How to improve Xantia braking system?

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Thunderbird

Post by Thunderbird »

Comparing Xantia HDi 2.0 110 (which is representative of every other Xantias) and V6 Activa, we have:
- Same tyre size (205/60R15)
- Same discs size (288mm discs in the front)
- Different suspension (Activa vs hydractive II)
- Different ABS (Xantia Mk2 V6 Activa has a more expensive and sophisticated ABS)
It is known that a more fast/effective suspension increases breaking distance; as well as a better ABS. The conjunction of these two factors leeds to a much improved breaking system, despite the tyres/discs.
Since it is not possible to change non-Activa's Xantias suspension, and may also be difficult and rather expensive to replace the ABS, I believe it won't be possible to improve dramatically the Xantia suspension. Still, there could be a way of improve it a little.
Breaking distances are the right way to measure how good breakes are. For instance, my wife's Renault Twingo breaks respond much faster than Xantia ones and give the idea they are superior, but they aren't - they make over 80 meters from 130km/h to 0km/h.
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Post by uhn113x »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">It is known that a more fast/effective suspension increases breaking distance; as well as a better ABS. The conjunction of these two factors leeds to a much improved breaking system, despite the tyres/discs.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Don't you mean <b>decreases</b>? You don't want to increase braking distance. What is a 'fast' suspension? [?]
BTW, it is 'braking' - 'breaking' is something you do not want to happen at all [}:)]
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Post by ItDontGo »

Firstly you shouldn't be driving fast on the road anyway. If you fry the brakes on a road car then you are driving dangerously. If you are talking about driving on the track then why on earth would you be doing it in a Xantia?
The Activa will brake better as you wont have to build up the load in the suspension for as long a period of time as in the non-active cars because the suspension as active.
To brake most effectively in a car with suspension you build up smoothly on the brake pedal and the rate of this is dependent on the stiffness of the suspension (this is why the Activa will be better). If you simply slam the brakes on you will not get the maximum performance out of the brakes - just like if you jerk the steering wheel you will not get the maximum amount of latteral grip from the car in a corner.
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Post by Thunderbird »

Sorry for the syntax and other english errors.
Consider just the two following example situations where better brakes would be welcome:
- All of a sudden someone/something appears on the road right in front of you (a person, a kid, a dog, other car, etc.).
- You are somewhat distracted and don't keep the safe distance from the car ahead and it brakes hard and suddently.
Etc. - there are many real life situations where you're not racing on the road and come to need better brakes.
From my experience, I've concluded that the easiest way of improving Xantia brakes is to keep the hydraulic system in good condition and use good tyres. Different tyres in the same car can produce differences in braking distances of 25%!!!
Citroen is not doing their homework - C5 seems to brake even worst than the Xantia!
I don't have the values, but I've seen a comparison between Renault Laguna mkII and Citroen C5 and the Laguna brakes much much better!!!
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Post by pwatson »

I'd be interested to know where you got your previous figures for braking distances from. Had a quick look on the web but can't find any
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Post by Thunderbird »

The figures come from a french magazine I have at home.
I believe it is L'autoJournal, but I am not sure as I buy several french car magazines. If you want I can give you the exact description and number. It's one of the french magazines that, in each review they made, indicate the braking distances.
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Post by alexx »

About Laguna mkII and C5, I also compared C5 2.016v with BX and Laguna on the same part of the road, braking from 80 km/h to 0, and C5 stopped 0.1s earlier (about 2m difference) than Laguna and BX. Laguna had Michelin Pilot Primacy 205/55 R16, C5 Michelin energy 195/65 R15
Laguna also had awfull feel on the brake pedal. After Megane mkII, most boring car I'v ever driven.
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Post by Kowalski »

I've had two Xantias and am happy with the brakes on both of them, they are vastly superior to any other car that I have driven. My brakes are all standard parts but I keep them in good condition, i.e. when I changed pads or discs I lubricated the front caliper pins etc.
The only reason that Xantia brakes would perform poorly compared to other cars would be that something was not right (i.e. not working properly to specification) with the brakes, tyres or suspension systems
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Post by tomsheppard »

Braking distance is proportionally inverse to traction. If the tyres don't stick then the ABS will come in and extend the braking distance, compared to good boots.
Xantias stop well, like all hydro citroens unless the pads are shot/cheap. Use the best pads you can get and good tyres of the right section.
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Post by Thunderbird »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by tomsheppard</i>

Braking distance is proportionally inverse to traction.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
What do you mean by traction? If the tyres have better grip they brake better, with or without ABS. If grip is zero, the car doesn't brake at all (like driving on ice with normal tyres!).
With ABS activated the car only brakes on the intervals when the wheels are locked. Better grip, lower braking distances.
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Post by Kowalski »

With a good ABS system, the wheels are never actually locked unless you are in very poor grip conditions. The ABS system detects wheel speed, and works out from the 4 wheel sensors how fast the car is going. If one wheel slows down beyond a threshold the system detects that it is about to lock and reacts before it actually locks by reducing brake force. If the system is fast enough, the wheel will never actually lock, it may be turning more slowly than the road is moving.
Fundamentally, although a wheel can go from full speed to being locked pretty quickly (without ABS), it does not lock instantly.
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Post by tomsheppard »

Yes, but a wheel with a low grip tyre is more likely to lock and thus trigger the abs than a wheel with good traction. The most important part of the braking system is the tyre contact patch. The ABS only works to release the wheels from the locked condition, so your understanding of what is going on is flawed. Incidentally, as many of our Scandinavian members will tell you, it is entirely possible to brake and stop on an icy surface. It just takes appropriate driving skills, that's all.
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Post by Kowalski »

An ABS system that lets wheels lock solid and reacts to that is not a good ABS system. An ABS system that pre-empts wheels locking by detecting the wheel slowdown and releasing brakes before the wheel has stopped turning is a good ABS system. If the ABS system lets your wheels lock you can't steer.
I found that in snow and ice conditions my ABS on my Xantia did absolutely nothing useful at all, it allowed the wheels to lock which meant no steering. The braking force available would be very big, I was travelling slowly and the wheels would lock extremely quickly, quicker than the ABS system could react, also the wheels would take a relatively long time to start turning again.
In better conditions, I have been able to brake and steer at the same time due to the ABS doing its job properly and preventing the wheels from locking rather than trying to react to locked wheels. There is a big difference between preventing wheels from locking and reacting to wheels that have locked.
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Post by Thunderbird »

Kowalsky,
I agree 100% with you! I've read a few articles about this subject that point the ABS as one of the most critical factors in braking distance.
ABS allows you to steer when you brake hard - that's its purpose - but a cheaper or first generations ABS increases braking distances a lot!
In tests, dummy driver (just pressing hard on the brake pedal) using expensive/last generation ABS beats a F1 pilot skills in the same car with no ABS, achieving lower breaking distances.
It has a powerful computer analysing and controling all the data (like speed on each wheel) provided by several sensors and actuates independently on each wheel (can be braking hard on front-left, not braking on front-right, slighty breaking on rear-left and breaking half-hard on rear-right).
Most cars just don't have it because it's expensive. Xantia's ABS (non-Activa) is a crap! Any skilled driver would prefer the Xantia without ABS and would pump the break pedal when necessary to steer while braking hard.
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Post by Kowalski »

The early Xantias have a different ABS system to the later ones, and the post facelift ones have a different system again.
The ABS my '97 Xantia has is very effective except if the conditions are VERY poor i.e. snow and ice. It works effectively even on gravel and that is far from being an easy surface to cope with.
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