|Steel versus Aluminum
A frequent question about structures generally and Sled decks specifically is:
“Why don’t you make it out of aluminum, so it’s lighter?”
The short answer for a sled decks is:
“It wouldn’t be any lighter, but it would be much more expensive.”
The short answer for structures generally, is:
“It wouldn’t necessarily be any lighter.”
The aluminum-vs-steel, strength-vs-weight issue is one that is commonly
misunderstood, especially among sled deck makers, sometimes even among
The misconception arises from the partial truth that there are aluminum alloys that
are as strong as, or stronger than, steel. It’s more accurate to say that there are
some aluminum alloys that are stronger than some steel alloys.
One of the most widely used high-strength aluminum alloys is alloy 6061 with T6
temper. Pound for pound, 6061-T6 is stronger than some steel alloys, but not as
strong as others. The fact is, for any given high-strength aluminum alloy, there are
higher-strength steels that outperform aluminum in strength-to-weight. Factors
other than strength and weight eventually dictate which material is a better choice
for a particular application.
For sled deck design, another of those other factors, a very important one, is
stiffness. And as we will see:
There is no stiffness advantage in using aluminum over steel.
Strength refers to the maximum load that a material can be subjected to without
Stiffness refers to how much a material bends when a load is applied.
Stiffness is quantified by a parameter called Modulus of Elasticity. Without getting
overly technical, we can look at the relative stiffness of steel versus aluminum by
comparing this parameter:
Aluminum’s modulus is about 10 million psi.
Steel’s modulus is about 3 times that: 30 million psi.
That means, for a common structural shape used in a sled deck design, (for
example, a 2-inch square tube), and for the same limit of bending with the same
load, the wall thickness of an aluminum tube would need to be more than 3 times
the wall thickness of a steel tube.
Steel is about 3 times heavier than aluminum. (Steel is about .3 pounds per cubic
inch, aluminum is about .1 pounds per cubic inch.) So the aluminum tube with the
thicker wall ends up weighing the same as the thinner-walled steel tube, for the
This means: For a typical sled deck design, it will not be any lighter if it's made of
aluminum than if it's made of steel!
Note that this is regardless of the alloys chosen--the modulus is pretty constant.
For steels, it varies from about 28.5 to 30 million psi; for aluminum it ranges from
9.9 to 10.3 million psi. So “high strength alloys” offer no stiffness advantage.
Clearly, the choice of steel or aluminum for a particular application requires
engineering scrutiny beyond the misleading "aluminum is lighter" assumption.