Roll bar angle:

[Rex Schimmer]

There is a guy on the HAMB that is building a banger tank and he is saying that the roll bar(s) must be angled to the top rail by 120 deg. or 30 deg from vertical. He says that he got his info from Kiwi Steve, who is an SCTA inspector but I do not see it in the rule book and I only have the 2009 rule book. Is this true? is it something new and in the 2010 rule book?

Rex

[doug odom]

My question would be   WHY?

[dw230]

The answer is no. No reply to Doug.

I do know that there is a restriction on seat angle for Top Fuel, etc. I am sure there is an optimum angle for cages, seats etc. to be safe.

The SCTA/BNI does not restrict cage angle.

DW

[NathanStewart]

He says that he got his info from Kiwi Steve, who is an SCTA inspector

Steve Davies isn't just an SCTA inspector, he's the Chief Car Inspector.  There's a certain authority that comes with that position so I'd leave it to him and the appropriate category chair person to institute certain build requirements for specific vehicles.

[Rex Schimmer]

Nathan,
I agree that if Steve says the angle should be 30 deg then you probably have to make it 30 deg to get past his tech inspection but if it has to be 30 degrees then it should be in the rule book and not dropped on you as a suprise when you roll into inspection.

Rex

[Nexxussian]

Are you sure he's not talking specifically about the front hoop, in the picture in the rulebook it looks like it's at least 30* from vertical.

[saltfever]

I agree with Doug, why? Why should there be any angle from perpendicular on the front hoop anyway? Anything other than perpendicular is a weaker design. I can see how it could evolve to a laid back style due to streamlining, driver visibility, or egress . . . but it is weaker and induces more stress on the weld than a vertical. Maybe the bail-out requirement is facilitated by such a design but it is certainly not to increase strength.

[Joe Timney]

Actually the angle adds strength as most hits will glance off the bar instead of digging...that explanation came from the NHRA chassis design team years ago.

[interested bystander]

Rex, Are you sure you're not misquoting the guy on HAMB, or did he misinterpret Kiwi Steve's comment? I'll betcha Kiwi SUGGESTED 30 degrees and a good suggestion it is.

The angles in the 2010 rule book on pages 25 and 26 ,using my trusty protractor, are
 pretty close to 45 degrees.

FYI, the current SFI chassis spec is 20 degrees from the vertical MINIMUM.

Somebody check with Kiwi Steve! And get a WRITTEN response.

[saltfever]

Actually the angle adds strength as most hits will glance off the bar instead of digging...that explanation came from the NHRA chassis design team years ago.

Joe and others, I respect the wisdom and experience that is being brought to this discussion but it simply doesn’t make any sense (which may be my fault  ). I really want to understand the engineering behind this and so far antecedents don’t make any sense.

NHRA? Turn the car upside-down on an asphalt track. Place a load of 4,000-8,000 lbs on top of it. Which is stronger a vertical upright or one laid back at any angle? You know the answer. Ok. Now drag the vehicle (on the asphalt) in the direction of travel. Same question, which is better? Same answer. Now if NHRA is considering sliding off-of guard rails, banging into grandstands and ricocheting away because of a deflection angle; that would be an interesting analysis and may give credence to something other than vertical. It has nothing to do with LSR and LSR has none of those problems. We need to be careful about applying standards or assumptions from one environment onto a totally different venue. Sometimes good intentions . . .

[John Burk]

I made my front bar laid back about 30 deg . I'm sure the stress on the roll bars is never just vertical . Free standing towers are always wider at the bottom .
John

[Gwillard]

Actually the angle adds strength as most hits will glance off the bar instead of digging...that explanation came from the NHRA chassis design team years ago.

Joe and others, I respect the wisdom and experience that is being brought to this discussion but it simply doesn’t make any sense (which may be my fault  ). I really want to understand the engineering behind this and so far antecedents don’t make any sense.

NHRA? Turn the car upside-down on an asphalt track. Place a load of 4,000-8,000 lbs on top of it. Which is stronger a vertical upright or one laid back at any angle? You know the answer. Ok. Now drag the vehicle (on the asphalt) in the direction of travel. Same question, which is better? Same answer. Now if NHRA is considering sliding off-of guard rails, banging into grandstands and ricocheting away because of a deflection angle; that would be an interesting analysis and may give credence to something other than vertical. It has nothing to do with LSR and LSR has none of those problems. We need to be careful about applying standards or assumptions from one environment onto a totally different venue. Sometimes good intentions . . .

In a modern, well designed roll cage neither column bucking nor bending is the primary concern. The cages are sufficiently braced and reinforced to prevent failure in these modes. The area of most concern is where the cage structure is welded to the rest of the car. In the case of a streamliner or lakester this is usually at the top frame rail.
In any structure stresses tend to concentrate at changes in cross section. The more abrupt the change, the greater the resulting stress. A cage hoop that attaches at a 90 degree angle has a more abrupt change in cross section than a more obtuse angle, hence a more severe concentration of stresses. The opposite side, where the angle is more acute, is virtually always gusseted. This aids in preventing stress concentrations at the weld. An additional benefit is that any attachment angle other than 90 will give more weld area, though this is rarely a primary reason for laying back the cage hoop.
Laying back the cage hoops serves several other purposes. Doing this gives a larger base of attachment to the car spreading out any imposed forces over a larger area, it gives a more triangulated structure when compared to a hoop attached at right angles, and is less likely to snag on objects it comes in contact with such as other cars, guardrails, and racetrack surfaces that may possibly penetrate (dirt, sand, salt).

[saltfever]

edit . . .deleted as redundant.

[saltfever]

Nice post Willard, thanks.

Rather than draw pictures lets use the new rule book (pg.25-26). BTW, there are two figures one, no figure three, and a figure 4. So for figure one I’ll use “top” or “bottom”.  

For simplicity, assume a 3,500 lb vehicle takes flight and lands with a 20 G force (70,000 lbs). Now apply that load to both configurations in figure one (bottom).  So you are sitting in there and have 70,000 lbs above your head. Which design makes your crotch strap pucker! Obviously, the bottom view is the best intrinsic design. However, using triangulation, strength of materials, geometry, either design can be made equal in strength. So strength is not the issue.  But why is inferior geometry, requiring mitigation, the one of choice?

I don’t buy into the roll bar deflecting or “digging-in” less than any other. Both a vertical post or an angular post have a horizontal cross-tube above the drivers head. That horizontal tube (on either style cage) will hit the ground with the 70,000lbs of force. Think about it. As soon as you hit the ground 3,500 lbs of vehicle mass will want to continue in the direction of travel. Reacting that force will be the horizontal tube’s friction (or “digging-in” if you prefer) against the surface. That reactive force is applying pressure to distort the cage! Look at Figure 4, pg.25 and apply 70,000 lbs of down force and apply some unknown longitudinal force (from the ground drag on the cross bar) to that cage. Tell me it doesn’t want to trapezoid! You don’t need FEA to see what is happening. Even a perpendicular tube will still want to trapezoid the entire cage. However, both styles can be triangulated to resist such a force. So why is one preferred over the other? NHRA, and NASCAR conditions don’t exist and are irrelevant. Since strength and ricocheting are not an issue in either design, one might be preferred over the other simply for egress.
 
I am not lobbying for a perpendicular upright or advocating any configuration. I just want to know why a weaker design was chosen. Strength isn’t everything and there are always trades. Sometimes, you defer to strength for a more important alternative. So far I haven’t heard why. Much of LSR is personal choice. I hope that part of the sport always exists. However, at the same time, in order to make an informed decision, you are entitled to good information.

[Gwillard]

That would be one example. Here's a link to a page on efunda.com, a web source for engineers. It explains the concept much better than I could.
http://www.efunda.com/formulae/solid_mechanics/stress_concentration/stresscon_intro.cfm?search_string=stress%20concentration
The transition of the geometry from a single tube (the upper rail, for example) to a roll cage structure can be thought of as a shoulder on a shaft. A sharp corner will fail at a lower loading than a smoothly radiused corner.