Increasing Body Rigidity

[randywebb]

What places should be welded or otherwise reinforced for reducing the body flex on a high performance street car? And, how easy are these places to get to?

Also, what cracks often develop with a lot of miles?

[DG624]

I have noticed more flexing under the jack points. Maybe not a good idea to use the standard jack to lift the car.

[86 911]

Originally posted by DG624
I have noticed more flexing under the jack points. Maybe not a good idea to use the standard jack to lift the car.

You've got that right! On my car if I jack it on one side, the doors will hardly close. Do you have a roll bar in your car, Randy? If not, I would say where the roll bar connects would be an ideal reinforcement. Also a horizontal bar in the trunck might help reducing flex in the front.

[widebody911]

Depends on the car. Targas and cabs are a whole differnet ball of wax.

All of them can suffer fatigue & cracks around the jack points, spring plate mounting area, sway bar mounts, and to a lesser extent, around the shock tower areas and front suspension pickup points. Cabs and targas also get weak in the door sill area, particularly on the lower-vertical part of the "L" of the forward door frame.

[TRE Cup]

if you are running coil overs in the rear- most definitely the shock bridge to the frame rails. and the shock mounts to the bridge. The rear torsion bar mounting pins (for the trapezoidal cups) reinforce those- the factory actually made an overlay plate of steel and welded it in. The sway bar mounting areas- the RSR's had "skids" that extended from the bar mounts to the chassis so in case of an "off" the car would not rip out the bars. See threads re: Wevo's new rear bar mounts and if you extend some triangular skids forward from their excellent mounts, you will have that section rigid. The front sway bar areas get reinforced , similar to the way 914 gt's had the rear arms boxed: flat plates with small holes for welding in multiple places to spread the load that large bars and modern rubber bring. Of course a proper strut brace in front is assumed. Check out the rack supports and add some extra welding- Tyson recently made some diagonal reinforcements for some of our cars that have since made this open area a lot more rigid. The original RSR's had a lot of reinforcement including most of what is listed above, as well as welded in tabs for the corners of the engine comparment and stitch welding (1/2" long welds along most of the panels on the tub)
Also some of the 935's had front trunk x members above the gas tank and a diagonal in the engine compartment. Coupled with the cage, i don't know how much stiffer one can get a production chassis once all these mods are done
Stitch welding can be done in the front and rear compartments w/o too much hassle (save paint prep and finish) but the interior is more work due to wiring/carpeting/ fuel lines/ controls. The engine should be out for proper reinforcement of the compartment there.

[DG624]

Matt, what do you use to jack up your car and where do you do the lifting? Did you weld a plate under the car for jacking it up?

[86 911]

Originally posted by DG624
Matt, what do you use to jack up your car and where do you do the lifting? Did you weld a plate under the car for jacking it up?

I use the jacking plate which goes into the side of the car for jacking. I use a 2.5 ton Husky hydraulic SUV/car jack. It looks like this:

[patkeefe]

Uh, weren't they designed for some chassis flex?

[Tyson Schmidt]

Originally posted by patkeefe
Uh, weren't they designed for some chassis flex?

Uh, sure. That's why Porsche, and every other manufacturer brags every time their new chassis comes out about how the new chassis is X% stiffer than the one it replaced.

Sorry if that came off sarcastic, but I get tired of people constantly second-guessing these proven mods that even the factory did when they went racing.

The Factory built the cars so they were as cheap and easy to manufacture as possible while meeting a certain design criteria and weight goal. That doesn't mean we need to leave everything stock because that's the way the factory did it. There's lots of room for improvement.


The stiffer the chassis, the softer the suspension can be for a given level of grip and handling. A stiffer chassis with more compliant suspension has a much better ride-to-handling quotient than one with stiff suspension and a flexy chassis.

[patkeefe]

It's cool, Tyson...
I ask these questions so I can hear the answers from the people who know the real truths. So, I'm not second guessing, I'm first guessing. I ain't learning anything if I don't ask.

This is the first thread in which I have ever seen the subject of chassis rigidity broached, save for some coilover questions and stiffening of the strut towers for camber control.

[jluetjen]

Originally posted by patkeefe

This is the first thread in which I have ever seen the subject of chassis rigidity broached, save for some coilover questions and stiffening of the strut towers for camber control.

PatKeefe;
We'll let you off on this one. It's come up a number times, often regarding Targas and Cabs. Mark Donohue's book -- The Unfair Advantage says that the coupes have a torsional stiffness of about 2000 lb-ft per degree. Given the race tires of the time, he thought that that value was more then adequate.

[Tyson Schmidt]

Hey no problem. I hope it didn't come off like I was picking on you.

Chassis flex is certainly taken into account when they set everything else up in the suspension at the factory. But ideally, there would be no chassis flex whatsoever, and the suspension would do all the work.


You really should drive a car some time that has all these reinforcements and a full cage. It's really quite a joy when the chassis is drum tight, and the suspension is dialed in. It's like the car is an extension of your body, and it seems to react intuitively to your inputs.


The real problems come when someone stiffens the suspension, eliminates the rubber compliance from the bushings, and stickier rubber on the car, but leaves the chassis alone. These cars are spot-welded form the factory, and were never meant for dealing with the forces we are putting them through today with modern tire technology.

[patkeefe]

Tyson:
No, I don't feel picked on. I was always the guy at school that asked the stupid questions everyone else was too shy to ask.

When I overhauled my suspension over the winter, I replaced all the rubber, Chuck's polybronze front, neatrix rear, 21/27 and bilstein sports. I intuitively felt that it would be unwise to go any stiffer on the suspension for a street car, but had not done the proper diligence in researching reinforcement options. I did get the tape measure out to check for chassis distortion, and found none at the time. However, as the subject of this thread points out, I didn't feel I should just take a WAG on where to reinforce, and whether it is really necessary for the street car.

Now that I finally signed up for DE, I figure maybe it's time to pay attention to these little details. I just want to avoid haphazard gussets or counterproductive results. If I had to guess, I'd go with cross bracing on the struts, bracing from the strut towers to the front torsion bar mount locations. and maybe a triangular brace from the rear torsion tube to the tunnel.

Another winter project. I'll get the research done by then. And, thanks for those links, John.

Thanks!
Pat

[Tyson Schmidt]

I'll take some pics of my car on Monday that'll show you the weakest point on the 911 chassis, and how to strengthen it. It doesn't require an engine drop, but does require removing undercoating, and a MIG welder. Oh, and some small triangular gussets.

It's all you should need with your set-up, since you didn't go overboard on the mods.

[craig001]

If Pelican doesn't sell them, I remember seeing a frame stiffening kit in the Vertex print catalog. In the later cabs, and maybe targas. Porsche went to a double layer floorpan set-up for added rigidity. Don't know exactly what year that started though.

[Brother]

I'm interested in seeing those chasis mods, Tyson.

[rdane]

Originally posted by Tyson Schmidt
I get tired of people constantly second-guessing these proven mods that even the factory did when they went racing.

The Factory built the cars so they were as cheap and easy to manufacture as possible while meeting a certain design criteria and weight goal. That doesn't mean we need to leave everything stock because that's the way the factory did it. There's lots of room for improvement.

The stiffer the chassis, the softer the suspension can be for a given level of grip and handling. A stiffer chassis with more compliant suspension has a much better ride-to-handling quotient than one with stiff suspension and a flexy chassis.

That TRE crowd is pretty sharp Tyson's and Dave's comments need repeating. Good thread Randy.

[Jeff Alton]

Good thread indeed! Can't wait for the pics of the mods. My car is heavily modded, about as far as you can go in the suspension department and I am dying to get out the miller 175 and start stiffening it up! This is great.

Jeff

[randywebb]

It'll help if you can separate the mods needed for ...

1. a hot rod coupe on the street (assume T-bars)
2a. same re track
2b. same re track but with coil-overs
3. targa/cab on street, again hot rod type
4. ditto re track

For example, I remember either Tyson or TRE (what is the difference, BTW?) told me the triangular R shock reinforcements wouldn't really add anything to my hot rod street driven coupe.

If you can break it down that way, then everybody will have an idea what makes sense and what doesn't.

[Porsche Doc]

Here is the the way we reinforce the front and rear for coil overs.
It a IROC look race car GT3R for PCA and HSR that we built last year
It stiffens it up real good.