NCJeepers

I installed new tow hooks to the front of my 1999 Cherokee. No way I want to be "that guy' on the beach, stuck with no recovery points. I have seen it too many times. :-)

http://www.jeep4x4center.com/rugged-...-11236-05.html

They came with 3" carriage bolts (with no markings) and nuts for the rear hole, but that was it for hardware. The speed original speed nuts cracked and were worthless, and the original three front bolts were not long enough to cover the extra distance added by the thickness of the mounting plate, so I had to purchase new bolts and nuts.

Question: Is 8.8 bolt grade good enough for this application? My tow hitch came with 8.8 grade bolts (which I used) so I ASSUMED that would be an appropriate grade to install the tow hooks. Besides, I did not see any 10.9 or 12.9 bolts at Home Depot anyways.

Thanks!

EZEARL

Only came with two bolts? Here's the C4x4 kit which comes with grade 8.8 hardware. Their holding up so far.

Cvail

Grade 8 and Metric 8.8 are NOT the same thing. An 8.8 has a proof load just a little below a US Grade 5.

Just remember with bolted connections, the only purpose of the bolt is to provide enough clamping force to keep the parts from sliding relative to one another OR to provide enough clamping force that the bolt preload is never overcome. The greater the strength of the bolt, the more tightening torque you can apply, thus the more clamping force you can generate.

For a general rule of thumb, the clamping force you can apply with a bolt can be approximated by F=Torque/(bolt diameter x .2) for a lubricated piece of hardware.

EZEARL

THANKS
Learn something new every day. Corrected my post. Only grade 8 hardware is on the hooks.

NCJeepers

Thanks for the reply. I read that as a "yes" 8.8 grade bolts will suffice for this application. They provide the clamping force to keep the mount from moving on the frame when pulling the vehicle forward. In the mean time, I will tell Home Depot to get some better grade bolts in stock!!! :-)

abodyjoe

here is a chart that explains metric and american grades on bolts..

https://www.boltdepot.com/fastener-i...ade-chart.aspx

Cvail

Lets figure up....

Assume you are really stuck (and the general rule of thumb on a winch is you need twice the vehicles curb weight). Lets say a 8000lb winch pulling on the first wrap. So that means the tow hook needs to withstand 8000lbs.

The coefficient of friction between two dry steel surfaces is roughly is roughly 0.7. So the clamping force required to prevent an 8000 lb force from overcoming the clamp load would be 8000/0.7=11428 lbs

Now assume an 8.8 metric zinc plated bolt. Based on the formula that says F=.2x Torque x diameter of fasterner we can calculate the clamp load that various size bolts with give (this is assuming we are torquing to yield).

M6 x 1.00= 1981lbs
M8 x 1.25= 3620lbs
M10 x 1.00= 6248 lbs
M12 x 1.25= 9144 lbs

So figure the number of bolts on each bracket and there you go. This doesn't take into account any jerking load so I'd try to double the clamp loading.

Disclaimer: These are rough numbers and are there are numerous sources for friction coefficients and effective diameters.

EZEARL

The installation instructions list mounting hardware as being M10. Is that Class 10.9 hardware? Bolt heads have 8.8 on them.

Cvail

m10 is the size (roughly 3/8") A couple of those and you'll be fine if you torque them correctly.

NCJeepers

Each tow hook is rated at 10,000 lbs. Each bolt is M10 or M12, I believe (same as stock, just longer).So, with four 8.8 grade bolts per tow hook, the tow hook would break before the bolts would.

Turbo X_J

YMMV, as the tow hooks are bolted to old rusty metal only really suitable for use on chinese washing machines. Cherokees as most things were designed to last through the warranty period only. Any life left in it from then on is an absolute bonus.

NCJeepers

The tow hooks were bolted to the tow hook mount. The tow hook mount contained the four holes for mounting to the frame. The frame looked to be in pretty good shape. No rust. We'll know for sure if/when I have to get pulled out. :-)

kickice24

Just an FYI to anyone who needs, or wants, to buy hardware and doesn't want to get ripped off by the big box stores. If you're lucky enough to have a Tractor Supply or Rural King near you, you can buy 8.8, 5, or grade in 8 hard ware by the pound instead of by the piece. WAY cheaper.