[cbattles]

Question for you guys building bumpers.

I get the whole stout thing, and usually in my opinion, the stouter the better. But I have to think about what's holding all of that heavy duty bumper. If the bumper itself isn't crushing and absorbing any impact, then it seems that the impact is just gonna start to move back until it hits a weak link.

Meaning, if you're not going to significant lengths to strengthen the unibody behind the bumper, is an extremely strong bumper really the best thing for these vehicles?

Just looking for some input...

[Matt Gertsch]

It's best to allow the unibody to absorb forces the way the engineers designed it to, but fortify the bumper to resist impacts and deflects as much stress away from the unibody as possible.

I have some prints for good XJ frame tie ins. PM me your email address and I'll send it to you.

[wingless]

Changes to the bumper can affect the force detected by the air bag sensor and change the threshold for tripping the system.

[cbattles]

Originally Posted by Matt Gertsch

It's best to allow the unibody to absorb forces the way the engineers designed it to, but fortify the bumper to resist impacts and deflects as much stress away from the unibody as possible.

I have some prints for good XJ frame tie ins. PM me your email address and I'll send it to you.

I agree and it sounds like you agree with me to a certain extent.

I'm actually currently building a light duty bumper for my ZJ. I plan to add reinforced tow points, and a reinforced bar behind the light duty bumper. I plan to add these as the project progresses and I'm able to reinforce some target areas of the front of the unibody frame.

The core aesthetic part of the bumper is actually boxed 1/16" steel - in my opinion strong enough to act as more protection than the factory styrofoam, but light enough to allow for the natural course of action during a collision.

PM Sent.

[Matt Gertsch]

Originally Posted by wingless

Changes to the bumper can affect the force detected by the air bag sensor and change the threshold for tripping the system.

The sensor for airbags on most Jeeps is under the driver seat and is tripped by a sudden change in G forces, rather than an actual impact to the bumper.

[wingless]

Originally Posted by Matt Gertsch

The sensor for airbags on most Jeeps is under the driver seat and is tripped by a sudden change in G forces, rather than an actual impact to the bumper.

The G force the sensor detects will be different if the vehicle structure is changed from the factory bumper, changing the trip threshold.

[cbattles]

For me the issue isn't the airbag. As far as I'm concerned if I get hit hard enough in the front it'll probably go off, but I'd personally rather it didn't. So I guess regardless of what happens I'll expect it to go off but be pleasantly surprised if it doesn't. lol

My concern is that by building say a 1/4" plate bumper, mounted directly to the subframe, the 1/4" plate is going to be transferring that impact directly to the subframe. So, if the subframe isn't also made to be more stout, and it gets tweaked as a result of the impact, the results are actually going to be much worse than if I never put the extra bumper on.

Now don't get me wrong, I completely see the need for some stout tow points and some extra protection up front. I just doubt that building a bumper stout enough to provide tow points is actually the best thing in the long run. That's why I'm going with 1/16" steel for the bumper, and then providing some additional mounts behind the bumper for tow points. The 1/16" steel, when boxed, should provide adequate protection for most standard bumps/bruises, but still be weaker than the frame and not cause the issue that I'm concerned about.

If I had a full frame I'd bring on the beef, but....

[Scotch740]

I see where your comming from but you might consider 1/8" vs 1/16". 1/16" just seams really thin. It would be a shame to put a lot of work into a bumper just to put a large dent in it from a rock. Kinda a trade off but I would think a 1/8" bumper would still crumple sufficiently in a crash and it will certainly hold up better vsrocks or deer.

[bigjeep913]

it doesnt matter how you build it, 1/16" is way to thin. you really need to reconsider what you are choosing for materials. atleast build it from 1/8" or 3/16". that 1/16" stuff isnt going to do anything for impact resistance, just crumple up and then allowing whatever is doing the damage to get to the frame.

[Matt Gertsch]

My email doesn't want to send .PDFs for some reason. I'll get those sent out to you guys when I can get it fixed. If you search XJ Tech, you can find a link to Great Lakes XJ, where the image is hosted.

[BESRK]

1/8" should be a good compromise between strength and "crushability". You'd be able to dent 1/16" with a whack from a broomstick. When you look at most aftermarket ZJ or XJ bumpers, they tie into the frame/subframe in more places than stock in order to spread the load out over a larger area. Most of us build our front bumpers leaning toward offroad worthiness. The frequent bumps and scrapes make a thicker-than-stock metal selection the common choice. Otherwise, it's almost easier to retain a stock bumper and just add tow point brackets.