Skip

FWIW, the design of the Weltmeister harness bar that attaches to the C-pillar (the end bits) is such that the angle is at ~45 degrees relative to the shoulder harness travel - if connected to the rear chassis mounts.

Also FWIW, the 75" standard length (long) harnesses are long enough to loop over the B-pillar (Brey-Krause type) and C-pillar harness bars (most others - Welt, KLA, etc) and be final-mounted to the rear factory chassis seat belt mounts.

Skip

smokey

Skip, the 45 degree angle would increase the force on the bar by about 87%, compared to attaching the belt directly to the bar, if my calculations are correct. Could someone check that? My arithmetic is not always reliable.

smokey

Sorry to keep this going, but at an included angle of 120 degrees, the force on the bar is equal to that of being attached directly to the bar, and at anything more than 120 degrees, the force on the bar is actually reduced. So there is a situation where the force is reduced: the angle has to be more than 120 degrees, and then the harness bar does actually become a guide bar. The bar thus has to be as close to the driver as possible, and the mounting point on the body as high as possible. There are probaly some bar designs where this is true, if the belt is mounted to the shoulder harness position in the back seat. My final word - I hope.

DB944T

"...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)"
___________________________________________

Just to try to answer skip's question - how does the force on the bar get larger in the "looped" situation?

Its a matter of mechanical advantage - the belt passing over the bar to a fixed point creates a pulley system (like block and tackle or engine hoist, etc.) pulling on the bar.

Like a lever multiplies force, a pulley system does the same for ropes, belts etc. attached to a fixed point. The force caused by the person(and on that end of the belt) is the same. It can generate a larger force through the rope/belt version of leverage.

Dave

Skip Wolfe

Great discussion. Making me use my atrophied mechanical engineering degree. It is correct that the force on the bar is increased by looping rather than mounting directly to the bar.

With that said it should be noted that the Brey Krause bar has 4 mounting points - both the B and C pillars rather than either/or like the Stable Energy style bars. The main bar attaches to the B pillar seat belt mounting point and then there are support rod that attach to the rear shoulder belt mounting points on the C pillars.

<img src="http://members.rennlist.com/skips951/Brey%20Krause.jpg" alt=" - " />
<a href="http://www.bkauto.com/porsche/r1020.php" target="_blank">http://www.bkauto.com/porsche/r1020.php</a>

Also the support rods attach to the bar in between each set of belt guide loops creating a triangular truss which will help keep the bar from collapsing.

<img src="http://members.rennlist.com/skips951/Stable%20Energy.jpg" alt=" - " />
<a href="http://www.stable-energies.com/stable/shoulder.html" target="_blank">http://www.stable-energies.com/stable/shoulder.html</a>

The Stable Energy bars on the other hand are only supported with two mounting points and does not have the secondary supports. I had a BK bar for a short while and was extremely impressed with its construction. I was also able to compare it against a buddies SE bar and there really is no comparison - the BK bar is far superior. I ended up selling it and getting a Redline rollbar which provides roll over protection as well as provide a stable harness mounting point.

<img src="http://members.rennlist.com/skips951/944RollBr1.jpg" alt=" - " />
<a href="http://www.homestead.com/redlinerennsport/SafeguardPage.html" target="_blank">http://www.homestead.com/redlinerennsport/SafeguardPage.html</a>

My feeling is that if your are going to regular track events then you should really go to a roll bar, but if you end up going with a harness bar, then the BK bar should be the only one you consider.

FormulaOne10

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"> "...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)"
___________________________________________

Just to try to answer skip's question - how does the force on the bar get larger in the "looped" situation?

Its a matter of mechanical advantage - the belt passing over the bar to a fixed point creates a pulley system (like block and tackle or engine hoist, etc.) pulling on the bar.

Like a lever multiplies force, a pulley system does the same for ropes, belts etc. attached to a fixed point. The force caused by the person(and on that end of the belt) is the same. It can generate a larger force through the rope/belt version of leverage.

Dave </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Actually a pulley does no such thing. All a pulley does is redirect the direction of a force vector. You are confusing force with work. There is nothing in the world that directly multiplies force. Now what pulleys and levers do is enable you to apply force over a longer distance to get work done. For example, if you use a fulcrum to lift a heavy load, you want the fulcrum as close to the heavy side as possible. However, now you need to move the lever a much greater distance.

When the strap isn't looped around the bar in an accident (mounted to the rear seat belt mounts):

The person pulls on the strap, the bar then redirects the force in a direction useful for the mounting point on the strap and safer to the person. We are talking about tension here so it works both ways.

When you connect the strap to the bar:

The person still pulls on the strap with the SAME force. However, now the strap grips the bar and pulls on it. Instead of the tension of the strap pulling on the rear seat belt mount it is now pulling on the bar. This isn't the best mechanical explanation but most people can visualize this.

The only reason you use a harness bar is to eliminate the downward vector you will get by mounting directly to the rear seat belt mounts. This will ensure the the strap is not pushing downward in an accident (possible spinal compression among other things). With a harness bar it will just pull horizontally.

IMHO, the best way to mount would be the rear seat belt mounts and then over a harness bar. I wouldn't mess with trying to place all the force on the harness bar. It just isn't worth the risks.

Coming from a mechanical engineering student...but I make mistakes too.

Skip

Okay, this is getting long - so, how about getting to the real reason why belts are not attached to the bar...

Belts should not be attached to the bar because it is a possible single point of failure. With each (of a possible 4 for 2 occupants) belt attached to it's own point, the risk is greatly reduced.

Skip

smokey

FWIW, after reviewing all the options, I originally installed a Weltmeister harness bar on the C-pillar, and for reasons I've explained, attached the shoulder harnesses directly to the bar. It's 1 1/2" roll bar stock (heavy), with flanges welded on the ends in tension, so it seemed strong enough - as strong as a cross-brace on a roll bar, although it's longer. The bolts should be good for 10,000 lbs each at least in shear, so they will not be the weak point. Its strength would be easy enough to calculate, but my mechanical engineering skills have atrophied. The BK bar is a better design, but I also considered cost-effectiveness. I used it for a while, and then realized that with the stock seats, the shoulder harnesses were useless, because they would slip off my shoulders in any complicated collision, so I installed seats with shoulder harness slots. I run with the BMW Club as well as the PCA, and the BMWC won't allow you to use a 5-point belt without a roll-bar, after displaying a photo of a BMW coupe with the roof flattened. Good wake up call. So I installed a redline roll-bar (no endorsement implied) after considering a weld-in cage or roll bar. Cost-effectiveness again. I have scheduled another 25 track days this summer (I'm a slow learner) so the next stage may be a weld-in bar or cage. At Mosport, the most impartant part of the cage is probably the door bars, because of the blind corners. My story to date, FWIW.

Z-man

Great thread. Thoroughly enjoyed the explanations on both sides.

At this point, I will continue to loop the shoulder harnesses over the harness bar and mount them to the rear seat belt anchor points.

I'll most likely be doing close to 20 events myself this summer! First event is in 2 weeks: Lime Rock! Whooot!

-Z.

Skip Wolfe

Zoltan,

Wow 20 events - where do find the time? With all of the DE events that you do, have you ever considered going to a roll bar. The way I look at it, the more events one does, the faster they get, and with the speeds that an experienced driver attains, the more saftey needs to be looked at. The potential for a wreck is diminished because of skill and experience, but the potential seriousness of a wreck is increased because of the speeds involved. Myself I went with a Momo Start seat and a Redline rollbar. The Redlines are nice because they are a true bolt in bar and relatively easy to remove.

Man I wish I could do 20 events in a year!!!

Skip Wolfe

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Skip:
<strong>Okay, this is getting long - so, how about getting to the real reason why belts are not attached to the bar...

Belts should not be attached to the bar because it is a possible single point of failure. With each (of a possible 4 for 2 occupants) belt attached to it's own point, the risk is greatly reduced.

Skip</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">I am wondering why PCA has the requirement. They are increaseing the risk not reducing it. If the bar fails in either case, it would be bad. If the belts are mounted to the bar and the bar fails then their would be no shoulder support, but if the belts were bolted to the rear seat belt points and the bar broke then there would be so much slack that their would still be essentially no support. So with that in mind why wouldn't PCA opt fo the option that produces less stress on the bar, therefore minimizing the risk of breaking.

Z-man

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Skip Wolfe:
<strong>Zoltan,

Wow 20 events - where do find the time? With all of the DE events that you do, have you ever considered going to a roll bar. The way I look at it, the more events one does, the faster they get, and with the speeds that an experienced driver attains, the more saftey needs to be looked at. The potential for a wreck is diminished because of skill and experience, but the potential seriousness of a wreck is increased because of the speeds involved. Myself I went with a Momo Start seat and a Redline rollbar. The Redlines are nice because they are a true bolt in bar and relatively easy to remove.

Man I wish I could do 20 events in a year!!!</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">OPPS! I did a typo: I meant 20 days total: about 7-9 events. Sorry.
I'd love to do 20 events, but I think I'd be in the doghouse then. But then again:
"You can always sleep in your racecar, but you can't race in your house!" <img border="0" title="" alt="[Eek!]" src="eek.gif" />
-Don't EVER tell my wife I said that!!!

But back to the subject: I do plan on getting a redline rollbar, but due to budget constraints, now is not the time. (Something about putting in a new kitchen...)

-Z.

Z-man

Crap: double post. Sorry.

So, here's another racing metaphor:

"To earn a small fortune in racing, start with spending a large fortune."

(Both quotes are shamelessly borrowed from a post in the Racing/DE forums!)

-Z.

smokey

Skip Wolfe, I checked with PCA re the harness bar requirements, and was told that there is no national standard for DE, but it is under review. One of the things pointed out to me in that correspondence was that belts will stretch up to 25% in a collision, so one of the desirable parameters is to keep the belts short. In their opinion, any belt attached to the back bulkhead will stretch too much to keep the driver's head off the windshield in a collision, which tends to rule out the c-pillar. FormulaOne10, work in equals work out. Both levers and pulleys multiply force.

Skip Wolfe

FormulaOne10,

I gotta agree with Smokey on this one. Draw yourelf a freebody diagram and do the vector math. Looping the belts will increase the force versus mounting directly to the bar.

I also agree that looping the belts will increase the length of the belt and therfore increase the amount that the belts can stretch.

The best way to mount the belts is directly to the harness bar.