Brake LIne Pressure Question

[sunman]

It was 1998 and I was copying the 1986 Mustang SVO/Lincoln bore sizes for the front calipers and 1 1/8” master cylinder. I had the rear brakes already.
At Fords 73mm front 54mm rear the D52s with a 2 13/16” bore size is real close.
I believe these large bores require a power booster tho, all my Camaros and Novas had power.

[6TNCRZY]

Chevrolet brakes from a 4000 lb Malibu should have no problem locking up the brakes on a 2600 lb Cobra. I felt the brakes on my ERA were horrible and took a lot of pressure with the pads supplied by ERA. I replaced the pads with Willwood BP40 pads and the difference was amazing, I now can lock up my sticky Avons at almost any speed. Highly recommend BP40 pads, high coefficient of friction consistent from cold to hot.

[sunman]

A Malibu has a 1” master cylinder with 11” booster.
The bigger 2 15/16” piston takes more fluid to move = more pedal travel.
I like the Willwood 12.19” dual piston brakes, they fit most 15” wheels.

[cycleguy55]

Originally Posted by Argess

I don't think you should have used the fact it's 4 pistons. Calculations should be one side of the caliper only, so base it on two.

Anyway, it's good to know yours work fine.

Please explain why calculations should be based only on one side of the caliper.

[Argess]

Well, using all 4 for piston area is OK, but you can only use the piston area of two of them to calculate the force on the pads.

Let's say you weigh 200 lbs. Put a piece of plywood on the floor and stand on it. You are putting a force of 200 lbs on the plywood.

Now put the plywood against a wall and push on it with 200 lbs of force. The plywood is being compressed the same... with 200 lbs of force.

Next stand the plywood up on edge in the middle of the floor. You press on one side with a force of 200 lbs. I press on the other side with 200 lbs. The plywood is compressed the same as if it were against the wall, or the floor.... with only 200 lbs of force.

Hope this makes sense.

Now 4 pistons are better than two as they take up small misalignments between the caliper and rotor, so it's a worthwhile change.

[Argess]

Well.... success!!! I managed 8.6 Gs this monrning. I think I might stlll have a bit left but cut the testing short as not only was I happy with the 0.86, but it was only 4 degC (39degF) outside.

Now previously all I ever did with the silicone brake grease was lube the guide pins which is typically almost all you do with most disc caliper systems. The other spot for grease in the little end ears on the pads. But these D52 pads don't have the tabs. The one against the piston does move of course, so I greased the edges of the backplates where the rubbed on the caliper as well as re-lubing the little spacers where the guide pins go through and also re-lubed the guide pins. Didn't lube anything on theoutboard pad as it doen't move in the caliper, but did add a little lube around the guide pin holes in the pad and re-did the hole in the caliper where the guide pin end goes through.

However, as can be seen below is a picture (not my car, but a pic I found on the net) there are abutment pints on the knuckles. The caliper slides on these in order to move the inboard pad against the rotor. Here's where I added lube that never had any before:



Now these D52 calipers have been used for well over 20 years in many GM vehicles, but nowhere on the net could I find anything about greasing the abutments. It is interesting to note that a picture I found of the Wilwood equivalent doesn't mdention it either but clearly shows where they didn't paint the caliper in order for caliper to slide on the abutments.



Of course those abutment areas are a great place for the grease to pick up brake dust dirt so I'm not sure if they were meant to be greased. However I suspect that those vehicles that used them also had a booster so extra pressure to overcome the friction was available.

My next step is to try the Yellowstuff D52 pads I bought. I already have them in the rear. I'm going to wait until Spring as they need to be run a few hundred miles to wear away the coating EBC puts on them that's supped to clean the disc of old pad material.

I amy aslo try the Wilwood calipers, but they don't recommend them with DOT 5. Of coursse Tilton doesn't recomend their MC with it either, but I found no evidence of seal problems with the old MC I took apart. 25 years of being immersed in DOT5.

[cycleguy55]

Originally Posted by Argess

Well, using all 4 for piston area is OK, but you can only use the piston area of two of them to calculate the force on the pads.

Let's say you weigh 200 lbs. Put a piece of plywood on the floor and stand on it. You are putting a force of 200 lbs on the plywood.

Now put the plywood against a wall and push on it with 200 lbs of force. The plywood is being compressed the same... with 200 lbs of force.

Next stand the plywood up on edge in the middle of the floor. You press on one side with a force of 200 lbs. I press on the other side with 200 lbs. The plywood is compressed the same as if it were against the wall, or the floor.... with only 200 lbs of force.

Hope this makes sense.

Nope. Overly simplistic explanation that doesn't account properly account for opposing forces.

Does anyone have any solid engineering rationale as to the use of only one side of a caliper in calculating piston area?

[Argess]

Here's what accounts for the opposing forces: The caliper with piston(s) on only one side SLIDES. That's what puts pressure on the pad that doesn't have a piston behind it.

I work out brake torque finding the drag from one pad, then double it to find the total brake torque. If you use piston area of both sides, you don't double it.

There is no difference in brake torque between a caliper with 2 pistons per side and one with only 2 pistons on one side.

[eschaider]

Deleted

[fintubi]

One more data point:

Hawk HPS pads have come up a couple times upthread.

Yesterday I swapped in HPS front pads on 3014, which has the GM front brakes and Jag IRS. After bedding in the new pads, I found a noticeable improvement in bite and an easier time probing the lockup threshold as compared with the Centric pads that came with the kit. I may need to add some more front bias now.

Bill

[strictlypersonl]

Originally Posted by fintubi

One more data point:

Hawk HPS pads have come up a couple times upthread.

Yesterday I swapped in HPS front pads on 3014, which has the GM front brakes and Jag IRS. After bedding in the new pads, I found a noticeable improvement in bite and an easier time probing the lockup threshold as compared with the Centric pads that came with the kit. I may need to add some more front bias now.
Bill

If you're getting too much bite (line pressure) on the front, shift the balance bar shims so that the bar free length is increased on the side toward the front master. On the next release of the manuals, I'll illustrate that more clearly.

See https://erareplicas.com/427man/brakes/index.htm

[Bobcat]

Bob , quick question . On your attachment on the bias bar setting , you say to set the front rod approx. 3/8" longer than the rear .
In the ERA manual I have for ERA 757 , it says to set the front brake cyl rod 3/4" thread to lock nut backside and rear brake rod to 1/2" thread to back of lock nut , which is 1/4" longer for the front brake link .
If this is a change , would it apply to my car ?

Thanks

[eschaider]

Originally Posted by cycleguy55

Nope. Overly simplistic explanation that doesn't account properly account for opposing forces.

Does anyone have any solid engineering rationale as to the use of only one side of a caliper in calculating piston area?

Brian,

Let me take a swipe at this…

We all recognize that when we exert a force on an object, if it remains static, i.e., doesn’t move (accelerate) in the direction of the force being applied, then there is an equal and opposite force being exerted on that ‘something’ which we are pushing/pulling on. That equal and opposite force not only equals the force we are exerting but, because of its direction, cancels the force we are exerting.

Let's say the force we exert is 5 pounds, and the counteracting force is also 5 pounds, but the direction of the force is precisely opposite ours — the result is no movement. If you exert a force of 5.1 pounds on the same object that is pushing back at 5 pounds, we will begin to move the object in the direction of the force we are applying.

In the case of a brake caliper, that would cause the pistons with the lower clamping force to be pushed into their caliper side. So why is the clamping force not 2X the observed caliper clamping force of just one side?

I will use another example we are all familiar with to answer that. Earth’s gravity exerts an attractive force on all matter. That gravitational attraction/force creates the phenomena we refer to as weight. We can measure this metric by stepping on a bathroom scale. When we do, we can see what our ‘weight’ is. Think of it as a measure of the attractive force exerted by the planet’s mass on our body.

To avoid being plastered flat against the planet's surface, our body’s muscles push against that attractive force with an equal and opposite force allowing us to stand upright. The bathroom scale measures that force in pounds. To remain motionless and upright, our feet and legs need to push back on the planet’s surface with an equal but opposing (opposite direction) force.

So, the pregnant question becomes when we stand on the bathroom scale, which says 200 pounds, do we weigh 200 pounds or 100 pounds? Is that apparent second 100 pounds from gravity acting on the mass of our bodies? We all know the answer, we weigh 200 pounds, and no less an authority than our wives has confirmed it! Not just the 200 pounds — but the fat part also.

Kidding aside, the attractive force of gravity and the muscular force applied by our legs to remain upright are exactly equal and cancel each other out. Still, the scale only shows 200 pounds, not 400, and we definitely do not weigh 100 pounds. The same phenomenon occurs in the brake caliper.

As someone has already observed, the benefit of multiple pistons is to enhance the brake pad’s ability to conform to irregularities in the surface of the rotor, optimizing surface contact with the rotor for better braking performance. By using pistons on both sides of the rotor, we extend this braking performance enhancement to both the front and back side of the rotor. Significantly we do not increase the clamp load on the disc any more than gravity increases our weight when we step on the bathroom scale.