Ello all, after going out to check over the hibernating CX and to try its front spheres on the BX I fitted the CX fronts on the BX at the front. Previuosly I'd fitted the 'old' CX spheres to the BX and they were way too soft and underdamped. The new, 5 mnth old pair are CX Turbo spheres and are the same pressure as standard but with a slightly firmer damping. The spring rate of the front is now better but I have noticed a characteristic that's changed.
The ride is better overall but when the wheel has to go down a pothole or recessed drain cover there's a real harshness to it. I have a feeling that the sphere pressure is too high at 75 bar for the light bx and that the gas is hardly compressed under the car's front axle weight. If so it wouldn't allow enough downward movement at the right speed.
I was wondering if anyone knows the diameter of the hydraulic piston of the front struts so I can work out the static pressure and how much the sphere gas is compressed - anyone happen to know? I think I once heard that it was 25mm but need to confirm.
BX front suspension
- Aerodynamica
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BX front suspension
Graeme M
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CX 2400 Pallas LPG
2CV6 dolly (SORNed)
Mk1 Xantia 1.9TD SX
'c'est hydropneumatique'
- Rob_e (UK)
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I haven't actually got a strut in pieces to be able to measure the piston itself. But I do have a strut that isn't fitted to the car, and trying to measure through the holes at the top of the cylinder where the LHM goes in and out; I believe the internal diameter of the cylinder, and therefore (hopefully ) the piston as well is 17mm.
I remember from having discected a frount strut in the past that the piston is domed at the top not flat like the ones in the rear cylinder; so I guess for your calculations you should probably assume it is half a sphere when working out the surface area.
Do you know what proportion of the BXs weight is on the front wheels?
One thing that has always puzzled me a bit, sort of on the same subject is; the pressure in the suspension should be proportional to the weight on the suspension, so why when the car is on the intermediate height is the suspension harder? surely there is not the same pressure in the suspension just more volume of LHM?
Rob
I remember from having discected a frount strut in the past that the piston is domed at the top not flat like the ones in the rear cylinder; so I guess for your calculations you should probably assume it is half a sphere when working out the surface area.
Do you know what proportion of the BXs weight is on the front wheels?
One thing that has always puzzled me a bit, sort of on the same subject is; the pressure in the suspension should be proportional to the weight on the suspension, so why when the car is on the intermediate height is the suspension harder? surely there is not the same pressure in the suspension just more volume of LHM?
Rob
BX19 GTi 8v 89 227k
- jonathan_dyane
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Coincidentally, this thread and the myth or reality one are randomly crossing over... If my understanding is correct the stiffness difference you mention is due to the geometry difference of the different height, which will increase stiffness with extra height, whilst soften and roll more with lower height (due to the distance 'twixt roll centre/centre of gravity being compromised)Rob_e (UK) wrote: One thing that has always puzzled me a bit, sort of on the same subject is; the pressure in the suspension should be proportional to the weight on the suspension, so why when the car is on the intermediate height is the suspension harder? surely there is not the same pressure in the suspension just more volume of LHM?
Rob
"Boring damned people. All over the earth. Propagating more boring damned people. What a horror show. The earth swarmed with them." -Charles Bukowski
That sounds an excellent summary.jonathan_dyane wrote:the stiffness difference you mention is due to the geometry difference of the different height, which will increase stiffness with extra height, whilst soften and roll more with lower height (due to the distance 'twixt roll centre/centre of gravity being compromised)
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- DavidRutherford
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... or not: The shape of the top of the piston isn't relevant, only the cross-section area.Rob_e (UK) wrote:I remember from having discected a frount strut in the past that the piston is domed at the top not flat like the ones in the rear cylinder; so I guess for your calculations you should probably assume it is half a sphere when working out the surface area.
Another reason the suspension gets stiffer in intermediate is that the rear suspension is beginning to press on the top bump stop, and the front suspension is now loading up the inner wishbone bushes. It's noticeable that in intermediate, the suspension is bouncier, which is because there's more rubber coming into effect.
this might be a signature
- maxgreenwood
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the intermediate height stiffening puzzled me also. I enjoy learning!
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'90 16TZS (m), White, 86k
'89 19TRS auto, Olympic Blue, 133k
'88 Saab 900 8v Turbo (auto) 107k Red with Tan leather, lovely drive and well maintained.
'07 Hyundai Santa Fe 2.2 CRDi 85k (m). Practical family wagon