Fired Up the Sawzall - New Project Pics!
#226
Race Director
Originally Posted by RedlineMan
Thanks Joe;
A couple of points:
- We have very well established that this is not a conventional cage. However, I don't think that necessarily makes it unworthy for race class certification.
...
The only place that it may seriously fall affoul is in number of chassis "contact points." ...
- Bends and strength. To make the blanket statement that bends in a tube make it weak is incorrect, or at least potentially erroneous. You would have to study what forces that tube were likely to see before you could make such a judgement.
...
If you put some arch in that tube,...
A couple of points:
- We have very well established that this is not a conventional cage. However, I don't think that necessarily makes it unworthy for race class certification.
...
The only place that it may seriously fall affoul is in number of chassis "contact points." ...
- Bends and strength. To make the blanket statement that bends in a tube make it weak is incorrect, or at least potentially erroneous. You would have to study what forces that tube were likely to see before you could make such a judgement.
...
If you put some arch in that tube,...
My point on rules was certain classes limit attachment points on cages. This is to prevent super stiff cage work that they consider to be beyond the intent of the class. PCA GT however only cares if the cage is safe. Attachment points are a minimum rather than a maximum. That is were I was talking about the "no rules" aspect of this cage.
Bends in general make the structure weaker. I don't consider an arch to be a "bend". Sure it curves, but the bend so gradual to have very different properties that a normal bend which is more often a much tighter radius. An arch is a wonderfully strong design element when loaded in the proper direction. Rather weak in other directions. The tough thing about putting arches in a roll cage is that there are so many different directions of load that benefit in one direction weakens the other and weakend is just bad.
Most upper cross bars in roll cages are not arches anyway. They are straight or have two sharp bend on each end. This structure does not perform the same way as a proper arch does.
I STILL wood attach that bar direct to the side of the chassis. You are so close and by doing that you have a SIGNFICANT increase in strength of the cage as well as chassis stiffness.
#227
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Now, that I'll buy...
A much less sweeping generalization. You certainly can't predict exactly where a load will be coming from, but you assume very generally and make the best of it. These are not true archs, indeed, but they possess some of the basic properties of one. If the first impulse of the cage is to transfer force down any of its lengths in compression, it has performed the valuable service of spreading load away from the impact, even if only for a split second.
Because of this, I try to put the bends in my header tubes nearer the center than the ends.
A much less sweeping generalization. You certainly can't predict exactly where a load will be coming from, but you assume very generally and make the best of it. These are not true archs, indeed, but they possess some of the basic properties of one. If the first impulse of the cage is to transfer force down any of its lengths in compression, it has performed the valuable service of spreading load away from the impact, even if only for a split second.
Because of this, I try to put the bends in my header tubes nearer the center than the ends.
#228
Mr. Excitement
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Rather than bend and rebend and rebend I use PVC plumbing drain pipe for mockups. Heat it with a heat blanket that is used to bend elec conduit, a heat gun, a cheepo tank top propane heater(my winter time favorite as I get to heat the shop too) or a torch fittied with a flame spreader. while warm and pliable you can bend it to the shape you want with your hands and then cool with a wet rag to freeze it in place. With little effort and little cost you have a full size template to match up in metal. You can mock right down to the fish mouth rough outs. You can keep the templates for the next time as well.
Or do the bling tunner thing and skip the make it from metal part completly. I saw a fake roll cage kit made from PVC. Cut to fit, glue in place and wrap in roll bar foam to finish. Mount little fire bottle on pass side A pillar and you are ready to cruz wit d boys. (it might not pass Tech)
Or do the bling tunner thing and skip the make it from metal part completly. I saw a fake roll cage kit made from PVC. Cut to fit, glue in place and wrap in roll bar foam to finish. Mount little fire bottle on pass side A pillar and you are ready to cruz wit d boys. (it might not pass Tech)
#229
Race Director
Originally Posted by RedlineMan
The only place that it may seriously fall affoul is in number of chassis "contact points."
Originally Posted by RedlineMan
Frankly, I don't think that any great advantage is gained in chassis stiffness at the amateur level relative to things like weight, driver ability, HP, etc.
Originally Posted by RedlineMan
Other than that, the only thing I see as a problem is any myopia of the inspector in question.
Originally Posted by RedlineMan
Look at a header tube (top of the windshield). The loads that it is likely to see are downward in a roll over. It will not be a linear compression load, but one entering tangent to its length. If you make that tube straight, if the force is enough to cause a yield, that tube will almost immediately start pulling in on its end points, they being the side hoops. You have now begun a sequence of collapse in that structure; everything imploding to the center.
If you put some arch in that tube, and a force is applied in tangent, that force will be transmitted in compression into the end points (side hoops). Unless the tangent force is far removed from the plane of bend of the arch in question, that arch will support more force than a straight piece of tube. It may still ultimately fail, but before that it has dissipated a lot of energy throughout the structure, hopefully enough to minimize intrusion.
Or... so it seems to me. We need an engineer! Mike?
If you put some arch in that tube, and a force is applied in tangent, that force will be transmitted in compression into the end points (side hoops). Unless the tangent force is far removed from the plane of bend of the arch in question, that arch will support more force than a straight piece of tube. It may still ultimately fail, but before that it has dissipated a lot of energy throughout the structure, hopefully enough to minimize intrusion.
Or... so it seems to me. We need an engineer! Mike?
I'm really surprised you're not welding the upper side tubes to the coachwork. You've already stated you weren't building for any rule set. You've weakened the coachwork by cutting it out. I thought you were cutting it out so you could weld the tubes into it. When I first saw it I thought "how clever" but if you're not going to weld them in, I think you're doing yourself a disservice. Welding them in place would make the structure much stronger overall.
#230
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Originally Posted by Geo
John, that's potentially a scary statement. While there may be some green inspectors, I'd wager most of them have seen more cages up close and personal than you or I ever will.
I'm not an engineer, but I'd wager that "arched" tube would collapse much more quickly. The tube is already bent! An arch is only stronger parallel to its axis.
I'm really surprised you're not welding the upper side tubes to the coachwork. You've already stated you weren't building for any rule set. You've weakened the coachwork by cutting it out. I thought you were cutting it out so you could weld the tubes into it. When I first saw it I thought "how clever" but if you're not going to weld them in, I think you're doing yourself a disservice. Welding them in place would make the structure much stronger overall.
One aspect of this project has been trying to limit it so that I could actually complete it before things get busy. Having said that, I have in the end done a lot more at every juncture than I intended to. It may yet happen.
#231
Mr. Excitement
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Originally Posted by RedlineMan
OK, assuming that to be true, then explain the "NASCAR door bars" that everyone seems to think are God's Gift? How are they not also blighted by this statement?
Point 1 - While I am not being **** about meeting every single point of a rule set, neither do I want to stray too far. Point 2 - The close proximity of the tube and roof structure render them virtually one to a large degree, even though they may not be directly intigrated
Originally Posted by M758
John,
Bends in general make the structure weaker. I don't consider an arch to be a "bend". Sure it curves, but the bend so gradual to have very different properties that a normal bend which is more often a much tighter radius. An arch is a wonderfully strong design element when loaded in the proper direction. Rather weak in other directions. The tough thing about putting arches in a roll cage is that there are so many different directions of load that benefit in one direction weakens the other and weakend is just bad.
Most upper cross bars in roll cages are not arches anyway. They are straight or have two sharp bend on each end. This structure does not perform the same way as a proper arch does.
I STILL wood attach that bar direct to the side of the chassis.
Bends in general make the structure weaker. I don't consider an arch to be a "bend". Sure it curves, but the bend so gradual to have very different properties that a normal bend which is more often a much tighter radius. An arch is a wonderfully strong design element when loaded in the proper direction. Rather weak in other directions. The tough thing about putting arches in a roll cage is that there are so many different directions of load that benefit in one direction weakens the other and weakend is just bad.
Most upper cross bars in roll cages are not arches anyway. They are straight or have two sharp bend on each end. This structure does not perform the same way as a proper arch does.
I STILL wood attach that bar direct to the side of the chassis.
#232
Race Director
Originally Posted by RedlineMan
I am not questioning the knowledge of the inspectors. I am wondering about those that might stick so resolutley to the rules as to be zealots. If it is a highly competitive group like National/Regional SCCA, then I can see the need for being more stringent. If it's just local weekend warrior stuff, I think that is a bit silly.
Originally Posted by RedlineMan
OK, assuming that to be true, then explain the "NASCAR door bars" that everyone seems to think are God's Gift? How are they not also blighted by this statement?
Actually John, I fear that folks will think NASCAR bars are the ticket and try to mandate them. I don't want them. And I'm very safety minded.
Originally Posted by RedlineMan
Point 1 - While I am not being **** about meeting every single point of a rule set, neither do I want to stray too far. Point 2 - The close proximity of the tube and roof structure render them virtually one to a large degree, even though they may not be directly integrated (I will be attaching at the main nodes, which I KNOW will make you happy ). Point 3 - The strength of the unmodified roof relative to the tube structure is paltry, and I therefore do not see huge gains to be had in integrating them. Point 4 - It makes it a lot harder to pad!
Do it John.
Do it.
#233
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You know, G...
When I first envisioned this, I had in mind to completely hide the side hoops by fairing them in with sheetmetal. You may note that the NASCAR boys do this. I imagine it is for safety (nothing to snag), and aerodynamics (window opennings create a TON of drag. There is also a lot to be said for having a more uniform flatter surface for safety reasons.
On the other hand, I thought the scruts would never go for it, so I let it go.
You're a pain.
When I first envisioned this, I had in mind to completely hide the side hoops by fairing them in with sheetmetal. You may note that the NASCAR boys do this. I imagine it is for safety (nothing to snag), and aerodynamics (window opennings create a TON of drag. There is also a lot to be said for having a more uniform flatter surface for safety reasons.
On the other hand, I thought the scruts would never go for it, so I let it go.
You're a pain.
#234
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Regarding bends;
The main reason for the bends in the side hoops and header tube is to tuck things in tight to the chassis for headroom, space efficiency, and to reduce the uneven shape of the impact zones. This latter point is not addressed often enough!
Funny... I note that the formed sheetmetal beams that these tubes are complimenting (door openning (or aperture) and header beams) are not straight, but arched.
Hmmm....
The main reason for the bends in the side hoops and header tube is to tuck things in tight to the chassis for headroom, space efficiency, and to reduce the uneven shape of the impact zones. This latter point is not addressed often enough!
Funny... I note that the formed sheetmetal beams that these tubes are complimenting (door openning (or aperture) and header beams) are not straight, but arched.
Hmmm....
#237
Mr. Excitement
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Originally Posted by RedlineMan
You know, G...
When I first envisioned this, I had in mind to completely hide the side hoops by fairing them in with sheetmetal. You may note that the NASCAR boys do this. I imagine it is for safety (nothing to snag), and aerodynamics (window opennings create a TON of drag. There is also a lot to be said for having a more uniform flatter surface for safety reasons.
On the other hand, I thought the scruts would never go for it, so I let it go.
You're a pain.
When I first envisioned this, I had in mind to completely hide the side hoops by fairing them in with sheetmetal. You may note that the NASCAR boys do this. I imagine it is for safety (nothing to snag), and aerodynamics (window opennings create a TON of drag. There is also a lot to be said for having a more uniform flatter surface for safety reasons.
On the other hand, I thought the scruts would never go for it, so I let it go.
You're a pain.
I am going to be a pain too.
Dont drill holes, use a high bond contact cement to fasten the padding.
All kidding aside I understand the desire to have everything tight and to have as much clearance as you can get but to me from here it looks like you compromised one system to install another that does the same thing. Rather than gaining 2X you had to start with a sawsall induced deficit and much of your added metal is only making up for what was lost to the sawsall. There must have been a way to have a nice tight clever photogenic install without negating the original strutures value. Complement rather than compromise.
#238
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Originally Posted by kurt M
No need to make flashing, just slather on some Bondo.
I am going to be a pain too.
Dont drill holes, use a high bond contact cement to fasten the padding.
All kidding aside I understand the desire to have everything tight and to have as much clearance as you can get but to me from here it looks like you compromised one system to install another that does the same thing. Rather than gaining 2X you had to start with a sawsall induced deficit and much of your added metal is only making up for what was lost to the sawsall. There must have been a way to have a nice tight clever photogenic install without negating the original strutures value. Complement rather than compromise.
I am going to be a pain too.
Dont drill holes, use a high bond contact cement to fasten the padding.
All kidding aside I understand the desire to have everything tight and to have as much clearance as you can get but to me from here it looks like you compromised one system to install another that does the same thing. Rather than gaining 2X you had to start with a sawsall induced deficit and much of your added metal is only making up for what was lost to the sawsall. There must have been a way to have a nice tight clever photogenic install without negating the original strutures value. Complement rather than compromise.
The drill holes suggestion was in the sheetmetal fairing around the tube, not the tube itself, I'm sure.
I hear what you are saying about the structure, but I feel you are over playing the importance of it. I could take a sledge hammer and with one blow cave in the side of the roof to some degree. I'll get my teeth chipped to Hell if I try hitting the cage like that. To compromise a structure that is so relatively weak is not a big loss in my opinion.
If I fair it all in with sheetmetal, it will be stronger than either alone.
Now... if I could find a really long lasting contact cement...
#239
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Larry Herman
2016 Ford Transit Connect Titanium LWB
2018 Tesla Model 3 - Electricity can be fun!
Retired Club Racer & National PCA Instructor
Past Flames:
1994 RS America Club Racer
2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
CLICK HERE to see some of my ancient racing videos.
Larry Herman
2016 Ford Transit Connect Titanium LWB
2018 Tesla Model 3 - Electricity can be fun!
Retired Club Racer & National PCA Instructor
Past Flames:
1994 RS America Club Racer
2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
CLICK HERE to see some of my ancient racing videos.