Index Home About Blog
Subject: Re: Variable ride height possible?
From: (Andy Dingley)
Date: Nov 03 1996

The moving finger of Puddin' Head <> having

>Is it possible to have a variably controlled ride height on a car with
>MacPherson struts?

No. Because of the arrangement of MacPherson struts, any attempt to
put an adjustable ride height cartridge into the strut would need to
compress the spring as well. You'd not only be adjusting the ride
height, you'd be moving the spring out of its design region. Chances
are you'd get a horrible rising spring rate effect, and you'd
certainly need a huge force in the hydraulics.  Moving the strut
itself would be awkward too, as move struts are mounted in a way
that's not easily moved. If you really wanted to do this, fabricating
an entire non MacP ssupension is probably simplest.

>I'm just wondering because I would like to lower my car about 1" to
>1-1/2" to even out the wheel well gap, 

A fixed lowering is possible by changing the strut. Dont know about
your particular car, but it has been done on others. You can even
cut-and-shut the strut towers themselves.

>I know that Citroen made a few cars that have a computerized hydraulic

Only one I think, the latest Xantia.  The others (even the XM, SM, DS
and base model Xantias) aren't really "computerised", just good
hydraulic design.

>that's so good that the car hardly lean in turns and drives
>so smooth that it feels like you're floating, even over railroad tracks
>at 60mph.  

They're not _that_ good.  Some people even get carsick in them,
because they don't like the XM's usual ride.

>It's even supposed to out-corner a Corvette, 

That says more against Corvettes than it does for Citroens. Most
European cars can out corner a Corvette.

>and ride height
>is adjustable with a joystick-like device which can jack the car up real
>high looking like a 4WD, and lower it to where it looks like a
>lowrider.  How did Citroen do this?  

Cheaply !  The much acclaimed Citroen ride height adjust is a gimmick
with little practical use. Sure, you can jack it up until it _looks_
like a 4x4, but you're hard on the bump stops at this point and
there's no suspension travel left. Even trying to park it like this
will shake your fillings out. It's handy for tyre changing without a
jack, but little else. The low rider position works in Germany, but
most country's motorways are too bumpy to make real use of this fuel
efficient mode without getting a bad ride quality.

The XM also has adjustable suspension modes, a standard mode with
softer damping and a harder "sport" mode. Sport mode is selected by a
manual switch, or automatically if you do any sharp cornering. The
solenoid valve that controls this isn't well sealed and can fail after
a few years. It's a couple of hundred quid for the valve (unless you
have a hydraulics catalogue handy), but it also tends to break the
output transistor stage of the electronic podule controlling
suspension stiffness. Time for a new podule, at around 600 quid, or
you can do some soldering yourself and replace the 67p transistor....

>How did they do it without batteries 

The XM has a "Mister Fusion" hidden in the boot. - at least that's
what the servicing bills suggest. I know at least one person who
bought an XM on the basis of its outward resemblance to Ed Straker's
car in UFO  (obscure reference to tacky '70s Brit TV sci-fi).

How does it really work ?   First of all, damn near everything on a
Citroen is hydraulic. There's an engine driven pump that provides the
main energy supply and for transient loads (short bursts) where the
power needed is more than the pump an supply immediately, each
subsystem has its own hydraulic accumulator. These are the fabled
Citroen green spheres, a 4" diameter sphere with a rubber diaphram
inside, pressurised with nitrogen. The nitrogen is compressed inside
the diaphram by hydraulic pressure, which it can release on demand. On
an XM there are something like 6 of these spheres, one for each
wheel's suspension, one for the brakes, and one for the soft
suspension mode. Citroen brakes are hydraulic, but the fluid is this
funny green Citroen stuff (another UFO reference ?) and instead of a
vacuum booster it uses the engine pump.

Citroen brakes also turned up in Maseratis (owned by Citroen), Rolls
Royce and Bentley. They're not so bad now, but they used to be noted
for having a rubber button instead of a brake pedal. This button
didn't move either, it was just force sensitive. Not a bad system when
you're used to it, but a lot of early drivers pressed too hard on it
to try and make it move, and ended up spinning the car.

There's no handbrake in most Citroens, instead it's a fourth pedal.
They just like being weird for the hell of it.

Down at the wheel stations, the usual spring and shock absorber
arrangement is replaced by a hydraulic cylinder and an accumulator
sphere. The restoring spring action comes from the pressurised gas in
the sphere, the damping from tiny nozzles which the hydraulic fluid
must pass through, and the ride height control by a quite simple
mechanical lever and valve arrangement that lets pressurised fluid in
and out of the wheel circuit.

>the car trimmed real low when parked and raised up when the ignition was
>started, then it would adjust and finally settle.  

They do that. There are few things sadder looking than a Citroen left
parked overnight. The big CX (probably what you rode in) looks like a
dead fish in the mornings.

If your Citroen can't "get it up" on starting, then it's time to take
it and have its spheres twiddled with. They need to be repressurised
every few years.

>It seems like the ULTIMATE performance suspension would be the
>Citroen's computerized hydraulics.  

The ultimate is probably the Australian Kinematic system. Take a
Landrover 110 and fit Citroen-like technology to it, only with _real_
suspension travel and _real_ height adjustment.

I like a lot about Citroens, but I'd never have an XM myself. It's as
complicated as a space shuttle, with the build qualities of a Vostok.
Heater matrices last about 6 months, obscure little hydraulic valve
things that my own car doesn't even _have_ keep going wrong in a
bizarre fashion and never giving much change out of 500 quid. 

Mind you, the Citroen garage people (not a Citroen dealer) have a
great sense of humour. Every time it costs a packet to fix it, they
have a tale about the guy who was in the week before where putting the
wrong kind of air in the tyres of his XM had left him with a four
fgure repair bill (XMs built on Tuesdays before 1993 need a different
sort of air in the tyres from ones built after Bastille day 1992).

Index Home About Blog