Manual disc brakes

[ntsqd]

I've posted a lot over the years on this topic. I've an older thread on the IH8Mud forum covering the whole topic of brakes. I'm far from a real expert, but I did spend two years working in the industry at wilwood and several decades since working with and observing other's brake system results. There is at least one person here whom I know easily exceeds my applied knowledge on the topic.

In this thread: https://classicbroncos.com/forums/t...n-manual-disc-disc-brake-combinations.324668/ I mentioned that my mentor while at wilwood developed a simple formula to set-up a manual disc brake system. It should be noted that this formula does not consider the rear brakes. That said, I spent the next 10+ years watching how well the formula agreed with the system configurations that people arrived at, whether they used the formula or got there by some other means. The formula has proven to be a reliable predictor of how the system will feel to the operator.

The formula itself is simple. The combined hydraulic and mechanical leverage is ideal at 95:1. Seems simple enough? What's important are the boundaries, and this is where I've had to expand beyond what my mentor originally proposed. As most know, brake pedal feel is very subjective. Most can agree on a narrow range of pedal feel being OK, but what it ideal to one person isn't even close for another. These boundaries have been established by watching the topic for literally decades and aligning the results with to the boundaries of the formula. In the direction of a hard pedal that still stops the vehicle about 93:1 is as low as most are willing to go. In the other direction there seems to be a bit more tolerance. Can get out to 100:1 before most all will complain about the pedal feeling mushy.

How to figure out your total ratio requires knowing the pedal's mechanical ratio, the bore size of the caliper piston(s), and the bore size of the master cylinder. Total Ratio is the ratio of the hydraulic piston areas multiplied by the pedal ratio. For hydraulic ratio that means the areas of both front calipers are added together, then divided by the m/c piston area.

I'll start with the standard Ford sliding caliper. It has a 2-7/8" piston OD. I've no idea what the actual ratio of an EB's brake pedal is, but it is in my notes that it is 6:1 so I'm using that. With that pedal ratio a Ø1-1/32" m/c will have a Total Ratio of 93.3:1 That's a little low on the ratio. The pedal will be rock solid, but the truck isn't going to stop very easily.
If a Ø1.00" m/c is put in the system then the Total Ratio is 99.2:1 and it takes 157 lbf on the pedal to generate 1200 psi in the system. That's a bit high in ratio. More than a few will think that the pedal is too mushy.
Let's say that the T-Bird calipers are used to replace the std. Ford calipers. Those have a Ø3-3/32" piston in them. Still using the 6:1 pedal ratio combined with an Ø1-1/8" m/c the Total Ratio is 94:1. That's closer to the theoretical ideal of 95:1 and I suspect that most would be happy with that pedal feel.

Hopefully this shows how this is simple, yet difficult at the same time. The calcs are easy, but finding the right combo of parts is not. Setting up a spreadsheet to examine all of the combinations is really helpful.

 

[.94 OR]

So, your title "Manual disc brakes" leads me to believe that this would be sufficient without any power boosters.
Does the math change if a booster is added, vacuum or hydraulic (or electric)?

 

[Yeller]

So what your saying is when I was running a pedal ratio of 200.28:1 the brakes were very mushy, I can confirm scary mushy! It would lock up the tires and suck your eyeballs out of your head, right as the pedal slapped the floor and you could push it there with your pinky. The easiest to push, most powerful brakes I've ever experienced that made you need some new fruit of the looms every time you drove it . I did adjust and accepted it, but didn't let others drive it. It wasn't long I developed a power assist that works very well.

 

[ntsqd]

So, your title "Manual disc brakes" leads me to believe that this would be sufficient without any power boosters.
Does the math change if a booster is added, vacuum or hydraulic (or electric)?

The heavier the vehicle the more a booster makes sense. I think that EB's, weight-wise, are on the edge. Comes down to how much do you want to leg press every time you need to stop the vehicle? I suspect that for an EB driven only ever by one person that it's very doable. If SWMBO or others are going to drive it, then probably not. An interesting math exercise is to use the spreadsheet to also calculate what the pedal force needs to be to generate 1200 psi, which used to be considered to be the "panic stop pressure". This will really drive home the leg press aspect.

This formula has no application when a booster is involved. Obviously with a booster the harder that you push the harder that they work. However, from my observations over the years the input force vs. output force boosters are not a linear relationship, Which is how the family truckster model of some vehicle can have quite a bit different brake feel vs. the sporty coupe version. All of the parts, except for the booster, look and are the same. The Sporty car booster may look the same but has a different boost curve in it. There kind of be dragons there, I've never opened one up to see if I could figure out how that is done. In spite of the problems that I had with my own H-B conversion (that had nothing to do with the H-B part of the system, I'm firmly of the opinion that H-B should be the only booster considered if a booster is going to be used.

So what your saying is when I was running a pedal ratio of 200.28:1 the brakes were very mushy, I can confirm scary mushy

! It would lock up the tires and suck your eyeballs out of your head, right as the pedal slapped the floor and you could push it there with your pinky. The easiest to push, most powerful brakes I've ever experienced that made you need some new fruit of the looms every time you drove it . I did adjust and accept it, but didn't let others drive it. It wasn't long I developed a power assist that works very well.

Yeah, exactly like that. When the Total Ratio gets that high you can easily distort things like brake tubing and flex hoses. Even braided SS hoses will expand enough to feel them do it.

 

[ntsqd]

Rear brakes are usually subject to lower operating pressure. The standard proportioning valve, once past it's "knee point", limits the rear brake pressure to ~57% of the front brake pressure. Below the knee point the pressures are 1:1, pressure in = pressure out. Note that a p-valve is not like a typical regulator. It doesn't limit to a specific pressure. It limits to a specific percentage of the input pressure. I recall an R&D exercise where the Engineer involved was looking at trying to make this percentage adjustable and it proved to be not really feasible.

I've coined the term "Natural Balance" to describe how well the front to rear brake balance is able to operate without a proportioning valve. Perfect Natural Balance would be no need for a p-valve at all. It's a nice goal, but other factors are more important.
Can always put a lot rear brake capacity in a vehicle and then dial it down with a p-valve. The only reason that I can think of to do this is when static clamping performance is most important. That's a pretty small segment of users. More often braking at speed is much more important. Putting calipers on the rear axle with huge pistons only means that you've got to adjust the knee point on the p-valve to a very low pressure or the rear tires lock up way too easily. If you were fortunate enough to buy one of the LBS valves when those were available from either BCB or the sequence of mfg's who made it, then running large bore rear calipers may not be a big deal. But even then you're carrying around weight and brake capacity that you just can't use. The Explorer rear discs seem to be a pretty good match for stock rotor sized front brakes.

 

[.94 OR]

Maybe some day I will get a chance to drive an EB with H-B to see what I'm missing.
I've got the standard front disc upgrade but powered with my own 9" Mustang booster that I assembled 30+ years ago. Since I don't know any better, I think it works great!

 

[ntsqd]

It likely does, but that doesn't mean that it can't be better. You're welcome to drive my FSB with H-B in it. Maybe at a WH event in Lodi?
My EB with H-B is non-op and has been for a while. I'm not getting around to it, it may be someone else who puts it back on the road.

 

[ssray]

I can relate to the rules of a braking system you have laid out. I still have my 87 Buick Regal Turbo T though it’s waiting for some finish up work on the brake system at this point. It had the infamous power master braking system that turned out to be not so great. Electric pump, accumulator and pressure switch to cycle the pump, all to supply power to the booster on the master. It quit suddenly and I found the brakes almost unusable. It wasn’t like a vacuum booster where you still have manual brakes if something with the booster system fails. I could put 2 feet on the pedal and barely slow it down. No way I could even come close to locking anything up and definitely not safe to drive. At some point they became unobtaniam, probably because of that safety reason. Must of been well below that 93: 1 ratio in that system overly relying on boost pressure.

Using the ratio as a guide makes it easy to figure and if you want to go power at any point should keep you with usable brakes should you lose the boost portion. i look forward to using it at some point.

 

[ntsqd]

Boosted brakes typically do have much smaller Total Ratio. Adapting a booster to a dialed-in manual system won't yield the desired results. Have to start with the booster, of whatever type, already in the system.

For a turbo powered anything there's the old school easy button and the new one. Old is to go Hydraboost. Why they didn't do that originally is beyond me. The parts were there, they could have just used them. From what I've seen, there is a significant number of H-B equipped Pro Touring cars out there. They've got the same problem, little to no intake vacuum to operate a vacuum booster. Should be able to find a sportly tuned H-B unit.
New would be to adapt an electric booster off of an EV.

 

[DirtDonk]

I wonder what my ‘04 Buick Regal used for boost? Well brake boost that is… I know what it used for engine boost!
Funny though, as often as I worked under the hood, I thought for sure I remembered seeing a vacuum booster.
I was fairly familiar with the brake system I thought, but now that you mentioned Non-vacuum assist, I’m second guessing myself.

But I do know that the one time the engine died from a faulty ignition switch, it could’ve been a disaster. Luckily, I slowed myself down from freeway speed well enough with the first application of the brakes, that I was able to finally stop in a field off the side of the freeway.
I literally stood on that pedal for all I was worth. Thought I might break something, I was pushing so hard!

Once I got it running again and got home, I fixed the electrical fault and never had the problem again. Well, the fault came back, and was a known defect to GM W-body owners. But it never killed the engine again after that.
Thank goodness!

 

[ssray]

Thanks, Thom, That does make sense with the Powermaster as you needed to bench press 400 Lbs. Seems like I have towed dead vac boosted cars in the past and been able to stop them. Maybe there’s hydraulic restriction going on in that Powermaster and the hydraboost units. I’ll have to try it dead when I get it fixed. (See below) I’ve not seen any drawings or been in one either so I don’t know whats in there either. Maybe you saw it but there was a post up just this week about CPP coming out with an electric booster. I couldn’t find it for sale on their site yet. Found it online for $749. And a ton of other boosters as well!


CPP electric booster

Paul, The Regal has a hydraboost in it now. Need to do a bit of plumbing, get the gas tank in (new pump) and finish a conversion to R234. The 84/85 Turbo cars used hydraboost so that was I think the only option that bolted right in. Had to get a couple hoses as well to connect to the steering. There is an accumulator on it so I suppose you get help for 2 or 3 stops if it’s in good shape. The round ball accumulators on the Powermaster were known to go bad as well and the pressure switch and the pump. Real winner huh? Your experience sounds like mine!

Soott

 

[ntsqd]

What I have read & heard, but fortunately not experienced is that with H-B that has an accumulator is that you have boosted brakes with a dead engine or PS pump so long as you don't take your foot all of the way off the pedal.

I've been known to remove the vacuum check valve in a vacuum booster when we need to move a non-running vehicle. That reduces the pedal effort. Sometimes significantly.

 

[nvrstuk]

Correct TS, do NOT remove your foot or you might be in for a ride if the engine died. HUGE drawback for what we do off road with the HB.

Something not addressed yet (unless I missed it) is tire size, TS hit vehicle weight but tire diameter and weight of the tire & wheel combo makes a monstrous difference in pedal feel with the same pedal ratio. Huge isn't very scientific but let me tell ya if you hold a splitting maul by the head at arms length and move your arm in a circle it's easy.

Try holding on to the end of the handle of that same splitting maul and let the 8# head be appr 30" from your clenched fist... try holding it then try rotating your arm while fully stretched out. GOOD LUCK! HUGE lol

Same difference with tires when braking or accelerating with 140#'s of tire/wheel combo compared to stock at 45 or 50# (guessing).

 

[ntsqd]

I'm not seeing how having huge tires for flywheels can affect how the pedal feels, but it sure would affect how effective the brakes are - which would affect how hard you have to stand on the pedal. Which would shine a light on whether or not you need bigger brakes. Even just 35's on Snowball's stock brakes really made them marginal for KA commuter traffic. Having the issue with the rear's gaining air in ~250 miles sure illustrated to me just how much of a flywheel just those tires are. With air in the rear, the fronts were having to slow down all 4 flywheels er.... tires and the weight of the vehicle. Bleeding the rears at least got them to slow down the rear tires if nothing else, and the difference is startling!

 

[phred]

I’ve been following your math and it makes sense based on the H-B systems I have built from parts. Several on EB’s with 33” tires. Pedal effort minimal and a very firm consistent pedal with little mush/fade in a panic stop. Almost too good as the sweat on your forehead will hit the windscreen. Now the same set up in a full size 78 that weighs 6500lbs, 4 wheel disc, 35’s and heavy bead locks will stop but as the need for more stopping power in a panic situation increases the pedal will go almost to the floor.
I too am searching for a combination of master cylinder and p-valve that will eliminate the as Steve put it “fruit of the looms” moment.

 

[ntsqd]

I'll venture out where most fear to tread and say that there isn't such a combo. The reason is because the hydraulics are scalable - they have to be, meaning that once you have a m/c bore size and caliper piston size that give a firm pedal with good modulation you can increase the caliper piston area by 20% if you also increase the m/c piston area by 20%. If you don't increase both by the same percentage then you fall out of that Total Ratio that gave you the desirable pedal feel. Can scale the areas up and down together, but their hyd. ratio can't change or you loose the pedal feel. The same thing happens if you mess with the pedal ratio, it messes up the Total Ratio.

To get more braking torque there are two options. Increase the hydraulic pressure by pushing harder. Welcome to Hydro-Boost territory.
Or you can increase the OD of the rotors. This is aftermarket Big Brake kit or 8 lug conversion territory.

Both is what I did on Snowball (my FSB). I did the GM 3/4t conversion on the TTB, replaced the 8.8 with a D60 sporting Expedition rear disc brakes, and added a Hydro-Booster. Changing the front rotors took it from a rotor with a 11.7" OD to a rotor with a 12.5" OD. I kept the stock m/c, but in going to the GM calipers it gained a little in caliper piston area. I'll try to show how this works w/o getting too heavy on the math.
First assumption is that the pad Centroids (the center of the pad's friction area) between the Ford pad and the GM pad are the same. Location of the centroid is .75" in from the rotor OD
Second assumption is the both pads have a friction coefficient of 0.5
At 1200 psi the pads have a sliding friction force of 600 psi.
Apply that at the pad centroid. A rotor of an 11.7" OD has a radius of 5.85", minus the centroid distance that makes the torque arm 5.1" long.
600 psi * 5.1" equals 3060 lbs-in torque.
Now apply that same force on the larger rotor. 600 psi * 5.5" equals 3300 lbs-in of brake torque.

 

[nvrstuk]

Phred, I think I'm dialed in for what you mentioned in your post.

Tim got to try the combo first since his rig was actually running lol but with the combo of parts we are running now it's the best setup I've run since the installing the HB designed Chebby mc, large JB6 discs & and adjustable bias valve.

Great feel in all situations. Also perfect for idling in low gear and trying to hold back stooopid low gear ratios w/o too much leg effort.

I've tried 6 different m/c's, 3 different sized calipers up front and 4 different in the rear.

This works on 40"s with 134# tire/wheel combo's (flywheels) on as TS put it, 8 lug territory. Trying to slow them down takes a LOT more pedal pressure however- not nearly as much as a doubler in low, your t-case in low, your trans in low. Try stopping that at idle w/o HB. lol

I can attest that in an auto with a tight converter you can't hold it back. I was kinda shocked and set up an HB right away. Popping your trans into neutral to stop halfway down a nasty ledge is not fun!

 

[Yeller]

Try front wheel drive on that ledge if you can’t hold it, you still can’t stop but the rear not pushing is a game changer. Steers easier with the lockers on too.

 

[spap]

On my first bronco a 76, the po removed the booster I think, but brakes were pretty good. Stock master cylinder.
I Bolted on junk yard rear disks brake s from a mid seventy’s Lincoln continental , 9 inch with disk brakes, and everything bolted on even the disks. We’re 5x5.5 stud spacing. Never changed the the prop valve. And it was the best stopping bronco I every had.
33 inch tires, e fronts would lock before the rears.. I don’t remember pedal effort, but don’t remember it being really hard to push the brake pedal, but I was younger then too.

 

[DirtDonk]

The 76 and 77 Broncos were available with manual disc or power assist disc brakes.
Depending on what was ordered. With manual brakes, there was a spacer between the master cylinder and the firewall. 1 inch thick if I remember.
So yours may or may not have been swapped out at some point. Might’ve come from the factory that way.