Front bumper build

[JAS]

Here is an update from this weekend - got the light hoops in and added the support structure. Here is what it looks like on the truck...



still a lot to do but it is getting there. I have updated the Photobucket album link from the first post with the details.

[studum]

Looking very true to your CAD design

[Morpheus]

Bumper's looking great so far! Looks very professional.

One thing I would question, if you intent on using the shackles for recovery, they don't look like they're very well supported (at least in the CAD drawing)

If you're pulling straight, will they support pulling a vehicle stuck in mud?
if you're pulling at an angle, will it twist the entire bumper assy?

/2c

[JAS]

Hey Mike - Here is what I am planning for the recovery point. It is 1/2" bar that is inserted from the back of the tube, welded on the back and front, with a physical feature that will prevent it from pulling through. The tube it is going through is notched at both ends and gusseted to perpendicular tubes.
I did spend a lot of time thinking about this as I don't want to cause any injuries or damage, and I don't want to deform the bumper with a recovery attempt. I would appreciate your (or anyone’s) input on this since this is the first time I am doing this.

[Jayman]

I'm no engineer, but it looks to me like you have the area fairly well supported, at least for a straightforward pull. The angled piece that you have running back from the top of the bumper back to the truck should support any forward tilting motion that you'd encounter from a pull in that direction.

Tough to say how it would do on a sideways pull, but none of the current bumpers available for the Xterras, be it first or second gen, are particularly good at sideways pulls. But you've covered this bit already in your original thread.

Anyways, looks awesome! Can't wait to see the finished product!

[Morpheus]

I'm not an expert on metal works by any means, but I would wonder, the tubing you are using is hollow, so there's got to be a point at witch the tube would start to collapse on its self in a pull situation. The trick would be to make sure there's no way that you wil exceed that threshold.

I would suggest that you put your material to a test, not test it on the truck. Take a scrap, set up your shackle mount the way you plan, mount it to a solid surface and hook up a winch and give it a pull.

[JAS]

Hey Mike – you have given me a lot to think about over the last 12 hours. I am definitely not an expert on this and I wish that I could consult an expert, but I do have a degree in Engineering Physics so I feel that I am at least competent to look at this problem from that perspective.

On the practical side I have consulted an expert who builds tubular race car frames, roll cages, and has even built stunt vehicles for movie applications. He looked at the design and material selection, chuckled, and said that I have absolutely nothing to worry about.

On the theoretical side I looked at it this way…
The material in question has a tensile strength of a conservative 20,000 psi. The dimensions of the recovery point (4.5” is the long rear dimension) spread the delivered force over a conservative 2 square inches suggesting that I could deliver 40,000 pounds of force before reaching yield strength. Now when you do a pull/yank, the force will be distributed throughout the structure of the bumper, meaning that the whole thing will flex to some extend and absorb the energy of the yank. When I look at how much force a recovery vehicle can actually produce I think the limit is the yank strap with breaking strength of 20,000 or less. I have never seen/heard of a new strap in good condition snapping during a recovery so I will assume that the bumper will never actually be exposed to 20,000 lbs, which would be about ½ of what I think the bumper will be able to take. I would definitely feel sorry for the drive train of the vehicle that is actually delivering 20,000 lbs of force at the far end of a yank strap. The breaking strength of 3/8 synthetic winch line is 19,000 lbs about twice that of what a off-road winch can produce. I just don’t see how a recovery vehicle can deliver enough force to the bumper which would take it beyond its yield point.

Now I am not sure about how my welding is changing the properties of the material, but I would assume that I am making it weaker. The design of the bumper is such that it takes advantage of physical features and relies on the welds to hold it in place, so I hope that I am safe there.
This is by no means a professional opinion, but the more research I do the better I feel about it. Doing the dynamic calculations is beyond my scope being 18 years removed from my formal education so I will not play around with the conservation of momentum and impulse calculations.

I would definitely appreciate if anyone out there has access to an expert that could weigh in on this.

[Scotto]

I think you'll be fine, buddy. Your degree in Engineering Physics, combined with your buddy's practical knowledge, in conjunction with some pretty straight forward math suggests to me that it's A-OK. Impulse calculations probably aren't necessary. I'd just skip over the conservation of momentum too.

You've built something that's probably stronger than the junk we've attached to our trucks. I doubt anyone who built my bumper ever tested it for anything more than bumping into small trees.


Scott

[Morpheus]

You seem to have thought it through really well.

I just thought it looked pretty!

[Bow_Tied]

The material in question has a tensile strength of a conservative 20,000 psi. The dimensions of the recovery point (4.5” is the long rear dimension) spread the delivered force over a conservative 2 square inches suggesting that I could deliver 40,000 pounds of force before reaching yield strength. Now when you do a pull/yank, the force will be distributed throughout the structure of the bumper, meaning that the whole thing will flex to some extend and absorb the energy of the yank. When I look at how much force a recovery vehicle can actually produce I think the limit is the yank strap with breaking strength of 20,000 or less. I have never seen/heard of a new strap in good condition snapping during a recovery so I will assume that the bumper will never actually be exposed to 20,000 lbs, which would be about ½ of what I think the bumper will be able to take. I would definitely feel sorry for the drive train of the vehicle that is actually delivering 20,000 lbs of force at the far end of a yank strap. The breaking strength of 3/8 synthetic winch line is 19,000 lbs about twice that of what a off-road winch can produce. I just don’t see how a recovery vehicle can deliver enough force to the bumper which would take it beyond its yield point.

You almost certainly know the following and most certainly are smarter than I am if you have a degree in physics, or anything for that matter. But I will say this for those following along that may not know:
Tensile strength refers to small specimen of material that is put into a test fixture and pulled apart until failure or necking. Along the way there is measurement of elastic deformation (the force that will stretch the material a little but not so much that it won't return to its original measurement) and plastic deformation (permanently deformed) called yield strength. This is over simplifying perhaps, but you get the idea.

Now with that said, Jas, your point referring to the tensile strength of the tube may be over-simplified. What may be called into question is not the tensile strength but rather the buckling strength of the tube considering the uniformly distributed load over that 4.5” length. To be overly simple about it, it is more of a question of beam deflection (of which the material property is a constituent of the beam the strength).

__________
^^^^^^^^^^^^^^^^^^^^^^

The carats ^ are the beam which represents the length of the tube. The underscore is the 4.5” pc applying a uniformly distributed load over its length to the beam. A free body diagram would be needed to determine the support points of the beam. I say this is over simplified as it really doesn’t account for the adjacent supporting structure.

Now I am not sure about how my welding is changing the properties of the material, but I would assume that I am making it weaker.

Properly applied, the weld will be stronger than the parent material. Properly applied includes proper joint preparation, weld technique, and weld materials/equipment among others. What can happen and I think you are alluding to it is the parent material may become weaker in the heat affected zone adjacent the weld due to heating. It happens, but proper prep work can help minimize it.

I would definitely appreciate if anyone out there has access to an expert that could weigh in on this.

I am no expert. But it looks like the bases are covered to me. You could further do weld calculations but in the end it is a pretty complex shape to assess with great accuracy. If you have access to finite element software it could give you a good idea of what you have for deflection.

If you were worried about it, and I wouldn’t be, you could add a support from the top of the 4.5” pc back to the frame mounting point. This tension member would make the effective length of the beam shorter. I don’t think you need to do this at all, the upper member you have there now seems close enough. Out of curiosity – that member appears smaller, is it also tube or is it solid bar? That member is in tension for extraction, but compression if you hit something which is very different (see also “Euler column”).

I am always amazed at how strong a few flimsy pieces of thin metal are when welded together in the right way.

Great work, keep us posted.

[JAS]

Not much to report from the last few day. I wish someone would have told me how reasonable laser cutting is - I just spent two nights hunched over 3/16 flat stock with a sawzall and a grinder making gussets and cutting my fog light brackets. A buddy of mine did me a huge favour and got my recovery points laser cut for me (he snuck it in on an order he had at a laser cutting place and didn't charge me for it). I picked these up today and when he told me how much they cost I just about crapped - told the wife that I had just spent that much just in beer the last two nights. Next time I am making anything out of flat stock I am definitely getting a quote for laser cutting.
I also checked cost and availability of 3/16 Black ABS with P&A plastics in Hamilton - $120/sheet (4 x 8) this is a lot more than I need but they only sell it by the sheet. If anyone has any need for it then let me know - we can split the cost. This will be used as the webbing between the tube structure. Thanks Ron for the tip on ABS - it definitely looks like the way to go for me.

I hope to have this all welded up and sent out for sandblast by the end of next week.

[JAS]

Hey Ron - that is a good writeup and is along the lines of what I was thinking. In the beam deflection scenario, the beam will fail on the tension side of the member, not on the compression side, so the tensile strength of the material is the best number that I can think of using in my evaluation. Since I am underestimating the tensile strength at 20,000 PSI and the area at 2 square inches I am taking the liberty of over simplifying the analysis. I also wanted to consider what is going to be delivering the force for the recovery and what is connecting a stuck vehicle to a recovery vehicle.

The tension/compression member from the top tube to the mounting bracket, as well as a few other pieces supporting the bumper shape from behind are 1"OD X .125 wall CREW tube, so yes it is smaller. This piece is heavily gusseted along its axis.

I totally agree with you on being amazed at how strong something can be made with such thin materials. Today's unibody vehicle are extremely strong yet you can bend the sheet metal with your hands, or the engineering students that are making popsicle stick bridges spanning over 12 inches which are supporting over a 1000 lbs.

[Morpheus]

Have you checked what a sheet of aluminum would be worth instead of the abs plastic?

Wouldn't abs plastic be brittle in the winter if you got a stone hitting it on the 401 ?

[JAS]

Ron actually turned me onto ABS and I am quite excited to try it out for a number of reasons:
- I can work it using my woodworking tools (table saw and router)
- I have no experience or tools to work with thin metal/aluminum and I don’t think I could do it justice welding it into position.
- from what I have read it is suppose to have excellent impact resistance, is heck of a lot stronger then I would have thought and is used extensively in automotive applications
- I don't have to finish it and if it scratches the color is homogeneous throughout the material (no rust)
- It is suppose to be easy to thermoform. I am hoping to use a heat gun and bend it to the shape I need.
-I am also hoping it will be quite light

Let’s hope that all of these points are true – I am convinced I want to try it but still quite nervous. At least if I screw anything up I will have plenty of spare material.

[Rev]

Here is an update from this weekend - got the light hoops in and added the support structure. Here is what it looks like on the truck...



still a lot to do but it is getting there. I have updated the Photobucket album link from the first post with the details.

This is going to be fantastic. I'm reading this thread with great anticipation.

[Morpheus]

If you were using an aluminum sheet, you'd probably pop-rivit it in place, not weld it.

and you can always swap out the filler material if you change your mind down the road too.

[JAS]

Rev - thank you for the compliment. There will be significant progress next week with pictures to follow.
Mike - I have not thought of riveting the panels into place - I will give that some thought this weekend as I will be stepping away from the project. Once again you have given me something to think about and I really appreciate your input.

I have updated the Photobucket link at the start of the thread with a few more pictures.

I am also very excited about the fact that I just called in another favour and a friend of a friend is going to bend up a rad skid plate for me for free. There have been a bunch of friends that have made this build go smoothly while controlling the costs. They will all get a plug before the build is over.

Here is what the frame attachment looks like with a gusset tacked in...

[JAS]

This job is becoming an exercise in patience. I wish I had more of an update as I have been working on this most of my available time, but the bumper is pretty much looking the same. The latest challenge was getting the recovery points into place. The rough opening was cut with a grinder and filed to final dimension by hand. I got my plastic and have also started playing around with that. I was hoping the end of this week for sandblasting but that will be delayed by about a week.

Here is what the recovery points look like at this point…

[HolidayNation]

Farty process or not, the thing is looking awesome man....

[Bender]

Farty process or not

Farty?