Manual seatbelt swap

[e1_griego]

More n00b questions from me (no. way).

I am thinking that I'm going to finally replace my automatic seatbelts in my s13, and wondering about the safety of using the longer bolts + spacers/washers that seems to be the preferred method of bolting in the pivot.

This is the hardware


So it looks like this:


Is this safe? Dangerous the have that much spacer?

Dumb questions, but I figure more engineer types here that I trust.

[Air]

im not a engineer type (yet), but my gut tells me that would work. I would think the forces that your body would exert on that seat belt in a crash would not even come close to exceeding the limits of that bolt. doesnt a grade 8 bolt have somewhere in the neighborhood of 20,000 lbs shear strength?

I dont have the internal plastics on my car, so i just used the stock bolt from that location to mount my pivot.

The thing i would be more worried about is to properly mount the bottom (where the reel is). I had to build a metal bracket to extend the reels bolt holes in order to mount it in a manner i felt was "safe", but even now it could be safer imo, i have added another bolt and piviot location where none was before.

[Jordan.Best]

I agree. Grade 8 bolt and a good amount of threads used to secure it is all you need.

[djsilver]

I learned something when looking for hardware to swap seatbelts. Metric cars use M11x1.25 and SAE cars use 7/16" fine thread bolts. The interesting thing is that they're close enough to interchange so if you can't find the metric version you can use the SAE versions.

[2Fass240us]

Seems like that bolt would see shear stress from some portion of the load being in the radial direction (perpendicular to the long axis). Making it longer increases the moment arm length and thus the stress. Jacob is more versed in than I and would be better-equipped to answer.

Even given the above, I think you'll be OK.

[Def]

The only issue is you're adding bending load by spacing it out like that, the shear stress remains the same. A class 10.9 bolt(metric bolts don't come in "grades") has an ultimate tensile strength of 150 ksi, and shear strength is usually about 60% of that for relatively ductile bolt strengths, so about 90 ksi.

Given you're using a M12 bolt, and the minor diameter probably has a rough area of 0.17 in^2, your 250 lb fatass stopping at a chest bruising 20 G's gives a 5000 lb load for a shear stress of 29 ksi(less than 90 ksi obviously).

The bending/torque/moment assuming you hang it out about 1.5" would be 7500 in-lbs. Now the complicated part is that the spacers here will take some of this bending moment away, but assuming just the bolt is holding it you get 7500 in-lbs with a tensile force of approx 32 ksi at each bolt. This is complex loading(bending and shear), so it gets much more complicated here, but suffice it to say, when you do all the calcs things are well within the capability of a M12 bolt.

I'm not a structural guy, so my very "back of the napkin" analysis is not 100% correct and doesn't use all the Von Mises formulae etc., but it lets you know you're not "getting close" to what the bolt can hold.

FWIW, I ran a similar setup with stock S14 belts. Not spaced away quite as much, but pretty close. I need to sell those belts now that I think of it, great upgrade from the stock S13 head choppers.

[Tower240sx]

Def...give me your seatbelts

Thanks,

Mike

[2Fass240us]

Bending load, not shear stress...thanks Gaycob!

[e1_griego]

Alright, sounds great! Thanks guys.

[Def]

Def...give me your seatbelts

Thanks,

Mike

Sounds like an even trade for some pimp brake prototypes.

[Solo_S14]

Bumping an old thread, but have you guys ever ran into issues with the seat belt coming back out after it's fully retracted (once removed).

I've got S14 belts, and they've been sitting in a box for about 2 months.
I want to re-install them, but when I tried this weekend, they won't come out of the roll more than half an inch before locking.

If I repetitively pull them to lock, I can get them to come out about a 1/4 inch at a time or less, but this is obviously not what I'm looking for.

Anyone know if they lock for some reason, and how I'd go about unlocking them?

[logr]

Hold them with the right side up, the way they a mounted in the car.

[e1_griego]

Yup.

btw, I got my s14 belts in. Ended up getting some bnib s14 belts from a friend for cheap. I can't believe I waited this long to get them installed. Best money I've spent on the car in a long time.

[B-Random]

<---looking for a set of s14 seatbelts if anyone wants to sell theirs.

[Def]

I've got a set. Black, good shape. I might still have the covers over the b-pillar bolts, but not 100% sure on that.

Shoot me a pm.