5 point harness with stock seats?
10-31-2002, 09:05 PM
#31
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There is an old trick question from an interview for an engineering position - " What is tha allowable load on a 1/2" bolt in single shear". Usually it leads to "depends on the type of bolt," etc. when the real answer is that you should never load a bolt in single shear.
The problem witha single shear load is that it puts a bending moment into the parts it is joining and that can make it difficult to design the parts so that they don't flex. Also, the offset between the bolt tthreads and the load point actually puts the bolt in bending, anothe not so great condition.
The problem witha single shear load is that it puts a bending moment into the parts it is joining and that can make it difficult to design the parts so that they don't flex. Also, the offset between the bolt tthreads and the load point actually puts the bolt in bending, anothe not so great condition.
11-01-2002, 08:27 AM
#32
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In addition to the PCA Club Racing guidelines <a href="http://www.pca.org/pca/clubrace/docs/pca_rules_2002.pdf" target="_blank">here</a> on mounting harnesses and seats, there is some additional information at the OMP site, with FIA drawings to illustrate<a href="http://www.ompusa.com/harness_notes.htm" target="_blank"> here</a>
11-01-2002, 06:16 PM
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I apologize for pursuing this shear question, but I have a roll bar in my car held by bolts in shear, so if my calculations are incorrect, I may change my equipment. So here goes.
A value of 0.6 is listed for the ratio between shear strength and ultimate tensile strength (UTS) for Grade 8 bolts in SAE Technical Paper 770420, "Analysis and Design of Threaded Assemblies." A Grade 8 bolt's UTS is 150,000 psi. A 1/2 inch bolt has an area of 0.196 sq. in.
Therefore, the shear strength of a properly torqued 1/2" Grade 8 bolt is
150,000 x 0.6 x .196 = 17,640 lbs
Two of these bolts in shear can thus carry 35,280 lbs before breaking. With a car weight of 3,000 lbs, the bolts can withstand a 12 G deceleration, in addition to the strength of the roof itself. I actually have four bolts in shear, so I'm being conservative.
As always, I stand to be corrected if these calculations are incorrect. Sorry about the length of this post.
<img src="graemlins/yltype.gif" border="0" alt="[typing]" />
A value of 0.6 is listed for the ratio between shear strength and ultimate tensile strength (UTS) for Grade 8 bolts in SAE Technical Paper 770420, "Analysis and Design of Threaded Assemblies." A Grade 8 bolt's UTS is 150,000 psi. A 1/2 inch bolt has an area of 0.196 sq. in.
Therefore, the shear strength of a properly torqued 1/2" Grade 8 bolt is
150,000 x 0.6 x .196 = 17,640 lbs
Two of these bolts in shear can thus carry 35,280 lbs before breaking. With a car weight of 3,000 lbs, the bolts can withstand a 12 G deceleration, in addition to the strength of the roof itself. I actually have four bolts in shear, so I'm being conservative.
As always, I stand to be corrected if these calculations are incorrect. Sorry about the length of this post.
<img src="graemlins/yltype.gif" border="0" alt="[typing]" />
11-02-2002, 03:48 PM
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This problem, as with most design problems, is not a matter of simply bolt strenght but rather the whoel system - bolts, roll bar details, detail of the atttachm,ent point to the car, etc. There are dozens of single shear connections in a car - seat belt attachments, for instance. The key is to look carefully at each point and make sure the whole connection can take the load, not looking just at the bolts.
11-03-2002, 11:20 AM
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Smokey:
Read my post carefully. I'm not talking about the seat belt bolts, which are 7/16-20 pitch UNF thread...I'm saying that the entire rear-end of the DAS bar is held in-pace by two ( 2) 8mm x30 bolts. In shear. A cab will have no coupe roof to act as a "skid" plate if you go on your head backwards. Possibility of having the top of the bar "dig-in", with a large moment arm to the rear mounting points.
Do the math on THIS scenario..and report back if you like...would be interested in your findings.
---Wil Ferch
Read my post carefully. I'm not talking about the seat belt bolts, which are 7/16-20 pitch UNF thread...I'm saying that the entire rear-end of the DAS bar is held in-pace by two ( 2) 8mm x30 bolts. In shear. A cab will have no coupe roof to act as a "skid" plate if you go on your head backwards. Possibility of having the top of the bar "dig-in", with a large moment arm to the rear mounting points.
Do the math on THIS scenario..and report back if you like...would be interested in your findings.
---Wil Ferch
11-03-2002, 11:26 AM
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OH...
And to the ever-patient MikF...i can't comment on the strength of the Targa roll bar. Perhaps others with Targa experience can ( and should ) chime in. Ny recollection is that it is a strong structute, and certainly not a decoration...but whether or not it's OK as a roll-bar substitute, I can't say. Even if it is, you should consider the overall height, and whether or not your head extends beyond the horizontal plane between the front pillar and the Targa bar...no good if your head ( with helmet) is too tall.
---Wil Ferch
And to the ever-patient MikF...i can't comment on the strength of the Targa roll bar. Perhaps others with Targa experience can ( and should ) chime in. Ny recollection is that it is a strong structute, and certainly not a decoration...but whether or not it's OK as a roll-bar substitute, I can't say. Even if it is, you should consider the overall height, and whether or not your head extends beyond the horizontal plane between the front pillar and the Targa bar...no good if your head ( with helmet) is too tall.
---Wil Ferch
11-03-2002, 02:58 PM
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Will R,
I agree with you completely about the danger of having only a roll bar for track driving in an open car. If you go off track, soft ground or a gravel trap will flip the car, and then the bar will dig in. Whether it's strong enough at that point is probably academic because with a full harness, your helmet will dig in as well, and I don't even want to think about the consequences.
There are many opinions out there, and the only reason I did the shear calculation was because I thought maybe I had missed something in my previous calculations for my roll bar. At the track for DE events, it's everyone's personal decision as to what equipment they use.
The other topic is the question of neck protection in a front-end impact with a tight harness. We had a minor off-track at UCR a few years ago, and the car became airborne and came down nose-first in soft ground. The driver had cracked neck vertabrae from the neck flexion on impact. I haven't gone to the HANS device because of the cost and because I'd look ridiciculous wearing it in DE, but I do wear the large Bell neck brace with the major padding under the chin, where you need it in a frontal impact.
I agree with all the comments on the need to analyze any restraint and roll-over system in its entirety. It's only as strong as the weakest link. My own system is a compromise, and as my speed increases, I will probably upgarde it: roll cage, etc.
The only time I've ever commented at the track on someone else's restraint system has been when I've seen four-point belts with the buckle in the middle of the driver's chest, and shoulder harnesses bolted to the floor behind the driver's seat. In those cases, the driver would have been far safer with the standard three-point.
I agree with you completely about the danger of having only a roll bar for track driving in an open car. If you go off track, soft ground or a gravel trap will flip the car, and then the bar will dig in. Whether it's strong enough at that point is probably academic because with a full harness, your helmet will dig in as well, and I don't even want to think about the consequences.
There are many opinions out there, and the only reason I did the shear calculation was because I thought maybe I had missed something in my previous calculations for my roll bar. At the track for DE events, it's everyone's personal decision as to what equipment they use.
The other topic is the question of neck protection in a front-end impact with a tight harness. We had a minor off-track at UCR a few years ago, and the car became airborne and came down nose-first in soft ground. The driver had cracked neck vertabrae from the neck flexion on impact. I haven't gone to the HANS device because of the cost and because I'd look ridiciculous wearing it in DE, but I do wear the large Bell neck brace with the major padding under the chin, where you need it in a frontal impact.
I agree with all the comments on the need to analyze any restraint and roll-over system in its entirety. It's only as strong as the weakest link. My own system is a compromise, and as my speed increases, I will probably upgarde it: roll cage, etc.
The only time I've ever commented at the track on someone else's restraint system has been when I've seen four-point belts with the buckle in the middle of the driver's chest, and shoulder harnesses bolted to the floor behind the driver's seat. In those cases, the driver would have been far safer with the standard three-point.
11-03-2002, 03:16 PM
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I did a quick calculation of the 8 mm bolts. If they're Grade 8, they would have a shear strength of about 7,000 lbs each, so two of them would be good for 14,000 lbs. I'm not sure you can get 8 mm bolts in Grade 8, and I believe all the seat belt bolts in a Porsche are SAE, not metric.
But let me take off on another tangent. Has anyone worked out the forces on a roll cage carried out to the suspension at the corners of the car, into the crush space? That could be interesting.
But let me take off on another tangent. Has anyone worked out the forces on a roll cage carried out to the suspension at the corners of the car, into the crush space? That could be interesting.
11-03-2002, 04:19 PM
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Smokey:
You then see that 8 mm bolts in shear are not very strong, relative to the weight/speed of the car.
BTW...in my DAS bar ( coupe application) I'm using grade 12.9 for the rear 8mm bolts. i don't think you can apply USA grading standards to metric bolts. As a rough guide, US grade 5 is about metric 8.8..... US grade 8 is about metric grade 10.9, ... and metric grade 12.9 is better than US grade 8.
Lastly, correct.... all cars use 7/16-20 pitch UNF thread seat belt bolting...even Porsche.
---Wil Ferch
You then see that 8 mm bolts in shear are not very strong, relative to the weight/speed of the car.
BTW...in my DAS bar ( coupe application) I'm using grade 12.9 for the rear 8mm bolts. i don't think you can apply USA grading standards to metric bolts. As a rough guide, US grade 5 is about metric 8.8..... US grade 8 is about metric grade 10.9, ... and metric grade 12.9 is better than US grade 8.
Lastly, correct.... all cars use 7/16-20 pitch UNF thread seat belt bolting...even Porsche.
---Wil Ferch
11-03-2002, 06:28 PM
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While I am not certain of this, I think that the "standard" seat belt bolts are something beyond grade 8, and I agree that a 8mm bolt in shear is not something I would want to trust with my fate in a crash.
As far as the forces, check the link in one of the earlier posts in this thread. It is a worthwhile read, and estimates the decelration forces in a tire wall impact at about 40G. Obviously all of that is not taken by the roll bar, especially in a coupe, but it does point out how easy it is to underestimate the forces.
As far as the forces, check the link in one of the earlier posts in this thread. It is a worthwhile read, and estimates the decelration forces in a tire wall impact at about 40G. Obviously all of that is not taken by the roll bar, especially in a coupe, but it does point out how easy it is to underestimate the forces.
