OK, so I'm being a bit melodramatic and it's more about me being curious as to how they work...

I get the front ARB, as it pivots about a point on the subframe (i.e. the chassis to all intents and purposes), which means that when one wheel's suspension is compressed it acts to compress the other side's too and therefore help reduce roll:

(Thanks to Tom for this one...)

The rear ARB OTOH has me scratching my head. It runs along the rear suspension cross-beam, and at no point is it attached to the chassis:

(This one is Sarah's)

Therefore, AFAICS despite the pivots, the ARB can only move in the same plane as the suspension beam and so I see no way that it can act to reduce roll unless the suspension arms flex at some point between the cross-beam and the hub (which I can't imagine they do to any extent...) Sure, there are droplinks at either end etc. just like on the front one, but I honestly can't see how they do anything to alter the way the rear suspension moves... :confused:

Somebody care to set me straight...? :ne_nau:

I've thought the same thing, hence the lack of one on my car.

What I'm looking at is a "U" shape, with the springs acting at the bends in the U, the ARB along the base, and the "droplinks" connecting the arb to the trailing arms, yes?

In which case the Furbie would appear to have a live rear axle! Is that right!?

The rear 'torsion beam' is exactly that in that it acts as an anti roll bar.ie if the suspension on one side is compressed when cornering, the beam twists and tries to make the other side (which would try to raise) compress as well cancelling out body roll. Most rear beams of this type have an anti roll bar inside them,so adding another externally will stiffen up the anti roll action, thats why people fit them,not just to look good.

Oooh

There is some clever people on here

I could have not said it better myself

Thank you

Sarah

Twist-beam rear suspension - Wikipedia, the free encyclopedia

It says there "no need for a seperate sway bar"?

Well we all know they work so why do we need to ask the question?

It says there "no need for a seperate sway bar"?

No there isnt as in practice the whole rear beam is an anti roll bar. However the rear beam is designed to flex a given ammount, making this stiffer will effect the handling. Really the rear anti roll bar is more a rear beam brace.

I did a practical experiment on this, based on reading the Wiki, using 2 40cm long plastic rulers.

Hold each end of one ruler in a hand, rest your elbows on something, move one hand up and the other one down simulating the action of suspension in roll, and watch the ruler twist.

Now hold the second ruler back to back with the first, and repeat. You will feel the difference in the twisting effects, which is an increase in roll stiffness, equivalent to the Furbie's additional rear ARB.

In answer to Ken's earlier question, here's a schematic I found of the Furby's suspension - the beam's sort of like a flattened H with the axles at bottom left and right and the pivots at top left and right:

I buy the stiffening argument in principle, but then that begs the question of if they were purely functional bits of kit, why bother with the droplinks? A nice beefy section of I-beam held on rigidly with U-bolts would surely be far more effective than something that has the cosmetic appearance of a 'proper' (chassis-mounted) ARB, thus avoiding the whiff of turd polishing that caused me to post this in the first place? I mean the OEM cross-beam is about four times the diameter of the RARB - can it really add all that much more to the rigidity?

I guess a lot of it also comes down to reports of 'lift-off oversteer' I saw in a number of write-ups, which I reckon is more to do with the placebo effect causing drivers to chuck their cars harder into corners, meaning that tyrewall flexion under high steering forces causes a momentary delay between steering input and direction change...

ISTR the Whiteline (and possibly others) being adjustable. That is probably where the droplinks come in.

Well, I found Decron's thread for the Roomy, which shows the details nicely:

http://briskoda.net/roomster/whiteline-rear-arb-fitting-1-9-scout/96521/

...but I still don't get the point of the pivots / droplinks. :confused:

From Decron's pictures, the droplinks allow you to attach the 'arms' of the RARB further or closer to the pivots, which will affect the amount of leverage applied, and thus how much the RARB will twist under load. But then seeing how it's only attached to the suspension and not the chassis, the action of the suspension twisting the RARB 'shaft' can only be the same as the reaction of the shaft pushing back against the suspension so the actual moment applied will be the same AFAICS wherever the droplinks are set. In fact, I'd go so far as to contend that the distance of the pivots from the car's centreline would have more influence on the adjustability, as the closer they were together, the more freedom there would be for the shaft to flex as well as twist... :ne_nau:

That is, of course, if they act to prevent roll by bracing the suspension as has been suggested, rather than having the lever action of the FARB which they obviously don't (to my eyes at least). Maybe the only way this'll be settled is by me sourcing some U-bolts and a length of girder...

Or we could just call the whole thing a ruddy nice chap and-it-doesnt-matter-what-the-race-is

From Decron's pics, moving the droplink to a more rearward (Roomster fit obviously) increases the length of the arm, which will increase mechanical advantage, but also increases the amount the bar has to bend for a given wheel movement.

Of course the whole "better handling" thing will be somewhat offset by the fact that the ARB is unsprung mass!

I am in the Lummox camp as it is more of a "beam stiffening" device.

I can assure you 1,000,000% that they work on the Roomster, it is not snake oil. The idea of the drop links is in essence to be able to stiffen or soften the effect of the bar.

On the Roomster even changing the ARB "holes" provides a noticeable difference in handling and whilst the Roomster (of course this is a furby thread) did not suffer from "excessive" body roll, mine now corners as flat as a pancake and the car is transformed on flowing alternating bends, it is much more controlled.

The Roomster pretty much shares the same rear axle as the Mk1 Octy / MkIV golf.

Neuspeed ARB on mine for ref .....

the idea of the droplinks is simply to 'join' it to the chassis or beam, that said, a position of a conventional droplink would noramally control the length of the 'lever' effect of the main part of the bar but in this appliciation it would make no difference because the pivot point of the arb is roughly on he same axis as the beam.

bascially the rear beam is not solid, they do flex a little, so in essence is is a large linear spring with a preset tension/poundage, so all you are doing in adding a rear arb is making that 'spring' stiffer..

i'm not denying or confirming the whole snake oil argument, but based on testimony from people who've fitted them they do appear to do something... weather or not it actually makes the car handle better or worse is another story?? after all, by stiffening the rear beam you are affecting the wheel rates or the rear suspension and thus affecting the handling, so... yes you can feel the difference but is it better or worse??

maybe if he fabia has softer rear suspension springs it would actually be better

Ok Tom, I see where you're coming from, but if we could move the normal spring or the beam fore and aft along the trail arm (and chassis rail for the coil), that would change the wheel rate without changing the the spring poundage, yes? I'm thinking that moving the droplink fore and aft will have the same effect (remember that an ARB is effectively a torsion bar spring designed to maximise its decoupling from the spring rate when both wheels move in the same direction in bump or rebound).

Ok Tom, I see where you're coming from, but if we could move the normal spring or the beam fore and aft along the trail arm (and chassis rail for the coil), that would change the wheel rate without changing the the spring poundage, yes? I'm thinking that moving the droplink fore and aft will have the same effect (remember that an ARB is effectively a torsion bar spring designed to maximise its decoupling from the spring rate when both wheels move in the same direction in bump or rebound).

The only thing with that is the RARB is attached with bushes to the beam and is not clamped. AIUI there's little or no rigidity there, so if the droplinks were removed, the arms would just fall to the floor...

The only thing with that is the RARB is attached with bushes to the beam and is not clamped. AIUI there's little or no rigidity there, so if the droplinks were removed, the arms would just fall to the floor...

It's designed to act in twist, so of course it just drops if you take the droplinks off. The same thing would happen with the OEM front units if you took both drop links off at once.

i don't think the roll bar would drop to the floor if the drop links are undone , the Jabba rear bar is clamped tight onto the rear beam axle as is the OEM front ARB

i can see the point that the drop links don't seem to do anything when the whole ARB is mounted on the rear beam , if the ARB was mounted onto the body, then adjustable links would work

and i can assure the sceptics that the Jabba RARB for the Fabia works , the car is so much more predictable and controllable when fitted , it is almost a case of night and day in the handling (yeah i know the Fabia is the best handling car) department

Briand posted some pictures demonstrating the RARB in action, and a normal vRS

OK, so I'm being a bit melodramatic and it's more about me being curious as to how they work...

I get the front ARB, as it pivots about a point on the subframe (i.e. the chassis to all intents and purposes), which means that when one wheel's suspension is compressed it acts to compress the other side's too and therefore help reduce roll

The other way of looking at it is that by linking either side with a sprung bar, you are allowing the uncompressed side to add its force to the compressed side and thus stiffen it up. Although as you point out the front arb pivots about a point attached to the chassis, it does just that - pivot. There is little 'leverage' being applied to that mount, so does it matter what it's attached to?

Looks similar to the back suspension setup on the Astra. I've a Whiteline rear ARB on mine, it mounts differently in that the bracket is bolted to the pivot point. I think I have a pic here somewhere.

Front hanger:

Fitted:

As someone said, it braces the beam so try and stop it flexing too much. Really helps bring the Astra more towards neutral handling, rather than it's understeer to snap oversteer tendancy as standard.

Snake oil??!!

Tell that to the identical fabia that was following me around a curve a couple of years ago - my only handling upgrade at that time was a rear ARB. He followed me exactly matched in speed and line and lost control (rear end came out). Must've been a shock for him, very nearly going off the road...

Bas