another roll cage question: Bar under dash needed by PCA rules?

[TimT]

The testing you quote above -- was that a destructive test?

No all the testing I have done has been on computer models. I and a few others have built cage models on FEA programs.. Once you have the model built in 3 space, its easy to add/remove fixed points,etc.. then you can go nuts with trials, i.e. place different loads at various places on the cage and see the reactions.

FWIW I had to go into our transport trailer today... and while in there took a look at the cage our GT3RS which was a podium finisher at Le Mans.....

There is no kneebreaker on that car... which was built by Porsche Motorsports.

[Astroman]

Originally Posted by TimT

FWIW I had to go into our transport trailer today... and while in there took a look at the cage our GT3RS which was a podium finisher at Le Mans.....

There is no kneebreaker on that car... which was built by Porsche Motorsports.

Tim,
Is there no equivalent bar hidden behind the dash?? Is there no bar that connects the front right and left portions of the cage at all?? Very interesting... I guess I'll have to eat my words on this one . My apologies...
Jack

[ninesixfour]

Originally Posted by TimT

FWIW I had to go into our transport trailer today... and while in there took a look at the cage our GT3RS which was a podium finisher at Le Mans.....

There is no kneebreaker on that car... which was built by Porsche Motorsports.

But aren't the A pillar bars tied into the front shock towers on a GT3RS? Porsche has used the knee breakers before such as in the 964 Cup where the cage is not tied into the suspension.

[TimT]

But aren't the A pillar bars tied into the front shock towers on a GT3RS?

Yes! on the GT3RS that we own, the cage structure continues from the A pillar to the shock towers. There is nothing connecting drivers and passengers side.

What I feel (and I am apparently in the minority) is that the knee bar is kind of a placebo effect... Since it has always been done it must be right!!

No need for anyone to eat words.....

I think I'll have some free time coming up soon... I'll redo those roll cage models.

[gtu935]

After reading this thread, I went out to look at our 2003 RS as well and I was suprised that there is no knee bar and no other supporting bar/s that attach the Left side cage to the right side roll cage in that area. The only attaching bar is the over head/ windshield bar running left to right and of course the construction of the main hoop section over the drivers head. As we are, at this time, reconfiguring my 935 copy (not a factory car) and looking to cut as much weight from this car as possible I am now seriuosly considering cutting out the kneee bar in this car.

Thanks for the leg work Tim.

[gtu935]

THis is a roll cage we did here some 12 years ago. One of our first.

[gtu935]

Not pretty purely functional. This car was built from a street car in stages.

[gtu935]

Here's the right side door bars. The cage had been painted as well.

[gtu935]

Another roll cage in our latest customer project.

[gtu935]

More pictures of our latest 911 roll cage install can be seen at

New project.... a clean slate

[stownsen914]

Originally Posted by gtu935

As we are, at this time, reconfiguring my 935 copy (not a factory car) and looking to cut as much weight from this car as possible I am now seriuosly considering cutting out the kneee bar in this car.

I'll look forward to seeing the turbo at the track next year in full 935 bodywork. Sounds like fun.

From Tim's work it sounds like it would not negatively impact stiffness if the knee bar is removed .... I'd be concerned that a heavy side impact without a knee/dash bar could cause the cage to collapse laterally on the driver ...

Scott

[mistertate]

Doesn't the firewall/dash structure protect against a side impact? If so then the 'knee bar' is a redundancy. Seems like the mac-daddy thing to do would be remove the dash, weld the a-pillar bars to the metal dash structure, then reinstall the dash. For some reason the SCCA thinks this is cheating.
That red 964 picture looks more like a failure of bolt-in cage feet and insufficient door bars. Do Safety Devices cages come with door bars?
I like those last few cages where the builders put a ~3 degree bend at the crown of the main hoop, header bar, and roof bar if it has one. A lot of people haven't figured that out yet.

[gtu935]

THis car did get a knee bar and the rear horizontal bar both have bent ends. The entire chassis and bar/cage had been powder coated.

[Tom W]

My 964 with the factory dash removed and a cross bar installed into the cage to serve the purpose of the knee bar but higher up (the steering wheel mounts to it and a dash was fabricated over it (much lighter than factory).

[gtu935]

very nice.

[stownsen914]

Originally Posted by mistertate

Doesn't the firewall/dash structure protect against a side impact? If so then the 'knee bar' is a redundancy.

Maybe ... but I would think the dash is not nearly as strong in resisting lateral cockpit crushing (such as might be seen in a significant side impact) as a lknee/dash bar. If we used the logic that the unibody is strong enough to withstand various kinds of crashes, then in many cars much of the roll cage would be redundant (hopefully we can agree that roll cages are not redundant).

If anyone has any destructive/crash testing showing that a knee/bar is redundant (does the FEA testing mentioned earlier in this thread take that into account?), I'd be very interested -- I'd remove the knee bar from my track car and be happy to be rid of the weight.

Scott

[Az911]

Factory Porsche race cars don't have a knee bar

[jluetjen]

Hi TimT! It's nice to see that someone validated my original analysis. In the picture of the red 911 on the first page, I wonder what the car looked like inside? Did the cage pull-up from the floor? Keep in mind that Safety Devices cages are usually bolted in, and generally mount to the floor in Showroom Stock legal fashion. Did the tubes bend? Where? The fact that the car was punched in below the A-pillar is not significant in and of itself until the details are examined.

[Joe Ricard]

Very interesting stuff. I am building a car now.

914 in fact it is a retired Grand Bayou Series car (local track series). It had a Knee bar that bent out towards the knees. Sawz-all make quick work out of it. I removed the dash and put a straight cross bar between the cage a pillar bars. and then went ahead and tied the front shock towers to the A pillar cross bar junction.
I like the diagonal bars in the grey car pictures. going from drivers side cage to passenger side shock tower. definately stiffen up the chassis.