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Unread 01-22-2010, 02:47 PM   #1
pulsepb
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My front tow point/ winch subframe build *pics*

I figured I'd show off my latest creation. As you can see for now I have shackles mounted through it for front tow points. Later this year I am going to build a front winch bumper and this will be my base. The bumper will slide over the outside of the two vertical bars and be held in place with two 7/8" bolts per side with a 1/2 inch bolt in the center of that tube

as far as contruction the base plates are 1/4 inch, the vertical plates are 1/2 inch and the tube tying them together is .120 wall DOM. The hardware is grade 8 with 1/2" and 7/16' diagonally from each other.

original concept

changed to due to constructability issues
what i actually went with












material cost was about $20 and the grade 8 hardware was about $20


Last edited by pulsepb; 01-22-2010 at 02:54 PM.. Reason: bad image tags
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Unread 01-22-2010, 03:54 PM   #2
CJ7-Tim
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Creative design. Nice machine work and welding.

Consider adding some gussets from the .120 wall DOM to the clevis plates.
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Unread 01-22-2010, 04:07 PM   #3
WhiteOut
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When you build the bumper and mount it you should really change the mounting. 3 bolts per side is no where near enough. The ARB had 4 through the frame rail, 2 through the front and 2 on the bottom per side, them 3 bots per side holding the bumper to the mounts. All 3/4" grade 8 hardware IIRC.
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Unread 01-22-2010, 07:18 PM   #4
pulsepb
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Quote:
Originally Posted by CJ7-Tim View Post
Creative design. Nice machine work and welding.

Consider adding some gussets from the .120 wall DOM to the clevis plates.
Thanks

funny you should mention that, my roommate and I discussed the merits of running the DOM bar at all.

Quote:
Originally Posted by Fraser_Brown View Post
When you build the bumper and mount it you should really change the mounting. 3 bolts per side is no where near enough. The ARB had 4 through the frame rail, 2 through the front and 2 on the bottom per side, them 3 bots per side holding the bumper to the mounts. All 3/4" grade 8 hardware IIRC.
do you have a picture? I dont quite follow the placement of the bolts as you described?
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Unread 01-22-2010, 08:39 PM   #5
kwsamms
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That is a really cool design...may have to try something like that for my ZJ! Got any drawings? Haha
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Unread 01-23-2010, 12:42 AM   #6
ratmonkey
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You only mounted to the bottom of the rail? If so that's not nearly good enough for a winch bumper. You need to "box" the rail with your mounts.

The 7/8 bolt hole is too big for the plate you used. You'd be much better off using 4-5 1/2" bolts per side to mount the bumper to your frame plates. It'll spread the stress concentration over a wider area. You'll have more peaks, but they'll be smaller.

Its all about spreading force over the widest area possible.
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Unread 01-24-2010, 07:55 PM   #7
pulsepb
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Quote:
Originally Posted by ratmonkey View Post
You only mounted to the bottom of the rail? If so that's not nearly good enough for a winch bumper. You need to "box" the rail with your mounts.

The 7/8 bolt hole is too big for the plate you used. You'd be much better off using 4-5 1/2" bolts per side to mount the bumper to your frame plates. It'll spread the stress concentration over a wider area. You'll have more peaks, but they'll be smaller.

Its all about spreading force over the widest area possible.
If you refering to my 7/8 shackle pin holes. Id have to disagree. Ill calculate the bearing stress later but im confident it will come out to be more than sufficient.

Quote:
Originally Posted by Fraser_Brown View Post
When you build the bumper and mount it you should really change the mounting. 3 bolts per side is no where near enough. The ARB had 4 through the frame rail, 2 through the front and 2 on the bottom per side, them 3 bots per side holding the bumper to the mounts. All 3/4" grade 8 hardware IIRC.
as far as my mounting to the frame Im confident it is sufficent as well. The 1/2 bolts are torque to a spec of 106 lb-ft and the 7/16' bolts to 69 lb-ft. this creates a sufficient pre-load where the friction of the plate on the frame is so great that the bolts will never see any shear effects as long as it it under the calculated load.

granted i used a rudimentary equation, I made up for this with conservative numbers. As you can see the preload applied creates a very large friction force. Also notice that that is per side.



As i mentioned Ill calculate the bearing stress on my 7/8 holes later.

various resources I used to make my major assumpions
Coefficient of Friction Reference Table - Engineer's Handbook
Coefficients Of Friction
Standard Metric Bolt Shank Dimensions

kwsamms:
I dont have any drawing but i could take some pics to give you ideas. Ive never looked at the frame of a zj so im not sure what you could do. maybe check out kevins zj shackle mount
http://www.kevinsoffroad.com/zj/rad%...t/f9baed38.jpg
Radiator Support for ZJ Jeep Grand Cherokees by KevinsOffroad.com Hard-KOR Products
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Unread 01-24-2010, 08:04 PM   #8
tarfin
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Nice work!
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Unread 01-24-2010, 09:15 PM   #9
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Looks good. Id agree that the bar between is beneficial - Id think itd work to strengthen the whole setup by essentially acting as a crossmember. Plus, if your pulling from just one of those shackles, i think it'd help to distribute the load.

Do you have it bolted through the holes for the factory tow hooks? Or where exactly did you bolt it to the unibody? - its hard to picture without looking underneath.

I know everybody here is critical of the strength of recovery point mounting. You definitely make a good point with the friction, that the bolts would be squeezing so hard that it shouldn't matter. I believe that the thin sheetmetal of the unibody is the main concern, as opposed to the shear strength of the bolts. Either way, i still think it'd be damn near impossible to rip that thing off.

Post more pics if you do any more fab work.
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Unread 01-24-2010, 11:37 PM   #10
ratmonkey
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I doubt you'll rip the mount from the frame rail. I'd bet good money you'll deform the crap out of it with a good tug or two with the single mounting plane like that though.

Your jeep, good luck.
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Unread 01-25-2010, 11:50 AM   #11
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The only way to find out is to lock the wheels on asphalt and tug away...preferable uphill. I bet that bad boy will hold without deformation......if the calcs are indeed correct +\- 10%.
Mike
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Unread 01-25-2010, 08:11 PM   #12
pulsepb
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Quote:
Originally Posted by nick62087 View Post
Looks good. Id agree that the bar between is beneficial - Id think itd work to strengthen the whole setup by essentially acting as a crossmember. Plus, if your pulling from just one of those shackles, i think it'd help to distribute the load.

Do you have it bolted through the holes for the factory tow hooks? Or where exactly did you bolt it to the unibody? - its hard to picture without looking underneath.

I know everybody here is critical of the strength of recovery point mounting. You definitely make a good point with the friction, that the bolts would be squeezing so hard that it shouldn't matter. I believe that the thin sheetmetal of the unibody is the main concern, as opposed to the shear strength of the bolts. Either way, i still think it'd be damn near impossible to rip that thing off.

Post more pics if you do any more fab work.
I think the cross-member is of great help.

its mounted through the factory holes although i drilled out the square holes to give me additional mounting points.

I plan on fabbing up some stuff in the near future. Ill definitely be posting it up.

Quote:
Originally Posted by ratmonkey View Post
I doubt you'll rip the mount from the frame rail. I'd bet good money you'll deform the crap out of it with a good tug or two with the single mounting plane like that though.

Your jeep, good luck.
I understand the concern of the lower sheet metal, but its all about proper loading. How much money you looking to bet? I used these on Saturday got a few tugs in the snow after i slid into a ditch.

I think that large point that is being missed here is the plate has 21 sq inches of contact with the frame. This distributes the load very well. I calcd out the stress on the 1/16" steel making up the bottom.

At the bottom is the shear stress exhibited on the steel in psi. Ill let you look up the yield strength of mild steel. If you dont reach the yield point there wont be any deformation. I calculated it with a force of 10,000 lbs on one side so i think its sufficient.

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Unread 01-25-2010, 11:27 PM   #13
ZJ5point2GCL
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looks good, too bad its not on a ZeeJay so i could copy them
Nice Work
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Unread 01-26-2010, 01:47 AM   #14
pulsepb
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Quote:
Originally Posted by tarfin View Post
Nice work!

Quote:
Originally Posted by ZJ5point2GCL View Post
looks good, too bad its not on a ZeeJay so i could copy them
Nice Work
thanks!

10 char
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Unread 01-26-2010, 02:45 AM   #15
ratmonkey
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10k pulling force is pretty conservative for a pull on a good stuck.
You want it to fail at a higher rating than your pulling strap. Calculate your forces at 30k and add in another 20% for safety.
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