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?

 

[LittleInch]

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.

 

[BrianPetersen]

"... 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.

 

[3DDave]

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.

 

[TugboatEng]

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.

 

[gruntguru]

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

 

[IRstuff]

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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[LionelHutz]

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.

 

[gruntguru]

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

je suis charlie

 

[GregLocock]

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.

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[3DDave]

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.

 

[Lou Scannon]

Graduates of the Bob Newhart Bus Driving School

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[geesaman.d]

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.

 

[romychaj]

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).

 

[gruntguru]

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

 

[romychaj]

didnt i say that?

 

[gruntguru]

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

je suis charlie

 

[SwinnyGG]

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

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.

 

[gruntguru]

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

 

[BrianPetersen]

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.