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Old April 29th 2010, 17:54
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Post Building the Perfect Street/Track VW IRS Suspension System

by Adrian Pillow
Dec 03, 2002

Notes and Assumptions:

This is theoretical based off of information gathered from different sources on the Internet and literature. Please feel free to add to or correct any information that might be less than clear or accurate. The sources for information are given proper credit wherever possible.
You Super Beetle folks have to draw your own conclusions, as I do not know jack about the MacPherson strut setup in the cars.
Any omissions of available products from EMPI and KYB are on purpose, as I am not overwhelmed with their stuff.
This article is aimed at creating a suspension system that is aimed for street and track events (mostly PCA and SCCA auto-cross) based off an IRS pan with a light Beetle top body. The Beetle body will be in a stock configuration using metal fenders and such, essentially little to no "lightening" with Lexan windows, fiberglass, or stripped out interior.

I am going under the philosophy that the early 911 will be a close match to the Beetle in terms of weight bias and handling characteristics. Of course given that the front suspension is different we have to allow for some creativity in this area and infer things that are similar.

According to Bruce Anderson in his book Porsche 911 Performance Handbook here is the optimal early 911 setup for street and occasional track:

Front Torsion Bar: 21mm (stock 18.8mm)
Front Sway Bar: 22mm
Rear Torsion Bar: 26-27mm (stock 24.1mm)
Rear Sway Bar: 22mm

From here I will take each item in a sectional fashion as to try and avoid running issues together and hopefully make for a quickly referencable document that is easy to follow.


Front Suspension Torsion Bars:

First off drop the Link and King Pin setup (pre-66 models), no discussion about that here. We will assume a ball-joint front end as it seems to be the most common and logical way to go. The following information is derived from the VW Beetle Performance Handbook by Keith Seume.

The early VWs front ends came with four (4) torsion leaves per tube. Later King and Link pin models had four (4)large leaves and four (4) small leaves per tube. The ball-joint models had four (4) large leaves and six (6) small leaves for dampening.

Apparently a solid torsion leaf gives a harsh ride and the more separate leaves per torsion tube the more dampening effect you have. Being as the Beetle front end is very light as it is I would just go ahead and stick with the later ball-joint ten (10) leaf setup. No real areas of improvement the way I see it in this area. Granted if an Avis/Puma adjuster beam is added in I am not certain how this might affect the torsion beam action.


Front Suspension Sway Bar:

I am not even sure what sway bar is given in a stock Beetle configuration, but I can surely say that with any improvement in the rear suspension it will be too soft for use and need upgraded. Currently I am not aware of any other option than getting an aftermarket sway bar from such vendors as SAW - www.swayaway.com. The only product they have listed is a 3/4" unit for the standard Beetle front end. 3/4" = 19.04mm, this of course falls short of our goal of 22mm as earlier stated. Remember that we do not want the rear sway bar to be bigger than the front due to the sudden overstear that can be caused by this type of setup.


Front Shocks:

This is easy in my book, Bilstein ball joint shock PN is B46-0620. Granted Koni has early VW shocks as well that I will need to research more, PN 80-1787. Again Bilstein will be a tough product to beat, as they seem to be the longevity with retained performance champ a true 100,000 mile shock. www.shox.com has the best prices I have found and a very helpful staff.


Rear Torsion Bars:

This is clearly the toughest area of selection and setup/installation you will face in this whole suspension system. I am not even going to get into the proper adjustment of the torsion bars here, it is complicated and I would advise going to a professional race shop to get this worked out and corner balanced right, which is a great advantage when on the track or if you like to carve through twisty mountain roads.

The best VW rear torsion bar offered seems to be in the T3 Squareback wagon with 23.5mm torsion bars (thanks to Chigger for that info). The base sedans torsion bars seems to be 22mm units, but I am not sure of that. Basically any VW torsion bars that are 26 9/16" long are a direct fit/swap in an IRS pan.

SAW has many varieties to choose from in almost any combination you can imagine from 20-30mm. Here is a chart showing the spring rates of each SAW torsion bar, VW bars 26 9/16" long:

22mm (stock) - 464 lb
23mm (stock) - 554 lb
25mm - 773 lb
26mm - 904 lb
27mm - 1062 lb
28mm - 1212 lb
29mm - 1400 lb
30mm - 1603 lb


That was pretty dry, now on to the Porsche stuff! Any 924 or 944 (968?) rear torsion bar will fit into a VW IRS pan with no problems at all.

924 1976-1979 22mm
924 1978-1979 Optional 23.5mm
924 1980 + 23.5mm
931 (924 Turbo) 23.5mm
924S TBD
944 1983-1985 23.5mm With M030 = 24.5mm
944 1986+ 25.5mm
944S 26mm (according to KDanie)
944S2 24mm With M030 or M031 = 25.5mm
951 (944 Turbo) 23.5mm, With M030 = 25.5mm
944 Turbo S 25.5mm


Plenty of selection here! The 25.5mm torsion bars are a perfect choice and will match up closely to the 26-27mm that we were originally shooting for.


Rear Sway Bars:

The big choice here is between SAW (www.swayaway.com) and Porsche 924/944 parts. First lets cover the SAW products available... Which is one, the IRS 3/4" (19.04mm) sway bar kit. This again falls short of our 22mm goal, but is an easy match for the front 3/4" SAW kit. This might be the only shot of having front and rear sway bars of an equal size.

On a side note, I used to own a '69 Beetle and bought a 3/4" rear sway bar kit for it but never installed it due to the poor mounting hardware provided. The best way to mount these bars is by a clamp around the torsion tube or a weld on clamp that is on the torsion tube. There are some odd setups out there so just be careful with what you buy.

Now lets get creative with some Porsche parts! Most of the Porsche 924/944 has been gathered from this website: www.924.org/techsection/technical.htm under the "suspension" area. Basically any rear sway bar equipped 924 (1978+) and 944 can be used on the VW IRS pan with aftermarket clamps to the torsion tube or welded on mount/clamps. There should be a kit available through Weltmeister (Automotion or Performance Products) which clamps onto the torsion tube and provides mounting points for the sway bar. Granted it is $219 at www.performanceproducts.com, so maybe SAW has a cheaper kit or it is time to whip out the welder and make some.

924 1987+ 14mm, do not use earlier 18mm
931 (924 Turbo) 14mm
924S TBD
944 All None standard
944 1983-1985 Optional 14mm
944 1986+ Optional 18mm
944 M030 20mm
944S None standard, optional 18mm
944S M030/M637 25.5mm
944S2 1989 18mm
944S2 1990+ 16mm
951 (944 Turbo) 18mm
944 Turbo S 16mm or 18mm (Not sure which)
968 All 16mm, optional 20mm


There is no telling what is available from the Porsche aftermarket in this area as well. I bet if you need a custom sway bar one can be had through some good Porsche race shops... It will cost though. Since we have a 3/4" SAW unit up front at 19.04mm (3/4") then it is best to stick with the 18mm sway bar the way I see it. If you have fears of sudden oversteer or nasty road conditions then stick with a 14mm or 16mm unit.


Rear Shocks:

This is easy in my book, Bilstein IRS PN is B46-0034. Granted Koni has early VW shocks as well that I will need to research more, PN 80-2149. Again Bilstein will be a tough product to beat, as they seem to be the longevity with retained performance champ a true 100,000 miles shock. www.shox.com best prices I have found and a very helpful staff.


Bushings:

Some people swear by the urethane units and some by the stock German rubber units. There are advantages and disadvantages to each type. You be the judge on what works for you. Personally I will try the urethane units just because I like the long-term value due to the lifespan and tighter control of the suspension.

It seems that Rocky Mountain Motorworks has a good selection of the blue urethane bushings that are supposedly self lubricating. Opposed to the red ones which as far as I know do not have this feature.


Control Arms:

Of course the control arms are part of the suspension system and many people like to swap in the aluminum 944 control arms or other 944 components on the rear. This is very dependant on what you plan to do with the car and what rear track width you plan to run. I will leave this up to you all to decide on what works here in your situation. Personally I will stay with the stock VW IRS components in this area since I want a slim track to retain the stock fenders.


Conclusion:

As we have seen here it is close to impossible within reason to achieve the early 911 setup as Bruce Anderson laid out, but we are really close to a stock 911 setup with these easy add on swaps. All in all pretty good and keeps the budget within reason as well. In summary here is what we/I are ending up with:

Front torsion bars: Late model vw stock 10 leaf
Front sway bar: SAW 19.04mm (3/4") dia
Front shocks: Bilstein
Rear torsion bars: Porsche 25.5mm
Rear sway bar: Porsche 18mm with weltmeister clamp on mounts
Rear shocks: Bilstein


Hopefully this is a bit educational for you all as it has been for me compiling and researching all this information. Some areas I wished for more available products and in some areas we have almost too many options. There is a lot to think about when designing your system that relates to how you want to use your car, just err on the side of caution as an extreme setup will be a real downer in a daily driver or Michigan pothole dodger.

Good Luck
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