mklaskin

Who's on first?

winders

Damn, I said I was done. After this I really am!

I guess you don't get the physics....and you have said something like above several times now.

Increasing rotor size will not decrease the amount of heat the rotor/pad combination has to generate to slow the car.

Veloce Raptor

Correctamundo!

mark kibort

You know i get it. Sure, the KE is the KE you are transferring it and have to dissipate it. BUT, where im going with that, besides the increase in thermal mass, is that with 15% diameter change, less pedal pressure due to more leverage. (Even though the outer diameter speed is higher, right!

EDIT: Now, go back to the statement which you read incorectly. you said " increasing rotor size willl not decrease the amount of heat the rotor/pad has to generate". I said that the larger diameter rotor will dissipate more heat. two different statements.

I get it.... same KE change to the slower target speed ) BUT, because you are using less pressure for the same deceleration forces the pads are not pressing as hard on the rotor. This means any fade you do have due to temp , you are not pressing as hard for the same decel force.....and if you do need braking force during fade , extra pressure will result in more stopping power as well. (because there is still quite a bit of stopping force during fade, but just not as much as at the optimum temps.
And, the thermal mass and surface area for the larger diameter rotor is larger which is a big help as well as more ease for brake application modulation due to less pedal force for the same braking force.

KaiB

Nah....

Done also.

mark kibort

Funny, you are all over this particular Power-cetric thing, when actually , you have more force on the rotor at the greater diamaeter! power = Force time speed. very funny, due to your position of the "torquey" engine race car discussion vs same power which is so analogous. Ironic to say the least.

But, even though the same KE is dissipated, with less pedal pressure, less power is going up in pad deformation, vibration, and grooving the rotors, and more of it is going up in heat. thats the main reason why the larger rotors work better, besides you cant ignor the greater thermal mass , thats traveling faster, AND is taking more advantage, and creating brake air cooling through lower pressure zones at the edges due to the higher surface speeds. But, you never would think of that would you now! your a funny guy Dave!

Veloce Raptor

Mark,

It's not that you don't know anything. In fact, you are quite intelligent.

It's just that you know so much that simply is not so.

Done also.

mklaskin

http://en.wikipedia.org/wiki/It_Pays_to_Be_Ignorant

mark kibort

You feel we need to be aware this?
you say, "Nah"? funny.... funny how the responses are like my 8 year old.
If you have a thought, post it. whats the problem? its a discussion!
why do you think they use larger diameter rotors, and why do they act so different than smaller diameter rotors? Many times, its not the thermal mass because many of the 2 piece larger diameter rotors that are 20% larger diameter, have much less mass than the solid 13" rotors.
I would say its due to to the things i just posted. do you not agree, lets hear your story as to why.
im thinking the greater surface area at the pad contact area is key.
even though the speed is 15-20% higher at the contact area, there is 15-20% greater distance all the way around. so the pads contact the the same point in a revolution, in the same amount of time, even though the speed is much higher. there is 15-20% more mass in this direct area, and the braking force is 15-20% greater. sure the KE is the same for the same rate of deceleration , at a lower pedal force, but the heat can dissipate for the these reasons.

mark kibort

Thanks for that .... i guess. ah, com'mon, dont take your balll and go home.
its just a discussion Dave. Actually, it makes you think a little. Admit, you have thought about what ive posted more than you have in a while....even after talking to all the "experts".

What it really boils down to is, im searching for answers to a problem i have. the list has given me a ton of advice. some good and some with suspect foundation. Im going the path of consensous for the cooling and bias. pretty excited to try.
BUT, when it comes to the raw physics, i dont think you can argue the point of the one thing i was fiighting. the lower weight scenaro. its not intuitive, but if you pass it buy someone that knows, and frame it in the right set up, you will get the same answer i gave. you say im wrong, but i dont think you have checked your work.
go ahead.... take a break .. go check out what i said. (and the only thing im really fighting back, is the information to support why lightening the car will make it easier on the brakes ) remember, Its not easier on the brakes, due to the higher top speed before braking , and thats the point.

all the other things ive brought up as far as cooling, my tests, and thoughts, are educated guesses.

My last post about why larger brakes then, if the KE needing to dissipate is the same, is something that is interesting and can be discussed. I would be willing to bet, my reasoning is not too far off but would love to hear an opposing set of reasons for why larger brakes work better.

In the end, the problem is what it is. brake fade only at turn 2 Laguna.
can it be solved with bias or cooling, or am at the physical limitation of the 12.6 ferrous solid rotor and do i require a larger diameter rotor?

linzman

This thread has been quite entertaining. Mark, aka Sheldon, you are indeed overthinking this. The bottom line is a lighter car uses less brakes. Let's look at practical versus theoretical here. When I took my 87 911 from 2800 to 2300 lbs, my rotors and pads lasted 3 times as long. I could do over 20 DE days on a set of Pagid pads, and my rotors would last 3 seasons minimum. The brake fade I would rarely experience at the end of a session no longer happened, EVER. The brakes did not get as hot. And yes, I was going faster. After all, if you believe the 10lbs weight loss is equivalent to 1 HP, I had the equivalent of 50 extra HP. Did I asd any cooling to my brakes? No. Did I change rotor size or calipers? No. Are there other factors? Sure, my driving got better over time, the suspension setup changed as a result of the weight loss, etc. but the single biggest factor was the weight loss. Aside from that, cooling is the biggest factor, either by direct cooling via ducts or by having a bigger heat sink via bigger rotors and different pads. Instead of arguing the physics of it all, just go out and try all the things that have been suggested over and over again. Or continue to have problems. Just my .02 cents. Keep it simple.

mark kibort

Thanks for the KE discussion agreement! wheeew!!!!

yes, the 5mph was just for the sake of example. i did add 50hp and got a full 10mph faster, so i figured i would split the difference. but the theory still remains solid. as was mentioned by someone else. the net effect of lowering weight, will have a near offsetting effect to KE, negating its ease on the brakes if there is a heat problem to begin with.
i can go though the effects of 200lbs on a 3000lb/375rwhp . I think i have a simulator that can give a rough estimation.
as far as entering the turns more quickly with 200lbs... i bet the cornering force change is in the same gain range as acceleration. also we can plug in the numbers and see. but good point, as i mentioned before. the fact that you might be able to turn in earlier, would be an advantage. How much... probably slight, and would help lap times, but not the braking over-temp issue.

bias and ducting tools on the way.

thanks

mark kibort

Thanks for the post. Im giggling a little. my story. when i went from 3700lbs to 3000lbs, i used to go through a set of pads over a year, then at the lighter weight, i went through a set of pads in 3 weekends. you do seem to get better as a driver as you start improving the cars we drive and race!

The point is, and the physics clearly state, that if you are having a braking issue, and using the brakes to their limits, by losing weight, you will not solve the problem , and wear should be the same because KE will be close to constant. if you see otherwise, OTHER things might have changed. you might have been driving a little easier in your DEs after lightening the car. quick story. i remember, adding the 50hp to the holbert racer. i thought, with its HUGE torque, i could short shift and save the motor and still do lap times ive always done..... in the end, i was 2 seconds slower. then i started driving like i did before, with no regard for engine or brakes, and my lap times improved instantly 1.5 second a lap, and fully utilizing the hp as i got more skilled, was 2 seconds as lap faster.. Point is, you might have been subconsciously driving more conservative. if you were driving a 87 911 where pads lasted 20 DE days, you were in no way driving the car and its brakes to its limits. what you should have been looking at, that 50hp gain equiv, should have given you near 10mph faster terminal velocity before turn ins at the faster track. if you didnt, you just lightened the load on the car and were having the same fun and less wear. I would bet that i can drive my race car with absolutely NO brakes and run a faster time than many spec 911s at laguna. Thats just a product of really wringing your car out and good amount lot of HP (and using front tires to push to brake! ) . Remember, i could also cook a set of holed rotors in 3 weekends until they looked like this: thats a lot of heat. getting the weight out, just shifts the weight for speed in the KE equation.

But, i get your point too! yes, i need to get the cooling and bias thing going . and like i said, im excited to have a project to do this . its been a long time since i have made any significant changes to the race car.

flatsics

..

333pg333

Is it presumptuous to assume that you would like to 'cure' your braking issues? I mean surely the bottom line is that you'd like to stop fading at the end of this one particular straight on the track that you visit occasionally? I know you have an inquiring mind Mark but in the end I think you want your car to improve under brakes? If so, then why don't you just do some/all of the pretty simple changes that have been repeated ad infinitum and live happily ever after. The benefit will be noticed on all the tracks you run at which is nice too. As for the lightness thing, I'm not going to argue physics with you and I don't think you're going to go about any major weight shedding anyway, but lighter is always better for everything...just is. Also, I thought you used to run a squarer tyre setup but seem to remember that you went back to a narrower front? Obviously if you can increase your front contact patch that would also assist braking. We went up from a 30% aspect ratio tyre to a 35% with same width and noticed an improvement. Our cars are similar in that they're transaxle with approx 50/50 weight distribution. We are at 2550lbs wet with 510whp on a conservative dyno. Braking isn't an issue for us.