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Lightweight Fabrication Basics, Build Tips, & Techniques

Posted in How To on August 30, 2013 Comment (0)
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In the four-wheeling world, strength and reliability are key to survival. Getting out there and back over rough terrain requires a vehicle that can handle the rigors and abuse. Factory production vehicles incorporate strength and stiffness in body panels, frames, and suspension components. This can be derived from intricate sheetmetal stampings combined with welding or epoxy bonding to build yet more complex and rigid assemblies.

But, we seldom leave our vehicles stock, or we may fabricate portions or whole vehicles for off-road use. Choice, size, and shape are all considerations for building structures. We want to increase strength but keep weight down. Lower weight means more horsepower per pound, better vehicle handling, and typically better overall survivability when hitting the dirt.

Here, we’ll look at some of those materials and how they’re used to help us get more performance out of our rigs. Some knowledge of fabrication materials and their use can be helpful if you do your own work, or hire someone to do custom work for you.

Steel vs. Aluminum
The two most common metals used for automotive manufacturing or fabrication are steel and aluminum. With cost-effective methods and stronger types of aluminum casting prevalent today, many vehicle manufacturers have replaced some former steel components with aluminum. These include wheels, A-arms, some body panels, and soon to be frames! Modern casting methods and efficient CNC machining can create more complex 3D shapes designed to maximize strength, while keeping the weight down.

Steel weighs approximately 480 pounds per cubic foot, or 0.28 pounds per cubic inch. So, a 12x12x1-inch-thick steel plate weighs about 40 pounds. Aluminum is about 170 pounds per cubic foot, or roughly 1⁄3 the weight of steel, by volume. So, in the right applications, the use of aluminum can offer considerable weight savings over the use of steel. It can be machined at far faster rates than steel, but does require more sophisticated welding techniques.

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Tube Structures
Structures built from tubing are common in our world, be it in the form of a chassis, cage, bumper, or suspension links. Round steel tubing serves many purposes. Carbon steel tubing is available most commonly as HREW (hot rolled electric welded). A step up in hardness and strength is DOM (drawn over mandrel) tubing. 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 greater rigidity and may allow the use of lighter wall tubing in some chassis areas. However, use of chromoly requires more specific welding techniques.

Round tube is most commonly used for chassis and cage building and it provides equal resistance to bending anywhere along its circumference. When considering bending moment strength of tube for structures such as tie rods or suspension links, know that the bending strength is a function of (D4-d4), where D is the outside diameter (od) of the tube and d is the inside diameter (id). Run some quick calculations and you’ll quickly see that increasing outside diameter typically has a larger impact on bending strength than simply increasing tube wall thickness. Applications require balance between rigidity desired, fitment room, and resistance to damage from obstacles the tube may contact.

Tube OD (in) Wall Thickness (in) Relative Strength Weight per FT (lbs)
1 0.120 1 1.1
1 0.250 1.4 2.0
1 0.500 (solid rod) 1.5 2.6
1.25 0.120 2.1 1.4
1.25 0.250 3.2 2.6

The table above shows the relative strengths of several tie rod size examples. It’s easy to recognize that strength improves 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 od more than doubles the strength, from a relative strength of 1 to 2.1. However, using solid rod over tube provides little added strength compared to the significant increase in added weight.

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Bracing and Plating
When building tube structures, careful layout and design can maximize strength. This is most commonly an important goal when building rollcages. Here again, structural layout should be considered in all three-dimensional planes. Light-duty sport cages may be more simply designed. But, it’s best to bisect square or rectangular tube rollcage structures with another tube. This turns the four-sided shape into two triangles, which are far more robust shapes.

Additionally, rollcage tubes are stronger when their ends can be made to converge together in a common “node.” Each tube ending here contributes support to all the other tubes. Short tube braces or plate braces added to corners that could be impact points in a rollover can further strengthen these areas.

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Dimple Dies
When building steel or aluminum sheet structures, rigidity and strength can be improved with dimpling. Circular dimple dies come in a male/female pair. Once a hole is cut in a piece of sheetmetal, a press is used to push the dies together onto the sheet and flare the hole.

We all know a flat sheet of any material flexes relatively easily, but by pressing dimples into the sheet and giving it some height, we can add much more rigidity to a sheet metal structure. It’s one thing if you need sheer metal thickness for something like a skidplate or other armor. But, for chassis areas where you mostly need to box some structure, you can usually use a thinner sheet of metal that has dimples in it in place of a thicker, flat sheet. You can get the same structural strength while shedding some weight.

Here’s a pair of dimple dies used to flare circular holes in sheet steel or aluminum. Starting with a cut hole (left), a press is used with the dies to form a flared hole (right) and provide added rigidity to the sheet of metal. Here’s a pair of dimple dies used to flare circular holes in sheet steel or aluminum. Starting with a cut hole (left), a press is used with the dies to form a flared hole (right) and provide added rigidity to the sheet of metal.

Fastener Grades
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. Bolts are specified by SAE, ASTM, or ISO standards from automotive and engineering governing councils. The tables on the next page list ratings for common SAE and metric sized bolts.

A grade 5 bolt (center) will have three radial marks on the hex head. Grade 8 (right) will have six marks. Grade 2 (left) will be unmarked or have some other designator. Stick with Grade 5 or Grade 8 bolts (or equivalent metric grades) for automotive use. Tensile strength is greater than yield strength for a bolt. A common misconception is that it’s 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. When in doubt, use Grade 8 or better. fw A grade 5 bolt (center) will have three radial marks on the hex head. Grade 8 (right) will have six marks. Grade 2 (left) will be unmarked or have some other designator. Stick with Grade 5 or Grade 8 bolts (or equivalent metric grades) for automotive use. Tensile strength is greater than yield strength for a bolt. A common misconception is that it’s 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. When in doubt, use Grade 8 or better. fw

SAE

Bolt Grade Nominal Size (in) Min. Yield Strength (psi) Min. Tensile Strength (psi)
2 1⁄4 thru 3⁄4 57,000 74,000
Over 3⁄4 thru 11⁄2 36,000 60,000
5 1⁄4 thru 1 92,000 120,000
Over 1 thru 11⁄2 81,000 105,000
8 1⁄4 thru 11⁄2 130,000 150,000
18-8 Stainless 1⁄4 thru 5⁄8 40,000 Min. 80,000 – 90,000 Typical 100,000 – 125,000 Typical
3⁄4 thru 1 40,000 Min. 45,000 – 70,000 Typical 100,000 Typical

Metric

Class Nominal Size (mm) Min. Yield Strength (Mpa) Min. Tensile Strength (Mpa)
8.8 All Sizes below 16mm 640 800
16mm - 72mm 660 830
10.9 5mm - 100mm 940 1040
12.9 1.6mm - 100mm 1100 1220
A-2 Stainless All Sizes thru 20mm 210 Min. (450 Typical) 500 Min. (700 Typical)

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