Chassis stiffness coupe vs targa

[Nico911]

Wondering if there's a big difference (or any) in chassis stiffness between a coupe and a targa. Does it get worse with age ? Let's say we're talking about a '84-'89 chassis, all stock.

Thanks

[Dave Colangelo]

OH YES, when I jack my Targa up in the back I can barely open the doors. The roof on the coupes provides a huge structural role in the chassis. The Taraga's flex in all directions the weight is on either side connected by a flat bit of metal. Im not sure if it gets worse with age but I can tell you that the Targa's warp with age, I can see it in my car.

Regards
Dave

[Nico911]

Originally Posted by Dave Colangelo

OH YES, when I jack my Targa up in the back I can barely open the doors. The roof on the coupes provides a huge structural role in the chassis. The Taraga's flex in all directions the weight is on either side connected by a flat bit of metal. Im not sure if it gets worse with age but I can tell you that the Targa's warp with age, I can see it in my car.

Regards
Dave

Really !!! I was suspecting that, but not that much...
Thank you Dave.

[SchnellSchweitz]

Interesting solution corvette owners can use.

http://www.vetteweb.com/tech/0209vet_c4_coupe_anti_flex_support_bar_install/viewall.html

[McLaren-TAG]

Corvette coupes are flexi.

[Dave Colangelo]

I will offer some points to the contrary,

Our host does offer a solution that I have yet to try, they are support bars that go on the interior sides of the car and they do supposedly offer some solution to this issue.

This flex is not an issue for me, I drive the car for fun, I dont use it on the track, I dont race it, thus this flex is what it is in my opinion. Having the targa top is awesome and I accept the flex as part of that trade off.

Unless you are going to race the car, dont let this scare you off a Targa or cab over a coupe, they are just as awesome cars.

The flex I was talking about most likely happens to all cars, its just the nature of the body experiencing torque.

Regards
Dave

[nineball]

Originally Posted by Dave Colangelo

I will offer some points to the contrary,

Our host does offer a solution that I have yet to try, they are support bars that go on the interior sides of the car and they do supposedly offer some solution to this issue.

This flex is not an issue for me, I drive the car for fun, I dont use it on the track, I dont race it, thus this flex is what it is in my opinion. Having the targa top is awesome and I accept the flex as part of that trade off.

Unless you are going to race the car, dont let this scare you off a Targa or cab over a coupe, they are just as awesome cars.

The flex I was talking about most likely happens to all cars, its just the nature of the body experiencing torque.

Regards
Dave

the targa body braces by jm pro are nla, and while they did help quiet the targa top down they don't do a huge amount for flex. they only cover about a foot or so front-to-back, which is where the targa needs help. don't get me wrong, they still help, but they are no substitute for a coupe.

there is a good chance i will have a couple proto-type targa body braces available in a couple of weeks. working out the details now.

[911pcars]

That chassis flex solution for C4 Vettes might work as intended, but it also places a rigid, non-deformable structure right next to the driver and passenger's head. Okay if wearing a race helmet. Not too if not.

911 coupes are stiffer than Targas, but the rigidity is only relative. Early coupes were pretty flexy. However, through the years, the 911 unit-body chassis became progressively more rigid due to thicker metal and improved chassis design.

A non-visible structural improvement for any 911 would be the installation of a layer of sheet metal along the interior rocker area and spanning the A to B pillar on each side. PAG added metal to those areas on the RSR race cars and later on factory Cabs.

Sherwood

[KTL]

Piggybacking on what Sherwood said, the Targas, much like the Cabrios, have additional chassis stiffening to address the loss of the roof rail. Look in the forward footwell and you'll see a different shape to the vertical wall area at the outboard corner of the car. This area was further reinforced to address the reduction in bending stiffness when you cut the roof. That's why coupes vs. Targa/Cabrio have different floorboards too.

[tdw28210]

Originally Posted by nineball

there is a good chance i will have a couple proto-type targa body braces available in a couple of weeks. working out the details now.

Nineball..are these your design or are you working with ToddC on these?

[nineball]

Originally Posted by tdw28210

Nineball..are these your design or are you working with ToddC on these?

with todd

[johonmirae]

That's why coupes vs. Targa/Cabrio have different floorboards too.

[1QuickS]

To get a simple comparison between Targa and Coupe structural stiffnesses compare a soda can stiffness (torsionally and longitudinally) before and after flattening the can. The amount of material in the can is unchanged but the geometry of where the metal is located has changed. To return the flattened can to the stiffness of the unflattened can you would need to add quite a bit of material to the flattened section. The longitudinal members return some of the stiffness that was lost when the roof was removed but still an inefficient structural member compared to a roof. The soda can example is not an exact comparison but it does convey the fundamental nature of the issue.

I find Targas to be irritating due to their torsional flex compared to coupes. They remind me of the ladder framed American cars of the 60's and 70's. I did see a vast improvement in the C2 Cabriolet I drove compared to the 73 Targa I'm familiar with but still not to the stiffness of my 67 coupe.

[Rusty 356]

Anyone own a 67S hard window targa? I was told back when that they would pop them out from flexing and offered the S with soft window. Any witnesses to that thinking. Targas do open and close at the gap and have a bit of twist in them as well. Still kind of nice.

[Shaun @ Tru6]

Brey Krause harness bar is one of the best bolt-on things you can do to reduce flex in a Targa.

[javadog]

Originally Posted by Rusty 356

Anyone own a 67S hard window targa? I was told back when that they would pop them out from flexing and offered the S with soft window.

I've never heard of that one, although I have owned targas that would pop open an improperly latched passenger door in a left hand turn.

A 914 seems vastly stiffer in comparison. Coupes are on another planet entirely.

JR

[nineball]

Originally Posted by Shaun 84 Targa

Brey Krause harness bar is one of the best bolt-on things you can do to reduce flex in a Targa.

they are nice but don't address the problem anymore than the jm pro versions did. the support is needed front-to-back, not side-to-side. the crossbar is used as a harness guide.

[redridge]

its simple... the front and rear of the targa are held together by the floorboards. The front and rear of the coupe are held together by the floorboards and the roof. The coupe is way stiffer...

[daleflesburg]

My 2 cents worth. Every convertible suffers from flex. The solution is a cage that welds in at least with 6 points. On a 914, tying the F&R shock tower tops along with a roll cage in effect makes the car into a coupe. Targas that I have ridden in on the track with an 8 point cage are as stiff as a coupe. Anything short of that is not really adequate to keep the tire contact patches on the pavement. I have ridden in a 911 cabrio with finger tips riding at the top of the door gaps, and the car moves 1/8" just driving around town. The Targa is just a cabrio with a targa bar.

[Shaun @ Tru6]

Originally Posted by nineball

they are nice but don't address the problem anymore than the jm pro versions did. the support is needed front-to-back, not side-to-side. the crossbar is used as a harness guide.

Hey Kris,

I get the physics involved, roofs are great structure members, but I can say from my experience with a very high mile (read worn-out) 84 with ER bearings and 22/29s, the BK bar did a lot to stiffen up the car. Anyone wanting a bolt-on solution, I'd highly recommend one.