Early announcement for the 86BUTTDROP lowering brackets for OEM seats

[timurrrr]

Made more progress on the project, and I'm crossing out a few geometrical challenges with each iteration. Got a new prototype from the manufacturer, took some measurements and figured out what to do next. At this point I pretty much have only one element left to iterate on (which I totally messed up in the current revision ).

Also, I temporarily bolted up the OEM seat rail to my seat to take some photos and measurements. While doing so, I found the limiting factor: the plastic trim on the door side of the seat is only ~22 mm from the floor reinforcement near the rear door side corner of the seat.

That dimension is depicted on this photo, where the silver part is used as a reference to demonstrate where the floor reinforcement is when the seat is bolted to the car:


So lowering the seat more than 20 mm from OEM will either require some cutting, or will cause damage (+squeaking?) to this plastic panel.
I might be able to design a "butt drop plus" version for customers who would be ok to cut some parts just to get another 5-10 mm, but for now I'll focus on the mild design that requires no cutting.

[timurrrr]

To the people who are interested, please please please answer the questions in the 2nd post, in particular "the distance" thingy.
It's important for me to understand how different people set up their seat rails so that I can optimize the geometry accordingly.

Originally Posted by timurrrr

If you're interested in buying one, please post a response, including:

1. If your seat is "fancy, heated" or "basic, not heated"
2. The distance between the front of the slider and the front of the rail on the door side of the seat in your normal helmet-less driving position (see picture below), in millimeters
3. If you're interested in helping me test near-final prototypes, or only interested in the "set and forget" product
4. If you have mechanical engineering background and can provide design feedback / suggestions
5. If you're located in the SF Bay Area

[Ultramaroon]

Originally Posted by timurrrr

If you're interested in buying one, please post a response,

Basic
All the way back (I'll measure after work and edit here)
Happy to help
Second gen ME. Let's do the math before we make chips.
We have Oregon between us.

[biggie]

Just to make sure I understand correctly, we (might) gain ~20mm headroom but we lose the ability to move/adjust the seat back/forth?

Also, how about your idea to remove the "post" between the bottom of the rail and the floor, that's still an option?

[timurrrr]

I was meaning to post an update last weekend, but the forum was down.

After some issues with the fabricator and FedEx, I finally have the first fully usable set of parts.

Here's a teaser:



There are a couple of minor millimeter-level tweaks I want to make to the geometry.
(I should probably cover the gaps in the floor lining?)

But overall I'm happy with it the way it is.
The kit is now installed in my GR86 that I semi-daily drive.
I can finally put my seat back high enough for me to comfortably put my wrist on top of the steering wheel and still have decent headroom left.
(I will probably still need to slouch a bit with the helmet on, but not nearly as much as with the stock seat)
If it wasn't for all the other tall people who might want this, I'd stop at this point and enjoy it by myself

I believe was able to achieve a ~20 mm drop.
Here's a photo taken from the rear left passenger footwell in an unmodified car:



You can see there's a ~20 mm gap between the bottom of the plastic trim panel on the left (door side) of the seat near the seatbelt anchor.
(The plastic can move slightly, so maybe there's a couple more mm like on the other seat I posted earlier)

Due/Thanks to the lowering, I physically can't put the camera in the same spot on my car.
To demonstrate how much lower my seat is now, I put a flashlight under the seat, and as you can see the same plastic panel completely blocks the light when the seat is in the lowest position.
That means it's touching the floor (I also verified it by touch), and confirms that the seat is as low as it gets without cutting the plastic:



This week I plan to meet my first guinea pig to have him sit in my car and let me know if any legroom adjustments are needed, and then I'll be make another kit for him.

Originally Posted by biggie

Just to make sure I understand correctly, we (might) gain ~20mm headroom but we lose the ability to move/adjust the seat back/forth?

Mostly. There's still a little bit of back/forth movement that happens as your lower/raise the seat, but probably only 1-2" total.
My taller-than-most wife said she would need some padding on the seat back to be able to safely drive my car.

Originally Posted by biggie

Also, how about your idea to remove the "post" between the bottom of the rail and the floor, that's still an option?

That might be an option to gain a bit of headroom.
However, having experienced a ~15 mm drop and then a ~20 mm drop, I personally would not settle for <10 mm that cutting off the post would give you.

[Ultramaroon]

Hate to be that guy but it might save your life. Based on the one pic, looks weaker than stock. Long thin tongue will bend/tear if front of seat is pulled vertically. If you want more input, need more info.

I'm always open to being wrong.

[timurrrr]

Originally Posted by Ultramaroon

Hate to be that guy but it might save your life. Based on the one pic, looks weaker than stock. Long thin tongue will bend/tear if front of seat is pulled vertically.

Thanks for flagging, I appreciate the feedback.
I had similar thoughts myself and I have already decided to make that area wider just out of an abundance of caution.

As a data point, the bracket gets wider under the seat (not visible in the teaser photo), so it's significantly stronger elsewhere already.
My friend and I did some tests on an earlier prototype, and we were able to shake the whole car through the floor bracket.
But once again, I'd rather make it wider and be safe.

That being said, I believe that the more critical area in case of an impact is the seat belt attachment, and you won't believe how flimsy the stock one is:




That U-shaped bracket is only 2 mm thick and is mostly flat. It's then welded (?) to the rail, and then the rail is bolted to the floor with a significantly oversized hole for the bolt (to make it easier to install?).
The other end of that U-shaped bracket is used for one of the corners of the seat, so that weld shares the load between the seat and the seatbelt.

The current version of my seatbelt attachment is 3.4 mm thick (+70%!) and it's part of the big bracket that bolts directly to the floor. I used minimal tolerance for the hole for the floor bolt, which causes a bit of a headache when installing the seat, but probably* provides extra rigidity.

* -- full disclosure: I can't possibly make any definitive statements about the safety of the brackets I designed, as I don't have the budget to do crash tests, etc. That's why I'm starting with local guinea pigs I can show all ins an outs of the bracket before they decide to get it, and furthermore I'm choosing guinea pigs with fabrication experience so that they can provide design suggestions if they see anything they are uncomfortable with.

[timurrrr]

Originally Posted by Ultramaroon

Based on the one pic, looks weaker than stock.

Another data point: the stock seat mounts on four arms via 3x M8 bolts and 1x M10 bolt.



The arm in the top right corner of the photo is the one on the door side back corner of the seat, and it doesn't look strong to me.
The other three don't seem to be that much stronger either, and the strength of the overall structure is limited by the M8 bolts.

For comparison, the tongues you're worried about are already 40+ mm wide and 3.4 mm thick.
I have to reuse the M8 bolts to attach the seat to my brackets, but all other bolts I use are M10 or larger.

[Ohio Enthusiast]

I not an engineer, but isn't the seat strength moot in frontal and side collisions? Sure, you wouldn't want it detaching and getting tossed around the cabin, but the actual occupant load is being handled solely by the seatbelt. Only in a rear-end collision will the occupant exert force on the seat (and those tend to be fairly low speed).
With that being said, it should be fairly easy for a mechanical engineer to guesstimate the strength of the original brackets based on thickness and bends and spec comparable flat bar dimensions.

[Ultramaroon]

Originally Posted by Ohio Enthusiast

I not an engineer, but isn't the seat strength moot in frontal and side collisions? Sure, you wouldn't want it detaching and getting tossed around the cabin, but the actual occupant load is being handled solely by the seatbelt. Only in a rear-end collision will the occupant exert force on the seat (and those tend to be fairly low speed).
With that being said, it should be fairly easy for a mechanical engineer to guesstimate the strength of the original brackets based on thickness and bends and spec comparable flat bar dimensions.

Yup. Agreed. Without knowing what the expected forces are, I'm just looking at the differences between stock and mod. The front mounting features of the rails are carefully designed for stiffness in exactly that mode you mention, to resist the bending moment induced by the upper body mass being accelerated into the seat back.


It's easy for me to express concern based solely on intuition. Answering it takes work

[Ultramaroon]

@timurrrr, looking more carefully at your photos, I agree with you. The factory front mounts are only spot welded to the rail. Your solution is easily stronger.

[timurrrr]

Update: the install for the guinea pig went mostly well.

Only "mostly" because the parts manufacturer messed up one of the parts, and I had to fabricate a temporarily replacement from an incompatible earlier prototype. Now I know a few more things to keep an eye on when I QC the parts.

I'm now looking for another guinea pig in the San Francisco Bay Area who has the seat rail measurement in the 30-60 mm range.

Originally Posted by timurrrr

The distance between the front of the slider and the front of the rail on the door side of the seat in your normal helmet-less driving position, in millimeters. See the picture below, in this example the distance is 60 mm.

[timurrrr]

With Ed's permission (he is my first guinea pig / customer), below is his detailed review.
I'll start working with my second guinea pig (different leg length / seat position) soon.

[Shneegle]

I've only recently gotten a 2024 Trueno Edition so seating setting is still moving around a bit. Definitely feel like I'm sitting on the car instead of in the car compared to the Gen 1.

1. Fancy, heated
2. 90mm
3. I'm in to help if needed
4. Yup have knowledge in engineering
5. Dallas, TX