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Aug 18, 2017 15:42:56 GMT
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I like clever people General consensus seems to be to add two extra tubes vertically (one per seat) behind each seat with saddle fixings to the lower cage bar. I will use M8 Allen bolts (8 in total - 2 per saddle fixing). This should make the vertical tubes take nearly all the loading so the bolts themselves will not see so much. As a guide the belt from the inertia reel to my shoulders is approx 900mm and the bar deflects the belt approx 50mm from a straight line. This appears to be ok by people cleverer than me, would you agree Peter?
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Aug 18, 2017 16:13:12 GMT
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I agree, that is a very good solution. Hadn't thought about the strength of the bar itself which without the 2 vertical tubes could also be an issue but now solved.
Peter
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Aug 18, 2017 17:29:21 GMT
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Cool, vertical bars were my first idea, but when I saw I could get the tube tight to the cage I thought they were not needed. Bar is 25mm x 3mm CDS tube, I did think of going bigger but it looked clunky Had a play with the 8 & 10mm inserts, 8 was a fail bit the 10 worked ok. I only had one 8mm to trial so will see if I can get more tomorrow to see how well I can install them with a home made rig.
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Darkspeed
Club Retro Rides Member
Posts: 4,864
Club RR Member Number: 39
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Aug 18, 2017 18:10:55 GMT
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The manual gives an expectation of 7/16 8.8 and if the tester see's that then there will be no further questions - neither would there be any questions if the "harness bar" was a welded part as it would typically be.
As regards the MSA and cages design etc - it's not relevant here only what is in the manual and what the tester is asked to test against - If it's not clearly defined in the manual they typically fall back on "What would be expected to be found in a production car" . So would a bar retained by "x" number of M6 bolts likley be found in any production car! I would say not. And even if you had design calculations to prove it there is not system in place for the tester to accept it - just what is given in the manual.
To avoid any questions I would have the cage sleeved and the harness bar retained with a 7/16 8.8 or higher specification bolt each side in double shear although single shear would be adequate.
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glenanderson
Club Retro Rides Member
Posts: 4,316
Club RR Member Number: 64
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Aug 18, 2017 19:15:37 GMT
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But the M6 bolts (or M8/10/whatever) aren't taking anything like the loads the 7/16" ones securing the end of the belts are subjected to. The bar is just there to raise the belt above shoulder height, the ends of the belt are still secured in the same place. Yes, the belt would exert a downward force on the bar in the event of a crash, but nothing like the load on the bolts securing their ends.
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My worst worry about dying is my wife selling my stuff for what I told her it cost...
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Aug 18, 2017 20:02:42 GMT
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I'm in agreement in Glen, however I have no engineering background to back it up. Force applied to the belt would try and push the bar down and forwards, the main point of attachment for the belt remains unchanged.
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Darkspeed
Club Retro Rides Member
Posts: 4,864
Club RR Member Number: 39
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Aug 18, 2017 20:05:03 GMT
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But the M6 bolts (or M8/10/whatever) aren't taking anything like the loads the 7/16" ones securing the end of the belts are subjected to. The bar is just there to raise the belt above shoulder height, the ends of the belt are still secured in the same place. Yes, the belt would exert a downward force on the bar in the event of a crash, but nothing like the load on the bolts securing their ends. If drawn out as a diagram the bar would be viewed as no different in its function to the loop that is also "just there to raise the belt above shoulder height" in any other car. You would not expect to see that fixed with a couple of M6 / M8 bolts. So at which point is it okay for the tester to say - even though the manual and every maufacturer uses 7/16 / M12 fine 8.8 - this is okay as it looks like the loads will be less. It's easy to pass - you use 7/16 8.8 at every fixing the belt passes over or is attached to - just as in a production car and in the manual - using anything less and you may well be returning with a fail to do it that way anyway.
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Aug 18, 2017 20:38:34 GMT
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But the M6 bolts (or M8/10/whatever) aren't taking anything like the loads the 7/16" ones securing the end of the belts are subjected to. The bar is just there to raise the belt above shoulder height, the ends of the belt are still secured in the same place. Yes, the belt would exert a downward force on the bar in the event of a crash, but nothing like the load on the bolts securing their ends. The M6 (or M8,M10) will face depending on the angle approx 60% of the load on the 7/16th bolt. I don't think the person in the seat is an average person. If you are dealing with a short fat person, the angle will grow rapidly and so will be the load on the bar holding bolts. Peter
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glenanderson
Club Retro Rides Member
Posts: 4,316
Club RR Member Number: 64
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Aug 18, 2017 21:18:31 GMT
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But the M6 bolts (or M8/10/whatever) aren't taking anything like the loads the 7/16" ones securing the end of the belts are subjected to. The bar is just there to raise the belt above shoulder height, the ends of the belt are still secured in the same place. Yes, the belt would exert a downward force on the bar in the event of a crash, but nothing like the load on the bolts securing their ends. If drawn out as a diagram the bar would be viewed as no different in its function to the loop that is also "just there to raise the belt above shoulder height" in any other car. You would not expect to see that fixed with a couple of M6 / M8 bolts. So at which point is it okay for the tester to say - even though the manual and every maufacturer uses 7/16 / M12 fine 8.8 - this is okay as it looks like the loads will be less. It's easy to pass - you use 7/16 8.8 at every fixing the belt passes over or is attached to - just as in a production car and in the manual - using anything less and you may well be returning with a fail to do it that way anyway. That "loop" holding the belt above your shoulder in a normal car, is also only 5mm thick. Holding it in place with a 7/16" bolt is more about standardisation of mounting hardware than the strengths of the fasteners concerned. I take your point about using 7/16" fasteners to remove ambiguity/argument with testers, but it's still overkill IMHO. Edited to add, in a normal car, the shoulder mounting is above the reel, and so the mount takes all the forward force applied to the belt and turns some of it through 300° or so vertically down to the load bearing sill. In the Pop. The shoulder bar isn't going to be subjected to any forward forces, because the anchor point is so far behind the bar as there is no wrap of belt around it. The whole point of the belt being above your shoulder line is to stop these downward forces being borne by your skeleton.
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My worst worry about dying is my wife selling my stuff for what I told her it cost...
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Aug 18, 2017 22:03:11 GMT
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Glen, that was my thinking too. What seems odd to me is if I were to fit some highback racing seats (plastic) with harness holes higher up it would be ok, but this seat would be vastly inferior to my bar - but this would be fine to pass the test. This is tonights progress, the bar is only resting together with all of the parts but is very solid without any welding or bolts fitted as yet. Lower support bolts should not see any stress as the bar will be in compression under load. Top mounts as they are angled forward should also see little stress as they will also be in compression under load. In total the bar will be held in with 8 x M10 12.9 grade bolts, personally I think that is over engineered so ought to be fine. M10 bolts actually fit better in a test peice. I will use grade 12.9. If I really have to sleeve the bars & use 7/16" bolts I will but I just done see the need.
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glenanderson
Club Retro Rides Member
Posts: 4,316
Club RR Member Number: 64
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Aug 18, 2017 22:07:56 GMT
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As you've shown in the latest pictures, all the load is taken by the frame; in compression now that you have the two vertical bars. The bolts aren't taking any of the load, they're just there to stop the rattles. 😉
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My worst worry about dying is my wife selling my stuff for what I told her it cost...
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Aug 18, 2017 22:16:33 GMT
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As you've shown in the latest pictures, all the load is taken by the frame; in compression now that you have the two vertical bars. The bolts aren't taking any of the load, they're just there to stop the rattles. 😉 appologizes i misunderstood you. I thought you were speaking about the situation without the vertical bars. You are right. with the vertical bars, the bolts hardly face any load Peter
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Aug 18, 2017 22:45:33 GMT
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Good, glad we are in agreement on this.
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Darkspeed
Club Retro Rides Member
Posts: 4,864
Club RR Member Number: 39
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A seat with harness holes higher up would not have passed either as the seat belt mount position does not change, the seat is not the mount. These types of installations fail as you cannot have loads taken by the seat unless you can prove the seat is designed to accept these loads - You could fit recliners - It makes no difference to the mount height how high the seat back is. The measurement is there for a position for the minimum setting around the a range of driver sizes.
Aside from that, with the 2 x M10's it would be a pretty hard to fail as the assembly looks part of the cage design and the bolt sizes in keeping with the nature of all safety related components.
The height is there for good reason because in a crash you as a bag of water compresses- If the seat belt retaining angles are incorrect, as you compress you go further and further forward - If the shoulder harness attachment position was on the floor ( so all attachement were floor mounted) and retained by a plastic seat in an accident the belt should smash the seat, and you would propel forward to the extent of the belt lengths.
If we apply some actual data.
The shear strength of the shank of a 7/16 8.8 bolt in single shear - how it would be applied - is 50kN. Thread is 36kN The shear of the thread of a M6 is 4kN and M8 is 8kN - they don't look very equivelent do they? The shear of an M10 10.9 thread is 34kN and shank is - 51kN
As the M10 10.9 is easily proven to be more or less equivelent to the 7/16 and you have 2 there is no question about the suitability.
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Thanks for the confirmation. Good to have it backed up by the maths. In reality though the tubes will see the load rather than the bolts?
PS, I was never happy with the 6mm bolts hence posting the idea for comment's.
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Darkspeed
Club Retro Rides Member
Posts: 4,864
Club RR Member Number: 39
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Aug 19, 2017 12:38:47 GMT
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The reality is that its BIVA - and it is what the manual states that actually counts - and the manual makes it quite clear that a suitable fixing for mounting is a 7/16 8.8 or one of equivalent strength. The tester only has that criteria to test against and this bar is classified as a mounting. What anyone "thinks" is strong enough is not relavent unless it can be proven to comply with the requirements of the manual. But I'm repeating myself.
And for a correction above - is 9kN and 16kN for 8.8 M6 and M8 respectively but still woefully poor.
Forces - "They just go down"!
If you get it wrong and turn left or right up a strip chances are you go into a wall at an angle and then roll, during that situation, or being T boned by a Volvo or just get plain old unlucky do the forces just go down? Or does the cage lozenge and shear off all the bolts? I suspect that with the M6/M8 even with the other cross bar that would be a very likley outcome and a scaffold pole will be dancing about loose in the car with you - The M10's would not be the weak link, the fixing method into the tubes would be.
Any one for the vector forces on the fixings in a head on followed by a barrel roll into a end over - apologies - but some things in BIVA are "overkill" for a good reason. To some forces just go down and M8's look more than enough! It's not a one dimensional universe.
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Aug 20, 2017 21:20:40 GMT
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Errr......yeees. I don't think this cage will lozenge in the event of an accident unless it was a catastrophic one - if so I'll likely be dead anyway as I will be the weakest link by far. Having a tiny car with a massive cage/chassis/tyres/engine is not really the safest thing to drive anyway, lol. Steel threads are welded in - took bloody ages to do without damaging the paint (used cold front putty) and also damaging anything else in the car. Seatbelt bar now finished. I'll need to check if the cap heads of the bolts are a fail - I can use mushroom heads instead if so but they are only 10.9 grade whereas the capheads are 12.9. Either grade is ok I believe. And checked over by Skye who said it was fine. We all know cats are fussier than biva's so that means a job well done.
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Aug 21, 2017 13:11:26 GMT
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Looking good. It might be that you taking these rules very serious (and express that during the test) that they overlook a point that may not be 100% oke. After you pass the test (and i bet you will) you could start a DVLA consultancy agency. There have been a lot of changes in the rules when i look at the rear mounting position of the belts in my MGB roadster. These are fitted to the body 40-50cm below the top of my seat..... Do you have some pieces of pipe left over??? Peter
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Aug 21, 2017 16:20:38 GMT
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Good work! Have you been able to get the paperwork sorted and a date for the test yet?
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Aug 21, 2017 19:17:52 GMT
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Not yet Phil, mending the car is easy - but paperwork I'm a bit phobic on
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