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oprn
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PostPosted: Wed Feb 28, 2024 3:19 pm    Post subject: Re: Brakes Reply with quote

EVfun wrote:
You would want to disable the check valve to run disc brakes, like Teuton did. If your rear brakes are still drum you would want to add an external residual pressure valve on that 1 line.

I took one of the residual pressure valves off my junk '67 master cylinder and am trying to clean it up now. I'm want to figure out how it works. I don't think it would fit in a modern replacement master cylinder.

Well Sir if my MC has any RPVs in it they are not working. I can stomp the brake peddle and immediately afterward all 4 wheels spin like tops. No residual drag what-so-ever. I really have no idea if this is a stock MC, what year it is or what car it was intended to be on... for certain. The last owner coated it with black undercoat when he did the pan.
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PostPosted: Sat Mar 02, 2024 4:28 am    Post subject: Re: Brakes Reply with quote

I am looking around for master cylinder repair kits. Not much available. Nothing so far in fact.
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PostPosted: Sat Mar 02, 2024 12:27 pm    Post subject: Re: Brakes Reply with quote

My master cylinder sizing quandary has been keeping me up at night. A few years ago when I bought the .88 dia master cylinder, the guy at Wilwood thought it would be appropriate for my system. Then, as I mentioned, my recent calculations seemed to produce excessive clamping forces, and then the guy I recently spoke to at Wilwood seems to think the .88 MC wonít work, that Iíll need to go two steps up to the 1.0 inch MC (because the .88 may not have the area to produce enough volume for my four calipers). Incidentally, the 1.0 inch bore will reduce the clamp force quite a bit.

Last week I put together a spreadsheet with formulas in the tables to determine where I stand and to plug in different options (parameters) to evaluate what I might do to reduce the clamping forces to levels that are considered necessary and normal. Considering that my CNC pedal is not of a normal length, one option is to shorten it to reduce its ratio (my initial setup all came from the now-defunct CNC Brakes company in San Diego). The other option is to increase the MC bore to redue those clamping forces, albeit with a potentially stiffer pedal. However, as Wilwood has stated, it may in fact be necessary just to get the system to work, and that's the part thatís really nagging at me at the moment.

So what I decided I will do is mock up my braking system on the bench and just see for myself whether the .88 (7/8Ē) MC is able to build pressure and what the pedal stroke is like. After all, the last thing I want to do is install all this crap under the Bus, fill and distribute the whole system with fluid, crawl around on my hands and knees bleeding it while somebody pumps, only to discover that the damned MC needs to come out for a bigger one. Screw that. Letís find out now while everythingís in armís reach on the bench. Iíve looked around the web and havenít found anybody whoís done this, other than labs with scientific machines used to analyze brake characteristics and collect data. This should be interesting Ė if not exciting Ė to say the least. I will post up some pics of the mockup as soon as itís done, Iím waiting for the tubing, nuts, and tools to show up.

Meanwhile, hereís the summary of the spreadsheet, I used colored cells to highlight the association of values and how theyíre applied downstream. I know itís a bit of an eye chart, but one can deduce how I arrived at certain values by following the color codes (e.g. first figure pedal ratio, then MC area, then determine resulting line pressure with those values; then calculate the caliper (piston) area, and finally, multiply those values by the MC line pressure to arrive at the clamping force). I didnít include the results for the 15/16Ē MC, and this is just a ground level assessment and doesnít factor in anything else such as vehicle weight, rotor diameter, speed, torque, or any other factors that are used in brake system analysis to properly design a brake system such as the OEMs do.

The only other thing Iíd be interested in trying is to hook up pressure gauge to see if the force on the pedal actually produces numbers that are in family with the calculations. Of course, a load cell on the pedal pad would be nice to know just how much real pedal force is being applied, but I canít bring any load cells or data acquistion equipment home from work to play in my science fair.

That said, I hope I didnít screw up any of these numbers or formulas, or Iíll have to ask Everett to delete the image and redo it with corrections! Smile

Edit: I've already discovered an error, so I'm off editing my image.

Okay, here is the corrected spreadsheet.

Image may have been reduced in size. Click image to view fullscreen.

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Last edited by Busstom on Sat Mar 02, 2024 12:44 pm; edited 1 time in total
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oprn
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PostPosted: Sat Mar 02, 2024 12:35 pm    Post subject: Re: Brakes Reply with quote

Please explain to me the basis behind your fear of too much clamping force. I am at a loss to understand why that would be a bad thing! My fear is the opposite, that I would build something that my wife and daughters would not have the physical strength to stop in an emergency.
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PostPosted: Sat Mar 02, 2024 12:47 pm    Post subject: Re: Brakes Reply with quote

oprn wrote:
Please explain to me the basis behind your fear of too much clamping force. I am at a loss to understand why that would be a bad thing! My fear is the opposite, that I would build something that my wife and daughters would not have the physical strength to stop in an emergency.

My fear is that, with an egregiously excessively overpowered brake system, the slightest touch of the pedal will result in overbraking. After all, that's what the calculations begin with - foot-to-pedal braking pressure, typically based on the average person applying, say 70 - 75 pounds of pedal pressure. Some companies like Wilwood use 100 pounds force as a basis. But you get the point.

A panic stop, where somebody rapidly throws the foot down on the pedal, would assuredly result in wicked lockups with little travel/range in the pedal for modulation.
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PostPosted: Sun Mar 03, 2024 12:19 pm    Post subject: Re: Brakes Reply with quote

We are at the opposite ends of the worry spectrum! 😁

I donít know if is is possible to get non boosted brakes that sensitive given the confines of our cars. The larger the mechanical advantage the more room you need to swing a leg. I think we might run out of room and reach before reaching that point. For sure we are likely to run out of stroke on the master cylinder.

Donít forget that the height of our tires are going to have an effect on out braking power too.

Interesting calculations you have going there I will try to follow them through and see how my system stacks up too.
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PostPosted: Sun Mar 03, 2024 2:00 pm    Post subject: Re: Brakes Reply with quote

Busstom wrote:
My fear is that, with an egregiously excessively overpowered brake system, the slightest touch of the pedal will result in overbraking. After all, that's what the calculations begin with - foot-to-pedal braking pressure, typically based on the average person applying, say 70 - 75 pounds of pedal pressure. Some companies like Wilwood use 100 pounds force as a basis. But you get the point.

A panic stop, where somebody rapidly throws the foot down on the pedal, would assuredly result in wicked lockups with little travel/range in the pedal for modulation.

I would be more concerned about excess pedal travel. More multiplication means more clamping force at 75 pounds on the brake pedal, but more fluid movement for a given amount of brake pad movement. I ran the numbers for a stock later Karmann Ghia with front disc brakes and got 1653 lb. of clamp force. They have a single 1.575 inch (40mm) piston of clamping force at each front wheel. The master cylinder is 0.75 inch (19mm) and by my measurement the pedal is 5:1.
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PostPosted: Mon Mar 04, 2024 5:59 am    Post subject: Re: Brakes Reply with quote

Math is not my strong point but I am coming up with 4,389# front and 3,568# rear clamping forces for my brakes. That is calculating using purely mechanical advantage not pressures.

Really though that is only part of the picture as brake disc diameter and wheel diameter need to be figured in too. Although my front clamping force is larger than the rear, the rear discs are a larger diameter than the fronts so that will tend to even out the braking force between the two. Then I am running 25" rubber front and 28" on the rear so that will take the bias back a bit in favor of the fronts.

We haven't discussed rubber compounds or weight shift either so the end results could vary quite a bit from our calculations. I suspect that is why engineers are employed at the factory!
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PostPosted: Mon Mar 04, 2024 6:14 am    Post subject: Re: Brakes Reply with quote

So now going back and calculating the change in braking effort from the stock peddle and EMPI discs compared to my lengthened peddle I get a 32% reduction in peddle effort. That seems worth while!

That of course is without taking into account the change in the rears from drum to disc so... the jury is still out...
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PostPosted: Mon Mar 04, 2024 9:36 am    Post subject: Re: Brakes Reply with quote

oprn wrote:
I think we might run out of room and reach before reaching that point. For sure we are likely to run out of stroke on the master cylinder.

EVfun wrote:
I would be more concerned about excess pedal travel.


You may just be right. Iím sure there are factors missing from my numbers that explain why some of the figures are off the chart. Maybe the combination I currently have is unrealistic which may bear out soon (physically configurable but incorrect and impractical).

I can tell you one interesting piece of criteria that I must satisfy with my setup, per Wilwood:

ďThe master cylinder push rod must bottom out in the bore before the pedal stops against the floorboard. The inherent safety feature of tandem master cylinders is the ability to still build pressure in one circuit if the other fails. In the event of a circuit failure, the pushrod (and pedal) may travel 50% - 80% of the total stroke before starting to build pressure in the other circuit, allowing emergency braking to stop the vehicle.Ē

This means my brake pedal will have to be much closer to me than the accelerator pedal, which I was hoping to avoid for ergonomic reasons Shocked The Wilwood MC has a 1.1 inch stroke, which amounts to some 7 or 8 inches at the footpad. This is attainable in my Bus, given how the MC mounts below the floor board, but hopefully I can shorten the pedal a bit to reduce the effective travel without stiffening up the brake feel too much.

Which brings me to this: I found the solution to my wish for a force sensorÖa fish scale! (Thanks Amazon!) This is essentially a ďtensionĒ load cell. I will use this to pull on the pedal in my mockup. I could even rig it to simulate foot pressure, because I just donít know what 75 or 100 pounds foot pressure feels like, using one foot only against a movable object (the pedal). This makes my desire to attach a gauge to the setup all the more valuable, I have two hiding somewhere from the original CNC setup, but Iíll be damned if I can find them.

I've hung a 20 lb barbell plate on this scale and it is spot-on at the low end of the range.

https://www.amazon.com/Outmate-Digital-Handheld-Ha...E&th=1
Image may have been reduced in size. Click image to view fullscreen.

Image may have been reduced in size. Click image to view fullscreen.

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PostPosted: Mon Mar 04, 2024 3:31 pm    Post subject: Re: Brakes Reply with quote

Busstom wrote:
I can tell you one interesting piece of criteria that I must satisfy with my setup, per Wilwood:

ďThe master cylinder push rod must bottom out in the bore before the pedal stops against the floorboard. The inherent safety feature of tandem master cylinders is the ability to still build pressure in one circuit if the other fails. In the event of a circuit failure, the pushrod (and pedal) may travel 50% - 80% of the total stroke before starting to build pressure in the other circuit, allowing emergency braking to stop the vehicle.Ē

I wonder about that with some brake upgrades. If someone tosses 1.75 inch piston 4 pot calipers on the front and then stuffs front wheel cylinders into their Bug rear drums to improve brake balance does a stock 3/4 inch master cylinder still provide dual circuit brake function? A quick check is to do a test bleed after bleeding and readjusting the brakes. Do one front wheel and insure the pedal doesnít hit the floor and the master cylinder does not bottom out. Repeat with one rear wheel. You need to insure a half failure still leaves you with half brakes.

4 wheel manual disc brakes, with an effective dual circuit brake system, might require a really light car or the expectation of high pedal effort. I shaved 500 lb and 2 seats by making a Bug into buggy and consider that my braking upgrade. I mostly just needed to rebalance toward the rear.
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PostPosted: Tue Mar 05, 2024 7:13 am    Post subject: Re: Brakes Reply with quote

EVfun wrote:
I shaved 500 lb and 2 seats by making a Bug into buggy and consider that my braking upgrade. I mostly just needed to rebalance toward the rear.

Yes and if I recall you have a pretty much stock 1200cc engine so you have a nicely balanced package there. Power, weight and braking are all very well matched.
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PostPosted: Tue Mar 05, 2024 8:03 am    Post subject: Re: Brakes Reply with quote

oprn wrote:
EVfun wrote:
I shaved 500 lb and 2 seats by making a Bug into buggy and consider that my braking upgrade. I mostly just needed to rebalance toward the rear.

Yes and if I recall you have a pretty much stock 1200cc engine so you have a nicely balanced package there. Power, weight and braking are all very well matched.

I run a largely stock 1600 single port with a 40 horse transaxle.
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PostPosted: Tue Mar 05, 2024 7:35 pm    Post subject: Re: Brakes Reply with quote

Sorry, my mistake!
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PostPosted: Fri Mar 08, 2024 9:46 am    Post subject: Re: Brakes Reply with quote

Here is an idea that keeps coming back in my mind and I cannot shake it. On the subject of the caliper pistons retracting and needing a residual pressure valve to deal with that...

What would be wrong with putting a light coil spring inside the caliper piston? Size it so that it is pushing just enough to keep the piston in place. So 2 psi on a 36mm 944 piston would be 3.16 pounds of force. On a stock EMPI caliper it would require 3.9 pounds to equal the same effect as a 2 psi residual valve.

I just checked out a few appropriately size springs. Interesting...

Image may have been reduced in size. Click image to view fullscreen.


It looks possible, the trick will be to wind a spring with as many coils in as room allows as the pressure on the brake pads is going to decrease as the pads wear.

To do the job with residual pressure valves will mean adding 3 valves, 6 adaptor fittings, flaring 6 lines and buy 6 flare nuts for those lines. Lots more places for leaks. The up side is the pressure will not change with pad wear.
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PostPosted: Sun Mar 10, 2024 5:55 am    Post subject: Re: Brakes Reply with quote

No thoughts?
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PostPosted: Sun Mar 10, 2024 9:20 am    Post subject: Re: Brakes Reply with quote

oprn wrote:
Here is an idea that keeps coming back in my mind and I cannot shake it. On the subject of the caliper pistons retracting and needing a residual pressure valve to deal with that...

What would be wrong with putting a light coil spring inside the caliper piston? Size it so that it is pushing just enough to keep the piston in place. So 2 psi on a 36mm 944 piston would be 3.16 pounds of force. On a stock EMPI caliper it would require 3.9 pounds to equal the same effect as a 2 psi residual valve.

[snip]

To do the job with residual pressure valves will mean adding 3 valves, 6 adaptor fittings, flaring 6 lines and buy 6 flare nuts for those lines. Lots more places for leaks. The up side is the pressure will not change with pad wear.


Why do you want a residual pressure valve with disc brakes? Usually that is only needed if you mount the master cylinder reservoir so its fluid level is below a caliper. It could also be needed with drum brakes in that situation. When that is the case a 2psi residual pressure valve is used.

Hmm... VW moved the reservoir up to near the top of the hood at the same time the switched to front disc brakes on some models. The later VW dual circuit master cylinder doesn't have any residual pressure valves.

I did quickly find a 2psi residual pressure valve option that doesn't require adapters. Empi part number 17-2928 is a 2psi valve with metric bubble flare. If Empi is selling it there are likely other sources.
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PostPosted: Sun Mar 10, 2024 9:41 am    Post subject: Re: Brakes Reply with quote

Would a 2 lb RPV hurt anything (on 4 wheel discs), even if reservoir was well above calipers? I have two 2lb RPVs with my old CNC setup, and I'd be tempted to use them to reduce slack from piston/pad relaxing.

And yes, many 2 lb RPVs out there with both flare types.
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PostPosted: Sun Mar 10, 2024 2:30 pm    Post subject: Re: Brakes Reply with quote

I donít think would hurt, but isnít the setup youíre working on using 4 piston calipers with 1.75 inch pistons? 2 psi is about 10 pounds of clamping force on those brake pads. Light drag like that probably wonít generate any real heat.

Most modern cars with manual transaxle have a clutch that doesnít need adjustment. Do you know why? The throwout bearing remains in light contact with the pressure plate. It spins under very light load for 100k plus miles without failing. I suspect just 9 or 10 pounds on the rotor would be similar, not enough load to generate significant heat or wear. But what do I know?
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PostPosted: Sun Mar 10, 2024 7:12 pm    Post subject: Re: Brakes Reply with quote

No the '86 944 N/A rear discs are single piston.

I will keep searching for a RPV with the correct flares and metric threads. All I have found so far in universal ones with a 1/8 npt thread then you have to select adaptor fittings.
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