Why increase Brake Pad Friction

[tk90]

I suppose this question first hinges on another:

In general, are a car’s brakes strong enough to lock the wheel?

Given that locking the tire (skidding) is detrimental in terms of stopping distance, and given that a brake system can effectively lock the wheels, is there a benefit to increasing CoF between pad and rotor?

 

[SwinnyGG]

If they're capable of going into ABS, they're capable of locking wheels if the ABS weren't there to stop it happening.

For sure. I think it's conceptually a little deceptive, because maybe the thought of 'locking the wheels' assumes near-instantaneous locking of the wheels immediately upon smashing the pedal to the floor.

At high speed there's a lot of KE in the wheel/tire for the brake to absorb before it can lock the tire, but it'll still lock the tire once the KE is absorbed.

@romychaj consider this.... engine torque available to the wheels reduces as speed goes up because of gear reduction, but full brake torque is available 100% of the time. It would probably be possible to design a brake system such that at high speed, the KE in a wheel and tire assembly was enough to fade the brakes to the point where once that energy was absorbed torque sufficient to lock the wheel was no longer available.... but in a car capable of high speed, that would be a really awful (and really dangerous) design.

 

[gruntguru]

At 150 mph there will be a lot of tyre smoke if you suddenly slow the wheels to even 100 mph. No need to "lock" the wheels.

je suis charlie

 

[MintJulep]

Brake pads are optimized to be made from low-cost materials using low-tech manufacturing processes in low-wage countries.

Nobody has a business incentive to change this status quo.

 

[TugboatEng]

That's true if you buy your brake pads from AutoZone... That's not true if you buy them from the OEM.

 

[SwinnyGG]

Uhhh… yes it is. There aren’t 9,000 brake pad factories in the world.

 

[gruntguru]

Its not the factories, its the R&D facilities of the prime contractor. I am sure that Brembo, Bendix et al focus on more than just cost.

je suis charlie

 

[GregLocock]

I was once involved in discussions with a brake pad manufacturer. There's an awful lot of by guess and by god in the mix.

The radar chart for brake pads has dry and wet friction, cost, wear rate, green fade, the other fade (mountain descent), progressivity, grabbiness, and of course everybody's favorites creep-groan and squeal. (from memory)

Some of those are installation dependent.


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Greg Locock


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[MintJulep]

Brake pads R&D is mostly altering the proportions of ingredients.

It's rare that something new is added to the mix.

 

[3DDave]

My buddy worked for Wagner Brake in R&D. They had a glitch of sorts - thought that the drum brake market for cars would stay good so he ended up with me in defense work. He had a similar story - tweak to hit all the requirements. They even tried pecan shells as an ingredient. Ohh noooo! I've said too much.

I've been reading about Russian 7X7 jets and new requirements to cut down on braking to save pad life. Apparently the carbon pads wear much faster if lightly applied cold than when up to temperature.

I expect that regenerative braking and active control of the torque to drive a force/decel will simplify the requirements.

 

[MintJulep]

Ohh noooo! I've said too much.

You've given away the secret sauce!

Pecan and walnut shells are both used actually. They are abrasive, and present sharp new edges as they fracture. This helps to keep the disk or drum clean. Also, as I noted earlier - cheap.

Since we've already started to spill the beans (not used as far as I know).

Stuff used in brake pads:
[ul]
[li]Phenolic Resin[/li]
[li]Rubber[/li]
[li]Metal Particles or Powders (iron, steel, copper, brass, bronze, others)[/li]
[li]Kaolin Clay or other Diatomaceous Earths[/li]
[li]Cotton, Glass and Synthetic Fibers[/li]
[li]Walnut and Pecan shells[/li]
[/ul]

 

[gruntguru]

What - no asbestos?

je suis charlie

 

[SwinnyGG]

Its not the factories, its the R&D facilities of the prime contractor. I am sure that Brembo, Bendix et al focus on more than just cost.

No doubt there are other factors.. but for OEM applications everything is made to a price point.

 

[romychaj]

"i would think that most passenges cars cannot lock the wheels at high speed (150 mph+)."

sorry, i meant to say 150 kph+.

not talking about high perf cars but the average car. so lets say we are looking at a $25k car, from 5-10 year ago (lets not discus this). take it up to top speed, 180-190 kmh probably, and hit the brakes. by the time you get 100% pressure the car is at 160 kmh and im convinced most of them cant stop the wheels/activate abs at that speed.

some other remarks:

- probably over 100 different materials are being used, up to 20 in one compound
- surprisingly most of them are cheap: glass, clay, cellulose, vermiculite, "rust", silica, carbon black, fly ash, al2o3 etc.
- sisal and coconut fibers give good results
- high perf/race pads start at $1000 (both axis), so considering the steel backing plates the compound is quite expensive stuff per wheight in the end
- mos2 and other friction modifiers go in there as well
- most exotic: banana peels

what i still cant grasp is the use of resins (epoxy, phenolic, silicone, cyanate ester) with a stability limit of 300-350°c while the pads (at least the surface, depending on conductivity) get up to 600-700°c easily.


edit: oh, "At high speed there's a lot of KE in the wheel/tire for the brake to absorb before it can lock the tire, but it'll still lock the tire once the KE is absorbed."

so what do you figure this might be in numbers? im guessing the equivalent mass of tire, wheel, nuts and disc might be 10% of vehicle mass. so deacceleratig this in e.g. 1/10 of the time will indeed double the torque neccessary. good guess?

 

[IRstuff]

Burning off the pad is one way to dissipate KE.

The rotors are fixed to the wheels and the pads are fixed to the frame. The wheels cannot lose KE unless the entire car loses KE.





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[SwinnyGG]

high perf/race pads start at $1000 (both axis), so considering the steel backing plates the compound is quite expensive stuff per wheight in the end

Uh... no they don't, unless you're talking about a vintage sports car or something crazy? You've got some weird sources. Or something. Full set of pads for the car I use for track days, which is a mid-90s sports car, is about 200 bucks.

good guess?

Not particularly. What you're trying to learn here is no longer very clear.

If you want numbers, calc them out. The math is not super hard.

 

[gruntguru]

The rotors are fixed to the wheels and the pads are fixed to the frame. The wheels cannot lose KE unless the entire car loses KE.

The wheels have KE of translation but also rotation. They can lose the rotation bit without the car slowing.

je suis charlie

 

[IRstuff]

They can lose the rotation bit without the car slowing.

Not unless they slip, which is where this discussions started, i.e, wheels locking. If they aren't slipping, then the rotation is strictly a function of linear motion.

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[romychaj]

swinny, a ferrodo front set of race pads for a corvette is $750. the rear will be a bit cheaper. the rest of your post is not very helpful either.

https://www.essexparts.com/ferodo-frp3133g-ds3-12-brake-pads-c7-corvette-camaro-56-cts-v

i now see that correction factors in the brake energy formula are given as 1.15-1.60 depending on the gear. that seems to imply an engaged clutch. so my "best guess" of 1.1 seems a good one. anybody?

edit: for clarity, this concerns the "equivalent" (=rotating) mass. again, average car.

 

[SwinnyGG]

swinny, a ferrodo front set of race pads for a corvette is $750.

....ok?

If this post is related specifically to corvettes with carbon ceramic brakes, this would be the first mention of it.

i now see that correction factors in the brake energy formula are given as 1.15-1.60 depending on the gear.

correction factors for what? What 'brake energy' equation?

If you clearly state what the actual problem is that you're trying to solve, you're likely to actually get answers. You have yet to do that.

 

[LionelHutz]

No, there were the first set found to kind of prove a point...