Constant-rate or variable-rate springs?

[Lugnut64052]

I'm doing a basically stock fix-up on a '72 C20. Preparing to put some new rear coils in. You can get constant-rate and variable-rate springs.

I understand variable-rates supposedly ride a little smoother, and get progressively stiffer as you load them up. I'm planning to use this truck as a daily driver/work truck. Probably haul some fairly heavy stuff around occasionally, and tow a pretty good-sized car trailer once in a while.

Opinions? One better than the other? Experiences? Recommendations? Which would you guys install?

[Lugnut64052]

. . . and I see BATurbo down below asking more-or-less the same question.

[BATurbo]

Hearing nothing but the crickets on this one.

I'm going to find a place around here in Detroit to try and get the old springs measured. While I'm there, I'll see if they can tell me what the spring rates should be, or maybe talk them into measuring a new set for me. If I get any info on the differences and recommendations of variable rate and heavy duty springs, I'll drop you a line...

[Lugnut64052]

Sounds good.

I was going to give it a few days, bump it once or twice, then go over and ask on the main board. Over on the Moog website they have a short paragraph or two on them. They make an original equipment spring, and the variable rate ones. They call the VRs an "upgrade." Meanwhile, since I posted the question I googled around some more and found this. I think it came out of an automotive technology textbook, and probably answers most of my questions.

COIL SPRINGS: Two basic designs of coil springs are used: linear rate and variable rate. Linear rate springs characteristically have one basic shape and a consistent wire diameter. All linear springs are wound from a steel rod into a cylindrical shape with even spacing between the coils. As the load is increased, the spring is compressed and the coils twist (deflect). As the load is removed, the coils flex (unwind) back to the normal position. The amount of load necessary to deflect the spring 1 inch is the spring rate. On linear rate springs this is a constant rate, no matter how much the spring is compressed. For example, 250 lbs. compresses the spring 1 inch and 750 lbs. compresses the spring 3 inches. Springs rates for linear rate springs are normally calculated between 20% and 60% of the total spring deflection.

Since heavy-duty springs are designed to carry 3% to 5% greater loads than regular springs, they are somewhat different. The first difference is wire diameter, which can be up to 0.100 inch greater than the regular-duty spring for the same application. The other difference is free length. A heavy-duty spring is up to 2 1/2 inches shorter than a regular-duty spring for the same application. The important factor to remember is load-carrying capabilities.

Variable-rate springs are characterized by a combination of wire sizes and shapes. The most common ones have a consistent wire diameter and unequally spaced coils. This spring is called a progressive rate.

The spacing gives the spring three functional ranges of coils: inactive, transitional, and active. Inactive coils are usually the end coils and introduce force into the spring. Transitional coils become inactive as they are compressed to their point of maximum load-bearing capacity. Theoretically in this type of design, at stationary loads the inactive coils are supporting all the vehicle's weight. As the loads are increased, the transitional coils take over until they reach maximum capacity. Finally the active coils carry the remaining overload. This allows for automatic load adjustment while maintaining vehicle height.

The latest design of variable rate springs deviate from the old cylindrical shape. These include the truncated cone, the double cone, and the barrel spring. The major advantage of these designs is the ability of the coils to nest or bottom out within each other without touching, which lessens the amount of space needed to store the springs in the vehicle.

Unlike a linear spring, a variable spring has no predictable standard spring rate. Instead, it has an average spring rate based on the load of a predetermined spring deflection. This makes it impossible to compare a linear spring to a variable spring. Variable springs, however, handle a load of up to 30% over standard springs in some applications.

NAPA has four different rear springs for a '72 C20:

Standard spring--
Wire Diameter:.765"
Load Height:8.75"
# of Coils:6.1
Design Load:1700
Spring Rate:389
Free Height:13.12"

Heavy duty--
Wire Diameter:.906"
Load Height:8.75"
# of Coils:5.89
Design Load:2900
Spring Rate:718
Free Height:12.54"

Standard variable-rate--
Wire Diameter:.812"
Load Height:10.75"
# of Coils:7.43
Design Load:950
Spring Rate:445
Free Height:13"

Heavy duty variable-rate--
Wire Diameter:.937"
Load Height:10.75"
# of Coils:6.82
Design Load:1335
Spring Rate:820
Free Height:12.38"

BATurbo, here's the specs on their available '70 C10 springs (there are only two):

Heavy duty--
Wire Diameter:.765"
Load Height:8.75"
# of Coils:6.1
Design Load:1700
Spring Rate:389
Free Height:13.12"

Variable-rate--
Wire Diameter:.812"
Load Height:10.75"
# of Coils:7.43
Design Load:950
Spring Rate:445
Free Height:13"

Looks like the heavy-duty 1/2-ton spring is the same as the standard-duty 3/4-ton spring. The 1/2-ton variable and the standard 3/4-ton variable are also the same. The variable-rate springs all have heavier wire, and a lower design load but higher spring rates, which lends some legitimacy to the statement in the text about comparisons being impossible . . . I guess.

[BATurbo]

That's good info. Thanks Bret.
I emailed Eaton Detroit Spring and they responded with the stock spring rate of 382 lbs and a free height of 13.19 inches. I'm going to go and talk with them about the advantages/disadvantages of variable rate vs. linear rate springs. See what they tell me. I happen to talk with a coworker who used to work with the Skip Barber racing school and in his opinion, if you're looking for handling, you won't be happy with variable rate springs. I don't know how much "racing" I'll be doing with my truck, but with the freeway speeds around here, a little handling can go a loooong way.