50/50 vs. 38/62 brake master cylinder

[PJ Grady Inc.]

38/62 refers to the weight distribution not braking bias afaik. There are only 50/50 BMC's for Delorean application unless a proportioning valve is utilized. DMCL decided one wasn't needed due primarly to weight transter which puts 70% of the braking force loaded onto the front wheels by default in most applications. The only significant difference between the Saab unit and the DMC is bore size which is a tad larger which if anything gives a bit more stopping power. Without a P.V. however both units are 50/50. I know this because I switched to rebuilding using brass sleeves due to the poor quality of most recent offerings. Urban legends die hard.
Rob

[David T]

The reason for the 50/50 bias BMC is because for a while it was difficult to get a cross/over with 68/32 so they just used the 50/50 because it was close enough and no one would notice under most driving conditions. Probably a panic stop in the rain on a turn with bald tires at high speed and you might notice a difference.

[PJ Grady Inc.]

The reason for the 50/50 bias BMC is because for a while it was difficult to get a cross/over with 68/32 so they just used the 50/50 because it was close enough and no one would notice under most driving conditions. Probably a panic stop in the rain on a turn with bald tires at high speed and you might notice a difference.

I'm not sure you understood what I just explained. Without a P.V. you get 50/50 unless you utilize different sized pistons in one bore. Unlikely.
Rob

[Elvis]

38/62 refers to the weight distribution not braking bias afaik. There are only 50/50 BMC's for Delorean application unless a proportioning valve is utilized. DMCL decided one wasn't needed due primarly to weight transter which puts 70% of the braking force loaded onto the front wheels by default in most applications. The only significant difference between the Saab unit and the DMC is bore size which is a tad larger which if anything gives a bit more stopping power. Without a P.V. however both units are 50/50. I know this because I switched to rebuilding using brass sleeves due to the poor quality of most recent offerings.Rob

That's a very good explanation, thanks for clearing this up

Urban legends die hard.
Rob

I know I know...

[81dmc]

I'm not sure you understood what I just explained. Without a P.V. you get 50/50 unless you utilize different sized pistons in one bore. Unlikely.
Rob

Never took apart a stock master to find out, but that definitely makes sense. So Saab unit for easier braking and stock unit for hard braking?


So even DMCH is spreading false info?

Did you know the original brake master cylinder was internally proportioned uniquely for the 62/38 weight distribution of the DeLorean? That's why we got away from the common "cross-reference" 50/50 proportioned part and decided to do it the right way - we went to our library of original factory drawings and had new parts made.

[Ron]

If the front and rear circuits have the same size bore, one way they get bias from the MC is to add 2 compensating ports. The port for the front circuit is closer to its piston (cup) than the port for the rear circuit is to its piston, while at rest. This causes the rear circuit to have a shorter (effective) stroke...the fronts activate first and have more pressure... Widely used for decades.
Another way- Some (or all? I can't remember ;-) tandem bore ABS systems don't have proportioning ports, but use a valve in the rear circuit that is closed by pressure created in the front circuit, giving it (the 1st) a head start, so to speak... Same idea basically.

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MC bias is only the tip of the iceberg. Tires (diameter, coefficient of friction), weight (front & back), mentioned earlier, as well as, center of gravity, caliper piston size (front & rear), rotor diameter (front and back), pad coefficient of friction (front & back), and even aerodynamic down force, just to name a few, all need to be considered to calculate the bias needed.
$.02: Stick with stock.
IF everything is correct and the brakes feel week/hard to YOU, try a MC with a smaller bore, but original proportioning.
Spongy/too much travel -> bigger bore...

[DMC-81]

I took apart my original BMC to see if it could be rebuilt and to know what made it tick. Maybe I'll dig it out and take a picture of the internals and post it. There is one bore, and as I recall, the proportioning is achieved with separate chambers (one behind the other), separated by a piston, and springs that deliver the fluid to the 2 circuits. Stay tuned...

[David T]

I think the bias is built in by using springs and compensating ports as Ron describes. The bore size is the same for the front and rear brakes so that can't be it. Never tried to measure it.

[DMC-81]

I think the bias is built in by using springs and compensating ports as Ron describes. The bore size is the same for the front and rear brakes so that can't be it. Never tried to measure it.

Yes, I remembered the springs, but as shown below, I didn't remember that there were 2 pistons, not one...

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I found my original Girling Brake Master Cylinder and took a few pictures of it.

Here it is disassembled:


Close up of the rearward piston that powers the rear circuit, (showing small holes in the piston - I think these are the compensating ports):


Close up of the forward piston that powers the front circuit, (showing small holes in the piston):


Here is how the two pistons are arranged in the bore:


Here is a comparison of the pistons. The front on the top with the heavier gauge spring, and the rear one the bottom with the lighter gauge spring. It looks like the 62 rear/38 front proportioning is perhaps accomplished by the different thicknesses in the spring gauges:


I'm not brake engineer, but it seems to me that the rear piston would compress easier (and therefore more) than the front piston, delivering more fluid displacement. Feel free to disagree as this is a guess.

[Ron]

Dana, compensating ports are bores in the housing. I haven't looked at a stock D MC, but many like this: For rear favored bias, both pistons retract, both chambers in the bore fill, when the pedal is pressed, both pistons pass the fill ports (purging ends), then the rear piston passes its compensation port and begins to create pressure 1st, THEN the front piston passes its compensation port and begins to create pressure too...
There are other ways it is done e.g, with a valve (similar to a push-up valve in the bore), as previously mentioned. I haven't seen/noticed one before, but, with the difference in spring strength you pointed out here, I could see how the position of the compensating ports could be equal at rest and the weaker spring causing/ensuring the rear piston passes its compensating port first...Or, just using equally spaced fill ports with a weaker spring (much like a biased ABS MC w/o compensating ports).
But, I don't see the springs affecting the pressure once the pistons have passed all ports, regardless of whichever port setup it has....

Very rough, but might help:
MC.jpg

Look in the bore. If there are 2 incoming ports only, it is the spring doing it. 3 or more it is compensating port(s)...or a combination.

Interesting stuff!

EDIT: Just a thought...do we know that the 38-62 is a bias rate for the MC itself?