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Two “new” Compass reviews

T4OG: Trans Four Owner’s Group

Mopars of the 2017 NYC Auto Show
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Car platforms and bodies

Dodge Aries in CAD diagram (before CATIA)Retired product planner Burton Bouwkamp wrote: “We (Chrysler engineers) defined a platform by the horizontal and vertical dimensions from the centerline of the front wheels to the cowl (base of windshield) and to the H point (the hip joint). Said another way, the cowl position and driver position relative to the front wheels defined a platform. The K and E bodies were the same platform.”

He added,

In general, we called it a different body if the windshield and the surrounding “A” pillars were different. A different platform had a different front track and a different position (“H point”) of the driver relative to the front wheels. By these definitions, the M Body was on the same platform as the F Body; and if it had the same windshield and cowl, it was a stretch for us call it a different body.

Bob Sheaves wrote that this method continued until the use of CATIA, phased in around 1990 with the T3000/BR (1994 Dodge Ram) program; existing projects used the proprietary Chrysler CAD/CAM system, with the older platform definition, while new projects started with a definition of the platform as the suspension, driveline, and (the single most expensive component) the pan stamping, which was the floorpan and firewall.

At the same time, the platform’s starting point was moved to one meter forward of the front wheel, rather than the center, to end the use of negative numbers.

In 1998-2001, the term was again redefined to exclude specific components, and referred solely to dimensions.

Bob Sheaves added, “By the logic some people express, such as the Prowler using the LH platform, one could also say the Viper was built from the Dakota platform, or the minivan shares the Neon platform.” But, Bob continued, a platform is defined by a set of dimensions, not the powertrain, architecture, or appearance. Thus, the Plymouth Reliant was not on the same platform as the Plymouth Acclaim, though they have numerous similarities in other ways, and the Acclaim is based on the Reliant.

A platform only defines the space required to assemble the parts and nothing more.

A chassis is only the structural parts that attach the suspension, brakes, powertrain, steering, fuel, and ride control systems to the body.
— Bob Sheaves

This means that a Dodge, Fiat, and Jeep can have the same platform, yet have entirely different personalities. Parts sharing does not mean platform sharing, and platform sharing does not mean parts sharing.

The Reliant, 600ES, and Limousine, despite having major differences in appearance, handling, length, height, etc., were on the same platform (as defined at the time), and would be even if they did not also have very similar suspensions and engines. Very different vehicles can be on the same platform and very similar ones can be on different platforms — the first generation Neon and Stratus being an example of the latter.

As time went on and technology changed, new definitions were adopted. A platform is now defined as a collection of fixed hard points, so that different vehicles with the same points can be built on a single assembly line, with similar crash characteristics. This greatly cuts development costs and increases factory flexibility, so that the product mix can be altered very rapidly without additional cost as customer tastes change.

AutoTechnician added:

The platform is a set of dimensions. It dictates the physical maximum/minimum size some parts of the vehicle have to be in order to fit on the conveyors, lifts, jigs, etc... It dictates the locations of where various hardpoints have to be so the robots can grab, manipulate, machine and weld the various body parts and hoist components into place.

You could have two wildly different vehicles on the same platform, not sharing a single common component. If you had a sub-compact and a big mean solid-axle pickup truck on the same platform, and you'd be able to build them in the same factory on the same robots because the critical hardpoints and dimensions are the same.

The architecture is what goes inside those dimensions. That is the chassis, floor pan, body, engine, transmission, suspension, etc...I suppose in theory you could even have an architecture with hardpoints that could work on two different platforms (such a thing might be useful for flex purposes?).

All manufacturers have a platform, and an architecture. A lot of confusion occurs because not all manufacturers use the same terminology as Chrysler, and it gets compounded over language barriers where some words might not have an exact literal translation to English. I've seen Volkswagen engineers talking about what is clearly an architecture as a “platform” and vice versa from other makes. Just remember, in Chrysler-land [and at GM and Ford]: Platform = Dimensions, Architecture = Parts that goes inside those dimensions.

Definition of a beehive

Bob Sheaves wrote:

The wheelhouse opening is determined by cycling the suspension and steering, with largest available tire from the product planning group parameters, in what is called a “beehive” to determine the clearance zone for tires, chains (if specified), offset of wheels, etc.

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Two “new” Compass reviews T4OG: Trans Four Owner’s Group Mopars of the 2017 NYC Auto Show