How to: Vader-e36 seat install

[chris4891]

ive got to admit when i first saw this, i really wasnt sure that i was ok with it. but i think ive managed to rationalize it. before i get into it, question.. what type of steel are the brackets made from? i would assume mild steel? this is more of a curiosity than anything.

also, im not familiar with the e36 seat, but i assume the bent rod behind the seat is there to attach one side of the seat belt. i think that particular feature is the key thing that makes me ok with this.

lets assume in the case of a front end collision theres no one in the seat. id imagine those brackets are strong enough to resist the inertia of the seat from wanting to keep moving forward. but if you were to put a person in the seat, things change a bit. now you have this rather large mass that you need to slow down.

for the original e30 seat, in the case of a front end collision, where your body weight is thrown forward, that loading is transmitted into the seat belt, then into the seat frame, and ultimately back into the main structure of the car.

In your case, and seemingly that style e36 seat in general, the load path is a bit different. instead of the forces in the seat belt being supported by just the seat frame on either side, now on one side youre tied directly into the structure of the car through that seat belt bracket. thats a much better design, imo. in this fashion, i would think that a much smaller percentage of the loading would be transmitted through your offset mounting brackets than if you were to applied this same concept to a regular e30 seat.

just a note though. while i've rationalized the design, i probably still wouldnt put it in my car without running a simulation in my finite element software at work, mostly for piece of mind.

regarding what golde30 said about connecting the mounts front to back on each side... you've pretty much already done that once youve attached the seat. that rail is pretty stiff. its geometry makes it quite a bit stiffer than if you were to just have a piece of 1/8 flat sheet connected front to back.

if you were going to beef up anything, do so on the set of inside brackets (towards tunnel). that inside rail, which is presumably where the seat belt attaches, is whats going to carry a percentage of the load from the seat belt.

A little background on myself.. im a mechanical engineer and do structural analysis most days. doesnt mean im infallible though.. if anyone sees anything wrong with my logic, speak up.

very clean install, btw

[AndrewBird]

I have never understood why people are scared about a seat moving or breaking in a frontal collision. In a frontal collision, the only stress your body puts on a seat is from the weight of your body down on it, no more. Inertia causes your body to move forward and forward only. Not down or back or up or any other direction. Your seat belt is the only thing that holds you back.

Now, that isn't to say force isn't applied in a roll over or rear end collision, but rear end collisions usually aren't nearly as bad as frontals and roll overs are going to be bad regardless of how your seat is mounted.

Also, as far as welding the seatbelt bar, as long as the weld is done properly, the welded section of bar will actually be stronger then the rest of the bar. And, with the shape of the bar, even if it did break at the weld, it would have to fold over itself before the seatbelt would come off.

[Toms73NovaSS]

before i get into it, question.. what type of steel are the brackets made from? i would assume mild steel? this is more of a curiosity than anything.

Yes they are made from 1/8 mild steel.

[chris4891]

The forces involved from just your body weight can be significant. Lets say you decelerate from 60mph to nothing in 100ms. That equates to roughly 27 g's.. we'll round up to 30.

For a mass that weighs 200 lbf accelerating at minus 30 g's, the equivalent loading is 6000 lbf.

I just wouldnt want my seat brackets to break or bend under that kind of loading and allow any part of my body to slam into the steering wheel or dash. At that point you're already having a bad day, but it could get worse if your seat were to break free. (i realize thats extremely unlikely)

[SamE30e]

The only thing you need to worry about is is breaking on acceleration. And I doubt your e30 makes enough power to do that.

And if you were in a 30g collision, you'd have a lot more to worry about then your seat popping loose.

[formula driver]

If you can get around to making some extra brackets, I would definatly take a set.

[e30serg]

chris4891: That bent rod is stock on my '88 325is. Maybe earlier models didn't have it. The load path with the e36 seats is the same as with e30 seats. In a front-collision crash, the 3 points being loaded are: the bent rod/bracket, the seat belt snap attached to the seat and the B-pillar mounting.

A person moving forward on the seat due to the crash will load the seat belt snap and therefore the seat frame. The bent rod/bracket will also loaded and if not welded correctly (with full penetration) could fail.

The way these seats are mounting is probably OK, I just wanted to clear things up a bit. I haven't done any FEA analysis myself, and it would be a good idea. Yes, I'm also a Mechanical Engineer.

[Chisum]

Why not just drill new holes in the floor and reinforce them with 1/4 steel plates on the underside??

[bostonbmw]

looks really clean. Thanks for posting the drawings!!!

[E30 Cabrio]

I have never understood why people are scared about a seat moving or breaking in a frontal collision. In a frontal collision, the only stress your body puts on a seat is from the weight of your body down on it, no more. Inertia causes your body to move forward and forward only. Not down or back or up or any other direction. Your seat belt is the only thing that holds you back.

Now, that isn't to say force isn't applied in a roll over or rear end collision, but rear end collisions usually aren't nearly as bad as frontals and roll overs are going to be bad regardless of how your seat is mounted.

Also, as far as welding the seatbelt bar, as long as the weld is done properly, the welded section of bar will actually be stronger then the rest of the bar. And, with the shape of the bar, even if it did break at the weld, it would have to fold over itself before the seatbelt would come off.

The seat belt receptacle is bolted to the seat, so if the seat breaks free then the receptacle probably won't do much good, nor will the seat belt.

[e30pwr]

Can you get more of an overview show of the seats in the interior, I think I just found my spring break interior mod.......

[fiercee30]

nice job.good write up but,you should post more pics of your interior.

[Uber E30]

so the plate for the seatbelt rod is under the car?

[dnguyene30]

if any of you guys wanted this, i can arrange a friend of mine make this (he owns a machine shop).
he charged me $100 for labor and material, the only thing you need to do yourself is the seat belt bar thingy (chop it off and re-weld it).

[Toms73NovaSS]

so the plate for the seatbelt rod is under the car?

Yes, I put a support plate under the car to spread the load.

Nut is welded to plate, plate to car....