Re: [T3] Engine Alignment

[Jim Adney <jadney@vwtype3.org>]

On 30 Sep 2003 at 11:11, BOB2TYPE3S@aol.com wrote:

> Jim wrote:

> >Here's the problem as I see it:  
> 
> >The subframe is mounted to the body in a number of places by what amount to 
> >thick (~1cm) rubber washers. The mount points of the subframe are captured 
> >between a pair of these washers and then a large bolt, with a washer under 
> it's 
> >head, is passed thru that sandwich and into the body. Originally this bolt 
> >tightened to a stop that left the washers slightly compressed.  
> 
> This is true, but there is a spacer sleeve that this gets tightened against. 
> This sleeve is trapped by the washer and the frame mount boss.

Right, that's what I meant by "a stop."

> >With the subframe captured between the 2 compressed rubber washers, the 
> >effective spring constant was pretty high, making this connection resilient, 
> >but quite stiff. Today, however, the 2 washers have shrunk a bit, so they 
> may 
> >no longer be under any compression. This completely removes any stiffness 
> from 
> >that joint, and it may even be loose.  
> 
> Well the lower one is supprting the body (all of the weight is on the lower 
> rubber bushing) and the upper is keeping some of the motion limited. Having had 
> the body off of several of these cars, I've seen this.

I think it's actually the upper bushing that's loaded by gravity, but that 
really doesn't change the argument anywhere. It's still under compression, but 
only statically. Once the car starts moving, there is the possibility for that 
bushing to become momentarily unloaded under dynamic conditions.

> >In addition, the rubber parts are probably harder now, so that, if the joint 
> is loose, the parts may "rattle around" against rather harsh stops.  
> 
> The rubber parts might shrink some, but they are still trapped by the 
> bolt-washer-sleeve combo, any movement might be very small at this point unless the 
> inner part of the rubber bushings are worn. This would allow lateral movement 
> too.

I was only thinking of up-and-down motion. I agree that lateral motion adds to 
the problem. With no preload gripping those bushings, it would seem like there 
would be a big loss of lateral stiffness. OTOH, if these parts were really 
loose, I'd expect them to wear rapidly from chafing. I haven't noticed this, 
but I really haven't looked for it either.

> >So, as I see it, a 1 cm rubber mount that shrinks by 2mm may only cause 
> about 2-
> >3mm of body sag, but that 2mm of compression set could completely remove all 
> >preload from the mounts, making them MUCH looser than they were originally. 
> 
> With all the load on the lower bushing, I think that if it shrinks that much, 
> it would have more upward movement than downward movement, after all, the 
> weight and gravity seem to be working on keeping the lower bushing in 
> compression, making it more likely to be compressed to your higher number. 

If we lose 2mm (vertical) from each bushing that lowers the car by 2mm, 
statically, and leaves us with something like 4mm of vertical play, 
dynamically. It's just that a bump in the road removes the otherwise 
"certainty" of gravity, for a moment.

> >Let me know what you think of this. I suspect that I'll want to see if I can 
> >replace any more of the bushings in my sun belt 69 with ones from some of my 
> >scrapped out, rust belt cars.  
> 
> Jim, this is very well thought out, and if anything you'd want to replace the 
> lower bushing half rather than both. I think one could build an spacer to 
> place under the lower bushing to make up for wear and shrinkage. This would 
> tighten the clamping force of the 2 bushing halfs, and keep the rubber isolation 
> that was originally there when built. What do you think about this? 

I think I really need to replace both. I've already replaced the lower ones, 
simply because that was easy. This removes half of the play, but not all of it, 
and I suspect that it's the play (or more accurately, the loss of preload) 
that's the important thing.

One of the results that made a big impression on me in freshman physics was a 
problem where we had an object "captured" between two springs with spring 
constants of k1 and k2. The question was, "What is the equivalent spring 
constant that results from this arrangement?" I expected the answer to be 
something like k1-k2, since the 2 springs seemed to be working AGAINST each 
other, but the correct answer, which was the one I came up with after working 
thru the math, was k1 PLUS k2. This means that clamping the subframe between 2 
rubber bushings with preload makes the connection stiffer than if we had just 
glued it to a single bushing, but that the stiffness suddenly goes to zero if 
the preload disappears. 

In our situation, where the 2 bushings are the same, this means that k1=k2. So 
the clamped with preload situation is twice as stiff as the situation where we 
have just one side preloaded by gravity. OTOH, that one preloaded bushing is a 
lot harder than it was when new, so maybe the difference isn't all that much.

If we start driving, however, we can occasionally unload the gravity preloaded 
bushings and then the stiffness goes to zero. Unfortunately, this is likely to 
happen at exactly those moments when stiffness is most important, ie bumps and 
corners. So I think we come back to the point of wanting to get rid of the play

Spacers to make up the lost length would be fine, but I think the surfaces here 
are not flat, so spacers would be somewhat difficult to make. Maybe we could 
make washers from stiff rubber sheet. You could put the spacers on either side, 
or split them up, but how you did it would affect the body height by half the 
total spacer thickness. You could also shorten the sleeve that you mentioned, 
but that would prevent you from going back to the stock geometry later on if 
this didn't work out for you. If you put any spacers on the top, you'd have to 
do this uniformly for all umpteen subframe bolting locations, too, or you'd end 
up putting a curse (stress) on the body/subframe assembly.

Either using spacers or shortening sleeves with old bushings, or replacing old 
hardened bushings with "new" ones, would remove any play, which I suspect is 
the real culprit here, but using the old hardened bushings would probably 
stiffen up things a bit while also making the ride slightly more harsh. Some of 
us might find some advantage in the harder bushings, while others would 
appreciate the quieter soft bushings.

I suspect everyone with loose bushings would benefit from getting rid of that 
play.

I should point out, however, that not all of these cars have a problem with old 
hardened shrunk bushings. This will probably be limited to sun-beaten cars, so 
if you find that all your door and window rubber is hard, stiff, and broken, 
you probably also have bushing problems that you hadn't thought of.

-- 
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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