Xantia Strut design flaws ?

Mandrake · Post by **Mandrake** » 01 Sep 2005, 14:22

Hi All,
During my ongoing attempts to get the best performance out of the suspension of my HA2 Xantia I've been doing a lot of thinking about the front strut design and come to a few nasty conclusions about the basic approach.
I don't think there is anyone that would disagree that a McPherson strut is inferior to the double wishbone design in the CX or GS, but there is a serious problem that I hadn't even considered until I started observing the behaviour of my car -
Because it is a slide bearing instead of a rotating roller bearing, any side thrust increases the inherant friction of movement in the bearing, making it more prone to "sticking" in the face of small movements.
This might not seem important until you realise that any time the car is cornering, accelerating, or braking, there is quite considerable side thrust being placed on the bottom half of the strut assembly.
Recently after greasing the struts I was quite amazed at just how much the harshness of ride reduced, showing that any parasitic friction worsens the ride and small bump stability dramatically. (Suspension doesn't move for small bumps, only bigger ones)
The only problem is that while the ride is fairly good during straight ahead driving, quite a bit of that small bump harshness returns as soon as you corner moderately hard, or accelerate or decelerate heavily.
The extra side thrust increases the static friction of the slide bearing and small movements can't be absorbed - thus harsh ride.
This phenomenon does NOT happen with the rotating bearings used in double wishbone designs, where side thrust has almost no effect on the amount of friction in the pivoting direction, unless the bearings are stuffed of course.
Partly I think this effect is due to insufficient lubrication being in the strut, but the rest of the effect would seem to be a fundamental limitation of the McPherson strut arrangement, unless extreme steps are taken in the precision of manufacture and lubrication methods.
So far I have been unable to find any detailed information on the internal construction of the strut, for example how many bush faces there are, where they're positioned, oil seals, etc... does anyone have any detailed diagrams of the internals of them ? (I've seen the BX ones but not the Xantia)
While getting grease in through the top by lubing the top shaft section of the strut makes a big difference, my feeling is that its not possible to get enough grease in that way to get a 100% satisfactory result.
The ride quality still seems to vary a bit from day to day, (improves after exercising the suspension up and down over its full travel, worsens after a few days of staying at middle height thanks to anti-sink) and also seems to worsen under side thrust as mentioned above.
I suspect that the grease sneaking in past the top seal can't reach the bottom bush face in any significant amount, however does anyone know exactly where the low pressure overflow port connects to in the scheme of things inside the strut design ?
Is it possible that a decent amount of grease could be applied through this hole to reach the bottom bush ? I was thinking along the lines of disconnecting the overflow pipe, using a grease gun to squirt some in, connecting a temporary pipe to a container on the ground, working the suspension right up and right down to squeeze out any excess grease, then reconnecting the normal overflow pipe.
Anybody tried anything like this or know whether it might or might not work ? Or have any other comments on reconditioning or otherwise improving the frictional performance of the struts ?
Regards,
Simon

406 V6 · Post by **406 V6** » 01 Sep 2005, 16:59

Here's a link for the strut design (it's from an XM, but it seems to be similar in the Xantia):
http://citroeny.cz/servis/xmser/predni_vzpery.gif
Liked you post a lot [:D]

guillermo · Post by **guillermo** » 01 Sep 2005, 17:42

Dear Friends,
What kind of grease do you use to lube the strut ?
and what is the best method?
The rear cilynder need also some kind of lube ?
regards
Guillermo

jeremy · Post by **jeremy** » 01 Sep 2005, 18:30

1. The rear cylinders have negligible side thrust and are much less prone to siezure.
2. All suspension is a compromise. In fact if you think of hitting a rut in the road the thrust really is backwards rather than upwards - so most suspensions provide for travel upwards. BMC suspensions were really rather good due to the deletion of the front wishbone with its stiff bushes and its replacement by a simple arm and a rubber bushed rod that took the thrust. I can remember being impressd by a very decrepid mini we had a few years ago which in common with all its kind would ride bad roads rather well but was uncomfortable on good.
My Triumph Stag had struts, a single transverse link like a mini and a forward facing control arm (opposite way round to a Mini) and I always felt its general refinement and freedom from unnecessary intrusions was much better than my XJ6 and probably better than even the ZX which in this respect is much better than the BX.
Don't BMW incline the struts backwards?
Jeremy

Kowalski · Post by **Kowalski** » 01 Sep 2005, 20:18

The Xantia front struts aren't so different to a steel sprung macpherson strut car, the coil springs don't carry much of the thrust, its up to the struts to do that. The struts on the Xantia are much better quality than your typical shock absorber too.
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by jeremy

Don't BMW incline the struts backwards?
<hr height="1" noshade id="quote"></blockquote id="quote">
Most cars do, but only by a couple of degrees at most, this is your castor angle. It makes the steering self centre and stops the steering from feeling nervous.
Trailing arm suspension (like the Xantia/ZX/405 rear suspension) also allows the rear wheel to move backwards as it moves upwards, as well as giving anti-dive geometry under braking. Trailing arms are old hat though, now it seems complicated multi-link set ups are in fashion.

Mandrake · Post by **Mandrake** » 04 Sep 2005, 06:16

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by 406 V6

Here's a link for the strut design (it's from an XM, but it seems to be similar in the Xantia):
http://citroeny.cz/servis/xmser/predni_vzpery.gif
Liked you post a lot [:D]
<hr height="1" noshade id="quote"></blockquote id="quote">
Hi,
Thanks for the link. It looks a lot closer than the BX strut design, but there are still a number of external differences to the Xantia struts (making me wonder how many internal differences there are) - the Xantia ones don't have the top rubber snubber attached to the strut housing as the diagram seems to indicate - they are attached to the top inside of the strut top assembly.
Also that diagram shows the oilway connecting to the sphere to be in line with the centre of the sphere neck, but in the Xantia strut tops the oilway hole is right near the top edge of the sphere neck.
I can't quite figure out that internal digram either. So which parts are sliding and which are stationary ? Is the sliding surface on the inner shaft with the bushes attached to the outer tube, (seems the most logical) or the other way around ?
Does the oil flow right out the end of the centre shaft into the bottom section, and if so what are the two small bores just above this marked with arrows for ?
Where exactly does the overflow port pick up the leakage from ? It seems to be positioned near the bottom end of the strut where the high pressure oil is, with nothing at the top end of the strut to collect the leakage from there ??
I'm confused. [:D]
Can anyone give a better explanation of the details or does anyone have an exact Xantia strut diagram ? [:)]
Regards,
Simon

Mandrake · Post by **Mandrake** » 04 Sep 2005, 06:19

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

Dear Friends,
What kind of grease do you use to lube the strut ?
and what is the best method?
The rear cilynder need also some kind of lube ?
regards
Guillermo
<hr height="1" noshade id="quote"></blockquote id="quote">
Hi Gullermo,
I believe Lithium L2 grease is a good choice of grease to use, however the jury is still out on the best way to apply it. If you search for posts by me mentioning 'strut' you'll find my explanations of the procedure I used to get grease in past the top seal and while this is very worthwhile and makes a big improvement, the whole reason for this current discussion thread is that I'm not convinced that grease is getting down into the bottom half of the strut, and therefore the greasing is not 100% effective.
As for the rear cylinders - at the rear there are trailing arms with seperate rotating bearings supporting the arms, the hydraulic cylinder there is only a hydraulic ram, and it does not experience any side thrust, so the problem doesn't exist.
The equivilent problem at the rear (excessive movement friction) is due soley to failed rear arm bearings, increased movement friction with side thrust on the arms won't happen when the bearings are ok.
It's only at the front with the McPherson strut design where the same unit is both hydraulic ram, and stabalizing strut that you see the problem I describe.
Regards,
Simon

Mandrake · Post by **Mandrake** » 04 Sep 2005, 06:35

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

The Xantia front struts aren't so different to a steel sprung macpherson strut car, the coil springs don't carry much of the thrust, its up to the struts to do that. The struts on the Xantia are much better quality than your typical shock absorber too.
<hr height="1" noshade id="quote"></blockquote id="quote">
While I'm sure the Xantia struts are superior to most coil-spring McPherson struts, I wasn't comparing them to those, I was comparing them to the double wishbone designs of earlier Citroen's, against which they definately fall short of the mark...
<blockquote id="quote">quote:<hr height="1" noshade id="quote">
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by jeremy

Don't BMW incline the struts backwards?
<hr height="1" noshade id="quote"></blockquote id="quote">
Most cars do, but only by a couple of degrees at most, this is your castor angle. It makes the steering self centre and stops the steering from feeling nervous.
<hr height="1" noshade id="quote"></blockquote id="quote">
This is one of the shortcomings of McPherson struts - the fact that the direction of travel of the suspension can only ever be inline with the caster angle axis limits your ability to optimize the geometry - and leads to a suspension that can suffer from diving under braking. (Among other things)
Contrast that to the double wishbone design in the GS for example where the caster angle and suspension travel direction can be adjusted independantly - in fact they lean in opposite directions in the GS which gives it it's anti-dive geometry. (Steering caster angle is tilted backwards as normal at the top, while the arms themselves are tilted forwards)
You can even engineer the caster angle to change with the suspension travel with double wishbones. (Which from memory I think the GS does, as both arms are not inclined forward by the same angle, although I'm not sure what purpose it serves...)
Although they try to disguise it by switching the Hydractive system to hard under hard braking, there is no doubt that the front of a Xantia nose dives under hard braking then lifts when you ease off, just like most conventional cars do, while the GS has NO front nose dive whatsoever, and no liftoff after releasing..(I think the CX is the same too)
Unfortunately the McPherson strut design in the BX/XM/Xantia is definately a cost cutting compromise compared to earlier designs, no doubt brought in by Peugeots control of Citroen... [V] (The first new model designed after the Peugeot takeover - the BX, had struts...)
Having said that, I don't see why they can't be made to work as well as possible, within their limitations, for example by finding out how to best lubricate them...
Regards,
Simon

Dave Burns · Post by **Dave Burns** » 04 Sep 2005, 16:02

The Xantia track control arm does not pivot at right angles to the suspension strut, if you take a look you will see that the rear bush of the arm is higher than the front one, a certain amount of anti dive has therefore been included in the design.
The vertical travel of the wheel is at right angles to the pivot line of the arm, not the strut.
Dave

FrenchLeave · Post by **FrenchLeave** » 04 Sep 2005, 21:28

The oil must be fed below the large piston at the bottom of the strut, otherwise feeding more oil in would collapse the suspension instead of raising it. I may well be wrong, but it seems to me that as the rubber bellows, and presumably the overflow collection pipe, is at the top of the strut, the probable scenario is that a controlled leakage takes place through the two holes just above the piston (arrowed). This would be specifically to lubricate the three(?) bearing surfaces. What I thought at first was hatching of the centre bearing seems, on closer examination, to be a spiral leakage path to conduct the oil to the top bearing and thence to the overflow collection pipe. The natural pumping action of the strut would assist in this process. If my reasoning is correct, Citroen have already covered the lubrication aspect - it would surprise me if they had not!

FrenchLeave · Post by **FrenchLeave** » 04 Sep 2005, 21:43

With reference to the, suggested, poor design of the McPherson strut; I remember that when it was first introduced (Ford Consul, 1951) one of the claimed advantages was the wide spacing of the attachment points, thereby spreading the load over as wide an area of the body as possible. It was never fitted before the advent of integral body/chassis construction.
I'm not sold on your claim of cheap construction - can you substantiate this? It is a fact that a large number of expensive cars, some with renowned roadholding, have opted for this design.

fastandfurryous · Post by **fastandfurryous** » 04 Sep 2005, 23:54

The main benefit that MacPherson struts give is a high roll centre, and wide load spacing. The higher the roll centre compared to the centre of gravity, the more stable the car is. Same is true of the width of the load spacing: the further apart you can place the load bearing points on the chassis, the less roll is experienced.
Never did a car make this more obvious than a Talbot Horizon. It had very well designed suspension, with upper and lower wishbones, full independent trailing arms (and twin-piston fixed calipers, but that's another story). Only problem was that the load was taken by torsion bars into the lower suspension arms at the front. This mean that it had a very low roll centre (about 18" lower than a MacPherson strut car) and quite narrow load spacing (again, possibly 12-18" narrower. Because of this, they had horrendous body roll, and you could easily loose control by throwing it about too violently.
But the point about cost is right. MacPherson struts are also cheaper than most other suspension systems.
It's worth noting that (as far as I'm aware) Mercedes-Benz DON'T use MacPherson struts.

jeremy · Post by **jeremy** » 05 Sep 2005, 13:57

Using McPherson struts for the front suspension is also quite handy when fitting a transverse engine with gearbox on the end! Remember the wonderful suspension arrangement of the old Rover 2000 (P6) where they tried to achieve a wide engine bay as they were dreaming of gas turbines at the time. (Steering box by the windscreen, backward facing springs feeding the loads into the shell behind the glovebox via bellcranks etc. - and if I recall correctly - single acting shock absorbers!)
jeremy