bolt in rollcages

[vennard]

are bolt in cages as strong as welded ones. From what I can seee they us e a kind of clamp system, would this not slide when the cage is under pressure.

[Frank]

Hi

First of all, are you looking for a race certified rollcage for such clubs like the SCCA? There are certified bolt-in rollcages on the market but they are rare. I don't know why, because on a structural point of view, if well designed they sould be as strength as welded rollcages. The thing is in structural engineering (like bridge building), designers are prone to use bolt assemblies instead of welding the whole thing together. I study mechanical engineering and have a teacher telling us this almost every course!! Welded structures are good but all resides in strong welds, and thats where it gets complicated. Bolted stuff is all standardised, welds are also but you need to get a certified welder, someone who can weld up to the standard needed and that costs a lot! It is also easier to predict what happens in a bolted assembly rather than welded (mostly because of the quality of the welds) If you find a certified bolt-in roll cage you only need to assemble it and respect the manufacturers instructions like required torque in bolts. It's easier and cheaper than certifying a welded rollcage. If it's only for looks, it's up to you!


Frank

[DORIGTT]

What is the best mounting points for putting a cage in our pan mounted cars?

How do the Heigo cages mount...to the body or to the pan?

[vennard]

Not just for looks. My main concern was that the cage may tear up from the mounting points on the body, as they are bolted through rather than welded to it. Now ive seen that in writing it sounds kinda lame
COuld you Or do you, Bolt through the body into a thick peice of metal on the other side so the relativley thin bodywork wont tear.

Cheers Karl

[Frank]

Hi Karl,

I don't know exactly where you want to bolt or weld the cage on your beetle's bodywork but a thing that is sure, body sheetmetal won't resist any big impact, witch means that bolted or welded you'll probably get the same result. Also it gets really hard to obtain strong welds on a thin sheet of metal, most bodywork is spot welded or mig welded, you'll never see a strong roll cage mig welded, welds are too weak. And Don't try to weld on thin bodywork with another welding process that makes stronger welds, you will probably warp the sheet metal which isn't better. The only places found on bodywork that are resistant are firewalls and suspsension mounts (strut towers) mostly because of their structural design. Firewalls are thick and rigid, it's the same for strut towers, they endure stresses caused by the suspension work. Look at any Rally car and you'll find that their rollcages are secured on strut towers, firewall and if the car is monocoque which is the case for most cars today, they are mounted on the structural parts of the bodywork. Since a beetle has no frame, it's probably considered as a monocoque, try to find out which sections of the body were designed as chassis or frame work, they are the strongest and thickest parts of the car. Seems to me that the floorpan is the strongest part of the car, but we shouldn't underestimate the rest of the it since Beetles had the thickest bodies available on the market. have you ever bent a fender on a beetle, try to pry it out to it's original shape, not an easy task!

finally, on your question on sandwiching (from what I understand) the body between thick plates of metal, it will distribute the forces on a greater area (diminishing the internal constraints in the mounting points) but these plates would only be secured on thin sheetmetal making this still weak.

Frank

[vennard]

oops. Did not mean 'bodywork'. reallly meant floorpan. The sadwhiching question was really meant in reference to the area behind the rear seat because only bodywork is available to get at easily.

Really should read these posts back before I carry on.

Thanks for the help


Kar

[3MCRacing]

you'll never see a strong roll cage mig welded, welds are too weak.

Mig welding has been used in race car chassis for many many years. it is more reliably done than Arc and less expensive then Tig that is why it is used so much. Look at any race car from class 11 to SCCA Trans-AM and i gaurantee that you will find Mig welds. When using a bolt in or weld in cage the main hoops need to have bases that will spread the load. ie... if you weld a 1 1/2" tube to the door jam where it meets the pan (not on the pan) then any impact will be spread out in the same dia as that tube but if you welded that same tube to a pice of 1 1/2 " x 3" flatstock then the impacting load has a much larger area to spread. If you really look at Rally cars you will notice that the tube going to the firewall does not end at the firewall it extends through the firewall usually to be used to support the suspsension mounting point.

And just in case you didnt know "monocoque" is an engineering term indicating a structural shape where loads are borne on the skin of the structure).

[Frank]

Hi Micheal,

I did a terrible mistake about the mig welding issue, when I wrote this I was thinking bodywork and mig welds from bodywork surely don't have the same strenght as welds used in framework. Sorry for that

For the part on the bases of the roll cage main hoops, I never said they were useless, I simply based my analysis on the fact that vennard was talking about body instead of floorpan:

"My main concern was that the cage may tear up from the mounting points on the body, as they are bolted through rather than welded to it" vennard

From what I understood, there would be some sort of a base plate installed on thin bodywork sheet metal, it could be anywhere on the inside of the car. Fortunatly vennard was talking about the floorpan.

Also, I know what monocoque means. Monocoques, with their ability to spread loads to their surface, considerably increased chances of survival to an accident over the traditionnal rigid rail frame designs used before. It enabled the car to absorb most of the energy on impact. Rigid frame rails didn't absorb that energy, the driver did. Race car design used in Cart and F1 racing pushed that concept on a higher level by creating monocoques that would literally "explode" (except the ****pit area) and therefore dissipating the energy gained during impact in the flying debris of the racecar. Unfortunately, that didn't save Greg Moore's Life back in 1999 .