it may be far more than you will put on it. Still, aluminum parts cannot be designed for infinite fatigue life like steel can.

Viscount made some aluminum forks many moons ago that were known for failing prematurely, so much so that they are referred to as "death forks". This example is well-known in the industry, though, and I would think that any currently available aluminum forks will have a very long fatigue life (just not infinite).
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Originally Posted by bragi "However, it's never a good idea to overgeneralize."
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Old 09-29-10, 08:57 AM   #7
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Aluminum rims are widowmakers, too; that's why you all ride steel rims exclusively.
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Old 09-29-10, 10:04 AM   #8
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Quote:
Originally Posted by Doohickie View Post

Viscount made some aluminum forks many moons ago that were known for failing prematurely, so much so that they are referred to as "death forks". This example is well-known in the industry, though, and I would think that any currently available aluminum forks will have a very long fatigue life (just not infinite).
These forks were cast alloy, and shattered easily.
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Old 09-29-10, 10:26 AM   #9
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Quote:
Originally Posted by Doohickie View Post
The thing is this: Steel has a stress threshold that, if you never exceed it, will result in an infinite fatigue life. Aluminum does not have such a limit. Apply enough cyclic loads to an aluminum part, no matter how small the loads, and eventually the aluminum part will suffer a fatigue failure.

That said, if you design enough margin into an aluminum part, the number of load cycles required to fail it may be far more than you will put on it. Still, aluminum parts cannot be designed for infinite fatigue life like steel can.
This is commonly believed, but it isn't really true - certainly not in the case of a bicycle frame which consists of *welds* as well as metal (which won't be pure alu or steel anyway, but an alloy with very different fatigue properties.) For a given weight, an alu frame may well be more fatigue resistant than a steel frame.

Somewhat technical discussion from Gary Klein:

Quote:
http://www.kleinjapan.com/at_klein/garys_speech.pdf

I think the “Aluminum Time Bomb Theory” demonstrates the lack of materials knowledge in the bike
industry. It should be common knowledge that most modern aircraft use aluminum exclusively for
their primary structures (internal frames and bulkheads) and 95% or better of their exterior surfaces,
including load bearing skins. The aircraft industry has been using these alloys for several decades. I
have recently been a passenger on some planes that I estimate were made no later than the 60’s. So
aluminum alloys have certainly proved their long term durability and high performance in the aircraft
industry. The occasional failure that has occurred has typically been due to a design or manufacturing
defect or improper maintenance.

The aircraft companies have picked aluminum because it offers the best combination of material prop-
erties and processing capability in order to create high performance, lightweight, robust aircraft. Prior
to the widespread use of aluminum alloys in airframes, I believe that Chrome Moly Steel was used in
many cases for structural members and coated fabric was used for skins.

Aircraft see high levels of shock and vibration. They are subjected to high G loads, both positive and
negative in severe turbulence and positive when landing. The most severe repetitive G loads occur on
aircraft carriers. The planes are accelerated and decelerated at high G’s by mechanical linkage with the
carrier.

The example given of repeatedly bending a small piece of metal is not relevant to the durability or relia-
bility of a bicycle frame. When you permanently deform the material as in the example you are yielding
it. This is not what fatigue strength or fatigue life refers to or is about. It has no relation to fatigue
strength. Some of the highest fatigue strength materials I have used are carbon fiber and boron fiber.
They will not take a significant permanent set, breaking instead at a high force level. So these extremely
high fatigue strength fibers would rate near zero by the repeated bending test.

The example is comparing the material property to withstand reversing yield conditions. On a bike,
this would be like crashing the bike, smashing the head tube back and buckling the top and down
tubes. Then straightening the tubes back (to the degree possible) and repeating the cycle. While this
might be useful in a few circumstances, I would rather my frame did not yield so easily in the first
place. The optimum material for this reversing yield property might be a low carbon (low yield
strength) or mild steel alloy which has not proven to be a good choice for high performance bike
frames.

The statement “Alu has a shorter fatigue life than steel.” also demonstrates shortage of material
knowledge and understanding. Sure, a high strength steel alloy will exhibit a longer fatigue life at a
high fully reversing load level than a high strength aluminum alloy. These numbers always reflect per-
formance for a unit volume. But it also weighs 3 times as much for the same volume. If the mass of
the bike is unimportant, then I guess steel is the better material. If density is factored in, aluminum is
actually stronger.

Failure of a structure due to repeated stress cycles has two main components. They are crack initiation
and crack propagation. The obvious thing to design for is to prevent crack initiation. In theory, if no
cracks can start, then we don’t need to worry about fracture toughness, or crack propagation. But this
does not work in real life. I think that all metal bike frames have millions of small cracks. It is inherent
in their metal structure. Most metals are made up of very small metal crystals or grains stuck together.
There are inherently a lot of flaws in the micro structure. The concentration of cracks is higher where
the metal has been welded or brazed, such as at the joints. I believe this is true for steel, aluminum
and titanium alloys. A tough material will allow the bike to perform adequately for a long time with a
crack in it that is below a certain crack size. The tougher the material, the larger the allowable crack.
Below this critical size, the crack will grow so slowly that it will not become a problem.

Crack initiation behavior is measured by fatigue tests.

Fatigue behavior of a given material is not at all well defined by any single number. Fatigue behavior for
a material is more accurately portrayed by a series of curves. The behavior (and number of cycles it can
withstand) will vary considerably depending on whether the load is only applied in one direction, both
directions, or is applied in addition to a static or constant load. For each type of loading condition
described above, the material will exhibit a range of fatigue cycles to failure depending on the load
level applied. The most commonly used test is the fully reversed load without static load. It is a simple
test to perform.

The fatigue life increases as the stress level is reduced. Common steel alloys and common aluminum
alloys have differently shaped curves.

The curve for steel under fully reversed loading is approximately a constant downward slope (plotted
on a logarithmic cycle’s scale) until about one million cycles, where the curve abruptly becomes hori-
zontal. It has a well-defined corner in it. This is called the endurance limit for steel.
The curve for aluminum does not have this sharp corner. The curve continues to decrease very slowly
well past one million cycles and becomes horizontal at five hundred million cycles. So the fatigue limit
for aluminum alloys is typically measured at 500 x 10^6 cycles, where the curve is no longer decreas-
ing. This is way more high stress cycles than a bicycle will ever see.

I should also add that there is typically a lot of scatter in fatigue data. Often a thick band showing the
range of cycles that the material withstood may represent the curves.

The shape of the curves sort of gives aluminum an advantage in the fatigue mode. I think the real high
stress cycles that a bike sees are more likely to be around 10,000 cycles during its expected lifetime
(about 20 years). As aluminum’s published data is typically measured at 500 million cycles, it is con-
siderably stronger at lower cycles. Steel is also stronger at lower cycles, but since it was measured at
one million cycles, the strength improvement at 10,000 is probably not as great as in the aluminum.
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Old 09-29-10, 10:28 AM   #10
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My '99 Specialized Allez had aluminium fork & frame. I hit a car bumper (thanks for turning in front of me, jerk!) hard enough to crack the frame where the down tube meets the head tube. No visual damage to the fork (not that I would ever use it knowing what happened to the frame).
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Old 10-03-10, 11:44 AM   #11
fuzz2050
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it might go back to that viscount 'death fork'.
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Old 10-03-10, 12:34 PM   #12
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Quote:
Originally Posted by Trakhak View Post
Aluminum rims are widowmakers, too; that's why you all ride steel rims exclusively.
I believe spokes are typically still steel and most of the hub is steel (especially bearing surfaces). Cute sarcasm though.
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