Xm Spheres

i tried using bx spheres on my xm as they were new and i was scrapping my bx they worked fine has anyone else tried this, the bx spheres have a slightly larger hole in the ctr of the damping valves

Terry,

When the manufacturers design the spring/damper ratio on the drawing board (read computer) they take several different parameters into account. This results in three outcomes :

Sphere size
Sphere gas static pressure
Damper hole diameter

These three variables are calculated and fixed at values which work in harmony to achieve a given end result. Each of the parameters can be changed but the combined effect will alter the car's handling characteristics.

I'm not saying don't do it (I must have experimented with nearly everything in my life ) just be aware that each time you make a change the suspension characteristics have changed. Without understanding the maths (did it at college but forgot all the formulas concerning sigma) then its a bit of a lottery. For the most part the car may behave perfectly well but just when you need the suspension to behave properly (ie braking going into a corner towards a stationary object, hitting a pothole with the outside wheel) it may exhibit a very unexpected response.

That's the problem with experimenting with the suspension ratios. It might feel fine when you do it but you'll never know the real effect until you need it most.

I'm no trying to be a killjoy, just being practical and safe.

An XM with Hydractive has very small damper holes in the wheel spheres but an additional damper hole in the electrovalve assembly. If the electrovalve works properly then all three holes are open and give the smooth Citroen ride. When the hydractive activates the middle hole closes and leaves the two small holes at the wheel thus making the suspension much stiffer (ie loads of damping). Changing the wheel spheres (and hence damper hole) for the BX spheres will result in the combined effect of a much larger damper hole (much less damping). It may be like having duff shock absorbers in a normal car.

Cheers

noz

i do see what you mean about alterd damping but did not know about the third hole in the e/v i used the bx spheres because they were brand new and at the time could not afford new XM spheres once i get sorted out whith my claim with my local council and get my car fixed i will be replacing all the spheres with genuin cit ones,,"£23.oo each at euro car parts.. the bx spheres do give an exceptional ride though and you can still feel the diference in sport mode......terry

Terry,

Don't doubt the exceptional ride in terms of the 'flying carpet' as it used to be known. The electrovalve in the centre will still close when told to do so and the overall damping will be increased so you'll still feel the difference.

Cheers

noz

I've put Xantia SX 2.1 TD Spheres on the rear of my XM V6 12 valve. Its quite soft, but the damping is still good. On my previous Xantia, I overgassed some BX spheres, and fitted them to the rear of the car... the result ...... So soft, you could BOUNCE the rear end just by engageing the clutch at idle speed... it was great fun in traffic jams, the rear end Bounced all across from top the bottom bump stops..

BUT....

this did not affect the handling of the car, it still stuck to the road like glue. It was more hairey when I was towing another car on my tow frame, only then did the back end feel isolated and the chance of getting pushed sideways by the towed car when cornering..

Regards

Paul

This post has been edited by citroenv6si on March 02, 2005 06:21 pm

Paul,

Firstly, welcome to club-xm. I hope you will find it useful to share your XM experiences with others for mutual benefit.

When you describe the amount of movement as being "from top the bottom bump stops" and the result as being " this did not affect the handling of the car", I'm finding the combination of both of these statements very hard to believe. After all, in a 'normal' car, that lesser mortals are resigned to driving, this condition would normally be the result of knackered shock absorbers. The lack of damping causes instability in the spring/damper setup because the damping ratio is way out of the working range. Whilst cornering, if you hit a pothole with suspension in this condition you will end up in a field.

In your overgassed BX example I assume the damper hole stayed the same. In this case you changed the K-value of the spring (in the analogy with a normal spring/shock absorber setup) and the damper value stayed the same. The overall effect on the suspension depends on the spring/damper ratio. Changing either one will affect the ratio and hence the handling characteristics.

I'm not trying to be awkward but if you think of every vehicle in the world which needs high stability whilst cornering ( F1 race cars for example) the suspension is stiff to non existant. American cars on the other hand wallow about like hippos and corner like s**t.

I will admit, however, that the original car designers probably came up with several different options for spring/damper ratios. Each option would have been evaluated against several criteria. No one option would have satisfied all criteria so consequently compromises must have been made. Certainly the usual compromise is between ride quality and handling (read soft and hard suspension respectively). The spring damper ratio would of course differ significantly between a road going car for public use versus a rally car set up for offroad racing. Clearly there is room for manoever in the selection process. Indeed there's a huge aftermarket of boy racers bolt ons which include shocks and springs. However, nearly all modifications tend to make suspension stiffer not softer.

Are you really convinced about this? No offence intended.

cheers

noz