Using Adjustable Drop Links to Modify ARB Characteristics

[Bobulon]

Howdy! I'm basically looking for a way to soften the effect of the Eibach front ARB on my GTA as it is too stiff for bumpy country roads. I recently came across some info on how adjustable front drop links work and thought I'd ask if anyone here has used them for this purpose. I'm thinking of raising the angle of the ARB to 'dial out' some of it's effect and then reverse that for track days and such.

[johnl]

Assuming that I understand correctly what you are asking;

Within the range of what may actually be physically possible (i.e. physical constraint would possibly / probably limit how much the angle of the ARB arms can actually be altered), I doubt that changing the static angle of the ARB arms would have a significant affect on the effective ARB stiffness. But if it did, then I suspect the affect would be the opposite of what you want (no data, just thought experiment).

Imagine the extreme example of the ARB arms being raised / rotated from horizontal to vertical. The 'lever arms' would no longer have any leverage on the middle of the ARB (that twists in torsion), and the effective ARB rate would increase massively. In theory the rate would become infinite, though in reality something would still deflect (or break...). The ARB would no longer be twisted as the suspenion rises / falls, but would instead deflect in bend, which I expect would take a lot more force than twisting it via 'lever arms'.

So, if my conjecture is correct, the ARB would be 'softest' with the arms at 90° to the vertical motion of the suspension travel (at normal static ride height). The more vertical the arms, the greater the effective ARB stiffness would increasingly become, so any change away from the arms being effectively horizontal would result in at least some increase in effective stiffness. I suspect that there may also be significant non linearity in this effect...?

I could be wrong...

Regards,
John.

[bazzbazz]

No, you're right. As you say, it is the opposite of what he really wants to do.

[Colin Edwards]

Hi Bobulon,

Are you using Eibach part # E40-10-001-01-11?

If you are using the above Eibach part # disconnect one of the front drop links and see what you think of the balance.  If the balance goes towards oversteer (very likely!) soften the rear bar by using the outer pick up point.  The above kit provides adjustable rear roll stifness.  Given the GTA is front heavy, it would be typical to run the rear full hard. 

Or leave the drop links in and try softer / lower duro bushes at the front?

[Bobulon]

Thanks John that makes sense. I have lowered the car quite a bit so the adjustable links might still come in handy to bring the ARB back to being parallel with the road as they are probably tilted up a bit now anyway.

Colin that's pretty extreme but I might try it just to see how it feels with no front ARB.

[johnl]

Re ARB bush hardness / softness / rate / linearity, my understanding (based on conjecture, thought experiment, limited knowledge...) is as follows:

Softer 'D' bushes will tend to make the effective initial rate of the ARB 'system' softer, but probably not in a linear manner, i.e. softer bushes will have less of a softening affect the more load is placed on them (and the load ranges from very light to quite heavy).

If the ARB is fairly stiff then the bushes will probably be the softest part of the anti roll 'system', but only in the initial / early stage of deflection when loads are quite low. Rubber will compress fairly easily at first, but the 'rate' will rapidly increase as load / compression increases. The bushes are in effect little 'springs, but unlike a steel spring, the rate of the rubber tends to increase substantially with deflection (which is why it is hard to design a linear rate rubber spring, but easy with a steel spring). The rubber will initially deflect fairly easily, and then abruptly become very resistant to further deflection. In this case the deflection equates to compression of the rubber as the ARB heaves upward at bush A and downward at bush B. This non linear characteristic of rubber is quite noticable with things like rubber bump stops.

So, the total system resistance to load / deflection will be initially dominated by the relatively soft rate of the bushes, but as the rubber further compresses the bushes' rate will exponentially increase in a not very linear manner. During deflection there will be a point at which the bushes no longer dominate the total 'system' stiffness (the ARB itself then does), and the transition may occur fairly abruptly.

With a stiff ARB mounted in soft bushes, a loading imparted into the ARB will initially compress both bushes, but the ARB itself will deflect (twist) very little. When the bushes reach the point at which their rate suddenly increases, the ARB will then start to significantly deflect. This is likely to see an abrupt increase in 'system' rate. With a soft ARB, the efect will be 'blurrier'.

The effect can be felt if you disconnect one of the drop links, get under the car, grab the loose end of the ARB and manually heave it up and down. A stiff ARB will be very rigid relative to human strength, try holding the one end in a vice and then try to twist the ARB manually, you might move the end of the bar a few MM. With typical standard rubber bushes, the disconnected end of even a stiff ARB can be moved a few CM as the soft bushes deflect (i.e. moved manually). Then, as the bushes stiffen up, the end of the ARB will become much harder to move. That it is possible to move the disconnected end of a stiff ARB when fitted to the car in soft bushes demonstates the degree to which soft bushes allow easy deflection of the ARB 'system', until the bushes stiffen.

So fitting softer bushes might be a good thing for softening ride quality on rough roads, or it might cause a significantly non linear and possibly abrupt change in the front weight transfer, or both. If front lateral WT suddenly and significantly increases after X roll has been reached (which implies an equal sudden lessening of rear lateral WT), then at that point there could be a relatively sudden transition from fairly neutral to relatively understeery some point after turn in.

So my long laboured point is, a stiff ARB mounted in soft bushes could concievably disrupt lateral WT (linearity), and so perhaps go some way toward creating a 'twitchy' handling car. This could range from very minor to something quite nasty. When upgrading from a standard ARB to one substantially stiffer, it may well be advisable to also increase the stiffness of the D bushes, as this would help preserve linearity. Stiff bushes would allow the chassis to more immediately 'see' the inherent stiffness of the ARB itself, and for the ARB system to operate in a significantly more linear manner, with all that this might imply for linearity of lateral WT (big deal for predictable handling).

Or, I could be way overthinking this...

Regards,
John.

[johnl]

Thanks John that makes sense. I have lowered the car quite a bit so the adjustable links might still come in handy to bring the ARB back to being parallel with the road as they are probably tilted up a bit now anyway.

It would be good to restore the horizontality of the ARB arms, but I tend doubt that you will experience a lot of difference on the road...

Colin that's pretty extreme but I might try it just to see how it feels with no front ARB.

I tend to think that you will experience a big difference on the road...

Years ago I was working on an early Mazda 323. I had to remove the front ARB for some reason and didn't get around to refitting for some days. The effect was a bit startling. It didn't really become oversteery, but the understeer reduced substantially. This might sound like a good thing, and it was fun in its' way, but oh the body roll and loss of actual steering response wasn't...

Regards,
John.

[Colin Edwards]

Hi Bobulon,
Are you running "track days" tires on the "bumpy country roads"?

Tire side walls too stiff for road use?
Are your front dampers ok?
You may need to increase the rebound damping with that fat front bar in use!

Colin

[Bobulon]

So my long laboured point is, a stiff ARB mounted in soft bushes could conceivably disrupt lateral WT (linearity), and so perhaps go some way toward creating a 'twitchy' handling car. This could range from very minor to something quite nasty. When upgrading from a standard ARB to one substantially stiffer, it may well be advisable to also increase the stiffness of the D bushes, as this would help preserve linearity. Stiff bushes would allow the chassis to more immediately 'see' the inherent stiffness of the ARB itself, and for the ARB system to operate in a significantly more linear manner, with all that this might imply for linearity of lateral WT (big deal for predictable handling).

I'm running the poly bushes that came with the Eibach ARB kit so hopefully they designed them to work with the stiffness of the bar, but I will try what you said and disconnect one end and push it up to see just how much movement is in them.

Hi Bobulon,
Are you running "track days" tires on the "bumpy country roads"?

Tire side walls too stiff for road use?
Are your front dampers ok?
You may need to increase the rebound damping with that fat front bar in use!

Colin

Yokahama AD08R tyres so probably pretty stiff suspension in KW V3 on pretty much the softest compression and mid range rebound.

[bazzbazz]

So, remembering what happened last time you had the GTA in Melbourne, and now your running it on track days while there as well!! 

Brave man . . . . brave man! 

[rowan_bris]

I have the same tyres on my GTV6 (Alfetta).  I have had them for 4 years I think and they are quite unforgiving.

[johnl]

I'm running the poly bushes that came with the Eibach ARB kit so hopefully they designed them to work with the stiffness of the bar, but I will try what you said and disconnect one end and push it up to see just how much movement is in them. 

As I said, on a human scale a stiff ARB can be very stiff and very difficult to manually twist (at all). On the car, if you can move a disconnected end up or down to any significant degree then most of that movement is IMO very likely to be deflection / compression of the bushes. On the other hand, a soft ARB can be very soft and easy to twist, the 147s useless 14mm rear ARB being a perfect example.

The Eibach bushes are likely to be harder than the stock rubber (I would expect). Even if the PU were of a similar hardness to the rubber bush material (some PU is relatively soft), the PU bushes would lose at least some compliance with the thgicker ARB simply because the ARB is thicker, i.e. the bush walls would be thinner to accomodate the ARB (for a given material, the less the depth / thickness of bushing material the softer stiffer the bush, and the wider the bush the stiffer the bush).

The non linear nature of a stiff ARB mounted in soft bushes, while I'm sure it is real effect, I'm not at all sure how big of an affect it may actually have in the real world. It would I expect differ from car to car, and what that car is being used for. My concern is theoretical, not a substantial real world problem that I have had to deal with, but when fitting a stiff ARB I do whatever I can to stiffen the mounting to avoid a potential issue. My 147 (road car) has a custom made 20mm rear ARB (a very good thing) mounted in rubber D bushes. However the bushes have been stiffened by means of shim plates placed under the base of each bush, but not beneath the clamp brackets. The clamp brackets pull the D bush blocks down hard onto these shims and considerably compress the rubber, making it in effect stiffer. Poly bushes would be better (probably), but the ARB surface isn't very smooth so not a good candidate for being mounted in PU.

I do note that serious racing cars never retain the stock rubber bushes (that I have noticed). A soft rubber D bush (or soft PU bush) will at first compress relatively easily and allow some degree of easy roll motion and inhibit weight transfer at that axle, then sudenly the bush won't compress much further. So with a stiff ARB in soft bushes, the %age of the total roll stiffness created by the axle line ARB 'system' will initially be relatively less, then after a small amount of roll has occurred become abrubtly quite a bit more stiff. This will impact the linearity of WT and may or may not be problematic, depending. It would I expect tend to occur shortly after turn in and well before mid corner.

Regards,
John.

[Bobulon]

So, remembering what happened last time you had the GTA in Melbourne, and now your running it on track days while there as well!! 

Brave man . . . . brave man! 

Not yet Baz, can't wait to do Philip Island though

I have the same tyres on my GTV6 (Alfetta).  I have had them for 4 years I think and they are quite unforgiving.

I still have a good set of Bridgestone RE003 on the OEM wheels so I might chuck those wheels on and see what difference the tyres are making to the setup!