Seat repair and safety


Recently, I found that the “reclining” function of my driver’s seat had failed, causing the seatback to fall freely, rendering it unusable. I fixed it in a desired position (no more recline function, sorry) via use of 2, 3/8", grade 5 bolts.

Now, for the safety check: grade 5 bolts offer shear strength of roughly 70,000 psi/in^2. Using 60,000 as a “fudge factor” for the minor diameter, I find my repair should be able to withstand 16,900# force.

Now: I weigh 210#; I’m guessing 160# of that is on the seatback, centered at 20" or so from bottom (I’m 5’10"). The bolts were drilled into the seat about 1" from the pivot point. Crunching the numbers suggests I’m good up to 5.2g, meaning (I guess) that I can squirm around all I want, but if I get rear-ended good and proper, the seatback might fail.

Some questions:

1. The 5.2g, I believe, is based on max repeatable load. What happenes if this load is exceeded? Meaning, just how hard do I need to be hit for the bolts to actually snap (vs. just “smush” a bit) and the seatback to move freely?

2. What g-force are seatbacks typically engineered to? (I mean, there’s no point in making the repair stronger than the rest of the seat.) It’s a '98 Ford Contour, if that matters.


. What g-force are seatbacks typically engineered to?

There was a big to-do a couple of years ago regarding rear seat passengers being injured or killed by forward seats failing in exactly the manner you describe. It came to light after a number of children were killed by the front seat collapsing down upon them in the case of a rear-end collision.

I would describe your efforts as more of a seat modification than a repair. I suspect that the shear strength of the bolt is only one factor in the equation. You’ve also compromised the seat frame and will need to factor that into your calculations as well.


I’m inclined to agree with TT. And suggest looking into an aftermarket replacement seat. Or one from a boneyard.

Or start looking for a good deal on a used wheelchair just in case. I’ll assume from your posts that you’ve run enough design validation tests to have learned that the results of a theory can be highly inpredictable when tested. In your case the “test” would be an auto accident with you in the seat.