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Discussion Starter · #1 ·
Assuming there are two identical cars that differ in the method of braking.


Car one brakes at a a pedal pressure of A for a time of 4 seconds.

Car two brakes at a pedal pressure of AX2 for a time of 2 seconds.

Meaning one brakes twice as hard, but half as long.


Which would allow for the pads to last longer?
Which would be cooler in the long run i.e. not overheat?
Would variables like slotted/crossdrilled rotors effect cars 1&2 in a big way?

discuss.
 

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Well, theoretically speaking:

resisitve force of friction = normal force x kinetic coefficient of friction

So in the second case, you'll have twice the amount of friction force for half the time.

If that force is constant throughout the entire time, than:

work = force x time

1/2 force x time = force x 1/2 time

So the amount of work is the same in both cases. That work will equal the change in kinetic energy. That change in kinetic energy is being converted to heat. If the change in kinetic energy is the same, it will produce the same amount of heat.

If you produce the same amount of heat in both cases, than I would think that braking for two seconds at twice the pressure would be better. This gives you two more seconds for your brakes to cool down. This is assuming that your tires have enough traction.

I think having slotted and drilled rotors can help the rotors cool down faster, but I'm not sure if they'd help in one instance more than the other.
 

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On the slotted/crossdrilled effect. Its not not so much that they are cross/slotted but the overall mass of the rotors which obsorbs the heat. Most people if not all would tell you that slotted/cross drilled is useless and more for show. In extreme driving instances, this is true due to it not having as much mass as a blank rotor.
 

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the math is correct on that but i think that the applying 1/2force over a longer time would be better for brake pad life even though the energy lost as heat is the same. that's why they tell you to brake gently to get maximum brake life. also heat would be different, in case 2 with force over 1/2 time your maximum temperature would be higher (has to do with power) and the fact that with case 1 your rate of energy being converted to heat is lower hence the rotors are able to keep cooler and hence the peak temp is lower.

but seeing as this is in the autocross/racing thread to get best times you should go with case 2 and yah, slotted/drilled rotors will help with heat, but for autocrossing you won't need them, only if you plan on tracking this summer.

hopefully someone with experience can confirm this
 

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how in any way will slotted/drilled rotors help with dissipating heat? It has less mass which in return is not as efficient as absorbing heat. slotted rotors are ment for the wiping action on the brake pads and drilled rotors reduce rotor mass, negates any heat dissipation.
 

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how in any way will slotted/drilled rotors help with dissipating heat? It has less mass which in return is not as efficient as absorbing heat. slotted rotors are ment for the wiping action on the brake pads and drilled rotors reduce rotor mass, negates any heat dissipation.
If a brake rotor was a single cast chunk of steel, it would have terrible heat dissipation properties and leave nowhere for the vapourised gas to go. Because of this, brake rotors are typically modified with all manner of extra design features to help them cool down as quickly as possible as well as dissapate any gas from between the pads and rotors. The following diagram shows some examples of rotor types with the various modification that can be done to them to help them create more friction, disperse more heat more quickly, and ventilate gas. From left to right.
1. Basic brake rotor. 2. Grooved rotor. The grooves give more bite and thus more friction as they pass between the brake pads They also allow gas to vent from between the pads and the rotor. 3. Grooved, drilled rotor. The drilled holes again give more bite, but also allow air currents (eddies) to blow through the brake disc to assist cooling and ventilating gas. 4. Dual ventilated rotors. Same as before but now with two rotors instead of one, and with vanes in between them to generate a vortex which will cool the rotors even further whilst trying to actually 'suck' any gas away from the pads.
 

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I believe if thats the case then all performance cars would have them as an option. For anything less of AutoX, blank rotors will more than suffice.
I agree with you that blank rotors seem more than enough. And I'm not really sure how effective these slotted/drilled rotors are at cooling. I'm just quoting what I've read.

I'm currently trying to find if someone has actually done a scientific study on blank vs. slotted/drilled rotors. I'd like to see what the real difference is.
 

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Discussion Starter · #10 ·
I've also read and heard about slotted rotors from track junkies, saying its passage for the air, so that the brakes will be compressing the pad to the rotor, instead of pad+rotor+gases.


Thats why I threw them in. This is in regards to track and performance, so pad life isnt an issue, just brake fade/time.
 

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Erg, Blank vs slotted vs drilled has been discussed numerous times.

drilled rotors have less "mass" than blanks, this means that there is less material to disperse heat. The "drilled" bits are for outgassing, which certain pads require to work effectively.
 

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Higher applied force to the pad will create higher peak temperatures at the contact surfaces of both the pad and the rotor. This will produce greater wear. Additionally, these higher flash temperatures will warp the rotor sooner, which is the real enemy of brakes.

All theory aside, the less pedal pressure you apply the longer brakes last. Many forms of racing run brakes for hours on end and there is no doubt what 'saving the brakes' means in terms of driving tactics.
 

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i think drilled rotors also help dissipate heat more evenly over the mass of the rotor which has its on benefits as well as keeping the rotor from warping as fast. as far as slotted i would think that the extra friction helps braking but would likely warp quicker than drilled. my thinking is they both have a specific purpose for an application, ie. autocrossing with drilled and quarter mile runs and slotted. though in most cases unless your doing one or the other there really isn't much of a difference between either hard braking or soft braking due to the fact your are likely not doing this for hours on end at a time, because the same amount of energy is being applied in both instances. the caviat is that if you are braking at high speeds this changes as it takes more force to slow down an object at a higher rate of speed than a lower rate of speed obviously but then you should probably be applying engine braking at this point to reduce your momentum first before braking.
 

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Here's a powerpoint from an actual study, I believe by the University of Miami. We can actually get away from people's opinions and see some cold hard facts.

They tested pad and rotor materials, along with perforated vs. notched rotors.

From their conclusions:

"The perforated disks produced better temperature distribution as well as heat flux compared to notched disks."

IIRC, their peak temperature were the same. But we can see, perforated (drilled) disks do perform better than notched (slotted) disks.

They did not compare these to plain rotors.


Thermoelastic Instability in Disk Brakes: Simulation of the Heat Generation Problem
 

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Assuming there are two identical cars that differ in the method of braking.


Car one brakes at a a pedal pressure of A for a time of 4 seconds.

Car two brakes at a pedal pressure of AX2 for a time of 2 seconds.

Meaning one brakes twice as hard, but half as long.


Which would allow for the pads to last longer?
Which would be cooler in the long run i.e. not overheat?
Would variables like slotted/crossdrilled rotors effect cars 1&2 in a big way?

discuss.
They should be near equal until you reach the limits of adhesion of the tires cars 1 and 2 are running then this no longer will be true because you neglect to mention the upsetting of the chassis that is going to occur with abrupt braking .

Smooth on the brakes and smooth on the gas is the fast way around a track .
 

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Yes i disagree with the study.

For one, they have not stated any variables.
Same car,
same fluid,
temp ranges of said fluid,
friction coefficient between tires and the road(surface),
temps of the tires/road,
temp ranges of pads,
brand of rotor,
process of manufacturing,
slots going to the edge or not

heat a 12x2 rotor to 600degrees and a 12x6 rotor to 600 degrees.. which one has the most heat dispersion properties?

The one with the most thermal mass. ( rhetorical question as you don't catch on to my sarcasm)

12x2 blank rotor
12x2 drilled rotor - which has the most thermal mass?
 

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Thermal mass (Cth, also called heat capacity) is the capacity of a body to store heat

The one with higher thermal mass will dissipate less heat.

Surface area also has a drastic effect on heat dissipation. Drilling and slotting rotors increases surface area. It doesn't matter if the slots go to the edge. The only difference that will make is increased surface area.
 

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sorry - i put the wrong term in my question. When you are applying the brakes in a race situation - you are heating up the pads and rotors due to friction. You cannot expect the rotor to act like a heatsink or any stereotypical heat dispersion device. If there isn't enough thermal capacity ( as you stated) then a situation of overheating (brake fade) occurs.

Drilling and slotting REDUCES the material.

they are not heatsinks, as i'm sure you know.

Yes it does matter if the slots go to the edge. The slots are to prevent glazing of the pads. Drilled were to provide outgassing properties for pads that needed it.
 
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