Brake and Clutch Friction Material

[Ian Anderson]

I could just be confused but follow my logic and see if it makes sense or if I have it wrong please let me know why.

On brakes running steel discs and friction pads - these work best once uo to a reasonable temperature - in my simple mind this means the friction increases between the surfaces at a higher temp. (until too hot when it all goes a bit wrong)

So on a clutch a sumlar process works - friction material and metal surface. A slipping clutch is bad and will heat everyhing up but will it to a point increase the friction / available thrust?

Now taking this to the nest step a clutch in a race start situation / drag race should bite better if it has been slipped and the surfaces "pre heated"

OK possibly not going to make any major difference but is the logic sound for kitting the drag strip? Or with a Renault 21 box which is marginal on power could this be sthe slip required when cold to not break input shafts and once hot perhaps cause more damage?

Please put my brain out of it's misery of trying to decide what is correct

Thanks
Ian

[Randy V]

Back in the 70's on my old TF Dragster, I used a clutch called a Crowerglide that was a centrifugal multiple disc (metal) clutch. It could be adjusted to slip various amounts until a specific RPM was reached where the combination of the heated plates and centrifugal force would lock it up..
Since there was no transmission involved, we did not want it to lock up too early or we would not be up high enough in the Torque band to make a decent pass..

That said - the slipping of the clutch was not really a torque multiplier like a torque convertor, but it would allow more torque to be imparted to the driveline at a lower speed.. I don't know how you would apply something like this with an R21 box or if you would even want to.. We had to rebuild the clutch every 2-3 runs back then. Today they rebuild them every run..

[Jac Mac]

A lock up converter with manual control over the lock-up function is essentially what you require Ian--& a dog engagement system, not much point in having synchro when the TC will absorb the change period.
Early Honda Civic Auto was a manual trans with Torque Converter & IIRC Rootes Group had Easi-drive for some models & I think Rambler/AMC had a similar setup at some point.

[Ian Anderson]

Sorry guys - seem to have thrown the ball too far out of the field! I'm not trying to design / fit a different clutch just trying to find if the friction material would perform bettter hot or cold.

Question is with a standard friction plate clutch would it work better or worse if the system was heated up by slipping it before something like a drag start.

Not a practical question as I'm not into drag racing (the GT40 sat on the line at the Santa Pod track spinning the tyres / lack of traction off the line anyway at a run what you brung day) just want to know if my theoretical assumption would "assist"

Ian

[Randy V]

Originally Posted by IanAnderson

Question is with a standard friction plate clutch would it work better or worse if the system was heated up by slipping it before something like a drag start.

Ian - Sorry - Mis-judged what you were going for...

A Std friction plate works best when warmed only slightly. There are metallic friction plates that are almost impervious to the heat (to a point)...

Metallic Discs:




I think that brake lining material might have just a little too much friction for initial engagement much like the metallic discs do when they are hot.

[Dave P]

Hi Guys:

First off I am no an engineer. But have worked with many types of brake systems and clutches.

Generally today performance brake systems use a carbon and metal based brake pads. Friction coke which is carbon is mixed with metallic compounds and other various friction modifiers along with a phenolic binder resin. The matrix is put into molds with the backing plates and subjected to pressure and heat in high tonnage presses.

The matrix is formulated in such a way that it has different properties under given conditions. Ie: some pads have a lot of cold bite while others have to get heat to opertate properly.

Street pads usually have to have alot of cold bite to work in your ford f150 when it is -30 degrees F.

Rotors are generally made from a gray cast iron. The castings made by some companies are made with a lot of care as to the makeup of the cast iron. Quantities of various compounds can have a dramatic effect on rotor wear as well as how the casting responds to the transfer layer of frition coke put down by the racing brake pad.

Brake pads for motorcycles are made differently as they are generally sintered material and do not have the phenolic binders.

The new Carbone Lorraine automotive brake pads are sintered which is a new application for the sintered type pads.


Clutches are a bit different and usually have alot of copper in the matrix

some still have organic based friction materials

some are ceramic metallic that is usually preformed pucks.

I used to know the process that most racing clutches used but have forgotten (maybe they are sprayed with molten friction material)

But any way the standard racing 7 1/4" clutch is not designed to be slipped. The plate start at .106 thick and are worn our at about .090

Loading a car on the trailer slipping your racing clutch is a bad idea.

If you look at racing clutches the plates generally do no have springs on the hubs that is why the chatter some times on take off.

The best wearing clutches I personally have seen is a carbon clutch. They are expensive but they require less to maintain over the life of the clutch.
But many sanctioning bodys outlaw them.

Oh the coolest clutch award goes to the F1 teams it will fit in one hand
but only cost 30K or so.

Hope this helps

[Cliff Beer]

Ian, incidentally, your brakes go soft when they're overheated not so much because the pad material or rotor can't handle the heat, rather, you're boiling the fluid within the pucks/pistons. That said, rotors will start to warp and pads will lose effectiveness beyond certain heat ranges, but typically it's the brake fluid itself that's the first to go off.