Eibach springs tech article

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Eibach Springs

by Paul Haney

A car needs springs to isolate the occupants from road irregularities and maintain tire/road contact. The springing medium can be steel, air, fiberglass, or anything that contributes a predictable and repeatable force as the spring deflects through some wheel travel.

Most early automobiles used flat-steel, multi-leaf springs because they were easy to make and had some interleaf friction built in that helped damp out unwanted oscillations. Some manufacturers added friction dampers as in the photo above. A few manufacturers, mainly Porsche and Chrysler, used torsion bars. Air and rubber/air hybrid springs have been tried also, but the most popular type of spring for both racecars and road vehicles is the steel, helical-coil spring

Because a mass attached to a spring tends to oscillate, a damper is required, and a helical coil spring with a damper mounted inside is a compact package. This, of course, is the familiar "coil-over" spring/damper unit that is widely used in road cars and all but universal in racecars.

We see coil-over spring/damper units in road racing applications on both open-wheel and stock-bodied racecars. Springs designed for use in coil-over applications come with inside diameters of 2.0 inches, 2.25 inches, and 2.5 inches. Off-road racers use 3.0-inch springs. Some oval-track stock cars, including NASCAR racecars, use bigger coil springs of 5.0- or 5.5-inch diameter. NASCAR rules do not allow damper mounting inside the coil spring.

Spring Basics

A spring provides a predictable force for a given deflection. That force, called the spring rate, is expressed in pounds of force per inch of deflection: 200 lb./in. In conversation the inch is generally understood, and racing people talk about "200-pound springs" or "thousand-pound springs."

A coil spring is really a torsion spring--the wire twists as the spring compresses. The material's reluctance to twist is what supplies the resisting force. A common misconception is that a coil spring "sags" losing spring rate over time, but a look at the components of the equation for the spring rate of a coil spring will show you that is not likely if the spring is properly designed.

Spring Rate = F/S = Gd4/8ND3 where:

F = spring force.

S = spring deflection.

G = torsional modulus of the material.

d = wire diameter.

N = number of active coils.

D = mean (average) coil diameter.

If we peer at this equation a little closer we can figure out some basic characteristics of coil springs. Look at the variables that are on the top of the division sign on the right side of the equation--G and d. Those are the variables that, as they get bigger, increase the spring rate. G, the torsional modulus, is a property of the steel used, meaning spring manufacturers should use the highest-quality steel and heat treat it properly and consistently.

Because d, the wire diameter, is raised to the fourth power a small change in this dimension dramatically changes the spring rate. Since we want all the coils to deflect a predictable distance, precision springs have to be made from wire that has a constant diameter along its length.

Look at the spring rate equation again and notice the variables below the division sign--N and D. These characteristics make the spring rate decrease as their values increase. A larger number of active coils and coils wound to a bigger diameter lower the spring rate. The number 8 comes from the basic geometry of a helical coil spring and is a constant for all springs of this type.

Having said that springs can't sag, it is possible if the spring was designed poorly. If a spring is designed (usually to lower costs) with marginally small wire or too few coils, and the spring is fully compressed, the material can be stressed beyond its elastic limit. The material doesn't fully recover fully when the stress is removed. Over time, as the underdesigned spring is repeatedly overstressed, permanent deformation can build up.

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Eibach Springs, Page 2

The Company

In 1951, Heinrich Eibach founded a small production shop in Finnentrop, Germany that has grown over the years into a leading manufacturer of advanced suspension components. Eibach is an original equipment supplier to Ferrari, BMW, Volkswagen, Ford, and other auto manufacturers. Racing customers include Alfa Romeo, AMG, BMW Motorsports, Ferrari, Ford, and Lola. Eibach products appear on race cars all over the world competing in Formula One, Indy Racing League, CART, Sports Car, NASCAR, German Touring Car, GT Cup, off-road racing, and Sports Car Club of America professional and club racing. Eibach also supplies replacement springs and engineered suspension kits to people wanting to modify their road cars for a distinctive look and/or improved handling.

Eibach has manufacturing facilities in Germany, Japan, England, and the United States. Their U.S. plant is in Irvine, Calif. just south of Los Angeles. This facility houses manufacturing, engineering, and sales in a building of 50,000 square feet. Another building nearby provides 15,000 square feet of warehouse space. Monthly spring production in Irvine is 30,000 to 40,000 units.

Making Springs the Eibach Way

The process starts with the best material for the product, precision-drawn wire.Most Eibach racing springs are made from a steel high in chromium and silicon which is slightly more fatigue resistant. For a product that is subject to cyclical stresses as is a spring the fatigue strength of the steel is the most important characteristic.

Eibach stocks precision wire in small increments of diameter so their designs can be optimized, allowing the lightest spring for the customer's needs. Chrome-Silicon wire is stocked in 1/4 mm (0.010 in.) increments in diameters above 9 mm. Below 9mm the increment in diameter is 1/10 mm (0.004 in.).

Some springs such as those specified by NASCAR rules are made of large-diameter chromium-vanadium steel wire and are cold wound over a mandrel as in the photo above.

Automatic Winding Machine

This is a computer-controlled coil winding machine that is very impressive to see in action. Wire comes into the machine from the left and the new coil spring seems to grow out of the hydraulically-controlled tools. The following three photos show the machine producing a barrel-shaped spring.

That's a powerful chisle at the middle top of the photo which cuts the finished spring from the wire stock.

Machine operators check parts coming off the machine as they adjust the tooling for a run of barrel-shaped coil springs.

Eibach engineers decided years ago that hot-forming springs causes hydrogen embrittlement and reduces fatigue strength, so all Eibach springs are cold-formed. This extremely robust, computer-controlled winding machine uses wire at a rate of 180 ft/min. producing roughly 400 parts per hour.

After cold-forming, the springs go into this oven for a heat treatment process that increases strength and fatigue resistance.

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Eibach Springs, Page 3

All springs are peened with steel shot which increases fatigue strength by about 40%. The machine above grinds the coil ends flat and perpendicular to the spring center line according to DIN class 1 quality specifications. This translates to less than 0.050 in. (1.2mm) across a typical CART or IRL spring that is 2 in. in diameter and 4 or 5 inches long.

Finally each spring is pre-set. By design the winder produces a spring with excess free length. During pre-set the spring is compressed to block (coil bind) to stress relieve the spring and set it to the final free length. After this process the spring is block resistant, meaning that it can now safely travel to coil bind and back to the design free length with no loss of that free length.

As a check on the manufacturing process 10% of the springs go through a final inspection also using DIN class 1 specifications.

Each spring is phosphate sprayed and powder coated for corrosion resistance and the spring rate is printed or tagged on each coil. The result is a brightly-colored, precision spring.

Progressive Springs

Eibach designs and manufactures both linear and progressive springs and progressive spring systems. Progressive systems use two or more linear springs of different spring rates called main and tender springs. The tender springs have a lower spring rate than the main spring and are made from a special wire with a trapezoidal cross section. After winding, the coils of the tender springs can collapse flat against each other providing a stable platform for the main spring to bear against.

Off-road racing vehicles use this type of suspension for soft spring rates rising to higher rates as suspension travel increases. Race cars with aerodynamic downforce can utilize the main spring to support the car at speed and the tender spring in slow corners. Formula Ford suspension tuners install small tender springs in the rear suspension to prevent lifting the inside rear tire in slow corners.

The barrel-shaped spring seen snaking out of the winding machine on Page 2 is a clever device. A look back at the helical-coil spring-rate equation on Page 1 reminds us a coil with a larger diameter has a lower spring rate. The middle of the barrel-shaped spring deflects more than the stiffer ends. This design allows the smaller coils to nestle inside the larger ones at full deflection providing more suspension travel than a conventional spring.

Design Software

Stan Hortinela, shown on the left with Motorsports Manager, Beau Kelly, is an engineer at Eibach Springs with the additional title of program manager. He explains the Wizard software used to design the multiple-spring systems, "We enter the spring material and the input values. The program needs spring internal diameter, static load, deflection before transition to the main spring, initial and final spring rates, and the amount of suspension travel from the static position in both the bump and rebound directions. The software calculates suggested output values, picks Eibach part numbers, and shows us a force vs. deflection graph.

"We also have our own spring design software. We get the input numbers from customer requirements and the software suggests a solution, but the engineer has to look at the trade-offs and make adjustments to optimize the product. We put a lot of emphasis on the ends of the spring being parallel. We want the lightest, most reliable spring system that meets the customer's needs. Stocking all the many wire diameters helps a lot."

"There are no super secrets here," Hortinela continues. "We specify the best materials and we've learned a lot from experience. This is a family-owned business and there's a lot of pride in the product. We tell people about our manufacturing process because we're proud of it and it helps sell the customer."