BX front suspension

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Aerodynamica
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BX front suspension

Post by Aerodynamica »

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.
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Rob_e (UK)
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Post by Rob_e (UK) »

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 :lol: ) 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
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jonathan_dyane
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Post by jonathan_dyane »

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
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)
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Way2go
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Post by Way2go »

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)
That sounds an excellent summary. :D
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DavidRutherford
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Post by DavidRutherford »

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.
... or not: The shape of the top of the piston isn't relevant, only the cross-section 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.
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maxgreenwood
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Post by maxgreenwood »

the intermediate height stiffening puzzled me also. I enjoy learning!
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