NHRA-legal roll bar

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Contents

[edit] 5 or 6 point rollbar construction (NHRA legal)

Required beginning at 11.49 with a closed car/truck, 13.49 with a convertible.

This topic will cover a 5-point roll bar as opposed to a roll cage. Most fellows these days will go ahead and build a cage while they have the welder and their tools fired up, but we'll cover the bar for those of you who don't plan on going 9's (or quicker). With an otherwise unaltered floor and firewall, a 5-point bar will take you to 10.00 flat. If the floor is altered in excess of 4 square feet (not counting wheel tubs or stock removable floor panels which are bolted, such as transmission removal panels on a pickup truck) or the firewall is altered in excess of 1 square foot, then a roll bar will take you to 11.00 and a roll cage as well as additional SFI equipment is required at 10.99.

If the floor or firewall are altered as described, then it only makes sense to get yourself an NHRA rulebook and install a cage because you will only pick up 1/2 second with a bar. (11.49 to 11.00). A "point" refers to a connection point of the end of a bar at the car floor or frame. A 5-point roll bar will have connections as follows:

  • The main hoop, or B-bar, will connect to the sill/floor/frame/frame crossmember at two points just behind the driver.
  • The two rear braces will make two more connections somewhere in the trunk area.
  • The fifth point will be in the area of the driver's left foot, made by the driver's "side bar".
  • Most of the time, racers will go ahead and install a sixth point "side bar" on the passenger side of the car, helping to stiffen the car up.

One note of interest here, minimum O.D. of the tubing for a roll bar is 1 3/4", for a roll cage it's 1 5/8", so it may be cheaper to buy all 1 5/8" tubing and build the cage from the start.

[edit] Roll bar rule

2008 NHRA Rulebook, General Regulations 4:10 ROLL BARS:

All roll bars must be within 6" of the rear, or side, of the driver's head, extend in height at least 3" above the driver's helmet with driver in normal driving position and be at least as wide as the driver's shoulders or within 1" of the driver's door. Roll bar must be adequately supported or cross-braced to prevent forward or lateral collapse. Rear braces must be of the same diameter and wall thickness as the roll bar and intersect the roll bar at a point not more than 5" from the top of the roll bar. Crossbar (lateral bar between the two upright sections of the B bar) and rear braces must be welded to main hoop (B-Bar). Side-bar must be included on driver's side and must pass the driver at a point midway between the shoulder and elbow. Swing-out sidebar permitted. All roll bars must have in their construction a cross bar for seat bracing and as the shoulder harness attachment point. Cross bar must be installed no more than 4" below, and not above, the driver's shoulders or to the side bar. All vehicles with OEM frame must have roll bar welded or bolted to frame; installation of frame connectors on unibody cars does not constitute a frame; therefore it is not necessary to have the roll bar attached to the frame. Unibody cars with stock floor and firewall (wheeltubs permitted) may attach roll bar with 6-inch x 6-inch x 0.125-inch (1/8") steel plates on top and bottom of floor bolted together with at least four 3/8-inch (0.375") diameter bolts and nuts, or weld main hoop to rocker sill area with 0.125-inch (1/8") reinforcing plates, with plates welded completely. All 4130 chromoly tube welding must be done by approved TIG heliarc process; mild steel welding must be done by approved MIG wire feed or approved TIG heliarc process. Welding must be free of slag and porosity. Any grinding of welds prohibited. See illustration. Roll bar must be padded anywhere drivers helmet may contact it while in driving position. Adequate padding must have minimum 1/4-inch compression or meet S.F.I. Spec 45.1. All cars running 180-mph or faster, S.F.I. Spec 45.1 mandatory.

[edit] Analysis

Now, let's examine the rule:

[edit] Dimensions

"All roll bars must be within 6" of the rear, or side, of the driver's head, extend in height at least 3" above the driver's helmet with driver in normal driving position and be at least as wide as the driver's shoulders or within 1" of the driver's door."

Some fellows will build their roll bar only to protect the driver if their car is a purpose-built race car that will never have a passenger (unlike a car which will also be run on the street), so they build the bar to look sort of like a pyramid, except rounded on the top where it protects the driver's head. They will run the main hoop (B bar) up from the driver's side sill to the roof, make a near half-circle around their helmet and then take the bar down to the sill at the other door instead of running it to the other side of the roof of the car and then down to the passenger side sill. This arrangement is just that much more work when you decide to go faster and make a cage out of it. The part of the rule that says "at least as wide as the driver's shoulders or within 1" of the drivers door" is meant for the "pyramid" guys. The part about being within 6 inches of the rear of the driver's head is to prevent a builder from installing the bar back at the rear window or up by the windshield with no other protective bars in between. You might have to picture the roof being caved in. If the bar is near the driver's head, he will be protected. By the way, let's get the terminology in order. The main hoop is normally referred to as the "B" bar, as that's where the B pillar is on the car. The windshield frame is the "A" pillar or A bar and the rear window frame is the "C" pillar.

[edit] Cross-braces

"Roll bar must be adequately supported or cross-braced to prevent forward or lateral collapse."

Forward or rearward "lozenging" or collapse is prevented by the two diagonal (as viewed from the side of the car) bars (rear braces) which connect to the top of the B bar and run down through the package tray and into the trunk area in the rear, or through the cab rear window/rear cab sheetmetal onto the floor of the bed if we're talking pickup truck, forming a triangular support to the B bar when the car/truck is viewed from the side. Lateral (sideways) collapse is prevented by the cross bar which runs from the left (or driver's side) of the B bar to the right (or passenger's side) of the B bar. This cross bar is installed at the driver's shoulder height with the driver seated in his/her driving position and also serves as the shoulder belt anchor point. It should be noted that many builders also install a diagonal "D" bar (as viewed from the front or rear of the car) from the "B" bar corner at the driver's helmet down to the other side of the car at the bar end/floor to further prevent structural failure.

[edit] Rear braces

"Rear braces must be of the same diameter and wall thickness as the roll bar and intersect the roll bar at a point not more than 5" from the top of the roll bar."

"Same diameter" means minimum 1 3/4" (1.750") "Same wall thickness means 0.118" for mild steel and 0.083" for chromoly "Not more than 5" means do not move the rear braces over to the side of the car to the vertical part of the B bar any further down than 5" as measured from the top of the B bar. The closer to the top of the B bar these rear braces are, the better they can prevent the B bar from forward or rearward collapse (lozenging).

"Crossbar and rear braces must be welded to main hoop."

The main hoop is normally called the B bar in tech lingo.

[edit] Side-bar

"Side-bar must be included on driver's side and must pass the driver at a point midway between the shoulder and elbow. Swing-out sidebar permitted."

This bar welds at the intersection of the B bar/cross bar at the driver's left shoulder and runs downhill diagonally to a point near the driver's left foot. Its purpose is to keep the driver in the car in the event he/she is out of the belts (yep, it happens). If you're planning a swing-out side bar, get yourself a NHRA rulebook. There are several ways to do it, all of them too lengthy to address here.

[edit] Cross bar

"All roll bars must have in their construction a cross bar for seat bracing and as the shoulder harness attachment point. Cross bar must be installed no more than 4" below, and not above, the driver's shoulders or to the side bar."

Caution: do not install this bar until the driver is seated in the car in his/her normal driving position in the car. And, do not install the side bar until this cross bar is in place.

Maximum protection for the driver will be afforded by installing the cross bar with the top of the bar even with the top of the driver's shoulders. That's why you don't want to weld this bar in until you have the driver in position. Now, with the cross bar in position, you can weld the driver's side-bar in at the intersection of the B bar/cross bar and be assured the geometric line of pull will be proper on the shoulder belts and the side bar will be high enough to retain the driver in the car in the event of upset, passing the driver midway between the shoulder and the elbow on its way to the floor close to the driver's left foot as dictated above.

[edit] OEM frames

"All vehicles with OEM frame must have roll bar welded or bolted to frame; installation of frame connectors on unibody cars does not constitute a frame; therefore it is not necessary to have the roll bar attached to the frame."

For instance, a Chevelle has a full frame, a Camaro is a unibody. If you put subframe connectors on a Camaro, it's still a unibody.

[edit] Unibodies

"Unibody cars with stock floor and firewall (wheeltubs permitted) may attach roll bar with 6-inch x 6-inch x 0.125-inch (1/8") steel plates on top and bottom of floor bolted together with at least four 3/8-inch (0.375") diameter bolts and nuts, or weld main hoop (B bar) to rocker sill area with 0.125-inch (1/8") reinforcing plates, with plates welded completely."

You can "sandwich" the floor of the car with these "plate pairs" bolted together, one on top of the floor and one underneath for each connection point and weld the bars to the top plates or you can use only one plate at each connection point and weld the plate to the floor or sill of the car, then weld your bar to the plate. This increases the square inch area of the installation and reduces loading on the floor so the bars will not punch through the floor sheet metal like they would if you just welded the bar straight to the floor or sill. When installing the B bar, you'll want to install it on each side as close to the door as possible, so that the driver will be "inside the box". This will probably require moving the B bar tubing up onto the sill of the car, at least partially. Bend up your 6 x 6 plate to weld it in, so that you are welding it partly to the sill and partly to the floor. Do not reduce the size of the plate to less than 6x6. You may have to grind the end of the B bars to make them conform to the curve of the plate at the sill and that's OK; you may have to heat and bend the plate to make it conform to the sill/floor and that's OK too, just do what you have to do to make it work according to the rules and you'll never have a problem at tech. Maybe you'll end up with part of the bar welded to the plate at the sill area and part of it welded to the plate at the floor, again, whatever it takes. Grind the end of the bar to make it work.

[edit] Welding

"All 4130 chromoly tube welding must be done by approved TIG heliarc process; mild steel welding must be done by approved MIG wire feed or approved TIG heliarc process. Welding must be free of slag and porosity. Any grinding of welds prohibited."

If you're not an expert welder, then hire a professional. It'll be the cheapest wages you'll ever pay for a nice roll bar installation. Nothing is cheesier than bird-crap welding. And don't even think about grinding any welds on the car. Any grinding on the cage material or the welding joints will turn the entire thing into junk. Hire a pro. If you fit up all the bars in the car and tack them in, the pro welder won't be there long doing a proper job. If you have fit the bars properly, there won't be much room at the roof to weld completely around the B bar/rear brace connections, so you may as well plan on gusseting these. Use 1/8" (the rule calls for 0.110" minimum) chromoly or mild steel flat plate of a triangular design with the long (unwelded) side of the triangle measuring at least 1 3/4". Use two gussets at each rear brace (four gussets in total) and you'll be bulletproof. By the way, NHRA now allows these rear braces to be bent (one bend only) to a maximum of 30 degrees (needs confirmation) so that the guys with street/strip cars can retain the back seat in the car for double dates and such. Most guys will run the rear brace bars from the B bar intersection back at a shallow angle to the roofline and then make a turn down and through holes in the package tray to the trunk floor, where the bars are terminated on 6 x 6 plates which are welded to the trunk floor. Now, you know what's going to happen don't you? Some guys from down off the farm are going to hear about this new lax interpretation of the rule from a buddy who didn't tell them about the "one-bend-only" part of the rule and they'll run the rear braces back from the B bar right up against the roof and make a 90 degree turn through the package tray and then another turn to terminate the bars on their plates somewhere on the trunk floor. Multiple bends in the bars will allow them to kink-up and will render them incapable of preventing the B bar from "lozenging" forward or backward. And the problem will be that there is little or no tech inspection at most drag strips, so they'll get away with it until they get to Firebird and get trailered. Then they'll raise a stink with the front office about how they passed tech at Podunk Dragway. You either have a race car or you have a street car -- make a decision!

[edit] Roll bar padding

"Roll bar must be padded anywhere drivers helmet may contact it while in driving position. Adequate padding must have minimum 1/4-inch compression or meet S.F.I. Spec 45.1. All cars running 180-mph or faster, S.F.I. Spec 45.1 mandatory."

Whether you think so or not, seat and shoulder belts stretch when you're flippin' and rollin', so make sure you've covered all your bases here with every possible place your helmet could make contact with the bars.

There is no restriction on improving a rollbar into a rollcage, even though you must use 1 3/4" minimum diameter tubing for a rollbar and can use smaller 1 5/8" for the rollcage. Just build onto the existing rollbar material with your new 1 5/8" tubing. Also, there is no restriction on using 1 3/4" material for the entire rollcage. There is just a 1 5/8" minimum for a rollcage and 1 3/4" minimum for a rollbar. You could even use larger material if you want to, you just can't use smaller than the rules call for. Of course, there has been much hoopla about the choice between mild steel and chromoly. Mild steel is less expensive than chromoly and can be migged or tigged in. Chromoly is more expensive to buy and must be tigged in. A car with a moly bar or cage may be easier to sell.

This article was written pursuant to the 2008 NHRA Rulebook and will be updated in the future.

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