The flywheel is a key part of your manual transmission vehicle’s driveline. Flywheels are not only important because, when combined with a clutch, they create the main transfer point of horsepower and torque from your engine to your driveline, but they also help balance the engine’s rotating assembly and help carry it’s momentum through the combustion cycle. Flywheels come in various materials and designs, including cast iron (most stock flywheels), nodular iron, chromoly steel, billet steel and billet aluminum. No matter what your flywheel is made of, whenever you change your vehicle’s clutch, you must also resurface or replace your flywheel.
Here is a little more info on flywheel materials to help you choose a replacement that’s right for your needs:
Most OE or stock flywheels are made out of cast iron (commonly called “grey iron”) similar to what cast iron frying pans are made of. “Grey” cast iron is formed with graphite flakes in the casting material that help it resist straight-line cracking as you would find in “white cast iron,” but also makes the material more brittle. Grey iron has less tensile strength and shock resistance than steel, but a comparable compressive strength to low or medium carbon steel.
*(Ok for a stock flywheel, but will not take much additional stress or impact, also does not have the tensile strength to endure high rotational stress – high rpm. Can be re-surfaced a few times in most cases, except in the case of Dual-mass flywheels, before requiring replacement.)
Nodular Cast Iron-
Nodular cast iron (also referred to as ductile iron) is the next common step-up in material and is characterized by having the graphite content of the iron form nodules during casting rather than flakes, this nodular formation of the graphite further inhibits the formation of stress cracks and strengthens the material against impact and fatigue but does little for tensile strength, surface wear or heat dissipation.
*(Better for a flywheel, but will still not handle impacts or high rotational stress well. Can be re-surfaced a few times in most cases before requiring replacement.)
Steel is an alloy of iron when combined with carbon and other trace elements. The amount of carbon in the mix can vary the hardness, ductility and tensile strength of the steel and can improve the iron at an atomic level to overcome what makes pure iron so ductile and weak. Steel therefore is stronger than cast iron, but only by sacrificing ductility and thereby requiring significant machining and handling. The quality and characteristics of steel can also vary widely dependent on the mix of carbon and trace materials, as well as, how the steel is heated, mixed and cooled as it is formed. The term “billet” steel, actually refers to the form the steel is created in. A billet is a long square or round bar of material that is then cut into “slices” before machining. The primary benefits of “billets,” are that they are more consistent in metallurgy and that their outer material is often machined away removing stress areas from the metal and creating additional strength. Weighing in at about the same as a cast iron counterpart, the benefit of a steel flywheel is its increased tensile strength and reduction in brittleness.
*(Better choice for a performance flywheel – can be SFi approved, but still very heavy. Does not dissipate heat well and still requires resurfacing when changing clutches. Can be re-surfaced a few times in most cases before requiring replacement.)
(aka. Chromoly steel) is an alloy of steel created primarily by the addition of chromium and molybdenum to high carbon steel. The addition of these two materials makes the steel stronger by weight, which in turn allows for a lighter flywheel to be made by removing some material from the dimensions of the part or creating “lightening holes” where sections of metal are completely removed. Re-dimensioning or adding these “lightening holes” to the flywheel design must be done carefully though to avoid creating any weak spots or stress points that may fail at high rpm or aggressive clutch engagement. Chromoly also often has a higher surface hardness or durometer that may be sacrificed when re-surfacing is required.
*(Better choice for a performance flywheel – can be SFI approved, but still heavy. Lighter weight than OE these flywheels can add some performance, but still do not dissipate heat well and will require resurfacing, if possible, when changing clutches. Can be re-surfaced a couple times at most before requiring replacement.)
(aka billet aluminum, 6061 T6 aluminum) is a non-ferrous metal that is lighter weight than iron or steel alloy and has a higher rate of thermal dissipation allowing it to cool and transfer heat more quickly. Aluminum also has higher ductility and is more easily machined and formed than steel alloys. “6061” aluminum is a hardened aluminum alloy containing magnesium and silicon to give the metal a higher tensile strength, making it stronger and able to handle higher rotational and impact stresses. The T6 reference in “6061 T6” refers to a tempering process that further increases the aluminum’s tensile strength to at least 42,000 psi. Since aluminum is malleable, and will not hold up well to direct contact with friction materials (as used on clutch discs), most aluminum flywheels incorporate a replaceable wear surface or friction plate made of high carbon steel and mated to the flywheel’s friction surface area with bonding material, rivets or bolts. Aluminum flywheels also usually feature a separate hardened steel ring gear/starter ring mated to the aluminum flywheel body by various means including interference fit, lock bolts or welded tabs.
*(Much better choice for performance flywheels – can be SFI approved, and is much lighter in weight providing additional performance, quicker throttle response and faster acceleration. Aluminum also dissipates heat better allowing for increased clutch efficiency and longer clutch life. Replaceable friction surface means flywheel is re-buildable and will not require periodic replacement with a new flywheel once re-surfacing tolerances are exceeded, just simply replace the friction surface.)
Choosing the “right” flywheel for you/your vehicle:
In simple terms, it depends on what type vehicle you have and how you’ll be using/driving it as to what would be the best choice of flywheel to purchase. Here are a few guidelines: