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Originally Posted by rwhite692
Anyone know the story about the brackets or fittings holding the downbars/door bars and roll bar together on Brett's car?
I'm wondering what the various sanctioning bodies would have to say about these...Since I see Brett has had the car out on the track quite a bit, maybe it has not been an issue...they look plenty stout.
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Lets start of with the fact that this is not a structural "roll" cage meant for rollover or impact protection. This product was designed for 'stiffening' up the chassis... at that is about all it can do. I have doubts that it will get any certification from the FIA or SFI... or the NHRA. Maybe the SCCA will give it something for low speed events but that itself would be questionable.
Current rules mandate:
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All bolted structures shall have at least two bolts, 180 degrees apart, through support pads and roll cage structure brace connections.
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So unless it is welded or utilizes two perpendicular fasteners, its a no-go. Since this clamp is rated to 450 lbs/ft of torsional loading, it doesn't seem to be much more useful than as to stiffen the chassis. The fact of the matter is that even in a low speed crash (45 mph or less) the impact load of a 3200+ lb car is about 10 times more than that of the clamps capacity... which is only rated to 450 lbs/ft. Basically... the clamp becomes the failure point in the event of an accident.
Moreover, the materials being used in this products construction, stainless 304, come's under some real scrutiny. Merely looking at a material from it's 'Ultimate tensile strength' alone is bad Ju-Ju... in consideration for a structural or safety aspect. Sure it does read nice that in the advertising that the product compares 304 stainless to 1020 steel... but the specifications that are actually being compared, means very little. What is important here is not the 'Ultimate tensile strength'... but the specific materials 'yield strength.' Also to take into consideration is that the UTS numbers deal with tension, not shear and its modulus of elasticity which are more characteristic in its failure mode, and more prevalent in roll cage modeling.
Let's define terms.
The "Ultimate tensile strength" of a material is the maximum tensile (stretching) load per unit of cross-sectional area that the material can withstand before rupture; strength represented in psi.
The "Yield strength" of a material is the tensile load per unit of cross-sectional area at which the material exhibits a specific limiting permanent set or elongation. It is thus different from, and greater than, the elastic limit; strength represented in psi.
The "Elasticity" is a property that a material exhibits by returning to its original size and shape after it has been deformed or strained by an applied stress. Each material has a definite point termed its 'elastic limit,' beyond which the material cannot be stressed without undergoing permanent distortion. The object in designing any structure is to keep the stress level at all times below the elastic limit of the material used. The elastic limit of a material is described as the maximum unit stress that the material can withstand without undergoing a deformation which will remain after the load has been released; strength represented in psi.
Lets compare what is really important (the following is from ASTM doccumentation):
304 stainless: UTS 73,200,
yield 31,200
1020 steel: UTS 82,000,
yield 70,000
4130 ChroMo normalized: UTS 97,200,
yield 75,000 (
Note: Only when it is properly heat treated will it yield over 100k psi. Nor will you heat treat a cage in a unitized chassis construction.)
So... what is shown here is that the 1020 steel has a more than a 100% increase, in the more important 'yield' strength, over that of 304. Also, this shows that using 4130 ChroMo for roll cage material is a waste of money. Because unless you actually heat treat the entire structure, it will not yield an amount over that of 1020 to make any kind of difference in this application. Basically, if you use 4130... you have a really expensive cage with the yield strength of 1020 but with brittle weldments.
I wont even go into the tubing size explaination used in the advertisement, as it doesnt hold it weight either.
Not to rain on the product Paul... but your advertisement is technically mis-leading.