Front/rear Weight Distribution

Hi George,

Following on from my last post, whatever the constant pressure that is needed to raise the car, that pressure will of course be applied to the hydractive sphere as well as the corner ones. If the pressure is greater than the hydractive charge pressure then the hydractive diaphragm will be displaced until that pressure is reached, but all that requires is extra fluid flow.

I have assumed that the hydractive sphere is only slightly, if at all, compressed under static conditions at the lightest vehicle loads, which I took as a 70kg driver plus 5 gallons of fuel (13 gallons now to make use of Dean's work). This seems logical to me as the corner spheres are capable of dealing with leg extension and for max. comfort on bump absorption you need largest sphere volume, so why compress the centre sphere at static loads? I may be wrong but it makes sense to me.

Aerodynamica wrote "and if it was higher than the 70 odd bar pressure of the middle sphere but unconnected to it in sports mode then as soon as it connected the car would fall to the floor and then be replenished with more fluid from the H corrector." Doesn't that support my theory?

Derek

This post has been edited by DerekW on November 30, 2008 07:48 pm

Kenhall1202. Hi Ken, a belated thank you for the info about the location of the leak off point, something else I didn't know.

Derek

Hi Derek

Have found a bit more information in the back of the english version of M&S XM book.

Weights (lbs) XM 2.0 Carb / XM 2.0 Inj / XM V6 / XM 2.1 D / XM 2.1 TD
Kerb, running order - 2822 / 2888 / 3131 / 2877 / 3009
Front-rear distribution - 1728-1093/1768-1120/1960-1171/1788-1089/1888-1119
Total laden - 3880 / 4067 / 4210 / 4012 / 4178
Payload - 1058 / 1179 / 1080 / 1135 / 1169
Max towing train weight - 6764/6933/7518/6878/1169(?)
Max trailer, braked/unbraked (cwt) - 12.6-25.6/12.9-25.6/14.0-29.6/12.8-25.6/13.4-29.5
Max trailer hitch download - 220 for all
Max roofrack load - 176 for all
Engine (PSA standard) - 302.8 / 314.3 / 369.3 / 339.4 / 385.8
Gearbox - 78.3 / 78.3 / 101.0 / 78.3 / 101.0
Unsprung weight,
front-rear - 199.7-123.0/202.6-125.7/202.8-75(?)/198.0-122.8/202.6-125.7
Power : weight ratio kW EEC/ton - 66.7 / 72.9 / 88.0 / 46.7 / 58.1
Power : weight ratio kW DIN/ton - 91.3 / 100.8 / 121.6 / 64.6 / 81.9

I stuck in a couple of (?) for figures I thought were typos.

Rather surprisingly the french version of the book has the same table in lbs.

Will look through the RTA books next to see if there is anything of interest there.

John

This post has been edited by xmexclusive on November 30, 2008 08:09 pm

Interesting topic chaps,

I can throw a bit of information into the arena that may help with these calculations.

I had a hoke through some of the old magazines I have with XM road tests.

The weight distribution of the 2.0SEi tested by Autocar and Motor in Sept 1989 was 62/38, more or less confirming Derek's earlier calucuation of 63/37

Because the suspension also needs to be able to absorb *quickly* any hole or depression on the road.

You say the corners can do this job but I doubt it: I think they are too damped.

2 observations from using the car:

1. when I (70kg) get out of the car (now that it is soft), it raises significantly, often as much as to activate the height correctors, front and rear. If 2 passengers are in car, the correctors are activated *every* time and the height correction is around 4cm, very visible to the eye. This tells me that the middle sphere is rather well used (compressed), even with 1 person on board.

2. if u leave the traffic lights somewhat swiftly, the rear crashes down, then the "hard" mode is activated and the rear will remain "bottomed" for the 1-2 secs it takes to revert to "soft". It won't move till the "soft" comes back, you can well see it in the mirror. This tells me that the rear corner spheres are so damped that they are totally unable to push the car up, even if u give them 1-2 secs.


Anyway, is it not possible to calculate how many bars would the static pressure be in the suspension legs if one knows the wheight data of the XM? This value will be the same whether the car stands over 2 or 14 spheres.

cheers
G

Thank you, John and Robert for that information. I think I'm almost at the point where I can start doing sums for the front suspension, I just need to measure the distance the car rises from fully down to normal ride height and to full extension.

Can't do for the rear until I know the ratio of wheel movement to piston movement, I had intended to measure it but it's too cold and damp for me to slide under the beast and I'm not sure there's sufficient access to measure accurately. Does any kind forum member - John, Phil, Andrew for example, have a rear subframe or trailing arm conveniently laying around so they could get the measurements for me? Pretty Please?

Looking at the drawing again I'm not convinced that I'm right and John is wrong. There is another annular object shown immediately above the upper seal of John's piston that could possibly be a support bearing, in which case the leg as shown would be on full extension with the "piston" hard up against that bearing. Against that suggestion, why hasn't the draughtsman filled it in to show it's a solid bearing? I think the jury is still out until John does his sectioning, although the calculations may show that one or the other possibilities - piston or ram - is impossible.

I never thought when I opened this discussion that it would turn into such a can of worms. But we should all be wiser at the end.

The saga continues.

Derek

This post has been edited by DerekW on December 01, 2008 06:29 pm

Sorry Derek, I still don't understand the idea of the distance from low to normal and high etc, - this is a statics question isn't it? If it's to determine the volume of LHM used to carry out the action, remember that the height corrector and it's operating linkage are not very accurate! there is a short amount of vertical movement of the hub that has zero effect on the corrector's in/output due to the inner dimensions of the valve spool. Also, I've never seen a Citroen yet that doesn't have some free play in the corrector linkage in either of the 2 normal bearing points.

Could you outline the idea of this measurement? coz I'm too thick to see how this relates to working out your optimal sphere pressures.

Hi Derek

so are you going to base those on the assumption the middle sphere stands empty of LHM at static load?? Just so we know.

cheers
G

Hi Graeme,

1. Drop the car to its lowest setting, measure ground to a convenient point - I will use edge of wheel arch as my reference.

2. Select normal ride height; when there, lift up front of body and hold it until too heavy to hold any longer then release. When it settles, measure ground to reference point and record.

3. press down on front of car and hold until the car rises, release load and when it settles re-measure.

4. Take the average of the two readings.

5. Select full up and when it gets there, measure ground to reference point.

That should give you the range of movement and the position of the normal ride height.

If you want to check your figure against correct normal ride height, there is a published measuring position under the front of the car which you should be able to reach within arm's length when the car is fully up, take the measurement there and then deduct the extension you calculated from steps 4 and 5. Compare this with the published ride height measurement.

Incidentally, some while ago I initiated a discussion on an easier way of checking normal ride height, it was essentially the same as this except that measurements were taken from the centre of the wheel rather than the ground, so as to take tyre differences out of the equation.

Hi George, no. First calc is to establish static load pressure, this will immediately tell me whether the hydractive sphere is being compressed.

Incidentally, I've decided that my basis of a 70kg driver and 5 (or 13) gallons of fuel is invalid. What if the driver is an impoverished small anorexic female running on the fumes in the tank? So I'll use published kerb weights.

Derek