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Old 15-09-2013, 09:14 PM   #1
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Default IRS Torsion Bars and Spring Plates

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.
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Old 16-09-2013, 08:52 AM   #2
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Quote:
Originally Posted by H-G View Post

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.
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Old 16-09-2013, 11:05 AM   #3
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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
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Old 16-09-2013, 03:47 PM   #4
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Thanks for the information which make perfect sense.

Following on, are all IRS (and by default S/A) torsion bars at a set rate or can they differ? If the latter, is there a way to tell a softer TB?
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Old 16-09-2013, 04:55 PM   #5
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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
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Old 16-09-2013, 09:58 PM   #6
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Thanks to you all.
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Old 17-09-2013, 11:32 AM   #7
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Quote:
Originally Posted by al_kaholik View Post
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
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
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Old 17-09-2013, 11:53 AM   #8
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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.
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Old 17-09-2013, 12:27 PM   #9
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Quote:
Originally Posted by evilC View Post
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.
I'm still in two minds to sell them, I got them for a steal at the time, much cheaper than I could find any uprated IRS ones from a Type 3/4 or a 944. Best get buying and selling again I guess.

H-G look for someone breaking a car, that is probably the best guarantee of gettign what you want. Ask someone to measure them
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Old 17-09-2013, 01:11 PM   #10
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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.
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