Friday, July 20, 2018

Why Get a Lightweight Flywheel?

By Matthew Eddy - 2018-07-20

First, Ill explain why a lightweight flywheel is beneficial. Obviously, any racer knows, that reducing the weight of the vehicle is always good. In fact, nothing improves a car more than reducing weight because you can accelerate faster, stop quicker and turn harder. Basically it checks all the boxes when it comes to improving performance, so just on weight reduction alone it provides some advantage. Additionally, there is another benefit when it comes to what is called “parasitic power loss.” What this means is the engine uses some of its power to move internal components (pistons, crankshaft, etc) and driveline parts (driveshaft(s). A lot of this is unavoidable, but some of it can be reduced be eliminating as much mass in the system as possible.

They flywheel is one of the heaviest single component in the drivetrain – depending on the vehicle. It serves a few purposes such as helps to start the car, stores energy to smooth out the engine, and ease shifting by keeping the RPMs from dropping quickly (or at least that is some peoples opinion). Essentially, its to make the driving experience more pleasant. However, for a more race/performance centric car this is less of a concern and instead the focus is on speed and power. So taking this heavy flywheel out (for perspective the one in my E36 BMW weighs about 24lbs) and replacing it with a lighter one will reduce the overall weight of the car and more importantly reduce the amount of mass the engine has to turn and that means more power to the wheels.

There are a few different lightweight options available depending on the application. For lower power cars such as my stock BMW that produces 190hp, a good option is an aluminum flywheel. As I mentioned above, the stock flywheel weights 24lbs, the aluminum flywheel is only 10lbs. Now, an aluminum flywheel is not 100% aluminum, the ring gear that the starter engages will be steel, and the face the clutch will grab will be a hardened steel plate riveted or bolted on because the aluminum will just get torn up. You can see the pictures of the flywheel I just purchased for my BMW below for reference. Another claimed benefit is for some cars with dual mass flywheels, the single piece lightweight flywheel improves reliability since the dual mass flywheels can fail when you ham-fist your shifts during some spirited driving.

There is a steel plate bolted to the face of the flywheel where the clutch engages

Other side of the flywheel

Now aluminum is great but some engines produce a lot of horse power and the aluminum may not be able to stand up to it. You choose to go with something that is a bit stronger such as one made from chromoly. The weight savings are not as good but it’s still fairly significant. One for my car would be 11-14lbs (depending on brand) which is a 10-13lb weight savings over stock but still 1-4lbs heavier than the aluminum one. What is chromoly? It’s a particularly high strength steel alloy with relatively high amounts of chrome and molybdenum. It’s from Chrome and Molybdenum that the term chro-moly comes from and is designated as 4140 steel. Most flywheels are made from cast iron which isn’t particularly strong but its cheaper to make. Using high strength steel the weight savings come from the flywheel being a lot thinner and holes are added to remove additional material as well. I am not going to tell you which is best to go with, you may have to do research on your particular car and what makes the most sense for your application. Fidanza has an interesting article that you might light to read for some additional information.

What is 6061 T6 aluminum? 

There are multiple alloys of aluminum out there but the most common “billet” type is 6061 T6. The 6061 designates the alloy, and the T6 is a reference to how it was heat treated. This allow also comes in tubes, bars, sheets etc. So pretty much any aluminum flywheel will be made with 6061 T6, but its also a very good alloy of aluminum. Billet simply means it was made from one big chunk of metal. There really is not any other way to make a flywheel, but it sounds cool to say “6061-T6 billet aluminum flywheel.”   

OK Cool But How Much POWER do I Gain?

This may vary significantly per engine and how much of a weight savings you are gaining over the stock flywheel but if the test done below is any indicator – it may be 1-2% gain in peak horse power and 2-3% gain in peak torques at the wheels. This may not seem like a lot, and its hard to say that this directly correlates, but if we say this could translate to 1-2% reduction in lap times that’s not too shabby.

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