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.pdfI'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