Since he asked about PCA I don't see how the SCCA GCR is relevant even if they have it right.

 

True...

But right is right and better is better, and establishing a precedent is important in these forums. I'm a little surprised that SCCA does not also talk about where the belts must run as they move from mount to release point, but they have it right. It is not under or over written. Either is a mistake in many cases.

If I am looking at the pictures and interpreting them correctly, this is the proper way to configure this type of setup. It is not ideal and it is not the best, but it is a reasonable compromise, given the limitations at play. Unfortunately, in some instances, PCA does not know this.

Everyone will have to check with their sanctioning PCA region, because PCA National has provided so little guidance on these matters that eveyone is free to be all over the map, and they are!

 

Thanks for posting the pics.

Personally I don't like that sub mounting set up because, assuming it's a stock racing seat, the sub straps as they pass through the seat bottom will contact and load against the seat shell compromising the integrity of the shell. Under extreme conditions a disitengrating shell could even cut the strap.

The bar itself is a great idea (that's how I built mine ) but I would move it forward several inches.

 

Hey All;

One aspect of this subject that really sticks in my craw (and it applies often) is that people will guffaw, sniff loudly, discount a setup such as the one that we are discussing, then jump right in their car that has sub straps bolted through a flat tin floor, and feel perfectly safe. Why? Because "everyone has always done it this way?"

If they took one measly second to think about it, they should be scared stiff. My standard test for them to mull over is; "what do you think would happen if you put a floor jack under the floor pan - right where your sub mounts are - and jacked the car up?" "Well... no one would DO that!" Yeh, DUH. What are you doing putting 1000+lbs of load into it with your sub straps when you hit something? DOINK!

It's such a simple concept, yet so elusive to so many.

Case in point. I recently changed out a seat in a customers new-to-him 993 Supercup. These cars have factory installed threaded insets in the floor pan for the sub straps. No reinforcement of any kind. This car has not been crashed, yet the floor was deformed upward from sub strap loading.

It really shouldn't take a rocket scientist like me to point this stuff out. Blindly accepting scriptures when your safety is at risk is not always a good way to promote it!

End of Sermon.

 

Thanks John.

Can you take us through how we get 1000lbs of load? Not saying I disagree. I would just like to understand the numbers.

 

I'm surprised that Porsche missed this. Maybe they didn't miss it and it's not as big of a deal as we think it is.

I'm with you though. Here's my sub mounting system.

 

Turning a 200 pound person into a 1000lb weight only requires a 5 G stop. I don't know the numbers on how it translates into the harness but 1000lbs seems like a small number considering we often see 50G+ hits in the top series and I remember one hit the Milwaukee Mile that was well north of 100+G.

 

Agreed but we're talking about sub straps here. The shoulder and lap belts take the majority of the load.

And there's still the outstanding question of why Porsche didn't reinforce the floor pan if the load is so high.

 

Just an example not sure of the exact numbers but...

Say the substraps takes 15% of the 'weight' of the driver, a 30G hit gets you up there.

1000 pounds just doesn't seem that unreasonable.

 

sub-strap keeps the lap belt down no? So it may not see 15% of the load. It resists the force of the lap belt pulling up. As the driver moves forward in an impact I bet the sub actually loosens a little because its attachment point (at the camlock) is moving forward and that shortens the distance to the mounting point.

 

Circuit is right on it!

Chris, two of the greatest crash testers extant - Melvin & Schroth - have seen loads in the sub straps nearly equal to the (EDIT) a shoulder belt. Schroth states (I have heard this right from his mouth) that "on average" lap belts see 2800lbs and shoulders see 1500 each. The usual dynamic used for quoting such data is a 40g hit, in my experience.

So, let's play that on a bit. The sub is more or less directly opposing shoulder strap movement. It is holding the lap belt low, but it is ACTING AGAINST the shoulders. The shoulders pull up, the sub pulls back down. Cancel. The lap belt stays in place by default. Even though the entire load is not rendered in a directly linear fashion (the torso flies forward, not upward), this clearly shows the potential for a significant load in the subs, no?

Interestingly, your sub setup might not pass all scriptures. I've seen some that say all belts must be attached to the chassis, and have no relation to any seat mount. In a perfect world, that may be the ultimate. however, considering how tough (and invasive) it is to build a good custom reinforced sub structure in a car, and what is considered the "norm," your setup is OUTSTANDING!

 

John, I agree 100% with your load path analysis assuming the lap belt mounts are directly behind the camlock and the lap belt doesn't ride up at all. Since lap belt mounts are lower than the camlock then the lap belts carry a lot of that offsetting shoulder load because they have a downward pull. The lap belt will move slightly and as it does more of the load is carried by the sub but it's still not a lot. I would think that the better way to think about the sub is that it's a piece supporting the load carried by the lap belt but not totally taking all the opposing load from the shoulders. In your numbers you need to know how heavy the driver was and the G's to know how much is being carried by the sub after the 2800 through the laps and 1500 through each shoulder (5800 total). If it were the 40g hit, a 175 lb driver would generate 7000 lbs. In that case doesn't it mean 1200 is going through the subs, not 3000 (1500x2) as you proposed (2800+1500+1500+1200=175*40)? Of course this all depends on the driver size. If the driver was less than 175lbs then the sub is seeing less. With a 150 lb dummy the total load is 150*40=6000 which would then be 200 for the sub (2800+1500+1500+200). Any idea how heavy the dummy is in those tests?

Am I thinking of this right?

 

One of the biggest problems I had when installing my 6pt sub was getting enough free clearance for all the sub hardware and fab a stout mounting right under the sub hole all when you are trying to get the seat to be as low as possible in the car. I have yet to see a really trick sub mounting. You can mount your seat very close to the floor if you route the sub anchor posterior but then the forces of impact are on the sub hole of the seat. This is a really bad thing to do. So you either have to tilt the seat to get some clearance and or kluge an additional bar under the sub hole so that if you do route the subs posterior the forces of impact are not on the seat subhole but on the extra "sub bar".

 

What if the seat manufacturers provided an anchor point under the sub hole, assuming they could demonstrate adequate structural integrity?

 

Chris;

Let me restate and embellish. I made an error as well, in the previous statement (see edit). I have heard Mr. Schroth state that the load in the sub belt is often nearly equal to that in a shoulder strap. I chose a number of 1000 to be conservative. He has stated numbers upwards of the same as the shoulder straps. Since the sub is almost literally an extension of the shoulder straps, this makes perfect sense to me.

Gregg; Why ask a question that you already know the answer to?

I can see it already; The Isaac Throne!

Interestingly, I'm way ahead of you. I have had my sub mounted to me seat for some time (mine is at right). I have been contemplating further revisions to strengthen the seat base, because I am aware that I am putting the load into that area, and I'm not at all confident it is up to the task... totally. I am not worried about the load to the aluminum itself, as I have it rendered in tension/shear. I am more worried about the structural ability of the seat base to resist bending upward. I also plan to free up more space and remove some clutter by switching to Sparco seat rails to eliminate the transfer cable and locks underneath.