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

hunterpeaks · 11-27-2008, 05:00 PM

I have a 88- 911, that has a full roll cage and was raced for 4 years, but last race was in 2004. I plan on racing it again in 2009. I was at a recent NASA event and a couple of the 911 guys there mentioned that I needed the bar that goes below the steering wheel, just under/at the lower dash edge level. I can't find any rules about this in the PCA rulebook. Does anyone out there know for sure?
Thanks.

Hunter

Astroman · 11-28-2008, 11:32 AM

It is not required for PCA or NASA racing. However I would recommend one.

hunterpeaks · 11-28-2008, 04:13 PM

Thanks Jack. Appreciate the confirmation.
Will see if my local guy can weld one in anyway as it does seem like a good idea.
Hunter

TimT · 11-28-2008, 08:14 PM

Quote:

However I would recommend one.

Yikes recommending a knee breaker.....

If the rules don't explicitly say you need a bar such as that, don't add one

there have been multiple threads here and in rennlist regarding roll cage design...

consensus is that bar is best for breaking knees... not much else..

cstreit · cstreit's Garage

Tim, got a link to those threads?

Astroman · 11-29-2008, 07:00 PM

Quote:

Originally Posted by TimT

Yikes recommending a knee breaker.....

If the rules don't explicitly say you need a bar such as that, don't add one

there have been multiple threads here and in rennlist regarding roll cage design...

consensus is that bar is best for breaking knees... not much else..

I respectfully and whole-heartedly disagree. What you suggest is far from consensus.

If you remove the lower dash "lip," the bar can be located in it's place... therefore posing no more risk than the stock dash itself.

And believe me, I used to be a paramedic and I saw plenty of femur fractures in UNrestrained drivers in front-end collisions. So I know that the theoretical risk exists... just not with a properly constructed cage with a properly restrained driver.

Poll any of the reputable cage builders around and you'll see that a good cage includes this "dash" or "knee" bar every time.

TimT · 11-29-2008, 08:24 PM

whatever

Quote:

My concern is this: I'm only 5'8" and I sit fairly close to the wheel/pedals. There isn't a lot of room between my knee and the dash as it is (maybe 3 inches?)... It seems a dash bar with padding will be nearly touching my knee... and this can't be good...

jluetjen did some thorough FEA using GRAPE analysis of a cage with and with out a knee breaker.. I then did some modeling using another FEA program and found that Johns results were correct.

Some of the same questions are bandied about in this thread

that bar doesn't see alot of stress, it really doesn't do much in the grand scheme of things.
btw one of the cages I have built

that knee bar is there because the customer wanted a knee bar

TimT · 11-29-2008, 08:35 PM

*

Quote:

This came up a couple of years ago, and I had some time on my hands to try modeling it out in Grape's FAE software. I posted my findings starting on page 2 of this thread. The short form answer is that the loads on the "knee bar" never seem to be very high, and if you take it out, it didn't seem to make a huge difference in the results. Keep in mind that loads are generally distributed around the perimeter of the cage rather then across the cage. Given the the negligible impact on the loading, and the very real risk of impact on the knees, I'm now of the belief that I'm going to leave the knee bar off of my upcoming race car cage.

*

stownsen914 · 11-30-2008, 02:16 AM

Quote:

Originally Posted by TimT

whatever

jluetjen did some thorough FEA using GRAPE analysis of a cage with and with out a knee breaker.. I then did some modeling using another FEA program and found that Johns results were correct.

Some of the same questions are bandied about in this thread

that bar doesn't see alot of stress, it really doesn't do much in the grand scheme of things.
btw one of the cages I have built

that knee bar is there because the customer wanted a knee bar

All makes sense as long as the car isn't hitting anything. But what happens in a massive side impact when there's no knee bar (or through-the-dash bar) to stop the cockpit from collapsing laterally?

Scott

Astroman · 11-30-2008, 06:03 AM

Ah clever, Tim. Dig up a quote of my own that seems to contradict what I'm saying. It's true that I once questioned the safety of a dash bar. People I trust and respect reassured me.

My cage builder was able to install the bar so that it's about 6 inches from my knees, which I'm very comfortable with. Again, it's tucked away in the exact location of the previous lower dash. If it was in front of the dash and with a pad, it would be close.

Not that it makes it right, but I'd say that, for some reason, 9 out of 10 custom cages have this bar (and even several of the bolt-in ones, too)...

vmax · 11-30-2008, 07:29 AM

This car had a Safety Devices cage installed. This cage did not have a knee bar.
Look at the driver's side a-pillar at how far the cage has been pushed in. I think a knee bar would have strengthened the cage, and limited the side intrusion. This pic also shows why a good door bar set-up is important.

TimT · 11-30-2008, 09:34 AM

Actually I found that quote when I was searching for some of the FEA stuff jluetjen did. I also did some models which were on a laptop that was stolen, and haven't taken the time to rebuild them.

Its show that if you removed the knee bar the stresses in the cage remained the same, or in other words,the cage performed the same with or without the knee bar

If you get that bar tucked way out of the way fine, you carry around a few extra pounds.

I think one overlooked spot in cage construction is the floor. I've seen a car that was balled up and the pedal cluster was pushed almost halfway to the seat

I saw pictures somewhere of a car with some floor bars, Gunnars sight maybe?

TimT · 11-30-2008, 09:36 AM

I still cant find , but there was an epic roll cage thread a few years ago. Dont remember if it were here or rennlist

zzwhm · 12:27 AM

I was at the track when the #38 car Doug posted above was hit and after seeing it added a knee bar to mine before I raced again. Without a cross bar at the A pillar to transfer the impact the cage simply moved inward on the driver.

chrisp · 11-30-2008, 05:33 PM

I'm not sure what PCA would think of it (because it reinforces and stiffens the chassis) but you would probably gain a lot of side impact structural integrity if you could tie the center tunnel and the side sills together. The knee bar is doing this to some extent but not as effectively because it's up and away from the sill. Perhaps you could add some rectangular section tubing laterally at the floor level between the sill and the tunnel. You would have to reinforce the ends to protect against punch through but I bet it would keep the sill from bending in towards the center of the car in a low height side impact.

I also like the idea of adding high density foam (like roll bar padding but in sheet form ) to the left side of footwell area.

chrisp · 11-30-2008, 05:34 PM

http://www.rollbarpadding.com/prod_SFI452.html

I tried to post this link in the above post.

stownsen914 · 12-01-2008, 06:13 AM

Quote:

Originally Posted by TimT

Its show that if you removed the knee bar the stresses in the cage remained the same, or in other words,the cage performed the same with or without the knee bar

Doesn't the testing you refer to relate more to the stiffness of the cage, not the resistance to being crushed? I can see how the lack of a transverse bar such as a knee bar wouldn't contribute much to stiffness, but wouldn't it significantly help resist lateral crushing of the cockpit in a heavy side impact?

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

Quote:

Originally Posted by TimT

I think one overlooked spot in cage construction is the floor. I've seen a car that was balled up and the pedal cluster was pushed almost halfway to the seat.

+1 on that. I am guilty of it too. I've thought about adding tubing to the floor of my track car ...

Scott

CRNT918 · 12-01-2008, 03:55 PM

Wondering aloud, new cars are made with a "crush zone" to absorb energy during an impact. Is it possible to design/make a cage that is too stiff?

juanbenae · juanbenae's Garage

looking to cage up too in the very near future and was wondering if some photos might be shared here? not to highjack, but you know.... on theme..

thanks

stownsen914 · 12-02-2008, 06:33 AM

Quote:

Originally Posted by CRNT918

Wondering aloud, new cars are made with a "crush zone" to absorb energy during an impact. Is it possible to design/make a cage that is too stiff?

I don't claim to be an expert on cages, but I believe the answer is a conditional yes. If you think about the parts of a newer car that are designed to crush, they are usually mostly in the front and rear of the car (not the cockpit). Since a roll cage, strictly defined, is to protect cockpit area, you probably don't want it crushing much (if at all) or you'd risk injuring the driver in an accident.

With that said, cars that have tubing outside the cockpit for stiffening purposes can be designed with smaller diameter or thinner tube in the front and rear portions of the tube frame. This allows for some crush in the extremities while ideally transferring less stress to the roll cage (cockpit portion of the tube frame).

Scott