Is there a reason modern trucks don't come with fully sealed frames to prevent rust?

Why are frames made of hollow tubing? Why not C-channels or I-beams or H-Beams? They would allow the whole frame to be painted with things like POR-15 and no places where water could stand for long periods of time.

Related to this: Is there any reason why I shouldn't weld shut every hole in the current frame (after thoroughly letting the frame dry in a low humidity environment?

 

Cost.

Yes, when water gets in it won't be able to get out.

 

The old Toyotas had fully channeled frames but they cost more, weighed more and did less.

 

Did less as in provide less strength?

 

Fully boxed frames are better, and rust out less than current Tacoma/Tundra C-channel frames that have multiple internal strength plates welded and riveted inside it, the rust grows between the plates and frame.

Just get a case of Fluid Film, coat inside your frame ASAP, it will be good for 20 years+

Check out my video series on How to Save Your Toyota Frame

 

You need flex in a truck. If you don't want flex, go unibody like a Honda Ridgeline. Pick.

 

People have very short memories... Does anyone remember when trucks (and cars) had bodies that would rust out but they would keep going until the drivetrain wore out? Because the frame was the last thing to give any problems? You look at any vehicle built in the fifties or sixties, parked in somebody's back yard or out in a field... or in a wrecking yard... and more than likely the frame on that once pristine vehicle is still usable.

Anybody ever heard of "planned obsolescence"?

 

There’s a YouTube video about this. Fully boxed frames are too rigid and give a very bouncy ride and the amount of body body roll increases. The c channel frame gives a balance of rigidity and flexibility.

 

The Tacoma has basically a hybrid frame as its boxed in the front and C Channel under the bed, probably a good balance of flexibility and rigidity as you said.

 

My T100 has a boxed frame and it definitely isn't too rigid. If I'm rough on the clutch the bed will literally hit the cab.

I don't buy that boxed frames are too stiff, because you can easily change the dimensions of the box profile to get the same deflection and yield strength for less weight, but more cost.

 

Geography makes a big difference, here in the Northeast a car from the 60s that has not spent most of its life in a garage will be lucky to not have the entire back seat rusted out.

 

As a structural engineer, I can tell you that ductility can be beneficial in a structural frame of any kind, especially a truck frame.

In general, load follows stiffness. Having a moderately ductile truck frame in the right spots is a good thing when it comes to making sure loading is distributed correctly while ensuring that certain parts of the frame deflect where you need them to. Engineering in a nutshell lol.

Personally, I think Toyota did a fine job engineering the Tacoma & Tundra frames for load distribution and deflection. Good balance between cost and strength while allowing the frame to deflect in the back to allow maximum traction on the road.

Now, as far as the corrosion issues go, no comment. But if I were to stamp a frame design for load bearing & frame flex for these trucks, it would probably look pretty similar to Toyota’s current design.

 

You nailed it tho - a boxed frame requires more material per section foot than a channel frame.

Yes, you can play around with the geometry of the boxed frame to scale down the stiffness (in^4), but a channel frame can be manipulated to utilize cross bracing to stabilize the top & bottom flanges of the channel itself. Gives you a lot more options for stiffness control and changing the effective unbraced lengths of the frame members IMO.

 

A few years ago someone posted an interesting crash test of heavy duty pickups. One had a C Channel Frame and the other two had fully boxed Frames, in a frontal collision the two boxed frame trucks had the bed slam into the back of the cabs whereas the C Channel trucks bed never touched the cab.

 

Doesn't ductility imply yielding without breaking, specifically in tension? You only want that in a collision to absorb energy. Otherwise you want just regular flex, to some extent.

 

Ductility in this sense is capacity for deflection of a member before yielding of the member material (ie - bending the material past a point where it won’t bend back). A highly ductile member will be able to bend quite a bit before being permanently damaged. Ductility is a function of the member cross section and material yielding strength. Doesn’t matter if it’s in tension, compression, shear, torsion, flexure, etc. I think malleability is related to ductility but I don't use it in design.

A common misconception is that rigidity and strength are the same thing. Rigidity is resistance to deflection where as strength is resistance to force loading per unit of area.

IE - a #4 piece of rebar will sag much more than a DF2 2x4 under load. The 2x4 will break way sooner than the rebar will if you increase the loading on each piece.

 

Galvanized frame would help.

 

Really depends on the vehicles wheelbase length, and suspension. I've ridden in full size trucks that owners totally boxed in the frames which did not ride bad, on the other had been in cj5 jeeps that owners had boxed in the frame, and they rode like a lumber wagon.

 

I use this (or similar) inside the channel on my 68 beetle. Maybe it would be a good thing to use against corrosion in modern frames.

Eastwood Internal Chassis Frame Black Coating 14 oz with Spray Nozzle Remove Rust Corrosion

$40 Eastwood Internal Frame Coating Aerosol | Internal chassiss Frame Rust Remover | Rust Converter and Prevention Aerosol Spray Undercoating for Vehicles |14 Oz, Black

https://www.amazon.com/dp/B00MROX1J4/ref=cm_sw_r_cp_api_glt_fabc_DZ5G16CEEY79X95RR3AW

 

Found nothing like cosmoline rp-342 better to coat a new rustless frame. Year later after a tough salty winter mine still looks like it came out of the toyota plant. Could never get the same results on my gen1 tundra when using fluid film.