Interesting XM review / test

[russ92xmsed]

What about wheelbases. That must dictate boot size. The XM has a very large wheelbase. Almost 3000mm, clearly interior space was more important to Citroen than boot space...although I think it is plenty too.

I did have somewhere a print out of the XM measurements from when I was at Uni. There is a company called Auto Graph I think, who take new cars and critically measure everything about them. So you get very accurate drawings of the cars with a list of stats. See if I can dig it out.

[casalingua]

The average boot volume in the 1991 "Car" Giant Test was 12.56 cubic feet or 495 litres.

[russ92xmsed]

I found the info I was on about. But only the back page, but this is the most important. Lost the front which had large ergonomic drawings of the car which is a shame. It'll be about, if found i'll post it up!

[casalingua]

Thanks for that. I can't quite read the luggage capacity. What does it say?

[russ92xmsed]

Sorry it is a bit faded on the print out. This is what is says.

Lift over height 632 mm / 24.9"
Luggage Capacity 341 CU DM / 12.0 CU FT

Luggage capacity measured with boxes...
Box number Number used
1 1
2
3 2
4 1
5
6
7
8
9
10 6
Not sure what the box sizes are. Doesn't say!

[casalingua]

The system used here only makes sense in the context of other vehicles. The figure for the XM works out as roughly 340 litres. If you use standard boxes then most tests will be an underestimate. But there is the possibility some makers might have optimised the boot to take lots of standard boxes and leave no odd gaps. Possible? Yes. Probable, no. The only reliable method is to measure all the dimensions and calculate the volume mathematically. The box system has the appearance of objectivity and repeatability but seems to me to run the risk of underestimation. The 460 litre figure is quoted every where so I will go with that.

[White Exec]

Front suspension performance over sharp dips is as much affected by tyre type, wear and pressure as it is by sphere orifice changes.
The tyre sidewall stiffness is the first vertical spring in the suspension.
I would be interested in Dean's thoughts about how much difference thin sidewall tyres make to an XM.
In my opinion drilling out spheres is delicate engineering job best avoided because of the risks from residual swarf.
Particularly so where spheres to the chosen altenative spec are available new for around £25.

John

Have just read my way through this old, interesting and extremely rangy topic. I think John is absolutely right with respect to tyre choice and condition.

The tyre is the first item of "bump absorption" in the system, and is affected by make, tyre-model, age, and pressure. In my experience, new tyres always feel more impact-absorbing than old ones (rubber presumably more flexible and less sun-baked).

Tyre fitters usually over-inflate (as well as over-tighten), and inflating to textbook pressure seems essential. Even a couple of psi over will bring on unpleasant low-speed/ridge discomfort in the XM, and reducing this to textbook usually fixes it. Important to inflate to correct figure when tyre is cold (20degC is usually quoted for pressure), and to avoid inflating to this figure in the depths of winter. Slight under-inflation, by just a couple of psi, although not recommended for tyre-wear or lateral stability, does seem to improve crashiness.

I've always found the XM to be very happy on Michelins: first Primacy, then Energy. Both have very supple (but none too gash-resistant) sidewalls.

After the tyre, sudden impacts are next fed to the "unsprung lump" which is the wheel/hub/strut assembly, and there isn't much we can do about any of this. Most of us ride on alloys, and I don't know whether a heavier steel wheel (not that I'm suggesting it) would behave differently. A heavier wheel would, in any case, just make a supple impact-absorbing tyre all the more important.

What's left of the impact is then transmitted to the suspension sphere, where the decision about volume/pressure/orifice comes in. I agree with John about the risks of drilling and debris and possible problems. I guess that increasing sphere volume would provide extra "comfort" - i.e. a small amount of additional suspension travel - possibly if coupled with a reduction in sphere gas pressure. But isn't this what Hydractive provides by switching-in the centre spheres (greater sphere/gas volume and elasticity) ?

So, are we just back to the old question of some of us wanting to be able to switch in Comfort (i.e. NOT Sport) at will, or maybe semi-permanently? [ Back to the electronic problem, then. Can anyone provide exact details of the waveform and voltage of the solenoid driver? ]

None of us complains much about the benefits of the car being in firm/Sport/4-sphere mode when it's being thrown around a twisting road, cornering, or braking hard. This is just all very welcome "stability", so being permanently in 6-sphere soft mode is probably not to be recommended.

Back to the damping orifices . . . Enlarging these too much would be akin to driving a conventionally sprung car with ineffective or no dampers. Given a car in this condition, hitting a sudden ridge/manhole/pothole (the sort of thing we complain about on the XM) would still produce an unwelcome thud, UNLESS THE TYRES COULD ABSORB IT.

Several of us have commented that some extra weight in the car definitely helps, be it passengers, cement bags, heavier engines, or full tanks.

So why was the BX such a good ride? Michelins - yes. Heavy car? - absolutely not. Lots of roll? - possibly (by XM/Hydractive standards). Unacceptably so? - no. Macphersons? - yes. Long-travel suspension - definitely.

Still without a definitive answer, but I think the tyres are very much a key.

Chris

[Dieselman]

BX use large spheres with large damper holes, the XM has tight suspension to stop excessive roll, thus transmits more bumps.

XM corner sphere damper orifices are between 0.5mm to 0.7mm on the rear and 0.6mm to 0.8mm on the front. BX are1.4mm rear and 1.7-1.8mm, front, so plenty of additional damping on an XM, even allowing for the 1.2mm centre dampers.

Having just been checking sphere tables, and performing a calculation, I think a Hydractive XM has the same gas volume/spring rate as a CX across the front axle, but smaller damper orifices, a non-hydractive car has less volume and significantly smaller damper orifices.

The unknown is whether the sharp bump damper springs are different between different spheres, or is just the centre hole sized to suit.

[White Exec]

It would be interesting, then, to see what might happen to the ride if the corner spheres on an XM were significantly enlarged - say to 2mm - just for experimental purposes. I presume the Hydractive control system would still operate normally to prevent body roll, but we could find out exactly what would happen to the ride quality, particularly on a sharp bump. This would also allow further investigation of the contribution of the tyre.

Someone must have a spare S2 to play with . . .

Chris

[captainhaddock]

Uhm, my xm came with comfort spheres and it wasn't a good idea. Front ones are replaced now but rear are still on it. After loosing some of it original softness it's just about OK now I think, but the Diesel drives better even though the hydractive is disconnected.
Talking about diesels: why is the tyre pressure on the Diesel the same as on the much lighter 2 liter? You can see the tyres with the heavy diesel lump between them, look much softer than the petrol. That doesn't really make sense to me. What is the tyre pressure for the even heavier 2.5 or V6 engine, stil 2.3 bar?