• From: Shakopee, MN
  • Registered: 2009-10-05
  • Posts: 136

Topic: Rollcage main hoop design issue

I have a design issue on the roll cage for the Chevette we're building. The car has a frame rail that terminates by the B pillar and I'd like to attach the main hoop to it. Here's the problem, it's located inboard quite a ways. I know it's safest to have the rollcage tight to the body, but I'm not sure the best way to accomplish that.

I figure I have two options. Weld box tubing or build a platform of plate steel out toward the b pillar and weld the hoop to that. I'm not sure there's enough of the car next to the frame rail to weld anything to, this car is basically a tin can and a rusty one at that. The other option is to weld the hoop to the existing frame rail and bend it out to meet the b pillar. I'm not sure how good of an idea it is to have a bend near the legs of the main hoop though.

Pictures so you guys can see what I'm talking about.

Rear view so you can see the space between the frame rail and b pillar.
http://pics.roomsapp.mobi/RS2AHEmegD.jpg

Side view of the frame rail. I plan on bending those tabs down and using plate steel to cap that off.
https://pics.roomsapp.mobi/C7idEHW7YW.jpg

  • JThw8
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  • From: Mullica Hill, NJ
  • Registered: 2009-06-10
  • Posts: 1,781

Re: Rollcage main hoop design issue

weld plate between the "frame rail" and the rocker, rebuild/reinforce rocker as needed.

The bend at the bottom of the cage would be a fail point

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  • From: Toronto, Ontario
  • Registered: 2009-05-03
  • Posts: 1,114

Re: Rollcage main hoop design issue

Consider a box or channel section floor crossmember both for mounting and side impact, tied to the subframe..

Jim "Endo" Anderton
30 years of racing and still not Brambilla.....
  • OMGuar
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  • Registered: 2010-09-01
  • Posts: 320

Re: Rollcage main hoop design issue

Spread the load!
  A single point  load is limited by the total strength in that location.
However if it's spread along side the body to mutliple locations the load is taken by more of the chassis than one point. Decreasing it's chance of failure.. (engineering 101) Braces spread the load to various sites. when building the cage do not think roll over, think impact.  Sure it's possible to impact the roof in a roll over..  The reality is far more likely to come at the front since that is the direction most of us are trying to go.. (sorry about the obvious)
  The trade off is crumple zone..  if the whole cage impacts something and stops instantly the load on the human body will exceed what the human body is capable of dealing with.  Result death!
If however a gradual collapse  is designed in that collapse will occur over time time to slow the chassis down less abruptly  Possibly saving the human inside the body.. (pun intended,....... I'm sorry)
If that is confusing to you behaps you shouldn't be designing your cars rollcage but rather leave it to someone who does understand  the energy and forces involved.
You may know how to weld and fit tubing, but that is not the only consideration involved..
  Look to those who design and build rollcages that are actaully designed to deal with impacts.. NASCAR, SCCA, NHRA, etc..  Every time you deviate from their designs you assume risks..
  Now I will grant that a car with a track top speed of a little over 100 MPH needs less design consideration than one with the potential of over 200 MPH.   Yet those walls and other cars  are just as hard.  While your mass may be less than that of  NASCAR the requirement to remain alive isn't decreased..

  • Registered: 2010-01-21
  • Posts: 221

Re: Rollcage main hoop design issue

Hey fellow Chevette enthusiast.  We're also building a Chevette and we decided that the structure of the entire middle of the car pretty much sucks, so we put in 2"x3"x.120Wall box tubes the full length of the rockers.  They're completely plated into the rocker and floor and tied in to the front and rear subframes so we have a ton of added body rigidity and a very solid mount for our roll cage.  Our car isn't nearly as rusty as what I'm seeing in that picture, and we still didn't feel it was sturdy enough to support a safe rollcage.  Plus we will also have much better side impact protection from that added rail.

I'm not at my own computer so I don't have any pictures here that I can share.  If you want to remind me with an email, I can send you a few when I'm back home.

You're in Minnesota I see. Are you planning to run the Chicago race? If so, you're not the only 'Vette running.

Tom

6 Reply by Mkotzias (edited by Mkotzias 2010-09-11 08:12 AM)

  • Registered: 2009-05-27
  • Posts: 361

Re: Rollcage main hoop design issue

I have a chevette for a street machine.
v6 with a turbo....

the 6 point bar I installed runs along the b pillar. It bubbles out and then back in and ties in at the rails.
the rockers are all the strength in these econoboxes.
the forward struts have the spreader plates welded to the side and bent to the floor to tie the rockers to the cage.
the main hoop is tied to the "rails".. spreader bent over it and welded to it.
the rear struts go to the rear rails.

I would also plate the rear floor pan as this is the worst problem area for Vettes.

  • OMGuar
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  • Registered: 2010-09-01
  • Posts: 320

Re: Rollcage main hoop design issue

Too bad none of you have the original Chevette.
The one designed and built by Bob McKee. Designed and built in the winter of 63-64 and winning many, many, races..   Small block chevy rear engine.
Looks like an early Can Am car..
hey maybe that means you can make it rear engined with a small block Chevy??? wouldn't that be fun?

  • From: Atlanta, GA
  • Registered: 2009-09-01
  • Posts: 561

Re: Rollcage main hoop design issue

I hope one of you is running a d-fens theme.

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  • From: Shakopee, MN
  • Registered: 2009-10-05
  • Posts: 136

Re: Rollcage main hoop design issue

Great, we have a bunch of 2x3 tubing leftover from our last circle track chassis build. I guess I was dumb for not thinking of using that. I had incorrectly planned on using plate steel and making box platforms to weld to. The square tubing is a far better idea.


TKing66 - Most likely we will not attend the Chicago race. I'd also appreciate any pictures of your cage.

Thanks for the constructive comments, I appreciate them.

  • From: Shakopee, MN
  • Registered: 2009-10-05
  • Posts: 136

Re: Rollcage main hoop design issue

Alright, reviving this thread to see if what I did will work.

Here's the plan. I tied the 2x3 tubing to the rear subframe with the plate and butted the tubing to the front subframe. I plan on running the rear hoop to the 2x3 tubing at the b pillar and the a pillar bars will terminate at the front of the tubing.

That should work as far as I see it. Am I right? Thanks.

http://pics.roomsapp.mobi/E2vXAuwA1i.jpg

  • From: Columbia, South Cackalacke
  • Registered: 2009-05-02
  • Posts: 602

Re: Rollcage main hoop design issue

The 'after' pic looks a lot better. Put your hoop on top of the square tubing as close as practical to the outside of the car and you should be good. Keep in mind when bending your main hoop that the total of all the bend angles can't exceed 180 degrees.

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