Detroit's muscle cars have a reputation as thundering rockets in a straight line but ponderous tubs in the corners. The 1999 Ford SVT Mustang Cobra attempts to smash the stereotype.
The Cobra's new independent rear suspension is far more sophisticated than the solid-axle rear ends that have held up every other Mustang model since its introduction in 1964.
Developed to sharpen the handling and improve the ride of the 320-hp Cobra, the independent suspension also represents an engineering feat.
The tightly packaged module fits into the same underbody cavity as the solid-axle rear suspension of 1999 Mustang V-6 and V-8 GT models, without a single change to the sheet metal. Even the springs and shock absorbers attach at the same body points as the Cobra's solid-axle siblings.
'Because the Cobra is low volume at 8,000 units (a year), we had to bring the suspension into the car for a reasonable number of dollars,' explained Paul Giltinan, Ford Motor Co.'s manager for Mustang vehicle and chassis engineering. 'That meant no tear-ups of the body.'
The Cobra went into production March 1; its base price is $28,532, including a $535 destination charge. The suspension module is assembled, aligned and delivered to Ford by Benteler Automotive Corp. of Grand Rapids, Mich.
BIG ADVANTAGES
Independent rear suspension means the rear wheels follow contours in the road independently of each other. They are supported by individual sets of control arms that pivot on separate attachment points to the body.
In a solid-axle design, both wheels are linked by one rigid tube that also houses the drive shafts and differential.
A solid-axle suspension is sturdy and cheap, but the independent rear end has big advantages for performance on curvy roads:
Improved handling because it better maintains the tire's contact patch in position for maximum grip in corners and can reduce nose lift during acceleration and dive during braking.
Greater stability because the independent suspension has less mass, or 'unsprung' weight, in its moving parts. The Cobra rear suspension's total unsprung weight is 125 pounds, vs. 250 to 270 pounds in the other Mustang models.
Smoother ride because bumps at one wheel do not upset the other wheel.
The Cobra uses short upper and long lower A-shaped suspension arms - generally the best suspension configuration for handling. The big lower arms are cast aluminum using a permanent-mold process that allows for thin but strong walls. The result is less weight.
The key to the suspension's adaptability is its tubular subframe, which bolts into the same mounting points as the solid-axle suspension. The subframe provides the attachment points for the A-arms and the differential.
PRECIOUS SPACE
Many key structural and clearance problems were worked out before metal ever was cut. Designers created and tested 60 different subframe models on their desktop workstations, said suspension systems engineer Eric Zinkosky. Of those, only five were constructed for real-world testing.
Even so, the suspension's development job took 21/2 years.
The biggest challenge: packaging the subframe to avoid hitting the fuel tank and the spare tire well. To gain precious space, engineers used an oval cross-section tube for the rear brace instead of a round one. The design also required a new exhaust system that passes below the axle rather than above it.
Squeezing the bulky module into the Cobra required trade-offs; packaging issues took priority over handling and ride quality in some cases, Zinkosky said.
'I've always thought suspension design was like juggling,' he said. 'You have to keep all of the balls in the air, not just your favorites.'
