Matt Marks

Karl,

In a nutshell (pardon the pun), the 5-point has a single, usually 2-inch wide anti-submarine (crotch) belt, while the 6 point has 2 (narrow) anti sub belts.

Can't say that I know that either kind is safer, but the 6 point belt is usually used in smaller open-wheel formula cars, as your legs are too close together to get the 2-inch wide sub belt comfortably between you.

The 5 point sub belt goes straight down and moints to a single point on the floor. The 6 point runs in an inverted "V" shape to two differnt points on the floor of the car

dave120

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Skip:
<strong> </font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by smokey:
<strong>Looping the belt over the harness bar and then connecting it to the seat belt mounts increases the stress on the harness bar by about 40 to 50 % compared to simply attaching the belt to the harness bar.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">I need to see an engineering example of this - anyone? I don't know enough to disagree, it just doesn't feel right...

Another item to consider is the strength of the bar itself holding the harnesses compared to the strength of the rear seatbelt mounts holding the harnesses. While the harnesses would be far less usefull if the bar were to collapse, is the fear more that the bar may bend, allowing it to lose it's strength and snap free - allowing the harnesses to come free. I just can't picture the forces of looping the harness over the bar being as strong and direct as the forces if it were attached directly to the bar... I almost picture the load of looping over the bar being in more than one direction, thus reduces the load relative to that of it being in one point (as in tied to the bar)... I'm probably missing a lever theory in here somewhere?

Skip</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Good points here. The reason HARNESS bars are never recommended to mount harnesses to directly is because the bar may bend/break. They aren't made out of material strong enough to hold the harnesses in some cases. IF the harness bar were made out of the same tubing material as certified roll bars, you could mount to them no problem.

As far as forces go, I don't see how looping them over can be worse than being mounted directly to it. From the physics I've learned (and I am still in school with more to learn, however) it doesn't quite make sense for the reasons Skip covered. This is a good discussion I like it.

wjk_glynn

Thanks Matt for the explanation.

Karl.

Z-man

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by smokey:
<strong>Looping the belt over the harness bar and then connecting it to the seat belt mounts increases the stress on the harness bar by about 40 to 50 % compared to simply attaching the belt to the harness bar.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">This is simply not true. In reality, the force on a bar where the harnesses are looped around are move evenly distributed than a harness bar where the harnesses are attached directly to the bar.
Where the harnesses are attached directly to the bar, ALL of the force on a frontal impact will be in one direction: toward the front. In worse case scenario, if the bar breaks away from the mounting point, it will more directly toward the front of the car, ignoring any gravitational forces acting upon it.
Whereas: if the harness is looped over the bar and attached to the rear seat belt mounting points, there are two separte forces acting on this bar, in two different directions, 90 degress from each other. If the bar breaks free in this case, it would follow a path roughly 45 degrees from the horizontal, causing the bar to probably hit the back of the front seats.

Regardless of how much force is acting on the harness bar, these bars do not have sufficient anchoring to attach a harness bar directly to it: two bolts ain't gonna hold it. That's why most PCA / DE venues do not allow direct mounting of the harnesses to the bar.

smokey: what makes you believe attaching harnesses directly to the bar yeild less force upon the bar? I'd like to see your reasoning.

-Zoltan.

smokey

In a 50 G collision, the tensile force acting on your shoulder harnesses is about 50% of your weight times 50, or, say, 5,000 lbs. If the harness is attached directly to the harness bar, that is the force acting on the bar. If you run the belts over the harness bar and then attach the belts to the seat belt mounts, then there are two forces acting on the harness bar: the 5,000 from your body surging forward in the collision, and the 5,000 lbs from the belt pulling on the seat belt mount. That is just basic Newtonian mechanics. You effectively have the harness bar acting as a pulley. If the angle between the two parts of the belt is 90 degrees, the total force on the harness bar is 5,000 times the square root of two, or 5,000 x 1.41, i.e. the force is 7,050 lbs - 41% higher. The sum of these forces is acting at a 45 degree angle to the belt connected to your shoulder, and the harness bar, if it breaks, will break at this angle. You will then have a shoulder harness attached only to the rear seat belt mount, and the shoulder harness will be useless in arresting your forward motion due to the slack in the belt. If the angle between the two parts of the belt is 60 degrees, the total force is 73% higher. All you do is add the force vectors, as per freshman physics. As to why the PCA says you can't attach the belt directly to the harness bar, I'd love to hear an explanation. Preferably from an engineer rather than a person with strong opinions but no understanding of basic Newtonian mechanics.

Z-man

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by smokey:
<strong>In a 50 G collision, the tensile force acting on your shoulder harnesses is about 50% of your weight times 50, or, say, 5,000 lbs. If the harness is attached directly to the harness bar, that is the force acting on the bar.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Yes, but the harness isn't attached directly to the bar, even in the case that you argue: it is looped over and attached to itself. Now as you clearly stated, I am 'STUPID' when it comes to such matters, but YOU NEED TO consider that in the equation. See here:
<img src="http://boards.rennlist.com/upload/harnessbar.JPG" alt=" - " />
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"><strong>
If you run the belts over the harness bar and then attach the belts to the seat belt mounts, then there are two forces acting on the harness bar: the 5,000 from your body surging forward in the collision, and the 5,000 lbs from the belt pulling on the seat belt mount. That is just basic Newtonian mechanics. You effectively have the harness bar acting as a pulley. </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">HOWEVER: if the harness is attached dirctly to the bar, the harness bar is STILL acting like a pulley. The angles are much less, but they are still evident. Remember: to attach a harness to a bar, it is still looped over the bar. They may be connected together, but they essentially NOT connected to the harness bar, just looped over it!
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"><strong>
If the angle between the two parts of the belt is 90 degrees, the total force on the harness bar is 5,000 times the square root of two, or 5,000 x 1.41, i.e. the force is 7,050 lbs - 41% higher. The sum of these forces is acting at a 45 degree angle to the belt connected to your shoulder, and the harness bar, if it breaks, will break at this angle. You will then have a shoulder harness attached only to the rear seat belt mount, and the shoulder harness will be useless in arresting your forward motion due to the slack in the belt. If the angle between the two parts of the belt is 60 degrees, the total force is 73% higher. </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">What if the angle between the two parts of the belt is almost 180 degrees, as when the harnesses are attached to the bar? Or is that only 5 to 10 degrees? Sorry, I'm not into physics, so please explain. Also: please explain how attaching a harness directly to the bar is NOT a pully.
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"><strong>
All you do is add the force vectors, as per freshman physics. As to why the PCA says you can't attach the belt directly to the harness bar, I'd love to hear an explanation. Preferably from an engineer rather than a person with strong opinions but no understanding of basic Newtonian mechanics.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">I don't appreciate being insulted like this. I haven't studied Newtonian mechanics in over 15 years, so I may be a little rusty with it, but that's no reason to bash me.
I've heard that harnesses should not be attached to the harness bar from several people in PCA, as well as people who sell such safety equipment. If you want, give Bob at Stable Energies a call: perhaps he can shed some light on this.

And refrain from the insults: that's not a way to win an argument, and it's not appreciated.
-Zoltan.

smokey

Zoltan, I apologize. I didn't mean to insult anyone, and my comments were meant to be general. It's a matter of personal choice as to how one mounts a harness, and I was simply voicing my opinion. I will keep my opinions to myself in the future.

Z-man

Smokey: apology accepted. Your opinions matter to me, but I just felt I was insulted.

Does the diagram I 'drew' make any sense? Is your claim about less tensile still true, in your opinion, even though the harness IS looped over the bar? I'd really like to see this explained.

Though it is a matter of choice how harnesses are mounted, but it is also a safety issue. If mounting to the bar makes more safety sense, then that should be what PCA adopts. If looping the harness over the bar and mounting it to the rear seat belt anchor points is better, than that should be the rule.

Thanks,
-Zoltan.

smokey

Peace, Zoltan. I don't know how to do graphics here, so I'll verbalize. Your drawings are both correct. In your first example, the force on each strap would be 2,500 lbs, but 5,000 lbs where the two belts join into a single belt again forward of the buckle. In the second example - for instance if the belt were clamped to a fitting on the bar and did not pass around it - the force would be the full 5,000 lbs on the single belt. However - and this is really the critical difference - if the belt passes around the bar, and then is doubled back and attached to another solid object rather than to itself, the force on the bar is doubled. Let me explain. You're standing in the garage, pass a rope around a rafter, and lift yourself up using the two ends of the rope passed around the rafter. The two ropes together support your whole weight, and each rope supports half your weight. The downward force on the rafter equals your weight. Correct? Now hold one end of the rope and tie the other end to an engine block weighing the same as you do, on the floor beside you, after passing the rope over the rafter. Now lift yourself up holding one end of the rope. Your weight equals the weight of the block. The rafter is supporting both you and the block, or twice your weight. This is the difference between a belt looped around the bar and attached to itself, and a belt looped around the bar and attached to another solid object.
I apologize for this extremely long post. I'm just trying to explain, and not to offend anyone. It's a very important issue, and I'm willing to be corrected. If I could add a smiley face or beer mugs, I would do so, but with the new system, I haven't figured it out. Cheers.

hrk

It depends from angle of these two forces from harness bar.
If we assume the seabelt is horisontal to shoulders and vertical down to rear seatbelt bolts. Angle within them would be 90 degrees and force in accident to be F resultant to harness bar would be sqr(2)F=1.4F

hrk

smokey

hrk - 100%.

Porsh-uh

Excellent demonstration, Smokey. I think that it is very easy in this issue to confuse the two important factors, although they are completely different. One is the force acting on the HARNESS and the other being the force acting on the HARNESS BAR. In the demonstration Smokey gives, the piece of rope (the harness) is completely capable of holding the complete weight of the body. Hence, when it is doubled over rafter, each side of the rope is capable of holding the weight of the body and it's equivalent. Because it is hung over the rafter, each side of the rope is bearing the weight of the body and the equivalent (the engine block) independently. So, the threat of the rope breaking is the same in either scenario. However, the load on the rafter (the harness bar) is doubled in the scenario where the body is supported, as well as it's equivalent. So, it seems (if I am following this correctly) that the belt breaking is not anymore of an issue in either case, but the harness bar coming under additional stress in the "two-fixed-point" example is the real issue.

smokey

I have put this question to PCA to confirm their DE policy and the rationale for it, because it's important.

Z-man

So, my question is this: how does the anchor points of the harness bar behave? In other words: it is better for the tensile force to be in one direction, or two?

I bet their response will have these two points:
1. The bar is engineered to withstand even the higher forces.
2. Given that the forces in the "looped over" setup are not in the same direction, but in a close to 90 degree separation, the bar may have less of a possibility to shear off and break away.
3. If the bar does break away, it is less likely to make it's way into the driver's compartment area, since it will essentially drop down at a 45 degree angle from the horizontal shoulder strap.

Well, I'm not the PCA tech expert, but that's my guess at what their response will be.

Smokey & all: I've enjoyed this thread.

-Z-man.

Skip

Sorry, folks - I still don't get it...

In the example of a person hanging from a rafter and then a person and a block hanging from the rafter - didn't you just increase the load by 2x? And, if the block were heavier than the person, then it would be a fixed point - similar to mounting the seatbelts to the rear chassis mounts. The issue is not if the belts (rope) can handle it - or even how the single belts distribute the load (because the moment is one-point on the bar), but what force is being exacted on the bar (beam). In a collision, the human body in motion will weigh X amount - this amoutn needs to be held in place by 4, 5, or 6 mounted belts. The force on the shoulder harnesses chould be evenly distributed (within reason) - these are attached to the bar, or to the rear seat belt mounts with the belt "resting" over the bar. How is it that the force is multiplied, and somehow more than the load exerted by the human body in motion (in the example of a person and a block from a rafter)?

Again, there may be a key element I am missing - but, when I sit here and play with my pencils and dental flos to illustrate this point to myself - I still don't get it. Looping the belts over the bar to the rear chassis mounts should decrease the load on the bar as compared to attaching the belts directly to the bar... I think

Skip