Subframe bracing for a Targa?
 

I am in the process of building up my Targa for mixed track and agressive street driving. I have some concern over the "twisting" I sense in the chassis that I believe is related to the targa. I am currently building my own triangulated strut tower brace (No offense Chuck) and I would like to possibly add subframe bracing as well. I am aware that Euro Tek used to sell rocker stiffeners, but given that my 86 Targa had factory stiffened rockers, I don't know how much benefit there would be. In addition, the flex seems to be longitudinal twisting, not a compression at the roof level.

I know subframe bracing is commonly used in Muscle cars to keep them from twisting themselves apart, would this type of bracing be useful on a targa? If so, where should the braces be connected?

Thanks!

I put in a harness brace and that helped but what really made the car feel different was when I hooked up the rods from the seat belt harness brace that then went down to the seat belt mounting bolts on the outer floor. Could not believe that these made a difference but they did.

Might look into this...

JoeA

i think that the best thing to do along those lines is a full cage tied into all the suspension pickups and B piller like Chuck did.

Hey Joe, Any news? You've been awfull quiet.

Since 911's don't really have subframes...

Did those targa stiffener bars really do anything? They seem like hype to me <shrug>. Did they reduce the twist in the chassis or just the flex (i.e. gap changes in the targa top and roll bar)?

Thanks guys,

Joe, I already have the the "targa braces" and I was also surprised at the amount of flex they removed between the roll hoop and A-pillar. However, they did nothing for longitudinal flex. In addition to the targa braces and front triangulated strut brace I will also be adding a harness bar across the back which also has a lateral brace that ties into a seat belt mount.

Bill, I agree that a full cage would be the best solution, but I have a grouchy wife that I need to appease by making this car as friendly to her as possible. Forcing her to climb over cross bracing will not fly. In addition, this car is primarily a pleasure car that I would like to keep as stock-looking as possible.

Steve, I understand the 911 does not have subframes, but something like a subframe brace that can stiffen the floor and tie the front structure to the rear structure of the car in absense of the roof.

Any thoughts on that X-brace under the floorboards? Maybe made out of narrow rectangular-section steel welded near the jacking points?

I had wondered about something similar to what you are talking about, except inside the car on the floorboards. Seems they are flatter inside, and you could run the bracing daigonally between the center tunnel and the sills since both of those parts are for structural rigidity.

I like what Thom did (widebody) with his cabriolet - I think that would work well in a Targa and still provide for easy engress.

Steve, you're right about the inside being flatter, the problem is what happens to the floor and seat mounting after the braces are installed.

Underneath you may be able to avoid interfering with too much of the mechanicals. Also, you should be able to limit the welding to a couple of points around the center tunnel and at the four corners of the X.

Jeremy, I am not familiar with what Thom has done with his cab. I saw discussions that mentioned its stiffness, but not exactly how Thom tightened things up.

Here you go, the thread with many cool ideas and pics
http://forums.pelicanparts.com/porsche-911-technical-forum/133980-more-cab-stiffening.html

Roamer:
I didn't think your wife was *that* grouchy...at least not to me! :)

Ahem...anyway.... look at the low-mount longitudinal tubes that connect the door hinge area to the latch area...like Thom did. I know of your reluctance to do a cage and keeping the car street friendly...I would mimic the Thom Fitzpatrick work...

--- Wil, your friendly neighbor !!!

Thanks Steve. However, I don't know that without the cage that those braces would do anything for the longitudinal twisting. I will have to contact my local body expert to discuss options.

Just a thought, what do the 914 guys do if not using a cage?

Wil, she is always better behaved when company is around . . . ;)

There is only so much you can do to the car to still allow space for occupants while also limiting yourself to "minor" modifications for what still is ... a street car.

The longitudinal braces ( a la Thom Fitzpatrick) adds fore-aft beam stiffness. Combined with the fact that the exisiting door sills create a sort-of depressed box within the cockpit...this results in a low-level "cube" structure that should add considerable resistance to both longitudunal twisting...and fore-aft flexing as mentioned.

Sure...Ideally...the longitudinals should be higher....and you really need to add a diagonal cross brace ( or "X" brace) for proper twisting resistance, but this is not a stripped-out race car.

Altogether...without putting in a false sub floor that reaches up into the front and rear "firewalls" ...I don't really know what you'd do to get 3 dimensional stiffening in a drop top street car.

Some here may recall the 962-engined '89 Speedster special built by Bruce Canepa that was showcased in Excellence a few years back. They too fishplated a sub floor along the entire bottom cabin to get the desired torsional stiffness for the tub. They mention that ( just like a race car)...as you angle up steep inclined drives...the inside wheel actually lifts off the ground. Impressive....but not a minor conversion .

--Wil Ferch

If not going with a full cage then this was my favorite answer

http://forums.pelicanparts.com/uploa...1067821102.jpg

Wil, if the x-brace is welded on the underside of the floorpan and attached in the vicinity of the jacking points and in some manner to the center tunnel, would this alone be effective? The areas that you would be attaching to are already the stiffest portions of the chassis and you would be effectively tieing in all of these sections with the brace.

Wil, Do you happen to have that Excellence issue that detailed what they did with the speedster, I would be interested to see what they did.

Does someone have a photo of the underside of a 911 that they could post for discussion purposes?

Roamer....in my view this wouldn't be effective since you need "height" to form a 3 dimensional body-of-stiffness. Adding "X" bracing that hugs the exterior/bottom of the floor will strengthen a plane that is already strong laterally.
I can dig up the article but they didn't go into much detail of how they did it other than providing a few more words than I did here. Someone here ( or I, later) can dig up Canepa's phone number. They should recall the work ...but be advised..it seemed extensive.

---Wil

Does anyone sell a door sill type brace? I like that design, but it is beyond my capabilities to make one myself