Smaller brakes?

Tyre road friction is highly variable depending on temperature, road surface, tyre friction factor and applied load on the tyre. For maximum breaking you need to get to about 15% slip. Not lock up but you need to get close.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

"... are a car's brakes strong enough to lock the wheel"?

Answer, It depends. Exclude the effect of ABS for this discussion.

If you are driving on ice with a layer of water on top (temperature right at the freezing point ... slipperiest condition) with summer tires, it will prove very difficult to NOT lock the wheels no matter how lightly the brakes are applied.

On the other had, if you put a good set of racing slicks on some 1960s car with 4 wheel drum brakes and go out on a dry racetrack with an abrasive pavement formulation and try to drive fast such that you overheat the drum brakes ... or you overload a tractor-trailer and drive down a big mountain without making use of the "jake brake" ... you are going to have trouble stopping. There's a reason for those "runaway truck" ramps in mountainous areas.

So ... you had better clarify your question and the circumstances surrounding it.

Higher coefficient of friction between pad and rotor will reduce the amount of effort that has to be applied by the driver for a given amount of deceleration.

It's all a balance. Greater CoF could mean greater wear rate or using a material that will melt or ignite from the energy the pads dissipate. On a bicycle the use of rubber pads was fine for rim brakes, but not for disc or coaster brakes.

The force creates by the brakes needs to be controllable. The higher CoF will have a steeper slope on the force/pedal pressure curve. This will make it more difficult to modulate the force which will make the brakes feel "grabby" and more prone to locking the wheel.

Not sure if I agree with that TbE. if higher CoF pads were viable at OEM, the braking system would be recalibrated to obtain similar pedal effort at lock-up with similar results in terms of modulation.

OP. One significant advantage of higher CoF pads would be possible elimination of the brake booster.

je suis charlie

One reason for NOT increasing brake pad friction would be the knowledge that after-market pads with lower friction might get installed, resulting in a decreased ability to stop.

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Not requiring a lot of pedal pressure to lock-up the brakes makes it easier to modulate them as long as there is enough pedal travel.

Interesting.
So do drivers modulate braking by varying pedal force or displacement?

je suis charlie

Brake pedal feel is complex, people don't like a very stiff pedal, which I would have thought is the optimum. The curve is non linear, with a generally rising rate.

Cheers

Greg Locock


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I used to really enjoy overboosted self-energized drum brakes. Give them a tap and practically slap the bumper on the ground. So, so touchy.

The ratio of force to effectiveness needs to be closer to linear and not function as an on-off switch. Though tell that to some city service bus drivers I was exposed to on daily commutes who apparently had a full acceleration and full brake buttons to press.

Graduates of the Bob Newhart Bus Driving School

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

Quote (tk90)

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?

OEM brakes are designed to lock up a wheel. That's mandatory for ABS to work well.

Higher friction is certainly beneficial when you increase the performance of a car above normal street driving conditions. This comes at a practical cost, as higher friction brake pads require some warm-up and perform terribly when cold. This makes them dangerous for street driving. So it seems to me that without giving up cold performance, OEM brake pads are already operating at maximum available friction. Then the size of the brake rotor and caliper are reduced as much as possible for cost and unsprung weight savings.

what has not been said:

- cof varies with temp
- cof varies with speed
- cof varies with pressure

(simple physics)

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

if the original question was: will a "wonder pad material" with e.g a cof of 1.0 benefit braking? i think yes, provided there are no of the usual trade offs (cold bite, abrasiveness, dust, noise, modulation, durability). on the other hand such a "wonder pad" would work as well with a cof of 0.1 if the braking system was designed for it.

edit: it might be interesting to note that the brake energy diminishes with speed and there is a temperature maximum of the sliding surfaces often roughly in the middle of the braking action (apply time).

Quote (romychaj)

on the other hand such a "wonder pad" would work as well with a cof of 0.1 if the braking system was designed for it.

Disagree. Something else would have to be compromised to compensate for the tenfold reduction in COF. . . . pedal travel, booster amplification etc.

je suis charlie

didnt i say that?

I disagreed with the bit in quotes and no - it says the opposite.

je suis charlie

I think you missed the 'if the system was designed for it' part

Quote (Gruntguru)

So do drivers modulate braking by varying pedal force or displacement?

I think it's more complicated than that, but I can tell you from racing experience that a car with small initial travel and then a hard 'wall' is MUCH easier to finely modulate brake pressure than one with a lot of travel at lower force.

If you think about it - if you're driving with your foot on the pedal and someone tells you 'increase the force you're applying to the pedal by 10%', you could probably get pretty close. If someone tells you 'push the brake pedal down an additional 1.5mm' thats a much more difficult task.

Not saying that racing drivers accustomed to really paying attention to what the brakes and tires are doing are analogous to everyone, or that road cars should be built like race cars. Just an anecdotal thought.

Quote (SwinnyGG)

I think you missed the 'if the system was designed for it' part

Not at all. You can redesign the system for a tenfold reduction in friction coefficient, but something will have to be compromised. Think about it.

je suis charlie

Quote (romychaj)

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

Why would you think that? (aside from that most passenger cars cannot reach that speed ... but the ones that can, are high-performance vehicles with good brakes ... and nowadays, all such passenger cars have ABS)

All such vehicles are capable of going into ABS mode at any road speed that the vehicle is capable of. If they're capable of going into ABS, they're capable of locking wheels if the ABS weren't there to stop it happening.

Quote (BrianPetersen)

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.

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

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.

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

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

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

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.

Cheers

Greg Locock


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

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

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.

Quote (3DDave)

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:

• Phenolic Resin
• Rubber
• Metal Particles or Powders (iron, steel, copper, brass, bronze, others)
• Kaolin Clay or other Diatomaceous Earths
• Cotton, Glass and Synthetic Fibers
• Walnut and Pecan shells

What - no asbestos?

je suis charlie

Quote (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.

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

"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?

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.

TTFN (ta ta for now)
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Quote (romychaj)

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.

Quote (romychaj)

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.

Quote (IRstuff)

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

Quote:

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.

TTFN (ta ta for now)
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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-...

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.

Quote (roymachaj)

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.

Quote (romychaj)

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.

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

Meh. Pads and rotors for a Veyron are something like $50,000.

That has nothing to do with where track pads 'start', which still isn't $1,000.

nothing to do with carbon or ceramic discs. why are you disputing the obvious?

here is another populal race pad ($1100 both axis):

https://www.bar-tek-tuning.com/endless-brake-pad-e...

Quote (romychaj)

why are you disputing the obvious

Providing examples of brake pad sets that cost $1,000 does not confirm that what you said, which was:

Quote (romychaj)

high perf/race pads start at $1000

How about we get back on topic, and you explain what the actual problem is that you're trying to solve, or the question you're trying to answer, we so the forum can potentially help you solve your problem or answer your question.

Well, I've always been disappointed in the amount of "bite" in my pickup trucks 4 wheel disc brakes. It seems to take quite a lot of force and pedal travel to get serious braking at or near ABS activation. I don't try to do that often but when traffic comes to a complete and unexpected stop I would like more aggressive brakes. So at 80,000+ miles it's finally time for new rotors and pads all around and I'm going to pay about double what OEM rotors and pads would cost for upgraded aftermarket parts. Note, there will be no increase in size because that would involve considerably more cost. All the reviews on multiple sites say there will be a very noticeable difference, I'll let you know. Probably be a couple more weeks before I do it.

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Well, I would say the new aftermarket high friction pads, rotors and fluid flush didn't make much difference at all. Brakes seem about the same as before. Part of the bedding in process of the new pads was to do moderate 40 mph to 10 mph braking followed by 35 mph to 5 mph braking in rapid succession without letting the brakes cool. That was enough to get some fade. Should have saved some money and used OE parts or gone the more expensive approach with bigger rotors and new calipers. I was looking at the brake specs on a new truck and in 10 years they have not changed them one bit. I don't understand how tow ratings can increase year after year and the brakes never improve.

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I don't think the truck brakes are the limit for GCW, as the assumption is that any trailer over 750 kg (as an example for my SUV) has brakes. They are involved in GVM calculations, but I don't know how that calculation is done. On most vehicles I've worked on the actual limit was gearbox/diff temperatures up one particular mountain.

Cheers

Greg Locock


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