[s-cars] Rear suspension epiphany (Fritz und Hans invade engineering dept?)

[Theodore Chen]

tony,

i suspect the bolts are intended to provide clamping force only, and
audi deliberately undersized them so that they would not be loaded in
bending.  did the bushing sleeves have serrations on them?  it's common
practice for suspension bits to be done in this manner.  the bushing
sleeves are made of a hard steel, and serrated to bite into the mounting
lugs.  the bolts are undersized or necked so that they are loaded in
tension only.  all of the shear and bending loads are borne by the
sleeves.

i would be uncomfortable with the fact that the replacement bushings
had their sleeves sized down (instead of upsizing the bolt or just leaving
it alone).  your replacement bushings are aluminum, so that will help
reduce the bending loads being applied to the bolts, but i think it would
have been better to stick to audi's original design.

-teddy

--- Tony Guttmann <T.Guttmann at ms.unimelb.edu.au> wrote:

> Some weeks ago I wrote asking about bushes for the rear suspension
> cross-member.
> Dave Forgie put me in touch with Addict Motorsport Design, who sell these in
> aluminium for $75 a set. The originals are not purchasable--one needs to buy
> the entire cross member for (in Australia) US$2250. When the suspension
> people
> removed the cross member, they had to burn out the original rubber bushes,
> but 
> the steel core, through which the mounting bolts pass, was found to be 16.5mm
> in 
> diamter. The new bushes, and the factory bolts, are 10.5 mm in diameter, and
> so 
> the new bushes, made by AMD, fit perfectly.  
> 
> The suspension shop said that when they fitted the new bushes, everything
> lined up
> perfectly, compared to the slightly out of alignment bits before, and my rear
> suspension
> noise has disappeared, and the old bushes on other suspension components that
> appeared to be under strain are now perfectly aligned.
> 
> I pass this on for the possible interest of the list, no affiliation with
> AMD, just
> a happy customer. But the mystery remains--why did Audi supply bushes with a
> 16.5mm
> hole and then pass a 10 mm bolt through them? Did they want the rear subframe
> to wander
> and twist around or what? And before you speculate--it's not that they
> started life as
> 10mm and wore. The holes are perfectly round, and all four are the same
> diameter.
> 
> Tony Guttmann
> '94 S4 with 97,000km
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