Anti-roll bars - an admission of failure?

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Anti-roll bars - an admission of failure?

Post by Spaces » 06 Oct 2011, 14:51

I was going to add a comment on the most comfortable Citroen ever thread but thought it deserved one of its own as I can't remember reading anything similar on here. It reinforces my idea that modern cars are engineered for the single carriageway ring-road, where acceleration and roundabout cornering are the two most important features of a car's dynamic abilities. It neatly fits in with the current belief that for a car to be any good on the road, it must be able to lap a race-track to the satisfaction of a TV presenter.
Below, two very different cars' front suspensions - one (1974 CX) with a minimally-sized antiroll bar, the other (1983 Pug-derived BX) a bit bigger.

Image Image


From a suspension engineer's perpective, the much-used anti-roll bar is a huge compromise - an admission that mass centres are too high, tracks too narrow and roll couples too big. It allows cheaper suspension to work acceptably well and for softer springing. But it is as cheap a major suspension component as there is - one main reason for its proliferation - and with well-designed positioning, not too thick a bar and careful tuning into the spring and damper settings they can work reasonably well.

With a suspension with adjustable-length pushrods, it is quite possible to create a well-engineered system which counters roll to a greater or lesser degree. Citroen themselves had one up and running in the sixties, http://www.citroenet.org.uk/miscellaneo ... ics-2.html which was part of Mages' original design for fluid suspension, so minimising or avoiding altogether the need for antiroll bars. Although the Xantia Activa used a very simplistic system to reduce roll by twisting up the torsion bar with rams - instead of altering the length of incompressible liquid pushrods which would have been the elegant solution as well as Citroen's original. This cheap approach is the one used by Range Rover and BMW which are trying to improve their ride and stability - however Mercedes has ditched the antiroll bar completely and used the variable liquid pushrod approach with its ABC suspension on top models. In the past it has used Citroen's patents for its estate cars' rear suspension.
Tenneco's Kinetic suspension (below right) as used by F1 and Citroen's rally C4 also does away with antiroll bars - http://www.autozine.org/technical_schoo ... nsion3.htm but requires electronics and all the wiring and sensors which go with it. Citroen's own (original) technology was as simple and clever as the its old-fashioned hydraulic valves to control fluid flow.

Image ImageImage

:?: So, if Mercedes rejects the Xantia Activa approach and throws away the antiroll bars along with the best Formula 1 cars, perhaps they're not a good thing at all. We all know that an antiroll bar appears to improve the cornering of a car and that it doubles as a place to position the ride height correctors on hydraulic Citroens, but what are the downsides to this bit of suspension?

It is a very crude piece of suspension - an undamped spring (torsion bar), which seems in complete contrast to the fineries of a Citroen's underpinnings. For many years the 'big thing' was 'independent suspension' - ie wheels able to move independently of each other, unlike those linked with a solid axle as on a 4x4. Linking left and right wheels with a torsion bar destroys some of the benefits of this independence.
Since such proven and quality cars as Saab's 99 and 900 (developed from many years' rallying success) did without them completely in most of their variations, are they completely necessary - or desirable? Many cars today use the cheap-to-make MacPherson strut with its less-than-perfect geometries and which works best without body heel - which happily coincides with public demand for cars which corner flat.

Provided both left and right wheels rise and fall at the same time, there is no problem - the bar moves up and down with them and provides no resistance to travel. But few roads are like this and even though soft rubber bushes can absorb some very small amplitude irregularities, anything much more than a couple of centimetres will have one side of the car affecting the other.
:bouncy:

This is felt in the cabin as the now universal rock-roll, which is accepted and even ignored by drivers and passengers even though the lateral forces on their necks can be considerable. Follow a modern car down a less-than-perfect road and watch the highly-visible effects for yourself. It will highlight either the discomfort of this effect as it happens to you, or make your well-suspended machine seem even better as you pass over the same bit of road without any rapid, uneasy motion. It is most evident where the left side of a road has repeated sink-holes or sunken grids.
As most cars are engineered by the French or Germans for smooth, level roads, our lumpy British ones can highlight this effect. The discomfort is nothing to the destabilising effect this has on the car at higher speeds, as wheels are jolted rapidly in one direction then the other, the roadspring's damper and tyre trying to damp out the torsion bar's springing oscillations.
If irregularites are on just one side of the road, the effect is limited to the opposite wheel. If they occur on both sides but not at the identical time, both wheels are being forced one way then the other, as well as actually coping with what is happening with the road at their own side. It is testimony to modern tyres and damping that the effects are not worse than they are. Not only is the grip reduced on an imperfect road, but steering is affected also, as would be expected with such violent forces acting on the front suspension.
:lolhit: Lots of Loading


A strong crosswind can cause you to have to turn into it on an exposed road, which unloads the 'inside' or weather-side tyre. With a heavy car travelling at lower speeds this may have no effect, but go faster in a lighter car and the last thing you wish for in a strong crosswind is uneven grip on the front wheels. Combine that with the front drive car's use of a rubber-bushed self-steering rear axle and the effect can be quite surprising. A sudden high gust will force you to turn into it if you want to maintain your course, which makes the car's cheap bits think you're cornering.
Even the heavy Xantia felt very shifty in a stiff and gusty crosswind last winter, so out of interest I took a CX onto the same patch of motorway and drove it at the same speed I had in the Xantia. The noisier A posts and door sealing of the CX only served to enhance the contrast. I took it to the same speed at which I'd originally slowed from and no difference. Even at an additional 20mph the old car tracked straight and true, barely affected by the breeze. What had been a tiring journey in the other car would have been a pleasure in the CX, apart from the dodging of nervously-crawling BMWs and weaving HGVs. :chin: :naughty:

As you corner, these forces can do nothing to help the tyre's grip with the road - they reduce it. Additionally, an antiroll bar reduces the grip on the inside tyre while loading up the outside - that's one of the immutable 'no such thing as something for nothing' laws. The only way round this is to fit wider tyres, which in turn fits in well with the workings of the MacPherson strut. All very well on a smooth and level grippy surface, but it makes the differential between good and poor road condition (ie surface, foundations, grip etc) abilities significantly greater. One thing which is very noticeable in a CX is just how much people slow when conditions deteriorate - and how that particular car's abilities continue to work at very nearly the same high level. :yes:

Of course, most cars are more comfortable than ever before in the eyes of their owners. Often this is the case, especially if you compare Fords from the 1980s with a 21st century one. But imagine just how good a modern car could be with low mass centre/low roll couple and independent suspension. :yeah:
Last edited by Spaces on 06 Oct 2011, 15:25, edited 1 time in total.

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Post by Norlander » 06 Oct 2011, 15:13

Had a GS in the 1980s, cannot recall strong winds from any direction bothering it at all. Was that due to suspension or the body shape?

Afaicr, it was noticeably good compared to other vehicles on normal (modern) tyres in snow as well.

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Post by myglaren » 06 Oct 2011, 17:42

Norlander wrote:Had a GS in the 1980s, cannot recall strong winds from any direction bothering it at all. Was that due to suspension or the body shape?

Afaicr, it was noticeably good compared to other vehicles on normal (modern) tyres in snow as well.
I'll second that and add that it was by far the most comfortable car I have owned and was commented on by every single passenger.

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Post by Spaces » 08 Oct 2011, 16:21

Image

Can anyone see what's missing from the suspension of the new McLaren supercar?

I forgot to mention that the man behind the technology used by Tenneco is Chris Heyring, who I remember reading about in the 90s. As a sculptor (interesting link there) turned art lecturer, he also appreciated old Citroens and inspired by the simple yet effective hydraulics he developed a system which sold for over $50 million, now used by Toyota/Lexus among others and outlawed in motor racing and rallying for being too good. It can be a passive reactive system or an active one with the use of sensors and a pump.

The McLaren supercar uses it to do away with 'stabilizer' bars, as this site calls the antiroll bar. There is a bit more detail about the system here and here's a short Telegraph piece about Chris Heyring.

The US military are thought to be making good use of the technology too - it really is where Citroen could have been if they had continued with clear, logical thinking and maintained vehicle suspension as their unique selling point. They appeared embarrassed about suspension hydraulics by the 1990s - which says quite a lot about the corporate intellect. It's such irony that while the richest and most upmarket motor manufacturers are starting to make use of the Kinetic suspension, the company whose world-class suspension inspired an Aussie sculptor to refine - even simplify it - has tried hard over the last quarter of a century to begin to banish all memory of it from its products.

Chris Heyring is involved in other ventures now, notably with bringing old-fashioned Citroen-like stability to water craft - with a good video here, complete with fleeting picture of a Traction Avant. It appears he's slightly of the ilk of Mages and Lefebvre, although by all accounts he's a humble guy and wouldn't really enjoy the comparison with such giants of engineering.[/img]

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Post by Dommo » 08 Oct 2011, 17:04

I was going to mention the Toyota Soarer, from '91, it had fully active suspension, didn't require an anti roll bar from what I can gather. There's a video on youtube showing a stability comparison of an active and none active Soarer and the none active one really struggled to match the cornering ability of the active one which didn't struggle at all.

So (unfortunately) whenever someone says the Activa Xantia was the first active suspension car they're not right, the Soarer was, and was a much better system! (Not cheap though at the time) The Xantia was the first Active anti-roll bar car, but not active suspension.

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Post by Norlander » 08 Oct 2011, 18:29

Can anyone see what's missing from the suspension of the new McLaren supercar?
Realism about the condition of roads in present-day Britain?

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Post by Dippy » 08 Oct 2011, 23:09

Anti' - Auto'

profoundly hopeful prefixes that are actually hidden code for :-

" someday it's gonna' screw you over big - time "

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Post by andy5 » 09 Oct 2011, 00:45

Spaces wrote: Can anyone see what's missing from the suspension of the new McLaren supercar?
I'm wondering why there's so much metal. Surely they could use carbon fibre to save a bit more weight.

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Post by addo » 09 Oct 2011, 07:59

I'm not sure the mechanical properties of CF reinforced resins are suited to compact high stress parts like LCAs and track rod ends! Titanium, maybe - but at what price do you call time?

A slightly naughty experiment today; I backed off the front ARB saddle clamps on the Xantia to leave about 1/8" airspace under the saddle clamp's leading edge. The front end - only tested at lower suburban speeds - articulated like a Dee. Much softer than usual!

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Post by Dippy » 09 Oct 2011, 12:20

Unequal lenghth wishbones, ball joints + shocker through coil spring .

Exotic materials apart , it's not that far removed from the
H.B H.C Viva /Firenza

Which ( for their time ) handled superbly - front end wise .

Full Circle ?

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Post by reblack68 » 09 Oct 2011, 14:29

addo wrote:I'm not sure the mechanical properties of CF reinforced resins are suited to compact high stress parts like LCAs and track rod ends!
They seem to work fine on bikes http://www.bustedcarbon.com/ Oh. :shock:

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Post by Spaces » 09 Oct 2011, 16:51

Dippy wrote:Unequal lenghth wishbones, ball joints + shocker through coil spring .

Exotic materials apart , it's not that far removed from the
H.B H.C Viva /Firenza

Which ( for their time ) handled superbly - front end wise .

Full Circle ?
They were largely extinct by the time I started driving but I do remember the letters to CAR magazine back in the 80s and early 90s (ie when the mag was world-class) from Kenneth H Ross. He maintained there was no finer cheap car (back then) if you stayed off the motorway network, making similar journeys in new Fords and berating their poor road manners. His letters were generally of a very high quality, with a broad sense of humour to make him even more memorable. Anyone else remember him, or any of his letters?

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Post by aerodynamica » 09 Oct 2011, 19:03

reblack68 wrote:
addo wrote:I'm not sure the mechanical properties of CF reinforced resins are suited to compact high stress parts like LCAs and track rod ends!
They seem to work fine on bikes http://www.bustedcarbon.com/ Oh. :shock:
What scares me is how sharp the broken edges are!

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Post by Dippy » 09 Oct 2011, 19:16

Spaces wrote:
Dippy wrote:Unequal lenghth wishbones, ball joints + shocker through coil spring .

Exotic materials apart , it's not that far removed from the
H.B H.C Viva /Firenza

Which ( for their time ) handled superbly - front end wise .

Full Circle ?
They were largely extinct by the time I started driving but I do remember the letters to CAR magazine back in the 80s and early 90s (ie when the mag was world-class) from Kenneth H Ross. He maintained there was no finer cheap car (back then) if you stayed off the motorway network, making similar journeys in new Fords and berating their poor road manners. His letters were generally of a very high quality, with a broad sense of humour to make him even more memorable. Anyone else remember him, or any of his letters?
Don't remember the Journo's name , but CAR was a good mag as you rightly say.

Gerry Marshall was a noteworthy driver for DTM ( Dealer Team Vauxhall )in the 70's .
He was also down to earth , to the point. and suffered P.R men badly.

B.L & Car Mag'( I think it was car ?) had him test the Morris Marina 1.8 T.C on a track just prior to it's launch date , it was also filmed.

Car quoted Mr marshall proclaiming " the British car buying public can trust the Marina , they can trust it to roll over at very low speed ! " or words to the similar ....

B.L threw a hissy fit and the ad was withdrawn . unsurprisingly Gerry M. did not suffer repeat business from B.Leyland.

He did go on to repeatedly demolish Mustangs , Camaro's & Jags etc on UK Tracks in what we now call Touring Car Racing. All with Viva's & Firenza's 8-)

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Post by aerodynamica » 09 Oct 2011, 19:38

two very different cars' front suspensions - one (1974 CX) with a minimally-sized antiroll bar, the other (1983 Pug-derived BX) a bit bigger.
Minimally-sized? but what are the relative torsional values of each at the axle? That isn't known is it? Looking at both pictures; the CX has the end of the ARB linked at a much longer distance from the hub (and the arm of the roll bar looks shorter) so could have a much higher ratio of leverage bar-to-hub movement - that is not directly ratable to bar 'size' without knowing the actual torsional values of each.
It is a very crude piece of suspension - an undamped spring (torsion bar)
Not sure that I'd agree this is an undamped spring. Which of the ARB's forces are not attenuated to one degree by the dampers of the main springs? 'Crude' is a value judgement: is it more crude a device than a tyre? It too, it a spring, minimally damped too, all of the mentioned suspension components at any stage are subject to the tyres' cycles of motion. Worth considering.
For many years the 'big thing' was 'independent suspension' - ie wheels able to move independently of each other, unlike those linked with a solid axle as on a 4x4. Linking left and right wheels with a torsion bar destroys some of the benefits of this independence.
Think Independent suspension is a relative term i.e. relative to rigid axles and having some advantages over those whether 'destroyed' by ARBs or other. By being imperfect it doesn't always equal full failure of the end aim (of idealised or close to idealised suspension or other subject).


Since such proven and quality cars as Saab's 99 and 900 (developed from many years' rallying success) did without them completely in most of their variations, are they completely necessary - or desirable?
-for rallying? or road use? Who knows what's desirable without stating the aim. Is the aim success on a rally stage? Then Saab shows they are not desirable.
Many cars today use the cheap-to-make MacPherson strut with its less-than-perfect geometries and which works best without body heel - which happily coincides with public demand for cars which corner flat.
But what are 'perfect geometries'? Even the dedicated track car has its camber, caster, dive, degree of anti-dive, squat etc, all changed quickly and frequently by the engineering teams between events, races and the like. This cutting edge of suspension use shows 'imperfect geometry' manually adapted and they similarly use no MacPherson struts (or their hybrid forms). therefore, having MacPherson struts is not the cause for 'imperfect geometry' because 'perfect geometry' in an abstract state.

The ideal solution would be on-the-fly variable geometry to accommodate the approaching conditions as well as anti-roll systems and variable spring and damper combinations, all on top of self levelling.

No, anti roll bars are a suitable solution for road use on cars that carry passengers and loads. ARBs omitted from Saabs would be unfeasable without sports springing and damping as seen on the rally cars.

I still reckon the Xantia provides better suspension than the CX and I'm a CX fanatic and owner of a CX and several previous CXs (albeit each an early series one) the absorbtion gifted in the CX is massively marred (if it's intended for comfort) by silly body roll and a lacking in vibration absorbtion. The Xantia copes with these last pointers in its stride MacPhersons and 'large' roll bars all the same.