Terrible Xantia brakes

citronut · Post by **citronut** » 08 May 2005, 02:10

are you sure kowalski,C5 is hydraulic susp/system,getting back to mod panjandrum may have hit the nail on head,on BXS that do not have spring under pedle if there is air in system the brakes are all or nothing regards malcolm

littleredrooster · Post by **littleredrooster** » 08 May 2005, 02:54

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by JohnCKL

Original spring brakes - 1996 Xantia 1.9TD SX
Modified solid tube brakes -
2001 Xantia HDi
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AndersDK · Post by **AndersDK** » 08 May 2005, 03:06

The C5 brakes are the conventional master cylinder and vacuum servo. No more high pressure brakes.

Kowalski · Post by **Kowalski** » 08 May 2005, 03:53

They do have the hydraulic suspension still, it uses a different fluid (LDS instead of LHM) but its separate from the steering / brake system and uses an electric pump instead of a directly engine driven one.

FrenchLeave · Post by **FrenchLeave** » 08 May 2005, 03:58

Is the C5 able to regulate rear brake pressure according to load as the doseur cars do?

FrenchLeave · Post by **FrenchLeave** » 08 May 2005, 05:02

Mandrake, I am interested in sadism, bestiality and necrophilia, am I flogging a dead horse? I may well be on this subject but Kowalski and Panjandram are right. Any coil spring with equal coil spacing obeys Hookes Law and is linear in its compression/load relationship. As you probably know, they are categorised by that relationship, for example as a 100N/cm spring. Taking the example of a 100N/cm coil spring, if you apply a force of 100 Newtons to one end, the spring will compress 1 centimetre, a 300N force will compress it 3cm and a 10N force will compress it 1mm. At the same time, the 100N, 300N or 10N force will appear at the other end of the spring (in our case at the doseur valve). That is what springs do. If the spring becomes fully compressed (coil bound) before the maximum workable force is applied, there is of course no more movement available and you then revert to the old Citroen system. It would seem from your description that that is the situation that prevailed with your car and another solution would have been to fit a stronger spring.
For those who prefer the direct acting type of brake, you have provided a solution and have, I'm sure, earned the gratitude of a lot of people.

Kowalski · Post by **Kowalski** » 08 May 2005, 05:29

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by FrenchLeave

Is the C5 able to regulate rear brake pressure according to load as the doseur cars do?
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Cars with conventional brakes have been able to do this for years, they have a brake regulator valve, usually attached to the rear anti roll bar in the way a Xantia has a height corrector. I think this is a requirement of type regulation etc.
Brake regulator valves don't compensate as well as the system the Xantia has and they can stick too. You can also confuse some types by doing things like braking gently, then braking harder and then going back to braking gently without taking the brakes off completely. The one on my Dads Cherokee caused him grief, at one point it was braking just the rear wheels so he had some interesting lockups.

DoubleChevron · Post by **DoubleChevron** » 08 May 2005, 06:38

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by FrenchLeave

Mandrake, I am interested in sadism, bestiality and necrophilia, am I flogging a dead horse? I may well be on this subject but Kowalski and Panjandram are right. Any coil spring with equal coil spacing obeys Hookes Law and is linear in its compression/load relationship. As you probably know, they are categorised by that relationship, for example as a 100N/cm spring. Taking the example of a 100N/cm coil spring, if you apply a force of 100 Newtons to one end, the spring will compress 1 centimetre, a 300N force will compress it 3cm and a 10N force will compress it 1mm. At the same time, the 100N, 300N or 10N force will appear at the other end of the spring (in our case at the doseur valve). That is what springs do. If the spring becomes fully compressed (coil bound) before the maximum workable force is applied, there is of course no more movement available and you then revert to the old Citroen system. It would seem from your description that that is the situation that prevailed with your car and another solution would have been to fit a stronger spring.
For those who prefer the direct acting type of brake, you have provided a solution and have, I'm sure, earned the gratitude of a lot of people.
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Gee's it's been years since I've used physics. Without getting technical, I believe the before and after experiance/testing in this case is far more important.
Here's my highly technical explanation
Spring in place : Brakes absolutley sh!tfull, no brake feel, impossible to modulate braking, moderately heavy braking is hopeless/non existant (remember my Xantia is a slugomatic, so I'm fighting the torque converter as well).
Spring removed : Brilliant brakes, feel just like a DS's brake button, ability to modulate braking is not only possible, it's fantastic. Light, moderate, heavy and crash stop type braking is dramatically improved (infact moderate and heavy braking is now possible, where as it wasn't previously ...... Just a heart stopping ...... ssssstttttttoooooopppp you b@stard followed by a panic enforced full on crash stop completely compressing the spring [:0] [:0] [:0]).
If I was to buy another Xantia I'd remove this spring before even driving it home (yes I consider it that dangerous). the fact it exists at all simply defies belief. All it means is where you previously would have brake moderately hard to a standstill, you now must do a full blown crash stop ....
There's an explanation about the theory of stuff like this in statistics too. You know a human will be statistically perfectly comfortable with there head in an oven and feet in a freezer ....... Certainly isn't that way in practise though.
seeya,
Shane L.

fastandfurryous · Post by **fastandfurryous** » 08 May 2005, 06:53

So it would seem that after the retrograde step from CX/BX brakes to Xantia brakes, there is another (even bigger) retrograde step to the C5 brakes. It strikes me that Citroen are abandoning their heritage of being different and interesting, and making cars that blend in. what a shame.

rbruce1314 · Post by **rbruce1314** » 08 May 2005, 16:09

Just to add fuel to the fire, my current xantia with abs has a MUCH softer pedal than the previous one, which in turn was softer than the BX of course. However every system has been smooth and totally progressive and I have felt confident in any situation with any of the systems. To be honest I would not want to modify mine- if it aint broke dont fix it.

citronut · Post by **citronut** » 08 May 2005, 16:19

yep thats since pug took them over aint it,they are fast becoming euoro boxes

Mandrake · Post by **Mandrake** » 08 May 2005, 16:20

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by FrenchLeave

Mandrake, I am interested in sadism, bestiality and necrophilia, am I flogging a dead horse? I may well be on this subject but Kowalski and Panjandram are right. Any coil spring with equal coil spacing obeys Hookes Law and is linear in its compression/load relationship. As you probably know, they are categorised by that relationship, for example as a 100N/cm spring. Taking the example of a 100N/cm coil spring, if you apply a force of 100 Newtons to one end, the spring will compress 1 centimetre, a 300N force will compress it 3cm and a 10N force will compress it 1mm. At the same time, the 100N, 300N or 10N force will appear at the other end of the spring (in our case at the doseur valve). That is what springs do. If the spring becomes fully compressed (coil bound) before the maximum workable force is applied, there is of course no more movement available and you then revert to the old Citroen system. It would seem from your description that that is the situation that prevailed with your car and another solution would have been to fit a stronger spring.
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Sigh.
Before going any further, can I ask if you've actually tried this modification ? And not just for a drive around the block, but a week or two of driving ? Or are your objections based purely on theoretical ideas of how a spring should behave ?
If not, since its such an easy change (literally 5 minutes) and so easy to change back again, why not give it a try, and then try to work out a new theory to explain the observed behaviour.
(I wonder how many of the disenting voices in this thread have actually TRIED the modification ? None, is what I'm guessing...it's probably more a case of "well the brakes seem fine to me, what are you people on about?", without even giving it a chance and without realising what they're missing)
It's not just "my" car (it's my fathers in fact) but the other Xantia's I've driven are exactly the same. His is a 1994 2.0i SX Auto, another one I drove just recently (in mint condition) was a 1997 2.0i VSX Auto. Both cars brakes behaved exactly the same. I'm not saying that ALL Xantia's are the same however, and in fact it sounds like the Activa might have a much stronger spring or no spring at all. (Surprise surprise)
I didn't try modifying the brakes on the 1997 VSX as it isn't mine to modify [:D]
It's very simple - the spring introduces a highly non-linear travel vs pressure relationship. Your brain/foot expects a linear relationship between how hard you push and how far you push, and it calibrates your pedal effort based on that.
In particular when you push the pedal to the point that the spring is fully compressed and the housing can move no further you get a very abrupt change in the pressure for very little movement.
Besides, pratical springs only approach linear behaviour over a small range of operation, they are NOT linear over their entire range from unloaded to fully compressed, they are only approximately linear when the material is compressed into its elastic range. This is not the case with an untensioned coil spring. (Which is what is in this device)
Until enough force is applied to make the spring start to compress, it is not in its elastic range, and therefore does not have a linear force to compression relationship. Go look it up if you don't believe me.
This is precisely why torsion bar suspension has preload so that even in the fully extended position the bar is still operating in the elastic and therefore relatively (but not completely) linear range. If it was allowed to fully relax at the extension end of the travel it would become highly non linear at that end.
Sorry, but both your reasoning, and your application of Hookes law are flawed.
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For those who prefer the direct acting type of brake, you have provided a solution and have, I'm sure, earned the gratitude of a lot of people.
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Well I didn't come up with the idea at all, I'm just someone agreeing with the whole approach. You did read all the thread, right ? [:p]
Regards,
Simon

Mandrake · Post by **Mandrake** » 08 May 2005, 16:23

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by DoubleChevron
If I was to buy another Xantia I'd remove this spring before even driving it home (yes I consider it that dangerous).<hr height="1" noshade id="quote"></blockquote id="quote">
As I'm Xantia hunting at the moment I was just thinking today along exactly the same lines - making a copy of the copper tube we put in the pedal linkage of my Dad's Xantia and taking it with me when I go looking to buy one, as it will almost certainly involve a trip of 160Km+ to drive it back home, and I would be wanting to replace the spring before the journey home....[:D] (it's so easy to do on the spot too...)
Regards,
Simon

Mandrake · Post by **Mandrake** » 08 May 2005, 16:44

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Panjandrum

Much eccentric speculation here. The theory about springs that don't compress at all under light pressure, then absorb a lot of additional pressure without passing on any additional force to the doseur before becoming fully compressed doesn't make sense to me?
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Read my message again, I'm not saying that it can magically absorb force without passing it on. Of course it can't. Force in = force out, but thats not the issue.
The issue at hand is pedal travel vs force applied. And by the way, springs are not totally linear devices, as I mentioned in a previous post, an uncompressed coil spring is not in its elastic range, and therefore it takes significant initial force before the spring will START to compress. Once it is well into the elastic range, it will be somewhat linear. (But even then, a spring is never truely linear)
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But, if there is something catching, or sticking, somewhere that would be different.
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Ah, you mean like a dry, unlubricated plastic slide housing made to very rough tolerances that is under a great deal of pressure from the leverage of a brake pedal, and has no guide to avoid it from twisting sideways under pressure and binding ? [:D]
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Like I said before, my Activa goes through the full range of braking effect smoothly with minimal pedal movement - I had a good try today just to check I wasn't imagining things.
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How much pedal travel ? "Silly spring" Xantia's have well over 50mm of pedal travel from the brakes first applying to a full crash stop. Older Citroen's, or Xantia's with the silly spring removed have about 10-20mm.
It sounds like the Activa either has a much stronger spring than the standard models, or no spring at all. Care to pull out the spring housing and have a look for us ?
<blockquote id="quote">quote:<hr height="1" noshade id="quote">
Is there any chance that replacing the spring with a solid object might be disguising some other underlying problem?
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Such as ?
Regards,
Simon

goldfinche · Post by **goldfinche** » 08 May 2005, 17:19

Hello I apologise because this is about a BX but is I think topical.I have a automatic BX Diesel. The brakes have always been what I would call most un BX like.Brake gently fine, controlible,. brake harder and initial braking OK then almost like brake fade and the feeling that you were not going to stop, press harder HARDER and stop with nose against windscreen waiting for the chap behind to join you in the boot.
New doseur,pads,discs,LHM etc. Still frightening.
Pedal box breaks split down side as usual, it has a funny plunger thing and you can compress it with your hand, feels not quite right and variable, the plunger can tip and jam so you can have zero play or nearly a quarter of an inch.Cant be right remove plunger (metal and retained by peened in washer.)find a RUBBER block, this is removed with a lot of questions about it's parentage and a metal plug inserted in it's place.
Braking is now controllable and BX like. The pedal was most certainly original and as manufactured.(Known history.)
Was Citroen playing about for a period??!!Who knows.
Whats in your pedal?
Cheers
Geoff.