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GM Seat Design Workshop

Written by PFM Staff

During one of the biannual GM Law Enforcement Product Council (LEPC) meetings, GM Fleet invited two of its corporate seat engineers to have a two-way exchange on seat design with this select group of fleet managers. The GM goal was to develop the first production seat specifically designed—from a clean CAD/CAM screen—as a police car seat. The GM seat design workshop was a hands-on exercise involving the current production driver’s seat of the current Impala, the current Tahoe and a generic seat mounted on a buck. The result is the new seat in the 2011 Chevrolet Caprice PPV.

GM Fleet understands how important the seat is—it occupies 80 percent of the interior, it is the single largest interface with the driver, it is the soft side of safety (i.e., it is a definite component in occupant safety), it has the ability to warm or cool the driver and it has to be adjustable to fit a wide range of occupants. Due to all these factors, as well as the cost of development and testing, after the driveline components (engine, transmission and axle), the seats are the second most expensive component of the vehicle.

The GM seat is designed to fit the 95th percentile male on the large size and the 5th percentile female on the small size. This includes variable contact points for the shoulder, upper torso (chest), lower torso (lumbar, hips, tail bone) and thighs. All of these contact points are factored for erect, neutral and slumped-forward driving positions.

Above all, as the top priority, the seat should not produce pressure points as these will quickly become uncomfortable, if not painful. Pressure point mapping software is used to model these pain spots on all locations of the seat for a wide cross-section of driver sizes.

Comfort versus Support

The seat is a complex combination of foam and fabric. Foam is especially critical to both initial comfort and daylong comfort. The various layers of foam differ in the number of layers, the density of the foam and the thickness of the layers. These different layers of different kinds of foam are used in combination to achieve exactly the right feel. And what is that “right feel”? What is that elusive combination of comfort and support?

While the tactics from each manufacturer differ, the strategy is the same—incorporate the initial comfort of a living room easy chair followed quickly by the daylong support of a custom-molded racing seat. This is possible (or almost possible) through the use of different layers of different kinds of foam, different bolster designs and different fabrics.

The basic concept is that the driver “melts” into the seat until just the right amount of support is achieved. The layers of the seat then hold that form. These top layers may be “less dense” foam that easily compresses or foam that compresses more readily with body heat. The top layers of foam may be thinner with the lower layers becoming progressively thicker. This development is complex and quite involved.
The ideal seat will have the comfort of a flat seat with the side support of a bolster seat and be easy to enter and exit. The ideal seat should feel cushioned when the driver first sits down, but then firm once he is into position. The ideal seat may have three to six different densities of foam and two to four different seat covering materials.

Too much seat back bolster is as bad as too little. Too much seat bottom bolster is as bad as too little. High friction seat materials can substitute for side bolsters but make it difficult to enter and exit. Large bolsters can improve comfort and driver control but make it difficult to enter and exit.

Fabric or Bolster?

As difficult as it is to select just the right combination of seat foam, the selection of seat fabric is just as complicated. Different fabrics have different amounts of grip. For example, you can use smaller side bolsters to allow easier entry and exit. Then add back support and control with the use of a high-friction fabric that won’t let the officer slide around on the seat. Of course, a fabric with too much friction makes it difficult to get the right position behind the wheel and slows the exit from the vehicle. A balance, developed around police uniform fabrics, is the answer.

A high bolster on the right side of the driver’s seat may offer more control, but it may also interfere with the muzzle of the duty pistol. A high bolster on the left side of the driver’s seat may offer more control, but it may also make entry and exit slower or more difficult.

Next to consider is the nature of the fabric itself. It needs to feel warm when it is cold outside and cool when it is hot outside. The fabric needs to resist stains and dirt. In some cases, odor-eating fabrics are used in some of the seat panels. The fabric needs to breathe.

Finally, the durability of the fabric must be considered. More durable, heavy-duty seat fabrics have been a part of the formal police package since Ford developed the police package in 1950. The fabric has to be durable enough, in general, to withstand many times the entry/exit duty cycle of a retail seat. And it needs to be durable enough, in particular, to withstand the high pressure points like the buttplate of the pistol magazine.

Height, Length, Width

The width of the seat cushion and seat back is, of course, one of the most important aspects of seat comfort, especially as the seat bolsters are incorporated into the seat. Seat bolsters are supposed to add side support. However, if the seat is too narrow, the officer ends up sitting on top of the bottom bolsters and leaning back onto the back bolsters. That is bad. Of course, if the seat is too narrow, the thighs hang over the sides of the seat, and the side support from the seat cushion can be compromised. The same goes for shoulder support and comfort.

The seat bottom must be long enough to support the thighs of tall drivers but short enough for the comfort of short drivers. Thigh extenders are one possible solution. Adjustable, like the lumbar support, these are an expensive solution.

Sculpted for Police

Given all that seat design theory and practice, given all the things that make the seat one of the most complex compromises in the entire vehicle, to all of this comfort-versus-support complexity, police officers add the design problem of the fully-equipped gunbelt.

After the seat back and bottom have achieved the right balance of comfort and control for the specific vehicle, the development of the police-specific seat begins. This involves more than carving out foam to make room for the holster on both the left and right side. While this is part of it, it is only an easy and obvious part of a well-designed police seat.

The LEPC members first heard how complicated the seats are and how many options for seat designs exist. Then, in turn, the GM seat engineers heard how the gunbelt adds an entirely new dimension to seat design. They not only heard, but felt: One of the designers put on a fully loaded duty belt and sat down on the seat back. Talk about pressure points! He did not need advanced pressure mapping software! The real “hardware” pointed it out.

From pressure point mapping, high pressure points of the seat back are identified. The handcuff case worn near the center of the back is an excellent example. The seat back needs to fully absorb a 1-inch thick, 4-inch by 4-inch brick, while still offering firm support to the rest of the lower and middle back.

Less dense foam may be used in this area, or foam with holes cored out. At the same time, you can’t just hog out the area that the duty belt touches. The seat back with foam designed to accommodate the police duty belt must also work in conjunction with the adjustable lumbar support located in basically the same area of the seat back.

The lumbar support is recognized as one of the most critical aspects of both comfort and fatigue. The seat must support the driver regardless of stature, in erect, neutral or slumped driving postures. More than just pushing out to support the lumbar region of the spine, it may also wrap around the lumbar area to support it from the sides.

All that said, just as each automaker offers powertrains with advantages and disadvantages, the same goes for police seats. This is not just a comfort-versus-support issue. It is also a Worker’s Compensation issue. Lower-back pain from an unsuitable police seat is a common patrol officer complaint—and a common reason behind injury claims.

You are careful in selecting a patrol vehicle with a drivetrain that provides the best balance between performance and economy. Be just as careful in selecting a patrol vehicle with seats that provide the best balance between daylong comfort and enforcement driving support.

Published in Police Fleet Manager, Nov/Dec 2010

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