Hi guys, does anyone know any rough figures for the spring rates of any spherically suspended citroen?
Im doing a vehicle dynamics study on the citroen BX for one of my dissertations and i have all the information i need other than this. The study involves creating a simplified beam axle virtual model of the bx using figures for the roll centres front and rear, centre of mass and roll bar stiffness that i measured, using a program called ADAMS. You can load a tyre model and road surface conditions model in and give the car a trajectory and speed and see what happens to body roll, yaw rate, lateral G e.t.c.
An educated guess compared to similar cars will do me if there arent any precise numbers about, just need a ball park figure at the least so i have something realistic to go on.
The pictures below show you what ive been upto just in case you were wondering
yeh, they are pretty special my neighbours are used to it all now, the guy across the road is the local area Mayor but he has a soft spot for the car, as much as it annoys him becaus ei get in his way when its parked on the road, because his first towing car was a bx diesel when he was a bit younger so its all good
I cant seem to find believable numbers for ANY car out there either! Some website had a comparison between a standard escort cosworth and a modified one saying the standard front spring rates were 21N/m and the upgrades were 75N/m, and apparently our uni impreza has 140N/mm, theyre not even in the same units, but if you correct eiether one, theres a massive differnce, and these two cars have mcpherson strut front sus and theres not going to be a massive difference in weight between these two vehicles so im lost. The standard cossie isnt exactly soft, so i dont believe the upgrades are as big a jump up as 75, which makes me doubt the units its in too. hmm
I would suggest you have a look at spring rates for standard pugs. You won't get a spring rate for a BX as there are no springs, obviously, so you would have to calculate the effective spring rate from basic principles.
It is not going to be easy but if you can get it right then it will hake one hell of an interesting section in the write up.
The way i would suggest going about it is as follows;
You have a sphere with a known volume.
Within the sphere you have a diaphram which maintains a pressure of nitrogen at a known level which you know. Remember this is based some what on an equal and opposite reactive pressure, ie if you have a sphere rated at 45psi then this will only occur when LHM is applied at 45psi.
You have the material properties of nitrogen (compresibility etc)
You have known values for LHM (viscosity, compresibility etc), if you don't fire off an email to Total Elf Fina (contact on there web address) and ask for the specific properties and or the MSDS (the later they have to legally provide).
You know the size of the hole in the sphere wall which lets the LHM in and out.
You now know the weight of the car.
So you have a basic fluid pressure system, you might have to make some approximations (like how much the acting pressure of LHM changes with respect to the HC opening/closing), and the influence of the diaphram (or rule it out all together), but it is a relatively simple damped responce system.
I think the biggest problem you might have is that the damping ratio and spring constant are combined (as the sphere does both!), so extracting just the spring ratio might be difficult, but if memory serves (and it has been a long long time) then ADAMS can be configured to use combined numbers?
I suggest you get hold of a book called 'the essence of engineering fluid mechanics' by Marcel Escudier (who is a part time retired lecturer at the University of Liverpool and used to work with Williams F1), this has some good pointers.
Some where i have a copy of an exam paper which pretty much asks the same question (albeit with air and not nitrogen).
Feel free to bat ideas about, and dont take any of this as gospel!
That is a fantastically helpful response vanny and i feel a bit bad about not starting this dissertation early enough as i could have made use of that info. What im planning on doing is using values for a similar sized car, 405 e.t.c. But im not having any luck finding values between sites that are coherant. people using lbs/ft lbs/in Nm and Nmm but not stating units means i cant compare numbers, spring rates sem to vary widly between similar vehicles so i have no base to go on at the moment. Im browsing spring manufacturers websites as a logical place to start, bilstein koni etc but no luck so far.
The report is quite basic and the data doesnt have to prove what a BX can do so much as show that i know how to interpret the data i get out so accurate figures for the BX are not so important. The pictures of the car up in the air are mainly for photographic evidence of me carrying out the experiment.
Ive just written 13,500 words and 75 pages on motorcycle jig design and im brain dead at the mo so this report isnt going to be my best work
Well, i'm more than happy to help where i can. I would suggest that you try looking on sites from the Netherlands. I don't really know how they do it, but the Dutch seem to always have more technical details than anyone else on there websites. Have a look at the mi16.nl website to give you an idea what i mean!
Will have a hunt about if i can when i get home, but you might want to check out the 10tenths forum, its mix or drivers, racers, marshals, scrutineers, and many industry experts, i can usually get an answer to the most complicated questions!
Also i seem to remember using a vehicle dynamics book at uni (pretty big bugger) that had a lot of example and thus example numbers in.
Volume and initial filling pressure equals the lenght and hardness of the standard coil spring. This is calibrated such that on the vehice at normal weight & height - the sphere will then have half the volume and double the pressure (the standard gas law applies). This locates the membrane approx mid in the sphere cavity for largest suspension excursion amplitudes. It then follows as the gas diffuses ("leaks") out over time that the gas volume keeps diminishing (as the loaded pressure will be held constant) making for harder and harder suspension. Finally you have the very jumpy Citroen symptom.
Every suspension needs a damper to avoid oscillation. So does a Citroen sphere suspension. The orifice bore diameter restricts the flow speed of the LHM in/out of the sphere cavity constantly. But at times (meeting a pothole f.i.) you get large energy suspension excursions which can not flow into the spring element. Then the damper valve opens to absorb the energy. Thus you may call the bore diameter "slow speed" damping - and the damper valve "high speed" damping. What ever you call it - simply think of it as a completely normal shock absorber found on all other cars. Because that's what it is. The damper valve is the odd circular plates with holes around the center orifice. The plates are spring steel with thickness and count selected for sphere application.
Exactly this damper valve is not fitted to the accumulator sphere - as this does not participate in the suspension circuit. Instead it is solely there as an accumulated reserve of system pressure should the engine (and then pump) stall during drive.
Note that all figures are calibrated exactly for the particular wheel on the particular model to match best (and safest !) performance.
Add to this the later type hydractive suspension systems which gives a whole extra range of dedicated sphere types.
A Citroen sphere is never "ohh - one of those green Citroen spheres. Got loads of them out back mate - just pick a couple"
Cheers
Luke
Please call me Luke
1989 BX 16valve White 70k almost up to scratch
Cheers for that Luke
This one is mainly for Luke and Vanny as i think you're the only ones who will know this, what pressures are the spheres on my car repressurised to? i know theyre XM ones so i know their cc's, and i also remember that they're also all either XM fronts or XM rears but pressurised accordingly. what that means is that the rear has alot more damping compared to the front, than standard, but its a brilliant combination as it handles like nothing else. Pushing the rear of the car down by hand - its clear to see how much its damped because it returns visibly slower than normal.
I think this information will be useful for Chris who now owns my 4x4, i think he would like to try this sphere setup on his car.
Fortunately I am responsible for the spheres fitted to your car Simran.
When Chris owned it he posted me a new pair of 3.0 XM front spheres that my mates at Langworth Motorsport regassed to 35bar, these were fitted to the rear of your car.
The current front spheres will either be the original front items which had the pressure dropped a little by Chris or he will have fitted his old rear set.
I run 16v rears on my front and the regasse XM items on my cars rear, if you lower the car using thee spheres you get a superb handling motor.
Cheers
Luke
Please call me Luke
1989 BX 16valve White 70k almost up to scratch
my neighbours are used to it all now, the guy across the road is the local area Mayor but he has a soft spot for the car, as much as it annoys him becaus ei get in his way when its parked on the road, because his first towing car was a bx diesel when he was a bit younger so its all good
Don't run your life on diesel