Shoulder Harnesses and Frame Tubes

[Drew Price]

Rule 3.4.1 (E) 'Lap Belt Mounting' states:

"To fit Drivers of differing statures correctly, in side view, the lap belt must be capable of pivoting freely by using either a shouldered bolt or an eye bolt attachment, i.e. mounting lap belts by wrapping them around frame tubes is no longer acceptable."

Rule 3.4.1 (F) 'Shoulder Harness' does not expressly forbid wrapping the shoulder harnesses directly around frame tubes, only that it must be mounted behind the driver to structure which meets 3.3.3, and that no loads my be fed into the main hoop supports.

I am interpreting this omission to mean that I can mount the shoulder harnesses by wrapping around frame tubes, so my question is:

Has anyone done this in recent years, and what was said about it? Am I interpreting this correctly? My searches only revealed extensive discussion about 3.4.1 (F) concerning the harnesses mounting to main hoop supports, but nothing about the actual mounting.

Best,
Drew

[fade]

yes your interpretaion is correct. many teams mount there harness this way.

[Anday]

yea, we mounted our shoulder belts that way on any car of ours I've seen. Just wrap the belt around the tube and use the bracket on the belt to secure it closely to the tube.
Just be sure that the lateral tube is connected to the structure of the frame and not to the main hoop supports.

Good luck on getting your car together there in Evanston.

[John Stimpson]

Don't know if this is common knowledge yet, but the shouldner harness bar will HAVE to be at least .095" wall thickness from now on.

Some now-alumni from Kettering did some crash testing on old chassis's and found that .065" bends after a couple crashes.

[Drew Price]

John,

Was this announced at Detroit, or is it in the rules change documentation for the '08 rules docs?

So by 'from now on' do you mean 'starting next year'?

Best,
Drew

[Steve Yao]

... 065" bends after a couple crashes.

A "couple" crashes?!
That's like saying "I hit a curb and bent my wheel, but I kept driving on it. I hit the curb a couple more times since then. This last one really put a good dent in the wheel. Conclusion: I need a stronger wheel".

Might as well beef up all the safety structures because I don't see any of them save for the roll hoops taking multiple impacts without deforming.

After multiple impacts your frame should not be seeing the track again ANYWAY.

Changing it to 0.095 is not necessarily a bad thing, but basing the decision off a test setup I don't know, and questionable failure criteria is poor engineering.

Please provide the test plan/setup/criteria used to obtain these results.

[markocosic]

Some now-alumni from Kettering did some crash testing on old chassis's and found that .065" bends after a couple crashes.

Bends can be ok depending on just what load you end up bending at.

0.065" in "0.1% carbon baseline steel" versus 0.065" in "nice steel" makes plenty of difference too.

Until such a time as it's written in as 0.095" - and justified/not safety-equivalency-able - I'd be inclined ot stick with the 0.065" and some numbers to back it up?

[Drew Price]

Actually, I interpreted John as saying that they took some old chassis (which can not be used multiple times at comp anyways, under the 2nd year car rule), and crash tested those, not current or cars that were to be used for competition, just getting the most out of old parts laying around.

Regardless, I would tend to say that as long as the tube is mounted stiffly enough and does not rip out, even during impact it may deform a little, but bending during impact, and drastic deformation is very different.

That said, unsupported length of the harness tube probably has more to do with this than anything else.

Best,
Drew

[John Stimpson]

You've got it Drew... Old chassis's were strapped to our decel crash sled, with a 95th male Hybrid III dummy strapped inside. An SAE paper was written, although I can't remember the name of it. The rules committee has met and discussed it, and .095" will be required for 2008.

On a sidenote - looking at the high speed footage of the crashes, I certainly learned this: You do *NOT* want to hit anything when you're in an FSAE car!

[Steve Yao]

I understood they used old chassis. It is still unclear to me why what happens after multiple impacts is of concern. No chassis will be continued to be driven after multiple impacts. At least not without careful inspection and replacement of deformed members. Something would get messed up enough to require attention if not scrapping.

Even an 0.095 tube can be bent given sufficient load/repetition/support geometry. If they want to revise, then they should include specific maximum spans betweeen supports...or require data/analysis/testing from each team as they do for the impact att.

And I certainly agree that you would not want to hit anything in an SAE chassis. We don't design them to hit anything more significant than a cone. In most cars, a major deformation of the front structure will result in broken legs or worse. Almost any penetration of the bodywork will meet with the driver. The simple fact that most of us have driver's legs forward of the front wheel axis makes SAE cars illegal in any other regulated auto racing; but then they are designed for autocrossing, and thats the only place they should be run.

[Horace]

Back on track with the original topic

Wouldn't it be okay to mount the shoulder harness on the main hoop bracing if you have additional bracing to prevent loads being transferred into the Main Hoop Bracing?

pg. 39 of rules

[mojo-racer]

I was the primary author on the SAE paper regarding these rules. No where in the paper did we suggest that the shoulder harness bar should be increased to 0.095" wall. We did two rounds of testing for this paper, the original round was run to replicate the impact as per the SAE spec and see what happens to the driver. We ran the same pulse three times then upped the speed to see if we could really hurt the driver. On the last run we noticed that the harness tube was bent significantly and torn slightly at the weld seem. We cut out the tube to replace it and found out it was 0.035 wall (the chassis was from 2004 and someone had put the wrong tube in). We then welded a 0.095 wall tube in its place and it was after 14 more decelerations that we measured the tube and noticed about an 1/8in. bow in the middle. I believe the unsupported length was about 22 in. Needless to say, I agree with Steve Yao that 0.095 is overkill. That actual rule change came from someone in the rules comittee who did a failure calculation and determined that 0.065 wasn't adequate. We actually never did a deceleration with a 0.065 wall tube in the shoulder bar. The SAE paper is entitled Analysis of a Frontal Crash in a Formula SAE Vehicle. Can't remember the number but you can search my last name (Rising) and it should come up.

[Matthew Giles]

2004 chassis? Oops. Well, it didn't bend when we hit a curb with the car the night before competition! One of the lower front tubes did though! What use it is to have an .065 shoulder tube when the whole front of the car can't actually transmit enought to bend it? Have the rules changed for the tubing in the front section of the car? I know for sure that the only way you could put that much load on that tube in our 2004 car would have been to drive into a wall and have the driver bend it backwards when he gets sandwiched between the motor and the wall.

For what it's worth I don't remember doing that on purpose.

[mojo-racer]

Yeah, I have a vague memory of discovering this a few days before comp when we were doing our pre-tech tech. I think we just hoped they wouldn't check it and luckily they didn't. The issue isn't transmitting the energy of the frontal impact through the frame so much as holding the driver in place when the body wants to keep going forward. Seating position can have significant impact on the stress on those belts and tubes. Think of strapping the driver down versus back. Although the point that the shoulder bar is the last thing you need to worry about in a crash is very true. Word of warning to all designers out there, watch were you put the steering rack. Good to hear from you again Matt!
Dave

[Dash]

Bringing this one back from the dead because it is my exact question. I would like to know if it is still legal to wrap the shoulder harness belts directly around the seat back bar. Our seat back bar is a straight bar that spans the main roll hoop. The rules don't indicate anything against it that I could find.

An answer would be greatly appreciated.

[RobbyObby]

Yes that is perfectly acceptable. Of course, as long as that specific tube meets the tubing requirements (ie 1in. x .095in.).

[Dash]

Thanks for the quick response! The tube does indeed meet the thickness rule.

[eric922]

If the shoulder harness bar is mounted welded to tubes that go roughly in the longitudinal direction in the side view from the the main roll hoop bracing to the main roll hoop (with the bracing behind the main hoop) do those tubes also have to be 1" x 0.095"? The bar between the two "longitudinal" bars would be 0.095 if course.