lightening flywheels vs lightening brakes

A high performance brake rotor is chock full of holes as is a flywheel. I understand that the brake rotor is designed to turn momentum into heat through clamping force and then shed the heat as quickly as possible. The flywheel on the other hand is trying to not heat up at all while providing as much clamping force as possible. The flywheel is large and heavy and when it is lightened I have never seen the contact surface itself drilled. That means that there is a limit to how much weight can be removed. I am wondering why the contact surface is always left alone unlike a brake rotor. Surely you need as much surface area in a brake rotor as you do in a clutch? My Triumph has a Tilton button clutch with about as much pad area as a set of high performance brakes. I have also had heavy trucks with button clutches, and they seem to clamp as well as the full contact organic clutches. So if material can be removed from the pad side of the clutch, why not the flywheel side?

From my understanding, the slotting and drilling of a rotor is actually go outgas the brake pads. You develop a gas cushion that can impair braking when the pads and rotors get too hot. Most of the TRUE racing rotors are slotted. Crossdrilling can and will crack if not done properly. I don't really see the benefit on a flywheel as far as slotting it because you're not building up that much heat and gas. Removing mass helps the engine spin faster and all that good stuff, but as far as drilling and all that, there wouldn't be much benefit.

Isn't there reason for the holes in brake rotors is to relieve gases formed during the hot braking process? Not for weight reduction. There is probably a loss of braking potential from the loss of 'mating' surface but is surpassed by relieving gasses trapped between the two surfaces by lowering the effective equilvalent friction coefficient.

there was an article in the June 2008 GRM that talked a bit about this.

It also increases the surface area of the disc (helps it cool faster)...point is it's not for weight reduction.

Cross-drilling reduces pad-to-disc friction (which any car with decent brakes can easily spare), improves cooling and decreases weight (although not by much), but the downsides are that it decreases durability (cracking problem) and increases pad wear (pads are fairly cheap and easy to change).

Now why would you want the same things done on a flywheel? Like brake discs, if lightening is the aim then different materials are a much better approach than drilling holes. If you could spare friction on a clutch, you'd do better to run a smaller diameter one. Heat generally isn't a problem at all. Clutches are not so cheap and a PITA to change, and finally, exploding flywheels are a very, very bad thing.

I could be mistaken, but from what Ive read, modern DOT approved (road approved) pads dont really offgass anymore since there are rarely compounds (organics?) in them that produce gasses when heated .

4cylndrfury wrote: I could be mistaken, but from what Ive read, modern DOT approved (road approved) pads dont really offgass anymore since there are rarely compounds (organics?) in them that produce gasses when heated .

Don't let fashion get pissed on by reality man.

To the OP, there are flywheels with replaceable friction faces which are generally screwed in. These screw countersinks effectively give the cross-drilled effect.
Another thing to keep in mind is that brakes are meant to be used for long periods of time compared to clutches, when it comes to slipping conditions.
The wear rate fro the clutch face is significatntly lower than brake pads, for purposes of servicability. There is also the difference in torque absorbed byt the two sytems.

I have found the holes and slots give a very nice spot for cracks to start.

But they look cool.

If you look @ supercars... not one comes with slotted rotors... drilled yes, slots no

From my understanding... drilled rotors has nothing to do with off gassing, they have everything to do with less weight, or so that was then intent on the Porsche 917

seems I remember reading that most high performance brake pad manufactures now recommend NOT using drilled rotors ... can't remember if they also caution against slotted .....

I read somewhere, here maybe, that a racing team stopped using drilled rotors due to cracking.