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Unread 02-04-2017, 02:56 PM   #1
cabman
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Default Custom Bumper Build on Stock Bumper Frame?

So I just found this thread but it sadly never got finished. I haven't been able to find any other 4th gen custom bumper threads: http://www.toyota120.com/forum/showt...?t=9094&page=4

I want to build a custom 4th Gen 4Runner bumper... here's the backstory: I bought my 2004 4Runner in October (moved up from a sedan that I beat up way too much) already outfitted with a lift and 33" tires and have spent the time since then wheeling it and trying to work out bugs that seem to suggest that the previous owner never really took it offroad.

When I bought the truck, the spare tire was just sitting loose in the back, so I immediately bought a hitch tire carrier from a friend with the intention of doing something else with it at some point down the road. It's definitely nice to not have the tire in the trunk, but the hitch carrier looks a bit goofy, kills the departure angle, and makes the truck feel really long when backing up / parallel parking. It sounds like it may be possible to get a 33" tire under the truck in the stock location without harming the departure angle much by deflating it and strapping it toward the rear end. If you've done that and are happy with it, let me know! The other obvious spot is the roof, but I like having space for a storage box and bike racks up there.

Anyway, I'm kind of thinking that the best solution might just be to put on a steel rear bumper with a tire swingout that I can store fuel and water on in addition to just the tire. But if I get a rear bumper, I have to also get a front winch bumper and I'm having a really hard time justifying the $3k bill that comes with all of that. I'm a mechanical engineer with access to fairly low-cost waterjet cutting, powder coating, CAD software, CNC milling, and high quality MIG welding equipment, so the obvious solution to me is to just build my own front and rear bumpers from scratch. It also gives me the opportunity to make the truck look really cool and unique.

The question I have with all of this, having never designed an offroad bumper before, is how to make it sturdy enough without completely killing all of the safety features of the car, without making it super illegal, and without building something that will crush in a way that totals the truck in a low speed accident. One of the main reasons I decided to get a 4th gen instead of a previous generation 4Runner was due to the newer safety features and airbags that come with a truck built this century, so I don't really want to compromise that. From looking around the market, it seems that ARB bumpers are at the safe end of the spectrum, having crash testing and crush cans to be airbag compatible. At the other end are some pretty rigid steel bumpers that mount directly to the frame. But has anyone ever tried just mounting a bumper directly to the existing factory steel bumper? Since the plastic bumper shell doesn't really do anything, it seems like that would give similar benefits to the ARB crush cans.

If you all can help me answer some questions, I will be happy to post pictures and build progress as well as any CAD and other design files I come up with during the process so that anyone with access to a welder can replicate it. Here are the main questions I've identified so far; if you think I'm missing stuff, feel free to add to the list:


Front Bumper:
1) Does anyone have CAD or dimensions already for the mounting locations, fender/hood base areas, wheel articulation, etc?
2) Is there anything that's actually special about the stock bumper in terms of crush characteristics? I haven't looked at it too closely yet. Is this project worth doing?
3) How much force can the stock bumper on the stock bumper bolts take? It looks like ARB adds a torsional support that helps when the bumper is being lifted or pulled on with a winch. Any ideas of how to do this without interfering heavily with the stock bumper/frame interface?
4) Should I contact my insurance company before trying to build a custom bumper?
5) Do all of these companies that sell custom offroad bumpers do anything to get their bumpers DOT certified or is it just a free for all?
6) What features and material thickness are important on an offroad bumper?
7) It looks like my stock bumper is moved up 1 bolt to account for the 1" body lift. Any ideas on dealing with this a better way?


Rear Bumper:
1) Is there a standard weld on pivot that supports the swingout action that anyone has used? What about something that makes the swingout lock at 90 degrees, 120 degrees, etc?
2) Should I try to maintain the stock hitch with the bumper going around it to avoid issues with making a custom hitch strong enough? Do low profile bumpers usually relocate the hitch or keep it in the same spot? Is there some sort of certification for trailer hitch mounting?
3) Has anyone tried putting really strong gas struts in the lift gate and putting the spare tire directly on the hatch?


Thanks for the help!
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Unread 02-04-2017, 04:15 PM   #2
cabman
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For reference, I just took a look at the stock bumper but forgot to take pictures. It looks like the cross member is made out of two independent steel tubes with a piece of sheet across the front. The connections to the frame have a small flare in similar to the ARB crush cans that looks like it would probably absorb some amount of energy in an impact.
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Unread 02-04-2017, 09:54 PM   #3
Jeffy
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Most people can easily fit a 33" tire in the stock location, the biggest issue is the panhard bar. I've got a 285/75/17 under there and it's fine. I have a 5th gen but many 4th gen guys have the same. You may need a ratchet strap to hold it tight to the rear and make sure it doesn't contact the panhard. Mine is only inflated to 20psi because that's more than enough for me to get off a trail or limp somewhere to get a real fix.

As for the bumper:
The stock bumper really isn't anything special, it's basically a big piece of plastic covering a fairly weak metal bar. The added protection from aftermarket/ custom steel bumpers more than make up for anything "special" about the stock bumper you take off. Also, they shouldn't affect your airbags any noticeable amount as the sensors aren't modified or changed at all when putting a new bumper on.

Here's 2 pics from my 4th gen wreck with a stock bumper. I'm no engineer but I really don't see how an aftermarket bumper would have made this situation any worse. The resolution isn't great but you can see where the frame bent under the stock bumper, which is right at the same spot that my aftermarket bumper now bolts on to on my new rig.

Just a note: Front airbags deployed in this crash but not the side curtains (head on collision)

https://farm1.staticflickr.com/428/3...c3e10a28_c.jpg

https://farm1.staticflickr.com/405/3...f5406691_c.jpg
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Last edited by Jeffy; 02-05-2017 at 01:47 AM.
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