A comment on tube work
bptactical
Jeeper
- Posts
- 84
- Thanks
- 0
- Location
- Commirado-Front Range
- Vehicle(s)
- '79 CJ7, 304, AT, Quadratrac HiLo w/ part time conversion.
A little background- I have been a welder/fabricator/machinist since 1978. I have have done fabrication work from titanium turbine exhaust systems to 18 wheel drive 110' reach AutoCar bucket trucks and everything in between. Early on I was a welder for a gent named Jerry Malloy who built some incredible circle track chassis and cars.
I have seen the unfortunate results of poor cage design and fabrication.
In perusing a lot of posts here and pictures as well I have a plea: if you don't know anything about structure design, stress loading both static and dynamic, proper material selection and correct fitment a rollover protective structure is not the place to learn.
It is also not the place to learn welding.
A few things that come to mind:
Exhaust tubing and water pipe belong in their respectively named places, they have no place in structural applications.
Plan your design carefully, extra tubing doesn't make a structure stronger. A properly designed structure will only have what is necessary to provide the strength required for the loads anticipated.
An intersection of tubing in the middle of an unsupported span is a failure point and serves no structural purpose.(seat mounting tubing aside)
Strength is in angles. Look at the frame for a fabric covered aircraft or a NASCAR car. You will discover that they are a series of intersecting triangles, one member bears on another to spread the load and stresses.
Bends in the middle of a span are a built in failure point.
Bends should never be in bracing structure.
About the only bends that should exist in a rollover structure should be in the front and rear main hoops.
Slight bends for headroom, door bars and such are acceptable but should be gusseted.
Bend radii- a good rule of thumb is to multiply the tube diameter by 3. This will typically provide a safe radius. Any less and in an impact/roll it will have tendency to kink and collapse.
Mounting points- poor mounting is just as dangerous as no rollover structure. What good is a structure when you go over on your lid and the structure just punches through the floor?
Again the "x3 tube diameter" rule holds well here, if the mounting plate dimensions are 3 times in length as the tube diameter you have a good plate size.
How many of us have seen a structure mounted to a body only to see just washers on the underneath side of the body?
You want a plate on the bottom that preferably is larger than the mounting plate on the top side.
If both the bottom and top plates are the same size all you have done is build a shear point. It will cut right through the floor in an incident.
Floor mount or frame mount?
I think we all can agree that frame mount is best right?
Maybe. It depends on the application.
For a mainly street driven rig correct floor mounting is acceptable, even preferred. This will still allow frame flex(which they are actually designed to do).
On a hard core off road rig with a soft and articulate suspension frame mounted structure is going to be best IF it is designed and mounted CORRECTLY!
The structure should mount on and if not, in close proximity to suspension mounting points.
Welding "standoffs" to the side of a framerail is a poor plan. You induce a weak point in a frame by concentrating a load in one area, especially the side of a frame rail.
You are far better served by welding a mounting plate to a rail again observing the "x3" rule, then welding the stand off to the mounting plate.
Spread the load!
The best frame plates I have seen are actually "saddle" plates that saddle the top of the rail and wrap around the frame. This transfers a fair amount of the load to the top of the frame, where it can take a downward load better than the side of a frame.
They are a pain to do and the body must be raised to install but they do result in the best design.
Tube fitting.
Don't squish a tube to get it to fit, fish mouth it correctly.
Tubes should be fit very closely, a pocket knife blade should not slip between a joint.
Nuts and bolts are NOT filler material!
Properly fitted tube intersections should require a minimum of filler material.
Avoid the weld and grind trap.
I have seen joints that had been welded, ground, welded, ground so much that the metal was essentially cast iron, the carbon had been burned out of it.
Drill a 1/16" vent hole above a closed intersection, it will allow the hot gasses an escape instead of blowing out the weld bead.
Tubular gussets weld all the way around, plate gussets leave the end unwelded- it is a stress point.
Not trying to be critical, I just thought of a few things regarding the subject.
Stay safe.
I have seen the unfortunate results of poor cage design and fabrication.
In perusing a lot of posts here and pictures as well I have a plea: if you don't know anything about structure design, stress loading both static and dynamic, proper material selection and correct fitment a rollover protective structure is not the place to learn.
It is also not the place to learn welding.
A few things that come to mind:
Exhaust tubing and water pipe belong in their respectively named places, they have no place in structural applications.
Plan your design carefully, extra tubing doesn't make a structure stronger. A properly designed structure will only have what is necessary to provide the strength required for the loads anticipated.
An intersection of tubing in the middle of an unsupported span is a failure point and serves no structural purpose.(seat mounting tubing aside)
Strength is in angles. Look at the frame for a fabric covered aircraft or a NASCAR car. You will discover that they are a series of intersecting triangles, one member bears on another to spread the load and stresses.
Bends in the middle of a span are a built in failure point.
Bends should never be in bracing structure.
About the only bends that should exist in a rollover structure should be in the front and rear main hoops.
Slight bends for headroom, door bars and such are acceptable but should be gusseted.
Bend radii- a good rule of thumb is to multiply the tube diameter by 3. This will typically provide a safe radius. Any less and in an impact/roll it will have tendency to kink and collapse.
Mounting points- poor mounting is just as dangerous as no rollover structure. What good is a structure when you go over on your lid and the structure just punches through the floor?
Again the "x3 tube diameter" rule holds well here, if the mounting plate dimensions are 3 times in length as the tube diameter you have a good plate size.
How many of us have seen a structure mounted to a body only to see just washers on the underneath side of the body?
You want a plate on the bottom that preferably is larger than the mounting plate on the top side.
If both the bottom and top plates are the same size all you have done is build a shear point. It will cut right through the floor in an incident.
Floor mount or frame mount?
I think we all can agree that frame mount is best right?
Maybe. It depends on the application.
For a mainly street driven rig correct floor mounting is acceptable, even preferred. This will still allow frame flex(which they are actually designed to do).
On a hard core off road rig with a soft and articulate suspension frame mounted structure is going to be best IF it is designed and mounted CORRECTLY!
The structure should mount on and if not, in close proximity to suspension mounting points.
Welding "standoffs" to the side of a framerail is a poor plan. You induce a weak point in a frame by concentrating a load in one area, especially the side of a frame rail.
You are far better served by welding a mounting plate to a rail again observing the "x3" rule, then welding the stand off to the mounting plate.
Spread the load!
The best frame plates I have seen are actually "saddle" plates that saddle the top of the rail and wrap around the frame. This transfers a fair amount of the load to the top of the frame, where it can take a downward load better than the side of a frame.
They are a pain to do and the body must be raised to install but they do result in the best design.
Tube fitting.
Don't squish a tube to get it to fit, fish mouth it correctly.
Tubes should be fit very closely, a pocket knife blade should not slip between a joint.
Nuts and bolts are NOT filler material!
Properly fitted tube intersections should require a minimum of filler material.
Avoid the weld and grind trap.
I have seen joints that had been welded, ground, welded, ground so much that the metal was essentially cast iron, the carbon had been burned out of it.
Drill a 1/16" vent hole above a closed intersection, it will allow the hot gasses an escape instead of blowing out the weld bead.
Tubular gussets weld all the way around, plate gussets leave the end unwelded- it is a stress point.
Not trying to be critical, I just thought of a few things regarding the subject.
Stay safe.
