brendon

Since the RPM drop is from 6k to 1k in under a second, I'd imagine the reduction in engine braking due to deceleration, especially at higher speeds would be very little. Especially in higher gears when the engine has the least braking effect. i.e. the engine braking is least when you need it most.

Any braking the rear can do reduces the heat input into the front brakes since it removes energy from the system in a different location. I maximize rear braking just short of instability and then adjust the bias as the front vs rear tire grip changes due to tire heating during the stint. I couldn't imagine deliberately leaving that extra performance on the table by ignoring rear braking.

No, the available "energy" is the rotational inertia delta of accelerating the engine (and whatever driveling is on the engine side of the clutch). The mass and speed of the car are unimportant provided the speed is high enough for this to matter.

mark kibort

why dont you explain yourself here. as i understand it, a bias valve puts a % pressure to the rears. in fact, with the valve, it tappers off from 70 to 100% of the pressure put to the front. meaning, even with a lot more pedal pressure to the fronts, (up to 100% before lock up) the rears wont get the proportional increase in braking pressure.
How is this "patently false"?

mark kibort

#1. again, you have to think about it by using extremes sometime. if you could reduce the decel time to match engine rev decel time on its own, you would have NO decel forces on the driveline, right?? conversely, if i was going down a hill and had terminal velocity (steady state speed). there would be a force due to engine braking and it would be at its maximum. somewhere in between its a mix of external forces and engine braking. or am i missing something. my commnent regarding the engine revs going from 6000rpm to 3000rpm in 1 second was out of gear. so , if somehow you could have brakes and tires that could do this , and maybe its in 1st gear, then maybe there is NO engine braking available, because the decel rate is matching what the engine rpm would drop to on its own, out of gear.

so you are saying, no matter how fast the car decelerates, the engine forces are always there..... hummmm. maybe your right. my example might be misleading my thinking. I would like to think that the engine forces are always there in full force, but i don't think they are. Care to elaborate and convince me otherwise?

2. yes, rear braking, not to maximum will not lighten the thermal load of the fronts. they will always be at max potential , BUT, you are partially right. the rears if not used for example, would lengthen stopping distances, so yes, the KE from brake application to brake release would be less with the rear brakes at capacity. however, i would bet in cars with more engine braking,and engines in the front, you would be pretty close to rear brake bias max, even if lightly biased due to the forces of engine braking, especially in the lower gears (3rd and 4th). again, ive measured the forces on the dyno. its a pretty easy calculation to show what they are during a straight line braking event at threshold levels on good tires.

3. I dont agree and if you think about it, i bet you will understand. you are right if you are talking "dumping the clutch" on a start or speed shift. the rotational energy is the source. HOWEVER, on a downshift, the source of energy is NOT the engine , its the chassis moving at a velocity which is the source. this is why you can infuse SO Much more energy into the driveline . if you can get a tire chirp with a downshift from 4th to 3rd, and a tire chirp takes lets say 2000ft-lbs at the rear axles in dumping the clutch speed shifting to 2nd gear . in 3rd, there would be 30% more force transfered to the driveline through the gear box. because its the 2000ft-lbs that gets the wheels to chirp . going through a 7:1 vs a 5:1 gear box ratio, multiplies this back to the driveline.
so, yes, it is the KE as a source, and it is in the form of reflected inertia (car's mass tied to the wheels) to the driveline through the gear box. HUGE point there.

Yes, you are also right that the engine is part of the equation, but think of it as a brake. not the source of the energy. Like hitting a wall. the wall is not the source... the car hitting the wall at speed is the source. In fact, think of the engine seized up. you have the KE of the car, ready to dump all the energy it can, via the grip of the tires. that is known..... 2000lbs of force lets say. depending on how fast and what gear, its a simple multiplication equation to see forces at the driveline past the gear box. ........ but its source is the cars velocity... much greater than the car could generate to the wheels by the engine alone on a speed shift.

Veloce Raptor

Mark, as an aside, i will be coaching at Laguna on October 17th. If you're there, it would be good to meet...

mark kibort

Sounds good. Ill try and make it out.... what event is it?
EDIT: I just remembered... thats the SCCA Runoffs.....with any luck, ill be able to be there racing...... either way Ill see you there.

KaiB

Now, as usual, you've altered your original statement. The above is more or less correct.

Please be careful throwing "facts" out.

Speaking of out, I am.

mark kibort

"as usual".... Im usually trying to give clarity. i just read what i wrote and my "clarity" statement and they say the exact same thing. (just one with more detail).
what is the problem. yes, at 100% threshold braking, there is some pressure biased by the bias valve. it was a response to brendan who thought i had some situation where no bias or no pressure was applied to the rears or fronts.

I didn't think i altered anything, but if you misunderstood me. that's fine. ask away and please dont hold back.

However, i for sure am not understanding you .... why is the original statement incorrect? my interpretation is what i wrote. how do you understand it?

mark kibort

after doing some fade tests for rapid decels at high speed, im pretty convinced its a KE change thing for a 12.6" rotor. SO, ive bought a 2 piece set up that is 3% larger diameter, (so 3% more themal mass) plus the aluminum hat should dissipate heat much faster too. but mainly, i think the rotor with the holes will work better. (but crack faster). after these wear out, going to the slotted.

mark kibort

so, what part of this is incorrect, or maybe unclear since my clarity statement seemed to be "more or less" correct. why was it patently false and why?
however, based on what brendan has said, im taking a second look at engine braking only being half or quarter of its forces under rapid decel. If he is right, then it makes the case even greater for the thought that engine braking is a lot more than folks give credit or tune for. makes sense for how many people ive seen skid off track and crash, due to rear wheel lock up or instability both in the pro races and club races. in my case, if my numbers are even remotely close to right, engine braking might be at the limit in 3rd gear and maybe even just over in 2nd, depending on the tires..

I wrote a few pages back You have to think about what you said about the ratio of "motor" and "pedal" in braking. Regardless of how much braking you are applying, if you are off the throttle, you are some % rear braking due to engine braking and its only in the rear (unless 4x4) It, in most cases is near the limit of the rear tire grip to the race track. if you then go to 100% threshold braking on the fronts, you have a bias valve that puts some rear braking in. and yes, you are right, they add up as the forces are in series. this is a key point. the rear decel force by the engine, at the rear tires might be 1/2 or 1/4 vs a non pedal braking condition (AKA: off throttle) this is because the greater the rate of decel, the less force the engine produces.

winders

Don't use drilled rotors. They have less mass than solid rotors of the same size and any cooling gain you might see from the holes will be offset by the reduced mass.

A 3% change in diameter isn't going to make a huge difference. It certainly would not solve the problem as described.

mark kibort

I know, you are right. i used the 964T rotors for about 3 sets, they would crack and crack bad after just 3 full race weekends. so, went solid. But, i did notice a little of that end fade of braking zone fade. dont know if i was imagining it, but it sure felt like the heat was buidling up and maybe the holes or slots i had used in the past, would help. im thinking the alimuminum hats, might dissipate heat much faster , and help more than the holes loose mass. I think i did a square area comparison with holes and with out. it wasnt much... maybe as much as the diameter increased.
im in the process to find the brembo slotted rotors that fit this configuation. dont like the holes!!!!!
thanks

TrackDays247.com

IF you can use 993TT rotors use our slotted sets - SOOO much better than other options available...

mark kibort

I KNOW!!!! remember i was asking about a 964T version of them?? I would have done that if available. Definitely my first choice.... I do think part , (not all) of the issue is the flat rotor when i used them. dont remember it being much of an issue with the drilled.
I go two piece... thats a brembo friction ring.... very pricey! but they last a long time too. (and weight savings, bigger diameter, and i can get the slotted)

brendon

I can agree that *IF* you can decelerate at the same rate as your revs drop due to engine internal losses... then there would be no engine braking. (In an F1 car you'd have to drive into a wall!) but at high speeds, the time difference between the RPM drop with the clutch in vs how long it takes to shed the same RPM means that it is an effect, but not a huge one.

I'm simply saying that under real braking conditions, this reduction isn't really that significant. Especially the more your engine is a "race engine" with lightened flywheel etc to make it more responsive.

It appears that you are saying that the retardation supplied by the rears doesn't impact the thermal load on the front brakes? I'm claiming that every bit of extra braking you can get out of the rears will cause the temperature rise of the front brakes to be reduced. You get two advantages, shorter stopping distances AND lower loads on the front brakes.

I did a simple test. Lift off the gas at speed in 3rd and 4th gear. Apply handbrake. All the force between no handbrake and locked rear wheels is excess braking capacity from the rear tires. None of my cars (that have handbrakes) seem to be remotely close to locking up the rears under engine only deceleration, even in 2nd gear.

The source of the "FORCE" is unimportant. In both cases you are doing the same thing, which is changing the rotational speed of the engine. In both cases the thing that is on the other side insisting that the engine change speed is the car/driveline. Here is a thought experiment: would the there be a significant change in engine acceleration if you had a truck pushing on your bumper as you drop the clutch? If so, why would the inertia of the engine change because a truck was on your bumper?

If you replaced the wall with a massive truck, then the energy released into your car would be identical if your car hit the truck at 100mph or your stationary car was hit by the truck doing 100mph. In other words, just like Einstein's relativity, changing the frame of reference doesn't change what happens.

brendon

Your original statement really implied that the bias valve adds braking after the fronts reach a certain % of the maximum pressure. I think it is clearer if you say it the other way: as the front pressure increases, the bias valve DECREASES the rear percentage of brake pressure from 100% to 70%.

The statement in fact, with the valve, it tappers off from 70 to 100% of the pressure put to the front seems to be contradictory. How can you taper off (i.e. reduce) while the percentage is increasing?