Ride quality...is it my imagination....

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Re: Ride quality...is it my imagination....

Post by white exec »

Peter.N. wrote: The 20mpg was tongue in cheek as most of the fuel was pouring out under the engine due to the leaking pump, they are not really as bad as that.
Thank goodness for that! Don't want the 2.5's to get themselves a bad reputation - they're an endangered species, you know! :roll:
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Re: Ride quality...is it my imagination....

Post by Peter.N. »

I've got four in the field - they don't belong to me though, should be able to make up at least one good one. :wink: Look at Google earth under 'higher spence B&B.

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Re: Ride quality...is it my imagination....

Post by Mandrake »

Peter.N. wrote:If you increase the sprung to unsprung weight ratio it will always improve the ride because you don't get the 'tail wagging the dog' effect which is why things like Rolls Royces ride so well as will reducing the weight of the wheels.
Quite right.

This is particularly important when you hit a large sharp bump like a ridge because the wheel's un-sprung mass will actually bounce up and down on the tyre - the more un-sprung mass there is the worse this oscillation will be. When the sprung weight is heavier the suspension is usually stiffer and heavier damped in proportion, this stiffer suspension and damping helps damp the oscillation of the un-sprung mass.

This is one reason why the DS and GS ride so well, especially considering that a GS is a very light car. The inboard disc brakes at the front dramatically reduces the un-sprung weight at the front.

Another factor that people overlook in suspension design for ride quality is parasitic friction - which is very high on a McPherson strut design like a BX/Xantia/XM/Early C5, with its sliding hydraulic ram that is also the top support for the hub which has to take all the side loads and not just the vertical weight of the car, versus the parallel wishbone arms with roller bearings in a DS/GS/CX.

Parasitic friction in suspension causes the ride quality to get worse with light loads and is probably the single biggest contributor to poor/harsh ride in lightly loaded cars. (Loading the car up negates the effects of high parasitic friction to a certain degree)

There are a lot of problems with this McPherson strut approach for ride quality:

1) Even under neutral conditions, (coasting in a straight line) the parasitic friction of the Xantia etc McPherson strut is quite high, especially considering that the top bronze bush runs completely dry.

This same high parasitic friction problem also exists in concentric coil spring design McPherson struts because the shock absorber units are themselves are directly connected between the top of the hub and the body, and internally are a hydraulic ram with parasitic friction. To get the very best ride quality hydraulic rams and/or shock absorbers should always be connected via a lever arm that will reduce the movement ratio.

2) In the McPherson strut design any side loads will try to twist the hydraulic ram and the ram must resist this. Doing so greatly increase the parasitic friction over and above the already high level that exists under neutral conditions, by probably a factor of at least 2x.

The things that will cause side loads on the strut in order of severity are braking, cornering and acceleration. Whenever you do one of these parasitic friction goes up so much that "binding" becomes an issue, where anything other than a fairly significant impact causes no movement whatsoever. This is what causes a good part of the "fidgety" or "harsh" ride that you can get in adverse circumstances.

Hard braking is particularly bad because all of the brake torque (which is typically a lot more torque than the engine can provide) is "winding up" the side load on the strut and the hydraulic ram has to resist this but it suffers from "binding" in the process. It's also a factor in cornering too - ironically this "binding" of the strut under hard cornering is actually one of the things that makes Xantia's etc roll so much less than older Citroen's around corners - as soon as you push it into a hard corner the struts tend to bind up a bit even without Hydractive 2 to switch to hard mode.

So there is some inadvertent roll resistance there, but it means that the car will ride more harshly around that tight corner and probably not grip as well over ridged surfaces as it otherwise might.

3) The geometry of the McPherson strut is significantly compromised - the strut itself slides almost vertically but the hub and lower ball joint are offset towards the outside by several inches tilting the steering axis inwards at the top. This means that when you turn the steering the strut top rubber has to twist and this twisting action places static side loading on the bushes in the hydraulic ram, so any time you are taking a sharp corner even at low speeds the parasitic friction goes up.

This is also the reason why a Xantia on full lock will often do a "merry dance" where it keeps correcting the height up and down constantly - the parasitic friction of the strut on full lock is high enough to cause stiction near the middle of the suspension travel and thus the height corrector keeps over correcting past the sticky point...

Furthermore and tangentially related to the McPherson struts bad geometry is that the roll bar link attachment on the strut itself causes the roll bar to be turned when you turn the steering in either direction - making the height corrector think the car is too low, thus the ride height always goes up on full lock, and this is usually enough to trigger the "merry dance" that then results from the increased parasitic friction.

Lets see how double wishbones (DS/CX/GS) addresses most of these problems and how inboard brakes (DS/GS) addresses the remaining problems.

1) Parasitic friction from the hydraulic ram is reduced by a factor of 3 by connecting the ram to a reduced motion part of the arm further inboard. So the wheel movement has "purchase" over the parasitic friction of the ram and doesn't see the full real friction. For a large suspension movement the hydraulic ram's piston isn't actually moving that much.

2) In the actual geometry part of the suspension (which excludes the decoupled hydraulic ram which has a pushrod and single point attachment to the piston) there are no sliding bushes only rotating roller bearings. Roller bearings have inherently far lower parasitic friction both due to being rolling instead of sliding, and also because there is a massive reduction ratio between the end of the arm attached to the hub ball joint and the diameter of the bearing shells. The ratio is probably something like 10 to 1. So the double wishbones with roller bearings design has massively lower parasitic friction.

3) Parasitic friction increases far less under acceleration, cornering and braking loads compared to a McPherson strut layout. In fact it hardly increases at all. None of these loads are presented to the hydraulic ram itself as it is fully decoupled, it only sees the up down movement of the top arm connected via a pushrod. The only increase in parasitic friction will be a relatively small perhaps inconsequential increase in friction in the roller bearings as more side loads are applied to them, but roller bearings respond to side loads with much more dignity than a sliding bush, especially a dry sliding bush.

One drawback of having such low parasitic friction even under hard cornering is you will get more roll, and that is where quite a bit of the famous roll of older Citroen's comes from - it's not all just about sphere and damper tuning. On the plus side, the suspension is still compliant and supple even under cornering loads meaning that ripples/ruts in the road will still be absorbed well, and it may actually grip rough roads during cornering better despite the increased body roll - at least that's what I remember from driving a CX - lots of body roll but plenty of cornering grip even on rough surfaces.

Inboard brakes:

4) Double wishbones with roller bearings can already resist braking torque quite well without too much increase in parasitic friction, but if you put the brakes inboard you remove the braking torque from the suspension arms completely...

5) Un-sprung weight is reduced quite dramatically. Together with reduced parasitic friction of the suspension, this makes it much easier to damp the "wheel hop" oscillation that normally occurs. If you watch old videos of DS and GS showing the front wheels going over steps in the road you can see that the wheel hop response of both is exemplary - there is very little oscillation and the wheel follows the bumps on the road very accurately. Not so on a modern car with big heavy alloy wheels and disc brakes at the front giving very high un-sprung weight! High parasitic friction when cornering, braking or accelleration also greatly reduces the ability of the suspension's damping to control the wheel hop oscillations.

Unfortunately from a purely mechanical/geometry point of view it's been all down hill since the DS and GS.... The GS has quite possibly the best front suspension geometry design ever used on a mass production car. There aren't any compromises in the geometry at all. Even the still very good CX has compromised geometry due to the offset ball joint to accommodate the brake disc. (But makes up for it in other areas like the full under chassis rubber block isolation to achieve a better overall ride quality than the GS, which lacks any isolation between the front suspension chassis and the body as they are just bolted together)
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Re: Ride quality...is it my imagination....

Post by GiveMeABreak »

Simon, thanks for this interesting comparison and good read. How does all this information relate to the C5 X7 setup specifically though, which would also relate to the C6 in some part, with which it shares the same running gear, albeit without the AMVAR system; as I understand it the setup is completely different from earlier C5s.
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Re: Ride quality...is it my imagination....

Post by Paul-R »

An excellent post Simon. It reminded me of so much that I had forgotten!

I am also interested in the X7/C6 analysis as well. Double wishbone front and multi-link (whatever that means) rear.

As a matter of interest, how long did it take to type up?
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Re: Ride quality...is it my imagination....

Post by Mandrake »

GiveMeABreak wrote:Simon, thanks for this interesting comparison and good read. How does all this information relate to the C5 X7 setup specifically though, which would also relate to the C6 in some part, with which it shares the same running gear, albeit without the AMVAR system; as I understand it the setup is completely different from earlier C5s.
To be honest I don't know enough about the C6 and C5 X7 suspension to really comment on them.

I did look into them a bit a few years ago and remember that they both use some kind of multi-link setup in the front instead of McPherson struts, and I did look at the AMVAR system a bit, but I can't remember much in the way of details.
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Re: Ride quality...is it my imagination....

Post by GiveMeABreak »

Thanks anyway - it has provided some really interesting comparisons. I recall the multi link systems was to improve the steering vibration - which I think is what the theX7 carried across from the C6 - from Julian Marsh's site:

Front and rear suspension
Double wishbone front suspension with a linked hub carrier
The double wishbone and linked hub carrier system separates vertical suspension travel and deflection.

As a result, the Citroën C6 steers a precise course in all circumstances, even on uneven, bumpy roads. This virtually eliminates steering vibrations while optimising cornering precision.

Multi-link integral rear suspension
The rear suspension features a longitudinal arm linked to the hub carrier to ensure longitudinal control and to minimise vibrations.
The suspension is assembled on a light alloy cross-member to provide good geometrical precision and rigorous dynamic control of each wheel, for optimum driving dynamics.
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Re: Ride quality...is it my imagination....

Post by lexi »

Even leaf sprung Land Rovers are more comfortable with 4cwt in the back.
Although the mechanics are different, it is akin to ballast in a boat. There is an optimal load where things handle good and are stable.
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Re: Ride quality...is it my imagination....

Post by Paul-R »

lexi wrote:Even leaf sprung Land Rovers ...
They have springs?
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Re: Ride quality...is it my imagination....

Post by myglaren »

I found my C5 estate rode far better when seriously overloaded and continued to do so for a few days afterwards.

Excellent write up Simon!
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Re: Ride quality...is it my imagination....

Post by lexi »

Paul-R wrote:
lexi wrote:Even leaf sprung Land Rovers ...
They have springs?
Parabolic springs on a leafer with weight in the back is not too bad Paul :lol:
Mind you, doing The Ring of Kerry with standard springs? My back never recovered from that! :shock:
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Re: Ride quality...is it my imagination....

Post by white exec »

A huge amount of thought was put into Rover P6 rear suspension. It featured in-board discs (right next to the rear diff), and a deDion tube. The latter kept both rear wheels perpendicular to the road at all times, so maximum grip. Coil springs all round, including a novel set up at the front, where the springs were mounted horizontally, acting against the bulkhead, and operated by a bell-crank. This was chiefly aimed at maximising available engine-bay width, originally with the gas-turbine in mind, but also, later, as it turned out, just right for the V8.

Like DS (and Rover dismantled several of these during P6's design stage), there was - by today's standards, and a million miles from XM and Xantia Activa - an enormous amount of body roll when the car was thrown into corners and through slaloms. But, the roadholding and security provided were simply impressive. V8 provided the Police with an extraordinary chase vehicle, which they were reluctant to lose. SD1, with its rigid back axle, just couldn't hold a candle handling-wise. But being made of tin-plate, it had a good edge on speed!

Allegedly, the inclusion of the deDion in the P6 cost Rover an additional £2.50 per car, and was nearly knocked on the head by the accountants, but heels were dug in. We had Wilks, King and Bache to thank for not letting that happen.
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Re: Ride quality...is it my imagination....

Post by GiveMeABreak »

Fascinating Stuff Chris. I recall as a teenager, an odd interest in the Austin Maxi, which my Father's friend owned. I used to stay with them on Exeat weekends or half term from boarding school when my parents were serving abroad. I recall him driving at speed over the sleeping policemen at the army barracks and it would fly over it keeping quite level. It was the Hydrolastic / Hydragas system IIRC. In those days though, Citroen's had so much style and individuality - there was simply nothing else like them on the roads..

It reminded me of my school French Teacher (who was actually French :-D ) - she had a Citroen GS Pallas. At the end of the day after school, she would literally launch herself out of the staff car park, straight down the tarmac ignoring any and everyone, including the 4 concrete speed humps. The car just soaked them up without a care!

Those were the days.
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Re: Ride quality...is it my imagination....

Post by white exec »

Nothing better than a pneumatic French teacher. Formative years, Marc! :roll:

Yes, Hydrolastic was quite extraordinary, and different to Citroen in that it hydraulically linked front and rear. Thinking was that as the front wheels encountered a ridge or rise in road level, the rear wheels would rise at the same time, thus keeping the car level. It certainly worked, in that during normal driving there was virtually no tendency to pitch, but the cars did have a characteristic vertical movement! An absolute gift to learners needing to do a good hill-start: with the handbrake on, you let up the clutch until it bit, and down went the rear end; release the handbrake, the rear end went back up and the car went forward - never failed.

Hydralastic was at its best on larger cars with a longer wheelbase. Fine on 1100/1300, Maxi, etc, and I think it was on the Austin 3-litre too. Not so good on the Mini, which always seemed a bit 'on wobbly stilts' with it. Moulton's rubber was better. Come to think of it, I think Maxi got Hydragas, or was that on the awful Allegro?

Someone once told me that Hydrolastic fluid was pretty much water, with corrosion inhibitors...
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Re: Ride quality...is it my imagination....

Post by lexi »

Sorry Chris, I got mixed up with Paul R there. P6 was special in suspension with what you mention. I had one but a friend way back actually waited for a Police spec with the high lift cam from the factory. It was at least a 6 month wait and about a 1973 model? It was of course the V8S model. Was awesome back then. On the BL stuff, I had a Morris 1800 too.That was a good ride when pumped up and working well. Guys used to advertise in local paper "Hydrolastic suspension pumped up at your door" :lol:
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