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A couple of times a week we are asked to give the plusses and minuses of composite vs. steel rear leaf springs. This is how we see it:
The TRW spring is the OEM replacement spring for the 81 and 82 Corvette. From 1963-1980, Corvettes came with steel rear leaf springs. In an effort to shave weight off the car to help with fuel economy, for the 1981 model year Chevrolet went with magnesium valve covers, a composite rear spring and a number of other weight saving substitutions. The spring change saved about 20 lbs; a huge savings for comparatively little money. To the end user, the twenty pounds is equivalent to about two and a half gallons of gas; not exactly earth shaking for the average Corvette driver.
Unfortunately the composite spring weight savings was right where the Corvette least needed to lose weight; low and behind the rear axle. The steel spring is like ballast in a racecar, right where it needs to be; low and behind the rear axle.
Composite springs don’t lose their spring rate over time. A 315 lb spring will stay 315 lbs until it breaks. Yes, that’s what sends composite springs to the dumpster; a rock in just the right place. Hit on edge by a road hazard, a composite spring will often de-laminate. It will turn a milky white at the point of injury and its service life is over. A steel spring will slowly lose its spring rate over time, but it will never break.
The softest TRW composite spring is 315 lbs. Most C2 and C3 Corvettes came with 196 lb. steel rear springs. After 20 years or so, when they are ready for replacement, their effective spring rate has eroded by as much as 40%. Replacing the original spring with a new stock steel spring makes a big difference; a 40% difference. Changing to a 315 lb spring would be a change of 270%, and you would have a completely different automobile.
The spring rate of the 78-80 OEM factory steel rear spring was 260 lbs. You can order a 315 lb spring for a 78-79, but for anything newer than that, you have to chose between 345 and 355 lbs. That’s a heavier spring rate than we run on our racecars, so you can imagine it’s not the softest ride on the planet.
One more thing that may or may not be a big deal to a street driven Corvette: The steel spring is at least somewhat progressive in its spring rate. In our custom steel springs that we use for our racecars, we individually arch every leaf to achieve an almost linier rate progression. This helps handling a lot, and we see it in our lap times. Obviously that’s not possible with a composite spring.
In summary:
Composite spring Advantages:
• Light weight
• Permanent spring rate
Disadvantages:
• Cost
• Durability
• Limited spring rate options
• Often much stiffer than stock
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