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Old 08-04-2019, 04:01 PM   #1
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Beefing up rear bumper for bike rack.

First, I am not looking to debate a bike rack on a rear bumper - I've read plenty on the topic. I am planning on this mod and looking for advice strictly on if I should extend the 2x4 box tubing that I am working with:


My trailer has an I beam frame and has a 1 foot piece of 2x4 box steel welded to the end for the attached bumper. A hair more than 6 inches overlaps the frame. My plan is to add safety struts to the bumper side and also add two stout bolts inside the 2x4 box tube going into the trailer frame (green dots). The reason is that upon inspection the box tube is only welded along the back and front edges and partially along the top (red lines) and I want to ensure this doesn't become the weak link. I will be carrying on the bumper my spare tire (already there) and a thule bike rack with 2 adult and 2 kids bikes. Net weight is around 150# - 200#s. Tire and bikes are all chained down for theft and also in the event that something were to fail - i would drag them instead of setting them loose on my fellow motorists. Additionally I would store a 100 lbs. generator on a bumper mounted carrier when parked (never while traveling).


My question is specifically since I am in here making the mod - I could without much trouble add another steel bar 4 inches by say 2 foot or so, into the box tube and extended out further down the frame, shimming once outside the tube. This would definitely add to the strength, however according to some online resources I have played around with, that 12" piece of tube has a load strength already over a thousand lbs., so should be sufficient. The reason for not extending the tube is the added hassle of drilling more holes into my frame if not required.


Thoughts? Thanks in advance.
Mike
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Old 08-04-2019, 04:04 PM   #2
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[QUOTE=
My trailer has an I beam frame [/QUOTE]


Real "I" beam or a manufactured I beam?
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Old 08-04-2019, 04:13 PM   #3
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Real "I" beam or a manufactured I beam?
Manufactured as in three pieces of metal put together correct? As best I can tell they look real, no seams or welds that I can tell.
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Old 08-04-2019, 07:11 PM   #4
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Looks like a real I beam to me! I'd run the attach part you are using forward on the beam as much as you have room/material for. If you attach it properly, it will spread the load out over that extended length and be less stress on the end of the I beam.
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Old 08-04-2019, 08:06 PM   #5
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Looks like a real I beam to me! I'd run the attach part you are using forward on the beam as much as you have room/material for. If you attach it properly, it will spread the load out over that extended length and be less stress on the end of the I beam.

That's the heart of my question. I am not going to grind off the welds as I would only gain 2 inches or so backward movement. So as it stands now I have half of the 12" tube against the frame and able to be bolted in... I can cut and slide another piece of steel into the tube and extend down the I beam, but I'm not totally sold that it is even needed.
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Old 08-04-2019, 09:15 PM   #6
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Put the bolts through. Drill bolt sized hole first then find spacer that the bolt will fit through and is just long enough to reach the back plate of the box tube and be flush with front surface. Drill front hole out so spacer just fits. Grind spacer so it does not protrude while in place.

Install a plate washer and spacer on bolt, insert in hole(s), secure with self locking nut.

Box beam won't go anywhere.
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Old 08-04-2019, 09:35 PM   #7
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It's hard for us to see any of the welds with the red marks on them but are able to see the one on the back side at the end of the I-beam so your going to have to make the call whether you trust the welds. If not, drill and bolt.

Adding an extended piece to what is there already will gain you nothing for carrying a load of bikes. That box tubing (and that I-beam frame) is good for way more just as it is. The attaching points are all you need to be concerned about.
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Old 08-04-2019, 09:48 PM   #8
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I smashed my tin foil OEM bumper flat against a light pole concrete base right after I told Liz I didn't need a spotter for this backup..... #$@[×%. Yep, I never say that any more
I went to my local welding shop and had them cut off everything to do with the bumper, supports and all. Then he replaced them with much thicker tubing and added more braces along with a receiver support welded to the frame for whatever rack I want. The rubber end caps still fit in the 4x4 square tubing, but it is three times thicker. Everything welded.
All that for around $150 including the steel.
Now if I back into anything the bumper won't bend, but don't know about the rest of the trailer
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Old 08-05-2019, 09:18 AM   #9
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Originally Posted by TitanMike View Post
Put the bolts through. Drill bolt sized hole first then find spacer that the bolt will fit through and is just long enough to reach the back plate of the box tube and be flush with front surface. Drill front hole out so spacer just fits. Grind spacer so it does not protrude while in place.

Install a plate washer and spacer on bolt, insert in hole(s), secure with self locking nut.

Box beam won't go anywhere.

Thanks for the feedback. I'm actually going to drill in from the I beam side and into the flush wall of the tube, and only that wall. I will be able to fish and bolt a 3/4 inch long bolt between the flush face of the tube and the I beam. through the back of the open tube. This + the existing weld should be more than enough to accomplish my goals. I do not want the bolt to span the steel tube - more hassle to deal with aligning a second hole and spacers as you noted.


Quote:
Originally Posted by 5picker View Post
It's hard for us to see any of the welds with the red marks on them but are able to see the one on the back side at the end of the I-beam so your going to have to make the call whether you trust the welds. If not, drill and bolt.

Adding an extended piece to what is there already will gain you nothing for carrying a load of bikes. That box tubing (and that I-beam frame) is good for way more just as it is. The attaching points are all you need to be concerned about.

Thanks for the reply. Those welds are sloppy, but so far have held my spare without issue - the bolts will be to supplement them. I will trust them once I back them up with a few bolts!

More so, you and TM seem to confirm my assessment that the box tube, with proposed supplemented bolting, can handle the task without needing to be extended.


And Wildcat - always an option to redesign the back, but if I can work with what I have, and the tube will hold, I can do it in my driveway for free!
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Old 08-05-2019, 09:58 AM   #10
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Some info that may be...or not be helpful to you. I have roughly the same set up on my bumper of our Cherokee. They have the folding cargo rack bolted to the added box frame with double "U" type bolts. Cargo capacity with that setup is 200 lbs. So that part of the frame is strong enough.
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Old 08-05-2019, 11:17 AM   #11
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Pics of my factory rack rated for 200lbs. If you look close there is minimal welds on the frame. Wildcat Maxx

https://photos.google.com/album/AF1Q...ZD1s1kEdoz-xJ6

https://photos.google.com/album/AF1Q...cEl8iKs67tGuuK

https://photos.google.com/album/AF1Q...Jy8VxkiLT_GQ-A
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Old 08-05-2019, 01:23 PM   #12
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Cheap bumper mod with 2'x6'x1/8' stainless, total cost $12.00Click image for larger version

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Old 08-05-2019, 05:51 PM   #13
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Quote:
Originally Posted by Kacz View Post
Thanks for the feedback. I'm actually going to drill in from the I beam side and into the flush wall of the tube, and only that wall. I will be able to fish and bolt a 3/4 inch long bolt between the flush face of the tube and the I beam. through the back of the open tube. This + the existing weld should be more than enough to accomplish my goals. I do not want the bolt to span the steel tube - more hassle to deal with aligning a second hole and spacers as you noted.




While the welds don't look especially pretty, I think a lot of people underestimate the actual strength of fillet welds. Every square inch of "weld" provides almost the same strength (per square inch) of the surrounding steel. With a 200# (or so) load on your rear bumper, centered about 1 foot back from the weld points is only adding 200 ft/lbs or force on the welds. Will these welds pass inspection for a marine vessel or skyscraper? No, but then again they don't have to carry the same loads.
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Old 08-07-2019, 05:04 PM   #14
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This may give you an idea. We added a receiver hitch, welded to the frame, and we have a Reese hitch haul, with a storage box.

I wasn't completely comfortable with the idea of the welds and the weight. So, I added the 9/16" dia ubolts on each side to secure the welds. They are both rated at over 3000 lbs with a solid steel plate on each.

I purchased these from Indiana U Bolts, you can access them on line and get whatever you need. They have about anything you would need to secure a hitch for bikes.
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Old 08-07-2019, 05:29 PM   #15
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One thing worthy of mention is the fact that the welds attaching rear bumpers are only part of the load carrying equation.

You have to take into consideration the load capacity of the "I beams" or "channel" that form the frame and how far you are from the rear most support point (spring bracket, etc.).

As ll kinds of reinforcement can be added to hold the bumper in place but too much added load might end up with the frame bending at the fulcrum point.

Common sense is a requirement.

This is MOST important if the frame is fabricated I- beam which uses lighter metal than extruded/rolled I-beam.
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