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From: Bill Putney on 7 Mar 2010 15:22 jim beam wrote: > On 03/07/2010 09:57 AM, Bill Putney wrote: >> Then we disagree. I said this was on multiple cars with no leaks. >> >> But in any case, it appears that we agree on more than 3 stabs. Except >> you still believe that there is sufficient vacuum in the plenum of an >> engine in gear under acceleration to charge the booster to usable level >> - and that is simply not correct. > > ok bill, give me your manifold vacuum numbers. even one inHg [a low > number btw] is ~2lbs... If you mean approx. 2 psi, no - it's not. 1" HG is approx. 0.5 psi. > for 63 sq ins, that's still 126 lbs boost in... Nope - you're off by a factor of 4. So you made a calculation error. I won't hold it against you - can happen to anybody. > addition to what the driver can apply. it may not be great, [assuming > the driver has wasted their previous vacuum on multiple brake > applications] but it'll still help stop the car. For normal braking, let's say the booster is charged by 15" Hg (7.4 psi). The math says that you then have 466 pounds of assist directly into the m.c. The booster has a reservoir for a reason - so it can store that vacuum and give a reasonable ability to brake even if engine vacuum is low (like if the engine cut off) for you to stop the car. If your vacuum is down to 2" Hg (0.98 psi) under whatever acceleration condition (even more than the 1" Hg that you threw out), you have less than 70 pounds of force into the m.c. So you're going to tell me that they designed it for 466 pounds of force into the m.c. piston, with some reasonable safety factor, and with less than 15% of the design vacuum available to the booster after initial depletion (by pumping), you will be able to overcome the engine? Sorry - I said earlier regardless of what the numbers said, I know from experience that the vacuum is not there to recharge the booster to any usable amount. Now we even have the numbers. BTW - I chose to ratio things (i.e., compare 466 pounds of force to 61 pounds of force) to show the ratios because we would have to talk in terms of what the actual forces are required to press the m.c. and the caliper pistons/pads, and we would have to back all that out to actual pressure required on the pedal by the driver - and you have mechanical advantage of the pivoting point of the brake pedal to factor into that - too many variables for the discussion at this level - so again - that's why I ratio'd it rather than keep in in absolute force terms like you did. Bottom line: Having only 60 pounds of the assumed 2" Hg compared to 15" Hg or greater design doesn't work out too well if your talking about overcoming a powerful engine with open throttle. Bottom line no. 2: Regardless of what the numbers say (and they make my point), in real world, the booster will not be of much help at all once depleted of initial vacuum charge if the engine is under acceleration (vacuum too low to be of much help). -- Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')
From: Bill Putney on 7 Mar 2010 15:42 jim beam wrote: > On 03/07/2010 09:44 AM, Bill Putney wrote: > then you're making generalizations from your own exceptional experience > that are not representative of most vehicle, and certainly not vehicles > with standard tuning. vacuum decreases significantly at w.o.t, is > sufficient to give brake boost. > > besides, how much vacuum do you think you need for a brake booster? with > a 9" diameter diaphragm, [on the small side by modern standards], that's > 63 sq inches. how much pressure delta do you think you need to double a > person's braking force? You'd have to know the mechanical advantage in the pedal from basic lever math and a bunch of other stuff that is beyond what either of us wants to get into here. Without the absolute numbers for all that stuff, it suffices to ratio things: When the booster is designed to properly operate at 7 times the vacuum that you'll get from an engine under acceleration, I say your assist is going to be close to useless - the engine will likely overcome the brakes in being able to sustain highway speeds (and this is before fade comes into the picture). Saying that 63 sq. in. is a big surface area and so even a tiny little vacuum will give you huge force means nothing. Fact is the assist force on the diaphragm with the very low vacuum under acceleration is less than 15% of what it was designed to use under normal stored vacuum conditions. > you're supposed to apply the brakes and keep them on. only if you > release do you lose vacuum. Right or wrong, many people were taught to pump the brakes. Some people will in fact pump the brakes, for any number of reasons. >> Plenum vacuum is just too low. Again - you >> don't seem to know that - if you want to argue that point, then give me >> some numbers for vacuum for a common engine of your choice at idle, and >> in gear under light, moderate, and heavy acceleration. If the numbers >> you come back with are honest, you will prove what I'm saying. > > see above. Yes - please see it. >>> if that is your experience, then i think you must have a vacuum leak... >> >> That is simply not the case. >> >>> even with wide open throttle, there is sufficient vacuum in the >>> manifold to create significant braking assist. >> >> That is profoundly incorrect - period. > > again, if that's your experience, i think you're working with something > not representative of most vehicles. certainly not anything post 1980. Nope. Your assumptions are wrong on both counts. >>> sorry dude, it's incredibly rare for a panicking driver to pump their >>> brakes... >> >> That's your *theory*, or you have something to back that statement up? > > where is your "theory" that people pump brakes? i saw someone on a > freeway in rain in los angles one time, skating along with their brakes > locked, car gently rotating, and the look on her face was that of > someone trying to break the pedal off she was pushing it so hard. > there's no way that person, as an average driver, was going to let off > that pedal, and thus, even though she was going to crash, she was not > going to lose vacuum. You keep throwing in scenarios totally different than what we're talking about. Again - you can't prove your claim that hardly anyone pumps the brakes any more than my claim that a lot of people would pump the brakes - so from that standpoint, it's what we each want to believe until someone presents some hard evidence. Your observations in life are no more or no less valid than mine are - so quit belittling mine and touting your just as anecdotal observations as more than they are. >> Admittedly, my theory that some percentage of drivers would pump the >> brakes is not provable as far as I know, but I do state it as >> conjecture, not as fact as you are stating your argument. > > with respect, you're projecting your own behavior as representative of > everyone. it's not true. Oh - I see - so no one pumps there brakes, either in accident avoidance situations or if faced with runaway acceleration. Got it. (I don't buy it.) >>> that's why abs exists - abs does that for them, and only in the event >>> of traction limit being exceeded.. >> >> You're not talking about the same situation. Trying to stop a vehicle >> with runaway acceleration is not the same setup as emergency accident >> avoidance as far as paniced driver psychology. (Yes - there are >> similarities, but don't turn that similarity into being equal.) > > indeed, but i'm pointing out the fact that refutes your supposition - > typical panicked drivers do not let off the brake pedal. That's your theory and your belief. No more or less valid than mine of the opposite. -- Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')
From: jim beam on 7 Mar 2010 16:19 On 03/07/2010 11:21 AM, Jeff Strickland wrote: > "jim beam"<me(a)privacy.net> wrote in message > news:F4udnU6SrZ0kZQ7WnZ2dnUVZ_vQAAAAA(a)speakeasy.net... >> On 03/07/2010 09:52 AM, Jeff Strickland wrote: >>> "jim beam"<me(a)privacy.net> wrote in message >>> news:xbWdnV8Xic4E3Q7WnZ2dnUVZ_rGdnZ2d(a)speakeasy.net... >>>> On 03/06/2010 08:36 PM, Bill Putney wrote: >>>>> Jeff Strickland wrote: >>>>> >>>>>> ...We have not explored the interaction of traction control and >>>>>> antilock brakes preventing the brakes from stopping a car with a >>>>>> throttle intent on being set to the maximum setting. We also have not >>>>>> looked at the issue of brake fade that comes from the brakes getting >>>>>> hot. If the brakes locked the front tires, the ABS would sense this >>>>>> and unlock them. The driver might keep his foot planted firmly on the >>>>>> brake pedal which has sunk to the floor, but the car still would not >>>>>> stop. The brakes get hot and fade badly, and the car wouldn't even >>>>>> slow down... >>>>> >>>>> Read my previous post. With throttle applied (even partial throttle), >>>>> the booster vacuum gets depleted *rapidly*. There is no way to use the >>>>> brakes to stop a car with an engine of any power at all with throttle >>>>> applied since, with throttle applied, intake vacuum drops to close to >>>>> zero. No vacuum = essentially no brakes. Try the experiment I described >>>>> in my previous post. >>>>> >>>> >>>> no dude. even with no engine running to replenish vacuum, there is >>>> still >>>> sufficient vacuum reserve in the booster to apply the brakes full-on >>>> three >>>> times. unless you have a leak of course, which could also be affecting >>>> your experience. >>>> >>> >>> >>> Doesn't matter. Even if the driver holds the brake pedal, the brakes get >>> hot >>> and fade, and this will cause them to stop doing the job they are >>> supposed >>> to do. >> >> if the brakes fade badly enough, even vacuum assist won't help much. >> >> > > Which is why a driver might not be able to stop a runaway car. You keep > saying that a driver can always stop a car by standing onthe brakes, the > facts are that this is not absolutely true. It's not even generally true, > but I'll give you the point that it's theoretically possible that a car can > be stopped while the throttle is held open and the brakes are held down. As > a practical matter, I don't think most people can manage such a situation. > maybe this isn't something you and i have discussed before, but underbraked domestics are a recurrent theme that's been done to death. [and i'm sure, literally.] just because someone has one experience with a certain vehicle doesn't mean it's representative of all vehicles. and it is generally true that most vehicles have a design spec of roughly 3:1 braking power vs. engine power. -- nomina rutrum rutrum
From: jim beam on 7 Mar 2010 16:27 On 03/07/2010 12:22 PM, Bill Putney wrote: > jim beam wrote: >> On 03/07/2010 09:57 AM, Bill Putney wrote: > >>> Then we disagree. I said this was on multiple cars with no leaks. >>> >>> But in any case, it appears that we agree on more than 3 stabs. Except >>> you still believe that there is sufficient vacuum in the plenum of an >>> engine in gear under acceleration to charge the booster to usable level >>> - and that is simply not correct. >> >> ok bill, give me your manifold vacuum numbers. even one inHg [a low >> number btw] is ~2lbs... > > If you mean approx. 2 psi, no - it's not. 1" HG is approx. 0.5 psi. > >> for 63 sq ins, that's still 126 lbs boost in... > > Nope - you're off by a factor of 4. you're right - i was reciprocated. > > So you made a calculation error. I won't hold it against you - can > happen to anybody. > >> addition to what the driver can apply. it may not be great, [assuming >> the driver has wasted their previous vacuum on multiple brake >> applications] but it'll still help stop the car. > > For normal braking, let's say the booster is charged by 15" Hg (7.4 > psi). The math says that you then have 466 pounds of assist directly > into the m.c. The booster has a reservoir for a reason - so it can store > that vacuum and give a reasonable ability to brake even if engine vacuum > is low (like if the engine cut off) for you to stop the car. > > If your vacuum is down to 2" Hg (0.98 psi) under whatever acceleration > condition (even more than the 1" Hg that you threw out), you have less > than 70 pounds of force into the m.c. > > So you're going to tell me that they designed it for 466 pounds of force > into the m.c. piston, with some reasonable safety factor, and with less > than 15% of the design vacuum available to the booster after initial > depletion (by pumping), you will be able to overcome the engine? > > Sorry - I said earlier regardless of what the numbers said, I know from > experience that the vacuum is not there to recharge the booster to any > usable amount. Now we even have the numbers. > > BTW - I chose to ratio things (i.e., compare 466 pounds of force to 61 > pounds of force) to show the ratios because we would have to talk in > terms of what the actual forces are required to press the m.c. and the > caliper pistons/pads, and we would have to back all that out to actual > pressure required on the pedal by the driver - and you have mechanical > advantage of the pivoting point of the brake pedal to factor into that - > too many variables for the discussion at this level - so again - that's > why I ratio'd it rather than keep in in absolute force terms like you did. > > Bottom line: Having only 60 pounds of the assumed 2" Hg compared to 15" > Hg or greater design doesn't work out too well if your talking about > overcoming a powerful engine with open throttle. it may not be great, but you can still do it. most healthy people can exert well over 200lbs force on the brake pedal. with approx 10:1 leverage ratio from the pedal swing arm, that's more than enough force to brake the vehicle. in normal usage, the vacuum booster is there to make braking effort light and easy, not to bridge the gap between the possible and the impossible. remember the days of un-assisted hydraulics? i do. strange how what we used to do before is suddenly impossible now. > > Bottom line no. 2: Regardless of what the numbers say (and they make my > point), in real world, the booster will not be of much help at all once > depleted of initial vacuum charge if the engine is under acceleration > (vacuum too low to be of much help). > 1. it still does help. 2. why would you pump the pedal repeatedly when you're trying to stop the vehicle? -- nomina rutrum rutrum
From: jim beam on 7 Mar 2010 16:39
On 03/07/2010 12:42 PM, Bill Putney wrote: > jim beam wrote: >> On 03/07/2010 09:44 AM, Bill Putney wrote: > >> then you're making generalizations from your own exceptional >> experience that are not representative of most vehicle, and certainly >> not vehicles with standard tuning. vacuum decreases significantly at >> w.o.t, is sufficient to give brake boost. >> >> besides, how much vacuum do you think you need for a brake booster? >> with a 9" diameter diaphragm, [on the small side by modern standards], >> that's 63 sq inches. how much pressure delta do you think you need to >> double a person's braking force? > > You'd have to know the mechanical advantage in the pedal from basic > lever math and a bunch of other stuff that is beyond what either of us > wants to get into here. Without the absolute numbers for all that stuff, > it suffices to ratio things: When the booster is designed to properly > operate at 7 times the vacuum that you'll get from an engine under > acceleration, I say your assist is going to be close to useless how did you manage before the days of vacuum assist? > - the > engine will likely overcome the brakes in being able to sustain highway > speeds (and this is before fade comes into the picture). fade is a feature of people that don't brake decisively and let the vehicle speed up again. or defective design/parts. > > Saying that 63 sq. in. is a big surface area and so even a tiny little > vacuum will give you huge force means nothing. Fact is the assist force > on the diaphragm with the very low vacuum under acceleration is less > than 15% of what it was designed to use under normal stored vacuum > conditions. how did anyone drive a car before the days of vacuum assist? > >> you're supposed to apply the brakes and keep them on. only if you >> release do you lose vacuum. > > Right or wrong, many people were taught to pump the brakes. Some people > will in fact pump the brakes, for any number of reasons. some people will hold their foot on the gas believing it to be the brake too. > >>> Plenum vacuum is just too low. Again - you >>> don't seem to know that - if you want to argue that point, then give me >>> some numbers for vacuum for a common engine of your choice at idle, and >>> in gear under light, moderate, and heavy acceleration. If the numbers >>> you come back with are honest, you will prove what I'm saying. >> >> see above. > > Yes - please see it. > >>>> if that is your experience, then i think you must have a vacuum leak... >>> >>> That is simply not the case. >>> >>>> even with wide open throttle, there is sufficient vacuum in the >>>> manifold to create significant braking assist. >>> >>> That is profoundly incorrect - period. >> >> again, if that's your experience, i think you're working with >> something not representative of most vehicles. certainly not anything >> post 1980. > > Nope. Your assumptions are wrong on both counts. why? why does my car come to halt when i brake hard at full throttle? why doesn't yours? > > >>>> sorry dude, it's incredibly rare for a panicking driver to pump their >>>> brakes... >>> >>> That's your *theory*, or you have something to back that statement up? >> >> where is your "theory" that people pump brakes? i saw someone on a >> freeway in rain in los angles one time, skating along with their >> brakes locked, car gently rotating, and the look on her face was that >> of someone trying to break the pedal off she was pushing it so hard. >> there's no way that person, as an average driver, was going to let off >> that pedal, and thus, even though she was going to crash, she was not >> going to lose vacuum. > > You keep throwing in scenarios totally different than what we're talking > about. no, i'm pointing out the flaw in the argument. for the argument to be valid, it has to work universally. otherwise you need to qualify it to specific circumstances. > Again - you can't prove your claim that hardly anyone pumps the > brakes any more than my claim that a lot of people would pump the brakes > - so from that standpoint, it's what we each want to believe until > someone presents some hard evidence. Your observations in life are no > more or no less valid than mine are - so quit belittling mine and > touting your just as anecdotal observations as more than they are. but dude, why do we have abs? it's not because people pump the brakes because then they'd be unnecessary! > >>> Admittedly, my theory that some percentage of drivers would pump the >>> brakes is not provable as far as I know, but I do state it as >>> conjecture, not as fact as you are stating your argument. >> >> with respect, you're projecting your own behavior as representative of >> everyone. it's not true. > > Oh - I see - so no one pumps there brakes, either in accident avoidance > situations or if faced with runaway acceleration. Got it. (I don't buy it.) abs. that's the reality, like it or not. > >>>> that's why abs exists - abs does that for them, and only in the event >>>> of traction limit being exceeded.. >>> >>> You're not talking about the same situation. Trying to stop a vehicle >>> with runaway acceleration is not the same setup as emergency accident >>> avoidance as far as paniced driver psychology. (Yes - there are >>> similarities, but don't turn that similarity into being equal.) >> >> indeed, but i'm pointing out the fact that refutes your supposition - >> typical panicked drivers do not let off the brake pedal. > > That's your theory and your belief. No more or less valid than mine of > the opposite. > you're entitled to your opinion, but preface them as such. "i believe that..." is quite different from the definitive statements you've been making like "the ... will absolutely *not* ... " etc., because it's not true. -- nomina rutrum rutrum |