944 Turbo Rear Suspension

[barks944]

I'm doing a refurbishment of the rear suspension on my 944 Turbo. Most of it is pretty clear either needs shot blasting and re-painting or replacement if it is a bush etc. But the parts in the picture above are rubber metal bondings and unless I am mistaken I would need to replace the whole part as the rubber doesn't seem to want to come off. Is this the case?

Theres a few more pics here: https://picasaweb.google.com/tombarker1804/944TurboSuspensionRebuild?feat=directlink

 

[Super_Marv]

You can cut off the bulk of rubber using a stanley knife, then remove the remaining rubber with a wire wheel.Some people use a powerful paint stripper to chemically dissolve the rubber.

Elephant racing sell replacement rubber bushes:

http://www.elephantracing.com/suspension/rubberbushings/944rubberbushings.htm

There may be other companies that sell them too!

 

[colin944]

Vw type 3 rubbers fit perfectly. That's what I used and all for under a fiver.If you need any shout and I can get them as my local VW guru is an expert at spotting the vw in your porsche. He was the tecnical man for the vw club for years and what he doesn't know about air cooled vw's is not worth knowing.

 

[barks944]

Thanks both, colin I'd certainly like to give the type 3 rubbers a go. Do you know the part code? I wonder if any of the other rubber mounts on the torsion bar tube are from the same car?

 

[os951]

Tom,

here's what we did to junior's project car's parts:

A friend of mine fabricated those bushings. Metal parts had a citric acid bath and afterwards i delivered them to platers shop to have new yellow passived surface (like factory did as well)

 

[barks944]

Plating looks like a good option. Are they rubber bushes Oli?

 

[os951]

Nope, they are made from Polyoxymethylene (POM)

 

[Jonny944CS]

I'm doing a refurbishment of the rear suspension on my 944 Turbo. Most of it is pretty clear either needs shot blasting and re-painting or replacement if it is a bush etc. But the parts in the picture above are rubber metal bondings and unless I am mistaken I would need to replace the whole part as the rubber doesn't seem to want to come off. Is this the case?

Just another option to confuse you [&:] :

IMHO the rubber on your spring plates looks to be in very good condition from your pictures. I have seen them much worse; replacement should be considered when the rubber has cold flowed, collected at one end, and or severely distorted the shape. When assembled the compression/interference on the rubber is huge, ensuring a nice tight fit and the are designed to twist, not rub/abrade.

For me, you can't beat the original bonded rubber for durability, NVH, ease of rebuilding and pain-free suspension set-up. The rubber provides a critical, friction free bearing/damper that will continue to resist years of abuse. Alternative materials or non bonded rubber will be compromised for normal use, in my view. If you are on a smaller Ø torsion bar, moving to a thicker 25.5 bar will give a similar improvement in wheel control (due to its higher bending stiffness) combined with a less harsh ride.

To clean the plates you could remove the fixings and give them a careful shot blast. The whole unit can then be plated, as rubber is not affected by the process (this is how they would have been manufactured originally).

 

[barks944]

ORIGINAL: Jonny944CS

I'm doing a refurbishment of the rear suspension on my 944 Turbo. Most of it is pretty clear either needs shot blasting and re-painting or replacement if it is a bush etc. But the parts in the picture above are rubber metal bondings and unless I am mistaken I would need to replace the whole part as the rubber doesn't seem to want to come off. Is this the case?

Just another option to confuse you [&:] :

IMHO the rubber on your spring plates looks to be in very good condition from your pictures. I have seen them much worse; replacement should be considered when the rubber has cold flowed, collected at one end, and or severely distorted the shape. When assembled the compression/interference on the rubber is huge, ensuring a nice tight fit and the are designed to twist, not rub/abrade.

For me, you can't beat the original bonded rubber for durability, NVH, ease of rebuilding and pain-free suspension set-up. The rubber provides a critical, friction free bearing/damper that will continue to resist years of abuse. Alternative materials or non bonded rubber will be compromised for normal use, in my view. If you are on a smaller Ø torsion bar, moving to a thicker 25.5 bar will give a similar improvement in wheel control (due to its higher bending stiffness) combined with a less harsh ride.

To clean the plates you could remove the fixings and give them a careful shot blast. The whole unit can then be plated, as rubber is not affected by the process (this is how they would have been manufactured originally).

Interesting perspective Jonny, they certainly aren't in terrible condition. Would I be able to achieve sufficient bond strength with new rubbers and an adhesive to give a part as good as factory new?

 

[Jonny944CS]

Would I be able to achieve sufficient bond strength with new rubbers and an adhesive to give a part as good as factory new?

My guess is you would struggle to replicate the factory bond.

Porsche acheive an incredibly high level of adhesion, (as anyone who has tried to remove them will testify) and they need to in order to resist the twisting force placed on them at the extents of wheel travel. As soon as the adhesion is lost, I would expect the rubber to wear away very quickly. I guess you also could argue that loads from an 850kg beetle with ~Ø20 bars and a 1300kg 944 with Ø25 bars give different operating conditions.

My understanding is that these parts are not service items, they are designed to last the life of the vehicle - hence the extreamely high cost of factory replacement parts.

If you are concerned they may have worn enough to reduced their performance, you could try to get hold of the replacement rubbers and compare the shape. Also re-fitting them into the housings will give you some idea of how much compression they are under.

Another trick is to have you torque tube housing powder coated. Any wear or distortion will be offset by the extra thickness of the Pcoat.

 

[colin944]

I would have to argue that vw rubbers would be every bit as good as standard porsche items. The friction between the rubber and the spring plate is achieved by the squeezing force of the end plate being bolted onto the torsion bar housing. The 944 rear end is only a version of of the vw irs system used in various air cooled cars and vans . The rubbers I used came from a vw type 3 (that's a fastback in common language) and with engine out back combined with a fairly heavy body I would suggest that it's rear suspension had to cope with alot higher forces than the rear of a 944.You could argue that a late vw type 2 rear end has to deal with even more force . Having the rubber bonded onto the spring plate is not going to make any difference to the twisting effect of the torsion bar and spring plate. The system relies on friction between the spring plate and the torion bar housing to work. Having one side bonded on is only going to transfrer the force to the outside of the rubber and if you rubber is worn the powder coating the housing is not going to make up for the wear as the powder coat will just wear off very quickly.

 

[Jonny944CS]

Having the rubber bonded onto the spring plate is not going to make any difference to the twisting effect of the torsion bar and spring plate. The system relies on friction between the spring plate and the torion bar housing to work. Having one side bonded on is only going to transfrer the force to the outside of the rubber and if you rubber is worn the powder coating the housing is not going to make up for the wear as the powder coat will just wear off very quickly.

Hi Colin,

It sounds as though our understanding of how the system works may differ somewhat. Also, I don't doubt that aftermarket type 3 parts will fit and be the same shape, my scepticism relates to how you would acheive a sufficient bond between the new rubber and a recovered spring plate.

My understanding of the VW IRS system is this:

All rubber bearings in Automotive suspension applications (or any engineering app. for that matter) are designed to work via elastic deformation. For this to work with maximum efficiency there must be no relative movement between:

rubber in direct contact with the housing
and

rubber in direct contact with the axle

As is the case in steel with a basic ball bearing, the balls are in many ways equivalent to the core section of the rubber that undergoes the elastic deformation. The torsion bar housing should not experience any relative movement to the outer parts of the rubber contacting it. I've had 5 of these apart over the years and in every case the black paint applied at the factory was still perfectly in tact in the area in contact with the rubber - hence the powder coating comment.

If for whatever reason relative movement between the rubber and housing/axle were to occur, the fiction in the joint caused by the two materials rubbing against each other would cause excessive amounts of heat and the vulcanised rubber would degrade very quickly. Just like the rubber on the end of a pencil does as you move it across a stationary piece of paper.

The assembly prevents the outer part of the rubber on the spring plate from moving via a combination of the huge clamping force created by 4 M10 bolts and realatively rough raw cast surfaces. However on the inside the forces trying to move the smooth steel relative to the rubber are much greater due to the lower effective radius. That is why, I believe, the factory spent more money on implementing a powerful bonded joint in this area. Otherwise they were at risk of cars reaching 20k miles and having to deal with some rather pricey warranty claims for sloppy rear suspension.

 

[u63af]

I have bought poly bushes for this. These will not be bonded does this mean they will wear quicker?

 

[Jonny944CS]

They should be OK. Poly bush replacements are designed for sliding surface contact with the steel spring plate axle. The material properties of polyurathane make this possible.

You can expect small amounts of internal friction and wear with age and debris ingress, but but it won't be anything like the same magnitude.

 

[George Elliott]

Johnny, your insight is very interesting, thanks. The original VW beam is a work of art when you consider the robust nature of the part and the modern equivalents today. I guess the NVH is better with the vulcanised rubber bush but the rigid plastic collars claim greater precision (if the wear which must occur is ignored)
I would stay original personally.
George
944t

 

[Jonny944CS]

I guess the NVH is better with the vulcanised rubber bush but the rigid plastic collars claim greater precision (if the wear which must occur is ignored)

Yes, exactly that. In addition, how much precision you require/need should also be considered against the usage for the car.

One of the main weaknesses with the VW IRS compared with modern 'multi link' designs is its tendency to 'toe out' slightly under transverse loading. Porsche tried to sort this on the 964 and 993 with increasingly more complex solutions. With, some would argue, only partial success. Particularly in the 964.

The beauty of the torsion beam design (particularly on the 911) was that the rear section of the bodyshell could be made very stiff whilst still packaging the engine/gearbox. For hard core racing cars with slicks, this was combined with rose joints, deleted TBs, short coil springs and a proper seam welded chassis to great effect by the factory and privateer race teams of the '70s and '80's.

For certain road cars on treaded tyres, firmly controlled rigid rear suspension geo is not necessarily the best solution. To achieve good balance, it must be considered in relation to the characteristics of the front suspension.

Peugeot Citroen produced a raft of fantastic handling hatchbacks throughout the '80's and '90's. Minimal under steer was achieved by cleverly designed trailing arms that also gave toe change under load.

PCGB race cars are constrained by the regulations to only use rubber bushings. Front running 944's and 968's stand up well to race after race of abuse and regularly show a clean pair of heels to the more powerfull 964's.

 

[barks944]

So still not really sure what to do here. Having had a better look at the original bushes I'm still not sure. They don't show any signs of the rubber giving up, no cracking etc however the rubber has been badly compressed from all its time clamped up tight. If the rubber has deformed plasticly is it exerting the same compressive forces on the outer housing as it used too?

 

[Jonny944CS]

From your pics it looks to me like the only areas of plastic deformation have occured close to the ends of the housing, where small amounts of excess rubber have been pinched. The outer rubber, where you need the compression, looks good to my eyes.

Mine were exactly the same. I took it to be due to the parts settling together after the initial clamping operation as oppose to wear during operation.


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Also, purely out of interest, what torsion bar index angle do you run with your KW V3's?

 

[barks944]

I'm not actually sure on the angle, I know they were re-indexed before the KW was installed. I can find out, I simply made a few measurements with a tape to make sure I put it back the way it was..

 

[sawood12]

Thom, the rear wasn't lowered that much, Promax reckoned they had already been lowered below standard height. In the maintenance history there is an invoice that says one side dropped (common problem) so the bars were re-indexed then to even out the rear so my guess is they probably lowered the higher side to match the lower side.