tube question

[big83chevy4x4]

im building a truggy for my friend, thinking about the tube work. the only die i have is 1.5" on 5" radius. is it safe to build a cage out of 1.5"? im thinking the thickest tube i can bend, i know baddog uses HREW, but with the smaller diameter, should i go with DOM? /forums/images/graemlins/thinking.gif /forums/images/graemlins/thinking.gif

 

[sled_dog]

I'd say 1.5" with a .120 wall would be safe. Build carefully and triangulate some critical corners(like on the main bar).

 

[ZZ4x4]

I would add that .120 tubing is strong enough structurally if you triangulate well, but for areas where you expect impact, like lower frame rails, sliders and bumpers, I'd go with at least .134 . Rock impacts will tend to dent the .120 .

 

[BadDog]

Warning: These are opinions and do not represent fact or actual engineering analysis.

Tube selection depends on the vehicle and must take into consideration any aesthetic or other limitations (exo, access requirements, etc) to the design.

Assuming ideal configuration properly using triangulation and support, I think 1.5 x 0.120 HREW should be fine for something less that 5000 lbs that you don't expect to severely beat on. What I mean is that it should hold up to protect you in a hard roll, but it wont stand up to frequent rolls. It will be too soft and dent/deform too easily to really beat on it. It will also take a bit more tube than usual to get suitable support for approaching 5000 lbs. 1.75" tube is considerably stronger than 1.5", more than you would think from just 0.250 larger diameter. I would tend to prefer 1.75" for anything over 4500 lbs or so (even considering an ideal design), and DOM or heavier wall tube for impact/abrasion surfaces. The new buggy I'm kicking around would (hopefully) be under 5k with 1.75 x 0.120 HREW cage/body and DOM for rockers and such that take lots of hits.

Like I said, these are just my opinions based on what I've seen, read, and discussed with others who know more than I do...

 

[jekbrown]

not trying to hijack or anything, but does anyone use heavier stuff like .134 DOM on the main cage hoops and rocker guards and crap and for the interior x-bracing and stuff something lighter/cheaper like .120 HREW? Just wondering if, from an engineering standpoint, it makes sense to do it that way or if the thinner wall HREW becomes a "weak link" negating the strength of the heavier stuff....

j

 

[BadDog]

Yes, it's not uncommon at all. I use 0.250 rockers because I know I'm very hard on them. I pivot on them, I fall on them, I often grind them crossing the lip on climbs. Normal tube wouldn't survive long at all...

Also, interior bracing (without bends) and gussets are often 0.092 or so. If kept in compression/tension then thin wall is still very strong. It just can't handle point loads and bending forces. That's where you switch to DOM, thicker wall, larger diameter, or a combination...

As I mentioned. My new buggy ideas include some DOM for the outer abused portions. Maybe 0.188 for the rocker tubes this time. The vast majority will be 1.75 x 0.120 in the outer cage. And I'll probably use thinner wall and smaller diameters for much of the inner supports too.

One thing you do have to think about is being careful with flat gussets or shear plates with thin wall tube. They can often cut through just like a can opener in a hard impact… Nice fat fillets on both sides or (better yet) using round tube gussets will help deal with that issue. I also like dual flat gussets mounted tangent for some applications.

 

[jekbrown]

cool beans... once I get my s-10 cab into autoCAD in 3d Im gonna post some pics of potential exo designs for you guys to critic. I know the super basics and have seen enough cages to know what tends to work... but Im not an engineer and my life may depend on it so I could use some feedback before I proceed.

In the mean time, are there any good books out there on cage building? seems like there should be something out there, prolly race car fab oriented... that could be usefull.

j

 

[ZZ4x4]

I am using .134 x 2 DOM for the entire roll cage and bumpers. I am using .120 x 1.5 for the seat mounting supports and triangulated gussets in a couple places. I've nailed the bumpers pretty hard on trees and rocks and they still look perfect. Of course bending the thick tubing takes some effort /forums/images/graemlins/wink.gif And the thick tubing cage will add a bunch of weight. But prolly safer in a severe roll.

 

[miniwally]

Most of us probably overbuild our cages.
Using a thicker material in a 1.5" to 1.75" for the exterior is a good idea. As most have said using a thinner wall small tubing for all internal bracing is the way to go.
My advice to anyone thinking about building a cage or buggy type frame is to keep in mind weight all of the time. I really belive that had I been more concious of this when building mine I could have saved a couple hundred pounds.
Also remember what you want the cage to do for you.

My opinion on internal cages is to create a cage that saves the occupants. It may deform and crush but needs to let the people inside walk away. The vehicle is going to have so much other damage to it if the role is hard enough to make the internal cage work that it will most likely be a total lose.

Those building a buggy type thing need to think about multiple times on its side/top and build accordingly. I know that I can take my buggy and put it on it's side or top many times and it will be okay to keep using. Thats why it's built the way it is.

 

[jekbrown]

yeah, I plan on making my cage pretty dang beefy. When you do all your own steering and brakes and crap ya wanna have some extra insurance just in case you crash at near freeway speeds. If it ends up weighing a lot... i can deal with that... I plan on building a super light toy-drivetrain buggy someday anyway, that can be my light rig. /forums/images/graemlins/grin.gif

j

 

[bga]

Just to throw a couple of reference points into this discussion:

NHRA and IHRA require a minimum of 1 5/8" x .120 tubing to be used in cars running 7.50 or quicker. A car running 7.50 in the eighth will be crossing the line somewhere around 90-100MPH.

Circle Track only requires 1.750" x .120 tubing for the cages. This goes for Pro, Amateur, Bombers, and Hoppy classes.

Considering this, I would consider 1.500" tubing to be too small for any main locations. Braceing and seat mounts would be fine with this small tubing. I would (personal opinion) prefer 1 5/8", or 1 3/4" CromeMoly.

I'll be ordering my cage from Jegs instead of building my own this time, and the 10 point setup will be made from 1 5/8" tubing.

Just my opinions and views. If you want a true professional opinion, give Competition Engineering a call and ask for Jeff or Mark. Those guys have always been more than happy to spend a little time on the phone with us on our past projects.