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squirebill

Diagonal, X , Cross Braking System

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Definitely going to dual chamber master cylinder on my '49 Plymouth Suburban and new brake lines.  Does anybody have any thoughts, pro or con,  on plumbing it up for a  diagonal, X, cross system?  Seems this was done on some cars after gov't mandated dual chamber master cylinder and before front disc/rear drums. Any thoughts?  Regards to all. 

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I wonder about braking balance on front wheels. As in one braking harder than the other and causing a pull to that side. Would it require a special master cylinder to allow equal braking on both front wheels? I know most vehicles I have had the front wheels do most of the braking. I don't have the answer, just a lot of questions. I would go front - rear vs X if it were me, unless I could definitively answer some questions. Just my 2 cents..............

Edited by oldodge41

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I'd suggest running the fronts and rears separate from each other, and use a proportioning valve in the rear brake line. Front brakes do the most work and rears are kind of along for the ride until you stomp on the brakes (panic stop). If the rears lock up before the front you can spin.

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I've had more than one modern car with diagonal braking.  Pre ABS cars too.  With an all drum or all disc setup it's not as difficult to balance.  Drum/disc combos need thought. 

 

There's a drawing of one way it's done

 

 

diagonal.gif

Edited by Sniper

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Anyone ever encounter a brake failure with an x system? It’s really hard for me to believe a hard, fast and straight stop would be available at least the first time.  

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In my internet reseach it seems "diagonal" , "cross", and "X" system are all the same thing. .....left front and right rear on one leg and right front and left rear on the other.

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Sniper.....I see proportioning valves for the rear brakes on the picture you supplied.   Did that vehicle have front discs and rear drums?  Since my '49 Plymouth originally had no proportioning valve I wasn't planning on using any on the split system.

From my research it seems the general thinking is that the front brakes supply about 66% of the braking force and the rear brakes supply the remaining 33%.  So in a front/ rear split system if something failed in the rear system I would still have 66% braking from the front brakes.  If something failed in the front system would be left with only 33% braking from the rear brakes.  On a diagonally split system,  if something failed on one leg, would have one half of the front system and one half of the rear system (about 48%) still available. That doesn't sound too bad but I would be concerned about steering with only one front brake working.  Anybody have any thoughts about this or my reasoning?

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To the best of my knowledge all production diagonal brake systems are on disc/drum or disc/disc systems.  Never seen or heard of a production all drum car having it. but I don;t claim to have seen everything.

 

But an all drum car shouldn't need proportioning valves either.

 

I think the reasoning behind production cars going to a diagonal system was safety.  If you lost the front brakes the odds of the rear end locking up and coming around on you were severely diminished by the diagonal split setup.  I don't ever recall losing my brakes on a diagonal split car so I don't know how they may affect steering but I have had misadjusted brakes that wanted to pull when they were applied, correcting the steering was never an issue.  Such was the case on my 51 when I got it.

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2 hours ago, Sniper said:

Not sure what you mean by an X system.  In the diagonal setup, it's not an issue

Diagonal = X system.  If you lose one front and one rear, I just can't visualize how the one working front is not going to cause a hard pull in that direction in a panic stop situation, assuming no antilock braking.  Of course you can correct by steering, but only after you feel the pull.  In a panic stop, or with some drivers, it may be too late.

 

I lost brakes on my box truck due to a line failure.  It still stopped but really low pedal and high effort.  Luckily I was moving slow and had some time to  adjust before hitting something.  Had it been in a 'I need to stop right now' situation, it would have ended differently.  This was a front /rear split so it stopped straight, just slower than normal

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At one point or another in my life I've had to clamp off the rear brakes, front brakes or just one front brake to get home due either to sticking calipers or cylinders or one broken line. 

 

In the case of having one front disc brake clamped off... that was the worst IMO, of course in the case of having only rears functioning I was never anywhere close to having to lock them up.

 

In any case I don't believe most master cylinders can pull off a lockup when one chamber has leaked all the fluid down to the reserve barrier between the two , at least in the cases I recall. 

 

Perhaps the most usual and most current configuration is probably best...?

 

Front rear split with a master cylinder whose chambers are joined down to a point which is calibrated so that it becomes incapable of a full lockup when either side loses too much fluid. At least this was typical GM setup in the 70s-90s.. pre abs RWD.

 

All else being just my opinions , I'd bear in mind x brakes are used pretty much exclusively on FWD. Apart from a few cars just after the requirement for dual chamber master cylinders I don't know of any RWD that aren't front rear split.

Edited by 50mech

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4 hours ago, Sniper said:

To the best of my knowledge all production diagonal brake systems are on disc/drum or disc/disc systems.  Never seen or heard of a production all drum car having it. but I don;t claim to have seen everything.

 

But an all drum car shouldn't need proportioning valves either.

 

I think the reasoning behind production cars going to a diagonal system was safety.  If you lost the front brakes the odds of the rear end locking up and coming around on you were severely diminished by the diagonal split setup.  I don't ever recall losing my brakes on a diagonal split car so I don't know how they may affect steering but I have had misadjusted brakes that wanted to pull when they were applied, correcting the steering was never an issue.  Such was the case on my 51 when I got it.

 

In 1964 SAAB replaced their single circuit system with what they called "Dual Diagonal" brakes. It used one front wheel and the opposite side rear wheel on the same circuit. Back then they had drum brakes on all four wheels. The fronts had two cylinders like a MoPar and the rears one. When SAAB went to disc brakes on the front they continued to use the same Master Cylinder. Their thinking was 80% of the stopping power was on the front and if you divided the system front and rear you might find yourself with only 20%. The diagonal system gave you 50% regardless of which circuit failed. The SAAB system was in response to a proposed government mandate and it was pretty well thought out.

As a mechanic I've encountered many failed master cylinders.

Both circuits exist in the same cylinder bore. One circuit is acted on by the pedal and it slides in that bore hydraulically pressing on the other circuit. Because fluid can not be compressed there is no "slop" between the circuits and action is as if there were only one circuit. The only connection between the pedal and the second circuit is hydraulic.

Should there be a loss of fluid in one of the circuits, then and only then is there physical contact either between the first circuit and the second or the second circuit and the end of the cylinder. Thus a driver would feel the pedal drop half way to the floor on stopping. That's if everything goes right.

On the early SAAB 900 models there were vents in the hood on both sides. On the driver side they put a plate under the vent to direct water away from the Master Cylinder. The passenger side had the HVAC intake there so there was a purpose for the vent. SAAB sold cars in England so they made right hand drive cars too and the design meant they only needed to make one hood. The problem was that when a car was involved in a wreck and the hood replaced, the body shops rarely moved the plate to the new hood. Water entered the reservoir of the master cylinder and over time rusted the bottom of the cylinder bore causing BOTH circuits to become inoperative. This lead to a recall which included an inspection to make sure the plate was installed and a plastic cover over the master cylinder. Later production cars only had the HVAC side vents and two different hoods had to be made.

So if you ever wanted to know if a dual circuit system can fail on both circuits, here is an example of such a failure.

 

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5 hours ago, Loren said:

 

 

So if you ever wanted to know if a dual circuit system can fail on both circuits, here is an example of such a failure.

 

 

That failure mode would have applied to a front/rear split system as well.  So it's not specific to the diagonal split system. 

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