Roll bar angle:

[saltfever]

Good point and thanks for the link.

Look at Pg. 25, figure 1 (bottom). A load applied to a vertical tube only has compressive stress. A load applied to an angled tube has both compressive stress and a bending moment introduced. For equal loads, the angle tube will deform first.  I’m being overly simplistic only to make a point. In a crash the loads are very complex.

[Gwillard]

In my response I am assuming that both styles of construction are designed and built to withstand equal forces with the only variable being how they interact with the rest of the chassis. We'll just have to agree to disagree about whether one would be more prone to dig into the salt than the other.
BTW, efunda has some great information and calculators that are free to use. Look up stress transformations and Mohrs circle, they are pretty cool. A subscription is available that gains access to huge amounts of references and sources.

[saltfever]

Yeah, very nice site. If not careful it could take lot of time away from this forum. 

That horizontal tube, sliding along the salt, doesn't know if it's support is angled or perpendicular. Now if digging-in is a possibility I could see where the support tubes would have an influence but we have no data. I can see the sliding friction would be a pure bending moment on a vertical tube. With an angled tube the force is less bending and some tensile stress. That may be the answer.

[SPARKY]

--survivability is about G loads, slight deflection--- dramatically lessens the G loads

[hitz]

The engineering types will usually shudder when I mention about design work: "if it looks right - it probably is."

harv

[Nexxussian]

--survivability is about G loads, slight deflection--- dramatically lessens the G loads

Yes.

[saltfever]

--survivability is about G loads, slight deflection--- dramatically lessens the G loads

Certainly agree, Sparky. "Crush zones" are a big topic of interest in all vehicle design. But do you want a crush zone 2" above your head? What is "slight deflection"? What is the certainty that it will only deflect 2" during and upset. We need to attenuate the forces but I believe there is so little room around the driver (in tight fitting vehicles) the cage shouldn't be the device to deform 

[NathanStewart]

I spoke to Steve at the last board meeting regarding this and he's recommending that all new constructions be built with the front hoop laid back at least 30 degrees from vertical.  It could be the predecessor to a new rule proposal that we might see in the future.  I say again, it COULD be the predecessor to a new rule PROPOSAL that we MIGHT see in the future.  There has not been any change to the current rules as written in the most recent rule book.

Again, this recommendation was made for a specific new construction vehicle and in no way implies a change to the current rules or rule book. 

In order for these type of conversations to go on it must be understood that the rules are always in a state of flux and are subject to change.  I think the SCTA does a very good job of notifying participants of any changes if and when they happen.

[SPARKY]

Nathan,  Any comments or insights on what might may be trying to be accomplish? thanks sparky

[saltfever]

Yes, I agree Sparky. Would someone please tell us the thinking behind a laid back hoop. Why is it better than a vertical hoop? I am not opposed to any style. I would just like to know why one is better than the other. Or even better . . .what accident investigation points in favor to one or the other.

If strength is an issue look at the Tractor-Pullers. Cages use vertical uprights.

[John Burk]

The most likely failure is where the roll bars are welded to the upper frame tube . Widening the stance lowers the force on the welds in a crash .

[saltfever]

Well you could just locate the vertical forward more. The uprights can be positioned in any location along the top frame rail. It doesn't matter whether the upright is vertical or leaned back. I agree with you John, the load is spread over a greater distance. But it can be accomplished with either style.

[racergeo]

  Not to make light of the passion that many are debating this topic with, but do you think a class for tractor pullers would bring some new interest to LSR. The sled would have to be removed and they would have to run on a special short course as they have no cooling system. Their roll cage doesn't look all that aero but at least they are strong.

[1212FBGS]

dont think i ever seen a tractor puller crash.... not a good example if ya ask me... if you are looking for examples for crash surviving structure... you will probably stop and study sprint car stuff.... pretty darn hard to hurt one of those frames
kent

[bbarn]

dont think i ever seen a tractor puller crash.... not a good example if ya ask me... if you are looking for examples for crash surviving structure... you will probably stop and study sprint car stuff.... pretty darn hard to hurt one of those frames
kent

Um...not to make light of it, but when they do it can be spectacular. With the wheel speeds they are running, even with a fuel cut-off they accelerate quite dramatically when let off the chain. The one that I saw had the tractor launching off of a dirt embankment at the end of the course and flying 20-30 feet in the air and cart-wheeling about 150 feet. The driver walked/limped away but the tractor was a total loss. It was as bad if not worse than most fuel/funny car crack-ups.