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Explaining Lift Kit Spring Rates

Posted in How To on July 1, 1999
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Photographers: 4WOR Archives
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Leaf-spring rates can be easily determined using one of the following methods. Intercomp Company has a spring-rate test fixture that uses a hydraulic jack to compress the spring and a force scale to determine the rate. The low-buck (albeit less-accurate) alternative is to have a friend stand on the leaf pack and measure the difference in the spring deflection (compressed arch height versus the relaxed height or free arch). The heavier your friend is, the more accurate your results will be. Divide your friend’s weight by the amount of deflection you measured. You can convert your findings to pounds-per-inch. Leaf-spring rates can be easily determined using one of the following methods. Intercomp Company has a spring-rate test fixture that uses a hydraulic jack to compress the spring and a force scale to determine the rate. The low-buck (albeit less-accurate) alternative is to have a friend stand on the leaf pack and measure the difference in the spring deflection (compressed arch height versus the relaxed height or free arch). The heavier your friend is, the more accurate your results will be. Divide your friend’s weight by the amount of deflection you measured. You can convert your findings to pounds-per-inch.
If you want to reduce the spring rate of your existing leaf-pack (or at least limber it up a little), take the pack apart and polish each leaf, grind the ends and tips to round off sharp edges, repaint (some people even wax each leaf), add Teflon liners between the leaves, and reassemble with bolt-type clamps or loosely crimped cinch clamps. There aren&#8217t many things that can be done to help coils or torsion bars to flex more. If you want to reduce the spring rate of your existing leaf-pack (or at least limber it up a little), take the pack apart and polish each leaf, grind the ends and tips to round off sharp edges, repaint (some people even wax each leaf), add Teflon liners between the leaves, and reassemble with bolt-type clamps or loosely crimped cinch clamps. There aren’t many things that can be done to help coils or torsion bars to flex more.
Some interesting dichotomies spring up when comparing leaf springs with different arches. All other things being equal, if a leaf spring is reformed for more arch, the spring becomes stiffer as its arch increases. Because new leaf springs with high arches are also given more length to place the shackles at the proper angles, they are sometimes softer than the lower-arched springs. Flat leaf springs are considered more desirable than high-arch ones for suspension flex because they let the axles droop equally as well as they compress. You can see in the photo that the high-arch spring will not allow much droop, but that high-arch gives you the lift and clearance you want for larger tires to get over larger rocks. Some interesting dichotomies spring up when comparing leaf springs with different arches. All other things being equal, if a leaf spring is reformed for more arch, the spring becomes stiffer as its arch increases. Because new leaf springs with high arches are also given more length to place the shackles at the proper angles, they are sometimes softer than the lower-arched springs. Flat leaf springs are considered more desirable than high-arch ones for suspension flex because they let the axles droop equally as well as they compress. You can see in the photo that the high-arch spring will not allow much droop, but that high-arch gives you the lift and clearance you want for larger tires to get over larger rocks.
A leaf spring in action! Note that the main leaf flexes more than the shorter ones and also before the others. These leaves show a tapered-end design with attached friction-reducing pads. The bottom leaf is for overload conditions and so is not tapered since its use is infrequent (only with heavy loads). The bolt has been removed from the spring clamp, which nets an extra smidgen of flex from the pack (the main leaf is now free to bend past the bolt hole). A leaf spring in action! Note that the main leaf flexes more than the shorter ones and also before the others. These leaves show a tapered-end design with attached friction-reducing pads. The bottom leaf is for overload conditions and so is not tapered since its use is infrequent (only with heavy loads). The bolt has been removed from the spring clamp, which nets an extra smidgen of flex from the pack (the main leaf is now free to bend past the bolt hole).
When deciding on a suspension kit, let the manufacturer know if you have any non-stock, weight-increasing equipment on your truck. Winches with their bumpers and mounts can add 100 to 200 pounds or more. When deciding on a suspension kit, let the manufacturer know if you have any non-stock, weight-increasing equipment on your truck. Winches with their bumpers and mounts can add 100 to 200 pounds or more.
If you&#8217ve swapped in a big-block or added a snowplow, the extra weight can cause soft springs to sag enough to negate the lift you wanted, upset your steering geometry, or even let your tires contact (read: damage) your fenders. Again, inform your lift manufacturer that you added weight to the front end. If you’ve swapped in a big-block or added a snowplow, the extra weight can cause soft springs to sag enough to negate the lift you wanted, upset your steering geometry, or even let your tires contact (read: damage) your fenders. Again, inform your lift manufacturer that you added weight to the front end.
Coil springs have the benefit of being produced with either constant- or variable-rate designs. This one is a constant-rate spring. Coils also have no internal friction, unlike the leaves of a leaf spring. Nor do they need to position the axle: They simply provide spring rate leaving positioning chores to trailing (or leading) arms and track bars. Coil springs have the benefit of being produced with either constant- or variable-rate designs. This one is a constant-rate spring. Coils also have no internal friction, unlike the leaves of a leaf spring. Nor do they need to position the axle: They simply provide spring rate leaving positioning chores to trailing (or leading) arms and track bars.
This is a variable-rate sprint. Note how the coils of the spring are wound progressively looser from top to bottom, creating the variable rate. This is a variable-rate sprint. Note how the coils of the spring are wound progressively looser from top to bottom, creating the variable rate.
Stone-stock, factory springs can be surprisingly supple. Especially if they&#8217re decades old and have lost most, if not all, of their temper. They are not, however, confidence-inspiring under heavy braking or hard cornering. Suspension manufacturers can produce flexible springs as well, but, since their springs usually also provide lift, they must have a higher rate than stock to compensate for the higher vehicle center-of-gravity, thereby keeping the roll rate under safe control. Stone-stock, factory springs can be surprisingly supple. Especially if they’re decades old and have lost most, if not all, of their temper. They are not, however, confidence-inspiring under heavy braking or hard cornering. Suspension manufacturers can produce flexible springs as well, but, since their springs usually also provide lift, they must have a higher rate than stock to compensate for the higher vehicle center-of-gravity, thereby keeping the roll rate under safe control.
Jumping, tough-truck racing, and mud bogging all require stiffer springs than the flexible twist wanted for high ramp-travel indexes and rockcrawling. If you use your 4x4 for these activities and day-to-day pavement pounding, a spring-rate compromise will be needed. This may involve trial-and-error, experience, a good discussion with a techline operator, and a fair assessment of your personal driving needs. Jumping, tough-truck racing, and mud bogging all require stiffer springs than the flexible twist wanted for high ramp-travel indexes and rockcrawling. If you use your 4x4 for these activities and day-to-day pavement pounding, a spring-rate compromise will be needed. This may involve trial-and-error, experience, a good discussion with a techline operator, and a fair assessment of your personal driving needs.

The next time you're shopping for a suspension lift kit, do yourself a favor and don't buy whatever your friends are running on their trucks. At least not before doing a little research (and perhaps soul-searching) on your own to decide how you'll be using your vehicle. Nowadays, even simple, solid-axle kits can cost hundreds of dollars--and that's without any optional kit equipment, shocks, or the new taller tires you'll want to install.

Along with proper steering and suspension geometry, spring rate is one of the most important design aspects of any lift kit. Although spring rate can be defined simply as the amount of weight necessary to compress a spring 1 inch, matching the correct rate to a specific vehicle and its usage is as much art and opinion as it is science and engineering.

Sources

Rancho Suspension
Monroe, MI 48161
734-384-7804
www.gorancho.com
Superlift Suspension Systems
West Monroe, LA 71292
888-299-4692
www.superlift.com
Trail Master Suspension
Chino Valley, AZ 86323
928-636-7080
http://www.trailmastersuspension.com
Eaton Detroit Spring
(313) 963-3839
www.eatonsprings.com
Explorer Pro Comp
800-776-0767
Skyjacker Suspension
www.skyjacker.com
Tuff Country EZ-Ride Suspension
West Jordan City, UT
James Duff Ent. Inc.
Sequim, WA
Intercomp Co.
Minneapolis, MN
ARB Accessories/ Old Man Emu Springs
Seattle, WA
National Spring Co.
El Cajon, CA
Rough Country, of Heckthorn Off-Road Products
Dyersburg, TN
Warn/Black Diamond
Clackamas, OR

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