December 30, 2016

Why E46 Subframe’s Fail

The BMW E46 3 series is notorious for its defective chassis design affecting all years and model variants. The most common chassis component being the rear axle carrier panel, more commonly referred to as the ‘subframe’.

The structural failure is evident in the case of cracks forming around the mounting points, wheel arch joins and can spread further throughout the car to the point it is even visible within the passenger cabin and boot space.

           

 

The documented structural failures are most evident in the M3 models. The differing factors between M3 models and less powerful E46 models is the engine and drive line. The M3 had a high power 3.2L straight six stated to have an engine output of roughly 342hp which is significantly greater than the lower range models.Subframe failure is often discovered when the chassis will begin to creak under on and off throttle input. The car will begin to feel numb, un-predictable and tend to wander on the road in extreme cases. This is not only a massive safety risk to the occupants but massively compromises the rigidity and integrity of the chassis.

The most common points of subframe failure are the front right and the rear left subframe mounting points. Shown is an example of the possible extent of damage at the time the car will begin to creak. The vehicle pictured was a 2002 model M3 with just under one hundred thousand kilometres (62miles).

   

 

In extreme cases the subframe material can break free from the chassis leaving cavities. In the instance shown below the female thread insert had no remaining supporting welds. Vehicle pictured is 2003 model M3 at 153,000 kilometres (94.9 miles).

 

In order to determine why these mounts are a common and catastrophic point of failure, we must first analyse the forces applied.

When isolating the analysis to the engine output forces, we can see that the torque about the engine axis and the perpendicular torque about the wheel axis develops a combined action on the common fail points.

The front right experiencing a combined compressive force (into the chassis) and the rear left experiencing combined tensile force (pulling out of the chassis) from the torque about the engine and resultant torque about the wheels. The forces applied to the remaining mounts are opposing one another. Although these forces are not equal due to the final drive ratio, it is why they are less prone to failure.

 

The resultant torque about the wheel axis is why it is often seen that the boot floor and wheel arch eventually separate from the chassis rails can be seen to move under engine load. The rear two subframe mounts are directly beneath the mid-section of the boot floor at the furthest edge of the wheel well.

The above analysis and prediction that the applied forces due to engine output are the result of failure is supported by many documented failures as well as why the higher output E46’s such as M3’s or even heavily modified E46’s seem to be the most common to fail.

The direction of the applied forces is also evident when analysing failures as the front right mount is occasionally lunged into the chassis slightly whereas the rear left will begin to peel away from the chassis rails. The images shown in the below where the force transmitted through the factory subframe bushings has developed an indent where it mates to the underside as it has deformed into a cavity between the female thread insert and the underside sheet metal.

 

It is believed that these forces simply exceed that which the chassis is able to withstand within its service life.

The causing of crack propagation is the result of metal fatigue. Metal fatigue occurs when the material is subject to repeated loading and unloading and thus flexing, causing brittleness. The cyclic loading experienced by the subframe would be the result of impulsive forces applied from varying throttle input, violent gear changes or other abusive driving. The more the subframe fails and deforms, the more shifting and flexing occurs, constantly accelerating the severity of the failure.

Several other predictions have been made as to what other factors contribute to the E46 subframe failure being so common.

From personal experience, we believe the front right subframe mount bushing design contributes to the failure. The bushing transfers the compressive loads into the chassis via small narrow lips protruding around the stud onto the underside of the subframe panel. These lips offer too small an area to transfer the loads experienced onto a metal surface that is too thin. The subsequent stresses exceed the material capability initiating the fatigue process.

As shown in figure 9, the stress is also localised directly atop spot welds causing them to fail and subsequently provide a weak point for the initiation of crack propagation. Once the crack is formed, it will grow perpendicular to the stressed experienced.

When designing for cyclic loading and impulsive forces, it is possible to design products to a theoretical infinite service life meaning, in theory should never fail. These products can fail due to uncontrollable variables such as material imperfection or abusive operating conditions going beyond the conditions intended during the design process however, failures would be extremely few and far between and if necessary could be quarantined.

Given the frequency and short lifespan till subframe failure occurs, it would be fair to assume that the E46 chassis has a limited life cycle, like most modern products. This life cycle is dependent on two variables; magnitude and exposure. The designed life cycle has proven to be shorter than anticipated and in extreme cases some failed within the new car warranty period.

In an effort to avoid withdrawing the model, remedies were developed such offering replacement rear axle carrier panels with additional spot welds (the latest revision being Nov-2004) and filling the cavities of the failure prone areas with an expansive epoxy foam substance to potentially limit elastic deflection and thus prolonged metal fatigue. Given the design itself has not been changed, it is inherently still flawed and prone to fatigue.

Without major design corrections, such remedies did not eliminate the structural design flaw or loads paths which are proven to be unreliable and merely extended the time till the inevitable failure occurs again.

In order to achieve reliable and ongoing rigidity and structural integrity at or above higher power outputs, specially engineered systems that utilise the necessary design corrections are required.

For further information, as to how the subframe issues occur and how our products are specially designed to compensate for the above issues as well as several other benefits resulting from their function please visit the technical section relevant to each kit.

http://cmpautoengineering.com/cmp-engineering-subframe-solution/