Seat belt mounting point rules

What is the minimum diameter tubing the crotch and shoulder strap can wrap around?

Not sure on the size but if you look up Bob Gliddens crash you'll probably want to build everything around you as strong as possible. He had just upgraded his cage and he used it all up. Like most of of us, I want to go fast but safety should be in our thoughts as we go. We lost a guy recently in a crash ...

Problem I have is my seat is sitting directly up against the crossbar and the belts get pinched as they go out of the seatbelt holes on the seat (too step and angle anyway) and down to wrap around the bar. Belts would end up getting cut from the seat over a long time of use.

I'm looking to see if I can weld another piece of tubing on top of the crossbar to allow me to raise the belt attachment point to be more in line with the seat holes. If it is legal to do what I want I need to know what size tubing to use.

I'm also using a piece of 1.25"x.120 tubing as the crotch strap attachment and need to know if that is legal.


all I can find in the rules is this that says --- All roll bars must have in their construction a cross bar for seat bracing and as the shoulder harness attachment point. In the sentence above is calls out the minimum crossbar size of 1.25"x.120



I figured I'd ask the guys here before I call the guy who cert'd the cage...

IIRC I have almost always mounded belts with brackets/tabs, or weld-in threaded spuds. I think the only time I did anything wrap-around(ish) was way back before the NHRA got all pissy about where you could use pull-pins. I mounted a crotch strap to the main frame rail wrapped around a pull-pin/sleeve deal. The whole pull-pin/bracket/sleeve assembly did double duty, it also mounted the front of the plastic seat as well as the crotch strap.

I see nothing wrong with mounting a wrap-around crotch strap directly around the 1.25" chassis seat bar under the front of the seat as long as the bar it's self is actually inside the car and/or above the floor (so it can't be cut by the track surface or guardrail during a crash). If you had a F/C cage you could weld a second short bar between the 2 rear F/C cage supports (at seat hole height) & loop the shoulder belts over that bar & down to wrap-around another bar (or the actual upper seat cross bar). With just a conventional upper seat cross bar (and no F/C cage) you might be able to do something similar if you add 2 uprights behind the seat from seat cross bar up to main hoop (so it looks like the 2 rear bars of a F/C cage). And then just add the short second cross bar between the uprights at the seat hole height.  

Another way might be to just take a small piece of tubing & bend two 90* bends just far enough apart to get both belts around it, & weld the two ends to the top of the upper seat cross bar. This could improve the angle the shoulder belts go through the seat hole. But I can see where an inspector might have a problem with this setup. If you use small enough OD tubing to get real tight bends, the small OD tubing could possibly cut through the webbing during a crash. Plus small OD tubing wouldn't have a very large weld surface area attaching it to the upper seat cross bar & would have a hard time standing up to the leveraging pull of the belts. An inspector might have less problem with doing it this way if bigger OD tubing is used. But the bigger OD the tubing is, the larger the bend radius will be (and take up more room on the seat cross bar).

My local guy mentioned using the few tubes of the FC Cage as well. What should the OD of the two uprights be? I have some DOM left around here somewhere. I'll have to dig out the notcher too.

jbozzelle wrote:My local guy mentioned using the few tubes of the FC Cage as well.  What should the OD of the two uprights be?  I have some DOM left around here somewhere.  I'll have to dig out the notcher too.

If it's only an 8.50 cage then it doesn't really matter what the bar sizes are on the funny car portion.

I don't recall any sizes listed for restraint mounting other than securely fashioned, no bolts going through the belts and cannot be wrapped around the frame outside the car.

The NHRA rule book does mention that with an 8.50 cage, if the driver's helmet doesn't sit in front of the main hoop (if sitting under and/or behind the main hoop) some additional tubing same size as the roll cage (at least 1.625" OD) must be added to protect the driver's helmet. So in theory any additional structure added behind the helmet/top of seat could be any OD tube you want if the helmet sits completely in front of the main hoop, or at least 1.625" OD tube if the helmet sits under/behind any part of the main hoop.

As for the OD of a tube that the belts themselves are actually wrapped around in a door car, IMO any OD 1" and bigger should be good from both a structural & belt webbing life stand point. Using 3/4" to 7/8" OD tube might also be good, but I don't think I would want to go that small. And I really can't ever see using smaller than 3/4" OD tube. During a crash the loading on the section of belt webbing actually wrapped around a tube should in theory increase as the tube OD is decreased (from the reduction of belt webbing surface area to carry the loading).

Thanks. My cage will cert to 7.50/slower with a few more bars so if I do this FC cage portion I want to do it right. Where can I find the rules that state the tube sizes? Are both the same size? Or can one be smaller, like 1.25?

jbozzelle wrote:Thanks.  My cage will cert to 7.50/slower with a few more bars so if I do this FC cage portion I want to do it right.  Where can I find the rules that state the tube sizes?  Are both the same size? Or can one be smaller, like 1.25?

I don't mean to come off as a jerk but sfi sells the books all day, Monday thru Friday. Buy the 25.5 book if you want 7.50.

whatbumper wrote:

jbozzelle wrote:Thanks.  My cage will cert to 7.50/slower with a few more bars so if I do this FC cage portion I want to do it right.  Where can I find the rules that state the tube sizes?  Are both the same size? Or can one be smaller, like 1.25?

I don't mean to come off as a jerk but sfi sells the books all day, Monday thru Friday.   Buy the 25.5 book if you want 7.50.  

That is a good point, having the SFI spec book in hand when you go to get the car certified can be a big help if the inspector decides to be a dick that day. You can buy the books here.....    http://www.sfifoundation.com/drag-racing-chassis/


And lets not forget the SFI books are copyright protected material. I do kinda doubt the SFI foundation would legally go after either party for sharing/receiving the copyrighted info & pics from their books. But they just might decide to blackball any SFI book owner that shares the copyrighted material. The SFI could refuse to sell him any of the other spec books, or refuse to give him any future spec updates/changes to any book(s) he might already have.

So the funny car cage rules are only in the SFI book?

jbozzelle wrote:So the funny car cage rules are only in the SFI book?

Yes. You don't need it for any other certs.

whatbumper wrote:

jbozzelle wrote:So the funny car cage rules are only in the SFI book?

Yes.  You don't need it for any other certs.

Yup 8.50 & slower the minimum is just what's shown in the NHRA/IHRA rule book door car cage diagram to pass the 8.50/slower cert. So in that situation the F/C cage plus other assorted extra bars & diagonals aren't needed.

But an 8.49 & faster car has to upgrade to a chassis/cage that will pass one of the assorted different SFI certs. And all the SFI door car certs require a F/C cage, plus some given number of assorted extra bars/diagonals (how many extra bars can depend on which SFI cert is used, & how the car is built). And the SFI certs also have a few extra tubing OD & wall thickness minimums to work with vs the limited number of choices the 8.50 & slower NHRA/IHRA (non-SFI) cert calls for.