DTFASTBEAR

I have looked at all the DAS bar information in the tech info area on Rennlist, but I have a question that no one seems to have addressed there...

How do you feel about having the bar only held into the back of the car by two measily 8mm bolts?? In a coupe, I think I'd be ok with this, but as one of the writers pointed out in the tech article, even having the accident impact of two 200lbs (big fat me and an instuctor) adults against the harnesses which I have attached to the DAS bar may be enough to sheer the 8mm bolts.

Here's the deal - I have a DAS bar in my 911 cabriolet. It fits perfectly. It looks great. But I'm getting ready to hit the track for my first time trial and the back mounting points frankly scare the crap out of me. With any sort of force towards the front of the car (the car upside down, going backwards...) those rear bolts would be in complete sheer (not tension like an Autopower bar mount) and there is only one on each side!

I'm considering getting some custom welding/fabrication done to beef up the rear mounts, which pretty much negates the purpose of having the bolt in bar. My other option is to just sell the DAS bar and get an Autopower or similar bolt/weld in bar that I trust.

Thoughts?

Thanks in advance,

Dean

john d 81SC

Dean,

Why not contact DAS (Dougherty Automotive Services) directly? I would think Bill or Colin would be able to answer your questions. Tele: 610-692-6039; e-mail: info@das-sport.com.

DTFASTBEAR

Hey John,

I should have mentioned in my original post that I did spend some time talking to Bill about this issue. He said that yes, the bolt in bar is by its very nature a bit of a compromise. He agreed that in a cabriolet, the bar could be subjected to a forward force that would be unlikely in a coupe, and that he didn't have any design or test data for such an impact.

I probably didn't make it very clear in my posting - I'm looking to see if anyone else has modified the mounts on one of these bars or has thoughts one way or the other about modifying this bar versus just going with an Autopower or similar bar that is not designed to be removed "without a trace". In other words, a bar that isn't designed as a compromise.

Bill and Colin were very helpful and very honest about what the bar is, and isn't, designed for. I wish I could afford a 911 coupe to "store" the bar in if I decide to purchase an Autopower bar...

Thanks!

Dean

Bill Gregory

Dean,

I had a DAS rollbar in my 911SC and I have one in my C2 (both coupes). I expect they'd do fine in a rollover, however, I agree that attaching the harnesses to them isn't awe inspiring. I run my harness thru the holes in the seat, over the bar, and down to eye hooks in the rear where the rear seat belts attached. If there's an accident the belts pull down on the cross bar. Otherwise, if you're worried about shearing the bolts, use grade 10 or 12 bolts.

DTFASTBEAR

Bill,

Thanks for the advice. I like your suggestion for mounting the belts to the tub rather than the bar, eliminating one of my two concerns.

I am having a fabricator cost out modifcations to the rear mounts later this week. I'll post my findings and, if I get it done, photos of the mods, back to the list.

Thanks,

Dean

Clark Griswald

Dean, I just dropped you an email. If you want to sell the DAS bar, I'd be interested.

Chuck

Paul H

Have you looked up the strength of an 8 mm SHCS? They are about 10,700 lbs in tensile load and 17,600 in shear (EACH!).

You should probably be more worried about something else failing, I think the limit you can survive is about 10 g, you should be okay for 50 g's plus (even at 200 lbs)

DTFASTBEAR

Paul,

I know just enough about shear, tensile, etc. to be dangerous... what is an SHCS? Where did you look that info up? Someone emailed me privately and suggested using 12.9 grade metric bolts. Anyone know of a source for these? If it is really that simple, that these bolts do indeed have that kind of strength in shear, then you are absolutely right, Paul. I should be worried out the tub ripping apart with the bolt still intact. That is after I have been suitably crushed by the G's, first, of course.

Trust me, I'd love nothing more than to find out that I am indeed worrying about nothing here. Poor Chuck is going to have to look for a DAS bar somewhere else, then, I guess...

Thanks,

Dean

Paul H

SHCS is socket head cap screw, you know the kind that needs an Allen (hex) key. I looked up the information in my Unbrako SHCS catalogue, don't you keep one beside your computer?

Go to a bearing or industrial supply house and ask for a grade 12.9 bolt. Get a reputable name (ie Unbrako, not an off-shore import). They can show you the markings to verify the bolt grade. Also ask for the recommended installation torque and use a torque wrench to tighten the bolt.

Remember each 8mm SHCS will support about 4 cars in tension, 8 in shear. Surprising, isn't it?

DTFASTBEAR

Thanks, Paul! I didn't keep that catalog beside my computer, but I do now. More accurately, I keep it IN my computer... I downloaded it from their site.

For those interested: http://www.spstech.com/nav.cgi?10082...unbrakodl.html

However, unless I am reading it wrong, for an 8mm (M8, right?) alloy SHCS, I see a single shear strength of 8800 lbs. each. Still a high number, but half of what you are quoting. All my Mechanical Engineering professors would be ashamed to hear me say this, but I completely forget how to do these calculations...

If someone can help, please step up, but I vaguely remember hearing that in a racing car accident, it is not uncommon for the car (or the driver) to sustain for VERY SHORT times (small fractions of a second) deccelerations as high as 100 plus G's. The forces associated with bringing a 200lb. body from 100mph to 0mph in the space of a few feet could generate these types of forces, I think. Can this be possible? Can someone give me a sanity check, here? If so, then these bolts are shearing, no doubt about it. I think that fact that the factory uses much, much larger bolts for each seat belt should be a strong hint...

Hate to keep beating a half-dead horse, but looking at an 8mm bolt, no matter WHAT it is made of or HOW it is heat treated, it just doesn't look to me like it could stand the forces that could easily be generated in a car accident. Isn't that why they use almost 2" tubing albeit hollow in rollbars?

Thanks,

Dean

J richard

Dean,

I think you are worried about too much.
The forces generated in a "survivable" car crash are much less than 100 G instantanious. That would have your body mass at 20,000lbs. Your neck would have to keep your 1500lb head on, you would turn to jelly and your 5 point racing harness would have failed. My little brother designs ejection and crash protection for military hardware, the standard is a 26G protection, and this assumes there is some injury. (spinal compression, fractures etc.)

The two parts of the equation:

1.If you figure that the shear forces are spread across FOUR bolts (two in the front, two in the rear) not just two. They may be oriented in different directions but the ones in the front will take a proportion of the load as compressive shear (relative to the orientation in the car) and the rear mounts will take tensional shear. The net result (assuming equal distribution and your 8,800 lb capacity) is 4x8800 or approx. 35,200lbs capacity. This could be slightly less due to the geometry of the bar and the exact direction of forces applied to it. Standard steel has a 36,000psi yeild strength, it has all but been replaced by 50,000psi, and heat strengthened hardware is available to 100,000psi. But be careful the harder the steel the stronger, but less ductile/brittle and more likely to break in an impact. This is typically why rollbars are Mild steel, not hardened steel.

2. The load. (you guys) If you figure that the Lap belt is around your center, it is going to take the majority of the load, not the H straps. The straps are simply going to keep your upper body from pivoting on your hips. So at the most they will take about a third to one quarter of your total body weight. (half your upper body above the lap belt) If you figure that is about 66lbs. (if it were me it would be less because I have a lower, mid section bias, but thats another story....) If you take the predictable load of about 30G max. It would be 66lbs x 2unlucky occupants x 30G's = 3960lbs. Way under the capacity of the bolts of 35,000lbs.
In additon you should consider that the cars crush zone, the stretch of the belt and your soft mid section will cause the load to be applied relativily slowly, not instantainously, softening the load (on the bolts, not you...)

My advice is to use the standard harware from the manufacturer, not a higher strength more brittle one. Then bolt the thing in and keep the greasy side down. The performance of the DAS bars looks pretty good to me, PCA and SSCA approves it. Guess whats in my car...

Hope this helps

Paul H

You are correct (I think). I gave you the catalogue 'double shear' value. I am not sure of your application, but it is probably single shear.

I will suggest you accept J Richards' eloquent and reasoned response that in several paragraphs sums up my thoughts a) someone a lot smarter than me has thought about this and b) be careful but not obsesive.

DTFASTBEAR

Thanks Paul and J, for putting up with my angst!

I am going to take the advice of Wil and Bill G. and fasten the harnesses THROUGH the bar's harness loops to a hard point in the rear of the car and then trust the high-grade bolts from Dougherty to hold the bar in place. Then, I only have to worry about the upside-down flying backwards crash situation.

If you guys ever saw me drive, maybe you'd know why I'm so worried...

Thanks again,

Dean

Wil Ferch

Guys:
Ahem...this one is going to Paul H's post somewhere in the string above....how can *any* bolt be 70% stronger in shear than in tensile strength ????

Clark Griswald

I spoke with DAS. They told me that their bars use SCCA legal tubing, but conceded that the mounting points do not meet SCCA requirements.

That doesn't make them unsafe or indicated they will shear. It just means they don't meet the letter of the SCCA requirements.

FWIW.