BX Suspension Tuning - Choosing Spheres

All aspects of tuning,modification and repairs to the BX 16valve.
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cammmy
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BX Suspension Tuning - Choosing Spheres

Post by cammmy »

Hi Guys

I've (amongst many others) always been confused by which spheres you would use to tune the ride of a BX.

One of the main points of confusion seems to be, does higher pressure = a firmer ride or a softer ride. I have always thought it would be firmer with a higher pressure but other people seem to think differently. Can anyone confirm?

Can anyone give advice on a range of sphere setups to use for configs that range between soft and wallowy to track day special?

Would like to make this a thread a point of reference for BX suspension tuning.

Ta
Cam
'89 BX 16v
'91 Peugeot Mi16x4 x2
'74 Triumph TC2500
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jayw
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Post by jayw »

There are two factors to consider here:-

The pressure: This is the pressure of the nitrogen surrounding the bladder of the sphere. The higher the gas pressure, the less the LHM filled bladder is able to intrude into the gas half of the sphere, therefore stiffening the ride as the bladder is not able to stretch as much as a low-pressure unit.

A heavier car appears to always have a higher pressure rating and a lighter one lower, this is with the exception of more "luxury" Citroens that inherently require softer setups i.e. high spec XM's. For this reason our light BX's would require the spheres from a heavier car to stiffen the suspension.

This is the opposite to normally sprung cars where a "heavier" spring would cause the car to ride higher due to not correctly compressing the first phase of the dual rate springs (and i believe this is where most peoples confusion over the subject arrises).

Of course, with our self-levelling that isn't an issue!

The other consideration is the bore of the hole which allows the LHM into (and out of) the bladder. The smaller the hole, the slower the fluid moves and thus reduces the speed at which the damper moves up and down (equivalent to "firmer" uprated shocks on a normal car). 0.5mm is the smallest available upto around 2mm.

Rating are given as 500/55/1.1 meaning:
Sphere Capacity (not particularly relevant) / PRESSURE (in bar) / BORE (in mm)

You are right that there are many schools of thought, none with any real evidence, just theory.

A few years ago I was told the best spheres for sporty use were those from the rear of a 3ltr V6 XM. At 30bar/0.5mm they are the lowest pressure/ highest damped (see above) spheres available. Suffice to say after the expense of fitting them all round the handling was completely screwed and I was convinced I must have been given the wrong spheres. After learning more about them over the years I now understand what it is that's needed.

You want high pressure for stiff springing, small bore hole for maximum damping and lower the car with the height correctors. Your combination of those is your choice but when choosing, always consider the weight of the donor car and it's "comfort" level. You can always re-gas to the pressure you want.

Dowload this, it will help you understand the choices: http://cid-6ce6986c07c4d5d4.office.live ... 0Table.pdf

Oh, and NEVER trust a motor factors word that the spheres he supplies you are for the car you asked for unless the pressures are actually printed on the sphere (this is because most factors think all spheres are the same and will take whatever the suppliers have in stock).

As I have brand new set of XM rears in the garage I was planning to get them re-gassed to 75bar and see what happens... mind you, i've been planning that for 2 years and it still hasn't happened...

Good luck and keep us posted with what you decide to go for.

JayW.
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ollie
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Post by ollie »

This all depends on what youre going to use the car for and where! the racers will have one preference for smooth tracks and others will have different choices for traction on the road! I personally find that having some suspension movement is usefull on real roads :lol:

The best handling one I persoanlly have driven was a 16v on a set of old and flat spheres in otherwise standard spec!
The less gas is in them the stiffer the spring rate (and the bouncier it is) - less gas to compress, the gas is the spring, the holes arre the dampers.

The standard damping on 16v spheres is ok, though for ultimate handling I feel the rears could do with being even more damped as it floats at speed.

Have driven a 16v with wider wheels and various sets of XM spheres as above but whilst it all felt very go-carty the reality was that the ultimate cornering speed on my favourite roundabout was almost exactly the same! only the tyres really helping the grip level, the spheres only ruining the ride!

As a teenager we had an assortment of diesel estate in good orig floaty spec, a TZD on flat Gti spec and a very early and light weight 1.4 on standard smaller spheres. Guess what, ultimate cornering speed through my favouriet S bends was absolutely identical as they all used the same tyres! and the one which felt the best was allways the fag packet light 1.4!

Remember that whilst on a flat circuit solid suspension helps with control, its not a fat lot of good mid bend when yours wheels pop down a drain cover or hit a badly repaired bit! and just for good measure once the stiffness gets past a certain point you just flex the shell!!
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jayw
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Post by jayw »

ollie wrote: The best handling one I persoanlly have driven was a 16v on a set of old and flat spheres in otherwise standard spec!
The less gas is in them the stiffer the spring rate (and the bouncier it is) - less gas to compress, the gas is the spring, the holes arre the dampers.
Flat spheres are simply NO spring, the only spring will be the VERY limited flex of the tyre's sidewall, hence the bouncing. This is much like driving around with the suspension on the lowest setting, particularly dangerous when hitting a bump mid-bend as you pointed out and definitely not recommended.

This also voids any damping value as the back-pressure formed against the diaphragm by the gas is completely lost meaning there is no rebound of the LHM through the damping hole. Additionally proved when you consider that she axle must be allowed to move both up AND down from it's resting position, a flat sphere would simply allow the diapragm to rest against the inner wall of the sphere (if it stretches that far without failing) making the contents of the sphere 100% oil, which as we know cannot be compressed. There would then be nothing but the axles own weight keeping the wheels on the road over a crest for example.

Again, we have the same stalemate with regards to low/ high pressure. The gas itself is not mechanically involved in the process, it's the compression of it by the LHM side that creates its value.

Low gas pressure would allow the diaphragm to stretch freely and easily into the gas half of the sphere, equalling very soft and pliable suspension.

High gas pressure would make it harder for the diaphragm to stretch, this would make the car bouncier, bouncier because you're stiffening the ride. Increase the pressure too far and the flex of the diaphragm is all but impeded and you have one very stiff car, akin to no gas at all.

The bounciness is then controlled by the damper, already explaind above.

Once more, height (lowering) is not covered by the suspension units whatsoever and isn't a consideration of either factors.

Like i said, I had XM spheres on my 16V, it was apalling.
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frog
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Post by frog »

I'd love to find some of the spheres mentioned in this thread:
http://www.bx16valve.co.uk/forum/viewtopic.php?t=786
ollie
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Post by ollie »

I didn't say totally flat :lol: Just older and lower in pressure than when they started!
what I'm now confused by is what does the springing? I was allways under the impression it was a combination of the gas compressing in conjunction with the shape of the diaphragm.
the logic being that if the pressure is slightly below normal (gas loss) then there would be less of it in the end of the sphere allowing slightly more intrusion of the diaphragm, and the smaller volume of gas being harder to compress as far for a given hydraulic movement.
i.e as spheres head towards totally flat the rate stiffens, untill the point where they have no give at all - then they get very dangerous and as you say no movement = no damping.

if what you say above is correct, when my 50 bar spheres drop to 40 bar the car should get more comforteable? and I can promise you it doesn't !! it gets stiffer and less comforteable, eventually after years of neglect getting to the dangerous bouncy stage. which is usually when the diaphragm splits.

Ollie
michael16v
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Post by michael16v »

frog wrote:I'd love to find some of the spheres mentioned in this thread:
http://www.bx16valve.co.uk/forum/viewtopic.php?t=786
The spheres in this link are mine. I got them from Peter McLeods workshop, left over from his race cars. I added the valve to easily adjust the pressures.
Lower the pressure, firmer the ride. Higher the pressure, softer the ride. This statement has its limits; if you way over pressure than the ride will firm up - just prior to a big BANG :roll: The spheres are accumulator spheres with heli coils fitted. There is NO hole in the centre (just a bolt) and this makes little difference to the ride.
The quantity and thickness of the valves either side of the centre spacer, (the one with holes in) makes the changes to the damping. As these work the same as any shock absorber. I.E. making it easier - less number/thickness of valve discs - or more difficult - more/thicker number of valves - for the fluid to pass by. Restrict the flow and firm the damping rate.
A lot of fine tuning is required to get the "right" setup. A lot of "flat" spheres have given up their valving to provide the parts for these adjustable spheres. The problem is I have had no setup information from the race cars to go on.
All good fun though.
Cheers
Michael
surjit25
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Post by surjit25 »

I use xantia frts on mine n I find. a bit stiffer try hydractive ones [quote][/quote]
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