Early VW Golf Mk1s had a similar arrangement, with the valve operated by the right-hand rear trailing arm - an arrangement doubtless designed with LHD in mind. One-up on RHD cars with the iron-block EA827 engine this could have some interesting consequences, due to the right-hand weight bias. On my first two Golfs it was possible to get the tail out on tight right-hand corners by dialling in some left lock to put more weight on the right-hand side of the car while braking hard, and then turning sharply right. This would induce a delicate clockwise pirouette until you ease up on the brakes. My current Mk1 won't do that.
Let's say you're running Gm metric calipers. ( you should say what they are thought) The standard size piston is 2-1/2 . There are calipers available in 2-3/4 up size for more front brake clamp pressure and also available in 2-1/4 and 2" for less. Rear wheel cylinders come in different sizes too. If you want to know what to change or how much to change you need to know where you are beginning from. A weight of each axle and the brake clamp or line pressure are probably the easiest to get. A trip to the scales and a guage or load cell. Then you can start guessing. Lots of late model stuff as a rear valve that adjusted by ride height. The lower the ride from set point height must mean that the load on the rear is heavier and thus the valve changes. Also if the front is diving from weight transfer that means the ass end is lifting and thus changes the valve's position.
Let me clarify this: You are using a mid 70's Ford M/C? or this brake combination is in a Mid 70's Ford Pickup? Without going through a lot of math, I would check out the plumbing that the GM Disc/Drum combo came from GM have already done the Math. A metering valve doesn't alter the front brake bias, and putting a proportioning valve in the front circuit is as insane as a block of wood under the pedal. If you have a big-n-little combo you'll need to up rear slave sizes to increase rear brake torque to overcome the larger rolling diameter. The best DIY way to test the front/rear bias combo is on a wet road where the wheels lock up without maximum pedal effort [ before the proportioning valve actually limits pressure] Get the combo correct first. With brakes you need to start at the tyre footprint and work your way backwards to the pedal
I don't think those conditions you cite would have much affect. Front end dive would not likely change rear end ride height very much and whatever movement there may be would be in the "unloaded" direction. Wet pavement affects the front end too and bumps from rough terrain are momentary displacements of the suspension. A load in the bed is constant. At least that's how it seems to me. Ray
Kerry, I'm using a non-power mid 70's MC from a Ford PU under the floor. I wish I knew the size of the cyl. I want to say 7/8", and a typical Speedway GM metric front disc kit and Speedway 11" Ford rear drum set up. I'm sorry, I don't know the piston sizes. Pedal pressure is what I would call normal. It's not like power, but I don't like power brakes because of no road feel. When it is wet out or sand on the pavement, if I'm too aggressive it will slide the front wheels easy. I never had the rear brakes lock up, but I know they are working, because I have adjusted them on occasion. The car runs and drives fine, I just thought if I had a little more rear brake in a panic situation it would set better with me. Also, my tire sizes are 185/75 R14 and 235/75 R15. The car (avatar) only weighs 2700 lbs. I don't know how much the axle weight is, but I'm sure the front is heavier than the rear. I don't have any need to reinvent the wheel here, I just thought I could improve my situation a little. Next time I have to do a brake job (which may be soon), I'll look into changing the rear cyl. size. Thanks for your help. That takes a lot of the confusion out for me. Thanks 1oldtimer for bringing this subject up. Sorry, I didn't mean to steal your thread.
In the late 60s some European makes had a height sensing valve too, even with teeny drum brakes in the rear. http://www.renault16.com/techniek7_en.htm
My understanding of the metering valve is it simply 'delays' the application of the front brakes momentarily to let the drums 'catch up' or to allow the chassis to 'set'. Another method for doing this is a 'split' master cylinder piston. Jaguar used this for a number of years (late 60s until '88). The master has a two-piece piston with a light spring between the two. The 'rear' piston moves first, then contacts the 'front' piston and then 'normal' brake pressure is applied to both circuits. One side effect of this is Jag couldn't fit a brake warning light using a pressure differential switch (the light would 'blink' each time the brakes were applied), somehow they got around this federal mandate by substituting a low brake fluid light. Adjustable proportioning valves are really race car pieces. If you have a badly unbalanced system with too much rear bias, they can correct that, but that's hard to achieve with the parts commonly used by us. Serious race cars are set up with a bit of rear bias, then the valve is used for fine tuning. I installed a couple of them (being young and ignorant), but lacking rear bias adjusting them only reduced rear braking, actually making the brakes worse, particularly on wet pavement.
Agreed. The only time you would want the front brakes to come on before the rear is if you're driving backwards. Try riding a motorcycle or bicycle and hitting the front brakes first. Just make sure you wear a full face helmet and have medical and dental insurance.
I really enjoy these topics. Like many non-professional mechanics/hot rodders, I spent my formative years making cars go faster with little regard to handling and braking. Great to be able to still learn new stuff. Carry on...
My dad, a professional mechanic and hot rodder drilled a few things into my head,,,,, Make it stop, make it go, make it pretty and always in that order. That stock is always the cheapest and easiest bet. if you just can't leave it stock you'd better get real good at making parts and being able to figure this stuff out. If it doesn't do what it's supposed to, it's not worth keeping Around. You'll never get your money back out of any modification unless you placed a bet and won the race.
What needs to be clarified here is "dive" is a suspension geometry, stiffness and load issue. Delaying the front brakes does not prevent dive or reduce front brake bias. There is no sane reason why you want the rear brakes to operate before the front brakes [try locking up the rear on a bicycle in a corner and you'll understand the painful way] The greater the decelleration [G Force] causes more front weight transfer. this requires more front brake bias. Now this is where novices get it wrong................................ Decelleration is limited by traction. Less traction = less decelleration = Less front brake bias This traction issue can be caused by tyres [as in big and littles, which also alter brake torque] But most of the time traction is limited by the environment which is the dilemma of the manufacturer's compromise of more rear brake bias and a proportioning valve. M/C size does not alter front/rear brake bias, only pedal firmness [poor mans ABS]. Front and rear brake bias is altered by Fluid Mechanics[piston diameters] and Brake Torque [disc/drum diameters etc] For any novice out there building a new project , I would suggest walking around the local "U-pull" with a tape measure and try and find a vehicle of similar weight and wheelbase. Then literally copy their brake combo. A Honda Civic can out brake most T buckets
I am sorry, but I have to say you have it exactly backwards. As a youth, I rode bicycles countless miles and as an adult, rode motorcycles for several decades. Aggressive rear wheel braking, without comparable front braking too, can put you down in a heartbeat. Both two and four wheel vehicles lose directional stability with rear wheel lockup. But when a four wheeler goes sideways, you have a better chance of recovery. A cycle will either lay down, if it's already leaning, or 'high side' the rider(s) launching them off onto whatever lays before them. Ray
I agree with you here. But...LoL...a little story that surprised me. My T...BBC and approx 1900-2000#...came to me with 4 wheel discs that worked poorly, needing a good pump to even get a pedal, every time you tried to stop. I hated the WS6 Trans Am rear brake setup, so I pulled it all off and used drums from an S10 pickup. Gave me an easy and reliable parking brake as well. I also used 2# and 10# RPV's (despite the famous Internet "brake guru's" warnings)...and got an excellent pedal with no brake drag. Anyway...with the big discs up front and the mid-size Chevelle calipers, the rear drums couldn't do the job and the front brakes continuously locked up on loose surfaces or in the wet. I researched the S10 wheel cylinders and I was able to source larger cylinders...which definately helped, but not quite enough...and I couldn't find any bigger. Going over some brake posts on a T bucket site, I discovered that "Spirit Industries", a T bucket builder, uses a Proportioning Valve in the front line! Seemed iffy at best. Found a brake video from Spirit where they show it was definately being done, so I figured what the hell...only got to cut one line to fit the thing and if it doesn't work I can change it out in an easy hour! Adjusted it all open and tried it. Brakes still locking. Turned it on 1 turn. Stops great! The front no longer overpowers the rear and things seem more balanced. I REALLY don't like the idea of limiting brake performance to coax the rear into doing more work...but on my car I have to admit it did make a bad situation disappear, and the T stops on a dime consistantly even on loose surfaces. Before the PV it could be a white knuckle ride if sand/grass etc was located where you were trying to stop!
I don't see any reason that shouldn't work. Yes, it goes against 'common knowledge', but that's kind of like 'common sense'; rarer than you think.... LOL
One method will cause brake induce understeer and the other method will cause brake induced oversteer. Manufacturers set cars up to understeer, not oversteer. Because the average "Joe Public, couldn't drive a nail into sand" [even though 99% of us are above Average drivers]
The reason it shouldn't be done is when a kid runs in front of your car, and you slam the brakes on. The proportioning valve will not allow any more brake torque to the front wheels . The 4 wheel equivalent of slamming the back brakes and laying it down on the pavement
So, explain how having the front brakes lock up and losing traction (and incidentally, steering also) is better?
The front brakes shouldn't lock up with increased weight transfer to the front [and the correct bias ] It is under these extreme conditions that have made manufacturers add a proportioning valve to the rear circuit [to prevent rear lock up] Adding patch up devices that limits the function of the front brakes should best be left with "cowshed engineers" [and have no place in this PC world where people have the tendancy to sue each other] If you cant find larger rear cylinders, then start looking for smaller piston callipers [it is quite simple to grasp,and probably cheaper than just bolting shit to the plumbing] Note: if the same car with "too much" front brake bias was fitted with a set of Sticky Road Race tyres, it would probably "stand the car on the grill"
"The front brakes shouldn't lock up with increased weight transfer to the front [and the correct bias ]" True...however there is no weight transfer at all on grass or sand/stone bits on pavement at 2 kph...and thats where my problem reared its head. "If you cant find larger rear cylinders, then start looking for smaller piston callipers [it is quite simple to grasp,and probably cheaper than just bolting shit to the plumbing]" I fully realize that my issue is mismatched front and rear brakes, but "fixing" the problem will mean either a full brake swap to smaller calipers and rotors to match, or total replacement of the rear system inc larger backing plates and drums....and potentially a master cylinder change as well. Thats hardly cheaper than a $50.00 PV and an hours work! LoL As for sueing each other...Well, I imagine SPIRIT has considered this already and came to the conclusion that stopping right away without the skinny front tires sliding out from under you is less likely to cause litigation than smacking into the car in front of you or sliding off the road because you braked a little on a gravelly turn. I'd generally argue WITH you on the thought of getting it right by cylinder sizes and still recommend it...but having tried this...? I gotta admit it works far better than I ever expected. I think its the total weight of the vehicle being so light that makes it work...along with the skinny tires being part of the equation. 50/50 weight distribution as well.
Its really important to study any problem and find then fix the cause. The obvious and apparent "symptom" isn't necessarily the thing that should be fixed, yes it needs to go away. How and why it's there is where the answer lays. An obvious and easy to understand example goes something like this. You come to your car and the tire is flat (that's the symptom) So You put air in it today, you put air again next week, next week, 3 days later and so on. The tire not having air isn't the problem, it's a symptom the problem of a leak. That example is sooo simple but the exact same process gets applied to many problems. http://www.speedwaymotors.com/Wilwo...Iron-Single-Piston-Floater-Caliper,24242.html These are Available in 2-3/4 or 2" bore. I've used them in 2" and the light weight car stops perfectly. Here's just one really great example of some symptom chasing type problem solving attempts. Looking at the car from the surface it had obvious deficits, taking it apart really gets the head scratching going from front to back this builder understood correcting symptoms but had no clue on what the problem really was. There's some sorta weird ass patch work from the same mind everywhere on this car.
This is why you need the correct front/rear bias [for gentle braking in less than ideal conditions] AND a proportioning valve in the rear circuit [to prevent rear lockup in emergency braking Until you really need to kick the brake pedal through the floor at a corner when some kid chases a ball across the road on a street corner. Get a competent "road racer" to test your brakes at the limit [as in "late trail braking" at corner entry] and it will be "backing itself in" like a sprintcar. If you cant balance it with hydraulics then you should consider 2 circuits with a balance bar. As for suing ,this what parents do when their little darlings are injured or killed and it is discovered it could've been avoided if there was correct engineering. One little "road racer" trick that is well kept secret On RWD cars with directional tyres, mount them backwards on the front[as in swapping L to R] [any body that has locked up tractor brakes going downhill should understand this] The loads under braking is opposite to loads under accelleration
Well, the confusion rages again!!! Hackerbilt, you described perfectly what is going on in my car!!! Granted, it's not like a T bucket, but it is noticeable. I still chalk it up to "bigs and littles" and this will make the adjustment needed! My initial thought was to switch the adjustable proportioning valve to the front line to stop the front wheels from skidding on loose pavement or panic stops, but all diagrams and opinions spoke otherwise. However, your experience proved my thinking was correct. BTW, I already have the 2" (metric?) calipers from Speedway. Plus, an after market adjustable proportioning valve that doesn't have a shuttle in it that does what Kerry is describing with factory proportioning valves. It's just like squeezing the brake line with a vise grip! With these old cars, we have a hodge-podge of mismatched parts that shouldn't even work, but the miracle of it is that they do a pretty god job in spite of it all! We get by with a lot of junk! Looks like I'll be going back to what I thought in the first place, in spite of all the hub-bub. I just want the car to balance out without reinventing the wheel. Thanks.
Keep something in mind. 31Vicky and Kerrynzl are correct! You first need to try to get the system as close as possible, preferrably fixed...by using calipers and wheel cylinders that work together. I'm completely convinced that running the PV on the front line is a potential bandaid for a VERY light car with extremely effective front discs/ thin tires ONLY. I'm not promoting it...just passing on my experience with something I actually doubted would work at all. I had a spare valve on the shelf so I figured why not? If my car were 3000# with sensible front tires instead of VW size pizza cutters I wouldn't have even bothered. Just that a company building the same basic car as mine was doing it made me give it a shot. For me it worked...but your experience might be vastly different! I do not believe this is a "fix all" for everyone.
It seems that a proportioning valve is not fully understood..It will always reach full pressure with enough pedal pressure...Period..
Ok, let's take this in pieces.... The key phrase here is 'correct bias'. This includes not only the brake bias, but also the effects of front/rear tire sizes. And a PV won't eliminate rear lock up under all conditions, it simply prevents the rears from locking before the fronts if adjusted right. Ok, installing smaller calipers will improve the bias, no question. But isn't what you're doing is reducing clamping pressure at the brake pads/rotors? Isn't that in effect what a PV does? Plus the possibility that small enough calipers may have less swept area, reducing braking effectiveness and heat dissipation. True, but that doesn't solve the problem if the owner is running 'big 'n littles'. If running bias-ply tires with this problem, switching the radials would no doubt help too. The key is making the combination work as well as it can. A PV is usually installed to adjust rear bias if there's a possibility of rear lock-up before the front (causing the car to swap ends) but still will not prevent lock-up under full-panic-stop conditions. If it does, you're leaving some braking performance on the table. Preventing any lock-up is why ABS brakes exist. Using a PV on the front and adjusting for lock-up just before the rears lock will (or at least 'moving' the lock point closer to the rears) allow you to retain control longer and I fail to see how it 'limits' braking. Once the brakes lock at either end, whatever's happening hydraulically no longer matters, now it's up to the tires...
Well How are you going to decide what to change around? You'll need to know all that you can about what's there now. Put antilock brakes on it Put bigger tires on it? They have kits that reduce the front calipers in increments all the way down to 1". What bore rear wheel cylinders you running now? No matter what though, you'll not get more braking than the available traction. Another thing to remember is that different bore sizes require different volumes of fluid to fill. You can jump up a bore size expecting greater pressure but wind up with less because the volume is off. The same holds true in reverse because you have pressure to evaluate.
Simply not true, at least by design. The only time I've seen rear pressure return to full master output is occasionally during heavy ABS cycling, or a very rare prop valve failure on a standard system. Prop valves initially allow full master pressure to the rears until the split point or knee is reached, then proportion any increased pressure. Common values are 27%, 43%, and 59%. The split point or knee is fixed on factory valves, but variable on adjustable prop valves, with a typical range of about 100 psi minimum to around 1000 psi. max.
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