Flywheels are necessary on piston engines because they preserve the momentum from one combustion stroke to the next. Early engines were so inefficient they had to have enormous flywheels to keep them running. Modern cars still derive some benefit from a heavy flywheel, but for racing, less weight is better.
To find out more about how lightweight flywheels work and why, we caught up with Dan Jenkins, a tech and product specialist with Fidanza Performance, a company that began by manufacturing lightweight flywheels for early Jaguars, then branched out to a full product line of flywheels and clutches for European, Japanese and domestic cars.
The first question is somewhat obvious, which is if modern engines are so great and don’t really need all the mass of a big, heavy flywheel, why don’t cars come from the factory with lightweight flywheels?
That’s where modern cars derive some benefit from flywheels that are heavier. Stop-and-go driving, silky engagement and reduced harshness are all key to a pleasant driving experience. In fact, cars now come with dual-mass flywheels, which are a two-piece design with a spring-hub center, not dissimilar from a clutch disc. The two-piece construction isolates noise vibration and harshness, which is necessary on a modern street car.
For racing, a pleasant driving experience means winning, and weight is often what stands in the way of victory. On a racecar, a little additional noise and vibration that comes with a lightweight flywheel is worth the tradeoff, but how much weight loss are we talking about?
“We average anywhere from 40 percent to 60 percent lighter than OEM,” Jenkins said. “It does vary per application, depending on whether you’re coming from an OEM single-mass or an OEM dual-mass flywheel.”
With a lightweight flywheel, right away you will notice the engine revs more quickly. You free up the horsepower your engine is already making and send it to the rear wheels rather than have it dissipate as parasitic losses. There are other benefits, too.
“It does help with reduced clutch wear and improved clutch efficiency. With the lightweight aluminum flywheels having better heat dissipation over a steel flywheel, it’s going to allow for a longer a life of the clutch itself,” Jenkins said. “The material itself will dissipate heat through the two surfaces compared with having one solid steel surface, which would hold the heat in that clutch area and overheat it and create more wear. It also helps with faster shifting and smoother shifting, with less rotational mass, it’s easier to get in and out of the gates and do rev-matching for the shifting process.”
For street applications where you might use the car as a daily driver in addition to HPDE, there is such a thing as too light. In addition to harsh engagement and the aforementioned vibration, flywheels that are too light can trigger check engine lights, hamper a smooth idle or create a situation where excessive rpm is needed to get the car to accelerate. But for racing, where you’re nearly always operating at the top of the rpm range in upper gears, the lighter, the better.
For momentum cars that don’t make a lot of horsepower, single-disc setups work fine with a lightweight flywheel. For faster applications, choosing a flywheel and clutch setup depends on horsepower, and if you’re making enough power that the clamping loads of the pressure plate are high and it resembles more of an on-off switch, it might be better to go with a dual-disc setup.
“The biggest thing when it comes to mating up the flywheel to a clutch is going to be the power that the vehicle is making,” Jenkins said. “So it’s not a matter of a methodology of designing it to a specific material. The material we have for our friction surface of the flywheel is designed to mate up with anything from an organic OEM style clutch to a ceramic to a Kevlar to a sintered iron. All of those will work with no problem. It’s just a matter of your driving habits you’re looking at using before you pick a clutch.”
A dual-disc setup has a lot more surface area, so you don’t need as much clamping force. However, a lot of times with a twin disc, it has power capabilities in excess of what the car produces, so it’s not necessary.
Fidanza CNC-mills its flywheels from 6061-T6 aluminum with 1050 heat-treated and flattened steel for the friction surface. The friction surface can be replaced by unbolting it from the aluminum portion and bolting on a new one with new self-locking fasteners. Before putting them on the market, Fidanza sends out all its products for SFI certification.
“They’ll spin it up to 150 percent of the rpm of the vehicle application it would be going into, so on average, they’re spun up to about 10,000 to make sure they’re not going to burst, to make sure they’re not going to come apart at higher rpm,” Jenkins said. “Some applications are spun up to 15,000 rpm, but the average is 10,000 to 12,000, so that way you know they’re not going to come apart in the middle of a race.”