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#31
January 29th 05, 03:07 AM
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MoPar Man wrote:
> Bill Putney wrote: > > >>That's because of the "self-energizing" property of drum brakes. >>The geometry of the pivot point is designed such that the small >>amount of friction applied due to the pedal pressure gets amplified >>by rotating the shoe into the drum harder (a multiplication effect, >>a mechanical "power brake"). > > > I thought it was because the 100% of the pressure in the brake line is > transfered to pushing both brake shoes into the drum (because the > wheel cylinder is pushing 2 pistons outward towards the drum out of > both sides of the cylinder) vs disk brakes (where half of the pressure > is wasted by trying to force open the calipers and the other half is > used to push the pads into the rotor). > > >>The downside of power brakes, which is a necessity with disk brakes >>because they do not have the designed-in mechanical amplification, >>is that after one or two pumps of the pedal while under throttle, > > -------------------------------------------------------- > > A condition I can't imagine happening in the field, unless it's one of > these hypothetical mysterious run-away > full-throttle-while-standing-still cases... I guess I have to ask why you snipped the rest of that paragraph where I essentially said exactly that. Continuing from where you snipped my post: "...your vacuum reserve is depleted, and you essentially have no (or extremely weak) brakes - this could become critical if a sudden acceleration situation arises (due to driver error, floor mat jam, or vehicle controls failure). The mechanics of the drum brake is totally immune from that loss of amplification." > ...Even in that case, you're > not going to be pumping the brakes several times (and depleating the > vacuum reservior) - you're going to plant your foot on the brake pedal > and keep it there. In that case, you're not going to depleat the > vacuum. A person in such a panic situation is not only possibly going to pump the brakes, but will very likely do so, only to find the brakes getting very weak by the first pump, and for all intents and purposes, totally gone by the second or third, and the remainder of the event, whatever it turns out to be, will be over in mere seconds. Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my adddress with the letter 'x') 
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#32
January 29th 05, 03:12 AM
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Nate Nagel wrote:
> ...but in a > typical disc with floating shoes and an anchor pin at the top, a lot of > the torque generated actually comes from the rotation of the drum > forcing the primary (trailing) shoe into the anchor pin. Obviously you meant "but in a typical drum with floating shoes..." 8^) Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my adddress with the letter 'x')
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#33
January 29th 05, 03:17 AM
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#34
January 29th 05, 03:34 AM
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Bill Putney wrote:
> Nate Nagel wrote: > >> ...but in a typical disc with floating shoes and an anchor pin at the >> top, a lot of the torque generated actually comes from the rotation of >> the drum forcing the primary (trailing) shoe into the anchor pin. > > > Obviously you meant "but in a typical drum with floating shoes..." 8^) > > Bill Putney > (To reply by e-mail, replace the last letter of the alphabet in my > adddress with the letter 'x') Sometimes I amaze myself. Sometimes even in a good way. Sometimes... well, I just amaze myself. nate -- replace "fly" with "com" to reply. http://home.comcast.net/~njnagel
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#35
January 29th 05, 03:39 AM
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Bill Putney wrote:
> A person in such a panic situation is not only possibly going to > pump the brakes, but will very likely do so I don't understand the thinking behind that. If you jam on the brakes, and you haven't locked up the wheels (because you don't hear the tires squeeling), then why would the typical person *let up on the brakes* and perform a second (or third, etc) application? In a panic situation where you know you haven't locked the wheels, I bet the typical person would keep his foot planted on the brake pedal until the desired degree of deceleration has been achieved.
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#36
January 29th 05, 04:57 AM
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On Fri, 28 Jan 2005, Bill Putney wrote:
> >>>The downside of power brakes, which is a necessity with disk brakes > >>>because they do not have the designed-in mechanical amplification, > >>Why do people keep saying this? > > The parrot effect, I'm guessing. > No. Rawk! Yes. > The reality of modern consumer vehicles that will be driven by > quite a range of ages, mental quickness, and physical strength. Right, 'cause women and little old men *never* drove before what you arbitrarily consider the "modern" age. Pfft. > I used to drive an International Travelall (similar in size to a Chevy > Suburban). Unfortunately the mfgr. figured it didn't need power > steering No, the original owner decided it didn't need power steering. In any event, that's irrelevant to the topic at hand, which deals with *brakes*. > I think Not hard enough, as it seems.
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#37
January 29th 05, 05:01 AM
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On Fri, 28 Jan 2005, Bill Putney wrote:
> I think it could be argued that for a commercially successful > main-stream consumer vehicle of today, they do. Arguability and truth are not the same thing, and the former does not imply the latter. Most consumers don't know or care what-all goes on under the hood and under the car. Give them a car that works (even marginally, viz. brisk-selling garbage from GM and Ford) and they'll buy it. > > ...I much prefer the feel of manual disk brakes to any other > > braking system out there. My '69 Dodge currently has stock Kelsey-Hayes > > front disks and stock rear-drums, activated by a MANUAL disk brake > > master cylinder and a MANUAL pedal linkage. The feel is just wonderful, > > and really only slightly higher pedal effort than when it had a power > > booster, MC, and pedal setup. There is much more pedal *travel* which > > allows finer control over braking with the manual setup. > > Yes - achieved with much smaller diameter 7/8" bore vs. 1-1/32" bore, so the "much" smaller diameter amounts to a whole five thirty-seconds of an inch. And that's only on certain vehicles. In some years and on some vehicles ('70 and earlier A-bodies for one example), the same 1" bore was used with or without a booster. > The reason that they don't have power boost is that (1) there is not > adequate vacuum to guarantee boost under all critical conditions Hydroboost > (2) The weight penalty of a separate electrically powered vacuum pump is > too high Hydroboost
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#39
January 29th 05, 04:05 PM
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Daniel J. Stern wrote:
> On Fri, 28 Jan 2005, Bill Putney wrote: > > >>I think it could be argued that for a commercially successful >>main-stream consumer vehicle of today, they do. > > > Arguability and truth are not the same thing, and the former does not > imply the latter. We're talking about the market place - not subject to rigid rules as are the laws of physics. That's why claims made about it are subjective. Certainly you aren't going to claim that you can predict what the "market" will do in response to some subtile technical change - yet there likely will be a repsonse to that change. There's just no formula to calculate the impact. > Most consumers don't know or care what-all goes on under > the hood and under the car. Give them a car that works (even marginally, > viz. brisk-selling garbage from GM and Ford) and they'll buy it. > > >>>...I much prefer the feel of manual disk brakes to any other >>>braking system out there. My '69 Dodge currently has stock Kelsey-Hayes >>>front disks and stock rear-drums, activated by a MANUAL disk brake >>>master cylinder and a MANUAL pedal linkage. The feel is just wonderful, >>>and really only slightly higher pedal effort than when it had a power >>>booster, MC, and pedal setup. There is much more pedal *travel* which >>>allows finer control over braking with the manual setup. >> >>Yes - achieved with much smaller diameter > > > 7/8" bore vs. 1-1/32" bore, so the "much" smaller diameter amounts to a > whole five thirty-seconds of an inch. And that's only on certain vehicles. > In some years and on some vehicles ('70 and earlier A-bodies for one > example), the same 1" bore was used with or without a booster. So that's a 40% increase in piston area = 40% increase in pedal-to-caliper piston force multiplication. Nice trick using a throwing out a linear parameter in an attempt to minimize what is a square law effect. 40% is a big difference when you're talking braking effort. > >>The reason that they don't have power boost is that (1) there is not >>adequate vacuum to guarantee boost under all critical conditions > > > Hydroboost Which does away with unobtainable vacuum requirements and gets the boost from an "always on" source: the power steering pump. Thanks for reinforcing my point that vacuum source for brakes is unreliable in certain critical situations like stuck throttle, which is what I believe we were talking about. Once again, my points about the difference between drums and disc brakes were in the context of the cars that 95+% of consumers and no doubt those here drive. I stand by what I said. The more commmercially viable vehicle will have power assisted disc brakes on the front if not on all four wheels - right or wrong, that's what today's market has determined. It has obviously not always been that way, nor will it likely always be that way in the future. > >>(2) The weight penalty of a separate electrically powered vacuum pump is >>too high > > > Hydroboost -- Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my adddress with the letter 'x')
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#40
January 29th 05, 04:11 PM
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MoPar Man wrote:
> Bill Putney wrote: > > >>A person in such a panic situation is not only possibly going to >>pump the brakes, but will very likely do so > > > I don't understand the thinking behind that. > > If you jam on the brakes, and you haven't locked up the wheels > (because you don't hear the tires squeeling), then why would the > typical person *let up on the brakes* and perform a second (or third, > etc) application? > > In a panic situation where you know you haven't locked the wheels, I > bet the typical person would keep his foot planted on the brake pedal > until the desired degree of deceleration has been achieved. People don't always behave logically in panic situations. That's why police find shooting victims with a bullet hole in their hands in their last desperate effort to stop the bullet from striking their face. Does that action on their part make sense? No - for if they survive, it just means an additional injury and/or permanent disability. Both of us are conjecturing what "most" people will do when they jam on the brakes and the brakes are not effective enough. Heck - many people will pump the brakes in any situation out of habit because that's what they used to be taught. I can't prove it, you can't prove your point - we're both speculating. Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my adddress with the letter 'x')
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