Seat Back bracing with Shells?

NoLift911 · 01-10-2007, 05:50 AM

Is there a need to have a seat back brace if you are running shells (Recaro PP's).

I was going to do this when I had my SRD's but with the Poles I am wondering if it is necessary...or what the benefits would be?

-Thanks

-Jeff

ErVikingo · 01-10-2007, 06:07 AM

I don't know what the pole position is but I say IMHO that with any seat there is an advantage when using a properly designed, executed and installed seat back brace.

I have regretfully seen/heard of seats ripped of from the floor as well as seat backs coming loose on impact.

I would not attach the brace to the seat but provide for a brace with a large "pad" close to the seat back (I've heard of the posibility of an attached brace going through the seat back in case of impact).

Do you use proper high density foam to line the pad on the brace.

Hope that helps and that you never need to test the efficacy of such brace !

NoLift911 · 01-10-2007, 06:34 AM

Some cars I have seen using a brace and others not - I know with reclining or two piece seats it is recommended, but not sure with a one piece shell what would the case be.

The brace mounts to the roll bar/harness bar and has a big pad that does not attach to the seat but rests against it...

jaydubya · 01-10-2007, 06:42 AM

Seat back braces are required for every major club racing organization (SCCA, PCA, NASA, etc.).

The only exception is when the seat is FIA approved and mounted with brackets provided by the seat manufacturer, specifically for use with that seat (this is for PCA, not sure about the other clubs). In that case, the seat is good for 5 years of use without a seat back brace. After 5 years the seat can continue to be used, but with a seat back brace. If the seat is replaced with another FIA approved seat, the seat mounting hardware must be replaced as well.

NoLift911 · 01-10-2007, 06:46 AM

Quote:

Originally posted by jaydubya
Seat back braces are required for every major club racing organization (SCCA, PCA, NASA, etc.).

The only exception is when the seat is FIA approved and mounted with brackets provided by the seat manufacturer, specifically for use with that seat (this is for PCA, not sure about the other clubs). In that case, the seat is good for 5 years of use without a seat back brace. After 5 years the seat can continue to be used, but with a seat back brace. If the seat is replaced with another FIA approved seat, the seat mounting hardware must be replaced as well.

Right - the seat is FIA approved with the requisite Recaro hardware so I should be good there...

I guess rules aside from a general saftey perspective what people were doing...I think the seat meets the specs but over and above that if folks thought it was a good idea. I am leaning towards it cannot hurt at this point.

jaydubya · 01-10-2007, 06:52 AM

I would probably opt not to use one if it weren't required. I don't like the idea of a metal spear pointed at my spine...

That said, you'll get plenty of strong opinions both ways on this topic. You might want to search in the "Racing and Driver's Education" forum at Rennlist.

Cheers,

Jeff

Formerly Steve Wilkinson · 01-10-2007, 07:55 AM

Well, the "metal spear pointed at my spine" is covered by a large steel plate that wraps horizontally around my Recaro seat and vertically all the way down my spine, so I actually have a "two-square-foot metal plate pointed at my spine."

which I prefer to the "two-inch-diameter steel baseball bat [the horizontal component of my roll bar] pointed at the back of my skull.]"

jaydubya · 01-10-2007, 08:00 AM

Quote:

Originally posted by Formerly Steve Wilkinson
Well, the "metal spear pointed at my spine" is covered by a large steel plate that wraps horizontally around my Recaro seat and vertically all the way down my spine, so I actually have a "two-square-foot metal plate pointed at my spine."

which I prefer to the "two-inch-diameter steel baseball bat [the horizontal component of my roll bar] pointed at the back of my skull.]"

Most people don't realize that the plate that wraps around your seat is really not part of the load path to your roll bar. Rather, the load path from the back of your seat is through the thick bar that attaches to your roll bar. Are you one of them?

euro911sc · euro911sc's Garage

A few years back there was a gent who passed at limerock going in rear end first. Apparently, the crossbar was at the same height as the holes in the seat for the harnesses. Seat broke there on impact with the cross bar so I hear...

Having a brace might be an advantageous preventative measure for things like that, though I also am not happy with a small plate to spread out the force from the spear pointed at my spine... best of two evils mayhap?

best regards,

michael

Formerly Steve Wilkinson · 01-10-2007, 11:14 AM

I'm well aware of the Lime Rock accident, since that's my home track. Which is exactly why I have a seatback brace.

I'm having a hard time understanding jaydubya's post. Am I one of what? And why is the plate that wraps around my seat not part of the load path? It is welded to the thick support bar that attaches to a collar on my roll bar and is an inextricable part of that support bar. Load path would seem to me to be spine to seat, seat to seatback plate, seatback plate to longitudinal bar, longitudinal bar to lateral rollbar, rollbar to car platform.

Oh, and what's behind my seat is definitely not "a small plate," like you see in the photos of some of these devices. It's about a foot and a half wide and two feet from top to bottom.

jaydubya · 01-10-2007, 11:38 AM

Quote:

Originally posted by Formerly Steve Wilkinson
I'm having a hard time understanding jaydubya's post. Am I one of what? And why is the plate that wraps around my seat not part of the load path? It is welded to the thick support bar that attaches to a collar on my roll bar and is an inextricable part of that support bar. Load path would seem to me to be spine to seat, seat to seatback plate, seatback plate to longitudinal bar, longitudinal bar to lateral rollbar, rollbar to car platform.

Oh, and what's behind my seat is definitely not "a small plate," like you see in the photos of some of these devices. It's about a foot and a half wide and two feet from top to bottom.

Are you one of the people who don't realize that the plate that wraps around your seat is really not part of the load path to your roll bar?

The seat back brace is designed to keep you head from contacting the roll bar if your seat collapses rearward, say when you back into a wall. It does that by keeping your seat from moving backwards. That is achieved by one or more relatively rigid posts that connect your roll bar to the back of your seat. See simplified top-view drawing below.

The problem is that you have now gone from a nice, flexible, energy-absorbing seat crush to having a relatively inflexible bar with no energy-absorbing capability restraining the rearward motion of your seat. Thus in a rear end collision all the force is transmitted from the chassis to your roll bar to the bar connecting to your seat to your spine. The large surface area of the contact pad does nothing to distribute the force since the big bar behind it has to buckle before that could happen. And it isn't gonna buckle before your spine takes the path of least resistance - above and below where that bar attaches to the back of your seat.

What would really be nice is a structure that would absorb some energy for say the first 2-3 inches of compression (enough to keep your head off the roll bar). There are a number of different ways that this could be achieved but no current seat back braces do this. The main constraint on seat back brace design that is required by the racing groups is minimum area of the plate against the seat, but if you still have a single bar connecting to the roll cage there will not be any distribution of load away from the center of the plate where the horizontal bar connects.

So the chance of hitting your head on the roll bar has been more or less eliminated, but the chance of breaking your neck or severing your spinal cord has been increased dramatically.

Perhaps "spear" was not a good word choice, since it is highly unlikely that the bar would cause a penetrating wound. "Rigid bar" would be a better description.

Cheers,

Jeff

jaydubya · 01-10-2007, 11:57 AM

To contrast the standard design depicted above, here is a structure that would distribute the load across the width of the plate behind your back. The length, diameter and boundary conditions at the joints of these rods would be chosen based on FE simulations to absorb the appropriate amount of energy as they buckle. Of course this design would have to be carried into the vertical plane as well and is probably impractical to manufacture or attach to a roll bar, but hopefully you see the difference between the load transfer path in this sketch and the previous one.

Cheers,

Jeff

Formerly Steve Wilkinson · 01-10-2007, 01:49 PM

Okay, I getcha, and I appreciate the excellent diagrams. I think the person who was killed at Lime Rock--he spun at the end of the front straight and backed into the tire wall--died not from hitting his head on the roll bar but snapping his neck against the roll-bar horizontal crossmember, or what some people call the "harness bar," which was at neck height, So I guess it's half a dozen of one...

ErVikingo · 01-10-2007, 05:17 PM

I've seen properly mounted, fresh FIA quality seats rip off the mounting base....

I use a large plate with high density foam. IMHO, in case of an accident (unless you use a deformable energy absorption contraption such as drawn above) thats better than no back brace pad.

I trust a bar better than what I trust the seats integrity. Remember that if the seat moves or the seatback breaks (or moves) the seatbelts will loosen up....

Be safe out there.......

Jeff Alton · 01-10-2007, 05:32 PM

Jeff, I l ike your idea, sort of like a steering column in design. I have seen a few also with some sort of high density foam between the plate and the seat but I am not sure how much, if any, energy it absorbs.

Cheers

ErVikingo · 01-10-2007, 05:39 PM

Jeff,
I saw one with a steering column shaft. Not tried (thank god !)

Hopefully will never know which one works better. Time for a sled test...

Jack Olsen · 01-10-2007, 05:46 PM

I took the Brey Krause brace, removed the thin low-density padding they have on it, and added a layer of 1-inch high density foam.

jaydubya · 01-10-2007, 05:51 PM

Quote:

Originally posted by catca
Jeff, I l ike your idea, sort of like a steering column in design.

Yep! It would take some work to put together a practical design though...

The cheaper option for those of us with limited time and money is probably to put a metric buttload of energy-absorbing foam between the plate and the seat, as you mentioned. How much is a metric buttload? I don't know, but if you imagine your seat pivoting at the bottom rear mount in a rear impact, draw a line from that point to your helmet. At some point as you go back your helmet may ( or may not, depending on many factors) contact the main roll hoop. I think it would make sense to have the full metal of the seat back brace engaged with your seat just before that point. So add enough foam - perhaps 3-6" thick depending on your seat setup? - such that the seat back brace loading surface is moved rearward, but not so much that your head would touch the main hoop after the foam is completely compressed. Most open cell foams will compress to less than 10% of their initial thickness when loaded in compression with sufficient force. After that they get very stiff.

Now the specific kind of foam to use is another subject...

I was just down working on my car, looking the main hoop and the seat back brace. My seat is so low that there is really no possible way for my head to hit the main hoop in a straight rear end crash.

Cheers,

Jeff

jaydubya · 06:06 PM

Quote:

Originally posted by Formerly Steve Wilkinson
So I guess it's half a dozen of one...

I think you're right - at least if we are choosing between no seat back brace and the commonly used ones.

The event that you reference was certainly a terrible tragedy. However I wonder whether seat back braces might have even more propensity for causing an injury than racing without one, since for many cars/seats/roll bars/drivers it may not even be possible to hit your head on the main hoop in a rear ender?

Formerly Steve Wilkinson · 06:58 PM

The main hoop is not my concern. It's the horizontal bar aimed at the back of my neck, well below the hoop. That's what killed the Lime Rock driver. Although as an EMS volunteer who spends 24 to 36 hours a week on ambulance duty, I do know that a body can go just about anywhere inside a crashed car.

Forgot to mention, for whatever it's worth, that I do have three-quarters of an inch of high-density foam between the steel plate and the seatback...I'm happy with what I have: a strong mounting to the rollbar, the foam, and a large, well-diffused steel plate cupping the entire seatback. A lot happier than I would be figuratively glancing over my shoulder at that two-inch steel tube a foot behind my neckbone. Or my daughter's, for that matter, since we share the car on the track and have two different mounting locations for the bar/plate.

jaydubya diagrams the ideal, of course, but I don't think it exists yet, at least not in our amateur world.