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az supporter
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Joined: Wed Sep 15, 2010 7:50 am
Posts: 228
Location: Cairns
Vehicle: 91 Tin Top, 04 Jimny

Post Posted: Mon May 21, 2018 9:38 pm 
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Hi all,

Looking at making up a 'bullbar' of sorts for the Jimny (non-vvt, m13a) to help in the case of an animal strike when out in the bush.

In terms of design specs I'm looking at the following key requirements:

1. Protect radiator in event of small-medium animal strike (e.g. kangaroo)
2. Have sufficient crumple as not to affect airbag action in the event of a serious collision
3. Be ADR compliant
4. Have sufficient airflow (at least equal to existing front end)
5. Have sufficient recovery options
6. Be lightweight
7. Be able to be laser-cut out of sheet and bent up easily

At the moment I am thinking of simply making two cross-braced A-pillars that extend from bullbar mounts attached to the chassis (see below). This would sit behind the existing plastic bumper but extend out of the top to protect the upper portion of the radiator.
Attachment:
001_ChassisMockup.PNG

Attachment:
002_BaseAttachment.PNG

So far I have started a mocked up the bullbar mounts (unfinished) and would appreciate some feedback or suggestions.
These will build off the pre-existing tie down mounts and use the tubular crossmember at the front of the Jim and retain the use of the tie down points - I am unsure as to whether it's wise to use these as a recovery option.
I was thinking 5mm plate for the two bullbar mounts, although this would obviously add a fair amount of weight. The alternative would be to add some extra geometry by bending and boxing some sheet although I do want to minimise areas that can trap mud/water.
Attachment:
003_BullbarAttachment.PNG

Ignore the 6 holes at the front, these are for me when I cut this out of cardstock to get a rough idea of where things line up as the chassis model is not 100%.

Cheers!


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az supporter
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Joined: Wed Sep 15, 2010 7:50 am
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Location: Cairns
Vehicle: 91 Tin Top, 04 Jimny

Post Posted: Fri May 25, 2018 10:36 pm 
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So as part of the design I'd like to have some failure points in the bullbar cross-members.
This would be so that the middle of the 'bullbar' could break away in the event of a serious impact and hopefully minimise effects on the intended crumple action of the front of the poor Jim.

I would like to try and calculate the impact force of an ~100kg object at ~100km/h and use bolts that would shear in an impact of this magnitude to attach the cross-members.

For example, in the below I have placed a 2.5mm folded sheet cross-member between the two (incomplete) bullbar uprights.
For those not familiar with the Jimny, the two large rails are representation of the chassis rails that extend out in front of the vehicle (bottom left of the image is towards the front of the vehicle).
The factory tie down points can be seen above in the first post.
Attachment:
004_BullbarAttachment.PNG


A quick back of the envelope calculation of 100 km/h (~28 m/s) * 100 kg gives 2,800 kgf (~27.5 kN)

If this calculation is correct (and I'm assuming I've done something horribly wrong) then I just need to use a single bolt that has a shear strength close to 27.5 kN.

Using the numbers provided in the pdf linked below, I'd likely need a single 8mm grade 8 bolt to roughly approximate the desired failure stress (with shear acting on the shank; 25 kN).
http://www.blacksfasteners.co.nz/assets/BoltShearCapacity_14-15.pdf

I'm not sure how multiple bolts affects the overall shear strength, but I'd assume it's not 100% cumulative although each successive bolt would increase the overall shear strength?
In my design, I currently have 3 bolts either side of the cross-member attaching to the uprights in addition to two bolts per side attaching to brackets (which are in turn attached to the chassis). Mounting holes for these are shown with red arrows below.
Two 10mm bolts per rail will be used to fasten the 'bullbar' to the chassis. I don't want these to shear off.
Attachment:
005_BullbarAttachment.png


I'm thinking this is probably a bit of overkill and I can remove these brackets as the cross-member itself would likely stiffen the whole front end considerably.
Attachment:
006_BullbarAttachment.png


If anyone here is a numbers wizard or deals with this stuff on the regular I'd appreciate a point in the right direction
I'm also open to criticism from the peanut gallery as I'd like to make this design as safe and efficient as possible

Cheers


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Joined: Fri Nov 13, 2015 9:08 am
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Vehicle: sj80

Post Posted: Sun Jun 03, 2018 10:17 am 
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Do you really want your bar to fall off after an impact with a kangaroo? Yuo have done your calcs on an impact speed of 100kmh which means that after the impact you then have to deal with running over your bar at 100. Not an ideal situation to be in. Forget crumple zones, make it as solid as possible.

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Joined: Sun Dec 27, 2015 3:07 am
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Location: Wallington, Victoria
Vehicle: 1993 Suzuki Sierra 1.3 SWB

Post Posted: Fri Jul 06, 2018 8:02 pm 
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you are thinking to hard, made the same mistake myself a couple of times, use the KISS principle, stock designs from local manufacturers are usually well proven, just copy them and add your own custom bits and pieces, easier and cheaper, (cheaper not necessarily less effective), just think why you want a bar and what you want it to do, regards.

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Location: Melbourne

Post Posted: Sat Jul 07, 2018 3:11 pm 
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I don't get it.

Marn has suggested the idea of the bullbat falling off. I don't think that's as much of a problem as some bit of it travelling backwards into the vehicle and potentially damaging whatever it is you were trying to protect. (radiator etc)

A genuine 100kg animal hit at 100km/h is a lot of force to dissipate. Look at the most successful bar designs (ARB) and you'll notice that the idea is to make the bar as rigid as possible in order to dissipate the force across a large area and across all of the mounting points as equally as possible. Consider your "designed to fail" bar and instead of the force being in the centre, now consider it under the headlight. This is now trying to peel the bar apart, separating the bar at it's "designed to fail" connection. Now all the force is transmitted into the wing and one side of the chassis, which will do a great deal of damage.

I don't think you can have it both ways - a bull bar isn't designed to dissipate force, it's actually designed to transfer it into the chassis, the theory being that's better suited than the panels to absorb it. I'm sure it's possible to design a bar to dissipate force if it becomes sacrificial, but what if you experience slightly more force than the design parameter? (As haas been suggested) does it fall off? does it travel into the front of the car wreaking havoc? what if the force is in an unanticipated direction? is it equally strong/dissipative in all planes? I think there's too many variables.

look at the reputation of Xrox bars and their bolted on wings and consider if that's how you want to head.

Personally, I'd buy an ARB bar and work out where you could reduce some weight with holes/slots/cutouts of you really want a project, rather than building a designed-to-fail bullbar.

Steve.

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az supporter
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Post Posted: Sat Jul 07, 2018 8:59 pm 
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Google bullbar testing. You need to airbag compatible bar, not just the mounting system. To be ADR compliant you need to have destructive testing. Good luck

_________________
greenzook89 wrote:
31zook wrote:
Makes me want something similar

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Joined: Sat Nov 05, 2011 7:55 pm
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Location: townsville

Post Posted: Mon Sep 09, 2019 7:03 am 
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Ive spent half my working life in the bullbar manufacturing business.
All that will happen is that the load will transfer back to the chassis section just behind your strengthened section.
Add in to your calculation the leverage at this point behind the strengthened section and it starts to unravel.
Putting bar work out front and above the lights add a lot to the leverage at the low chassis.
All you can do is work on is a bar to hold lights and push down trees.
If you hit a 100 kg roo at 100kph you will have chassis damage and then the insurance can take over.
All you can look to do is protect the radiator and maintain enough integrity to control the vehicle.
I have seen a patrol that lost steering due to a roo and ended up a total right off.

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Vehicle: MY13 2.4 SWB GV (02 Paj Io)

Post Posted: Tue Oct 01, 2019 4:56 pm 
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I want a reactive bullbar - like as in reactive armor on tanks.

you hit a roo - forward facing shaped charge goes off - DOUBLE WIN!!

yes i am joking

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