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Busstom
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Joined: November 23, 2014
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Location: San Jose, CA
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 Posted: Wed Feb 14, 2024 1:16 pm Post subject: Re: Brakes |
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| ...with drum brakes the leading shoe is partially self energizing. |
How do the shoes know which one is leading? Short of gravity, neither one seems aware of any forces other than the rotation. Is this related to the wheel cylinder and where it actuates against each shoe?
I ask because I've been delving into brake fundamentals recently and only just learned about the "self energizing" effect (also referred to as self-exciting).
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EVfun
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| Posted: Wed Feb 14, 2024 3:24 pm Post subject: Re: Brakes |
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I don’t really know to explain it well. A crude explanation is that the brake shoe is not being pushed out with each point perpendicular to the drum surface. It is being pushed out between perpendicular and the direction of the wheel cylinder. That leaves one shoe kinda being pulled into a shrinking space.
Here is a front brake picture from Buggeee showing the Bus double leading front shoe design.
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Busstom
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| Posted: Wed Feb 14, 2024 5:12 pm Post subject: Re: Brakes |
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Seems reasonable. I get how one end of a shoe relies on proper star adjustment for sufficient contact and the other end, the wheel cylinder.
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oprn
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| Posted: Thu Feb 15, 2024 5:17 am Post subject: Re: Brakes |
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That is true for drum brakes that only have one wheel cylinder. The example given in the Bus brake photo has a cylinder for each shoe. In this case if I am not mistaken, both shoes are leading and self energizing. This worked really well on the heavy Bus and enabled VW to avoid the use of a brake booster until they went to disc brakes in the later years.
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Abscate
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| Posted: Thu Feb 15, 2024 5:46 am Post subject: Re: Brakes |
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With the wheel cylinder in the leading edge of each shoe, the braking friction pulls the shoe into contact with the drum , thus self-energizing.
On a rear brake, the front shoes are self energizing and the rear shoes are not, if my recollection of the wheel cylinder being at the top of the brake plate is correct.
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oprn
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| Posted: Thu Feb 15, 2024 7:24 am Post subject: Re: Brakes |
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That is my understanding too.
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oprn
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| Posted: Thu Feb 15, 2024 7:30 am Post subject: Re: Brakes |
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| oprn wrote: |
| EVfun wrote: |
| You will notice that VW went to the low effort 17.5mm master cylinder in 1965 but just 2 years later when they switched to dual circuit brakes they jumped back up to a 19mm master cylinder. |
Aha! There it is! Thanks! In conversation with Flat on the weekend he mentioned there was a smaller MC in the older Beetles but he could not remember the year. I suspect it would be tough to find one of those today though! |
I just looked it up on RockAuto and they do list an 11/16" master cylinder. That would be the 17.5mm one!
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oprn
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| Posted: Sun Feb 25, 2024 4:56 am Post subject: Re: Brakes |
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Here is a bit of information gleaned from another thread that I think is of value to share here.
Racecougar, could you do me a favor please when you get a moment and measure the brake peddle from the bottom pivot to the center of the foot pad. Then again from the bottom pivot to the master cylinder rod pin. I am curious to learn what the peddle ratio is on those dual master cylinder kits is.
Thanks in advance!
jimmyhoffa
Oprn, I have the old Neal pedal set which the CNC is a copy of which the EMPI is a copy of, and they are all 6:1, give or take a fraction. My Wilwood forged pedals are as well.
racecougar
I'd expect it to be 6:1, but I can certainly double check it this weekend. It only takes a minute to pull the nose off.
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oprn
Thanks! I suspected as much, the stock peddle ratio I believe is 5/1 so you need a bit more mechanical advantage to push 2 master cylinders. Makes sense to me.
Wulfthang
I don't know what the ratio is but it does require more pressure with the duals. Not a whole lot but noticeable. It feels good though. More definitive.
racecougar
As Jim said, it's 6:1. Just measured tonight. Distances measured out to ~1-1/2" and ~9".
oprn
Thank you!
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EVfun
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| Posted: Sun Feb 25, 2024 9:25 am Post subject: Re: Brakes |
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| oprn wrote: |
| That is true for drum brakes that only have one wheel cylinder. The example given in the Bus brake photo has a cylinder for each shoe. In this case if I am not mistaken, both shoes are leading and self energizing. This worked really well on the heavy Bus and enabled VW to avoid the use of a brake booster until they went to disc brakes in the later years. |
VW used the dual wheel cylinder method for dual leading drum brakes used on the front of early Type 2 and Type 3 vehicles. There is another way to have 2 leading brake shoes, and old Bendix patent system used on '30's and '40's Fords (I see them on street rods) and many more American cars. They can be easily spotted because there is a pin above the wheel cylinder. This design makes the brakes dual leading in both directions. Motor Trend has a good write-up of the system.
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EVfun
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| Posted: Sun Feb 25, 2024 3:37 pm Post subject: Re: Brakes |
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| oprn wrote: |
Here is a bit of information gleaned from another thread that I think is of value to share here.
[snip]
racecougar
As Jim said, it's 6:1. Just measured tonight. Distances measured out to ~1-1/2" and ~9".
[snip] |
I went out and measured my clutch pedal, an EMPI (I think) single pedal with Wilwood single master cylinder. It is 5:1 as best as can be measured without disassembling it. I came up with 8.75” and 1.75”.
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Busstom
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| Posted: Sun Feb 25, 2024 11:39 pm Post subject: Re: Brakes |
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More data...
CNC pedal/bracket (bell crank) for dual M/Cs: 7.8:1 (11.75, 1.5).
Interestingly, Wilwood said this should work well with my Wilwood tandem M/C (.875 bore) and my current Wilwood piston/caliper arrangement (1.75 frt, 1.38 rr).
Time will tell 
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oprn
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oprn
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| Posted: Tue Feb 27, 2024 10:53 am Post subject: Re: Brakes |
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Another question here. It says here that their residual valves are for racing only, not for use on the road.
https://tbmbrakes.com/products/residual-pressure-valves
Do some of the car manufactures use them in production cars or not?
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EVfun
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| Posted: Tue Feb 27, 2024 5:48 pm Post subject: Re: Brakes |
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| oprn wrote: |
Residual pressure valves is a subject not covered yet in this discussion.
https://www.youtube.com/watch?v=DIVDjqniUX8
[snip]
Why did our cars not need one with the stock brakes but when we upgrade they seem to need them? What was VW's secret? |
For much of the Bug and Bus production VW did use a residual pressure valve. It was built into the older single circuit master cylinders. Thank Glutamodo for this picture:
The piston return spring is also the 10 psi valve. The smaller "stuff" at the left end is the little valve that lets fluid move TO the wheel cylinders. You can clip the stem on that valve and remove those parts to use the early single circuit master without a residual pressure valve, if needed in your application.
The early dual circuit master cylinders (VW made several) had different working lengths for the front and back halves depending on if you had a Ghia with front disc brakes or a Bug with 4 wheel drums. For drum brakes they had a screw in residual pressure valve. I couldn't find a good picture so I pulled this scrap '67 Bug master cylinder off my "junk shelf" of salvaged old VW stuff. You can see the extra part screwed into each master cylinder outlet and the cut remains of one brake line.
As for WHY VW didn't need the residual pressure valves for their later dual circuit brakes system it is something called a "restriction drilling." It is a smaller than traditional hole to let fluid from the master cylinder ports out-of or into the brake lines. The need for a drum brake residual valve was explained to me this way:
Inertia -- when a fluid gets moving it resists stopping. So if you let the brake fluid move back from the wheel cylinders, fast down the skinny lines, back to the master cylinder it won't want to stop when the rear brake springs stop pushing. The result can be a brief low pressure condition inside the wheel cylinder. Now look at how that wheel cylinder cup seals. It is an outward taper than creates a good seal if the pressure inside the cup is higher than outside the cup. It tends to suck air around the lip if the pressure inside the cup is LOWER than outside. SO, the brake system has to prevent a brief low pressure event inside the cup when the brakes are suddenly released. A residual pressure valve (stopping at 10psi) or restriction drilling (slowing the fluid movement) can both work.
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oprn
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| Posted: Wed Feb 28, 2024 4:25 am Post subject: Re: Brakes |
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First off I discovered that everything on the TBM site has the "not for street use" label. Just a disclaimer I suspect like the "known to cause birth defects and cancer" in California.
Thanks for the run down on VW master cylinders EVfun. This one I have is a dual on a '73 pan. I am not sure if that makes it early or late. I need to have a good look at it to see if it has the extra fittings you showed in the photo. It most certainly is not holding any back pressure on the rear brakes right now. Update: just checked and this MC does not have the extra fittings on the side.
I put a single Bus one on my sand rail so I am pretty sure it doesn't have any residual valve in it.
Interesting that cip1 lists the MC for a Beetle the same as for the Ghia but a different one for the type 3. Rock Auto has a different part number for each of the three. Concept 1 also lists the Beetle and Ghia as the same one but have no listing for the type 3. All of them are 19mm bore.
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EVfun
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| Posted: Wed Feb 28, 2024 9:56 am Post subject: Re: Brakes |
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The stock Bus single circuit master cylinder does have a residual pressure valve, like the single circuit Bug master cylinder. A couple of photos below. The second one shows the residual pressure valve design I am familiar with. It is being disabled as a part of his front disc brake conversion. The newer Varga kit has a slightly different style valve.
WildIdea posted this photo here:
Teuton posted this photo here:
[Edited to better source the photos]
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| Wildthings wrote: |
| As a general rule, cheap parts are the most expensive parts you can buy. |
Last edited by EVfun on Thu Feb 29, 2024 11:37 am; edited 1 time in total |
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oprn
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| Posted: Wed Feb 28, 2024 10:47 am Post subject: Re: Brakes |
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Just look at what I am learning here!
So then if one is to change to disc brakes on the front that needs to be altered?
Looks like the answer is yes! https://www.thesamba.com/vw/forum/viewtopic.php?t=298242
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EVfun
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| Posted: Wed Feb 28, 2024 1:54 pm Post subject: Re: Brakes |
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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.
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Busstom
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| Posted: Wed Feb 28, 2024 2:12 pm Post subject: Re: Brakes |
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| Busstom wrote: |
More data...
CNC pedal/bracket (bell crank) for dual M/Cs: 7.8:1 (11.75, 1.5).
Interestingly, Wilwood said this should work well with my Wilwood tandem M/C (.875 bore) and my current Wilwood piston/caliper arrangement (1.75 frt, 1.38 rr). That was from a young fella who apparently didn't really dig deep enough into my numbers and run calcs side-by-side with me 
Time will tell |
Damn, I made a mistake...I like to see the numbers and I got curious, so I started delving into the math to calculate/understand my clamping pressures at the calipers and estimate my front-to-rear bias.
In the process, I discovered I incorrectly measured my pedal, I used the wrong formula for my pivot point / pushrod orientation...so my pedal ratio isn't 7.8:1, it's 6.8:1! One of the reasons this matters is, after all was said and done, I calculated a whopping 4,700 psi of clamping force on the front calipers! 
I called Wilwood to discuss further, and tech support was confused by my stated 7.8 pedal ratio, so I revisited and had my wakeup call! Now, after recalculating, I'm still on course for 4,100 psi up front, which by most standards, is still about 4x too much 
Bottom line, after having all my latest specs, dude on the phone said I'm actually going to need more like a 1" bore MC, not my 7/8" bore He thinks my problem is going to be just "no brakes, pedal will go all the way to the floor!" Not sure I totally grasp that, but he and I both agreed, the only way to know where I stand at this point is just to build it all up, add fluid and test the damn thing. Period. Let's see what works!
That's what I get for buying crap way ahead of time, not doing the math, and mix-matching along the way. Still learning here too!
FWIW, my front/rear bias is currently 60/40 based on pressure calcs, and I do have a proportioning valve to adjust the knee point if/when testing indicates it necessary.
Edit: thinking about this further ("He [at Wilwood] thinks...the pedal will go all the way to the floor"), I think his suspicion is that the 7/8" MC just doesn't produce the volume needed to overcome the system slack inherent with my massive caliper piston areas.
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Last edited by Busstom on Wed Feb 28, 2024 4:48 pm; edited 2 times in total |
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oprn
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| Posted: Wed Feb 28, 2024 3:13 pm Post subject: Re: Brakes |
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Any of you remember the hay day of Ford power brakes back in the mid '60's? Some of those big old boat anchors would stand on their noses with the slightest thought of brake application. The first time I drove one I had to jackhammer my teeth out of the dash! Lord were those things touchy! I think it was a combination of a power booster on drum brakes that made them like that.
I don't think any of us are in danger of having that problem with our manual brakes.
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