Originally Posted by 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?

Originally Posted by 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 agree too. but the point is, if you could match this decelleration , then there would be no engine braking.. so i was assuming that if you had some where in between, wouldnt you have some reduced level of engine braking?? (or is it always there as a factor, full strength??)



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.

Well that is a factor that increases the engine braking anyway. being more responsive is one thing on the acceleration side. but those lighten flywheel factors are not really that much of a factor on the "in gear " acceleration side of the equation, nor on the engine braking side as well.... remember, i posted the equivilance of that factor? lightening the flywheel has a descending effect, depending on what gear you are in . 10lbs for example, has an effect of a couple HP in top gear and maybe 20hp in 1st gear. (reality is that the effect is as if 200lbs was removed from the car in 1st , or 10lbs was removed in 5th gear). point is , the engine braking is due to it creating differential pressure forces/ in the engine cylinders.
The point is, is the engine braking lessened by the rate of decel of the car? thats what im struggling to wrap my brain around. If its not, then its always there full strength. I guess if you thnk about it as a brake, then its always there.



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.

Yes, im saying this, only to the extrenet that the front brake application isnt shortened in duration. in other words, yes, if the time it takes to slow the same amount, is shortened, then the thermal load is reduced becuase the amount of KE/time (AKA HP-seconds) is less.
so, yes, if the rears can cause a rear decel rate INCREASE, and you havent maxed out the rear slowing capability, then yes. im saying that the amount of braking to the rear is low enough, in addition to rear braking that it cant significantly impact the front brakes thermal load. meaning this: you slow from 120 to 40mph in 4 seconds without rear brakes at all. we know that engine braking is helping in a significant way. what can the additional rear brakes help shave off the time. .1, .2, .3, .4 seconds?? if so that would be the reduction in the thermal load to the fronts.... the time at which they are applied.

KEY POINT: PLUS, dont forget, if you do have more braking capacity to the rear, that further increases weight transfer due to greater decel, ligtheng the rear, reducing their braking potential, giving more "bite" to the front tires, al'owing even MORE stopping force and heat to the fronts. so, its a complicated set of trade offs. nothing is for free.



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.

You missed a CRITICAL point of the discussion!! by lifting on the gas, you DONT have the weight transfer.. your cars with hand brakes DONT have slicks at temp. your cars with hand brakes are not decelerating from 6000rpm to 3000rpm. without the weight transfer, all bets are off. Does this make sense. you need the 1.5g of slowing to transfer the weight so that you only have 300lbs sitting on each tire. THIS IS THE POINT.

I serously doubt you did the test at 6000rpm, but if you did it at 3000rpm, you get half the braking force. a safe test might be 5000rpm to 3000rpm, as i dont think you are running around doing the test at near redline, are you??? and even if you did, you are no where near conducting a valid test, due to lack of weight transfer.... BTW, a front engine car vs rear??? (i.e. 911 vs 928 )
ill do a test for you. Ill video tape the engine decel in 3rd and then do it with the hand brake in neutral and compare effects. but ill do it on jack stands.



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?

Brendan, this is the knee jerk reaction to evaluating the situation.
you are thinking about it backward. under acceleration, the engine is creating the forces. its the sourse of the "KE". you drop the clutch speed shifting to 2nd, and the difference between the inertia of redline vs the actual drive RPM , in the time it takes to bring the engine RPM down,is dumped in to the driveline.... lets say you get a wheel chirp.
that chirp takes 4000lbs force at the tires. that means through 2nd gear, you had 8:1 reduction of gears, and thats 500ft-lbs at the engine driveline. (maybe the engine only produces 400ft-lbs of torque on its own and the extra 100ftlbs is the clutch dump of the inertia).

NOW, you are decelerating from 4th to 3rd at 100mph. the energy source is NOW not the engine, its the cars mass and velocity. you dump the clutch and get a chirp. sure, its fighting agaist the inertia of the engine to bring it up in speed vs down in speed in the first seanareo, but it doesnt matter. to get a chirp, we need 4000lbs of force at the tires. but NOW we are engaged to 3rd gear. only a 5:1 reduction. that means to get that chirp, we had to spin up the engine a little quicker than before it was brought down in RPM. Now, what do you think the driveline sees to get 4000lbs of force (4000ft-lbs) at the tires??????
YOU GOT IT..... now you have subjected the driveline to 800ft-lbs of torque. This is because its all reversed now. the sourse is the KE of the car, vs prior the KE was the engine sourse.



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.

you're misapplying Einsteins theory, which is true in your example, but not true to the situation we are talking about read above what i wrote and respond. in your example the sourses or KE are the same. in the example im talking about it's not.

.

Originally Posted by KaiB

Don't get him started again Brendon....

Originally Posted by brendon

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?

Originally Posted by KaiB

No Mark, THIS is precisely what you said:

Originally Posted by mark kibort View Post
if you then go to 100% threshold braking on the fronts, you have a bias valve that puts some rear braking in.

As I mentioned before you got nasty, the above is absolutely false.

Man up dude.

Originally Posted by mark kibort

I said that as the fronts reach 100% of their pressure, the rears get much less. the pressure is bled off. the bias bar doesnt "decrease the rear pressure, it holds it constant. (i.e. no more increase in pressure , for increases in pressure to the front part of the system)

Originally Posted by mark kibort

you can "taper off" by having the valve, diverting pressure, and thats what it is doing. At near 50% of the front pressure, this becomes a limit for the rear.... any additional pressure to the fronts, doesn't increase the rear pressure.. it stays relatively constant. thats what i meant by "taper off".

Originally Posted by mark kibort

I just thought of a great analogy for you, if what i wrote wasnt clear enough.

Yes, the sourse of the KE is a MAJOR factor, and here is an explanation and thought example for you.

First off , we are doing the same thing (accelerating or decelerating, the engine RPM, or it's inertia, in both cases, but the rate at which we are doing it is grossly different. (i.e. rate of KE change)

Picture a car that is tied to the track via gear and toothed belt. tires are the gear, and the belt is the road, so no slip.

you accelerate in 1st, push the clutch at 6kRPM to shift , floor it, take the RPM to 7000rpm and then dump the clutch.... from the moment you release the clutch , it takes 1/10th of a second. the engine inertia, and and power all dump into the driveline and the car lunges forward the power is aways there (and torque), but the 1/10th second of decelerating the engine is some KE value comprised of the rotating mass of the engine and flywheel, as the RPM drops from 7000 to 4500rpm.

maybe thats worth, 100ft-lbs.

NOW, take a car going 100mph, and you shift from 4th to 3rd and dump the clutch. 4500rpm to 7000rpm the engine revs up twice as fast 1/20th of a second. why? because the vehicle speed is so much faster (about double) the force acting on the driveline is now double through the gear box, and the wheels are tied to the road due to the "belt and gear" coupling.

The source NOW IS IMPORTANT, as the engine is being driven by the car's KE, and not the car being driven by the engine as it was in the first example. This is because of the velocity of the vehicle, the gearing forces the engine to accelerate at 2x the time, greatly raising the torque transferred to the gear box.

does that make it any clearer?

Originally Posted by mark kibort

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.

Yes, im saying this, only to the extrenet that the front brake application isnt shortened in duration. in other words, yes, if the time it takes to slow the same amount, is shortened, then the thermal load is reduced becuase the amount of KE/time (AKA HP-seconds) is less.
so, yes, if the rears can cause a rear decel rate INCREASE, and you havent maxed out the rear slowing capability, then yes. im saying that the amount of braking to the rear is low enough, in addition to rear braking that it cant significantly impact the front brakes thermal load. meaning this: you slow from 120 to 40mph in 4 seconds without rear brakes at all. we know that engine braking is helping in a significant way. what can the additional rear brakes help shave off the time. .1, .2, .3, .4 seconds?? if so that would be the reduction in the thermal load to the fronts.... the time at which they are applied.

KEY POINT: PLUS, dont forget, if you do have more braking capacity to the rear, that further increases weight transfer due to greater decel, ligtheng the rear, reducing their braking potential, giving more "bite" to the front tires, al'owing even MORE stopping force and heat to the fronts. so, its a complicated set of trade offs. nothing is for free.

Originally Posted by brendon

So are we talking about a brake proportioning valve or a balance bar? The latter between the two master cylinders provides an adjustable ration of front to rear pressure, but in no way limits rear pressure or reduces rear pressure. The front and rear pressures are both straight lines. If you press hard enough, you will lock the rears.

Yep..... i think the bias valve is good to have, if you dont have a racing abs, and have a lot of fade.. like i do. you can push to the limit and beyond, and not worry about more rear brakes possibly locking up the rear. I have to admit, i havent experimented too much with the bias. ive driven a car with too much rear bias, it was a joke.... kind of like a passenger was pulling the e-brake during some of the hard braking zones.

The proportioning valves tend to limit rear pressure rise after some pressure. That seems to be a less than ideal way to handle rear brake bias control.

I suppose the modern trend is to let the ABS hardware control rear brake pressure, so you can get optimal braking from the rear based on available traction, not pedal pressure.

If you have racing brakes, that's the trend. the old ABS is worthless, other than an early lock-up warning system.



Really what you want is to have the rears just short of locking up at max deceleration. Do you have the numbers handy for your front & rear wheel weights and your CG height? I think you said you get 1.5G deceleration, I'm curious to see the numbers wrt weight transfer to see what rear bias would be possible. I'm also interested to see your deceleration from engine braking alone.


This is true, and i agree, but what happens when you just have a little bit of steering input? you can unload that wheel and cause a lockup...
that can be a big issue on a high speed decel. I think its good that a car can have a lot of engine braking, as its more controlable, and is like a built in ABS.
PM me and ill send you over the engine braking dyno run.
as far as weight on each wheel. 830lbs on each front wheel, and 650lbs on each of the rear wheels.
CG height 19"
wheel base 100"

Originally Posted by brendon

I was assuming the same gear for both the acceleration and deceleration case. Not sure if comparing 3rd gear "engine acceleration" with 1st gear "engine deceleration" would make sense. In the same gear, using your numbers, the "at-the-wheel" maximum force is 4000ft-lbs regardless of the direction of acceleration. Anything more will result in the tires slipping. So regardless of the direction, the maximum load is limited by the tires attempting to change the rotational energy of the engine. i.e. acceleration vs deceleration is identical.

Originally Posted by brendon

Ok, this cuts to the heart of the matter. To the extent that you get any extra braking done by the rear brakes, you get a reduction in braking time because there is only so much energy loss required to reduce speeds for the corner entry. Even a small increase in rear brakes will increase deceleration Gs, reduce stopping distance and time. And the energy removed by the rear brakes no longer needs to be removed by the front brakes. If you are fading the rears at the end of the brake zone, you only need to remove a small amount of thermal load to reduce the maximum temperature to a manageable level. To that end, anything you can do to limit weight transfer under braking will also help improve your braking performance.

It's really a simple energy balance. If you need to remove 1.37MJ of energy to achieve your deceleration from 120 to 40mph, then you can choose to put that entire energy into the front brakes. Or you can put some in the rear. Even 10% rear braking would reduce the amount of energy in the front brakes by 10%. And if the added weight transfer causes an even greater RATE of energy absorption in the front brakes... then you win with both less overall energy in the front disks/pads AND a shorter stopping distance due to higher G forces. A double win.

Originally Posted by kurt M

Motor braking is like abs? WUT?

Motor braking is more controllable? WUT?

MK Please google bias bar and proportion valve. You seem to be blending them together and they do not function or produce the same outcomes.

Originally Posted by mark kibort

Kurt, yes, i was talking about the bias controller of my car, and many were talking bias bar which is an adjustable controller on a lot of race cars..... I dont have that, just a bias controller that has to be physically swapped out if i want to change bias. proportioning valve, does bleed off the fluild to reduce pressure to the rears..... and i agree, its different. sorry about that.

as far as motor braking..... yes, its like ABS, because what does abs do... it can get you max braking withouit wheel lock up. isnt that similar to engine braking in the rear.... you get near max braking, but the wheels cant lock up unless you have an engine seize .
its very controllable, from a perspective of driver feel and control, not adjustability as you might have taken from my post. rear braking effect through a curve can be applied with the throttle, and its very easy to control.

Originally Posted by kurt M

This is pure bunk.

Motor braking and ABS are about the same as football is to darts.

When you look up do you see the underside of a bridge?