Tyre blowouts and hydropneumatic suspension

ChevronMicky · Post by **ChevronMicky** » 04 May 2006, 20:11

In years gone by, Citroen used to advertise quite strongly that their hydropneumatic suspended cars could suffer a blowout without any loss of control. They produced impressive video footage demonstrating the point on cars like the venerable GS and BX.

Today, however, you never see this referred to, which surprises me. So, would my C5 react badly like ordinary cars to a blowout, or would it still retain full control like those Citroens of old? Xantia too?

(If appropriate) how come they no longer use it as a selling point?

deian · Post by **deian** » 04 May 2006, 20:23

I've heard this too, I look forward to seeing the replies from those in the know! According to a Wikipedia article about hydropneumatic suspension, they listed the advantages of this kind of suspension which included the fact that they can drive on three wheels, i was astonished, of course i assume it would be more drivable is the rear passenger wheel went!

andmcit · Post by **andmcit** » 04 May 2006, 20:29

McPhereson struts!! That's where it all went wrong...

The suspension geometry of the DS, GS, CX and Sm is engineered to give optimum centre point of contact of the wheel to the ground so any disturbance of the tyre on blowout doesn't have any 'pulling' effect off course on the cars steering.

No matter what point the wheel is steered at or travel on the suspension the simple effect of the double wishbone [as per racecar so optimum design] keeps the wheel purely vertical under all conditions giving that 'planted' sharpness and grip.

Combined with self levelling/anti dive etc it's simply a case of "thinking" the car round corners etc NOT having to actually make the steering effort!!

On the Bx, Xm, Xantia and C5 the whole lot is muddied and compromised to follow Peugeots shared floorpan and common parts and now the whole lot "is at sea".

It makes me laugh when I see these £60k plus Mercs with their wheel on lock and leaning drunkedly like a 2CV's. Downright sloppy...

Andrew

deian · Post by **deian** » 04 May 2006, 20:33

So it depends on the suspension set up? Does it not have anything to do with the height correction of hydropneumatic?

I know what you mean about those mercedez benz, 45o in both directions!

Why can't Citroen stay weird and do things differently!?! Damn Peugeot!

ChevronMicky · Post by **ChevronMicky** » 04 May 2006, 20:42

Andmcit, I'm certain the BX was advertised as one car not affected by blowouts.

andmcit · Post by **andmcit** » 04 May 2006, 20:43

I certainly wouldn't want to try a blow out on a Xantia or Xm at any speed but have had first hand experience of blowouts on Cx's and AT VERY HIGH SPEED...

Before the days of Gatso's tanking along on the M6 in a 2.4 GTi Cx there seemed to be a noise that started to intrude into the cabin. It took a while to determine what was actually wrong, and after a way it was getting a tad tiresome. It sounded like the exhaust had actually sagged onto a moving part OR the ground and fearing further damage and not prepared to put up with the noise any longer, I decided to stop on the next main straight where the hard shoulder looked wide enough...

Yes, Ok, imagine my surprise to walk around the driver's side of the car and then check the passenger side to discover the ENTIRE canvas had shredded off the rear wheel - I'd just minutes earlier been doing 90mph and only stopped as I was fed up with the noise.

Lets just say it was a 'tad' warm and the whole lot needed to cool down!

That was the rear. Imagine my feeling of being hard done by on my metric tyred turbo GTi in a similat scenario on finding an extra 'feel' through the steering - it's a generalisation to say the whole steering is fully artificial on these. Again I decided to stop where it was safe to do so and found a BIG gash on my nice recent 180quid tyre...

Can only guess these were down to road debris...

On a Xantia 6 months ago the tug on the steering was really dodgy and I had to stop before the thing fully deflated - big cow of a self tapper screw saw that Pirelli off.

Andrew

andmcit · Post by **andmcit** » 04 May 2006, 20:44

deian wrote:So it depends on the suspension set up? Does it not have anything to do with the height correction of hydropneumatic?

I know what you mean about those mercedez benz, 45o in both directions!

Why can't Citroen stay weird and do things differently!?! Damn Peugeot!

C6 has double wishbone suspension again!! Woohoo!!

$\:D/$

Andrew

fastandfurryous · Post by **fastandfurryous** » 04 May 2006, 20:46

ChevronMicky wrote: I'm certain the BX was advertised as one car not affected by blowouts.

I'm faily sure you'll find that was the CX, or the GS/GSA.

The BX, having McPherson struts and compromised suspension/steering geometry would be affected by a front blowout.

Try this on a BX... blow one front tyre up to 40psi and have the other at 5psi. The car will pull to one side quite dramatically. Do the same in a DS/CX/GS/GSA/SM..... No effect.

deian · Post by **deian** » 04 May 2006, 20:50

same for xantia too?

Post by **martyhopkirk** » 04 May 2006, 20:56

I have lost a back tyre on the Xantia and was able to keep the car going in a staright line with no pull and no discernable drop at the rear (whilst driving) but once stopped the car drooped - it would not have driven off again.
About a year ago I physically lost a back wheel on a BX, the car only started to drift when the wheel became wedged under the spat at an angle - I had rather foolishly thought the noise was down to the rear brake cover rubbing the rim

I wouldnt like to chance it with a front one.

fastandfurryous · Post by **fastandfurryous** » 04 May 2006, 20:57

Yes.

McPherson struts are a compromise. They're used because they are both cheap and very compact. Have a look at the complexity and size of the front suspension on a CX for comparison!

In answer to your other point about running on 3 wheels... this is possible, but only if the missing wheel is a rear one. By selecting intermediate or maximum height, the rear brake disc will just about miss the ground.

Mandrake · Post by **Mandrake** » 04 May 2006, 20:59

I had a flat tyre on the left rear of my GS many moons ago, in fact it was the very first day that I drove it on a trip out of town

I was doing 100Km/hr on a windy road and I could hear a very faint flapping noise, but I was sure it wasn't a flat tyre as the steering was normal, the ride was normal, and nothing seemed wrong apart from a very faint noise.

Eventually I pulled over to find a not only flat, but completely shreaded tyre.

There is a real risk of destroying a flat tyre on a Citroen simply because you don't notice something is wrong and pull over quickly.

On my Xantia I recently had a left rear tyre also go flat - it somehow got a large chunk of glass into the sidewall. I was only driving at 50Km/hr mind you but I noticed absolutely NOTHING wrong with anything, so I was pretty shocked to see a totally flat (and once again, ruined) tyre when I arrived at my destination and happened to glance in that direction.

As the Xantia has basically the same suspension at the rear as a GS I think it would probably cope with rear flat tyres just as well.

The only flat tyre I've had on the front of a Xantia the tyre didnt actually get a chance to deflate before I discovered it - it had a HUGE chunk of concrete somehow stuck through the middle of the tread and sticking out of it about 10mm.

Again I felt nothing wrong with the ride or steering but I could hear a noise that sounded like a classic flat tyre flapping noise, and as I was almost home I went home and checked the tyre in the garage - to my surprise it looked ok, but I could hear a faint hissing noise so I jacked that corner up and turned the wheel around and there it was...

Regards,
Simon

fastandfurryous · Post by **fastandfurryous** » 04 May 2006, 21:05

This is a good point... a rear flat will have about the same effect in just about any car, as there's no (or very little) steering associated with the rear wheels.

It's a front complete blowout that's the danger.

andmcit · Post by **andmcit** » 04 May 2006, 21:15

ChevronMicky wrote:Andmcit, I'm certain the BX was advertised as one car not affected by blowouts.

I'd be very surprised - the steering angles cannot be ignored - the issue is going to be muddied by the self levelling no doubt about it, but ths fact is, the axis angle of pivot acts through the strut and therefore the wheel totally differently to the older Citroens.

Andrew

Kowalski · Post by **Kowalski** » 04 May 2006, 21:31

andmcit wrote:McPhereson struts!! That's where it all went wrong...

I'd want rid of the nose heavy front wheel drive setup and get the weight distribution more even (rear wheel drive) before I'd consider swapping struts for wishbones. BMW's 3 series has what BMW calls a double-joint tie bar suspension setup (personally I'd call it MacPherson strut) and is generally thought to be one of the best handling cars in its class, it does also come with rock hard suspension, oleopneumatic would probably improve it.

The 407 is dual wishbone setup, the C6 is going to be too. I wonder whether you could do a little retrofitting of 407 or C6 suspension onto a C5, C5s are looking awfully good value...