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Old 04-13-2013, 10:02 AM   #1
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Default Engineer crew: Trailer hitch hardware help

This is my trailer hitch:
IMAG0060.jpg?t=1310741866
This is my bike rack:

The hitch weighs about 40lb, bikes weigh roughly 20lb each with water bottles and bags so we are looking at a combined weight of 80lb hanging out back. How do I know if the hardware I used to attach the hitch is sufficient? Sadly, I used the two long intake plenum bolts (four total) because they were the right thread pitch with a nut on the back-side. Should I drill the holes out and use some fatties in there or will these bolts work?

Thanks.
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Old 04-13-2013, 10:08 AM   #2
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Same set-up, can carry two bikes though.

I'm worried about sheering those four bolts.
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Old 04-13-2013, 09:53 PM   #3
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Size and grade of bolts or should we guess which intake bolts you were referring to? Lol.
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Old 04-13-2013, 10:03 PM   #4
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they are standard m8. Grade is whatever mazda has from factory. probably 5.8 or 8.8 I would think. I wouldn't change the size necessarily, but I would go buy some 10.9 or 12.9 grade just for peace of mind. I doubt you would break those.
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Old 04-13-2013, 10:31 PM   #5
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You could jump up and down on that and not break the bolts.
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Old 04-13-2013, 10:48 PM   #6
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Yeah, it would have to shear all 4 bolts combined in order for it to be a problem. I think the combined strength of the four should be sufficient.
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Old 04-14-2013, 12:07 AM   #7
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a 16 penny nail has 1000 lb shear strength. No worries.
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Old 04-14-2013, 12:26 AM   #8
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Grade 8.8 has the following properties:

Ultimate Stress (Tension) = 80Kg/mm^2
Yield Stress (Tension) = 80% of Ultimate = 64Kg/mm^2
Yield Stress (Shear) = 60% of Yield Stress (Tension) = 38Kg/mm^2

The minor diameter area of an M8 bolt is 32.8mm^2 (coarse) or 36mm^2 (fine).

Your bolts are loaded in shear. In that direction, each bolt (if it's an 8.8) can support 1,246Kg/2,741lbs (coarse) or 1,368Kg/3,009lbs (fine).

So, yeah, I think you're fine.
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Old 04-14-2013, 01:08 AM   #9
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Thanks, I feel better now. Unfortunately this think keeps flopping up and down and when I was loosening one of the bolts, the welded nut broke loose...so it's permanently stuck, lol.
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Old 04-14-2013, 10:56 AM   #10
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bolts should not be loaded in shear. They are loaded in tension and maybe bending in most applications. The point of a bolt is to hold the two pieces together such that the shear forces are only applicable to the joint interface by virtue of the normal force applied by the tightening of the bolt.

That's why your 15 ft-lb tight skinny clutch bolts don't shear right off when you do a burnout.
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Old 04-14-2013, 11:29 AM   #11
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Quote:
Originally Posted by y8s View Post
bolts should not be loaded in shear. They are loaded in tension and maybe bending in most applications. The point of a bolt is to hold the two pieces together such that the shear forces are only applicable to the joint interface by virtue of the normal force applied by the tightening of the bolt.

That's why your 15 ft-lb tight skinny clutch bolts don't shear right off when you do a burnout.
This guy gets it.
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Old 04-14-2013, 01:09 PM   #12
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I get it too, but not when drunk-posting.
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Old 04-14-2013, 05:20 PM   #13
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short version:

99% of all bolt applications you'll run into on a car will require a grade 5 standard or 8.8 metric bolt. Spending more for a grade 8 or 10.9 or 12.9 is a waste and in some cases, the additional brittleness may work against you.
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