Mike Eaton and I are walking through the show field at the Mustang & Ford Roundup at Silver Springs, Florida, when he spots a '66 Mustang that is obviously sitting like a grasshopper with rearend jacked up at the rear. "That car has definitely got the wrong springs on it, unless the guy wanted it that way," Mike says while pointing out the gap between the rear tire and the wheel opening. "It sits about 2- to 2 1/2 inches too high in the rear so it doesn't look right. That's what can happen when you buy the 'one-size-fits-all' springs. The springs fit, but the car doesn't sit right. A lot of people don't know that in 1966 there were four different rear leaf springs for Mustangs."
And Mike should know. His grandfather started Eaton Detroit Spring after buying the aftermarket division of Detroit Steel Products, a manufacturer of original equipment springs, in 1937. Mike began working there in 1977 and bought the company in 1985. As a spring manufacturer in a facility located on Michigan Ave. in Detroit, near the old Tiger Stadium, Eaton Detroit Spring has over 24,000 original spring blueprints for cars back to 1893, all catalogued by OE numbers, vehicle, and application.
The '65-'73 Mustangs used a combination of front coil springs and rear leaf springs. Eaton Detroit Spring is one of the few companies that manufacture both, either by following factory blueprints or custom. You can find application guides for '65-'73 Mustang coil and leaf springs at the Eaton website, www.eatonsprings.com. Follow the "Tech info" and "Downloads & Literature" links.
Our discussion with Mike revolved around the replacement of factory production springs, not aftermarket performance applications, although Mike says Eaton Detroit Spring can supply factory-spec Heavy-Duty or Competition springs for cars that came with the standard suspension.
Unlike today's Mustangs, vintage...
Unlike today's Mustangs, vintage Mustangs typically sat slightly lower in the rear than the front, as this factory photo of a '65 hardtop shows.
We see a lot of Mustangs sitting too low in front or too high in the rear, like that '66 out in the show field. Springs have a lot to do with that, right?
A car's stance makes or breaks the vehicle. And the only thing that changes a car's ride height is the springs. It's not like a headlight bulb or a windshield wiper; springs are specific to the vehicle, determined by the equipment on that vehicle and the total weight. That's why all the information we need-body style, engine, etc.-is so critical. In addition to setting the ride height, springs also support the car's weight and provide ride comfort.
MM: So choosing the right springs for a Mustang is a lot more complicated than just finding springs that will fit?
Eaton: Picking the right spring is very important. You need the year, make, engine, body style, whether it's air-conditioned or not, because all those things affect the weight of the vehicle. The rear leaf springs for all these years are the same length but they have different arches and different rates. Rate is how strong the spring is. For example, 85 pounds means it takes 85 pounds to deflect the spring one inch. Mustangs were arranged from around 85 to 153 pounds for the '69 Boss, which was the only time that spring was used. Ford used 32 different rear springs for the '65-'73 Mustangs. In '65, there were four different leaf springs; by '69 there were 12.
MM: Does equipment make the biggest difference in spring selection?
Eaton: Yes. With the front coil springs, you had different springs for air-conditioned cars because of the weight of the A/C. If you calculate how much weight is hanging over the front end, you can see that A/C adds about 300 pounds. The actual parts may only weigh 60-65 pounds, but they're out in front of the springs so you can figure about 300 pounds. Not so true for aftermarket air because it can be lighter, but with factory air it's about 300 pounds.