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Proportioning valve delete

Discussion in 'The Hokey Ass Message Board' started by Essex_29, Feb 10, 2010.

  1. Essex_29
    Joined: Dec 27, 2007
    Posts: 145

    Essex_29
    Member
    from Finland

    Come to think of it, the short wheel base Morris/Austin Minis used a brake limiter, so no matter how hard you pressed the brakes, a man could turn the rear wheels without too much effort.
    [​IMG]
    Morris Mini 1275 GT

    Guess this contradicts my earlier post a bit, but no rules without exceptions, eh?
     
  2. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
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    Essex_29, I am sorry but I disagree. I tried to explain this before and I propably used poor wording as I am not an expert on the subject terminology. I totally understand the concept that the fronts are built differently than the rears - meaning we can & do have varying braking power out of each end (front versus rear).

    If I take your statement at face valve ( the part about the Porportioning valve being used soley to match components because of various build conditions engine weight , wheelbase etc) then how do you explain the CURVE on my graph picture???? The if this is a STATIC problem and all the factors you mention are constant then the relationship between front and rear are LINEAR. This in reality is NOT LINEAR - at least not linear IF you are trying to optimize your brakes at various braking "g" load conditions. The relationship changes - ie NOT LINEAR. The proportioning valve attempts to duplicate this curve in a best fit arrangement.

    I'm sorry your point is NOT illustrated because the "problem" is not about driving at ballistic speeds - it is about varying conditions meaning trying to optimize balance at an entire range of braking pressure. Please look at the graph I posted earlier - notice the straight line illustrating the relationship between front and rear - changing components be it brakes vehicle weight cg etc won't change the fact that that line is FLAT. There is nothing DYNAMIC there. It is that shift in weight - which is of course different depending on how hard you brake that the proportioning valve is trying to compensate for. We always hear the statement that the front brakes do 75% of your braking - well as true as that statement is it is also slightly misleading. I refer to my origional fictional example of the car with all identical components & the perfect 50/50 weight split -Let's pretend we are approaching a stop and initially hit the brakes ever so lightly - under the lightest of braking conditions let's say there is NO weight transfer and therefore each tire is capable of supporting the exact same amount of braking capability. we would want the same brake pressure to both halves of our system. Notice the very beginning of our graph - it shoots up at a 45 degree angle - symbolizing equal pressure in both haves of the system. As we are approaching out stop we determine we are still going too fast and apply a little more pressure to our brakes. At this point we are experiencing some weight transfer (let's pretend it shifts from 50/50 to 60/40 - the result of the transfer means our front tires are carrying more dynamic load - the result (see note 1 below) is that it can now support more braking force before lockup than before - this is no problem for the fronts because under a mild condition we are still well within the limits of traction however for what we GAIN in the fronts ability to stop we also LOOSE in the rears (no free lunch) - this is where the problem begins. The solution is to reduce the pressure to the rear brakes to prevent lockup. So now EACH wheel is MAXIMIZING their ability to stop. AS we continue on we are noticing we are still going too fast and apply even MORE PRESSURE to the brake pedal. AS before the result is even MORE weight transfer now let's say it goes from 60/40 to 70/30. Just as before - the fronts having MORE LOAD can do more stopping - and also the rears can do LESS - again we must do something to address this - which as before is to drop the pressure to the rears even "more". And by "more" I mean it is even further from the LINEAR LINE than previously - so in anotherwords we haven't really dropped the rear pressure because in realality it is still increasing - what we have "dropped" was the RATE of pressure rise. Again let's refer to the graph - that curve is falling off quickly. These are varying conditions - we need to address them in some manner IF we want to optimize the system. So now granted we can design a system without a proportionign valve that is balanced at any single pressure point - however at the remaining points we will be "off" further from deal than IF we chose to use the proper proportioning valve.
    Now to adress your points about longer wheel bases and heavier motors etc etc. these all shift the CG of teh vehicle and YES I agree this certainly has an effect on the proportioing valve as it affects the WEIGHT TRANSFER.

    Note 1 - For years Drag Racers were always trying to maximize weight transer. The reason being that they realized to carry weight is a penalty - so making the cars lighter made them faster, but when they were restricted to a "small" tire they quickly realize that shifting as much weight to the rears tires on launch as possible they could also MAXIMIZE traction without adding overall weight to the vehicle . More tire load = equals more traction --> much the same way that weight shift forward means more tire load and increased brake ability.


    So my question to you is simple - if not the proportioning valve - how DID the factory deal with weight transfer?? Did they simply ignore it???? Did they design for one and only one braking pressure to be optimized?? What 's the scoop???


    EDIT: Since I was asked earlier for some "supporting documentation" and I did my best to comply (sorry those pics aren't all that great - if anyone is interested I can scan them next week and repost them so they are more legible) - what I now ask back is for all those saying I am full of crap - please do the same - I would love to see YOUR REFERENCES. If I have totally misunderstood my reference material I would appreciate some additional material so we can all get down to what's what. So far we've only had one and it wasn't very applicable to what is "common" in the US - especially the part about dual servo being only good for things like tow motors!! ha ha

    Essex_29 You're right we did neglect to answer your origional question. So the answer is "yes".

     
    Last edited: Feb 14, 2010
  3. henryj429
    Joined: Jan 18, 2007
    Posts: 1,063

    henryj429
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    HemiRambler, that brings me back to my earlier question. The Wilwood adustable valve has only one setting which probably will work best at one level of braking pressure and not as good at all others. An OEM valve - according to your very good discussion - should work well over a much wider range of braking conditions, except that it is optimized for a certain vehicle and could actually work badly when put on a car with different brake hardware, weight distribution and tires.

    So, for a hot rod that is a one of a kind vehicle, is the Wilwood the lesser of two evils?
     
  4. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
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    Henryj429 - that's a good question that leads us to discuss how the proportioning valve actually works. I hesitated to mention this before because I was afraid to muddy the discussion (fat chance - I know). Ideally we have a curve on our front to rear pressure graph. The proportioning valve doesn't magically duplicate this ideal curve - in actuality it can only be adjusted for a "best fit" scenario by changing the crossover or rather the Changeover Pressure point. See the attached photo. The dashed lines represent the actual pressure relationship out of the proportioning valve - notice in each example (the 60/40 and 50/50) both rates for the proportioning valve are the same. By the same I mean they change at the same rate - ie the 2 "slopes" are the same. So what we are adjusting is not those rates - although it'd be nice to - instead we are adjusting the point at which we changeover from one rate to another - trying to get the combination of those two straight lines to fit as best we can to the actual curve which represents the ideal braking condition.

    So it is quite possible in theory that a factory proportioning valve can have a different rate as well as a different changeover pressure - so maybe just maybe it could outperform an adjustable valve in some particular application. I think those are slim odds, but possible nonetheless. The adjustable one while far from ideal at least gives you the ability to "tweak" the realationship for a BEST FIT scenario.
    And just for the sake of discussion - please note these graphs are all in relationship to the "G" force. Sorta makes a fella wonder why?(hint hint for those who still think I am clueless) Also notice this example talks about the CARS WEIGHT being 50/50 and 60/40 - not the brakes doing 50/50 or 60/40 - obviously because that pressure relationship is changing as represented by the "curves".


    So the "one setting" we are talking about is in fact correct - depending on how you look at it. So it isn't really that "one setting" it's the various settings allowing us to shift our straight lines around to best fit our car's particular ideal curve. Without a proportioning valve the equation simply gets worse and worse as you go - represented by the initial actual pressure line continuing linear and getting further and further from that ideal curve.

    So in some sense of the word - calling the proportioning valve dynamic may be a bit misleading - what is dynamic is the brake bias versus the "g" force. So as the "g" force changes so does our ideal situation. The answer is dynamic - so tossing it (the proportioning valve) is about as useful as the aussies description of a tosser - it may make you feel good for a few seconds, but long term you accomplished very little.

    The factory proportioning valves are mostly combination valves - which simply adds multi purposes to one valve - I don't know enough about the rates and changeover points to say one is better than another across the board - at least when you are comparing apples to apples. For all I know they have the same rate - and in THAT case for a Hot Rod - the adjustable one makes more sense as I can tweak my "curve" to be as close as possible to the ideal. Now IF you have a situation where your rod duplicates a factory setup very closely - then unless you are REALLY dedicated & determined to find the "sweet spot" on your adjustable proportioning valve in THAT particular situation the factory valve may very well be the best bet. And in THAT case you get the added bonus of (assuming you're really using a combination valve) it's other features.

    Remember that "one setting" has adjusted more than just one pressure point.

    Does that make sense???:eek:

     

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    Last edited: Feb 14, 2010
  5. B Blue
    Joined: Jul 30, 2009
    Posts: 281

    B Blue
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    HemiRamler, I think you are correct in that the early GM valves with delayed proportioning were an attempt to optimize braking effort across the spectrum of braking conditions. But why? You increase rear braking effort under "normal" conditions when maximun controlled stopping is not the issue. Nintey percent of the time you really do not need the extra braking effort, all you gain is a small amount of front brake wear transfered to the rears.

    Also, the system is not dynamic in the sense it can react to current driving conditions. Put three people in the rear and a hundred pounds of salt in the trunk the braking split remains the same. The factory proportioning valve configuration is valid only when using factory spec components. Swap the front pads for some high performance items and the whole optimizing thing is out the window. For all its sophisication, the GM system was not adaptive to anything.

    Bill
    P.S., I note you addressed most of this while I was typing.
     
    Last edited: Feb 14, 2010
  6. Essex_29
    Joined: Dec 27, 2007
    Posts: 145

    Essex_29
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    from Finland

    B Blue, I think you are right again.
    The ideal brake bias may be a curve, but the only time we really need that braking in exact proportions is when braking as much as is possible, ie when the wheels are locking up.
    This is also evident whan we drive on slippery surfaces, as snow. With almost no weight transfer, the fronts lock up real easily, and you need to press real heavy on the pedal to lock up the rears.

    HemiRambler, You may be right about a proportioning valve being very important on production cars, but oddly enough the '68 Plymouth Satellite I robbed my brakes off didn't have one...

    Thank you for answering my original question.
     
  7. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
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    B Blue, I think the "why ?" wasn't nearly as much about pad wear as it was about ROAD CONDITIONS. On a slick surface you cannot stop as hard or you skid - so with LESS pressure to the fronts and therefore less weight transfer you can ADD (or not reduce as much) pressure to the rears when they CAN handle it - thus optimizing your braking under adverse conditions. Driving in the snow I get to experience this quite a bit. So I agree it ISN'T all about maximum stopping under ideal conditions - it is very much about that darn grey area in between!!!!

    With that in mind - I can see a system with no proportioning valve and matched components stopping under ideal conditions in some number of seconds or feet or what have you. Now near exact duplicate system WITH a proportioning valve and a rear brake capable of slightly more braking power - viola - it stops in exactly the same time and distance. No improvement - the excess rear power was too much and thus reduced by the proportioning valve. Does this prove you don't need a proportioning valve - (well to some I am sure it does - they argue that they matched their components "better" and eliminated this un-necessary part) and if ALL your braking is under this ideal set of conditions then I suppose you can make that arguement. This BTW is similar to the balance bar example I mentioned earlier. So with this example in mind - can we call these systems "equal" - I would say SURE, BUT!!!! (you knew this was coming right?) only under this one set of circumstances. So now same two vehicles - panic stopping in the rain - the proportioning valve system at this lower "G" load has the ability to INCREASE the REAR BRAKE pressure over it's non-proportioning valve counterpart. The proportioning valve car's rear brakes due to less weight transfer can carry "extra" brake load and thus THAT car stops better/shorter/quicker. Reduce the overall braking conditions even more and maybe you get below that changeover pressure and again the systems are equal in performance again. So can one make the arguement - it's not enough to matter - sure - I personally am not buying it, but to each their own. I also like to look at it this way - GM can and did match their components - they also got rid of 2 cent grease fittings and yet kept the proportioning valve. Why??? I can tell you one thing it wasn't public awareness!!!!!!! They kept it because it served a definite purpose.

    I agree to the point about it not being dynamic in the sense you describe - the system has no idea whether you have 1 passenger or 5 - but with 5 you DO have more weight - with more weight you get more weight transfer - and with that additinal transfer you get more brake potential from those front tires - effectively meaning you are simply operating FARTHER up the pressure curve - so now with more load - you are hopefully closer to optimizing than with out it. Now clearly - this is somewhat limited - the only truely dynamic proportioning valve are the ones I mentioned earlier the ones that were used in some pickup trucks - those sensed ride height (due to load) and adjusted the proportioning valve accordingly. So yes I agree and admit that the pickup truck example is not normal or even very common. I probably misused the word dynamic - but I was trying to emphasize the part about the pressure curve being non-linear.

    Yes the GM system was NOT ADAPTIVE - I didn't mean to suggest it was - rather it was designed to adapt to those changing conditions - once set - it didn't ever change that relationship - instead YOU the driver changed how hard you depressed the brakes - the only thing remotely "adaptive" was how it changed the rate of pressure drop. And YES I realize that was truely static. I suppose I chose some poor words - but what I was trying to emphasize was that the brakign conditions CHANGE with load and this system changed with YOUR LOAD - granted it did the same change everytime so calling it dynamic was probably confusing - I just don't have a better grasp of the english language to have come up with better more accurate and descriptive words.:eek::eek:


     
  8. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
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    Essex - absolutely agree LOTS of production cars did NOT have porportioning valve - brake systems have been refined to work better and better every year since their inception. Do you think that '68 Plymouth will stop as good as the latest hardware on the road today? Apples and oranges I know, but therein lies the problem - we ARE using older less sophisticated hardware than those around us - they have their antilock this and computer that etc etc. We are already at a disadvantage - plain and simple. Why would you give away even more???? I don't want a computer in my car - I'm not even suggesting that - I am only saying why not make your system as optimal as possible?? May save you some grief in some extreme downpour someday.



     
  9. ehdubya
    Joined: Aug 27, 2008
    Posts: 2,315

    ehdubya
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    Essex 29, you're right, 4 wheel drum bias is proportioned by wheel cylinder and lining area but all drums are self energizing to some degree in that when the linings contact the drum they wind out so pedal effort is not directly relative to braking force like disc brakes which have different pressure requirements.

    I think what you're describing they define as Duplex. I'm curious what specific pre vacuum assisted vehicles Bill mentioned used Servo brakes as I've never encountered them.
     
  10. B Blue
    Joined: Jul 30, 2009
    Posts: 281

    B Blue
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    I believe what I've called servo brakes is really Duplex, which employ a servo action to magnify the braking effort.

    Bill
     
  11. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
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    I'm not sure & I wondered the same thing initially because it sounded so bizzare so I read and reread it several times. What "they" wrote was quite confusing. Their version of duplex I could see as described as a DUAL SERVO but as described earlier would work crappy when rotated in reverse as you loose all servo action in that direction. I think I even added to the confusion because what I was describing as DUAL SERVO was what Bendix called DUO-SERVO which is what I think they called "servo" which again they went on to describe som ebizzare stuff like NOT using that for steer wheels and better suited for slow speed two motors.

    The real confusing part is that ALL DRUM brakes exhibit some SERVO ACTION - the question is how much. An old style single wheel cylinder bottom pivot twin shoe drum brake setup has one shoe leading and one shoe trailing - with ONLY the leading shoe benefitting from servo action (well when rotated in one direction anyways). I called this "single servo" but I think the proper term or at least the more common term is "leading and trailing Shoe" brakes.

    The Duplex version they had - I didn't even consider but can see how it could be thought of as DUAL Servo - since you can arrange both shoes to benefit from servo action in one direction but neither shoe benefits from it in the opposite direction.

    And what they called SERVO - I STILL (mistakenly?) think of as DUAL SERVO - but in reality Bendix Called DUO SERVO and was set up with a single wheel cylinder - NO shoe pivots - but floating shoes connected to one another and anchored in place by a large pin. In one direction you get both shoes to work together as they become one due to the connector link (aka adjuster link or self adjuster)- These get forced into (or better stated wedged into place)on one side of the anchor pin while the opposite side pushes away from that same pin. The entire operation is reveresed when the drum is rotated the opposite direction and therefore works very well in BOTH directions - giving you all the benefits of dual servo action and no reversing penalty. Clearly it was THIS setup they described so goofy. They also went on to say that most mechanical brakes (99% IIRC) were SIMPLEX - which must date this either to WAY WAY BACK or simply so different from teh stuff we use in the states that I can't even fathom it.

    So anyways - YES - whenever I wrote DUAL SERVO - what I really meant was the Bendix DUO SERVO and not a twin wheel cylinder twin shoe pivot getup.:eek:


    The really confusing part is the servo description - since they all benefit a certain degree from servo action - it is just plain confusing to say some are servo and some are not. They pretty much all are - just to different degrees - - let me frame that comment is about DRUM BRAKES!!!:D:p


     
  12. ehdubya
    Joined: Aug 27, 2008
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    ehdubya
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    yay, we're all on the same page! Yeah the terminology sure is confusing, let's hope Essex 29 gets some good brakes.
     
  13. Essex_29
    Joined: Dec 27, 2007
    Posts: 145

    Essex_29
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    from Finland

    Thank you Hemi Rambler for a lot of interesting and inspiring info on brakes, prop valves etc. I've been looking for info on drum brakes on the net, and there's lots of designs!
    This is what I found on what you call Dual Servo Brakes:

    "Duo-servo:A drum brake that has servo action in both the forward and reverse directions."

    That name makes sense.

    Never mind the name, Any case it's that kind of drumbrakes I have under my cars.
    The '68 Heavy duty 11" dia and 3" wide Mopar drum brakes are fine on my light weight Essex. They're not as good as some newer brakes, but as I'm sure lots of people building rods do, I chose them because they look cool.
    :cool:
    But that's another thing, and another car.
    The reason for taking out the internals from my proportioning valve is that the Ford rear drums seem to need more pressure than the GM drums once plumbed in the same place. The reason not to replace the prop valve is that I don't know for a fact that deleting the prop valve will get me enough pressure to the rear brakes.
    If this is the case, a proportioning valve will do no good before the rear brakes are rebuilt with larger diameter brake cylinders.

    As you may have seen in a previous post of mine, I have now deleted the valve by taking out the internals, and when some other work is done, and the brakes bled, I'll go for a test drive, maybe take it to a brake dyno, and I'll post the results. It may be a while though, because we have winte and snow and 20 below freezing... BRRRRRR!

    Haven't anybody tried to adapt a Ford 9" with drum brakes on a GM car, and had this problem with weak rear brakes?
     
  14. Essex_29
    Joined: Dec 27, 2007
    Posts: 145

    Essex_29
    Member
    from Finland

    Thank you Hemi Rambler for a lot of interesting and inspiring info on brakes, prop valves etc. I've been looking for info on drum brakes on the net, and there's lots of designs!
    This is what I found on what you call Dual Servo Brakes:

    "Duo-servo:A drum brake that has servo action in both the forward and reverse directions."

    That name makes sense.

    Never mind the name, Any case it's that kind of drumbrakes I have under my cars.
    The '68 Heavy duty 11" dia and 3" wide Mopar drum brakes are fine on my light weight Essex. They're not as good as some newer brakes, but as I'm sure lots of people building rods do, I chose them because they look cool.
    :cool:
    But that's another thing, and another car.
    The reason for taking out the internals from my proportioning valve is that the Ford rear drums seem to need more pressure than the GM drums once plumbed in the same place. The reason not to replace the prop valve is that I don't know for a fact that deleting the prop valve will get me enough pressure to the rear brakes.
    If this is the case, a proportioning valve will do no good before the rear brakes are rebuilt with larger diameter brake cylinders.

    As you may have seen in a previous post of mine, I have now deleted the valve by taking out the internals, and when some other work is done, and the brakes bled, I'll go for a test drive, maybe take it to a brake dyno, and I'll post the results. It may be a while though, because we have winter and snow and 20 below freezing... BRRRRRR!

    Haven't anybody tried to adapt a Ford 9" with drum brakes on a GM car, and had this problem with weak rear brakes?
     
  15. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,966

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    A couple of things really should be addressed, clarified and corrected. I'm not going to quote the incorrect information, as it has been repeated several times. Also, several magazine articles, as well as some internet info, are just wrong on some of their content.

    1-Drum brakes. There are two basic designs, servo and non-servo, period. When the shoes are isolated from each other, there is NO servo action, whether a fixed or sliding anchor design, and therefore are non-servo brakes. These can be found on most early cars/trucks, early Fords up to '48 (Lockheed) and on the rear axles of many late model front-drive, and a few rear drive vehicles. The fixed-anchor designs are not as efficient as sliding anchor designs, but they are all non-servo in operation.

    Bendix brought out the Duo-Servo drum brake in the late 30's, and it became standard on all Fords in '49 as well as many other makes, and, along with design variations made by other manufacturers, are still used today on the rear axles of many cars and trucks. Both shoes use a single common upper anchor, while being connected or linked together at the bottom by the adjuster. When the vehicle has forward motion and the brake is applied, the forward ( primary) shoe "serves" the rear (secondary) shoe through the adjuster, forcing the secondary shoe into the drum and back into the wheel cylinder. This "servo" action makes the brake much more effective than non-servo designs, along with requiring less system pressure and/or pedal effort. (Ford cars and light trucks, along with many other cars, did not need power assist until after the early 50's)

    There is/was also a Uni-Servo brake, found mainly on boat trailers with hydraulic actuation. These have one forward-acting cylinder, resulting in good forward servo braking, but down to about 10% while backing up, to ease launching.

    The only disadvantage of a Duo-Servo brake is complexity, and cost. Duo-Servos usually have different length and/or material pri/sec shoe linings, along with sometimes complex self adjuster assemblies, whereas non servos usually have common shoe/linings and simpler adjusting mechanisms, which makes them easier to manufacture, assemble, warehouse and install.
    Bob
     
  16. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,966

    V8 Bob
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    Proportioning Valves- PV‘s for short.

    PV's became necessary because of the higher line pressure required for disc brakes. Their purpose is to limit pressure to the rear brakes. There are several kinds; ie, fixed, load sensing and inertia, which are mechanical, and the later electronic PV's incorporated into ABS systems. I will keep the discussion to the fixed style, like most of us use or are familiar with, such as the Wilwood, and older Bendix and Kelsy Hays stand-alone valves.

    Front disc brakes became popular in the late 60’s, and standard fare by 1975 or so. The problem was there was not universal brake test requirements or standards until FMVSS “105-75” was adopted in the mid 70’s. Up to that point, a few states had tough lining requirements, Pennsylvania being one of the toughest, and most manufactures made sure their vehicles passed these independent state requirements. But it wasn’t until 105 testing became mandatory that vehicle brake systems became more refined. This is one reason why PV’s were not on all early disc-brakes vehicles.

    The “105“ (and newer “135”) tests require all vehicles to meet stopping distance requirements at loaded GVW and LLVW weights, full and partial braking systems (failing each half of the master cylinder), failing any assist (ie, vacuum ), several dry and wet fade tests, and parking on a 30 % grade. In a common vertical-split brake system (front axle/rear axle, as apposed to a typical front drive “cross” split LF/RR & RF/LR), the rear brakes have to be large enough to stop the vehicle within the required stopping distances, loaded and LL, from 60 MPH, and hold the vehicle on a parking grade, up and down for 5 minutes, along with all the other tests the vehicle has to perform. Now maybe you can understand why PV’s are necessary. If you knew how much test time was needed to match PV’s to each vehicle, with all the various wheel base, tire, front/rear weight change, engine size, weight capacities, etc, you would understand why using a factory or aftermarket “fixed” PV’s either alone or as part of a combination (combo) valve is a crap shoot at best!

    The PV's operate by allowing system pressure to be equal to all brakes until the "crack", "split" or "knee" point has been reached, and then reduces pressure out of the PV based on a pre-set fixed percentage, rate or ratio. Common rates are .27, .43, .59 and several others. If we take a .27 valve, with a “crack” point of 500, all pressure will stay equal to 500 psi, then the PV will limit the rear pressure at a rate of .27, or 27% of every additional 100 psi delivered to the fronts. So at 600 psi to the fronts, the rears will see 527; 700 fronts = 554 rears; 800 fronts = 581 rears; 900 fronts = 608 rears, and 1000 to the fronts = 635 to the rears. Important to note- these values are the same static or dynamic with a fixed PV, and the rear pressure never reaches the front pressure value after the crack point. Usual max crack pressure is about 1000 psi, so the adjustable Wilwood or similar will allow equal pressure up to this value with the knob screwed/turned to max.

    Bob
     
    Last edited: Feb 19, 2010
  17. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

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    Bob, It appears that my reference material contradicts your comments - Could you please elaborate on exactly what and HOW servo action is achieved.

    Also how does the mechanics of the Boat example you cited below work???

    EDIT: actually my reference material divides the drum brakes into 3 categories: Leading/trailing shoe, two leading shoe & duo-servo (while they do show examples of tri-leading shoe - it's basically a derriviative of the two leading shoe and therefore isn't "different" enough to make another category out of it)

    Anyways - in the 2nd picture (I just added) they are comparing the 3 main types of shoe arrangements to see how each one's differing SERVO ACTION affects PEDAL EFFORT. So again the material more than suggests each design benefits from servo affect - it plainly shows the results.

    I think as the dsigners understood this relationship and started to rearrange components to take advantage of it - they also started to use the terms - and at THAT time some were referred to as "servo" brakes while other (older) designs were maybe not so much, but to suggest some have it while others do not is really muddying the waters. IMHO

     

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    Last edited: Feb 20, 2010
  18. V8 Bob
    Joined: Feb 6, 2007
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    HemiRambler,
    I tried to explain the difference between servo and non-servo above, so I'm not sure what else I can add that would make it more understandable. The picture you show is a fixed anchor non-servo brake, plain and simple.

    There have been several names for various designs of non-servo brakes over the years. Some have one cylinder, some have two, along with fixed and sliding anchors, but the fact is, if the shoes do not interact with one another, they are non-servo. As I stated early on, there is a lot of mis-information on the net and published. I found one article on the net stating how to adjust a Bendix Duo-Servo, when in fact they were describing a Lockheed-design fixed anchor shoe adjustment! Go figure!

    The graph you presented shows the effectiveness difference of:

    1-single cylinder non-servo or single-leading; the least effective.

    2- twin cylinder non-servo or two leading, more effective, usually found on heavy cars and light-med. trucks, and

    3- Duo-Servo, king of the hill in drum brake design.

    The Uni-Servo trailer brake was/is used with hydraulic surge hitchs-the hitch telescopes and operates a master cylinder during vehicle braking (trailer dynamic inertia) but also while backing the trailer (trailer mass). The brake has a single piston cylinder that pushes the forward or primary shoe, which then serves the secondary through the adjuster, and provides near-duo servo performance forward, but has very little braking going backward, without the need to lock out the surge master. Later hydraulic disc equipped trailers with surge hitches use a backup-light-operated fluid (or other) lockout that effectively makes the hitch “solid” and allows easy backup without trailer brake apply.

    I agree with your last statment that design names get lost, mixed or confussed over time, intentionally or otherwise. What bothers me most are some of the statements made by people in the aftermarket brake supply business that can make some really nice adaptors, brake kits, pedal/cylinder assemblies, etc, but don't have a clue on how some of the products they sell actually work!

    Bob
     
  19. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,966

    V8 Bob
    ALLIANCE MEMBER

    Combination Valves-One more item that needs clarification.

    Combo valves became standard issue in the 70's through the 80's on most vehicles. They usually contain the prop valve (PV), metering (hold-off) valve, and pressure differential switch (diff. switch), although some only have the PV and diff. switch.

    The PV has been discussed above so we'll pass on that for now.

    Metering, or hold-off, was used through the 70's on and off until the 90's on disc/drum vehicles, mostly cars. Their purpose is to hold off the front disc brake pressure until the rear drum brakes reach about 100 PSI. The theory was that by allowing the drum shoes to overcome the strong return springs, shoe-to-drum contact would occur at the same time the front pads contacted the rotors, providing "even" braking. Sounds good, and worked Ok for some vehicles. The problem was that the rears could become too aggressive, promoting early rear slide during performance stopping distance testing, a big NO NO. The DOT "105" requirements allowed only one wheel to slide above 10 MPH. If you are backing off line pressure to keep a rear from sliding, the stopping distance will be longer, and possibly "blow" the stop by exceeding the requirement. So the metering valves disappeared for awhile, then came back in the 90's on a few vehicles because "pedal feel" and "benchmarking" became very important. Thankfully, ABS became the norm and did away with the need for metering and PV's! You really don't want one of these on your custom brake system, as it could cause early rear slide, and you want to avoid rear slide at all costs.

    The pressure differential switch was simply a shuttle switch that turned a failure light on in the dash when there was a system pressure difference of about 400 PSI or more. Of course, this applies to 1967 and up dual brake systems. This switch has NO effect on braking-it just lets you know if you have a serious pressure problem.

    It's not a bad idea to use a diff switch (newer masters with non-metal reservoirs have built-in float switches that monitor fluid level) but using a new or especially a used combo valve is not a good idea. Keep it simple by using an adjustable PV with a disc/drum system, along with necessary residual valves. It's easier to plumb in a PV and not need it vs having to add one later.

    Bob
     
    Last edited: Feb 20, 2010
  20. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
    Member

    V8 Bob, Yes we agree the graph shows the effectiveness difference between the various designs - why do you suppose there is a difference - the reason is servo effect.

    I'd like for you to describe the actual workings of what makes a servo brake a servo brake. I understand you description of what it LOOKS LIKE (2 shoes connected) but what I am getting at is what is actually happening to get us this "bonus" effect. My references clearly describe the servo effect being a result of the relationship of the shoe pivot point and the drum rotation direction causing the shoe to move towards the drum in one rotational direction, but AWAY from the drum in the opposite rotational direction. This can be applied to a shoe whether or not it is "connected" to another shoe or not.

    I am NOT arguing the generic terms and references of certain design being "servo" and others not - what I am suggesting is that they do ALL benefit (to some degree) from Servo Action. Thew fact soem are called "non servo" may be well and good, but I still suggest it is quite misleading. Now granted in some cases - it (servo effect) is admittedly very little and in others a great deal. See graph above.

    What I'd like you to describe is what mysterious feature do we gain by connecting the 2 shoes together - I see that as nothing more than making them "act" like a single shoe - just longer. Now teh ingenious part of anchoring them to a pin to allow them to work identically in both directions - yes that is what makes it so superior, but OTOH is a simple extension of a much simpler design and bound by similar features.
     
  21. B Blue
    Joined: Jul 30, 2009
    Posts: 281

    B Blue
    Member

    Bob, the popular adjustable Wildwood PV is described as adjusting pressures up to 60%, or some such number. Reading everything I could find on this valve, I got the distinct impression it operates at the set value throughout the pressure range, with no "knee" point. So I guess more to the point, do you know if it proportions by knee adjustment, pressure regulation after the knee pressure is reached, across the board or do the knee and percent regulation change in unision? Here is a case of the manufacturer knowing exactly what they have, but not making it clear to the buyer.

    Your description of the problems with the hold off valve sure helped to expalin what happened to me on the interstate about 25 years ago. Heard brakes squealing and when the traffic finished parting, I was looking at a mid sized chevy sliding backwards into my lane and the guy holding on for dear life. Not a good sight.

    Bill
     
  22. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
    Member

    B Blue, the old wilwood I bought 15 or 20 years ago had a graph that showed you the "Knee" point - it was THAT point which you were adjusting. Prior to that IIRC the relationship was 1:1.
     
  23. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,966

    V8 Bob
    ALLIANCE MEMBER

    Bill,
    As HemiRambler stated, the "knee" or "crack" point is what is moved up and down in pressure when the knob is turned or adjusted on all of the adjustable PV’s I’ve used or come across. The slope, rate or percentage of pressure to the rear is fixed, as I stated earlier. Common low end adjustment usually starts around 200 PSI, and stops at about 1000 PSI. Please go back and review my post on prop valves.

    HemiRambler,
    I will try to "splain" the functions of servo and non-servo drum brakes again, as I know I might not be clear at times…I can even confuse myself J
    Looking at the left front brake of a vehicle, let's start with a typical non-servo late model brake, with a double piston cylinder at the top, along with sliding anchors (allows the shoes to move up and down) at the top and bottom. When pressure is applied, the cylinder moves the forward facing or front shoe out against the drum, and the shoe tends to "wrap" into the drum because of the forward CC rotation, with the lower half of this shoe against it’s anchor, and providing the highest point of force, or retardation. The rear facing shoe is also pushed into the drum, but because of the CC drum rotation, tends to slide over the shoe with a lower percentage of braking or friction. With the vehicle going in reverse, the exact opposite will occur with the forces and movements on the shoes. Sliding upper/lower anchors, as opposed to fixed lower anchors, allow the shoes to move into the drum with higher forces, resulting in higher efficiency braking.

    Now let's switch the LF brake to a common Duo-Servo design, with a double piston cylinder and single anchor at the top, and the shoes connected at the bottom with links and an adjuster (we'll leave out any self adjusting mechanisms at this time for simplicity). As pressure moves the front or Primary shoe out off it's anchor, CC drum rotation "wraps" the shoe, forcing it down and to the rear, but instead of being stopped by an anchor, it "SERVES" the rear or Secondary shoe THROUGH the adjuster, forcing the shoe out against the drum, AND UP into it's anchor, creating a "wedge" effect with the rear cylinder piston forces. The upper half of the secondary shoe is where the highest retard forces are. That's why the primary shoes usually have a shorter/thinner lining than the longer and sometimes thicker secondary shoes, and why you have to use care not to mix them when assembling a Duo- Servo brake. Going in reverse, all the forces and movements are also reversed, resulting in braking forces close to those going forward.

    With a Uni-Servo brake, there is only one forward-facing piston, so the brake function and retardation are very close to the Duo-Servo, during forward braking only. When going in reverse, very little braking results, and that’s why this design was popular on surge-hitched trailers.
    Bob
     
  24. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
    Member

    Bob, Thanks for that - I think we are "tripping" over terminology. For the sake of arguement & simplicity let's look at a single brake shoe. When rotated in one direction it is more effective than the other. What causes this?? I say it is the servo effect.
     
  25. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,966

    V8 Bob
    ALLIANCE MEMBER

    HemiRambler,
    My definitions are based on info right out of my Bendix brake manuals, not generic or questionable reference material, and the many years of hands-on experience with brakes in general. A Servo drum brake is when one shoe serves the other, not the drum. Facts are facts.
    Why a shoe is more or less effective depending on drum rotation is a fact, and is the result of the brake design and physics, both of which I am limited in knowledge to try and explain. It definitely isn't magic!

    Bob
     
  26. B Blue
    Joined: Jul 30, 2009
    Posts: 281

    B Blue
    Member

    Bob, I think I understand your explanation pretty well. In fact, that as my understanding of how virtually all proportioning valves operated. Then I started to look for one to buy. This is typical of the descriptions I found:

    WILWOOD PROPORITONING VALVES FEATURES:
    Compact and lightweight forged billet aluminum construction has made Wilwood's proportioning valves the best available. Pressure adjustments range from 100-1000 PSI and provide for a maximum decrease of 57% in line pressure. This adjustment lets you fine tune the front to rear braking balance by proportionately decreasing the rear (or in some cases the front) brake line pressure. Can also be used to adjust individual front wheel braking in dirt track applications.

    My problem is I cannot reconcile this description, especially the part about using it to adjust individual front wheel braking in dirt cars, with the concept of the Knee or cracking point.

    Bill
     
  27. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,966

    V8 Bob
    ALLIANCE MEMBER

    Bill,
    The crack point or knee is simply the point when the the pressure stops increasing normally, and starts to proportion less to the brakes plumbed to it.
    Dirt track/roundy-round cars can use the PV to limit pressure to the left front since it's going to do the least amount of braking in the curves.
    Bob
     
  28. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
    Member

    Bob I am not questioning Bendix - simply the terminology. Much like the difference between an ingition "coil" and a magneto "transformer" basically they are the same animal - terminology was changed because they do operate in different conditions with different parameters and don't work very well when intermixed.

    Of course I realize there is no "magic" - I was simply trying to unravel the "mystery" attached to the term "servo effect" as related to drum brakes. You can question my reference material that is fine - I didn't write it. I used to work with a fella who worked for Bendix - maybe it's time to look him up again. He used to tell me all sorts of good stories about how Bendix would claim one thing - and the car manufacturers another - in the persuit of a sales edge.
     
  29. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,966

    V8 Bob
    ALLIANCE MEMBER

    HR,
    My statement about questionable reference material was about mine, NOT yours. Several of my Bendix manuals illustrate and describe the physical layout of many of it's competitor’s drum brakes, using the terms "servo" and "non-servo". Do you really think Bendix would put into print "claims" about competing brakes in a service manual if they were not true?

    You lost credibility when stating you may look up a fella, who worked at Bendix, who told you all sorts of stories... Just what the HELL does that have to do with this thread??? So now you are going to try and discredit the knowledge and experience I bring to the table, along with the very good and well known name of a company, from a “fella….with all sorts of good stories”. You obviously don't want to know what is fact, and I’ve obviously wasted a lot of my time. Hope you enjoy all the stories…
    Bob
     
  30. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,208

    HemiRambler
    Member

    Bob No - you misunderstood my intentions - I am not now nor before arguing the terminology - I am certain I have misused several terms and is why I attached lengthy explanations in an effort to make the points understood inspite of the terminology. And no - the mention of looking up an old buddy was certainly NOT to discredit what you have posted - rather it was an attempt to understand why the apparent disparity in the terms. Do I think Bendix would (or you) would "make up" anything to prove a point - no certainly not. I was hoping my Transformer / Coil terminology example would have served to make that part clear. To answer your question about what it may have to do with this thread is simply this: Imagine, if you will, you are bendix- you just design a Huge Improvement to drum brakes design - one that will truely set yourself apart from the competition - you would be foolish to not try to differentiate yourself from the "lesser" competition as much as possible. Making the world BLACK and WHITE makes alot of Marketing sense. Their brakes suck because ours are better - ours are "DUO SERVO" theirs are not!!! Think about it -wasn't the term Self Energizing used prior to Servo?? Bendix couldn't very well lay claim to that (marketing wise) because the term was already in effect - no they had to market the improvement and come up with a term to describe the improvement. THAT term was Duo Servo - which simply meant that their new design was self energizing in BOTH directions. - Is THAT what really happened??? I dunno - that's what I GUESS happened - that's what I wanted to call and ask my buddy about - if THAT makes me loose credibility with you then there's not alot I can do about that.

    I know where I work the marketing department is constantly trying to come up with new buzz words to differentiate us from our competition. I suspect Bendix and others did the same thing - I suspect in doing that - the waters have been muddied. The entire point of my posts was to try to get thru that mud (even though my poor use of terminology may have added to it) and see if we couldn't understand the function of brakes a little better. If my credibility is damaged for my effort - so be it.

    Oh BTW - I looked thru several old references I had last night - the trend in the older was ones WAS to use the term Servo - as one shoe serving another. My new references described them differently - as I tried to recall and post earlier. YMMV
     
    Last edited: Feb 23, 2010

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