GM X platform

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There have been two X-body automobile platforms from General Motors. All X-bodies were small entry-level models.

[edit] Rear wheel drive

The rear-wheel drive X-body underpinned the Chevrolet Nova and similar cars of the late 1960s and 1970s. It was also the basis for the Cadillac Seville's K platform. The wheelbase was 111 in and many components were shared with the contemporary F platform.

Applications:

• Buick Apollo (1973–1974; 1975 sedan only)
• Buick Skylark (1975 coupe only; 1976–1979)
• Chevrolet Nova (1968–1979)
• Oldsmobile Omega (1973–1979)
• Pontiac Ventura (1971–1977)
• Pontiac Phoenix (1977–1979)

1968–1974 GM X-bodies were rear steer (with the steering linkage behind the steering gear) whereas 1975–1979 models were front steer (with the steering linkage forward of the steering gear). Note: "Rear steer" does not mean that the rear wheels steered the vehicle. It strictly relates to the position of steering components in relation to the steering gear.

[edit] Front wheel drive

The front-wheel drive X-body was used for compact cars from 1980 to 1985. The X-body program was widely considered a failure at the time, but the derivative GM A platform, which was introduced in 1982, continued for over a decade. Interestingly, only the Skylark name was carried over to the next generation of GM compact cars, the N-body. The Citation was succeeded by the Chevrolet Corsica on the compact L-body for 1987.

Vehicles using the X-body include:

• 1980-1984 Oldsmobile Omega
• 1980-1984 Pontiac Phoenix
• 1980-1985 Buick Skylark
• 1980-1985 Chevrolet Citation

[edit] Braking problems

NHTSA sued General Motors Corporation over the safety of their X platform family (United States v. General Motors, 841 F.2d 400 (D.C. Cir. 1988)).

The cars were initially designed to be five-passenger models, with bucket seats and lever actuated parking brakes. However, a decision was made late in the design cycle to broaden the cars' purchasing appeal by offering six-passenger models with bench seats. This necessitated a change from a parking brake lever (mounted between the seats) to a parking brake pedal. The pedal, however, did not have enough leverage to apply sufficient pressure to the rear brakes to hold the car on an incline.

Without enough time to redesign the braking system, the decision was made simply to use brake linings with a higher coefficient of friction instead, to hold the car with the pressure that could be applied through the parking brake pedal. However, this in turn had an undesirable effect; the increase in friction of the rear brakes, along with the excess forward weight distribution of a front wheel drive car, led to a tendency for the rear wheels to lock up under braking, which led to the rear of the car slewing sideways and loss of directional control and/or spinning (see oversteer).

The Court of Appeals eventually ruled against NHTSA and for GM, however, on the grounds that NHTSA's case for performance failure was based only on circumstantial evidence. The Center for Auto Safety includes the case as one of its Safety Research Reports; the index of the report is available here.