Vintage Mustang springs were...
Vintage Mustang springs were square at the bottom, tangential at the top. These Eaton coil springs were made specifically for a '66 GT hardtop without air conditioning.
The transmission can make a difference in the front springs due to the weight, automatic versus manual. Even the seats-you'd think the weight difference between a bench seat and two buckets seats wouldn't make a lot of difference, but apparently it did. The location of the battery made a difference in '69 rear springs. Is the weight of the battery under the hood or in the trunk?
Also, the two-barrel or four-barrel engine in '71 made a difference. The 302 and 351 two-barrels used the same rear springs but when it went to the four-barrel, prior to Oct. 9, it used the same rear spring. After Oct. 9, the rear springs changed. That's common when it comes to springs.
There were only two or three different front coil springs for '65-'66 Mustangs, but it went to five for '67, seven for '68, and eight different front coil springs on '69-'70 Mustangs. Then in later years, they started standardizing things and didn't go so crazy. In '73, that generation of car was phasing out so they started settling down with the number of different springs.
MM: What about build date?
Eaton: When the car was built can make a difference if Ford made some change that calls for different springs. The rates and number of leaves are the same, but for some reason Ford made a change in the specification of the spring. For '69 Mustangs with the 390 engine, there was a change on Sept. 10, 1968. Prior to Sept. 10, they had an Improved Handling package, but after Sept. 10, they called it Competition Handling. The spring rate on the Improved Handling-technically heavy-duty-was 114 pounds, but after Sept. 10 the rate went up to 135 pounds.
Like the front coil springs,...
Like the front coil springs, rear leaf springs were specific to the car, depending on body style, engine, and equipment.
What if someone wants to step up to Competition or heavy-duty suspension springs? If they order for their car and their equipment, the car should sit OK, right?
Eaton: If they call us with a '73 convertible and the specifications call for an 85-pound spring but they want the Boss 302 spring that's 153 pounds and they want it sitting at the stock ride height, that's a piece of cake. We can do that for them. Chances are, if they order the regular Boss 302 spring, the car would not sit right. But if we know that they want that spring for their particular car, we'll make that spring so the car will sit right.
MM: So you can make custom springs?
Eaton: As a manufacturer, we can make anything. We have countless number of thousands of custom blueprints. Probably 85 percent of our production is custom. It's non-stop applications, so if someone wants their car to ride higher or lower, softer or harder, we can do that.
Only a handful of companies make coil springs. Most are high-volume guys and they won't even talk to you about custom springs. We're a low-volume manufacturer so we can make custom springs, providing the tooling is in place and we're making something like it already. In that case, making a one-off, custom spring is no big deal.
Leaf spring tooling is a piece of cake. For coil springs, we have about 450 sets of tooling; we stock or create somewhere around 650 part numbers. And out of those 650, we can alter each one of them a minimum of six different ways. The last time we counted leaf springs, we have something like 1,800 different patterns. Out of those, you can generally make somewhere between six and 12 different types of springs of each pattern, so we can make literally thousands of different springs. If you ask one of the other companies about a custom spring, they'll probably give you our phone number.
For owners who want better...
For owners who want better handling, Eaton Detroit Spring can supply Heavy-Duty springs that were used on high-performance Mustangs.
How can Mustang owners order your springs?
Eaton: They can call us directly, or a number of Mustang vendors, like National Parts Depot, YearOne, and others, handle a broad selection of our springs instead of the one-size-fits-all. Some vendors offer four springs at the most, where Ford might have had 32. The one-size-fits-all springs will fit on the car because they're all the same dimensions and the same number of leaves, but they can vary the ride height plus or minus three inches.