How should a BX ride?

BX Tech talk
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Re: How should a BX ride?

Post by RobC » Fri Dec 16, 2016 2:50 pm

Good theory that may play a part, but I think it's far more than just tyres
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Re: How should a BX ride?

Post by Defender110 » Fri Dec 16, 2016 3:03 pm

Tyres and tyre sizes do play a massive part on the very light BX ride thoughas I found out when I fitted wider 195 tyres to mine and ruined the ride.
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Re: How should a BX ride?

Post by Kitch » Fri Dec 16, 2016 5:54 pm

Defender110 wrote:Tyres and tyre sizes do play a massive part on the very light BX ride thoughas I found out when I fitted wider 195 tyres to mine and ruined the ride.
They are crashier on 195's, for sure. Best ride I ever experienced in a BX was my Dad's GT (and that's not a bearing front subframe car) on 175-section Michelins. Didn't grip much, but rode very nicely, and that was standard spheres and all old bushes/bearings etc. Having decent wishbone bushes and droplinks makes a difference, likewise the struts and the rear arm bearings.

Looking forward to the 16TRS. Bearing front subframe, and it'll be on comfort spheres 8)

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Re: How should a BX ride?

Post by callyy_f » Sat Dec 17, 2016 12:50 pm

I think mine had 185s and there were certain bumps that when driven over at certain speeds would make quite a loud bang in the cabin. If I remember correctly I was told it's because the nature of the suspension means it can't always rebound quick enough and so you get that shock, as one of the other posters alluded to on the bottom of page 1.

The main thing I noticed with BXs - even my relatively rough and sticky one - is that although they're still quite loud inside, the actual body movement is limited. You can go over bumps or into potholes and for sure you'll hear about it, but unlike traditionally sprung cars the occupants don't actually get rocked about much. You can still hold your coffee without it being thrown all over the place in a BX! Top Gear demonstrated that well when they used a Citroen to film the horse racing, the picture remained relatively still where all of the competition just couldn't do it.

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Re: How should a BX ride?

Post by Tim Leech » Sat Dec 17, 2016 9:44 pm

That's a good point, my mk1 xantia non hydractive with comfort spheres seem to absorb the bumps better than the bx, but that could be better insulation etc.

It has quite thick tyres which also helps.
Last edited by Tim Leech on Sun Dec 18, 2016 4:44 pm, edited 1 time in total.
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Re: How should a BX ride?

Post by kiwi » Sun Dec 18, 2016 10:40 am

RobC wrote:Good theory that may play a part, but I think it's far more than just tyres
Got to agree with that! All the BXs I have (four) ride differantly on the same size wheels and Tyres. I prefer the TXD for a solid ride! The Estate is a bit bouncy, The auto is well auto and the 19TRS manual in my wifes words a bit light!
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Re: How should a BX ride?

Post by RobC » Mon Dec 19, 2016 10:59 am

All this talk of tyres is all very well and good, why does the ride quality improve when the load increases?

My tuppence-worth is that with more load in the car, the gas in the sphere becomes more compressed so the sphere behaves as if the pressure's been increased. Higher pressure spheres = more comfort. Does this seem feasible?
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Re: How should a BX ride?

Post by KevR » Sat Dec 24, 2016 12:03 pm

RobC wrote:All this talk of tyres is all very well and good, why does the ride quality improve when the load increases?

My tuppence-worth is that with more load in the car, the gas in the sphere becomes more compressed so the sphere behaves as if the pressure's been increased. Higher pressure spheres = more comfort. Does this seem feasible?
Nope. Well, it might be a factor, but really what's happening is you're changing the ratio of sprung to unsprung weight. As the car hits a bump, it accelerates the wheel upwards, along with the unsprung weight (wheel, tyre, hub, brake, part of the driveshaft, lower half of the suspension strut etc, in the case of a front wheel). That in turn acts on the spring (in this case the fluid/gas in the system), which tries to transfer it to the sprung weight (the rest of the car). Some of it is damped out by the suspension, some of it gets through, and as has been explained above, short sharp shocks tend to be transmitted through the system more than long, slow movements, because the damping can't react fast enough. In normal shock absorbers/dampers the damping's provided by oil being forced either through holes or past flexible shims, and the usual analogy is to think about pushing the plunger down on a cafetiere - push slowly and it will move, push hard and it will lock. Citroen hydraulics work rather differently, but the effect is the same. So, the suspension has a certain force being put through it, and it can only absorb (or more properly temporararily store) so much, before it has to transfer some to the bulk of the car, which will make the shell try to move. The higher the ratio of sprung/unsprung weight (and by loading the car up with passengers or tat, that's what you're changing), the larger that force has to be to get through to the bodyshell, so the less you, the occupants, will feel. In the days of luxury private trains, they'd make them ride more comfortably by fitting bloody great boxes of lead under the floors to have the same effect!

Tyres do make a huge difference as well because they can absorb a lot of small shocks, or they can transmit the whole lot. In my experience the best tyres for BX ride quality were the old Michelin MXL in standard 165/70 14. They had very soft sidewalls which could filter out a lot of minor road imperfections. Most modern tyres are much stiffer, so more of the small stuff gets through.
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Re: How should a BX ride?

Post by Way2go » Sat Dec 24, 2016 3:03 pm

I agree with KevR, The best demonstration of this is speedbumps and we used to say the BX "laughs" at speed bumps. I say was true because the old style road wide "sleeping policeman" were slow rises and falls; not so with the later "speed cushions" which have sharp angles of attack and the BX needs to slow down for. However if you straddle the speed cushion that improves the ride as it changes the sharpness of the angle of attack at its edges.
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Re: How should a BX ride?

Post by RobC » Tue Jan 03, 2017 12:17 pm

Thanks Kev, I'm learning a lot here :)

So does this mean that all cars, even those which are conventionally sprung, also get more comfortable the more load they're carrying? I'ts been so long since I drove a 'normal' car I can't remember!
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Re: How should a BX ride?

Post by KevR » Tue Jan 03, 2017 12:33 pm

RobC wrote: So does this mean that all cars, even those which are conventionally sprung, also get more comfortable the more load they're carrying?
Only up to a point... Obviously you reach a point where the suspension can't cope with the extra weight, and on conventional suspension there's also the problem of excess weight changing the geometry (usually by making the car sag at the rear). A lot depends what its normal operating parameters are, because all suspension setting is a compromise. For example most passenger cars will be set up to ride at their most comfortable with three or four passengers, but be able to tolerate a fair bit of extra loading as well. But a panel van might be set up to ride and handle best with a ton of stuff in the back, and will be rattly and crashy when you drive it empty. My old Defender pickup has a payload of 1200kg, so if it's empty, it rides high and is relatively stiff, but with, say half a ton of gravel in the back, it rides quite nicely, going on to get progressively more wallowy as it gets more loaded.

It all comes back to that ratio of sprung/unsprung weight. In an ideal world you would pare the unsprung weight down to an absolute minimum rather than adding to the sprung weight. I suspect that most modern cars would ride far better on plain steel wheels and skinny tyres than the huge, ridiculously ornate (and relatively heavy) alloys and enormous tyres that are the current fashion. I wonder whether BX steel wheels are lighter than BX alloys....?
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Re: How should a BX ride?

Post by Casanova » Fri Jan 20, 2017 3:22 am

My BX mirrors the experiences described here - body control is absolutely superb, even on undulating or uneven roads (one in particular near me is a treat; really makes the suspension work, and nothing can keep up with a Citroen down that road!). But potholes, and the harsher square-edged type of speed humps, can cause even the BX to be uncharacteristically crashy.

The description of this video mentions the spec of the Dutch BX it features as "500cc spheres at 50bar, holes enlarged". Is this a mod anyone is familiar with?
https://youtu.be/_T_LKsLSyFY
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Re: How should a BX ride?

Post by KevR » Fri Jan 20, 2017 9:29 am

Lots of good information here and on related pages:
http://citroen.tramontana.co.hu/suspens ... nt-spheres
1990 BX TZD Estate ('the grey one', 1991 BX TZD Estate ('the white one'), 1982 2CV6 Charleston (in bits), 1972 AZU Serie B (2CV van), 1974 HY72 Camper, 1990 Land Rover 110 diesel LWB, 1957 Mobylette AV76, 1992 Ducati 400SS, 1966 VW Beetle, 1990 Mazda MX-5, 1996 Peugeot 106D, 1974 JCB 2D MkII, 1997 BMW R1100RS, 1987 Suzuki GSX-R1100, 1978 Honda CX500A, 1965 Motobecane Cady, 1988 Honda Bros/Africa Twin, 1963 Massey Ferguson 825, and a lot of bicycles!