Sphere specifications

[jonathan_dyane]

I got into a conversation with someone about spheres today, and ended up looking at a table I have which outlines the different spheres which were fitted as standard to different models of BX.

I understand that the size of the damping hole had a bearing on the ratio between soft and firm, but how does the volume of the sphere fit into this?

As an example, I believe that the BX GTI came with 400/55/1.65 spheres (volume/pressure/damper), and seeking a more comfortable and less sporting ride it has been known to fit the spheres specified for the 'cooking' 16 and 19 models, which were 500/55/1.80.

However the BX 14 (and early 16 and 19 models) were fitted with smaller volume spheres which employed the same pressure and damping hole (400/55.1.80). From this I (dangerously) assume that smaller volume also results in more 'float'. Is this correct or am I far off the mark? I cannot understand, if this is the case, why the GTI uses a 400 sphere...

God, I should get out more...

[kermit the frog]

It's not the volume it's the size of the damping hole the smaller the hole the more restriction to flow and therefore stiffer suspension.

On conventional dampers the hole is in the piston which has oil passing through it from one side of the piston to the other.

On adjustable/ race dampers these holes are variable giving some adjustment to the performance. (some race dampers have upto four seperate adjustments)

On Citroen's the pressure and volume are the spring and the hole is the damper.

So when the nitrogen leaks out the suspension becomes hard as there is no gas to compress therefore no springing action.

I hope that helps you out.

Obviously the calculations and specifcations for various types models etc are a bit more complicated than my simplified version.

Regards Kermit

[DavidRutherford]

the smaller the hole the more restriction to flow and therefore stiffer Damping

Corrected for what I think you meant.


The sphere volumes are important, as when combined with the pressure dictate the softeness of the suspension.

A 400cc 55bar sphere is harder than a 500cc 55bar sphere simply because there is less gas in the 400cc one (given that they are at the same pressure)

As a simple comparison, if you multiply the cc figure by the Bar figure, you get a "relative softness" figure:

400cc X 55Bar = 22000
500cc X 55Bar = 27500

A case in point is the CX safari rear spheres. They are "only" 35 Bar spheres, but at 700cc they actually hold a fair volume of Nitrogen:

700cc X 35Bar = 24500

So theoretically a 500cc X 55Bar sphere would be softer. They use 700cc spheres to ensure that there's enough suspension travel, and enough range of load capacity to account for an empty car, or one loaded up. If you were to fit a 500x55 sphere to the back of a CX safari, it would probably not compress the nitrogen at all, and so you'd have no upward travel on the supension when the vehicle was unladen.

In the same way that there's a relation between the stiffness of a spring and the damper, meaning that a system is either underdamped or overdamped, the same is true of the Citroen system. As the overall softness of the suspension increases (the multiplicaition figure above) then the damping hole needs to increase in size, otherwise the system will become underdamped. Vice-versa, when spheres start to go flat (and the suspension becomes harder) the hole is then too large, and the system becomes under-damped. This explains why when spheres go flat, the car becomes "bouncy"

[jonathan_dyane]

And now I understand! What had been throwing me was the fact the BX14 had 400/55/1.80's and the 16 and 19 500/55/1.80's and I had assumed that the smaller engined car would, if anything, be given a softer ride.

I suppose that there are other factors at play; perhaps the 14 had a thinner anti roll bar, or maybe even it was considered expedient to make the more expensive 16/19 models ride 'better'. Never having driven a 14, I don't know how they ride...

Thanks,

[kermit the frog]

Sorry david i was being a bit thick.
But i think i now understand the principles involved
Thanks for putting me straight.

Regards Kernit

[DavidRutherford]

And now I understand! What had been throwing me was the fact the BX14 had 400/55/1.80's and the 16 and 19 500/55/1.80's and I had assumed that the smaller engined car would, if anything, be given a softer ride.

There's also the weight of the vehicle to consider. The BX14 is significantly lighter at the front than a 16/19. As I'm sure many of us know from experience, spheres and height correctors mean that the suspension stiffness is related to the load being carried. Load up the car, and the suspension goes stiffer. Unload the car, and it goes softer. In the case of the BX14, if it used the same spheres as the 16/19 it would be too soft, so lower volume spheres are used to achieve roughly the same ride as a 16/19.

Theoretically, the BX14 ought to also have slightly smaller damper holes to go with the smaller overall gas volume, but that would introduce yet another sphere type.