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Metal Weights and Strenghts - Mastering Materials

Posted in How To on January 1, 2009
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Building vehicles in the off-road world is all about taking raw materials and fashioning them into a functioning machine. Part of that process involves the selection, fabrication, and use of metal structures. A little knowledge of the subject can go a long way towards understanding choice and use of these metals. Here we'll consider the weights and strengths of common materials used to build our work trucks, commuters, race rigs, and play toys.

Steel VS. Aluminum
The two most common metals in use on our vehicles are steel and aluminum, so we'll confine much of our discussion to them. Weight, strength, weldability, and machinability all play a part in deciding which of the two you may want to use for a specific application.

Steel weighs approximately 480 lb. per cubic foot, or 0.28 lb. per cubic inch. So, a 12 x 12 x 1 inch thick steel plate weighs about 40 lb. Aluminum is about 170 lb. per cubic foot, or roughly one-third the weight of steel, by volume. Knowing these numbers, you can easily calculate the weight of a designed structure.

When you go to a local metal supplier for materials you'll often find a dizzying array of products available. If you're simply fabbing up a simple bumper, metal choice may not be particularly critical. However, if you're building a cage for a high speed race truck, you'll want to know a bit more about the various material grades and properties.

Steel products are often sold in whole 20 ft. or similar lengths or can often be purchased by the foot at a bit of a price premium over full lengths. Most suppliers that do industrial cutting to order will have remnant pieces that are often sold at bulk prices by weight. This is a good way for the home fabricator to pick up an assortment of material sizes.

Sometimes it makes more sense to machine a component from solid aluminum than it does to build it from welded steel components. These UTV a-arms are solid material but the lighter weight of billet aluminum as compared to steel works well in this situation.

Here you can see a complicated suspension structure built from a number of steel sheets and tubes that have been welded together. This construction provides a very strong member without the heavier weight of a solid steel structure. Of course, cost to build this more complicated structure is higher than if it were one solid casting.

Weights
We can calculate the weight of a structure we're interested in by knowing a few simple facts. First, we need to determine the density of our material (weight per cubic inch) and then we simply multiply that number by the volume of the material in our structure.

Here are the approximate densities of some common metals for calculation and comparison. You'll see metals such as titanium and magnesium are considerably lighter then steel but offer excellent strength characteristics in competition applications where the higher cost of these metals can be justified.

When using aluminum, heat-treated alloy types such as 6061 (common for sheet goods) or 7075 (common for aluminum links) varieties are often used for automotive fabrication. Pure aluminum is relatively soft and of low overall strength. Alloys of aluminum are considerably stronger but can still be cut, drilled, tapped, machined and welded.
Material Weight per Weight per
  cubic inch cubic foot
Steel 0.28 lbs. 480 lbs.
Cast Iron 0.26 lbs. 446 lbs.
Aluminum 0.10 lbs. 170 lbs.
Titanium 0.16 lbs. 281 lbs.
Magnesium 0.06 lbs. 112 lbs.
Copper 0.32 lbs. 560 lbs.
Brass 0.31 lbs. 531 lbs.

Some common structure volumes can be calculated to find their weight per foot as shown below. The volume is calculated by determining the cross-section area (gray) and then multiplying by the length.

There are two basic types of structural steels used for fabrication: hot-rolled steel and cold-rolled steel. These terms simply mean the metal shapes were formed when the steel was hot or cold. I-beams, channel, angle, solid square and some round shapes are usually hot-rolled. Seamed round and square tubes are also hot-rolled and formed. Smooth rounds, squares and sheets are often cold-rolled.

Structure Volume
Flat Plate W x H x Length in inches
Box tube ((W x H) – (w x h)) x Length in inches
Round Tubing ((.5D)2 – (.5d)2) x 3.1415 x Length in inches


Material Grades
It's not enough to simply decide whether to use steel or aluminum for a project. Amongst those metals is a wide variety of grades that further designate their hardness or stiffness due to heat treatment, and also their machinability and weldability.

For instance, if you're making aluminum body panels for a race truck you could find aluminum sheet at your local home improvement store. But pick up a sheet and you'll find it to be flexy and easily creased or dented. By contrast, heat-treated grades found at an industrial metal supplier offer much greater stiffness and overall strength. Cost will be higher, but weight is essentially the same for both grades.

You can quickly see that the DOM and Chromoly varieties offer greater strength than the standard welded tubing. These materials are harder, will deflect less when impacted, and have more tendencies to spring back to their original shape. They are also harder to bend during fabrication as well. Heavy wall DOM tubing is especially common for use on suspension and steering links.

Tubing Types
Tubing is available most commonly as HREW (hot rolled electric welded) or ERW (electric resistance welded). A step up in hardness and strength is DOM (drawn over mandrel) tubing or CDS (cold drawn seamless). This tubing has undergone the additional dimensional forming where the welded seam is rolled flat and the entire structure is hardened. Finally, there is chromoly steel tubing which offers the most rigidity and may allow use of lighter wall tubing in some chassis areas. However, use of chromoly requires more specific welding techniques.

Strength rating for common steel tube types is listed here:

Tubing Material Yield Strength Tensile Strength
1010 ERW 32,000 psi 45,000 psi
1020 DOM 70,000 psi 80,000 psi
4130 Chromoly 70,000 psi 90,000 psi

Mild steel welded tube is often considered sufficient for building cages for non-race use. Some racing bodies require the minimum use of DOM or chromoly tubing and have specific requirements related to wall thickness and tubing layout.

The two most applicable specifications are yield strength and tensile strength. Yield strength is defined as the maximum load at which a material exhibits a specific permanent deformation. Tensile strength is defined as the maximum load in tension (or pulling apart) which a material can withstand before breaking or fracturing.

Tubing Weights
The following table lists weight per foot for common round tubing sizes.

O.D. Wall Thickness Weight (lbs.)
(inches) (inches) per foot
1 0.065 0.6
1 0.095 0.9
1 0.120 1.1
1 0.250 2.0
1.25 0.065 0.8
1.25 0.095 1.2
1.25 0.120 1.4
1.25 0.250 2.6
1.5 0.065 1.0
1.5 0.095 1.4
1.5 0.120 1.7
1.5 0.250 3.3
2 0.065 1.3
2 0.095 1.9
2 0.120 2.4
2 0.250 4.6

Bending Moments
When we're dealing with loads such as those placed on a suspension arm as a vehicle lands from a jump or the case of a steering tie rod being pushed into the face of a rock, we experience bending forces.

Strength is derived from two dimensions for a tube of a given material: outer diameter and wall thickness. Thicker wall translates to greater resistance to denting or other impact damage. Of course, as the diameter or wall thickness increases, the tubing increases in rigidity. However, size and weight restrictions limit inordinately large sizes in most applications.

Let's take a quick look at some relative strengths of some sample tubing sizes. Say you have a tie rod made of 1 inch diameter, 0.120 inch wall thickness tubing. We'll consider this to have a normalized strength of 1. This represents the force necessary to deflect the tube some distance or the amount of tubing bend as the result of some applied force, such as pushing it against a rock on the trail. The bending strength is a function of (D4-d4), where D is the outside diameter (o.d.) of the tube and d is the inside diameter (i.d.).

Tube O.D. Wall Relative Weight
Tickness Strength per ft.
1-inch 0.120-inch 1 1.1
1-inch 0.250-inch 1.4 2.0
1-inch 0.500-inch 1.5 2.6
(solid rod)
1.25-inch 0.120-inch 2.1 1.4
1.25-inch 0.250-inch 3.2 2.6

When choosing tube sizes, note that the large diameter tube has greater strength. But, space limitations may restrict the size that can be used. In addition, strength data shown above assumes the load applied to the tube does not distort the tube wall. In other words, the data assumes the tube wall is not dented. If you bash a tie rod tube out on the trail, it will begin to bend with a lighter load than if the tube was undamaged.

This table shows the relative strengths of several tie rod size examples. It's easy to recognize that strength rises with increases in either tubing outer diameter or increases in wall thickness. There are a few interesting numbers to note. First, observe that increases in diameter provide the greatest jump in strength. Just changing the 0.120 inch wall tubing from 1 to 1.25-inch o.d. more than doubles the strength, from a relative strength of 1 to 2.1.

Conversely, excessive wall thickness increases strength to a lesser extent. Added metal towards the center of the tube axis does less to increase strength than increasing diameter does. Hence, a solid rod is only slightly stronger than a heavy wall tube. Note the relative numbers of 1.4 for 0.25-inch wall versus 1.5 for a solid rod. There's not much difference in strength, but you can see the solid rod is heavier, adding unnecessary weight. Take a look at how a lot of race vehicles are built and you'll find large structures built from relatively thin materials to the extent the components can still take whatever contact beating they may need to endure.

Fastener Strengths
It's common to talk about stretching and breaking strengths of fasteners in automotive applications. Bolt strength is often expressed in terms of load (in psi) needed to deform or break a bolt.

Hardware should be chosen based on strength grade. Grades 2, 5, and 8 are common American standards. Grade 2 is low grade hardware and should not be used for automotive applications. Grades 5 and 8 are appropriate for automotive use with Grade 8 being used in most cases. This photo shows from left to right: Grade 2 (unmarked), Grade 5 (three radial head marks), and Grade 8 (six radial marks). Some aftermarket manufacturers also make even higher rated bolts with strength greater than Grade 8 for use in critical high stress locations.

Bolts are specified by SAE, ASTM, or ISO standards from automotive and engineering governing councils. The table below lists ratings for common SAE and metric sized bolts.

You'll see that the tensile strength is greater than the yield strength. A common misconception is that it is better to run Grade 5 bolts as opposed to Grade 8 bolts because a Grade 8 bolt is harder and more prone to snap and break where a Grade 5 will stretch more before breaking. However, the higher grade bolt is considerably stronger in both yield and tensile strength.

SAE Nominal Min. Yield Min. Tensile
Bolt Size Strength Strength
Grade (inches) (psi) (psi)
2 1/4 thru 3/4 {{{57}}},000 74,000
Over 3/4 36,000 60,000
thru 1-1/2
5 1/4 thru 1 92,000 120,000
Over 1 81,000 105,000
thru 1-1/2
8 1/4 thru 130,000 150,000
1-1/2
18-8 1/4 thru 40,000 Min. {{{100}}},000 –
Stainless 5/8 {{{80}}},000 – 125,000
{{{90}}},000 Typical
Typical
3/4 thru 1 40,000 Min. 100,000
45,000 – Typical
70,000
Typical
METRIC Nominal Min. Yield Min. Tensile
Class Size (mm) Strength Strength
(Mpa) (Mpa)
8.8 All sizes 640 800
below 16mm
16mm- 660 830
72mm
10.9 5mm-100mm {{{940}}} 1040
12.9 1.6mm- 1100 1220
100mm
A-2 All sizes 210 Min. 500 Min.
Stainless thru 20mm (450 Typical) (700 Typical)

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