21RBS bumper weight capacity????

[shovelhead]

I've seen some folks went for beefing up the CL rear bumper as well as adding a platform.
What did LL do to "beef up" the bumper?  Anybody have a feel for the standard 21RBS bumper carrying capacity if any?
I have a welder friend (does excellent aluminum work) that I'm thinking of asking him to beef mine up and add some
accessory capabilities. 

[charliem]

[font size="3"]Dave,

Here's what I had LL do when I purchased my 21RBS:

[a href="http://livinlite.proboards.com/thread/14/bike-rack-21rbs"]http://livinlite-owners.com/thread/14/bike-rack-21rbs[/a]

You'll find the drawing I gave LL. I was looking for a 4x4 cross section bumper that would withstand the torque of a bicycle rack.[/font][font size="3"] [/font][font size="3"]The placement of the added section and the method of welding shown is critical to this. [/font][font size="3"]LL followed my drawing exactly. [/font][font size="3"]At least one other forum member used this drawing when buying a CL and another forum member was able to duplicate this mod after purchase so it can be done afterwards. Let me know what you think.

[/font]

[admin]

The difficulty in doing this after production is with the strength. Since LL offers the rear deck rather than a rear accessory mount point all they need to do is extend the frame. I'd love to know if anyone has done the math to know what the doubled up 2x4 bumper will hold.

I didn't realize I basically did the same thing you had already done @charliem. Looking at the thread I remember now that you were doing exactly what I wanted but when it came time to get mine done I spaced out on the fact that you had already done it. The only thing I dont recall now is about the length of your extra 2x4 piece. On my rig, the frame is 2x3 and the bumper is 2x4, so I ended up indexing the extra 2x4 so it extends a little under each frame rail. Does yours do anything like that, or is your frame 2x4?

[charliem]

[font size="3"]Estimating the absolute strength of the beefed up bumper is beyond my limited structural engineering capability. The relative strengths are fairly simple: the vertical bending strength is slightly greater than 2X the OEM bumper; the fore-to-aft bending strength is slightly greater than 4X the OEM bumper. However, for our purposes, the real concern is the bumper's ability to withstand a torsional, or torque, load imposed by loads not centered over the bumper. In other words, loads from bike racks, carrier baskets, etc. I would not put too much faith in an exact calculation, such as a specific rating in pounds, because the dynamics of the ride would be impossible to guess.

I think the weak spot of the OEM bumper is the top weld holding bumper to the frame member as shown by the red arrow in the picture. Note: the picture has been artificially enhanced for visibility.

[attachment id="938" thumbnail="1"]

With a torsional load on the bumper this weld will tend to tear or peal, leading to the failure of the remaining welds. The additional parallel 2x4 member adds several welds to the frame in almost pure shear. That plus the added "twist" resistance of the resulting 4x4 member produces a very strong and rigid bumper. I can stand my 200+ pounds on the bike rack and feel no give. I also see almost no movement of the bikes from the roof mounted camera while traveling.

Sean was able to come close with his added shelf, but he was dealing with a 2x3 frame. Magoo had the factory duplicate my mod and they did it after he bought it so he proved it can be done afterwards. I suspect his 13QBB has a 2x4 frame like the 21s. I think a competent aluminum welding shop could replicate this if care is taken to protect wiring, lights, etc. from heat.

[/font][a href="http://livinlite.proboards.com/thread/683/bike-rack-back-bumper-13qbb"]http://livinlite.proboards.com/thread/683/bike-rack-back-bumper-13qbb[/a]

[geo92128]

Our new 21BHS has the standard back bumper with no additional support. I asked Steve Morgan the service manager what I should do if I wanted to add a bike rack, He said to get a piece of steel the same height as  the bumper and put a piece on each side buffered with a piece of thin rubber to keep the steel and aluminum apart. He said to get a bolt on system that does not require drilling.

[charliem]

[quote source="/post/8942/thread" timestamp="1428366222" author="@monted"]Our new 21BHS has the standard back bumper with no additional support. I asked Steve Morgan the service manager what I should do if I wanted to add a bike rack, He said to get a piece of steel the same height as  the bumper and put a piece on each side buffered with a piece of thin rubber to keep the steel and aluminum apart. He said to get a bolt on system that does not require drilling.[/quote][font size="3"]Go at you own risk, Friend. All your dealer did was slightly mitigate the compression load and the dissimilar metals issue. He did nothing to address the basic strength of the bumper and its attachment to the frame. If the manufacturer won't warrant the installation where do you think your dealer will be when you get into trouble? Don't spend much time looking for him.........he'll be elsewhere.
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[admin]

@charliem, great explanation of the concepts here. For some reason the words escape me when it comes to addressing all the different points involved with this type of project.