It's trying to nod!

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swiveleyedgit
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It's trying to nod!

Post by swiveleyedgit » 19 Nov 2014, 17:36

Something I noticed this morning, sitting in a queue of traffic I moved the gear lever from drive to neutral and the front end dipped an inch or so. Putting it back into drive made it rise up. Does it every time I tried it. Now, what's that all about?

Peter.N.
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Re: It's trying to nod!

Post by Peter.N. » 20 Nov 2014, 13:00

I would have thought that was normal, as soon as you put it into drive its trying to move forward, that would affect the suspension height.

Peter

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Stickyfinger
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Re: It's trying to nod!

Post by Stickyfinger » 20 Nov 2014, 13:18

Mine does not do that, not once started.

Hills and handbrakes can cause a "leveling" at the rear occasionally but the nose does not move during gear changes/selection at lights....just checked it on a drive this morning

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Mandrake
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Re: It's trying to nod!

Post by Mandrake » 20 Nov 2014, 14:41

Peter.N. wrote:I would have thought that was normal, as soon as you put it into drive its trying to move forward, that would affect the suspension height.
Not the front of a front wheel drive car Peter.

Engine torque on a rear wheel drive car lifts the front suspension but engine torque from a front wheel drive car compresses the rear suspension with no reaction on the front suspension.

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Re: It's trying to nod!

Post by Peter.N. » 20 Nov 2014, 19:37

Mmm, well if you brake it will alter the suspension height so why not if you do the opposite?

Peter

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Re: It's trying to nod!

Post by Stickyfinger » 20 Nov 2014, 20:00

not if your stationary....which is the question as I read it...

Why does the car front lift when I shift to drive from park whilst stationary in traffic in my auto?

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swiveleyedgit
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Re: It's trying to nod!

Post by swiveleyedgit » 20 Nov 2014, 22:55

Yes Stickyfinger, that's correct. I've got my foot on the brakes while this is going on so the car isn't pitching forwards or backwards at all.

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Re: It's trying to nod!

Post by Peter.N. » 20 Nov 2014, 23:11

Ah, right - no idea.

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Re: It's trying to nod!

Post by Stickyfinger » 21 Nov 2014, 00:30

OK, ?? just a guess, as it is Hydro-3, this system has a automatic - ride height/speed/bumpy road - system. Could it be that playing up ?

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Re: It's trying to nod!

Post by Mandrake » 21 Nov 2014, 10:57

Peter.N. wrote:Mmm, well if you brake it will alter the suspension height so why not if you do the opposite?
Acceleration and braking cause a different centre of rotation for the reaction torque...

When you accelerate on a front wheel drive car the reaction torque is centred exactly on the front axle - the entire car tries to rotate about the front wheels in a direction that causes the rear suspension to be compressed. Because the centre of rotation (don't know the fancy name for it) is in line with the front suspension movement axis (up down) there is no net up or down reaction on the front suspension. (Well maybe a tiny amount if the suspension geometry is poor, but in theory almost zero)

When you brake the situation is much more complex. Now you have the centre of gravity of the car (which is typically somewhere near of just below the gear lever on a front wheel drive front heavy car) being thrown forward. There is an imaginary line between front and rear suspension which has a point part way along called the "pitch centre" - this is the point around which the car rotates when it pitches forward when braking and the exact position depends on weight distribution, relative stiffness of front and rear suspension and suspension geometry, but it is always somewhere between the front and rear wheels. The centre of gravity being thrown forward tries to rotate the car around this pitch centre which imparts a downwards force onto the front suspension and an upwards force on the rear suspension.

A bit hard to explain without diagrams but accelerating and braking definitely aren't opposite effects when you look at the chassis dynamics and how it causes the body to react on the suspension...

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Re: It's trying to nod!

Post by Peter.N. » 21 Nov 2014, 13:03

Mmm, Thanks. It was much easer when we had king pins and bushes.

Peter

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Re: It's trying to nod!

Post by MikeT » 21 Nov 2014, 13:34

If it's any consolation, my Xantia will often cycle through a rise and bow/curtsey if I stop with a fair amount of steering lock applied (ie, queuing on a tight bend). I wonder if it's anything to do with torsion through the torsion bar as that's what the height corrector is attached to.

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Re: It's trying to nod!

Post by Mandrake » 21 Nov 2014, 15:02

MikeT wrote:If it's any consolation, my Xantia will often cycle through a rise and bow/curtsey if I stop with a fair amount of steering lock applied (ie, queuing on a tight bend). I wonder if it's anything to do with torsion through the torsion bar as that's what the height corrector is attached to.
It's caused by increased friction in the movement of the struts when you're on a steering lock - the geometry of the suspension and the rubber block in the strut top twisting puts a lot of side load on the bushes in the strut when you go on full lock, that increased friction causes the strut to "grab" and prevent the height corrector from smoothly correcting the height. (It keeps overshooting in a series of lurches because the strut is binding)

The thing that initially sets it off is that turning on either lock rotates the strut and with it the mount point for the roll bar link - this pulls on the drop links and rotates the roll bar making the height corrector think the front is too low so it tries to "correct" this!

It's normal for all Xantia's to rise at the front on full lock because of this. However if it keeps going up and down repeatedly that's a sign that the front strut rams need a lube...

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Re: It's trying to nod!

Post by MikeT » 21 Nov 2014, 15:23

Mandrake wrote:It's normal for all Xantia's to rise at the front on full lock because of this. However if it keeps going up and down repeatedly that's a sign that the front strut rams need a lube...
Ah, I gotcha. Sure I lubed the struts a few years back. Will do em again though. It doesn't always do it mind, but if it does start the up/down process, it usually repeats for that "session" of queuing.

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Re: It's trying to nod!

Post by Mandrake » 21 Nov 2014, 15:28

A few YEARS ago ? :lol:

I usually lube the struts on a Xantia at least a couple of times a year...

The top bush in the strut is a bit of a weak point in the strut design - it essentially runs "dry" and doesn't receive any natural lubrication... it's this bush that binds under side thrust. A bit of grease on this top bush (applied via the shaft) makes the ride immeasurably smoother but unfortunately it doesn't last forever as the lubrication works its way out again.

I once thought about modifying the strut to try to make the top bush self lubricating but never came up with a scheme I was completely happy with...