Citroen XM Forum

This looks like is the full version of the test. Looking at 15:29 maybe reveals more of what the driver is actually doing. Looks like he brakes a little, the CX looses the straight just a bit, then he releases the brakes and brakes again once on dry soil. Still looks unfair on the other cars -they seem to just slam the brakes on them. Am I missing something?

http://www.youtube.com/watch?v=oZfB2sNEsU0

I think it's showing the benefits of the Diravi. Left side is on slippy surface, right side is dry road, driver brakes, twist reaction is set up trying to rotate the car clockwise due to grip difference, the front wheels react and the other cars go straight into an uncontrollable skid. But the Diravi holds the steering dead straight so the reaction is absorbed and the driver can carry on braking to a safe stop.

Marc

The slewing is due to weight transfer and the rear wheels locking, neither of which happens in the CX.

The weight transfer is forward, the normal cars inmediately lose grip on the left side and the car starts to rotate clockwise because the front wheels don't resist the movement, whereas on the CX the hydraulics hold the front wheels exactly where the driver points them throughout. That's the benefit of a Citroen with Diravi steering.

marc61 wrote:The weight transfer is forward, the normal cars inmediately lose grip on the left side and the car starts to rotate clockwise because the front wheels don't resist the movement, whereas on the CX the hydraulics hold the front wheels exactly where the driver points them throughout.

We obviously watched a different video.

In the one I saw, the steering remained straight, or even slightly turned into the skid, on all cars The metal spring cars pitched forwards allowing the rear wheels to lock and induced a spin.
The CX didn't lock the rear wheels due to the nature of the rear brakes reducing weight transfer and the method of feeding the rear brakes being proportional to load.

Dieselman wrote: In the one I saw, the steering remained straight, or even slightly turned into the skid, on all cars The metal spring cars pitched forwards allowing the rear wheels to lock and induced a spin.
The CX didn't lock the rear wheels due to the nature of the rear brakes reducing weight transfer and the method of feeding the rear brakes being proportional to load.

The proportional load to the rear brakes is not limited to hydropneumatic Citroens. A 1991 Clio 1.4 I had, indeed didn´t have this feature from factory, but when I swapped the rear axle for a R5 GT one (with brake-discs), it had it -was just a little lever attached to the suspension, acting on a valve, as you´d expect. I´d be surprised if in large sedans of that era the feature was not standard.

Is it maybe a case of that CX having ABS as standard, while the other particular cars didn´t?

Having owned an XM with Diravi, I can´t see it having any effect in this test.

Also, you can see that the CX finally comes to a stop further than the other cars -the focus of the camera changes at the end. I´d bet my life that if I slammed the brakes on a non-ABS hydropeumatic Citroen, it´d behave exactly like the others. The effect as in the video is dead typical of an ABS Vs. a non-ABS situation.

Almost makes me want to go out on my XM and try myself! With the ABS fuse off

On each of the cars if you study the immediate subsecond period after the driver hits the brake, the front wheels react by turning slightly to the right - the car experiences a clockwise rotation due to the difference in friction/grip between the left and right road surface as the brakes are applied and in a further fraction of a second the normal cars get into a right pickle. Meanwhile the CX overcomes the reaction because the Diravi holds the front wheels on course as the driver feels no reaction at the steering wheel unlike on the other cars and so the CX can steer out of trouble with brakes full on. That's why Citroen advertised it as being totally safe if a front tyre bursts etc because the driver can rely on the front wheels being held hydraulically by the Diravi. The SM and XM 24V are the same.

I agree this test is unfair if all the cars dont have ABS, but it's the Diravi power steering (not power assisted steering) that's makes the difference. Would be good to put a non-Diravi XM in the same test alongside a CX to see the difference.

I give up...!!

The nature of the anti dive setup on hydropneumatic Citroens and the brake pressure being fed off the rear means you cannot normally lock the rear wheels on such cars and the wheels stay in contact with the tarmac better.
Having locked up my own cars, I can guarantee that a Citroen will plough on straight ahead on any surface I have encountered.


I am aware of brake proportioning valves on cars with regular braking systems. For the valve to work the rear of the car has to raise meaning the rear tyres are more prone to losing grip, the Citroen rear sinks on braking, ensuring less weight transfer and better stability and grip.