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Strengthening Your Off Road Truck

Posted in How To on February 1, 2010
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You're probably looking at the title to this piece and saying to yourself, "Bracing what?" Well, hold on a minute and allow us to explain. If you're like us, you like to get out in the sand or rocks or mud or whatever you wheel in and have some fun, while looking for a challenge. But before you leave pavement you'll want to ensure your rig has been built to take the abuse you're going to dish out to it.

This means adding strength where strength is needed. So, we'll discuss some of the areas that may need some attention, discuss some of the fabrication options, and show some actual examples of building to survive.

Much of our vehicles are made of metal and when choosing type and size there are several characteristics to consider. Material comes in a variety of shapes. For example you can have round, square, or rectangular tube, or flat strap steel or plate. Each of these has different uses and strength characteristics.

At first it may make sense to make structures as strong as possible to avoid breakage. However, this results in the excessive use of material, driving up both cost and weight. We'd like to minimize both of these where possible. Remember that weight steals speed and places greater strain on parts.

Also, recall that any grouping of components is only as strong as its weakest link, so leaving a substandard part in the chain can cause it to break too easily. Whenever one part is made stronger, you'll need to consider where the next weak point might be. If something breaks, rather than just patching it together and indiscriminately beefing it up, look to see how it failed and what may be the future consequences if something is changed.

Strength In Shapes
Some geometric shapes are inherently stronger than others and can better withstand forces before failure. For instance, when building a rollcage or other structure, there are several ways to configure the individual pieces to make the whole structure. In 'cages, triangles are inherently the strongest shape, and strategic bracing will help ensure vulnerable impact points can take a beating and distribute the force into the structure.

Tubing Strength
How does the size and wall thickness affect how well a tube resists flexing? Much is based on the outside diameter of the tube. 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
THICKNESS STRENGTH
1" 0.120" 1
1" 0.250" 1.4
1" 0.500" (solid rod) 1.5
1.25" 0.120" 2.1
1.25" 0.250" 3.2

This table shows the relative strengths of several tie rod size examples. It's easy to see 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 bet the solid rod is heavier, adding unnecessary weight. As we mentioned before, we want great strength but without making everything excessively costly or heavy.

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