TheSamba.com Forums
 
  View original topic: Roll cage building help Page: 1, 2  Next
thesatelliteguy Sat Jan 16, 2021 6:42 pm

This is my first time building a roll cage from scratch. Iím just kind of steeling bits and pieces of other roll cage designs I find and copying them.
What do you think? See anything I should change before it gets fully welded.
I should point out that there will be more bars added to the sides. But I want to weld all the over head stuff first, then drop the cage through the floor and weld the tops of the over head tubes, then lift it back up and add my side bars, cross bars, bars going to front suspensions and so on.





sailtexas186548 Sat Jan 16, 2021 8:40 pm

I would like to see the middle of the cage closer to the roof, bugs are small and every inch counts. Otherwise I think it looks great, nice fit ups

rayjay Sat Jan 16, 2021 9:04 pm

When you start welding the tops you will find that the vertical legs will move on you. You should make a fixture to keep them fixed into place in relation to one another.

dustymojave Sun Jan 17, 2021 12:34 am

That Vee over the front seats should meet at the B-pillar hoop. not be spread. If they meet, you have a nexus, where the loads can spread evenly in all directions. With them spread, forces will be concentrated at the middle between the Vee legs and tend to crack the B-pillar hoop.

Structurally, a diagonal bar across the B-pillar hoop is needed in spite of the diagonals overhead. If the car rolls onto the side, a lateral force will still bend the uprights sideways. There's a 5-16 Baja Bug in my back yard that got rolled and in spite of the diagonal bar, it wasn't done right and the cage failed to the side. They had held the bottom end of the diagonal up to clear the fuel cell. So the cage bent at the diagonal a foot above the floor. The entire cage went sideways.

It will need a shoulder bar for shoulder harnesses. That should be at a height so the bar is at or just below the tops of your shoulders when sitting in the driver's seat. Near the bottom of the side windows is usually a good height.





This shoulder bar in my Baja is removable with 1/2" aircraft pins for rear access.

thesatelliteguy Sun Jan 17, 2021 11:43 am

rayjay wrote: When you start welding the tops you will find that the vertical legs will move on you. You should make a fixture to keep them fixed into place in relation to one another.

That is excellent to know. Thank you. Iíll make up my cross bars first.

thesatelliteguy Sun Jan 17, 2021 11:51 am

dustymojave wrote: That Vee over the front seats should meet at the B-pillar hoop

I was afraid of that. That was the initial idea but one of them got cut a little short, so they all got placed like that. It took me a while to get those 4 bars notched right to fit in the intersections, so I think Iím gonna leave it. But perhaps a gusset in there will help disperse the load?

thesatelliteguy Sun Jan 17, 2021 1:44 pm

I started making some braces today and came to a cross road.
Should this tube be straight, or bent like I have it depicted with tape in the pic.
I plan to have shocks (and later on coilovers) come through the body and mount to the cage. Iíve noticed that most cars with this set up have the shocks/coilovers mounted between the roll cage and the body, but this set up will cause the shocks/coilovers to be mounted on the inside of the cage, closer to the center of the vehicle. Is that a bad thing? Will it cause some suspension problems? I havenít really gotten to suspension theory yet in my build but I will be soon. I would imagine that the closer a shocks upper mounting point is to the center of the vehicle, the less effective it is. But how much are we talking here? Is 1 or 2 inches going to make a difference?

i

Bad_chopper Sun Jan 17, 2021 5:11 pm




this is how i did mine, so they would clear the shocks, the problem i have with it is if rear ended it is an easy point to bend, so i will need to reinforce it i think. i waited till i figured out where my shocks came in threw the wheel well before i made that piece.

and as far as your over head cross i put gussets between the two bars on the front two and back two

L5wolvesf Sun Jan 17, 2021 5:47 pm

I'm wondering why the main hoop is as far back from the door pillar. On some cars the main hoop is gusseted to the door pillar.

RLFD213 Sun Jan 17, 2021 6:23 pm

My only advise is make sure the cage isnít wider then the body so you can take the body off. The bars by the B-pillar sometimes pudge out and the heater channels get hung up.

sailtexas186548 Sun Jan 17, 2021 7:23 pm

RLFD213 wrote: My only advise is make sure the cage isnít wider then the body so you can take the body off. The bars by the B-pillar sometimes pudge out and the heater channels get hung up.

If you make the cage narrow enough For the body to come off, it will be too narrow and bash your knees and the b pillar will be really close to your helmet.

dustymojave Sun Jan 17, 2021 8:44 pm

RE: The junction in the top center of the cage
Gussets will serve to fix to a certain degree the issue of the tubes not meeting in the center. Make the hypoteneuse of the gusset (the edge of the triangle away from the corner) a curved line so the ends of the gussets taper to the tubes.

RE: suspension theory

The shock mount bolts, lower and upper, should be parallel to the trailing arm pivot bolt. When you are mocking up the rear suspension and that part of the roll cage, the rear suspension should be "cycled". That is... with the springs removed, if torsion, remove the torsion bars and reassemble all else, including trans and axle shafts with CVs. Then move the suspension through its full travel, watching what the various components do as you move it. You don't want anything but the stops to go bump. When the shock mounts are in the right place, cycling the suspension through its full travel will hardly move the upper end of the shocks.

Don't worry about "this set up will cause the shocks/coilovers to be mounted on the inside of the cage, closer to the center of the vehicle. Is that a bad thing? Will it cause some suspension problems? " Put the mounts where they work for the shocks.

The rear trailing arms with stock pivot locations do NOT pivot in a straight line parallel to the centerline of the car. I see LOTS of cars with shock mounts built as if the suspension works that way. My son and Rick Crumb build Crumco Baja Bugs with straight pivot rear suspension. That will go in a straight line over bumps better than the angled pivots used on VWs (and Porsches, BMWs, Alfa Romeos, etc.), But it won't corner as well.

So the shock mount bolts should be at an angle to the centerline of the car. Shocks with heim ends CAN be mounted so the bolts are "square" to the car. But you can probably open a beer bottle with your teeth too. Just because you "CAN" doesn't mean you "SHOULD".

rayjay Mon Jan 18, 2021 7:19 am

The "spring base" is a little known aspect of car construction and tuning. A good bit of the article is circle track specific and has no bearing for our use but there is a lot of good basic info there that helps a person understand things that can be addressed during fabrication of shock mounts, etc.

The basic lesson about spring base is to put the upper mounts as far apart as possible. This will provide the maximum roll stiffness with no difference in vertical travel dampening.

Personally, if I was building a rail or baja with expensive shocks, etc I would probably convert it to coilover springing and get rid of the torsion bars.

https://www.hotrod.com/articles/ctrp-0912-what-spring-rates-to-use-in-your-race-car/

----------------------------------------------------------------

From the above article

ear Spring Installation In the rear for a solid axle suspension, the car "feels" the spring base as the distance between the top of the two springs. This is all the car knows and it is as if the chassis were sitting on a pair of springs that are resting on the ground. The rear solid axle assembly is a solid base for the springs to sit on and the car does not know, nor does it care, where the wheels are located.

Overturning moments acting through the center of gravity are effected by the resistance to roll created by the spring base width and spring split, the rear moment center height, and the rates of each spring. To effect changes to the rolling tendencies of the rear suspension, we need to look at altering the spring base (distance between the tops of the rear springs), the installed spring angle, and/or the moment center height. The narrower the spring base, the greater the tendency for the rear end to roll for a given set of springs and moment center height.

A narrow spring base can be a real problem for some types of cars. The four-link, Dirt Late Model cars sometimes have the springs installed at high angles with the top of the coilover positioned well inside the framerails. This severely limits what we can do to eliminate excessive rear roll in the car.

Many teams have moved the top shock/spring mounts out closer to the wheel to increase the rear spring base. The swing-arm types of dirt car have a very wide spring base by virtue of being mounted straight up, but suffer from another variable-motion ratio. The spring is mounted directly on the trailing arm, similar to the front spring mount, and that reduces the rate that the car "feels."

rayjay Mon Jan 18, 2021 7:34 am

Bad_chopper wrote:


this is how i did mine, so they would clear the shocks, the problem i have with it is if rear ended it is an easy point to bend, so i will need to reinforce it i think. i waited till i figured out where my shocks came in threw the wheel well before i made that piece.

and as far as your over head cross i put gussets between the two bars on the front two and back two

Did you run a tube crossways connecting the upper shock mounts ? I saw on your build page that you added a tube from above the shock mounts to the upper intersection of the real legs - main loop - roof hoop intersection which is perfect.

thesatelliteguy Mon Jan 18, 2021 9:34 am

L5wolvesf wrote: I'm wondering why the main hoop is as far back from the door pillar. On some cars the main hoop is gusseted to the door pillar.

I put the B pillar back a little bit bc i wanted more room for the drivers seat. I kind of want to move everything back about 3 inches so I'm not so close to the dash and windshield. Its a decision I'm starting to wonder if ill regret.

thesatelliteguy Mon Jan 18, 2021 9:52 am

dustymojave wrote: RE: The junction in the top center of the cage
Gussets will serve to fix to a certain degree the issue of the tubes not meeting in the center. Make the hypoteneuse of the gusset (the edge of the triangle away from the corner) a curved line so the ends of the gussets taper to the tubes.


Like such?
Im thinking 1/8" steel plate gusset on top of tubing and on bottom. Or do you think one 1/8" plate, sandwiched between the tubes?

thesatelliteguy Mon Jan 18, 2021 10:20 am

dustymojave wrote:
The shock mount bolts, lower and upper, should be parallel to the trailing arm pivot bolt. When you are mocking up the rear suspension and that part of the roll cage, the rear suspension should be "cycled". That is... with the springs removed, if torsion, remove the torsion bars and reassemble all else, including trans and axle shafts with CVs. Then move the suspension through its full travel, watching what the various components do as you move it. You don't want anything but the stops to go bump. When the shock mounts are in the right place, cycling the suspension through its full travel will hardly move the upper end of the shocks.

OK, I think I understand that. So basically, I need to get all my suspension components first (i don't have axles yet) before i can finish up that part of the cage.
It throws a wrench into things since I don't have the monies for the coilovers right now and only have some crappy ProComp shocks to use. I guess ill just revisit that part of the cage later on when i do have coilovers, and build it right.
Although that bring to mind a question.
Will 3x3's work on 22mm stock torsion bars? Is that enough spring?
Im not expecting amazing results from the torsion bar/ProComp suspension on the rear, but having something that is street drivable would be nice so i can drive it and enjoy my work until i can afford the coilovers.

As Rayjay suggested I'll get some temporary bracing on the vertical legs before i drop the cage and weld the tops of the overhead bars, to prevent the vertical legs from moving.

Bad_chopper Mon Jan 18, 2021 1:50 pm

rayjay wrote:

Did you run a tube crossways connecting the upper shock mounts ? I saw on your build page that you added a tube from above the shock mounts to the upper intersection of the real legs - main loop - roof hoop intersection which is perfect.

no i did not run a tube across, it dosnt seem like there will be much force to need that. now the lower tubes that go around the coilovers i think i need to put something there.

dustymojave Mon Jan 18, 2021 4:16 pm

L5wolvesf wrote: I'm wondering why the main hoop is as far back from the door pillar. On some cars the main hoop is gusseted to the door pillar.

The B-pillar of the cage is back from the B-pillar of the body because it's fastened to the torsion housing, the strongest place to attach it. I approve of that location. Although in some ways, such as it would be nice to gusset it to the body B-pillar, it's not as good or convenient. If I were building a new race 5-1600 Baja Bug, that's where the b-pillar hoop of the cage would land.

rayjay Mon Jan 18, 2021 4:32 pm

I've installed a lot of flat plate gussets but these are available cheap nowadays and are what I would use.

https://www.speedwaymotors.com/Roll-Cage-Saddle-We...10527.html



Powered by phpBB © 2001, 2005 phpBB Group