For reasons of convenience and operational efficiency, the vast majority of purpose-built racecars, except for some Active-suspension Formula 1 cars, are suspended by coil springs. For several years, most of the Indy Cars, NASCAR stock cars and so on in the USA, and several of the leading Formula 1 cars, have been fitted with the distinctive, navy blue coils supplied by Hyperco Inc. a company founded in 1987 and headquartered in Logansport, Indiana.
The suspension springs of the racecar serve the same three basic purposes as the springs of the passenger car. First, they isolate the occupants and components of the sprung mass from road shock. Second, they control the pitch and roll attitudes of the vehicle. And third, they keep the tires in contact with the road surface as constantly as possible, because a tire that is floating (or bouncing) above the surface cannot accelerate, brake or steer.
Racing, however, imposes other, more stringent, requirements – accuracy of manufacture, minimum practical size and weight, and absolute consistency of performance. In a passenger car installation, a spring that is a bit ‘off’ in rate, in free length, or in load at height – or that takes a small set or a slight sag in service can still be satisfactory. Not so in racing.
In motor racing, more than in most fields, time and money are interchangeable. We never have enough of either. We simply do not have the time to be fussing about with any component that is less than the very best of its type. No one will argue this point when it comes to the high-profile items – shock absorbers, valves, connecting rods and so on. It is equally true of suspension springs.
There are a couple of reasons for this.
First, both ride height and corner weight are a function of the load at height of the individual springs at each corner of the car (not, as often supposed, of the rate or the free length of the springs). If one or more springs sag or “take a set” in use, the pre-load of the spring, the corner weight and the ride height of the car will change accordingly.
We often hear a driver report that his racecar either got loose or started to push during a race. This is usually blamed either on changing track conditions or on the tires. The real culprit may be one or more sagged suspension springs.
Think about it. An Indy Car on a 2.5mile oval will be running a dynamic ride height of about 0.25in (6.4mm) and spring rates in the 4000-5000lb/in (700875 N/mm) range. One spring taking a very small set can change the corner weight enough to change a perfectly balanced racecar into an undriveable pig. The situation in road racing is better – but only slightly. Chasing this sort of thing wastes effort, tires and engines and can drive sane crews mad.
Second, race teams change springs frequently. Every time that they do so, the pre-load has to be reset and the car has to go back on the alignment platform to have the ride height and corner weights readjusted. The closer that the springs approach perfection, the less the margin for human error, the less time is spent on the alignment pad, and the less irretrievable practice time is lost.
Very few spring manufacturers are capable and willing to take the time and effort to design and manufacture suspension springs to the stringent specifications and tolerances demanded by professional motor racing.
Hyperco’s springs are produced by Matthew-Warren Inc (formerly the Mechanical Spring Division of Rockwell International), where the legendary Jim Fiedler has been making racecar coil springs since he was discovered by Team Lotus during its first invasion of Indianapolis in 1962.
There are several parameters to coil spring design. A spring is designed to support a specific load at a specified design height, to have a specified rate (usually expressed as the load in pounds required to compress the spring one inch from its design height), to fit inside a given dimensional envelope, and to offer both a maximum free length and a minimum solid stack height. The closed and ground ends must be uniform in both dimension and rate, and they must be square. Racecar springs should be designed for an infinite fatigue life and, of course, for minimum weight.
Coil springs can be either “hot-coiled” or “cold-coiled”. Cold-coiled springs are more precise, but only if they are wound from pre-heat treated bar stock. Many racing springs are cold-coiled from annealed “bundle stock”, then quenched and tempered after coiling. This method produces a less accurate spring and can result in surface imperfections.
To arrive at the optimum design and minimum weight for a given spring, it is necessary to determine the most efficient wire diameter. A spring that is overstressed at one diameter may well be under-stressed and heavier than necessary at the next larger stock wire diameter. For reasons of economy, most spring manufacturers inventory wire in standard mill run diameters with increments of 0.031in (0.8mm). Hypercoils are designed from an inventory of special mill run wire diameters in increments of 0.015in (0.4mm).
Both the fatigue life and the maximum load that a spring can support can be increased by inducing residual stresses within the material by carefully “presetting” the spring. The fatigue life of virtually any metal component can be increased notably by shot-peening (not to be confused with bead- or sandblasting). All Hypercoils are pre-set and shotpeened. Plating metal components often leads to hydrogen embrittlement and subsequent fatigue failure. Hypercoils are powder-coated with a distinctive, abrasion- and chip-resistant finish that does not affect the performance or life of the spring.
Hyperco springs with a wire diameter of 0.625in (16mm) or less are coldwound from pre-heat treated SAE 9254 chrome-silicon wire. Optimum wire diameter for the individual design is selected from the largest inventory of wire diameters in the industry. After coiling, the closed ends are ground, and the spring is thermally stressrelieved, pre-set and shot-peened. Completed springs are magnaflux inspected, powder-coated and checked for dimensions, trueness and rate.
Hyperco then tests each coil for actual rate, and packages matched pairs in permanent storage cylinders. This pairing of Hypercoils within the already tight manufacturing tolerances assures the racer of accurately rated springs within a toleranced range of +/-1%.
Springs of wire diameter greater than 0.625in (i.e. NASCAR’s prescribed 5.5in/140mm diameter springs) are hot-wound from SAE 5160 chromevanadium wire. The wire for these springs is centerless ground, both to remove surface imperfections and to arrive at the optimum wire diameter for the specific design. After coiling, they are quenched and tempered immediately (to avoid the creation of a decarburized layer), stress-relieved, shot-peened, and inspected. Hyperco hot-coiled springs are guaranteed to be within +/-2% in rate.
Some confusion regarding spring rate has resulted from the difference in measurement methods between the UK and the USA. In the UK spring rate is measured from free length – the spring rate is expressed as the load in pounds required to compress the individual spring exactly one inch from free height. In the USA, the spring rate is expressed as the difference between the load at design height and the load required to compress the spring one inch from design height.
Because the closed and ground ends offer less resistance than the main coils, the UK measurement is inevitably a lesser number. Because the closed and ground ends of many springs are not particularly uniform, the UK method may not give a true indication of dynamic spring rate. As a point of interest, an electronic load cell tester is required reliably to measure either the rate or the preload of racing quality springs – the popular hydraulic “spring rate checkers” are simply not accurate enough.
Hyperco has invested heavily in computerized testing and measuring equipment. The company can generate accurate graphs and tables of spring load per unit deflection in increments of 0.1in (2.5mm) from free length to solid stack. These also serve as preload tables. Full spring design assistance is available (with same day service when required). Hyperco will supply springs manufactured to a team’s design, or design springs to fit specified parameters.
In Formula 1, weight and package size are critical. Where weight is the paramount consideration, springs are designed to be as short as possible within the twin parameters of required deflection and fatigue life. When the last few grams of weight are not of over-riding importance, designers prefer springs designed to a “load at height”.
For cost considerations, most racecar springs are designed to a standard free length, rather than to a standard load at design height. This necessitates adjusting preload, ride height and corner weight with each spring change. If the full range of springs were designed to support the same load at design height, this would be virtually unnecessary.
Hyperco’s reputation for quality and service has generated impressive results. Every Indianapolis 500 winner for the past 33 years used Hypercoils. Fully 80% of NASCAR’s Winston Cup cars, virtually all of Indy Lights, SCCA Trans-Am, Toyota Atlantic, coilsprung USAC midgets and World of Outlaws Sprint cars are sprung with Hypercoils. The majority of Japanese Formula 3000 and Formula 3 teams are customers.
Both “anti-rattle” and helper springs are available but, since racecar designers have moved away from the variablerate coil spring, Hyperco supplies only linear rate springs. Any desired variable rate curve can be accurately and economically achieved by stacking linear springs in series. Hyperco can provide design service for these installations when required.
Future plans are to continue expanding the line of high-quality Hyperco suspension coils to encompass all forms of racing. In order to improve overseas service, SPA Design has recently been appointed Hyperco’s UK and and European distributor. With ever-increasing emphasis on suspension technology, and as more and more quantitative information becomes available through on-board data collection, Hyperco is confident that more racing teams and constructors will recognize the performance advantages provided by Hypercoils.
Reprinted from Racecar Engineering Magazine, 33 Banstead Road, Caterham, CR3 5QG, England, Tel: 0883 34 1551 © Q.Editions Ltd 1994