Modern simulation tools and digital design aids allow carmakers to sculpt exteriors without building prototypes or carving clay models for wind-tunnel evaluation. There are applications for creating and developing engines, transmissions, and chassis components. But when it comes to the fine art of designing interiors, the tool of choice is the 50-year-old contrivance shown here called an H-Point Machine (HPM).
The fundamental data point revealed by this human surrogate is the precise location of a seated occupant’s hip joint, hence the name. Why does that matter? Because humans touch more parts of the car than would a rabid octopus. The design process thus begins at the H-point with the noble aim of comfortably accommodating occupants, but it quickly encompasses roof height, door openings, passenger restraints, airbag operation, collision performance, outward visibility, and driver fatigue. The list goes on.
The HPM consists of two molded-plastic pans that mimic the backsides of a human’s thighs, buttocks, and torso. True to life, these components hinge about the H-point, which is precisely indicated by crosshairs. Steel components serve the skeletal role, providing a means of connecting lower leg, ankle, and foot proxies and placing weights at appropriate locations. Bubble levels and protractor scales facilitate installation and measurement tasks. The HPM is designed to compress the seat cushion and backrest exactly like a human; the beauty of this tool is that it never needs a restroom break or a day off.
The HPM mimics a 50th-percentile, 69.1-inch tall man weighing 172 pounds. Lower-leg components are adjustable, ranging from 10th- to 95th-percentile male lengths. In its standard J1100, the Society of Automotive Engineers (SAE), which manufactures and sells the HPM, defines dozens of interior measurements. They’re in wide use by the EPA (to define vehicle classes), NHTSA (to position dummies for crash tests), and ISO (International Organization for Standardization). The passenger-volume figures we report also come from the HPM via the SAE’s J1100 standard.
One of the most interesting dimensions revealed by HPM measurements is what SAE calls H5: the vertical distance from the ground to the H-point. While that statistic speaks volumes about the character of any car or truck, it’s rarely reported. To shine light on this shadowy area, Art Center College of Design authorities Stuart Macey and Geoff Wardle published an instruction book, H-Point: The Fundamentals of Car Design & Packaging. Here’s one of their survey tabulations:
Vehicle Type: H-point Height (range in inches)
Sports car: 11.8–13.8
Passenger car: 15.7–19.7
Minivan: 27.6–29.5
SUV: 27.6–31.5
Large truck: 27.6–37.4
Supporting our curiosity in this area, the SAE has assigned Hieronymus—the HPM shown here—to our garage. This new staff member will help compile our databank of H5 measurements. We’ll also use this handy tool to quantify exactly how much roof pillars diminish the driver’s outward visibility. Thanks, Hieronymus, for all that you reveal.
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