Functional Roll Bar Design Discussion

[JRinKY]

I spent part of the weekend removing some of the interior cockpit panels from my car to see what can be done to install a functional roll bar that does not completely ruin the interior. What I saw makes me think that it is possible, so I am starting this thread to gather requirements and constraints for the design.

My ultimate goal is to create a roll bar design that meets the SCCA Solo roll bar requirements, present that design to my local SCCA Technical Inspector for review, then proceed to SCCA National to get the design homologated. SCCA specs seem to be the most detailed and the most restrictive that I have seen, so I am designing to meet them with the thought that all other standards should then be met.

I do not plan to sell roll bars. I have talked with my attorney and my insurance agent, and my attorney was distinctly unhappy about my selling automotive safety equipment. My insurance agent was very happy about it, but made me unhappy with the liability insurance premium increase that he thought would be appropriate. If I can design an acceptable roll bar, and if I incur significant expense in the process, I may ask those interested to help share the cost of that expense. I also may not. The first requirement is to create something usable.

SCCA Solo Rules do not include any specific strength requirements. There is a minimum tubing size specified, and there are some pretty specific design criteria, but nothing about strength. The SCCA GCR does have strength requirements for Formula and Sports Racer roll cages, but they are not directly applicable, and, for a production car, are not achievable. For that reason I have decided to use the NHTSA’s proposed roof crush strength requirements for US market street cars. That requirement is for a load of 2.5 times the vehicle’s weight to be applied at a 25 degree angle laterally and 5 degrees longitudinally, without deforming the roof more than 5 inches. I have modified this slightly, using the same load at the same lateral angle, but changing the longitudinal angle to result in the load contacting the roll bar in the back, and the top of the fender well in the front. This seems to represent the actual loading that would occur in a rollover. It also puts a more severe load on the roll bar, which results in a more conservative design.

Initial design calculations indicate that 1-1/2 OD, 3/16 wall 4130 alloy steel tubing will meet the loading requirement. This tubing exceeds the SCCA spec of 1-1/2 OD .120 wall tubing for our vehicle weight of 2900 lbs.

The SCCA spec requires a single-piece hoop of no more than 4 bends with a total angle of 180 +/- 10 degrees. A workable design will require 6 bends and a total angle of approximately 200 degrees. The nature of the bends is such that an exception seems reasonable, and the possibility of anchoring the bar to the rear cockpit bulkhead makes an exception for total angle reasonable also.

The SCCA spec requires rearward-facing braces, but the design of our backlight is such that they cannot be used. A precedent exists in the Honda Del Sol and the Porsche 914, and is listed in the GCR, so it should be possible to be included in that exception. For competition use it may be necessary to add a “Petty Bar” which would run from the main hoop to the passenger side footwell. This would be removable, with a permanently mounted bracket on the passenger side floor.

The SCCA spec strongly suggests, but does not require, a lateral brace within the main hoop. This is possible, but interferes with the passenger seat. More on that later. The brace is not required to meet the NHTSA loading.

Installation of the roll bar requires modification of the lock pillar interior trim panel and elimination or relocation of the speaker that is mounted in it. It will also require a notch to be cut in the rear bulkhead cover panel to allow a support plate to be attached to the shoulder harness anchor bolt. The driver’s seat will be slightly restricted in rearward travel. To see where it must stop, fold the seatback forward, and adjust the seat for a 3 inch gap between the back of the seat bottom and the trim ring around the bottom of the cargo net. The backrest can still be adjusted far enough for the back of the headrest to contact the rear cowling. The lateral brace will require that the passenger seat be limited in rearward travel to approx. 2 inches forward of the driver’s seat position, with the headrest kept 2 inches away from the rear cowling. It may be possible to make this brace removable, so this limitation may be eliminated. As noted above, it may also be found that a lateral brace is unnecessary.

The roll bar will run up the outside wall of the cockpit with its inner face approximately at the face of the trim panel in the speaker location. The recess in the trim panel that makes room for the seatback angle adjusted is eliminated. The bar then makes two approx. 45 degree bends to follow the shape of the seat, with approx ¾ inch from the seat to the bar, and from the bar to the top. The top of the bar will be approx 6 inches above the peaks of the cowling humps, and will run level from seat centerline to seat centerline. This level is also approximately equal to the top of the windscreen frame. The bar will run almost directly above the headrests when they are back as far as they will go. SCCA specs require ½ inch of foam padding wherever the helmet can contact the bar, so there should be just enough space all around.

A twin-hoop bar is probably feasible, but would require massive reworking of the rear bulkhead cover panel, possibly including the body-colored waterfall area. I am not interested in doing that kind of modification, so I am not presently investigating a twin-hoop bar.

Using Aircraft Spruce and Specialty for prices, I am expecting materials for this bar to cost approx $400. Final cost of the installation will depend on the deal you make with a local fabricator, but would typically be another $400. Alterations to the lock pillar trim panels will push the total cost to around $1000.

Comments ? Suggestions ? Did I miss anything ?

[GhostRL]

3/16 tubing is excessive for 4130. It will be excessively heavy, even for chromoly at that thickness, a whole 50% thicker then what is needed. (.120)

[JRinKY]

Spinner : 3/16 tubing is excessive for 4130. It will be excessively heavy, even for chromoly at that thickness, a whole 50% thicker then what is needed. (.120)

That depends on what you consider "needed". The bending moment generated by the design loading will cause 98ksi of stress in 3/16 wall, and 129 ksi in 1/8 wall. The yield strength of 4130 is 105 ksi, the ultimate tensile is 122 ksi.

By my calculations, we need 3/16 wall tube.

[RedHeaded RedLine]

Originally Posted by JRinKY

My ultimate goal is to create a roll bar design that meets the SCCA Solo roll bar requirements, present that design to my local SCCA Technical Inspector for review, then proceed to SCCA National to get the design homologated.
Using Aircraft Spruce and Specialty for prices, I am expecting materials for this bar to cost approx $400. Final cost of the installation will depend on the deal you make with a local fabricator, but would typically be another $400. Alterations to the lock pillar trim panels will push the total cost to around $1000.

Comments ? Suggestions ? Did I miss anything ?

I would be very interested in defraying your cost (signing waivers). We really need to get the car onto Reobling Road! -J

[Nova-Exarch]

Are you planning to add harness equipment on it?

I am interested in a functional roll bar.

I'n not overly concerned with modifying the interior. I would rather modify pieces than loose seat travel though.

If I was to get one it would have to have harness attachments.There is no way I'm gonna do the sort of things I would need a roll bar for without a harness.

[JRinKY]

The objective at this point is to create a workable design for the roll bar. Details like harness attachment points will be relatively trivial after that.

By modifying the lock pillar trim cover, there is room for 1-1/2 OD tubing. The seat travel is not limited by the tube, but access to the seat back rake adjuster and to the release lever are blocked when the seat is within about 3 inches of the back of the cockpit.

I have contacted NHRA and NASA about variances to their roll bar specs. Both of them require 1-3/4 OD tube, with different minimum wall thicknesses for 1020DOM or 4130. Neither of them will allow 1-1/2 OD tube with any wall thickness or in either material.

My next step is to look harder at 1-3/4 OD tube, to see if it can be made to fit without causing too much problem inside the car. I am not confident at this point that it can be done.

[PoppyH]

You might try to get info from the Setup TV show to see what they did for roll bar or I suspect roll cage, but bet they did something that won't work with conv top as they had a hard top and would bet they built cage under the top which won't work with conv top.

[JRinKY]

PoppyH : You might try to get info from the Setup TV show to see what they did for roll bar or I suspect roll cage, but bet they did something that won't work with conv top as they had a hard top and would bet they built cage under the top which won't work with conv top.

The top interferes with the braces, but doesn't really affect the actual roll bar much. The biggest problem is interference with the seat function.