Lotus Europa: the chassis is a spine through the middle, and that's it. Kind of like this (except the engine is in the rear):
Could an acceptable cage be built similar to the above pic (with everything returning to the center, more bracing, etc.), or would it be necessary to go the more typical route (entire new pan, etc.)? Like this:
Note: the key words were "acceptable" and "necessary"
I seem to remember that the main hoop can't have more than 180 degrees of bend in it. That would make it hard to turn it back to center. I would hate to have the bottom bends get adjusted in a roll over and have the chassis end up in the cage with you.
Not an engineer BTW, though I have built some cages with Jensenman.
Is this just for your safety, or is this for a sanctioning body? If it's for a sanctioning body, that's easy - just ask them.
If it is for your own safety, I don't really know. I am an engineer, but haven't ever build a cage from scratch.
SVreX wrote:Lotus Europa: the chassis is a spine through the middle, and that's it. Kind of like this (except the engine is in the rear):
Could an acceptable cage be built similar to the above pic (with everything returning to the center, more bracing, etc.), or would it be necessary to go the more typical route (entire new pan, etc.)? Like this:
I have no knowledge of sanctioning body requirements, but I would be worried that it is basically unsupported vertically. If you're doing something like that, I would try and triangulate it vertically from those horizontal cross-bars back up to the top of the frame.
Like Toyman said, the total of all the angles in a main hoop cannot exceed 180 degrees. In the SCCA rule book, there are some allowances for backbone frame vehicles.
That top picture looks like an Elan chassis. The setup there does need more bracing IMHO, were it me in addition to what's on there now I would add a tube running from the inside of each of the 90 degree bends down to a plate welded to the top of the backbone. From the front or rear this would look like a V.
EDIT: A quick Google returned this, showing the V brace I am thinking of. It might be a bit more complicated than you are thinking but you get the idea.
Here's a prefab Europa cage:
Again, a bit more complicated than you are probably thinking but food for thought.
I don't like that prefab Europa cage.
All of the mounting points are side bolted to the backbone. In a rollover, the entire structure is dependent on the sheer of the bolts (if the outrigger tubing didn't bend- same results).
The main hoop design works well from a design standpoint for a Europa, but I don't think it is SCCA legal, because it is not in one plane.
The braces to the rear are not mounted to the chassis. They are either mounted to fiberglass, or suspension components.
There is only 1 side rail, not 2.
The diagonal brace on the main hoop is kinked.
I do, however, like the first pic.
Jman:
Can you give me a reference for the SCCA backbone stuff? I can't find it, or the 180* rule (which I thought existed).
davidjs wrote:I have no knowledge of sanctioning body requirements, but I would be worried that it is basically unsupported vertically. If you're doing something like that, I would try and triangulate it vertically from those horizontal cross-bars back up to the top of the frame.
I agree, that first pic I posted was a bad design- needs a LOT more bracing. But it clearly showed the returns to the backbone.
SVreX wrote:Jman:
Can you give me a reference for the SCCA backbone stuff? I can't find it, or the 180* rule (which I thought existed).
It's going to be in the GCR (General Competition Rules). If you are an SCCA member, the GCR is available as a PDF on this page: http://www.scca.com/contentpage.aspx?content=44
It's also repeated in the 2009 Time Trial and Solo 1 rules but IIRC it's not in the Solo 2 book. Also IIRC there is not a specific set of specs for a backbone chassis, there's just some general information regarding engineering.
That bright yellow D Mod Elan belonging to Mark Huffman which was on the cover of SportsCar last year had a roll cage (again IIRC) and there was mention in the article of the guy who designed and built the bar. If you can scare up a copy, maybe he can give you some pointers.
I noticed the weird bend in the Europa prefab cage main hoop as well and no I don't believe that's SCCA legal. IIRC the main hoop must be made in a straight plane as viewed from the side but the whole thing may be tilted up to 3 degrees (viewed from the side) from vertical as measured from the car's horizontal plane.
BTW, when picking roll bar tubing there are different specs for different weights. 1.750 x .095 wall is legal for just about all but the heaviest cars. Smaller and lighter cars in the 1800 pound range can use smaller tubing but with a thicker wall, 1.625 x .120 wall. Here's the thing: the 1.750 x .095 wall is at least 30% lighter that the smaller diameter thicker wall tubing. That may not sound like much, but on the average cage it can mean as much as 30 pounds.
I was facing similar issues when I started building my TVR.....the factory frame was 1.5"/.080 round and square tubing. It is primarily a tubular backbone chassis with small outriggers to catch/support the body. I acthally think that the fiberglass body might have been counted on somewhat to add some rigidity. The frame weighed maybe 200 pounds, tops.
If you are building off of the existing chassis, I would look into trying to include some sort of tubular outriggers. In your first pic, something like those lower black tubes that run out from the center spine and between the wheels. Instead of the small round tubing, go with something like 2x3 or 2x4 rectangular tubing. Triangulate the rectangular space that this creates and you should improve the torsional rigidity tremendously. This new structure will also give you a place to run the rollbar/cage members to.
For the rear-going legs of the bar/cage I would look into running a piece of square tubing between the chassis shock towers, then run the rear legs to it in an "X" instead of just going straight back.
Good luck! As I'm learning with my project, fabrication seems to be more about the thought and planning that goes into it than the actual work. I seem to spend more time at the shop talking about the project with the guy doing the work than I think he actually spends doing the bending/cutting/etc.
Too bad you aren't closer to DFW -- can't recommend the guy doing my stuff high enough. Very good attention to detail, good prices!
-jeff d
Jensenman wrote:Like Toyman said, the total of all the angles in a main hoop cannot exceed 180 degrees. In the SCCA rule book, there are some allowances for backbone frame vehicles.
That top picture looks like an Elan chassis. The setup there does need more bracing IMHO, were it me in addition to what's on there now I would add a tube running from the inside of each of the 90 degree bends down to a plate welded to the top of the backbone. From the front or rear this would look like a V.
EDIT: A quick Google returned this, showing the V brace I am thinking of. It might be a bit more complicated than you are thinking but you get the idea.
I'd use a V brace like this too. The strongest way to set this up would be to have a V like this, with two tubes running from the top of the main hoop to the backbone. The fewer the bends from backbone to top of the hoop, the better.
SVreX wrote:I don't like that prefab Europa cage.
All of the mounting points are side bolted to the backbone. In a rollover, the entire structure is dependent on the sheer of the bolts (if the outrigger tubing didn't bend- same results).
The main hoop design works well from a design standpoint for a Europa, but I don't think it is SCCA legal, because it is not in one plane.
The braces to the rear are not mounted to the chassis. They are either mounted to fiberglass, or suspension components.
There is only 1 side rail, not 2.
The diagonal brace on the main hoop is kinked.
I do, however, like the first pic.
One thing to remember- it may not be the cage relative to the car, but the cage relative to you. If you attach the seats to the cage, you are the one being protected- it really wont matter if the backbone falls out or not- as long as you don't.
Even if it was well put together, if you fall through the floor, the entire design is moot.
So consider how your seat is mounted WRT the cage.
E
Good point, but I keep picturing me inside my intact cage upside down while the chassis shears off those bolts and falls into the cage with me.
But, thank you. I will be looking at the seat mounting.
Huffman's Elan:
I don't see the V brace from the other pic I posted. Keep in mind he's running under Solo 2 rules, not Club Racing. Given what I know about Elan design, the V brace is the way to fly.
BTW, when doing your cage keep in mind harness mount points. I'd try to incorporate them into the cage, along with seat mounting if at all possible. That way if the plastic decides to disintegrate at least you'd be contained within the cage.
J-man, I think you are reading the 2007 rules.
As best as I can tell, the 2009 Solo rules have roll bar info, but defer to the GCR section 9.4 on cages.
The GCR 9.4 is IMHO written terribly. The 2007 rules read much better. 2009 doesn't have the 180* rule, doesn't have degrees on other componenets, doesn't refer to backbone applications, and prohibits the V-brace, by mandating that a main hoop brace must cross 75% or more of the car.
I hope I am wrong, but I haven't found better guidelines. That is what I am asking for.
