Some thoughts on the Design and Mechanical Properties of Mk2 Rear Beams

I have decided to have a look at the design of the Mk2 rear beams.

I do this more out of academic interest than because I intend to modify my own beam.

Having said that, I may well attempt to replicate what VWMS did. But, this will be discussed later in the thread, when I have developed some of the basic premisses on which I believe the beam is designed to work.

Also. Someone on CGTI, who shall remain nameless, but you know who you are, Mr Eyre, has refuted my saying that the beam will flex!

So. I shall develop this thread to look at stresses and deflections seen in the beam.

 

I tell you what happened there Dave. I saw the above prints on vw_singh's thread, read them quickly from start to finish, nothing I understood whatsoever and was hoping we'd get some clarification.

Onto the claim ref beam flex, it's purely flex horizontally across the '<' section I was referring to, and also the extent to which it might. Yes, it's open section, so it could if forced with unlimited effort, but are cornering forces enough?

Let me throw a link to a thread here Brian.G had started a while back, which may or may not be relevant!

 

i was surprised how from wheel to wheel ie across the car hwo flexible they are, i have wondered about how the beam would bend behind they pivot point of the beam, ie pushing the wheels together or apart.

i just cant believe the c shape of the beam doesnt flex, i guess thats why VWMS triangulate the beam.

 

Everything flex's... Question is, how much (and in what way...).

Rear beams flex a lot, might be fun to bring the rear beam bushes into this discussion too, esp the Corrado / Passat passive toe jobbies, there's a lot of BS talked about them

Cheers for now

Simon

 

Then can you do it for a Mk1 rear beam.....................................please

I consider that if I want to stiffen it I can box in (weld or bolt??) part of it, cheap as chips.

 

fyi ...

Re. Mk1`s ... jacking in the centre of the beam has been known to bend them.

Not sure if this applies to the Mk2 ?

 

Indeed, but (based on a 65-35 front rear weight split) this is ~350kg placed vertically in the centre of the beam through the horizontal 'V'-section.

In-use, the forces, an amount unverified, are lateral, horizontally across the V.

 

For what its worth,i did have a mad thought one day when playing with a "wide track" mk3 CL axle (no built in ARB) and it was SO SO wobbly,i thought it was closer to "independant" rear suspension than a GTI spec one! why i chose to use a GTI one the put ANOTHER anti roll bar on top of that one aswell i'll never know.
Just a thought.

 

I'm happy to do some CAD stress analysis on this if anyone needs - will give same info as above (polar moments etc etc) but also theoretical deflection based on material and associated density etc etc.

And yes, boxing in the V section with a vertical plate over the open side will stiffen it up incredibly. This is because the added part will be in tension and compression like the centre of an I-beam (the main bit that gives the strength). It'd mean you couldn't get to the standard ARB though - I'm not sure if aftermarket ARBs bolt in the same place or whether they're external t the beam.

Either way, let me know if I can help - would be good to put my motorsport engineering degree to good use!

 

Conjecture heh! That's good.

I have undertaken some rough calcs., which I will write up as time permits, but from an academic point of view I wonder how this design came about. It would be nice to be able to discuss the design with the Engineers who invented it, but as this is not feasible, the following thoughts are proposed.

Anyone else have thoughts on this?

I have tried to think why VAG would use a rear beam of this design, and believe it may have a lot to do with space, comfort and structural strength than handling and road holding. Certainly from the modifications which have been undertaken by both factory and private competition users, the standard beam can be improved where performance matters.

A little history regarding the VAG rear beam design.

I have undertaken some research, mainly on the web, and have come up with the following. I can't vouch for the accuracy, for, as we all know, there is a lot of inaccurate stuff published in all media. If anyone has definative information which sheds more accurate light on the subject, then please let us know.

As far as I can see, the first 2 modern FWD cars were from SAAB and DKW (AUDI). In the 1940s and early 1950s, SAAB were using independent rear supension with torsion bars on the 92. A bit more advanced than DKWs dead axle and transverse leaf spring set up.

In 1955, SAAB introduced the 93 with a rear beam similar to the Golf set up. But, it was a single tubular beam with the ends bent backwards. It was located with trailing links, at the ends, and a single resiliant rubber bush, in the centre. This beam differs from the Golf type in that it is very stiff in both torsion and bending. SAAB used this design on the 93B, 95, 96 and 97 series cars right up until 1978 when 96 production ceased. This is know as a 'Torsion Beam'. SAAB sold a Sport & Rally rear beam for the 96 which was even stiffer, reinforced at the stub axle joint, and had 1 1/2 deg. of neg. camber built in. Standard beams had zero camber.

In 1959, DKW introduced a rear beam which could possibly be similar, to that used on Golf Mk1s. I can find no pictures of this rear suspension, but I did find a reference to it being called a 'Twist Beam', and that is the nomenclature used when refering to the VAG rear beam. I guess it got the name due to the fact that it will tend to twist out of plane when torsion is applied?

I guess that DKW were trying to emulate the handling and road holding of the SAABs, but being a more comfort oriented product, the DKW designers probably sought more flexibility. At the same time, the SAAB design requires a very strong and stiff chassis, where the centre support is located, whereas, putting the supports at the end of the beam means loads are carried in areas that are relatively stiff due to the wheel arches being located close by. The trailing links also take up a lot of space and rear seat heights have to be quite high, reducing rear headroom.

The definite use of a 'Twist Beam' designs, in VAG products, is that of the AUDI 50 (POLO), Mk1 Rocco and Mk1 Golf.

I have found no definative information on this type of beam used in the early large AUDIs, 70, 80, 90 100, although one source quotes the 1969 100 having a twist beam? Certainly, in their 1967 design, SAAB switched to the use of a straight rear beam with trailing arms and a transverse 'Pahard Rod'. More modern large AUDIs have used this design on the FWD cars, but since the late 1990s, the A6 has switched to a twist beam very similar to the Mk2 Golf design. Other AUDIs may also use a twist beam these days, but I can't be ar5ed to check!

 

Well Jon. I'll leave it to you then!

I have already drawn the beam with a vertical plate , and was saving this for later, when I had posted results from a standard beam.

I was hoping to lay the details out methodically, albeit slowly, to develop ideas, and seek reactions. But you have pre-empted me!

 

http://www.clubgti.co.uk/forum/showthread.php?t=164051&page=29

 

As far as I'm concerned the history of the beams is overridden by the purpose, which is compact packaging and low cost.

It look years before the manufacturers threw away the beams and admitted they had to go independent rear - 2003 in Volkswagen's case - as they all raised the bar on each other and it became unacceptable to market beam axles to an ever more sophisticated - and informed - consumer.

The evolution from Mk1 axle to Mk2 axle is clear in my mind, having seen the way the Mk2 axle articulates off car. It does a far better job of giving independence in the vertical plane and yet resisting sideways force.

Etiquette suggests Dave goes first!

 

Of course, Dave, please continue! If you need some help with anything then let me know. I know from my uni days that hand calcs of Stress Analysis can take forever, and it's so easy to misinterpret something or use the wrong value and then have to backtrack through the method.

I'm not sure if it will yield any useful info but I have hard copy of Mk2 16v Group A full car spec at home somewhere so i'll flick through that and see what they actually did to the normal axle.

 

1988 Surely Chris!

 

If you insist Dave. Over to you...

 

Syncro!

 

Oooops

Guilty as charged but I do use an 18" timber to spread the load a bit

 

At the risk of appearing a little pushy, if this thread is to progress from its periodic off topic-nesss, a layman's explanation of the printouts in post 1 would help this reader certainly, and I suspect others too...