Citroen hydraulic brake pressure for Maserati cars

I have a question concerning the hydraulic pressures generated by the Citroen system for the Maserati cars that used it. It is my understanding that the working pressures for the brake system varied between 2200 psi to as high as 2600 psi. Since built up pressure already exists due to the Citroen pump does pressing the brake pedal merely opened a valve to release the flow of brake fluid into the brake lines. Thus, is the pressure in the brake lines always at between 2200 to 2600 psi from the moment you pressed the brake pedal or does the brake line pressure vary due to the amount of force generated when putting pressure on the brake pedal.

I am assuming that in a conventional braking system with a master cylinder, the line pressure is proportional to the amount of pressure being put on the brake pedal. I understand that a fluid is non compressible but I am not sure about the principals for the Citroen system verses a conventional braking system.

It seems to me that in the Citroen system the brake line pressure stays constant due to the hydraulic pump in the system and is not related to the pressure applied to the brake pedal whereas in a conventional system the brake line pressure changes with the force applied by the driver to the brake pedal.

I am contemplating changing the steel calipers on my Bora to aluminum ones but I am not sure that such a conversion would be safe. If someone would further my education I would be grateful.

Elliot

 

Well a lot of people "feel" that's how the brakes respond since they do engage with more gusto than most standard braking systems. But that may be do to the operator not being used to them or if there's gas (air or nitrogen due to a sphere leak) in the system and then the gas compresses first before the brakes fully engage. That can lead to a pull one side to the other or a more intense initial grabbing sensation.

To your basic point, yes the pressure does vary and if you're not noticing that then something is wrong.

Look around for an operational drawing of how the system works but basically it's pump=> regulator=>main accumulator then it branches off to the accessory circuit and the braking circuit. Front & rear accumulators => brake pedal controller => front & rear calipers. On some cars I think there's another component that limits fluid flow as well after the brake pedal controller. The regulator after the pump maintains the specified proper pressure in the accumulators. Your left foot does it for the calipers. I forget the exact number but even if the pump fails you should have enough residual pressure in the system for several full stops from speed.

On the track I never experienced brake fade coming down from about 130 mph and the modulation was pretty good IMHO. But it's not a natural race car.

The two Group 4 Thepenier Boras both initially had the LMH system.

If you buy Mark's new book I'm certain he must have covered that in those two cars. I believe both have now been converted to something else.

 

The brake-system in essence is easiest compared to the air-brakes used on trucks. The controller on the brake-pedal modulates the pressure going from the accumulators to the brake cylinders. When you let go of the pedal, the controller opens the return line to the tank and the pressure in the lines equals atmospheric pressure again, and the brakes dis-engage.

 

Elliot,

I think Bob's (staatsof) basic explanation of the system circuit is about right.

Also, I would advise that you find a Citroen expert for specific advice. These guys might be of help:
Brad Nauss Automotive
Dave Burnham Citroën - Welcome

Also note other LHM threads:
http://www.ferrarichat.com/forum/maserati/228217-citroen-hydraulics-khamsin-bora-early-merak-late-indy.html
and additional discussion in the Khamsin thread, (search by post, not by thread).

As to my comments below, take them in the context that so far my Khamsin has been a hobby, not a car, and it is still a way off from being a car.

"... my understanding that the working pressures for the brake system varied between 2200 psi to as high as 2600 psi...".

I don't think that's accurate. The engine driven piston pump might make that, (I don't know), but the next element in the hydraulic circuit is the pressure regulator. We've had the discussion around here in the past, and I recall that the system actually operates at about 65 or 70 bar (975 or 1050 psi). A Citroen technician should be able to adjust this.

"... Since built up pressure already exists due to the Citroen pump does pressing the brake pedal merely opened a valve to release the flow of brake fluid into the brake lines...".

That is my understanding. I believe the pedal actuates a spool valve.

"... Thus,is the pressure in the brake lines always (at between 2200 to 2600 psi from the moment you pressed the brake pedal or does the brake line pressure vary due to the amount of force generated when putting pressure on the brake pedal...".

I've never heard of complaints of slamming on the brakes with each activation. I've assumed that the Citroen engineers thought this out and use a needle valve type mechanism to allow some modulation with pedal effort/position.

"... I am contemplating changing the steel calipers on my Bora to aluminum ones but I am not sure that such a conversion would be safe...l".

Why? To save weight? This doesn't sound right to me. I think you need a racing brake specialist or mechanical engineer with subject matter expertise to make this call.

Carmine

 

Just so you're clear that there are no return lines involved with the calipers themselves.

BTW, when you go to bleed the brakes you can hold down that pedal and empty the entire reservoir tank pretty quickly.

 

I have to call Brad to see what the actual bake line pressure is and whether it is modulated depending on the pressure put on the brake pedal. My understanding that the Citroen system operates at a much higher level than normal brakes and that is one of the reason for the switches and the cylinders having problems with leaking because the Bosch equipment was designed for a working load of 1500 psi maximum rather than the higher Citroen. Supposedly, the reason that Bora brakes don't fade after repeated stops from 100 mph is due to the very high brake line pressure keeping the brake pads from being pushed off by the out gassing of the binding materials used for the pads, which is why some brakes are slotted today. However, it's time to contact one of the Citroen experts who work with the hydraulic system to get some professional advise. I appreciate everyone taking the time to help me sort this out.

Elliot

 

How could not think that the brake pedal modulates the pressure? Otherwise all stops would be like throwing a boat anchor.

You are correct about the higher pressure.

Aluminum rotors? Why not go for carbon fiber?

 

Because of this, someday, if I ever get to it, I thought that I would install a pressure reducing valve in the hydraulic circuit for the auxiliaries. It strikes me that we don't need 1000 psi to raise the headlamps or adjust the seat.

Attached is a diagram that I made of the Khamsin hydraulic circuit, based upon the manuals from MIE and observations in my own car. Recently, Bob (staatsof) and I talked and touched upon the subject of some of the differences relative to the Bora, so it's not exactly what you've got. But I expect other than the lack of steering and the addition of the pedal adjustment it should be pretty similar.

I'm no expert but I've always thought that brake fade was caused mainly by the disc's inability to dissipate heat quickly enough. The function is to transform the kinetic/mechanical energy of the moving car to heat energy at the brake and then discharge the heat.
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Bob, I think Elliot is talking about calipers -- not rotors.

 

Oh you're right. My error ...

So he's worried that modern aluminum calipers would flex or fracture under the extra load?

I have to wonder why at some of these changes?

 

I think the diagrams are in a couple of the issues of MIE's VCM. They may have even issued a separate poster of it?

 

The whole question has come up because a fellow Bora owner in my area of the country is in the process of commissioning new calipers from Wilwood brakes and they are comfortable doing them in aluminum. There is also consideration for changing out the rotors from steel to carbon-ceramic which are different than carbon-carbon rotors. As for the concern about the switch from steel calipers to aluminum, if the pressure is as elevated as some think then aluminum is most likely more venerable to failure than steel. As for the question of fade, heat dissipation is the question but vented rotors in steel do much better that solid rotors and the Bora had a much better record for lack of fade than many of its contemporaries and all of them were running about the same diameter vented rotors in steel. Out gassing pressure is a known problem and grooves in rotors help release the pressure generated. Another way to over come that type of pressure is to increase brake pad clamping pressure. The Bora brakes have always drawn comments from testers as being touchy and only with continued exposure to them does one learn how to treat them to achieve smooth braking. I have always found that a very light touch to the brake pedal is all that is needed for most normal braking. I assume that only a very small amount for brake fluid is being released when I open the valve controlled by the brake pedal which would cause a small movement of the brake pistons and less clamping of the brake pads. It's all speculation on my part and as I said previously, I will check with the experts to see if I can find the answers and as a last resort, I will put a pressure gauge on one of the brake line at a caliper and see what it says.

Elliot

 

Elliot,

See the brake control valve as a variable pressure reduction valve. When you apply brakes, you exert a certain pressure on the pedal, which sets the control-valve to regulate a corresponding pressure to the brake lines. The harder you press the pedal, the higher the pressure allowed by the controller. The pressure in the lines will thus never be higher than the pressure regulated by the main regulator, no matter how hard you push the pedal..You also need to distinguish between a small amount of fluid being allowed through by the brake-contoller or regulated/controlled brake line pressure: The latter is what takes place. As Bob mentioned in an addendum to my first post here, if fluid is allowed out of the brake system, the controller will allow a continuous supply of fluid to maintain brake pressure. This is all rather confusing until you have an understanding of how a pressure controller works.

Wilwood is a reputable company, there are plenty old Jags running them, a very popular upgrade. Mind, however, that the Maser calipers are not small, and have them assure you that the replacement caliper will be free of caliper-flex.
Also make sure that they know that the system is using LHM, not normal brake-fluid, to be sure that appropriate seals are used. I have no idea if a normal seal will work with LHM.

Best,

Jack.

 

Call Jerry Hathaway at SM World. If he ran one at Bonneville reaching 206 mph I'd bet he can answer a few of your questions. I think he's probably be interest in hearing about your project too. Hey, maybe he'd even join Fchat!

 

The citroen hydraulic high- pressure system uses a pressure of some 170 bar. The citroen calipers are aluminum, so no problem there. The brake is actually a valve with pressure feedback. The harder you press the pedal (open the brake valve) the more resistance you get. Thats the modulation. Because of the high pressure any air in the system gets compressed and because LHM doesn't absorb moisture you wont get brake fade because of that.

cheers, Göran

 

That sounds right, Jack.

Göran, could you please direct me to a Citroen manual or web link on this? Thanks.

 

Lots of manuals and nice stuff on the citroen sm wiki.

Pressure Regulator - Citroen-SM-wiki

cheers, Göran

 

The person doing the conversion is a prominent manufacturer of aftermarket racing products who has a very good reputation for his products. He is fully aware that LHM requires different seal materials compared to ordinary brake systems. He has sent Wilwood all four uprights, hubs, calipers and rotors for their review. I expect that they will come up with the correct replacements for the rotors and calipers. I believe that my friend, given that he manufacturers racing hubs himself and has the engineering and machining capability, may end up making replacement hubs in house. His aim, as is mine, is to upgrade the running gear of his Bora without changing the look of the car. I don't think he is as concerned about being able to reverse any changes as I am but the ones he will be doing lend themselves to upgrades and still can be returned to original specs without much trouble. All in all it is nice to have a fellow Bora own in my area to work with who has the same goals I have, even if we are taking slightly different paths to get there.

Elliot

 

Think of it not in terms of the hydraulic pressure in the braking system, but now much PSI is applied by the pads to the rotor. This is the force a set of calipers must withstand.

If you simplify it that way - regardless of the method by which hydraulic pressure is fed to the brake caliper - you can rest assured the force being applied by our Bora brake calipers, via the piston to the pads and rotor, is no different in terms of PSI than other high performance cars' brakes.

Which is a long way of saying the braking force endured by an SM, Bora or Khamsin's calipers is no greater than that of an Alfa, Porsche, BMW, Ferrari, et al.

In conclusion - aluminum calipers can withstand the same forces as steel calipers, as they have for years....
Best,
-Art

 

Art,

I agree with that approach, and from the same vein of thoughts I have been pondering friction-materials on the pads: A harder material could make the brakes easier to modulate.

Then I considered that, unlike in a traditional hydraulic braking system, the pressure in our brakes actually has a ceiling, and if the wrong material is chosen, you would diminish braking ability in a full-on situation.

Has anyone experimented with harder pads?

Best,

Jack.

 

Moderator,

Please merge this thread into Ivan's LHM thread, (just bumped).

Carmine