Friday topic for debate

decoupe

decoupe

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I've heard various points of view as to whether the rear shock tower/battery brace (TEP and others) has any real value as stiffening the chasis of the e9 - mostly to the negative whereas a front brace is considered very effective.

So has anyone considered fabricating a brace to connect the suspension points that live under the rear seats - possibly triangulated to the driveshaft tunnel/rear firewall (wrong term?)?

None of this would be visible of course and there seems to be a fair amont of room to work in so space should be a non issue. It could incorporate the twin motorcycle battery project the Tony mentioned as well.

Anyway, with any guidence the forum can give (other than "WHY?") there is a fabricating shop here that builds bobsleigh and luge sleds for the Canadian olympic team that is more than up to the task if I can supply the parameters.

Any issues with the points themselves being robust enough? Any reason not to connect to tunnel as well?

Can you tell it's Friday?
 
My opinion (basically a Why? answer!)

You are just going to add weight to solve a not very significant problem. Bigggest problems I know of in the rear body are breaking the tops of the shock mounts or tearing the differential mounts loose.

I think that the rear body is pretty stiff compared to the front. The whole car is kind of floppy compared to anything newer, even our 77 320i.

On the other hand, I've never heard of anybody doing any experimenting, so I'd like to see what you come up with.
 
In some situations a strut brace helps with handling, especially in motor racing where one tends to be "on the edge" as it were. I have noticed a difference especially on cars with strut type suspension, when doing stage rallying
However a strut brace or braces will only be of benefit if your prepared to do other work such as seam welding the car.
I actually did this on my 2002Tii and it certainly made a difference, but then again I re bushed the whole car with uaethane (spelling) bushes by "Polybush"

I think that strut braces on their own will make little difference, you would need to do the whole thing, and I agree with Bill that at the back end the diff mount to the body is most likely the weakest link and as the dear old bucket gets rebuilt I am going to seam weld and strenghen everywhere! :shock:
Malc
 
I agree that the rear strut brace is unlikkely to have much effect. In general strut braces have some quirly aspects. We had a debate on this on the BigCoupe Groupe a coupl eof years back. The issue is that most front braces are made with two shock tower mounts and a cross brace that is bolted to the mounts using a single longitudinal bolt (that is the bolt is running for and aft). The result is a hinge at the moungin point. This is handy since it means you don't have to custom buld the brace to fit the car's angular discrepancies, BUT it also means the brace does little for cornering stiffness.

Without a strut brace the shock towers are basically relying on the stiffness of the inner fenders supported by the front cowl and the firewall. Imagine a cardboard box withthe bottom cut out. Mark one end as the firewall, and the opposite end as the front cowl. Now push inward slightly on the top of two opposite sides about a third of the way from the firewall end. The box sides flex in. This is what happens when you hit a bump and the suspension flexes evenly on both sides. Now try pushing in on one side and OUT on the other (you may have to have someone hold the box for you). In this mode the box sort of twists.

Now take a piece of wood and tape it to the edges at that 1/3 point. THis is our experimental strut brace. THe tape provides no bending stiffness at the joint. Just like the hinge bolt on 95% of the commercial strut braces. Now do the experiment again. This time pressing in on both sides results in a VERY stiff box. However, when you do the opposite side pressing experiment, the result is the same as without the strut brace. In cornering you are twisitng the box. The outside strut is pressing up (and IN due to the camber angle of the strut), and the other side is being pulled down and out. If it is hinged, the strut brace does rather little to prevent this.

Now, if you were to somehow attach the wood to the sides in a way that it woudl not swivel or hinge at the joint, the box would not twist under an opposing load.

Ireland Engineering (www.bmw2002.com) makes strut braces for many BMWs (including the E9) that have no hinge bolt or swivels. I have not tried one, but the engineering concept seems to make sense.

As for the rear. Basically the rear bulkhead is doing the same thing in back that the firewall is doing up front. However, the back us under much less twisting load, and the bulkhead is closer to the shock towers. In addition the package shelf acts a bit like a brace, albeit a few inches removed from the real action. If you really want to strenghten the back, then I would put in an X brace. Put a brace across the top (without any hinge points) and then also run abrace diagonally down to the inner wheelhouse and trunk floor. You could, as was suggested, make this into a battery tray as well. I suspect the result would be an improvement, but only under very hard driving conditions.

S
 
The TEP brace mentioned does triangulate. I have one in my e12. It bolts to the sides of the towers and the floor, no hinges at all. It might not be needed, but it's safe to say it works. I bought the TEP rear brace mostly because it was on sale and it incorporates a battery holder. I have had it in the car almost from day one, so I can't tell you how it feels without it.

A triangulated rear brace would help the e9 feel a little tighter, but ultimately I think you need a cage to make any significant improvement in the flexible flyer.
 
What these cars really need is more torsional and bending stiffness between the front and rear suspension pick-up points.

I've been contemplating an x-brace running from the rear subframe mounts to the front "frame rails", and then another x-brace from there to the engine cross-member.

Modern frames for popular old cars like the shoebox chevy and 30's fords use these kinds of x-braces to stiffen the chassis. They have tunnels built into the cross-brace for driveshaft and exhaust.
 
De coupe!
Seam welding is a bit of a horror and really should only be done when the car is completely stripped/
Basically it involved going around every major body shell component and
laying down about an inch, (25mm) of weld, leave a gap of the same and repeat.
The worst part of the job is getting all the seam sealer off the joints before welding.

However that said it does make a major difference, with the 2002Tii and 3 rally cars under my belt I can testify to the benefits!!

Pm me for more info if required!
Cheers
Malc
 
Wondering about a chassis on a unibody...

I used to have a 5.0 mustang which was a unibody and had the car seam-welded with a k-member and about $5,000 worth of suspension work. The end result with that and a 6-point cage was a car so stiff not even hitting a curb at 60 mph would flex the damn thing. Needless to say I know the coupe can be stiffened enough to hold a huge amount of grip.

I was mulling ideas in my head about the coupe and saw someone who had built a custom tube chassis for a 2002, cut out the underside of the car and welded the body onto the chassis. I know this is extreme, but I normally dont go about making a car "half-ass" when it comes to performance. I was thinking of using coupeguy.com's new grp2 luigi flares which would allow 275+ width tires in the rear.

Anyway, the question is, is it possible to create a tube chassis for an e9 like somone did for the 2002? I wish I knew the guy to ask exactly what is needed.
 
A strut brace in the the back of the coupe in my mind would'nt do a damm thing, the rear "struts" as you call them are realy only shock towers and therefor only have to contend with vertical loads, all side ways loading of suspention is through the subframe and then into the body were the 2 x long studs bolt though, bracing these points together would possably be not a bad idea (are they under rear seats?) and if the brace could pick up on the top of the drive shaft tunnel as it went from side to side it would be even better.
Same for the front strut brace, in my mind they are 10 x better if they attach to the fire wall as well as going from strut to strut
 
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