so, a fellow user asked me if i had anything relating to time on brake pedal vs heat generated.

ok, so there are about 1,000,237,4xx variables to this "equation". lets keep it simple for general purposes, for the time being.

the two main things i see here:

1. amount of pressure. more pressure equals more heat, pretty easy. but i do not think this is linear, pressure vs heat.

2. amount of time. more time equals more heat. my thoughts are the same as #1.

i don't know/haven't seen the other discussion towards this subject. really haven't had a chance to go over much, just got to work and posted this to "not forget" about it. just hoping this stirs up some conversation opposed to what is better x, y, or z.

 

coefficient of friction plays into the heat part. so more pressure doesn't always equal more heat, as lower friction means less heat, regardless of pressure...

 

correct, this is over simplified for discussion purposes. compounds, materials, etc play a big part in this, but trying to keep out at this point. atleast until someone can start showing charts or studies etc.

 

How about this:

#2 amount of time.... the same speed and same mass, with a quicker decel makes more heat... so more time is not always more heat.

 

2 exactly the same cars with exactly the same braking inputs. also, with exactly the same parameters, ie. ambient temp, track temp, etc. how does that happen?

 

In a uniform system the only way to have quicker decel time in two identical cars/set (that means same pad material) is to increase pressure. Thus they are inversely correlated. (meaning if pressure goes up, time on brakes goes down)

Side note: I have always heard beginners are worse on brakes. In sticking with the this issue. I was under the standing that a "more" gradually braking (as in noobs) will lead to more fade due to heat then those with more experience and smash the pedal for a much shorter period of time.

If these is true, then the heat transfer away from the pad/rotor contact surface must not have time to heat the whole system before the pads are released.(Heat conduction, I believe is the physic term)

Look at it like this, if you touch a hot oven(this is a hard, fast scernio) you burn yourself instantly but if you notice your finger hurts but is not really hot. Conversely, if you try to carry an hot bowl (i microwave my soup) for a few seconds(slower gradual braking) the after effect is a pain from heat that last much longer.

These last examples are very generalized and obviously I can't factual support them.

 

When I think of brakes I think of heat issues. That is what ultimately cause fade, failure. You can select the strength of you braking by material, and give yourself a heat window that is best for your application.

 

from the first paragraph, yes. i should have said my #1 and #2 are "statements" towards those actions only. not, both at the same time. in some instances they could be correlated together.

good side note also. which could open up a whole other can of beans. "forward" braking comes to mind here.

also, i have no factual support at this time either myself. that is why i am here and to hopefully help him in the other discussion, or hurt him. lol.

 

This in theory seems like good reasoning, but the fact is you're dumping all that kinetic energy into the system over a shorter period of time, so the temperature spike is higher. And yes, a shorter stopping distance has to come from more brake torque and that in it self is what raises the temps. Its the dynamic torque at the hub that counts, you can have the same torque with higher Mu (coef of friction) or more clamp load (give the same brake system, disc size etc) but the end result is similar in terms of heat. So in a shorter period of time (higher decel rate) the system will be "working harder" and also have less time to dissipate the heat.

 

Right, so if you spread that logic to the finger example (I'm tryin to simplify) that sudden heat change will cause the physical damage of a burn. Now neglecting that we don't care about the damage done on the pad/rotor (in terms of wear) there will be a sudden temp spike mid brake. Then after the release, the heat that did not have time to dissipate outward now has not only the time (your next straight), but also the metal it can conduct to is physically cooler. Which would result in faster more drastic his dissipation.

The abrut hard breaking would have the greater fluctuations in temp changes, compared to someone who "road" their brakes. In terms of what is useful for a track I wonder which is more beneficial? If you are finished braking it really does not matter that your pads are overheated as long as there is sufficient time to cool down before you next braking point.

A constant high temperature due to "gradual" braking where the heat is transfered to the hubs, rims, etc. will not be able to cool down quickly between braking zones.

 

Your point is good, but riding the brakes longer means you're not accelerating more, so the higher speed at the end of the straight wont be as much there. (on paper, because if the car is not stable its a different story) So with higher speeds at corner entry = more heat. Riding the brakes creates temperature up to a certain point, it doesnt continually and gradually keep rising.

For example from some of the work I've done and looking at datalogs, this past week a hard stop at Road America turn 5 (hardest braking turn in the US) with Level 5 Motorsports "Hurricane Porsche" and test driver Pat Long, I saw on the Motec logs an approach of ~170mph down to ~50mph at a 1.5G of peak decel. That had a disc temp spike to 1600F. In some of the slower laps the driver was to braking earlier and a longer stop at decel rate of 1.0G the disc temp would only spike to about 1200F. Thats about 25% less heat to dissipate before turn 6.

Hope this helps.

 

this goes back to the forward braking idea. if you "ride" your brakes you are "asking" more of them when they are hot, maybe too hot for the pad to function "correctly". and like ferraeira said, from riding them, you are over slowing the car. time on the brakes is time you are not on the gas.


good info.

 

I am not clear after reading this. Short firm braking gives a higher spike, on the surface or whole rotor. Does it spike on the rotor pad interface and then dissapate and transfer. The burn analogy gives a worse surface burn, though depending on surface temperatures and heat transfer either fast or slow burn could be deeper, more heat into the tissue.

 

[QUOTE=FerreiraCompetitions;45982924

For example from some of the work I've done and looking at datalogs, this past week a hard stop at Road America turn 5 (hardest braking turn in the US) with Level 5 Motorsports "Hurricane Porsche" and test driver Pat Long, I saw on the Motec logs an approach of ~170mph down to ~50mph at a 1.5G of peak decel. That had a disc temp spike to 1600F. In some of the slower laps the driver was to braking earlier and a longer stop at decel rate of 1.0G the disc temp would only spike to about 1200F. Thats about 25% less heat to dissipate before turn 6.

Hope this helps.[/QUOTE]

EEEWWW, real facts, I like. My info is off my high school /college physics class and from procrastinating studying sleep disorders. haha
If I'm being critical for the sake of learning. I would comment on two things.
1.) You say riding the brakes to a point increase temp, that point would be dependent on when the brake pad fade and can no longer provide the mu level need decel thus halting the amount of applied friction.
2.) 1.5G of braking produced 1600F , 1.0G of braking made 1200F, a 25% decrease BUT thats a 50% decrease in braking force. Now obviously a million other factors come it, but there seems to be a non linear relation ship between force and temp. Force can double and temp will not double.

Also if you have that data (which is awesome by the way) what was the rotor temp before the next braking zone for each respective session.

 

My point was that in a very short braking time, heat generation will not have time to dissipate, and will cause a spike in the pad/rotor interface. Then will be able to dissipate to the other metal surface in direct contact. So a fast peak then a fast dip.

The burn analogy was to show the specific heat transfer. A hot oven burn and reflex withdraw will have more damage, but less heat transfer, then carrying a hot object (not oven hot) for a few seconds.

 

50% of 1.5 is .75 FYI. Thus a decrease from 1.5 to 1.0 is a 33.3% decrease.

 

So, does the rotor have more heat at the end of laps of hard braking an spikes or more moderate braking, not asking which spikes the highest. I understand that the spike could exceed the pad working temperature so is a problem in itself, but so is the build up of heat if high enough.

 

haha thanks, not sure if my math failed me or I was thinking 1.0 to 1.5 was 50%, either way brain fart on my part. Some more data points would be nice in order to graph.

 

Assuming you are on flat ground, heat energy dissipated is equal to the kinetic energy lost. So energy before braking is 0.5mv1^2 and final energy is 0.5mv2^2 where V1 is the velocity before braking and v2 is the velocity after braking. Part of the kinetic energy lost goes into heating the air due to aerodynamic drag, part of the energy goes into heating the brakes, and part of the energy goes into heating the tires and pavement due to deformation friction of the tire carcasses and to friction at the tire road interface. Some is lost due to friction in the bearings and other drivetrain components.

If you brake at the limit of the tires, say 1 G or more, your deceleration rate is pretty constant, and time to brake just depends on starting and final velocity. might get some spiking initially, but if you are on the brakes for several seconds, spikes will even out. And then heat depends on energy lost by brakes, on thermal mass, initial temp, and air cooling. So it is all a little complicated.

 

I always thought in general while higher pressure for shorter time produced higher heat at the rotor contact surface, longer time at lower pressure did more to heat the entire rotor even if at a lower ultimate temperature. More overall temp build up in entire rotor/caliper/hub/etc over repeated stops making it harder and harder to dissipate that heat.

Think of it as more area under the curve for more brake time at lower pressure than higher pressure for less time. This assuming all other variables being equal/eliminated.

 

While I have no logs of actual data to present but I have found that a gradual but progressive hard braking is much cooler on the brakes than a quick all out hard braking.

 

Do you have data of rotor temperature entering T6 before braking again? I feel like that might be a better indicator of heat absorption than a single spike, as several others have said.

 

Yes, entering T6 it was still at 1300F on hardbraking laps, and it was bellow the graph on the easy braking laps, which meant it was bellow 1000F.

Ok to your #1 point, riding the brakes will only get the temps up to a point, after that it will "plateau" and maintain even if you ran 50 laps with your foot on the brakes. The reason is because the thermal capacity of the whole system would still enough to dissipate that heat, and that heat is not that much because you're not asking the system to do too much work, and if the pads are good they wouldnt fade. Its when you ask the system to "glow" that things start getting crazy. (thats why they dont recommend dragging the brakes to "burnish" the pads)
Ok to the #2: Even though that math is not right, the thermal spike is not linear with torque rise. To a point in torque the thermal curve is at a steeper angle, and then because of convection it starts to flatten out.

descartesfool, the decel rate is not linear when at the limit of the tyres. The reason is because at higher speeds you have much higher downforce, to help against lock up, and more drag, so at a higher speeds the decel rate is normally quite high compared to the last 3rd of the stop, which is also when the drivers start bleeding off the brakes (again to prevent lockup and initiate turn in, etc).

Ok hope this answers some doubts, I'm sure I'm leaving some elements out and questions unanswered but I'll try to help.

 

referenced from the thesis link.

"Excessive Component Wear

The wear of frictional material is directly
proportional to contact pressure, but exponentially related to temperature, (Day and
Newcomb 1984); therefore more rapid wear will occur at elevated temperatures."

 

Hmm. So this seems to contradict my experience with noobs. That is, noobs ride the brakes like nobody's business and use weak pedal pressure (and consequently overheat the brakes, typically boiling the fluid), vs. advanced/instructor drivers who are hard and short on the brakes and do not overheat the system.

I'm wondering if there are more variables in the system than I account for in blanket theory, such as advanced/instructor drivers tend to have race pads whereas beginner/intermediate drivers tend to have stock or aggressive street pads. Not sure why there would be a difference in fluid boiling though, as both fluid systems would just be seeing X temperature, not a particular type of pad. Do the PADS absorb and hold a lot of heat that might otherwise transfer to the caliper/fluid? That also would not account for the definitely-slower entry speeds the noobs are seeing.