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Mechanical Engineering Consulting Firm Solves an Axle Failure

Attorney Requested Mechanical Engineering Consulting and Litigation Support

Glew Engineering provides mechanical engineering consulting to high technology and industry.  As is often the case, an attorney asked us to provide an independent 3^rd party review of the cause of repeated torsion axle failures on a semi-truck trailer that his client experienced.  We obtained the failure information and photographs from the client and researched the trailer and its axle configuration.  As mechanical design experts, we formulated an unbiased opinion on why the trailer’s axles failed multiple times despite being under the rated loads.  For the final phase of our litigation support, we wrote and expert report documenting our findings.

Litigation Support for a Mechanical Failure

The trailer manufacturer built the client’s trailer with three torsion axles each rated at 7,000 lbs of load.  When the first axle failed, the manufacturer installed three new 8,000 lbs axles.  Then one of the 8,000 lbs axle failed.  The photographs showed inordinate tire wear on the inside of the tires.  Numerous weigh slips proved that the trailer never had loads greater than 21,000 lbs.  Furthermore, normal use damaged the frame at the point of axle attachment.  This frustrated the client who needed her trailer in working order as her livelihood depended on transporting supplies to horse shows.  Despite multiple attempts to resolve the problem with the trailer manufacturer directly, she was unable to a get her new trailer in useable condition.  The client resorted to filing a law suit and we provided mechanical engineering consulting and litigation support.  Using a licensed mechanical engineer is beneficial for testimony in litigation because the courts give them more weight than unlicensed engineers. Advantages of using a licensed engineer

Mechanical Design Comparison of Torsion and Leaf Spring Axles

A leaf spring suspension system uses a series of leaf springs bound together which allows a rigid axle to move up and down.  With a leaf spring suspension, wheels move together with the axle, not independently from each other.  See Figure 1 Truck Leaf Spring Axle.

A torsion axle has a trailing arm mounted perpendicularly to the end of the axle upon which the wheels are mounted.  The arm relies on the torsion spring force between it and the axle, which allows each wheel to move independently.  Hence, the phrase “independent suspension.”  With torsion axles, the full load weight shifts to one axle (or one side) for short periods of time when traveling.  With a tri-axle system, when a person drives over a pot hole in a road, the first wheel that encounters the pot hole extends down into it, while the following wheels do not extend downwards until they reach the pot hole.  While one axle and wheel is down the pot hole, the other two axles must carry the full load.  Conversely, when one axle encounters a bump, it disproportionately carries more load than the other two.  Essentially, each wheel is challenged one at a time by the road conditions. Torsion axles generally produce less sidewall wear on tires than a leaf spring suspensions.  Due to Improvements in mechanical design,  most vehicles produced today use torsion axles. See Figure 2 Torsion Axle. Below are examples of torsion and leaf spring axles. 

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Figure 1  Leaf Spring Axle

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Figure 2 Image of a torsion axle, independent suspension.

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Mechanical Engineering Consulting Evaluation Rules Out Excessive Weight

The axle manufacturer rated the original axle at 7,000 lbs.  After the original failure event, the trailer manufacturer replaced the axles with new 8,000 lbs. axles made by the same axle manufacturer.  Photographs of the tires after both incidences on the CargoMate™ showed excessive wear on the inside of the tires, resulting in a smooth surface on one side.  

The manual indicated that this type of failure was due to loss of camber or overloading.  A mechanical engineering consulting investigation was able to rule out overloading.  The user provided copious weigh slips that demonstrated that the trailer was not overweight.  The loss of camber is consistent with torsion-axle failures.  This would generally be true regardless of the brand of torsion axle.

De-Rating of Multi Torsion Axles

Torsion axles must be de-rated when used in multi-axles configurations.   Another manufacturer learned this simple concept through hard experience.  According to the information provided within the Tie Down Engineering technical service bulletin,

“Tie Down recommends the down rating of each tri axle by a factor of 20% from the designated axle model rating.” (Emphasis added.)

Using this de-rating recommendation, the following formula was used to determine the GVWR (gross vehicle weight rating) of a tri-axle trailer:

• GVWR3= number of axles * de rating factor * axle rating = 3*(100%-20%) / 100%*W1 = 2.4 * W1
• GVWR3= 2.4 * W1 where,
• GVWR3: gross vehicle weight rating of a triaxle trailer, lb
• W1: Single axle rating, lb


The first column of Table 1, labeled W1 (lbs), shows the load rating for a single axle in a single axle configuration.  The second column of Table 1, labeled GVWR3(lbs), shows the load rating for a total of three axles in a tri-axle configuration.   

Table 1 De-Rating of Tri-Axles

Litigation Support Analysis and Mechanical Design Conclusion Regarding Tri-Axle De-rating

Since the client needed to haul 21,000 pounds of supplies, which the manufacturer claimed the trailer could reliably carry. Our mechanical engineering consulting efforts showed in the litigation support analysis that manufacturer should have equipped the trailer with three 9,000 pound rated axles, not three 7,000 pound axles.  Tri-axles need to be de-rated by 20% to accommodate for single tire excursions.  Or in other words, the sum is less than the parts!  

The manufacture made an error in mechanical design by not de-rating the axles in the tri-axle configuration. The specification and recommendation by the trailer manufacturer  resulted in the axle failure and excessive tire wear.

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