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Best Practices for Unibody Reinforcement


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#1 Mary Maxey

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Posted 21 August 2014 - 11:58 AM

Am putting unibody reinforcement on 2000 XJ. Instead of welding can I use auto adhesive (3M) with rivets or self taping metal screws for attaching the reinforcement to the body?

 

 

Mary



#2 Moses Ludel

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Posted 21 August 2014 - 02:19 PM

Hi, Mary, and welcome to the forums!  You bring a very interesting question.  If you're seeking a welding alternative for your reinforcement project, there are some choices.

 

In the body/collision industry, welding is the common choice for repairs or section replacements on a unibody frame/chassis.  Welding on sheet metal, even the heftier gauge of the XJ Cherokee's unibody, does have challenges, though.  The weld and heat affected zone (HAZ) adjacent to the welds is often troublesome.  Many repairs result in "embrittlement" caused by the heat-up/cool down hardening of the sheet metal.  This inevitably leads to cracking, especially on a trail Jeep 4x4 subjected to flexing, pounding and torque stresses!  Sometimes the sheet material, if originally hardened or metallurgically alloyed for more strength, can actually soften from the welding process or the introduction of the wrong welding filler material. 

 

Cautions:  For those welding on a unibody chassis, drilling holes in the add-on piece or repair section, then plug or buttonhole welding the new piece to the cleaned unibody, is considered an industry common practice.  Under no circumstances should a sheet metal component be "T" or butt welded to a structural sheet metal member.  Here is a quick overview and perspective from the American Welding Society (AWS).  Take the time to read this if you plan to work with automotive sheet metal:  http://www.aws.org/w...002/03/feature/.  Also see the Miller Welding guidelines at:  http://www.millerwel...G-TIG-GMAW-GTAW.  Miller follows the I-CAR standards.  I-CAR has strict procedures for automotive sheet metal welding:  https://www.i-car.com/.

 

So, to answer your question about alternatives, I can share that several suspension lift kit manufacturers have employed the use of self-tapping (quality grade!) sheet metal screws for attaching support members to the XJ Cherokee unibody frame.  On our XJ Cherokee's Full-Traction Suspension 6-inch long arm kit, the braces that anchor the front link-arm suspension to the unibody/frame each have a single large through bolt.  Industrial grade sheet metal screws, provided in the kit, are a large part of the remaining support to the unibody/frame.  

 

There is no denying the impact force of the XR Cherokee's front leading link suspension arms against this screw-attached crossmember!  The axle thrust and bumping against trail obstacles drive all of that force through the lower link arms and into these two braces.  Here is one instance where heftier, hex-headed sheet metal screws have performed well.  I've rechecked the torque on this hardware several times over the 50K miles since the installation.  Surprisingly, none of these screws have sheared, broken or come loose!  Here's that installation, note #11, #12 and #13 illustrations:  http://www.4wdmechan...nsion-Lift.html.  Again, there is a hefty bolt through the unibody/frame, plus some crossmember hardware, so these braces are not entirely supported by sheet metal screws. 

 

There is a technique that I really do prefer for sheet metal.  Trade products like "NutSerts" and AKV thread inserts are a remedy worth considering.  There are even "factory" examples of this type fastener, and they will go into any drilled hole with room behind it.  (Some even work with a "blind" hole.)  Here is an AVK example available from MSC Direct:  http://www.mscdirect...331760&fromRR=Y.  Note the MSC 'Big Book' catalog listings for Nutsert and AVK type insert thread nuts:  http://www.mscdirect...331760&fromRR=Y

 

Do not be discouraged by industrial size thread insert kits and installation tools costing over $400.  As you can see from the first link, a tool can be under $10, the insert nuts are available from MSC, Fastenal and many other sources.  This is the best "bolt on" solution for sheet metal, as you end up with a nut embedded and secure, close to flush with the sheet metal surface.  (The sheet metal can even be countersunk with the correct body tool for a precise flush fit of the threaded nut insert.)  One more example at MSC:  http://www.mscdirect...331760&fromRR=Y.

 

Rivets are similar if you can find properly graded, large enough rivets and an affordable installation tool to match.  The disadvantage of a rivet compared to the Nutsert or AVK type products is that the rivet is "permanent".  The only way to remove a rivet is to drill it out, and steel rivets, in particular, can eat up a lot of drill bits in the process!  I'd lean toward the thread inserts, and if this reinforcement is a stressed member, my choice of bolt diameter would be 6mm (1/4-inch) or 8mm (5/16") stud size.  A bolt-on reinforcement member can be removed for chassis and other service work.

 

"Structural member" is an important concept.  If you reinforce a sheet metal section that supports critical suspension, powertrain or body safety elements, be sure to improve on the structural integrity and stamina, do not detract from it.  A classic situation with unibody "stiffening" and bracing is to make a given section too rigid.  On an engineered chassis, added stiffening at one area can cause force to transfer to other areas, placing more stress on the zone that becomes stressed!  Be discrete with spreading the stresses evenly and allowing the unibody to behave as it was intended to do.

 

As for adhesives, there are OEM areas of these vehicles that often have members "glued" to sheet metal.  However, in looking at the best products 3M offers, they are intended for attaching body panels like door skins and gluing at other non-structural areas:  http://3mcollision.c...sive-38315.html.  This is obviously tough stuff, but not strong enough to be considered the same as a weld or proper attachment method at a structural section of the unibody—or in your case, for attaching a metal part intended to stiffen or reinforce the unibody frame/chassis.  If you were simply doubling up door skins, that might be different.  Here is a direct quote from 3M:

 

 "This product is not intended to bond structural components of a vehicle such as pillars, rockers, or frame members. If doubt exists as to whether a particular component is structural, then that component should be welded."

 

I trust this helps clarify.  I'd be pleased to take our discussion further.  Thanks for joining us at the forums, we look forward to your participation!

 

Moses



#3 Mary Maxey

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Posted 22 August 2014 - 05:16 AM

Thanks

 

I believe that I will use the thread inserts.



#4 Moses Ludel

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Posted 22 August 2014 - 07:05 AM

Good approach, Mary...Make sure the inserts, bolts and lock washers are graded hardware.  I would want a minimum of Grade 5 U.S. or 8.8 Metric, decent tensile strength and still ductile enough for twisting and jarring.  If the fit is more rigid, I would consider Grade 8 U.S. or 12.9 Metric.  If the thread inserts are Grade 5 or Metric 8.8, a bolt and washer match would likely work well.

 

When you drill the holes, hit the bare edges with a primer/sealer to prevent rust formation.  The inexpensive insert installation tools work fine.  Unless you're doing this work day-in, day-out, the heavier duty insert installation tools are not cost effective.

 

Let us know how this turns out.  It would be great to see some photo steps of your installation!

 

Moses




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