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IRS Torsion Bars and Spring Plates

Suspension 
2K views 21 replies 7 participants last post by  H-G 
#1 ·
I've just started out on a 356 build and have taken delivery of the chassis which is set up for an IRS back end. I stripped all of the parts out of an older IRS (twin spring plates) and also have a set of swing axle torsion bars and spring plates.

Both fit the chassis (I realise that the spring plates are different) and prefer the cleaner look of the swing axle setup.

My questions are;

- why do the IRS torsion bars extend beyond the spring plates? I'm sure that VW had a good reason to do this, but I cannot find any explanation to it.

- could the swing axle spring plates be re-profiled and fitted to the IRS trailing arms providing the clean swing-axle look?

My apologies if this has been covered before, but despite a few evenings looking, I have not turned anything up.

Thanks in advance.
 
#2 ·
My questions are;

- why do the IRS torsion bars extend beyond the spring plates? I'm sure that VW had a good reason to do this, but I cannot find any explanation to it.

- could the swing axle spring plates be re-profiled and fitted to the IRS trailing arms providing the clean swing-axle look?

My apologies if this has been covered before, but despite a few evenings looking, I have not turned anything up.

Thanks in advance.
VW softened the ride with the longer t/bars.
and yes, the s/plates can be made to fit the IRS trailing arm and use the short torsion bars.
 
#3 ·
Plus the later '03 had a single spring plate as well as being suitable for the longer torsion bar. With a speedster/356 I would recommend the softer bars as they will be more suitable for normal/fast road applications because of the lighter weight. A good set of damper will make it handle. Putting stiff springing at the rear on a lightweight shell will make it skip on the bumps, losing traction. On the S/A design the TBs are intentionally stiff to restrict the camber changes and the attendant loss of grip. The stiff TBs were more preferable than the wayward sloppy handling with soft TBs and S/A
 
#5 ·
You can usually tell by the thickness of the bars. For example type 3/4 (IRS) used 23.5mm bars, Porsche 944 (IRS) MO30 option was 25.5mm and IIRC 21.5mm was what was used in IRS beetles.

Couldn't tell you much about SA ones other than if the car had a Z-bar it had softer torsion bars, presumably by being a different thickness
 
#7 ·
Just to add to Al's info.
The 944 standard bar is 23.5mm (M030 options are like hen's teeth though and create an ultra stiff rear wheel rate in a heavy bug, let alone a lightweight 356.)
The standard IRS TBs I have Mic'd at 22mm and think of them as that rather than 21.5mm
 
#8 ·
Thanks evilC. I am going to try and trace some torsion bars from a Z-link rear end and compare with the standard swing-axle bars i have. If they are a little thinner, I may give them a go. If it doesnt work then I can revert back to a more standard IRS setup. It will be a while before I will be able to tell if they work as the build is likely to be a couple of year, however this is one of the first areas to bolt back together. Based on the info above, I think that i'll not bother with an ARB for the back and just look to ARB the front end.
 
#10 · (Edited)
I run a Chesil Speedster on a '72 S/A chassis, The donor Beetle had the Z bar, (would need some body mods to fit that on the Speedster). The Suspension feels nice, the slightly softer Torsion bars are good with the lower weight of the Speedster body, but doesn't feel as soft as an IRS rear end.
 
#13 ·
Al - I know where there is a beetle being broken (a mate) and pretty sure it had a z-bar setup. If all goes well I can pilfer the bars from him. I will be needing some other parts for the build (front beam, engine, steering...) so I am looking out for a suitable donor

Bada - thanks for the 'real-time' info.

All the above is making me think that a Z Bar/IRS hybrid may be the way forward.
 
#14 ·
An short update and another query. I have acquired a pair of adjustable spring plates which saves me converting a pair of swing axle spring plates to suit the trailing arms. I also bought the urathene bushes and a new pair of spring plate covers.

Spent Sunday mocking the back end up which made change from wire brushing and being up to my elbows in a de-greasing tank. I'm happy with the look and think that with the torsion bars from a Z-Bar setup, things should work out well at the back end.

Now the query I have is to ask if anyone can advise on the need for a spacer between the spring plate cover and the chassis when fitting adjustable spring plates? Now I've mocked up, I note that the covers seem to 'bend' around the springplate rather than remain true.

My feeling is that there should be as there effectively two plates at the torsion bar end, but would like to hear your thoughts/experiences.
 
#16 ·
While I haven't checked them against a new set of rubber bushes, they were as near a damn it the same as the ones that were removed.

I'd be interested in your thoughts about the urethane bushes Bada Ben, and why you're not so keen on them.
 
#17 ·
Urethane bushes are too hard for that application, and because of that, they do not work the way VW designed the bushes to work.
The Rubber bushes actually "grip" the end of the trailing spring plate, and "flex" with the suspension movement. This tight fit also means that they prevent the ingress of water and grit.
Urethane bushes do not flex, so the trailing spring plate has to turn inside the bushes, they can squeak, and wear rapidly, especially if water and grit gets in.
Hard bushes also transmit road noise to the chassis, and generally make the ride uncomfortable. I think they are a waste of money on a road car, an advertising gimmick with no benefits. Urethane bushes have their place, on cars with Wishbone suspension,(where they have advantages over rubber), but not on the Beetle rear trailing spring plates. IMHO
 
#18 ·
I have polybushes in the rear of my '03 and the stiffer rate they impart to me is a big bonus. Having compliance within car suspension loses control and the less compliance the more control. However, there has to be a compromise between soft rubber bushes for comfort and spherical bearings for control and where that compromise is is down to individual requirements.
 
#19 ·
I went with the urethane bushes after fitting them to the front of my T25 and the immediate improvement they made to the handling, but need to conceed that it's a wishbone setup so probably not necessarily a like-for-like comparison.

evilC - I get the compliance (or lack of) side of the argument which seems to be one of the biggest selling points of urethane; what increase in road noise/wear rates have you noted?

Bada Ben - I have to say that I had not read/heard about the wear rates with urethane but what you say makes sense in that application and I will bear it in mind. I was aware that they do transmit more noise, but I am willing to put up with that, especially as they are going onto a soft-topped car which isn't going to be the quietest of cars. That said, I am going to stay with the stock rubber gearbox mounts to minimise engine vibrations through the chassis. You do raise a good point about dirt ingress which is very relevant to my question - spacers on the covers. If they do need spacers, this would open them up and allow more crud in than would normally be expected; that said, they would be considerably less open that the original twin-plate set-up that I took off!

I get the impression that it's comes down to personal taste and what you want to do with the car and how you intend to drive it.

H-G's over simplification:

- Easy comfortable cruising then stick with the OEM rubbers (that's not to say that they prevent any spirited driving)

- Fast(er) road then the urethane starts to provide a more solid setup but with some compliance at the expense of noise and possibly wear and maybe need a bit more maintenance.

- Fast road/race then you would be looking more at a sperical bearing (uni-ball) set-up where you don't give a rats about the extra noise etc and need everything to be pretty much soild.
 
#20 ·
Yes, EvilC and I have had this discussion before, and we agree to disagree. It's personal preference. I'm happy with the compliant rear end on my Swing Axle Speedster. I don't use it for ragging around a track, but have Koni Adjustable dampers, and it feels fine.
If you still have them, don't throw the old Rubber bushes away, you may want to fit them when the Urethane wears out.
 
#22 ·
Bloody good point gojira!

That's one area I haven't looked at. I don't have the shell yet but when it arrives, it gets fully bonded to the chassis so there's no going back once its on.

I'm sure that this is something that Gary at the Speedster Clinic has thought about and made prvision for, but will check none-the-less.
 
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