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GTA Roll Bars vs Eibach Bars?

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eibach roll
11K views 63 replies 13 participants last post by  Fragoso 
#1 ·
Hi All,

Really debating at the moment whether or not to buy the Eibach roll bar kit for my 156, although I am not sure if the Eibach kits really live up to the price - £311.99 delivered with VAT direct from Eibach.

I need a new front bar for my 2.5 V6 either way and am interested in upgrading the bar, so thought maybe the GTA bars are perhaps nearly as good, and they also cost a little less than the Eibach kit, has anyone got any advice on this?

Many Thanks,
Tony.
 
#2 ·
I had a 1.6 Twinnie 147 and I went through this quandry myself, I chnaged the ARBs for GTA ARBs and was very impressed with the increased stability, My original review is here.

http://www.alfaowner.com/Forum/tuning-and-upgrades/107119-review-147-gta-arbs-upgrade.html

I have done a similar thing on the GTA but gone from standard GTA ARBs to Eibach and found the difference in a same order as going from standard on the twinnie to GTA ARBs.

Pascs also has some experience with different ARB setups on his 156s so good idea to get some input from him.

:)
 
#3 ·
I'd go for the GTA bars, but make sure you do front and rear to keep the balance.
If you just change the front it will make it understeer more.
 
#12 ·
I have concerns about the balance of the Eibach ARBs for a 156 SW 1.9 JTD16V. Those bars were for sure developed for optimal performance for a GTA which is more heavy at the front. The Eibach ARBs have 4mm more both at front and rear than the original ones. Isnt 26mm too much for a 1.9 JTD. I want less roll but also want the car to understeer less (which i already worked on raising the front tyres pressure a bit to decrease understeer, but i prefer to do this the right way with some arbs).

I'm with Bilstein B12 Pro Kit suspension and as some probably already know this suspension change the rake angle of the car. The rear gets a different front / rear height ratio than the original suspension. With B12 the rear gets lower in comparison to the front and that means that there is more weight on the rear, making the front lighter and of course making the rear more planted but the front gets more understeer. I want to reverse this effect a bit, so i'm worried that keeping the F/R stiffness ratio wouldnt work so well on a 1.9JTD due to the engine being lighter than the one on the GTA.

In the case of installing GTA ARBs the front as a 1mm improve and the rear 2mm improve meaning that the rear will get stiffer, which also means that the car will get more oversteer in comparison to the original setup.

Which are your thoughts about these variables?
 
#4 ·
If you are going to change the ARB's then I'd recommend going for the Eibach's if you can afford it - the change is not huge but the car is definitely flatter through the corners......also the rear Eibach is 'adjustable' so in theory you can alter the setup to give more or less rear stiffness

If you dont have a good set of dampers then I'd recommend you do this first as a change in dampers has a far bigger improvement than a change in ARB's

The GTA bars are only 1-2mm thicker than standard and the Eibach's are 4mm thicker than standard so there is quite a significant difference
 
#5 ·
Thanks for all of your replies.

I have the adjustable Koni Sport Kit (yellow dampers and blue lowering springs). Not sure how these compare to the Eibach/Koni spring and damper setup, although compared to what I had (standard springs and dampers) is a vast improvement, so Koni shocks and springs coupled with the Eibach bars would be a good choice you think?
 
#6 ·
The Koni kit is pretty low right -40mm ? how do you find that?

I'm very happy with the Eibach bars, you'll need to fit new genuine front drop links at the same time
 
#7 ·
I feel the Koni setup is pretty good really, I think it is -30mm, and has the benefit of being adjustable. I haven't touched this yet as the ride is still quite hard, but not spine breaking!

Thanks for your advice, I think I may go with the Eibach bars and I will fit new drop links at the same time. May I ask though, why genuine drop links?
 
#8 ·
I bought a set of non genuine Alfa drop links and the ball joint sticks out about 5mm more than the genuine ones. This is fine for normal ARB's but the Eibach's being a bit thicker or something, cause the non genuine ball joint to run very close to the suspension and appear to knock / rub against it.

Also I think non genuine ones can have a dubious quality and the Eibach's do put more force through the drop link
 
#9 ·
Hang on, that is exactly the problem that I have. One of my droplinks (nearside I think) is actually rubbing on the front suspension fork. So maybe this could be my problem? I can, with extreme pressure, move the anti roll bar from side to side, about 1/2 inch - would this be normal? The anti-roll bar I have has been fitted to the car for around a year and has covered hardly any mileage, so my thought in the roll bar bushes wearing out maybe wrong, as I have pattern droplinks (made by Firstline I believe).

Maybe if I was to try buying some genuine Alfa Romeo links and fit them prior to changing the bar, would this be an idea ???
 
#13 ·
If you have a Bilstein B12 kit, for a SW, I'd make up some spacers for the bottom of the rear springs to raise them 15mm from the strut spring perch. I think simply making spacers from a fibreglass paste should be strong enough. Just make a few drainage channels in the bottom. That should sort the aerodynamic angle of attack.

As the SW is heavier at the rear, use the holes for the rear ARB closer to the bushes. The JTD has more weight at the front so won't be significantly different from the V6. (the V6 is heavier overall but the JTD has a greater forward weight bias). The handling should be balanced at that as the variable rate rear coil springs should effectively self adjust the handling. As with any kit, the greater the roll angle, the greater the understeer effect. Therefore, if the rear is too loose in the wet or greasy roads, fit the rear ARB links to the outer holes.
 
#14 ·
So in your opinion i could stick with the Eibach ARB and just change from the softest to hardest setting according to driving conditions? I will have to change the front ARB for sure because mine got bent in a very hard pot-hole. We've take some time to find out what is making the front lean more to the left side than the right side, till the moment i've got the idea to unlink the ARB to see if the height gets leveled. And... surprise, it gets fully leveled, linked the droplinks again and it got uneven. So i will have to change it. Considering the labour cost i find out that is a good idea to make the upgrade of course with front and rear to keep everything balanced.

I have another concern that is the Xenon headlight leveling pieces. Hope the new ARB doesnt mess with those.

Raising the rear end seems a great idea.

The other issue with the Eibach ARB is that some guys seem to lose grip in tight corners. The thing is... how tight are those corners...? I love to do some twisties. My concerns is that i end up with a FWD car that gets much less progressive in terms of throttle control during corners.

Thank you for your input :rotate:
 
#16 ·
There was a question on here about the bracket for the Xenon self levelling. Apparently the Eibach front ARB doesn't have it but I think that some drilled and bent stainless steel and a stainless steel motorcycle exhaust clamp would sort that.

Yes, @Pud237, I'd expect no understeer with that set up. That said, the torque of the JTD is why the Q2 was created as it will always be able to spin the inner wheel.
 
#24 ·
Already fitted Eibach ARBs but noticed that the rear ARB doesnt sit centered. The right side (Uk driver side) drop link is pulled for the center of the car, while the left side link pulls toward the wheel. So it sits uneven.

What is even more weird is the fact that the bar has to limiter rings that fix around the bushing so... the only possibility is the bar being instaled inverted to fix the issue. Trying to figure out that with my mechanic. This was installed in another shop...
 
#25 ·
Assuming we're talking about the same thing, I also found that the Eibach 'limiter rings' were positioned in the wrong place to enable the rear ARB to sit centrally. From memory, my mechanic grinded the weld, re positioned the ring and re-welded. This enabled the bar to locate centrally and the drop-links to be vertical, not at an angle.
 
#26 ·
exactly it just seems that the limiter rings are positioned too far from the corner making the bar be positioned more to the left (looking from behing)

we’re also considering the possibility of the subframe being a bit shifted to the left side. Since the bar is attached to the subframe, the issue could be on the subframe instead of the bar. Will compare the Eibach with the old original one.
 
#32 ·
Misalignment of subframes is definitely possible though, but you need to get alignment of the wheels checked and look at SAI/camber and caster angles to work it out for the front, and camber angles for the rear. Can be a case of moving the subframe 2mm and re aligning wheels to get it fixed.
 
#35 ·
I have those differences before changing all the arms and dampers. I have B8 with Pro Kit all around everything with less than 2 years. In the meantime ive changed the dampers and springs from side to side in order to clarify if that would be the issue. The behaviour hasnt changed. Dampers, arms, antiroll bar, springs, everything was changed. The car started to behave like this after changing the antiroll 4 years ago. And at the time the subframe came out for the instalation. the car didnt in my hands and the behaviour changed after having the car, its its unlikely to be a bent chassis. Ive hit a pothole 2 years ago but it was on the opposite side, so if the damper was damaged wouldnt affect the opposite side in this way.

the subframe seems to me to be the only loose end.
 
#37 · (Edited)
I could be wrong, but, my suspicion is that it may have been around the time that your anti roll bar was replaced (which one?) that you first noticed a problem(?). To rule the suspect ARB in or out, perhaps try temporarily disconnecting one of the drop links, see if the asymmetric roll becomes unbiassed...

It seems like you have changed most things in the suspension, and the geometrical asymmetries you have given (camber, caster) are quite minimally different. I doubt it is a geometrical problem.

I have chased a subtle steering pull to the left in my 147 from soon after I bought the car. The steering felt a touch lighter steering left vs right. The car was more responsive right vs left. The handling was OK in either direction, but wasn't symmetrical in that it didn't feel the same in left vs right turns (either way was good enough, just felt like a slightly different car left vs right, a subtle affect, but I'm sensitive...).

I tried everything I could think of. Wheel aligned to symmetric settings repeatedly (I do my own alighments), to no significant affect. In desperation I also tried:

Asymmetric camber alignments where one side had up to 1° more negative camber than the other (I can adjust right side camber with slotted holes, to match whatever left side camber might be). Conventional wisdom says that the steering will pull toward the side with the least negative camber, but I was not been able to reduce the left biassed steering pull by making the left side negative camber greater than the right side camber. The tyres seem fairly insensitive to camber, at least how it affects a steering pull...

Asymmetric caster angles, up to 1° more caster on the left than right (I can change caster on the left side with slotted holes, to match whatever right side caster might be). Conventional wisdom says that the steering will 'pull' toward the side with the least caster, but this is not in my experience with this car where up to 1° less caster on the right side does not create a pull to the right, or even lessen an existing pull to the left.

Asymmetric thrust angle (to left and to right).

Aligned it with and without a driver equivalent mass in the drivers' seat. No substantial difference, but I do like it a bit better after aligning with driver weight accounted for (possible placebo...).

Corner weighted the car (as far as is possible, it can never be equally weighted left vs right).

Swapped tyres around, and new tyres.

Made adjustable ARB drop links front and rear, so all preload could be removed from the ARBs.

Made adjustable rear control arms to make rear camber adjustable, also with Toyota spherical rod ends and much stiffer Toyota rubber bushes (than the quite soft OE Alfa bushes).

Replaced any part which was obviously suspect or seemed possibly suspect. With one exception, the left side lower control arm seemed fine when on the car, ball joint was tight and both bushes seemed stiff when pried with a lever, but when I took it off the car the rear bush was in bad shape (damned thing did seem fine on the car...).

This turned out to be the main problem, at least the steering pull has gone since replacing the arm and it's offending bush. After all this effort and other things I have done, the handling is now amazingly good, for a fat FWD hatchback...

I have learnt some things along this journey;

Modern radial tyres seem quite 'numb' to camber angle. Setting my 147 up to have quite asymmetric side to side camber made no significant difference to the steering feel or handling. I suspect that car companies might request the tyre companies to design tyres that are relatively insensitive to camber, which would explain how the car companies get away with not providing camber adjustment these days. The stiffer the sidewall the more camber sensitive a tyre probably tends to be, but most radial tyres have pretty soft sidewalls...

Corner weight changes of quite a substantial nature have little discernible affect on how the car feels steers or drives, much to my surprise.

ARB preload has less affect than one might expect, i.e. very little.

At least some cars seem quite 'numb' to a significant difference in left and right caster angle. I suspect this may be related to scrub radius, i.e. if SR is near zero then the car will probably be relatively unaffected by unequal caster.

Regards,
John.
 
#38 · (Edited)
Speculative food for thought;

The driver sits offset well to one side of the chassis. As a result he / she experiences roll motion not only as an angular change in the 'levelity' of the chassis platform, but also as an associated rise or lowering of the seat height. The driver will subconsciously sense the dynamic rising as opposed to the dynamic lowering of the seat, not the height itself (i.e. is the seat moving up, or is it moving down?).

This is because the driver is NOT located on (or even close to) the same axis around which the body roll motion actually occurs (BTW this is not the 'roll axis' because there are hardly any circumstances in which body roll actually articulates around the geometric 'roll axis' as defined by the front and rear 'roll centres', but that is a massive other discussion...).

The car body will respond to lateral force by 'rolling' in an arc which is centred on an axis of roll (again this is not the more or less mythical 'geometric roll axis', but an axis that is at most only partially defined by the front and rear geometric roll centres). Consider that the driver is offset from the axis of actual roll motion, above and to one side of it, and to a different side depending on the direction of turn. Because the driver is offset to one side of this axis, he / she will be moved either up or down as the body rolls, depending on which way the corner turns.

In left and right hand turns the driver will either rise more than he / she lowers (when the car is turning in the ther direction), or lower more than he / she rises, because when roll occurs one side of the chassis gets lower to the ground, the other side gets higher off the ground, and this tends to be an unequal height change.

So all else being equal, because the driver is substantially offset to one side of the chassis, if enough roll motion occurs the driver will experience a greater change in height related movement (a rise or a lowering). The driver will sense either 'up' or 'down' motion, but also the 'up' will be more than the 'down' or vice versa, depending corner direction and the dynamic specifics of the particular chassis.

I suggest that with this non equal behaviour in left vs right hand corners (of any theoretical point being offset from the chassis centreline), that it may well be possible for a driver to percieve the degree of body roll and therefore the level of roll stiffness as being different in left vs right hand corners, even if the roll stiffness and roll motion is actually quite symmetrical...?

I suspect that as roll stiffness increases (so less actual roll motion), that the scope for such a possibly incorrect perception of roll stiffness being unequal left vs right would diminish.

So is it possible that your problem is only a percieved roll stiffness asymmetry caused by the laterally offset position of the driver?

As I said above, this is all just speculation on my part.

Regards,
John.
 
#39 · (Edited)
Speculative food for thought;

The driver sits offset well to one side of the chassis. As a result he / she experiences roll motion not only as an angular change in the 'levelity' of the chassis platform, but also as an associated rise or lowering of the seat height. The driver will subconsciously sense the dynamic rising as opposed to the dynamic lowering of the seat, not the height itself (i.e. is the seat moving up, or is it moving down?).

This is because the driver is NOT located on (or even close to) the same axis around which the body roll motion actually occurs (BTW this is not the 'roll axis' because there are hardly any circumstances in which body roll actually articulates around the geometric 'roll axis' as defined by the front and rear 'roll centres', but that is a massive other discussion...).

The car body will respond to lateral force by 'rolling' in an arc which is centred on an axis of roll (again this is not the more or less mythical 'geometric roll axis', but an axis that is at most only partially defined by the front and rear geometric roll centres). Consider that the driver is offset from the axis of actual roll motion, above and to one side of it, and to a different side depending on the direction of turn. Because the driver is offset to one side of this axis, he / she will be moved either up or down as the body rolls, depending on which way the corner turns.

In left and right hand turns the driver will either rise more than he / she lowers (when the car is turning in the ther direction), or lower more than he / she rises, because when roll occurs one side of the chassis gets lower to the ground, the other side gets higher off the ground, and this tends to be an unequal height change.

So all else being equal, because the driver is substantially offset to one side of the chassis, if enough roll motion occurs the driver will experience a greater change in height related movement (a rise or a lowering). The driver will sense either 'up' or 'down' motion, but also the 'up' will be more than the 'down' or vice versa, depending corner direction and the dynamic specifics of the particular chassis.

I suggest that with this non equal behaviour in left vs right hand corners (of any theoretical point being offset from the chassis centreline), that it may well be possible for a driver to percieve the degree of body roll and therefore the level of roll stiffness as being different in left vs right hand corners, even if the roll stiffness and roll motion is actually quite symmetrical...?

I suspect that as roll stiffness increases (so less actual roll motion), that the scope for such a possibly incorrect perception of roll stiffness being unequal left vs right would diminish.

So is it possible that your problem is only a percieved roll stiffness asymmetry caused by the laterally offset position of the driver?

As I said above, this is all just speculation on my part.

Regards,
John.
That would be valid and makes all the sense but i don’t think this is the case. The reason i think its not the case its because of the timing of the turn in is also different. And to me personally its a big difference. Its not only the motion, its the road feeling. I feel the outer front wheel to be very light during corner, not planted, which is strange. Normally i feel the stress on the body on the outer side. And at low speed that is the case. This happens mostly at high speed. Its like the car floats into the direction.

I’ve already wondered if it was something like tyre sidewall flex. I was running Toyo Proxes T1 Sport at the 4 wheels but recently put new Pilot Sport 4s at the rear and the rear Toyos came to the front. Will change them in the meantime because they’re also reaching the acceptable minimum. But if it was the case it would be equal in both cornering directions.

when i bought the car i felt it rolling more than now. The body roll now is substancially reduced when turning left. While turning right i dont feel the front as a block pointing to the direction but instead the front diving in an angular motion to the corner. The car grips but i don't feel it planted.
Never felt this on any car before so its quite disconcerting.

Now one thing that you said that made me wonder is about the left lower arm. The upper arms were changed at the same time but the lower ones didn’t. Being the left one, the oldest. It could be a possibility. I know... it should be changed on pairs. But at the time one of them was starting to knock and the other was new. I also noticed that when i’ve changed the lower arm, the caster value raised on the alignment.

It could be the case that if the arm bushings being worn, the arm could allow for a great camber only during compression...?

Another insteresting detail is that the left side is about 3mm lower compared to the right one. This after the proceidure we’ve done to the right damper lower fitting. We noticed that the right damper were incorrectly installed on the shop and it wasn’t properly sitting on the lower circunference. It was bolted in position but the damper has space to go down. We putted it right and the previous 9mm are now only 3mm. I wonder if driving for about 2years with uneven ride height (tilted to the left) could put enough stress to cause a premature wear on the left arm.
 
#40 ·
Semi random thoughts and blather arising from your statements and questions:

Front camber can be affected if the front lower control arm bush is badly worn. Camber is affected if this bush is compressed or deforms under load because these things will cause the lower ball joint to move in or out relative to the chassis. This bush (if worn / damaged) will have very little if any affect on caster angle, because the lower ball joint doesn't move backward or forward due to LCA front bush damage / wear.

Note that it is concievable for this bush to have become softened with age while still being correctly concentric when only lightly loaded. So, lack of unusual static camber doesn't necessarily indicate that the bush is OK. This bush if old / damaged / softened will likely suffer from excessive compliance and deformation under load, allowing erratic camber change with varying lateral loadings in use.

The other LCA bush is of course the rear one. If damaged / worn this bush will have little to no affect on the static camber, but is likely to have a significant affect on the caster angle. When worn / damaged / softened this bush allows the rear end of the LCA to move in and / or out. This doesn't allow the lower ball joint to move in and out, but instead allows it to move backward and forward, thus changing the caster angle.

The same applies to this bush as to the other, i.e. the rear LCA bush can be soft but still reasonably concentric, so correct static caster doesn't necessarily prove that the bush is OK. On the other hand, if the bush or entire arm is replaced with new and the caster changes, then it is a good bet that the old bush was not in good shape.

If one LCA has already been replaced quite recently then it should not be necessary to replace it again just because you are now changing the other one and it is considered 'best practise' to always replace suspension parts in pairs. If the LCA is still reasonably newish then the two bushes should still be perfectly OK, and ought not to have softened appreciably enough to cause a significant asymmetry, or need to replace what is not a particularly cheap part.

-------------

IMO tyre pressure seems very tyre sensistive. I have found that many tyres seem to work a lot better at quite high psi (or at least feel better, if by "better" we mean more responsive and fun to drive with), while others don't seem to need nearly as much. Tyres with less stiff sidewalls (i.e. most tyres) generally seem to require quite high pressure to be reasonably responsive, while 'sportier' tyres with relatively stiffer sidewalls (and treads to some degree) tend to be stiff enough at a lower psi, and higher pressure than that doesn't really seem to improve them.

Currently I have 44psi in my rear tyres and 39psi in the front tyres. Over the years of much experiment I have generaly found that I prefer it when I have more pressure in the rear tyres than the front ones.

I prefer higher psi in my road car tyres than most people, because I prefer a very responsive feel and behaviour. If I was racing on the same tyres I might not use quite such high pressure because as the pressure passes Xpsi the grip tends to decrease at least somewhat (while steering and handling response increases).

Horses for courses and everthing is a compromise. On the race track all that matters is what lowers the lap times, but on a 'sporty' road car we also want it to be responsive, sharp and fun to drive...

Regards,
John.
 
#41 ·
Semi random thoughts and blather arising from your statements and questions:

Front camber can be affected if the front lower control arm bush is badly worn. Camber is affected if this bush is compressed or deforms under load because these things will cause the lower ball joint to move in or out relative to the chassis. This bush (if worn / damaged) will have very little if any affect on caster angle, because the lower ball joint doesn't move backward or forward due to LCA front bush damage / wear.

Note that it is concievable for this bush to have become softened with age while still being correctly concentric when only lightly loaded. So, lack of unusual static camber doesn't necessarily indicate that the bush is OK. This bush if old / damaged / softened will likely suffer from excessive compliance and deformation under load, allowing erratic camber change with varying lateral loadings in use.

The other LCA bush is of course the rear one. If damaged / worn this bush will have little to no affect on the static camber, but is likely to have a significant affect on the caster angle. When worn / damaged / softened this bush allows the rear end of the LCA to move in and / or out. This doesn't allow the lower ball joint to move in and out, but instead allows it to move backward and forward, thus changing the caster angle.

The same applies to this bush as to the other, i.e. the rear LCA bush can be soft but still reasonably concentric, so correct static caster doesn't necessarily prove that the bush is OK. On the other hand, if the bush or entire arm is replaced with new and the caster changes, then it is a good bet that the old bush was not in good shape.

If one LCA has already been replaced quite recently then it should not be necessary to replace it again just because you are now changing the other one and it is considered 'best practise' to always replace suspension parts in pairs. If the LCA is still reasonably newish then the two bushes should still be perfectly OK, and ought not to have softened appreciably enough to cause a significant asymmetry, or need to replace what is not a particularly cheap part.

-------------

IMO tyre pressure seems very tyre sensistive. I have found that many tyres seem to work a lot better at quite high psi (or at least feel better, if by "better" we mean more responsive and fun to drive with), while others don't seem to need nearly as much. Tyres with less stiff sidewalls (i.e. most tyres) generally seem to require quite high pressure to be reasonably responsive, while 'sportier' tyres with relatively stiffer sidewalls (and treads to some degree) tend to be stiff enough at a lower psi, and higher pressure than that doesn't really seem to improve them.

Currently I have 44psi in my rear tyres and 39psi in the front tyres. Over the years of much experiment I have generaly found that I prefer it when I have more pressure in the rear tyres than the front ones.

I prefer higher psi in my road car tyres than most people, because I prefer a very responsive feel and behaviour. If I was racing on the same tyres I might not use quite such high pressure because as the pressure passes Xpsi the grip tends to decrease at least somewhat (while steering and handling response increases).

Horses for courses and everthing is a compromise. On the race track all that matters is what lowers the lap times, but on a 'sporty' road car we also want it to be responsive, sharp and fun to drive...

Regards,
John.
Yes, yesterday i was already wondering too much.

I have to change the longitudinal rear arms bushes. Its the only suspension part i haven’t touched since i’ve bought the car. I dont think that its what causing this issue but its about time to do it.

I also prefer higher pressures. At least on these soft sidewall tyres. The advantage of soft sidewalls is that you can change the car feeling just changing pressures, which is nice for older cars that don’t have adaptative suspension.
 
#43 · (Edited)
Yes, cold pressure. I've used quite high tyre pressures for many years now. Every time I get new tyres that I don't know, I experiment with the pressure until I find a psi at which that particular tyre feels to work well at (as I define 'well'), and usually end up at a fairly high pressure. IMO most tyres (especially the ones I can readily afford...) are pretty rubbish at manufacturer recommended pressures, but improve substantially with a bit of free air...

Most passenger car tyres have fairly soft casings (in particular the sidewalls tend to be very flexible, a lot more often than not the sidewalls are 'single ply' which almost always = soft). This helps make the tyre very compliant which assists the plushness of the ride quality, and most customers want a smooth and comfortable ride. Secondly it is cheaper to make tyres with more flexible casings, because typically there will be less material and less individual 'components' in the tyre, i.e. soft cased tyres are probably simpler in construction and cheaper to produce.

Higher performance 'sports car' tyres tend to have fairly stiff casings with more 'components' and a more complex construction, so tend to have a a harsher ride quality and cost somewhat more to make. Drivers of 'sports cars' tend to place a lot less importance on ride comfort, and a lot more on the dynamic ability of the tyre, and are often willing to pay a higher price for that.

When any tyre is inflated to a higher pressure, the casing will behave as if it was stiffer. The rolling elastic deformation will decrease, as will the lateral compliance when the steering is turned and when the car is cornering harder. A soft cased tyre inflated to a higher psi will feel and be more responsive. It will to some degree 'mimic' the behaviour of a tyre with a stiffer casing, because the higher pressure decreases the compliance of the casing.

'Sports' tyres with inherently stiff casings won't (in my experience) tend to respond as dramatically to raised psi as more 'ordinary' tyres with inherently soft casings (primarily intended as 'passenger car' tyres). 'Sports' tyres tend to already be stiff enough so more psi tends not to improve them much, and may even make them work less well, and certainly make the ride quality even harsher.

When I put a substantial psi into a cheaper softly cased 'passenger car' tyre, I am in some degree trying to make a silk purse out of a sows' ear. It is very unlikely that I will make the softer cased passenger car tyre behave as well as a 'proper' high performance 'sports' tyre, but it will be a lot closer to it than it is at 32psi...

You wondered about pressure being cold psi. I have found that the cold pressure doesn't increase more than a few psi as the tyre warms up in use, even on hot days, but I'm not constantly checking it.

The higher the initial cold pressure the less the pressure will rise as the tyre warms because the tyre will warm up less (despite having started at a higher pressure). This is because pressure increase is a product of temperature increase,and tyre temp comes from two sources, from road friction at the contact patch, and from elastic deformation of the casing.

Elastic deformation occurs with every tyre revolution as each part of the tyre 'rolls into' the contact patch and momentarily takes the weight of the car, and, when the casing deforms laterally when cornering. When the pressure is higher the elastic deformation is reduced both when just rolling along and when cornering. With less casing deformation the rubber molecules are not moving as much and there is less internal friction between the molecules, so the tyre casing generates less heat 'internally'.

My car handles very well with the pressures I use. The tyres wear very evenly across the tread (more evenly than when I used lower psi, i.e. below 40psi), and not any more rapidly. They are not going to burst like balloons, tyre 'burst pressure' is in excess of 200psi. I have maybe one flat tyre per year on average, which is no more than I ever had when I used lower psi, and many of the roads around me here are pretty bad (rural dirt, and a lot of badly patched bitumen...).

The subect of tyre pressures seems to have an ability to generate a remarkable degree of fear and loathing. In the past (on a Honda related site) I have been accused of being a completely mad fool and a stupid danger to all other road users and little children in the whole world because I don't stictly adhere to the car manufacturers recommended inflation pressures. Some people get quite upset about tyre pressures. I've never had a problem related to the higher tyre pressures I use (but plenty related to lower pressures...). This is just what I think and what I do, others can think and do what they want...

Regards,
John.
 
#55 · (Edited)
To me, personally, 44 psi is too much on the rear, at least with the suspension setup that i have on the car, which is quite firm. The max i've used on the rear of the 156 SW was 40 psi (cold) and i've found it fun, but dangerous on the limit specially on the winter at medium to high speed corners. I have a road section around the city area that connects to the commercial port, its a very flat twisty 2 lanes road, a total joy. With the 40 psi, i've had the rear slide at 110kmh on very light lift-off. To me its the point where the rear tyres clearly loses traction. Have in mind that i use 215/45/17 in 91 so its a XL rating, and its very likely that the fact of having a very firm suspension reduces the mechanical grip while aiding for some precision on the handling so i guess it might be different on your case. If it was only for low speed cornering i would stick to the high rear pressure, but since i've fitted the rear stiffer arb, i got that in a more balanced way.
Right now i'm using 2.6 (37.7 psi) Front and 2.5 (36.25 psi) Rear (with 2 passengers max) and i like it, the handling is well balanced across all the speeds. Alfa Romeo recomends 2.4 bar (34.8 psi) on all the corners after review, because the initial pressures they recomended were 2.2 (32 psi). So i'm using a little bit more. My rear bar is on the hardest setting of course. On the limit its balanced and very controllable with smooth lift-off when necessary.

Another thing i feel is that with higher pressures the road feel is firmer but less harsh. This sounds like a paradox but has a logic. Since there is more tyre damping due to the extra air, the vibrations dissipate more. So i feel more bumps, their vibration doesnt come through the struts and arms so much.

Tyres, i'm using Michelin PS4 at the rear and Toyo TR1 Sport at the front. I've had TR1 on the 4 wheels but recently the front ones came out and the rear ones went to the front and fitted PS4s on the rear. Will put new PS4s to the front as well. They're nice. Not the best handling, but very grippy and with high treadwear for what they are. In the meantime i guess the next time i will put P Zero Nero on all four corners. The Neros have stiffer sidewalls.
 
#44 · (Edited)
If i can ask a question unrelated to the current discussion, is it OK to replace just the rear ARB with something stiffer, and what would be the effect? Im considering ordering the eibach kit (as i feel like GT still has too much body roll in sweeping corners), and while im sure i can fit the rear one myself, i know replacing the front requires subframe removal, and might take a while for me to sort out, so i might end up driving with just the rear replaced with the eibach bar, and the front as stock.

I see people here mentioning replacing just the rear, but my understanding (mostly from dicking around with ARB stiffness in racing games) is that changing just the one will end up dramatically changing the balance, and will result in either over or understeer rather then balanced cornering.

I really want to start tuning the GT, but i dont want to suprise myself with sudden oversteer because i only replaced the rear ARB

EDIT: did some more digging, and from what i can find the difference between stock and GTA ARBs is much bigger in the rear, going from 14 to 16mm, with the front only going from 22 to 23 or so, and only replacing the rear is a common mod. Once i get the car through its MOT ill see if i can scrounge up either a GTA or a GT V6 rear ARB somewhere and put that in
 
#45 ·
My experience;
I haven't driven on any of the available aftermarket ARBs (Eibach and Ultra Racing?). I think they are 18mm and / or 19mm diameter.

The stock 147 rear ARB is 14mm diameter, and very weak. It is so feeble that when I ran the car with no rear ARB (temporarily) I did not feel any change in the cars' handling or steering. I can't see why Alfa bothers fitting it...

I have no experience with the 16mm GTA rear bar, but considering my experience with much thicker rear ARBs (than the GTA bar), I would expect the GTA rear bar to be quite feeble too, just not quite as useless as the stock bar.

I replaced the limp wire (14mm bar) with a 20mm ARB sourced from the front end of a Holden Rodeo (i.e. an Izuzu light truck with a Holden badge on it), very modified to fit the rear of the 147. Initially I left the stock 23mm front ARB alone. This customised 20mm ARB is much much much stiffer than the stock 14mm rear ARB.

As you would expect body roll was very much lessened. There was a substantial reduction in understeer, and a surprisingly minimal increase in 'harshness' (I would describe the feel as 'firmer' rather than 'harsher'). The steering became much more positive in feel and response.

Yes the balance is changed, which is a large part of the reason to "dick" with anti roll bars. The stock set up is very very undesteery. Increasing rear roll stiffness reduces understeer. It does this by increasing the amount of 'weight' which transfers from the inside rear tyre to the outside rear tyre, while simultaneously reducing weight transfer from the inside front tyre to the outside front tyre (this, despite no change in absolute front roll stiffness, but the front roll stiffness becomes a lesser % of the sum of total roll stiffness front + rear).

This is what changes the handling balance when the rear roll stiffness is increased (or if front roll stiffness is reduced). With increased rear roll stiffness and with lateral weight transfer occuring, the inside rear tyre becomes less loaded, so all of the rear weight is less equally shared by the two rear tyres thus reducing overall rear grip. Simultaneously the inside front tyre remains more heavily loaded loaded (weight transfer reduced), so the front weight is more equally shared by the front tyres, which increases overall front grip. So, with either increased rear roll stiffness or reduced front roll stiffness, the lateral grip balance moves toward the front of the car, and understeer reduces.

So, I can see no problems if the stock rear ARB is swapped for a much stiffer unit, while not also increasing the stiffness of the front ARB. I have done this, and it is good. Given my experience, my reccommendation is to fit the stiffest rear ARB you can find, whether or not you also fit a stiffer front ARB.

And more recently;
Initially the customised Rodeo 20mm ARB was not quite as stiff as the aftermarket 18mm or 19mm rear ARBs (according to the numbers spat out by an online ARB stiffness calculator, not actual measurement of the comparitive stiffnesses). This is because my custom rear bar had longer 'arms' than the stock and aftermarket ARBs.

A few weeks ago I further modified the rear bar, welding flanges onto the arms and drilling alternative drop link attachment points (holes) into the flanges, making the effective arm lengths adjustable. The bar is now set up with effectively shorter arms that make it significantly stiffer than the aftermarket 18mm / 19mm rear ARBs. This further improved the handling and steering, with very little increase in 'harshness'. Of course body roll has been reduced a bit more.

This worked so well that I decided to modify the stock 23mm front ARB, by welding flanges onto the arms with alternative drop link attachment holes (much as I did with the rear ARB). Using the alternative attachment points the arms can be effectively shorter than stock, for a significant increase in effective stiffness. At the effective arm lengths I am now using, the 23mm front ARB is now a bit stiffer than the 24mm GTA front bar.

As a result body roll is slightly less again, unsurprisingly. With all this increased substantial roll stiffness front and rear, the car corners very 'flat', though there is still a slight detectable amount of roll. This can be felt just as the car starts to change direction, but once it is actually turning it doesn't feel to roll any further. When pushed in a corner, the increase in front roll stiffness feels to have increased understeer just a bit, but increased responsiveness.

Despite the slight increase in understeer caused by the now stiffer front ARB, I think I prefer it because it feels even more stable on the road, but with excellent response. The car is still waaay less understeery than it is with the stock ARBs.

Regards,
John.
 
#46 ·
@johnlear thanks for the writeup, i have to say i dont think my problem is understeer so much as it just is that body roll when turning into a corner, the car doesnt feel very sharp to me. Im sure there is understeer out there somewhere, but given the cars current feel, and the roads here (most of the nice twisties are on top of a dike, so a mistake not only means going off road, it means dropping 10-15 feet), im not that comfortable pushing it to the edge.
 
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