Terrible Xantia brakes

Mandrake · Post by **Mandrake** » 06 May 2005, 11:30

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

DoubleChevron, I thought your description and illustrations (particularly the moving picture) were brilliant. I do, however take issue with your statement that instead of applying braking force to stop the car you are just compressing the @!*~ spring. As Newton said, for every action there's an equal and opposite reaction. Thanks to that action and reaction, the force you put in at one end must come out at the other. As XantiaV6 said, the same force on the pedal provides the same braking force. The only difference is the amount of pedal movement.
<hr height="1" noshade id="quote"></blockquote id="quote">
If only it were that simple. Yes in principle the force is the same on each end of the spring, but you introduce severe non-linearity into the action of the brake pedal, and you lose all sense of feel of the braking.
At very light breaking you are directly coupled to the brake actuator up until the point you press hard enough to begin compressing the spring. Now suddenly you have a lot of extra pedal movement with very little increased braking. Finally when the spring is fully compressed you're back to directly coupled again.
VERY indirect and non-linear. This is what causes the deadband mentioned. It's extremely annoying, and I find with the spring in place that you're constantly going from under correcting to overcorrecting, making a smooth pull up at a stop sign nearly impossible. (Especially on an automatic where you're fighting against the auto as well)
The second problem is the loss of feel - you have no feedback from the brakes as to how much pressure you're applying. The brake actuator is actually a feedback system - you press on the control shaft which opens a valve, which increases the pressure to the brakes, but the pressure to the brakes is also tapped and applied to the piston at the end of the brake actuator which pushes BACK at your foot again.
There is always this equilibrium between your foot and the hydraulic pressure pushing back at you, and thats one of the things that makes Citroen brakes so responsive and precise. With the spring in place you are decoupled from this feedback mechanism meaning that a given foot position allows the reaction from the control acutor to compress the spring rather that directly push your foot back.
<blockquote id="quote">quote:<hr height="1" noshade id="quote">
I understand that Citroen introduced the spring to make brake feel more acceptable to non Citroen drivers. As Andre Citroen would have put it "Chacun a son gout". (Loose translation, whatever lights your particular fire.)
<hr height="1" noshade id="quote"></blockquote id="quote">
Unfortunately in the process they threw the baby out with the bathwater, and turned one of the best braking systems ever invented into a pile of junk that doesn't feel as good as an ordinary Japanese car, thanks to the "silly spring"(tm) [:(!]
No, Citroen screwed up on this one I'm afraid, fortunately its very easy to fix...
Regards,
Simon

Kowalski · Post by **Kowalski** » 06 May 2005, 15:25

I have factory fitted springs under both of my Xantia brake pedals and have no problem braking smoothly. I can control and compensate with ease, stop quickly when required or gently. Frankly if you can't, you're either not a good driver or there is some other problem with your car, perhaps the plunger on yours was sticky or the spring tired / bent / broken. As for linearity, Hooke's law applies to springs, even on Citroens.
There is plenty of feedback, what is the difference between applying a force to a solid object and applying it to the end of a compressable spring? The spring moves as you apply force to it, i.e. there is movement which is a form of feedback. The feedback is designed to be compaitable with the way the human body works.
I feel feedback, for example holding the car with the brakes and putting the handbrake on, I can feel the handbrake going on through my right foot. It works for me, isn't broken and doesn't need fixing.

Panjandrum · Post by **Panjandrum** » 06 May 2005, 16:03

Ha, Kowalski, I'm with you.
I was wondering, reading all this, if my Activa has a stronger spring or something? I can get max brakes with a bare toe in summertime - totally smooth variation from no brakes to ABS on - hardly any pedal movement. The only time I get a kind of panic attack - heave on the pedal - is if I set off before the red light has gone off, or if driving any other car.

littleredrooster · Post by **littleredrooster** » 06 May 2005, 21:58

Just done this mod and would like to say that it has transformed the brakes. Modulating the brakes from a high-speed stop is now a doddle, whereas before the mod it was a jerky affair with that confounded spring compressing and releasing as body weight under deceleration took over.
Bloody brilliant.
From previous comments, is it possible that some have a stronger spring than others, making it less obtrusive? Mine was easily compressed between thumb and forefinger.

daktari · Post by **daktari** » 07 May 2005, 01:43

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

I have factory fitted springs under both of my Xantia brake pedals and have no problem braking smoothly. I can control and compensate with ease, stop quickly when required or gently. Frankly if you can't, you're either not a good driver or there is some other problem with your car, perhaps the plunger on yours was sticky or the spring tired / bent / broken. As for linearity, Hooke's law applies to springs, even on Citroens.
There is plenty of feedback, what is the difference between applying a force to a solid object and applying it to the end of a compressable spring? The spring moves as you apply force to it, i.e. there is movement which is a form of feedback. The feedback is designed to be compaitable with the way the human body works.
I feel feedback, for example holding the car with the brakes and putting the handbrake on, I can feel the handbrake going on through my right foot. It works for me, isn't broken and doesn't need fixing.
<hr height="1" noshade id="quote"></blockquote id="quote">
Quite possibly a fault with the spring/assembly then, as i dont have a problem with the braking on my other two cars. (Volvo & Toyota.) I did notice before i did this modification that when applying the brakes lightly with a constant pressure at slow speeds the feel of the car was similar to that of having a warped disc. This has now disapeared and the braking is smooth and constant. If i were so inclined i might well buy a new assembly to see if it were any different. But i`m more than happy with my brakes as they are now.

Mandrake · Post by **Mandrake** » 07 May 2005, 04:22

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

I have factory fitted springs under both of my Xantia brake pedals and have no problem braking smoothly. I can control and compensate with ease, stop quickly when required or gently. Frankly if you can't, you're either not a good driver or there is some other problem with your car, perhaps the plunger on yours was sticky or the spring tired / bent / broken. As for linearity, Hooke's law applies to springs, even on Citroens.
<hr height="1" noshade id="quote"></blockquote id="quote">
Nice try Kowalski [:D]
But I've driven lots of earlier Citroen's before (plenty of GS's, and CX's in particular) so I know what are good brakes and what are not. And standard Xantia brakes are NOT good brakes. It's nothing to do with my driving.
The first time I drove a Xantia my first comment to my father sitting next to me was "nice car, but I don't like the action of the brakes, they're terrible". He agreed with me as soon as he got to drive it, and he was a motor mechanic working exclusively on Citroen's as early as the 50's, and was around in the industry when the first ID19 with the high pressure brake system rolled off the lines...
<blockquote id="quote">quote:<hr height="1" noshade id="quote">
There is plenty of feedback, what is the difference between applying a force to a solid object and applying it to the end of a compressable spring? The spring moves as you apply force to it, i.e. there is movement which is a form of feedback. The feedback is designed to be compaitable with the way the human body works.
<hr height="1" noshade id="quote"></blockquote id="quote">
No its not, its designed to make the pedal spongy so that non-Citroen drivers won't be immediately turned off by the stiff sensitive brake pedal. The problem is that the spring makes the relationship between pedal travel and pedal force highly non-linear. Think about it.
It's no use invoking Hookes law without thinking through what is actually happening with the spring. For starters the entire range of pedal pressures are NOT within the linear action range of the spring.
Initial braking does NOT compress the spring at all, so the travel vs pressure relationship is that of the brake control unit itself, eg nice and sensitive with hardly any travel. Then you reach a threshold pressure where the spring starts to compress.
Because the spring is relatively soft compared to the leverage of the brake pedal, you now get a LOT of extra pedal movement for very little change in pedal pressure. This is the "deadzone" people complain about, and with the spring that was in my Dad's Xantia it starts right at the point where you're trying to do a gentle controlled stop at a stop sign.
Finally you reach a point where the spring is fully compressed and the housing can't move any further, and suddenly you're back to very little pedal travel for a large change in pressure again, and this is the crash stop point alluded to in the first message.
Sorry, but this is *NOT* a linear brake pedal action.
On a standard Citroen brake system the pedal travel is very short, but the relationship between travel and pressure is VERY linear, which gives very accurate control.
Even a conventional Vacuum servo brake system has a pedal travel whose action (movement vs pressure) is a lot more linear than a Xantia with a spring, and thats a sad state.
If you really think the Xantia's unmodified brakes are that great, go and drive a really good GS, or CX, then come back and tell us the Xantia brakes are great.
The CX brakes are in a different world. Not surprising when it doesn't have the "silly spring" and has 4 piston (2 per pad) arangement instead of a puny single piston caliper arangement. Or the GS with its 2 piston per disc on 11" discs mounted on the gearbox.
The brakes in the Xantia are definately a backwards step in technology compared to older Citroen's, but I will admit that with the spring removed they do still seem to work very effectively.
Regards,
Simon

Mandrake · Post by **Mandrake** » 07 May 2005, 04:39

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

Just done this mod and would like to say that it has transformed the brakes. Modulating the brakes from a high-speed stop is now a doddle, whereas before the mod it was a jerky affair with that confounded spring compressing and releasing as body weight under deceleration took over.
Bloody brilliant.
From previous comments, is it possible that some have a stronger spring than others, making it less obtrusive? Mine was easily compressed between thumb and forefinger.
<hr height="1" noshade id="quote"></blockquote id="quote">
From the differing opinions in the thread I would have to say yes, it looks like different years/models did come with different strength springs, and a much stronger spring which was not able to fully compress even in a crash stop would probably give a much better action than the original soft spring.
However I still hold that the very presense of the spring is a mistake. Citroen had high pressure braking systems whose control actuator went basically unchanged from 1956 to around 1990. Kowalski says "if it aint broke dont fix it", but I would argue that its Citroen who "fixed it" when it wasn't broken [:D]
Regards,
Simon

citronut · Post by **citronut** » 07 May 2005, 04:46

thats ok frenchleave with the amount of post gets sent to this forum there is bound to be some dupplicated dupplicated regards

Mosser · Post by **Mosser** » 07 May 2005, 06:17

Wow, what a difference this mod has made to my brakes, they were absolutely terrible before, and i used to dread braking from a moderately high speed as the non linear brake pressure required would make me look like a learner again, but the brakes are at least as good as my old trusty BX brakes were, i think it will add a bit more safety and enjoyment to my xantia driving now,
And, if you havent got a stack of washers handy then 15mm copper pipe fits perfectly !!, or you could just fill it up with araldite instead, both would lock it up solid

Mandrake · Post by **Mandrake** » 07 May 2005, 12:11

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Mosser
And, if you havent got a stack of washers handy then 15mm copper pipe fits perfectly !!, or you could just fill it up with araldite instead, both would lock it up solid
<hr height="1" noshade id="quote"></blockquote id="quote">
We also used thick wall 15mm copper tubing. Very easy to cut and file to the perfect length, and readily available. I wouldn't recommend the araldite solution, as it's rather permanent [:D]
Regards,
Simon

anybloodybody · Post by **anybloodybody** » 07 May 2005, 17:35

I did it! I normally struggle just to change a wheel.
Don't forget to file round the ends of the pipe. You wouldn't want the pipe cutting through the plastic ends.
This has got to be one of the most affective mods for time & money.
Suits my driving style

Panjandrum · Post by **Panjandrum** » 07 May 2005, 23:27

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?
But, if there is something catching, or sticking, somewhere that would be different.
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.
Is there any chance that replacing the spring with a solid object might be disguising some other underlying problem?

JohnCKL · Post by **JohnCKL** » 08 May 2005, 01:01

My 1996 Xantia 1.9TD braking is fine too with the spring attached. I can easily stop with controlled motions, even slow braking is in control. Those who changed their springs to solid tube, what year/model do you have and those that find their springed brakes are fine, what year/model are yours?
Original spring brakes - 1996 Xantia 1.9TD SX
Modified solid tube brakes -

fastandfurryous · Post by **fastandfurryous** » 08 May 2005, 01:20

Does anyone know if the modern C-model Citroen's have this spring arrangement? Certianly it seems like a horrible and very nasty modification to an otherwise excelent braking system.

Kowalski · Post by **Kowalski** » 08 May 2005, 01:31

The C5 has conventional brakes, i.e. master cylinder with servo, no doseur, no spring.