50/50 vs. 38/62 brake master cylinder

[hippieman9]

what is the difference in a 50/50 and 38/62 Master cylinder? I seem to have a 50/50 in my car. Looking to rebuild the brake system. should I go back to the stock 38/62?

[DMC-81]

what is the difference in a 50/50 and 38/62 Master cylinder? I seem to have a 50/50 in my car. Looking to rebuild the brake system. should I go back to the stock 38/62?

The stock front suspension linkage (LCA, sway bar) is relatively weak. The 50/50 unit is from a Saab and while it works, it increases the braking load on the front brakes from 38 % to 50%. The original design is 38/62 because of the rear engine layout. It's up to you if you want to increase the load on the front under these conditions. I chose to stick with the stock configuration.

[hippieman9]

The stock front suspension linkage (LCA, sway bar) is relatively weak. The 50/50 unit is from a Saab and while it works, it increases the braking load on the front brakes from 38 % to 50%. The original design is 38/62 because of the rear engine layout. It's up to you if you want to increase the load on the front under these conditions. I chose to stick with the stock configuration.

Oh ok, That is good to know. So am I to assume the 50/50 master Cylinders were all that were available for some time then? I see both are available from vendors.

[DMC-81]

Oh ok, That is good to know. So am I to assume the 50/50 master Cylinders were all that were available for some time then? I see both are available from vendors.

Yes, that is my understanding, while the crossover list was critical, and before DMC had them reproduced.

[DMC5180]

Also the front caliper pistons are larger than the rear pistons. By reducing the pressure to the front calipers and increasing to the rear, you end up with a more balanced braking force pressure of the pads to the rotor.


Sent from my iPhone using Tapatalk

[Elvis]

What's the difference inside the Master Brake Cylinder ?
How is the 50/50 vs 32/68 done ?

[81dmc]

I switched to a Tesla master cylinder with a 50/50 bias, and my braking is a lot better than the original bias master. Also, lockup occurs almost at the same time in the front and back, so no issues there.



The stock front suspension linkage (LCA, sway bar) is relatively weak. The 50/50 unit is from a Saab and while it works, it increases the braking load on the front brakes from 38 % to 50%.

IMHO, all DeLoreans should have LCA brackets installed, either from Owen (spittybug) or Ed @ DMCEU. If stability is a concern, save a few $$ buying a 50/50 master and reinforce the front end down the road....

[DMC-81]

I switched to a Tesla master cylinder with a 50/50 bias, and my braking is a lot better than the original bias master. Also, lockup occurs almost at the same time in the front and back, so no issues there.

IMHO, all DeLoreans should have LCA brackets installed, either from Owen (spittybug) or Ed @ DMCEU. If stability is a concern, save a few $$ buying a 50/50 master and reinforce the front end down the road....

Cool. I'm all for modifications if that's what you want for your car. My point was that if you're going to stay stock with the suspension, then my opinion is that the stock BMC is advisable as that's what the car was designed for.

If you want to upgrade & strengthen the front suspension then sure, the 50/50 BMC is an option. The front would still be light however, so I don't see what advantage the increased bias provides. A side by side braking distance test would be helpful on that question.

Although, after seeing the front wheels flexing backwards slightly under braking both on video and in person, I would suggest upgrading the suspension first before installing a 50/50 unit.

Cheers,

[David T]

if you change the tire sizes then changing the bias might be warranted. The reason the bias is so important is you do not, under any conditions, want the rear tires to lock up BEFORE the front ones. Because the Delorean is a rear-engined car, it has a VERY high polar moment of inertia. What that means is if the rear wheels lock up the rear end will try to swing around quickly. More quickly than most drivers would notice in time to correct.

[hippieman9]

if you change the tire sizes then changing the bias might be warranted. The reason the bias is so important is you do not, under any conditions, want the rear tires to lock up BEFORE the front ones. Because the Delorean is a rear-engined car, it has a VERY high polar moment of inertia. What that means is if the rear wheels lock up the rear end will try to swing around quickly. More quickly than most drivers would notice in time to correct.

Right, Because a skidding tire travels faster than a rolling tire.

I don't plan on changing tire sizes. I just was wondering why there were 2 different BMC. I have ordered a new stock style one and will be replacing it when it gets here. I have boxed in my LCAs just to help with stability too. as I have read that they are a weak point.

[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.

============

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... :wrenchin:

[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...

----------

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

Here it is disassembled:
http://dmctalk.org/attachment.php?attachmentid=57043&d=1530150403

Close up of the rearward piston that powers the rear circuit, (showing small holes in the piston - I think these are the compensating ports):
http://dmctalk.org/attachment.php?attachmentid=57044&d=1530150636

Close up of the forward piston that powers the front circuit, (showing small holes in the piston):
http://dmctalk.org/attachment.php?attachmentid=57045&d=1530151192

Here is how the two pistons are arranged in the bore:
http://dmctalk.org/attachment.php?attachmentid=57046&d=1530151314

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:
http://dmctalk.org/attachment.php?attachmentid=57047&d=1530151433

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:
57048

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?

[DMC-81]

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:
57048

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!



Thanks Ron. That makes sense about the compensating ports. I looked specifically at the BMC, and as you describe, there are two ports per circuit as described below. I also searched the port definitions (in parentheses):
1) a bigger by-pass port (to supply fluid to the master cylinder bore when the brakes are not applied)
2) and in front of that, the smaller compensating port (to allow the master cylinder piston to quickly return to the unapplied position, as well as compensate for fluid expansion or contraction from changes in temperature).

I looked again at the holes in the pistons and they must be there to help equalize pressure once released to the unapplied position.

Here is a picture of the ports:
http://dmctalk.org/attachment.php?attachmentid=57053&d=1530220512
The by-pass ports in blue, and the much smaller compensating ports in front of them. (And obviously the output ports to the brake lines in green.)
I lined up the rear piston in the position that it would be in the bore and the by-pass ports are behind the pressure seal, in the area of the necked down intermediate shaft of the piston:

http://dmctalk.org/attachment.php?attachmentid=57054&d=1530220641

It's hard to see, but the compensating port is roughly positioned 1mm ahead of the pressure seal:

http://dmctalk.org/attachment.php?attachmentid=57056&d=1530225361

So at least for the rear piston, it only has to travel very slightly forward to pass its compensation port and begin to create pressure. That would mean, as one continues to apply the brake, the 2 different gauge springs and the relative position of the front piston would impact the proportioning, correct?

I think the front piston passes the compensating port very quickly as well, as I can't imagine fluid under pressure escaping into back into the reservoir. I didn't observe any turbulence when I had the reservoir cap off as I was bleeding the brakes a few years ago.

Indeed, very interesting stuff. :wrenchin:




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

Well, DMCH states that "the original brake master cylinder was internally proportioned uniquely for the 62/38 weight distribution of the DeLorean" on the part description:
http://store.delorean.com/p-7478-master-cylinder-brake.aspx

[DMC-81]

^ Sorry, I meant to add that there was also the 38/62 red/orange band around the original BMC shown here (cleaned and fitted to my reproduction one), versus the black 50/50 band on some units. Hopefully that would signify a difference. If not, what does it mean? :dunno:

http://dmctalk.org/attachment.php?attachmentid=57042&d=1530150288

[Ron]

In the first pic, it looks like the forward circuit has 3 holes, one plugged, but in the last pic, it's not. Flash/reflection or ?? Anyway, good research there...
I think so...it looks like we have this one figured out, a combination of springs and port position. (...or convinced ourselves that we have ;-)

On the bands- I remember reading that all of the original D MCs had the orange band. I assume it's true, but don't remember the source...

[hippieman9]

In the first pic, it looks like the forward circuit has 3 holes, one plugged, but in the last pic, it's not. Flash/reflection or ?? Anyway, good research there...
I think so...it looks like we have this one figured out, a combination of springs and port position. (...or convinced ourselves that we have ;-)

On the bands- I remember reading that all of the original D MCs had the orange band. I assume it's true, but don't remember the source...

My current BMC has a black band and says 50/50 on it.

[DMC-81]

In the first pic, it looks like the forward circuit has 3 holes, one plugged, but in the last pic, it's not. Flash/reflection or ?? Anyway, good research there...
I think so...it looks like we have this one figured out, a combination of springs and port position. (...or convinced ourselves that we have ;-)

On the bands- I remember reading that all of the original D MCs had the orange band. I assume it's true, but don't remember the source...

Yes, the plug is a pin that holds the front piston in place. The pin is shown in the first photo in post 19.

Thanks. I'm sure there are some things we missed, but at least we know more about the anatomy of a BMC! I enjoy researching how mechanical things work. :)


My current BMC has a black band and says 50/50 on it.

Cool. If you can post a picture of it that would be beneficial to others reading the thread in the future.

[Mark D]

Really cool thread! I enjoyed learning more about the brake master cylinder :thumbup:

[hippieman9]

570645706557066

I know the pics aren't the best. But when I remove the BMC I'll take better pics. I have my new 38/62 BMC as well, I can put them side by side.

[DMC-Ron]

Here is the new master I purchased from DMC and installed about a year ago. It has a plastic band stamped with 38 62

57068

Ron

[hippieman9]

57117
57118
57119

Couple more pics of my 50/50 BMC now removes from the car.