DDK - Die Deutschen Klassiker

What about the compensation for the added weight in front of the axle line?
Recaro seats are heavy
Driver and passenger are heavy


Regards
Mike

Odd double post again...

The practical result will be interesting, and esp so if you did Targa v Coupe.

Doubt the factory troubled itself with this, but maybe they had a taper gap drawn from such a test?

When I worked with car manufacturers in B in W I never heard of anyone who did such compensation on volume production cars, but I was working with Rover, GM and Chrysler...

210 has a good point, luggage, tank of fuel etc etc

Hmm... would the reaction force from the ground not be via the torsion tube? If so the pivot point of the "moment" would be in front of the wheels.
?

Bruce M wrote:Hmm... would the reaction force from the ground not be via the torsion tube? If so the pivot point of the "moment" would be in front of the wheels.
?

You may very well be right. But we did consider that. In the end we concluded that it was correct to represent that force as upward through the rear axle centre line. Because, of course, it's not a simple system with only the upward force through the torque tube - there is a corresponding counter-force from the torsion in the tube. (And a bit of force through the shock mounts).
Our conclusion was based on imagining the rear torsion bar replaced by coilovers (as indeed can be done). We concluded that the net effect was to place that upward force at the rear axle. Hence we used that position as the fulcrum and considered the clockwise and anticlockwise moments of the engine and box around it.

210bhp wrote:What about the compensation for the added weight in front of the axle line?
Recaro seats are heavy
Driver and passenger are heavy
Regards
Mike

You are quite right. Indeed heavy. I'm getting heavier all the time. And those Recaros would make good boat anchors (which is why they are not going in my car).

We chose to make a (large) simplification. It was Barry and the anonymous painter's assertion that anything in the cabin (between the gearbox mounts and the front suspension) makes very little difference to the twist on the shell, hence to the door gaps.
Those forces are transmitted fairly evenly downward through the suspension mounts, are spread throughout the cabin and (on a coupe) the roof helps to stop it bending in the middle.

Full tank of fuel, and luggage, must make a difference (following the logic used thus far). But are obviously a lot lighter than the engine and box.
It would be better to model everything, but my pockets aren't deep enough for a full investigation! If anyone fancies taking this further and doing it better, please feel free to use our modest efforts as a starting point.

Tp81 wrote:Don't bin the spec until I print it off for reference.

Hi Tom,
Not binned, but I did take it down from my website following a fair bit of negative feedback.
If you really want a copy, PM me your email address and I'll send it over. Use with caution!
Cheers, Richard.

911hillclimber wrote: When I worked with car manufacturers in B in W I never heard of anyone who did such compensation on volume production cars, but I was working with Rover, GM and Chrysler...

Funny you should say that.
After a fair amount of gap discussion one day, Barry and I were standing around outside next to my (2014 model) BMW 5 series. For perspective, we had to conclude that the gaps on that car are absolutely shocking compared with the nearly-50-year-old Porsche inside. Paint's crap too - if my 70 car ended up with that much orange peel, it would be going straight back to be finished properly.

All relative!

My time with the big players was in the late 80s.
I was recently told the water based paints give a lot of peel, might be true.

We profile every car before paint and don't touch them again (most of the time!) after assembly. Door shells are ballasted for profiling but nothing for power train mass, and that's not inconsiderable...


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What do ballasting/profiling process' do Gary, interested in the details.

Richard, I think you think too much!

Imperfect is perfect.

Darren65 wrote:Richard, I think you think too much!

Yep!
I do have a tendency to get sidetracked by such things. Honestly, having had the thought, I would always have pondered if we hadn't done some kind of investigation.

Darren65 wrote:Imperfect is perfect.

Nope!
Deliberately imperfect is fine. Accidentally imperfect is incompetent.
Also I know you don't really think that. I've seen your cars, remember.

Cheers, Richard.

Gary71 wrote:We profile every car before paint and don't touch them again (most of the time!) after assembly. Door shells are ballasted for profiling but nothing for power train mass, and that's not inconsiderable...

Gary, that's super interesting, thanks. I imagine the door ballast must be quite a weight. Shudder to think what a W12 plus gearbox weighs!

Gapping - what about that big heavy lump in the back?
Results

So we tried it out. Using two imperial units of mass - the Barry and the Sandbag.

Barry's report on the experiment:
With the assistance of one of my workshop neighbours we measured, using a digital vernier, the door gap change just above door handle height.
I sat (feet dangling) on the rear slam panel with my sandbag in hand. This combination weighs in at 89kilos (72 for me, 17 for the bag). The centreline of the rear panel is around 125mm rearwards of the rear engine mount centreline, so we've gone quite a bit over the calculated weight.
...which you will remember was 80kg ish.

Results:
The total movement was .22 of a mm.
I have to say, when you lean on the rear slam of your car, you'd swear the rearmost brackets were still in. You just can't feel any notion of motion as one might say.

Conclusion:
It's always been Barry's practice to have the top door gaps a tiny bit tighter than the bottoms. They only ever go out at the top when loaded up with drivetrain, when well-used, etc. Apparently the official Porsche manual says that too.
So that's enough. The very small movement is taken care of by the usual procedure. No weightlifting action required at the painters.