In my opinion, it is not necessary for the reasons that you mentioned. Also, if you bolt the bottomes of the front legs down and then push up the back leg real tight, clamp it, then drill it, it will stay up tight against the roll bar without the top bolt.
Let me tell you about a Formula Ford I saw that spun and flipped and landed on its roll bar going backwards. Early technology car without a rear brace. The rollbar bent forward into the driver's helmet and broke his neck.
Don't assume the loads applied to the rollbar are always going to result with that joint being in compression.
What's your neck worth? A couple grade 8 bolts?
btw, no one ever said to me, "Cool car! Too bad that roll bar bolt makes it look so ugly!"
I could see the possibility of the tubes seperating in the event of a backwards sliding rollover. IF, the hoop bent tward the front of the car it could come off the rear leg if the bolt was missing.
I think this might also hold true for the Tangent insert, I doubt it has the shear force of the screw.
All that said the tubes are a very large diameter and I can't see the cars weight bending them.
Oh, one more reason the use the bolt... Rattle at that joint.
I actually have the blind rollbar kit with the intentions of installing them. But because I lowered the bars by 1" without moving the rear leg stub, the down tube is no longer exactly straight at the joint. So in order to get even spacing at the joint, the down tube needs to be grinded down at an angle. As a result, the blind rollbar kit may not line up where it needs to be to get an even joint.
I had at first intended to use Don's blind roll bar kit. I ended up selling them and decided to go with the bolt. Are the chances of your car rolling one in a million? Yes, but what if you are the one. I pictured myself being the guy that flipped. I don't know if you were around when Lew Comerford rolled his Superformance(?)at Thunderhill. That was enough to convince me that these cars can and will go over. Were the conditions extreme? Yes. But somehow the thought of that bolt showing just didn't matter anymore. For me the decision was to rely on the bolt rather than other untested methods of attaching the down tube.
The base of the hoop does not supply very much resistance to it folding forward in a crash. I would say it is highly likely to do so in any crash that tries to push it forward. The amount of extra strength the bolt provides is substantial. As ChrisH said, probability is low but the results would be catastrophic. Not acceptable.
Statistically, the odds of the missing bolt hurting you are astronomical. In the aircraft biz we work with a figure of 10 to the minus ninth for a single cause of a catastrophic event. If we keep it there or less, we're happy.
The failure of the joint is not a single event however. You first have to have the wreck and the wreck has to be of a very unlikely type.
So, statistcally, I wouldn't worry at all.
HOWEVER, wouldn't you hate to be that first test case?
To tell you the truth, I like the "industrial" look of the bolt. I put the hood rivets in cause I like 'em. Beauty follows function.
I was alluding to a risk assessment in my previous post. As I see it, it comes down to a couple of questions. Has there ever been a Cobra that crashed in such a manner that it would have caused the roll bar to WANT to fold forward (that wanted to not that did, because they probably had a restrained joint)? I think the answer is yes and therefore the probability is substantially higher that 10 to the minus nine. How many Cobras (of any sort) are there out there? So that would be 1 in how many. And I would say that if it does the bar will fold forward and the result would be catastrophic. As an aerospace engineer I would not design something for someone else with that risk level so I certainly won't do it for myself (+family/friends). Plus, if you ever want to sell the car you will not want to have compromised the designed in safety for liability reasons.
I'm not sure about the strength of my choice but I used three Stainless button head allen bolts. Tapped and threaded the inner rollbar tube. I didn't like the one bolt idea and this was before the blind rollbar kit came out.
Put the bolt in for the same reason you put seat belts in. For the same reason you have brake lights, same reason that you keep a fire extinquisher with you, same reason you put a battery switch in...Safety. It makes sense. Mess with that and you'll pay the price. It is just plain dumb to worry about the look of a bolt when it, statisticaly or not, might save your ass! But once again it is YOUR head. Keep it or not, it is up to you.
Believe me, I understand and agree that safety should come first. But on my car, with dual rollbars, I'm putting nothing at the top joint.
As has been pointed out, a backward slide at speed is the main issue. And I don't think there's a snowball's chance in Florida that a bolt would hold in that case. The flange off the hoop stub is less than 3/4". Splitting the difference and centering the bolt in the overlap (and taking into account the width of the bolt hole -- 5/16") means that there will be about 3/16" of material on the upper end of the stay and 3/16" of material on the lower end of the hoop stub to hold the joint together under stress.
In a crash bad enough to need the bolt, the bolt will rip the joint apart.
Maybe my number will come up while driving this thing, maybe it won't. If it does, I DO NOT want my last thought (as I'm sliding upside down and backwards at high speed, watching the rollbars fold in on me in slow motion) to be, "Gee, I wish I hadn't put that ugly bolt there if it wasn't going to do any good anyway." JMHO.
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