RPG's "Stage 2 Kit": Rear frame support and Fox Air Bumps

[Blind1]

Big J, Thanks for the excellent write up. I have learned a lot, and really have a good idea on what I will be doing to my Raptor in the future. First I just got to fill up the piggy bank.

 

[debate]

Grade 8 hardware!

I checked some Grade 8 fasteners and they're up on the diamond tip ( Rc ) scale. The F150 frame steel, think it's 945 YLK, typically returns about an 80 Rb ( 1/16 carbide ball tip ) value.

http://www.fastnetdirect.com/refguide/CapScrewtapbolts.pdf

 

[debate]

Its locked to the frame via that grade 8 hardware. The nut goes inside the frame and the bolt bolts down to it, clamping the support to the frame from the outside. Once torqued down, this design actually ends up pulling the frame into the support, and locking it there in place.



Questions? Fire away!

Do those two bolts contact the frame?

 

[BigJ]

The flanged nuts do; they go inside the frame and clamp the materials together.

Can you give some context for your comments? I'm not sure where you're coming from or going with them?

If it helps... I'm a fan of grade 8 stuff because of my experiences in other arenas and applications. Basically, I want the truck to fall apart around the hardware I use to secure things like this. Its WAY overkill most of the time, no doubt (I don't think its way overkill in this case though; the torque spec on that bolt in the pic alone is 250ft/lbs.) But given what I've seen in 'the real world' in other fields, grade 8 stuff for me is all about peace of mind.

 

[debate]

Sure. Tempered alloy bolts would be a good choice for winch mounting. Applications where they contact the open grain of our frame steel might be the place for a low carbon bolt.

Or a different way of mounting the frame braces, all-together.

 

[BigJ]

Hmm... what are your concerns? What issues have you experienced?

And did you see this?


I'm not positive, but that nut is probably coated in zinc. So its the zinc that's coming in contact with the grain of the frame, no?

 

[debate]

This brace kit is close to what I want/need for my Raptor. The hardened bolt crossing through the frame and creating a load bearing spot is my concern because it could "tear" through the frame steel under impact; if it is actually carrying a load?

The silver hardware is grade 5, zinc galvanized. The gold hardware is grade 8, zinc chromate, I believe.

 

[BigJ]

This brace kit is close to what I want/need for my Raptor. The hardened bolt crossing through the frame and creating a load bearing spot is my concern because it could "tear" through the frame steel under impact; if it is actually carrying a load?

The silver hardware is grade 5, zinc galvanized. The gold hardware is grade 8, zinc chromate, I believe.

Gotcha. I think you're probably right on the nut.

Just to be really really clear (and you may already understand this), the bolt does not cross through the frame. It passes through the outboard side of the box, and the nut is affixed to that bolt inside the frame; in other words, the bolt passes through only one wall of the frame (the outside), not two walls (the second being the inside wall). So what you have here is a clamping action between the RPG mount, and one wall of the frame.

To my knowledge and understanding, the bolt is not a significant load bearing part of the design; its primary purpose is to hold the mount on, and resist its twisting while under load.

Here, I've tried to draw what's going on to the best of my understanding. (I gots the mad photoshop skilz!). I'll reference the numbers in the pic within the text below



As load is applied (#1), the mount will want to twist up and inward (#2). It cant twist however because of the top plate (#3) and those bolts holding the mount onto the frame, and so the load is spread along the mount's top wall and the frame its in contact with. Those bolts we're talking about basically stop the mount from just twisting right off, and if I had to guess, that "sheering" stress is why grade 8 bolts were selected. That's where their strength comes in.

But as load increases, that twist stress increases. As the load pushes up (#1), that top wall presses down harder and harder (#4). At this point in the discussion, the only thing stopping that twisting stress from basically collapsing the frame on top (#4) is the frame itself.

And that's where that cross brace comes in. Before stresses get anywhere near the failure point of the frame, that brace has spread and transferred the load over to the opposite side (#5). So on the other side you have the outside wall wanting to twist down (#7) and off the outside wall (#6), but it is prevented from doing so because the bottom of the mount is now spreading the load to along the bottom of the frame wall its in contact with (#8).

I hope that makes sense. Its hard to describe, but as I understand it, the load that used to be focused on one silver dollar sized spot, under a crumple zone on the frame has now been spread out to like 48 square inches of surface area on top of that crumple zone, as well as like 96 square inches of bottom frame on the opposite side, and all the secondary contact points between here and there (totally rough guesses based on my eyeball measurements).

RPG, by all means please correct me in anything I've said wrong. I'm just trying to explain what was explained to me.

 

[debate]

That looks like the magnesium brace for the 2013 Raptor XT!

Thank for the explanation, BigJ!

 

[BigJ]

Glad to help! It took me a minute to wrap my head around it as Corey was explaining. I just hope I got it right! LOL.