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shackle bolt strength :grade-5 & grade-8

muddeprived

NAXJA Forum User
Location
PA
There's somewhat of a debate on my local forum about shackle bolt strength. I wanted to get more opinions on this.

I'm running relocation brackets for my shackles and it comes with a grade 5 bolt for the upper shackle mount. Does the sgear stress on the shackles exceed grade 5 bolt limits?

Every shackle i bought and these relocation brackets came with grade 5 shackle bolts so I would assume it's fine to use.
 
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To quote my BLAW teacher, "It depends"

It will probably be no problem. Are you doing jumps and Jeepspeed events with your
XJ? If so, grade 8 would be a good investment. If you aren't, I say grade 5 would
be fine. Alot of manufacturers are using grade 5 bolts with their hardware kits, and
you don't see a lot of drastic breakages involving these parts.

Another thing to think about: In the event of a lot of stress on your rear springs,
would you rather break the grade 5 bolt on your shackle first, or the main leaf?
Just depends on where you want your weak link to be.
 
In double shear, grade 5 is probably adequate. But I use grade 8 because it "feels better" to use stronger bolts and the cost difference is almost nothing. From a basic calculation you are putting less than 500 lbs of shear force on the bolt - grade 5 is probably fine depending on the bolt diam.

Estimated calc by a drunk engineer:
Vehicle weight is 3,000 lbs (don't count the axles or tires, just the sprung weight)
Assume 50/50 weight distribution: half vehicle weight on rear springs is 1,500 lbs
Two rear springs = 750 lbs per rear spring
Two bolts per spring = 375 lbs per bolt in double shear (assuming the axle is in the center of the spring, it isn't but it's close enough).

It isn't exact but it shouldn't be that far off. Look at the shear force for the bolt and the estimate for what you are applying.
 
You shouldn't be using grade 8 either way, you should be using property class 10.9 since all the leaf spring and control arm hardware is metric :lecture:

Unless you're doing longarms etc etc, in which case use PC10.9 or grade 8 depending on whether the manufacturer specs SAE or metric hardware.

I would use grade 8 simply because I'd really rather not have one of those bolts pop at the wrong time and the cost difference isn't all that big.
 
In double shear, grade 5 is probably adequate. But I use grade 8 because it "feels better" to use stronger bolts and the cost difference is almost nothing. From a basic calculation you are putting less than 500 lbs of shear force on the bolt - grade 5 is probably fine depending on the bolt diam.

Estimated calc by a drunk engineer:
Vehicle weight is 3,000 lbs (don't count the axles or tires, just the sprung weight)
Assume 50/50 weight distribution: half vehicle weight on rear springs is 1,500 lbs
Two rear springs = 750 lbs per rear spring
Two bolts per spring = 375 lbs per bolt in double shear (assuming the axle is in the center of the spring, it isn't but it's close enough).

It isn't exact but it shouldn't be that far off. Look at the shear force for the bolt and the estimate for what you are applying.

That's the force just sitting still, but going over bumps would drastically increase the force on the bolts
 
You shouldn't be using grade 8 either way, you should be using property class 10.9 since all the leaf spring and control arm hardware is metric :lecture:

.

The difference between a 14mm bolt and a 9/16" bolt is 0.012" (9/16 is a tad bigger) not enough to worry about and the cost difference between a 14mm bolt and nut and a 9/16" bolt and nut is significant.

I send grade 5 stuff out with my relocation brackets because I have been using grade 5 stuff on my leafs for 3 years with no issues, and I guarantee I am harder on them than 90% of NAXJA is on thiers.
 
Good point.

I actually used a 9/16" black oxide drill bit on the new LCA brackets I made for my d30 now that I think about it, and I had to use a file to open the holes out a little more so the bolts would actually fit through. Not sure if it was the drill bit, or the bolts being out of tolerance...

I stand corrected :anon:
 
Understood, but it aint like he's jumping the thing and getting 10x the load. The point was that the shear load on the bolts is tiny regardless of how big of a multiplier you include.

Edit: a 1/2" bolt(grade 5) is good for about 14,000 lbs of load before shear - assuming 120 ksi tensile strength and 60% for shear strength. Put it in double shear and the required load to failure doubles. There's no way that he'll hit those loads on a rear shackle.

That's the force just sitting still, but going over bumps would drastically increase the force on the bolts
 
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A lot of manufacturers are using grade 5 bolts with their hardware kits, and you don't see a lot of drastic breakages involving these parts.
On the other hand, how many grade 5(or equivalent) bolts do you see OEM?
 
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