Looking at The Whompy Wheel.

I am a mechanical engineer, and if I am going to look at the Whompy Wheel thing it is through the prism of what I know about designing mechanisms so prepare to meet some wonkish explanations on way out.

Firstly, let’s look at the way Tesla cuts cost.

Tesla Rear link One.jpeg

The dirty part is what was initially used in rear suspension assembly, the second clean part is the replacement.  When an engineer looks at the early part he sees that it was extruded and then sliced (sawed-off) to thickness.  This is as cheap as it can get in manufacturing.  Extruded parts are the most inexpensive to manufacture.  The part has multiple cavities in part to keep the wall thickness constant (as nearly as possible) and/or to lower the weight of the part.  When you are in engineering school they tell you that if you apply load to a part imagine that stress flows through the part as if it would be liquid. Any sudden change of direction, cross-section or sharp corner creates stress concentration.  Stress concentration is a condition where stress and the possibility of failure is larger than just the stress obtained by dividing the force by cross-section area (average stress at the point) would imply.  The replacement part seems to be nonplanar, it is probably die-cast or even forged so it seems that the initial part (which is in one plane only) uses rubber to adjust to the 3D mechanism, though this could be just optical illusion on my part.  The consequences of the using rubber at the pivot points was simply explained by the gentleman who had commented when posting the picture.  The new part seems to be a better design by shape and load-carrying capacity. It is plain from this example that Tesla’s philosophy of design is to shoot for cost-cutting as the primary goal in design and then leave the questions of reliability to be answered later.  

Another picture tells another 1000 words.

 

teskla drive two.jpeg

 

This is rear axle assembly I think from Model X.  We can identify here two links, most likely extruded, which failed. On first look, Tesla uses cast or die-cast parts for structural elements and some links in the suspension mechanism.  Cast or Die cast parts hold the punch of being inexpensive to manufacture relative to multiple features and complex shapes they can have built-in them.  The rub is the properties of materials as they are cast.  When you melt metals atmospheric gases and other inclusions (oxides) dissolve in the molten metal and when cooled they form bubbles and flaws in solid parts. Also varying rates of cooling build in stresses in parts. These problems are inherent in the process of casting.  In general, these cast materials are more brittle than materials processed by forging, cold-rolling and machining.  From the picture, I have the impression that the materials are probably alloys of aluminum.  Since aluminum alloys are 3 times more elastic than steel and 1/3 strength of steel designer has to create beefier parts ( more involved than this alone).  The danger of more brittle materials in suspension is that these materials are more likely to catastrophically fail (Whompy Wheel) then forged, cold-rolled or machined components, and without warning due to susceptibility to form cracks (leading to failures) because of inherent flaws, surface corrosion, fatigue (repetitive loading), shock loading (impact) and stress concentrations.

The first exhibit points to a tendency to place cost-cutting as the primary objective in taking design decisions.  As properties of cast materials can be improved by additional processing (but not entirely ameliorated) this adds up quickly to higher price of components.  For the explanation of the Whompy Wheel phenomenon, the adoption of cast parts for suspension design as a cost-cutting measure can be seen as the simplest one leaving other aspects of the design sound, though the extruded links seem to fail as they are indeed the weakest links.

 

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