Very interesting thread. I read all the points and some topic has never been addressed but might be important for some trying to get as much as possible to improve E9 body rigidity.
Despite the fact E9 body is flexible- no doubt - not many cars from that era (with such power plant under the hood) have reached such an age. Of course there are several reasons for. BMW was one of the few companies using shock station during body structural testing. We can see it even on YT for Neue Klasse. Not many are aware that over the years BMW was providing its services of testing body rigidity for many other manufacturers (including sport supercars from Italy ;-) ).
A friend of mine is experienced mechanical engineer and he told me once that improving rigidity as the target is not the best way as the other parts can be exposed for the strains not designed to be under such conditions which in the end can cause their premature damage.
Not sure single point approach is the best approach but for sure it is easier or only possible way. The car is the whole structure and should be treated in such a way during any structural modifications. That includes:
- drive train (power, mounts design...),
- suspension parameters (incl. unsprung mass, polyurethane in place of rubber ...),
- weight balance (front/rear)
- standard car load (usage)
- others
BMW E9 has been designed as grand tourer car with all the consequences. It was not for speeding or racing. We know how different E9 was for races where the lightweight was the step number one. But hey, race cars (any type) are not expected to pass 10k mileage or more as they are expected to generate the best possible result.
Question about introducing any reinforcements as serious improvements if the car will not be used in races or does not have serious modifications.
So many users with stock body E9 have not major problems where modern (CAD-CAE designed) cars are relatively delicate with several problems on for example glued windshield being teared off due to car usage (normal, street use). It shows that is the still a compromise.
I think modification are good in the parts most of users have problem with (rear differential, rear shock towers...). We know it was hand made with not the best quality control that time and restoration is a good way to improve many things (e.g. spot welds). Sometimes the problem is that there are several steel types in regards to doping impacting mechanical parameters. We have no idea (but can expect) what steel has been used for repair panels and how far it's parameters is in comparison to original one. There are specific requirements for structural steel or steel for the roof for example. It means that some of the cars being repaired might generate body stiffness problems due to the way they have been repaired over the years.
Going towards best rigidity we will easily end up on the structural cage with the E9 skin made of fiberglass and put in the and onto the frame. That would probably the the end of the range of structural modification and such solutions have been used in the past so often.
Using the car for cruising (as E9 is GT car) or concourse I can barely imagine rear strut bar in the trunk or strange shapes on the floor or under the car. For many its just does not look nice and making E9 out of "matching number" (but of course sporty
). All depends... ;-)
Maybe modifications has to be divided into some groups:
1. Improvements not noticeable for others but important according to experience of many - still classic and concourse car
2. Modification toward cars with powerful engines and used in racing - "uber" E9
3. Roll cage inside the car with lightweight solutions (plexiglass windows, no plastic interior parts - sport E9
4. Full custom structural cage with the E9 body shape as the cover of the hi performance frame - Dakkar or Speedracing E9
So far, rear suspension towers improvement (especially for "Bilsteins-like" option and rear differential reinforcement seems to be still for #1 where the others are coming towards #2+ but I'm probably wrong here.
What do You think?
