Structural Reinforcement

[doug_porsche]

Dumb question.

I have been looking at the "RSR rear shock mount" braces. On a non-coil over, (torsion bar), read suspension, is the shock mounts really a weak point?

It just seems to me, a non-engineer who did not stay at a Holiday Inn Express last night, that the forces pass into the chassis from the shocks alone just wound be that much to flex it.

Just curious.

Doug

[boba]

Originally Posted by doug_porsche

I have been looking at the "RSR rear shock mount" braces. On a non-coil over, (torsion bar), read suspension, is the shock mounts really a weak point?

The shock tower reinforcement on the RSR was a coilover related modification.

The shock towers were not designed to carry the sprung weight of the car thus the need to be reinforced.

The first use of coilovers were as helper springs added to supplement the torsion bars.

[RWebb]

DD - that surprises me (the cage not adding stiffness) - you might start a new thread on that.

Now, suppose somebody made a larger tunnel "shell" that fit over the existing tunnel? Would that add sig'ly more stiffness? It would look like a 964 tunnel maybe...
It would be less work as you wouldn't have to cut our 2 entire floor areas and swap them...

[petevb]

Originally Posted by RWebb

DD - that surprises me (the cage not adding stiffness) - you might start a new thread on that.

Now, suppose somebody made a larger tunnel "shell" that fit over the existing tunnel? Would that add sig'ly more stiffness? It would look like a 964 tunnel maybe...
It would be less work as you wouldn't have to cut our 2 entire floor areas and swap them...

He said his bar (bolted to the floor) doesn't add much stiffness. Big difference compared to a cage, especially welded, especially to the pickup points...

[safe]

Originally Posted by boba

Randy,

Ok, one thing I've been thinking about, is this brace needed if you go to a coilover setup?

[calling911]

Originally Posted by davidppp

Hmmmm.

To be ruthlessly truthful. I would strongly suspect bad repair work or rust if a coupe 911 shell distrted noticably when jacked.

Unfortuantely many cars I have seen have had new outer rockers tacked onto severely weakened inner sill structures which have either been poorly repaired, or simply left part-destroyed.

I have no twist data ( note, does anyone please), but have never seen noticable distortion in a nweish shell.

So far as Carbon is concerned, it will work, but its modulus is the same as steel of the same thickness, it is one tenth the weight, and hundreds of times the cost.

I'd use steel.

The weakest points IMHo are the front inner wings which not only tend to bend, but also are often rusted inside the boxes, the crossmember mounting bushes which are not very well welded into the structure IMHO, and the rear chassis box sections where the hot air tubes enter..

That said the 911 is avery strong shell provided all rust is replced propwerly....

And heavy too ..

Kind regards
David

I completely agree with this.. My 69 had both outer rockers replaced. When I removed them I was greeted to dust for inner rockers.. man, thats just wrong.. you guys that flip cars like this piss me off. I don't know if you realize how important the inner rocker is.. I would bet your are toast if you arent seeing any rust externally. There is really no way to tell if they are there short of drilling a bunch of inspection holes and rewelding.. which might not be a bad idea.

[scottbooth]

Hi Walt,

Thanks for the response. More good info.

My car already has the "eyebrow" supports from the center of the torsion tube to the center tunnel. That is part of the reason that I was curious about the added benefit of the extra shear plate from the pickup point. Lying under the car looking at things, it seems like the tube is pretty well supported as is.

Does anyone know if the factory RSRs had *both* the eyebrows and the plates from the pickup points to the rear seat buckets? Could these be two different solutions to the same problem?

Not trying to hijack, but as a bit of background, I am welding in the area to repair some rust holes in the torsion tube (in the hard to get to - but probably structurally less significant - areas where it passes through the frame rail box sections) and to add Wevo anti-roll bar mounts. While I am at it (famous last words) I am considering doing anything I can think of to strengthen the tube and its attachment points. This, even though I am pretty sure the repairs I've made so far are solid, if ugly.

The application is a DE car that may or may not ever get a coil over upgrade, but since the body is currently stripped of parts and undercoating, this is a great time to weld things in that I might wish for later. I am considering doing the shock tower reinforcements too, though I suppose that is also of limited benefit for a torsion bar car.

It is entirely possible that I am going off the deep end here and building more car than I need. This board does nothing to help those of us who tend toward overkill.

Scott

[dd74]

Originally Posted by petevb

He said his bar (bolted to the floor) doesn't add much stiffness. Big difference compared to a cage, especially welded, especially to the pickup points...

Correct. I started a new thread re. the roll bars:
http://forums.pelicanparts.com/porsche-911-technical-forum/386241-roll-bar-vs-roll-bar.html#post3690788

[DG624]

There is really no way to tell if they are there short of drilling a bunch of inspection holes and rewelding.. which might not be a bad idea.

911 Calling said that there is no way to inspect...How can the rocker repair be valid if there is still rust and no way to find out if old rust is still there? Should both inner and outer be replaced? I have a '86 Carrera that needs rocker repairs but will this be even worth the effort if I have to start to replace the entire outer frame? I don't want to spend 10,000 to fix a rusty '86.

[Bobboloo]

Originally Posted by safe

Ok, one thing I've been thinking about, is this brace needed if you go to a coilover setup?

The most important bracing for coilovers is of course the bracing at the top of the shock towers but any car that doesn't have the supports that tie the torsion tube to the seat bucket can benefit from the mod. This mod has as much to do with chassis stiffening as it does with torsion tube twisting.

Consider that the weight of the engine is over hung behind the rear axles and the doors leave a big hole on both sides of the chassis. This means this area of the chassis (rear seat area) turns into a big spring. Adding these pieces firm it up a little.

Originally Posted by DG624

911 Calling said that there is no way to inspect...How can the rocker repair be valid if there is still rust and no way to find out if old rust is still there? Should both inner and outer be replaced? I have a '86 Carrera that needs rocker repairs but will this be even worth the effort if I have to start to replace the entire outer frame? I don't want to spend 10,000 to fix a rusty '86.

Once you cut off the rusted areas you can see the extent of the rust in the inner rockers. There is however an area of the inner rocker that is exposed with the outer rockers welded on. It's about a 2 inch area that runs the length of the inner and outer rockers between the lip of the floor pan and the outer rocker. Just get under the car and poke it with an ice pick. If it's solid you should be fine. This exposed area is actually the area that is most prone to rust on an inner rocker.

[scottbooth]

More good info. Thanks Bobboloo.

I will definitely add the plates from the suspension pickups to the seat buckets. That makes sense and should be easy to do. There is one hard brake line that's kind of in the way, maybe I can work around it. (Or maybe it will pop free with no grief after 30 years .)

For now I think I will skip the shock tower reinforcements until I actually change to coil overs - at the moment it seems like extra effort just to add weight. In a torsion bar car, I suppose triangulating the cross member to the longitudinals might help overall chassis stiffness a bit, but the reinforcements to the shock towers themselves don't seem to be of much use. Someone please chime in if I am wrong about this.

And, good tip on checking the inner rockers from the exposed area near the seam.

Scott

[RWebb]

Here is something interesting:

http://forums.pelicanparts.com/porsche-911-technical-forum/398747-show-us-your-green-machines-post3833540.html#post3833540

scroll down to the post & pic by "Bobby"

He welded in later longitudinals from an 85' Carrera over the stock ones on his 73'

[DG624]

Warren thanks for the info...I will be getting my car fixed soon.

[EarlySport]

I recently completed a tunnel mod to my IROC build-up. Search 'Aussie IROC' for the main build thread, but here's a couple of pics of my new tunnel.





My motivation was to raise and relocate the shifter (4" up and back ). I've welded the new tunnel framework in, and it stretches from the sheet metal at the front of the passenger compartment to the rear vertical in front of the rear seat shells.

I could reasonably easily put some additional braces from the torsion tube to the other side of the rear seat vertical, joining up with my tunnel. I'd need to look at how to brace the front crossmember mounts. I could also add some diagonals to my new tunnel if necessary.

I wasn't sure though if by cutting out a portion of the centre of the factory tunnel ( to clear the shift rod ) I have lessened or improved stiffness.

I'd be interested in some expert opinion.

[911pcars]

"I'd be interested in some expert opinion."

Not an expert, just a seat-of-the-pants guestimate.

Good idea about moving the shift housing closer to the driver.

I agree. I think you could add/restore chassis rigidity by adding diagonal bracing to the ladder frame, even attach sheet metal panels to the framework via spot weld, rivet, etc. While access to all things within the tunnel is great, does this area have to remain open?

My $.02
Sherwood

[RWebb]

interesting static measurements on F strut towers

Lateral Stiffness of 911 Front Strut Tower

[gsmith660]

Originally Posted by 911pcars

I should update this thread to show an important piece that can be added for improved chassis rigidity.

Thom Fitzpatrick (Widebody) added these pieces to his track car, but I see the advantage in using them in any 911. The additional weight is about 18 lbs.





As it turns out, the factory RSRs used a similar treatment (except double thickness), in addition to a full cage.

Sherwood

I just finished my addition of longitudinal supports and did the jack test and as expected the door opened and closed without any dragging as it did before I installed the support. Now I am doing antirust coating and undercoating in the void spaces that I welded on.

[RWebb]

Some related threads (it appears some older threads have vanished; these are good as of July 30, 2008 at 10 pm Pac. Time):

Model of my RSR reinforcements - posted by Tyson:
Model of my RSR reinforcements

Increasing Body Rigidity:
Increasing Body Rigidity

Attempting Measurements of Lateral Stiffness of 911 Front Strut Tower:
Lateral Stiffness of 911 Front Strut Tower

Measuring Torsional Stiffness of 1988 911 coupe:
Torsional Stiffness of 1988 911 coupe

[jon sigurdsen]

I have a 1968 Soft window targa, it's...........soft

[hemeoncdoc]

Interesting observation re the top of the window frames rubbing against the top door sill. My 1968 911 does exactly that. My Dad bought it new - so I know the complete history. Original pan, never wrecked, etc. Did have fron suspension pan area replaced, as well as inner and outer rockers. Anyone have any ideas as to what is going on to rub in that location? I am thinking expansion of the door frame which might lower the top sill a few mm. I am doing a rotisserie restoration now, so I might be able to fix it... Any ideas appreciated