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Coil Springs vs. Leaves

Front View
David Kennedy
| Contributor
Posted December 1, 2001

Round or Flat, This is the Tech You Want to Know

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  • Stone-age simple. The leaf spring was around long before four-wheel drive, and their ability to both spring the vehicle and locate an axle makes them a proven piece that kills a whole flock of birds with a single stone.

  • Coilover shocks depend on coil springs to support the load. In this arrangement, two coils with different rates can be stacked to form a variable rate spring that will feather out the small bumps and still soak up the big air when necessary.

  • Here you can see how the jungle gym of trailing arms that must be used to locate the axle often outweighs the simplicity of a coil spring itself. When it comes to ride quality and axlewrap prevention, a coil spring/four-link suspension like this will arguably control the rear axle better than a leaf spring alone ever could.

Ever think about wheeling a sheet of plywood off road? Try it. Have your buddy tow a 4x8-foot sheet of plywood behind his 4x4 down the trail. Heck, even a graded dirt road would do, and you’ll develop an appreciation for why we fill so many pages of this magazine with suspension stories. That’s because a truck’s suspension is in a dead heat with the four-wheel-drive system when it comes to making our vehicles perform off-road. Think about it. Slapping a Dana 44 axle and a transfer case into a Corvette would not make it an off-road hero. Sure it might have four-wheel drive now, but the limited suspension travel will cripple it when you get to the first rough spot. The second half of the equation (dare we say more important half?) is the suspension.

Wheelin’ a Sheet of Plywood

You don’t even have to carry out this ludicrous experiment to know the torturous experience that it would be. You and the plywood would move together as a single unit and crash and smash over the slightest bumps in the road. You’d be hating life because the lumber you were riding on has no way to absorb the imperfections of the terrain without translating them directly to every square inch of the wood and subsequently your body. You would pitch, dive, and roll with whatever the road had to deal out because the plywood you were riding on has no way to absorb, move, or handle the terrain.

The experience would actually be even scarier because with no suspension, bumps and dips could cause the plywood to come off the ground. Yup, you’d get airborne, and the landings wouldn’t be soft ones. Imagine if you had to try and steer, accelerate, and push the brakes on that bad boy—it would never happen! The terminal velocity of your plywood “ride” would be quite low as a result, topping out somewhere in the neighborhood of 15 mph. Above that and the board would spend more than 75 percent of the time in the air. And that, ladies and gentlemen, is not driving. That kind of experience is more along the lines of what we’d call desert racing.

The Spring is King

If we wanted to drive vehicles without suspensions we’d all be sled makers or ship builders. Both of these types of vehicles rely on terrain that is almost perfectly smooth. But for those of us who like to travel where the environment is less than perfect, the spring is king. The spring, and we’re talking about any spring (coil, leaf, torsion bar, or airbag), permits the wheels and tires of a vehicle to bob and weave, scouting for traction and grip while the rest of the vehicle maintains a much more controlled and even keel. The springs of a vehicle are a compliant system that soak up the bumps and holes in the earth, sifting out most of the harsh crashing and bashing to let us, the drivers, concentrate on controlling the direction and speed of the vehicle. It is hard to drive when your eyes are watering from the vibration and you’re holding on for dear life. To get the proper education on all things springs, we called up Michael Eaton of Eaton Detroit Spring. Michael’s company has been manufacturing coil and leaf springs since 1937, and has springs for more than 100,000 applications. So it’s safe to say he’s a man that knows things about springs.

When designed properly, the springs force the tires into making contact with the terrain while supporting the weight of the vehicle. When choosing springs, your goal should be to maximize suspension travel. For our money, we’d try to error on a spring that was too soft. In other words, a spring with a low “spring rate.” If the springs are too stiff, they can become the limiting factor in your suspension travel, and make for a terrible ride. It is important to understand that all springs effect your vehicle’s ride height, chassis roll stiffness, pitch resistance, suspension compliance, handling balance, ride quality, and tire adhesion. With all those factors to manage it no wonder lift kits cost as much as they do.

Variable Rate Springs

In a perfect world your springs would have a variable rate. The best springs would have a soft initial spring rate to absorb the subtle irregularities of the trail, and a much firmer spring rate to handle the larger bumps and jumps and to control sway and axlewrap. It is possible to get this kind of variable rate with both coil and leaf springs. Coil springs use a different coil spacing within the coil spring to effectively change the spring rate. You can tell variable rate coil springs from a traditional progressive rate spring because they have some of their coils tightly packaged together, and the spacing between the coils gets progressively larger. Naturally the longer the spring, the more variable the rate can be.

Leaf springs vary their rates by changing the thickness of the leaves, or by adding “over-load” leaves that may only make contact with the chassis when the truck is loaded down. If you look at the rear leaf packs on dualies, you can see how the manufacturers can get massive load carrying capacities out of these trucks, without compressing your spine when you drive the truck around town with the bed empty.

Coils vs. Leaves

Sorry, but we aren’t going to declare a winner in the world of springs because there is no clear cut advantage to either coils or leaves. Coil springs cost less to make, are lighter, have no internal friction, and take up very little space. The downside with coils is that they do not locate the axles. A three- or four-link suspension must be constructed to work with the coil springs to do the same job that leaf springs alone can do. Leaf springs, on the other hand, are completely modular (if you want to add or subtract lift or load capacity, just add more leaves), do locate the axles, and can also help to reduce sway. The best leaf packs will use multiple thin leaves to provide load control yet still move freely. Yes, leaf springs are heavy, prone to axlewrap, and can require a lot of mounting space. But keep in mind that a suspension’s characteristics have more to do with how the whole system is set up (and the spring rates that are used) than it does with whether the springs are leaves or coils. Hey, as low-tech as leaf springs may seem, they can be made to work with maybe just a weight penalty over the coil spring.

Sources

Tokico Gas Shocks
Rancho Dominguez, CA 90220

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