Cross brace removal under rear bench seat for subwoofer install

E28 technical advice asked and given! Troubleshooting, modifications and more.
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sixseries
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Joined: Jan 25, 2008 2:17 AM
Location: Portland, OR

Cross brace removal under rear bench seat for subwoofer install

Post by sixseries »

Hey,

I'm looking to install a powered sub (see last pic) under the rear drivers side bench seat, and the drivers side cross brace is unfortunately in the way (see pics below for brace for reference). I searched the forum and found some threads about putting the sub in that location, but not much info about cutting the brace and pros/cons of doing that. Has anyone done and that suffered negative effects? What exactly is the purpose of that brace? My guess it would be to prevent the outward movement of the frame when weight is put on the seats, but not sure. I also took it to a metal fab shop and the guy suggested bending it so the bend went inwards instead of outwards.

Also, people are rarely going to be actually sitting in the rear seat.

Thanks!
Brad

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SlickDizzy
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Re: Cross brace removal under rear bench seat for subwoofer install

Post by SlickDizzy »

Those braces are necessary to provide tension so the rear bench "snaps" into place, otherwise it will loosely sit there and potentially rattle.

I was able to fit a similar powered sub under the passenger side one, FWIW.
travisj
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Re: Cross brace removal under rear bench seat for subwoofer install

Post by travisj »

I believe you can bend it the opposite way (upward) to provide a little more room. I think that's what I have done.
sixseries
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Re: Cross brace removal under rear bench seat for subwoofer install

Post by sixseries »

SlickDizzy wrote: Oct 13, 2020 9:53 AM Those braces are necessary to provide tension so the rear bench "snaps" into place, otherwise it will loosely sit there and potentially rattle.

I was able to fit a similar powered sub under the passenger side one, FWIW.
I don't really get this, wouldn't they be held into place by the seat backs pushing down, and then that hook like thing on the front side that kinda 'snaps' down on the sheet metal support, along with the middle hook thing that you bolt to the sheet metal support?

I'm not trying to say you're wrong, just better understand.
sixseries
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Re: Cross brace removal under rear bench seat for subwoofer install

Post by sixseries »

travisj wrote: Oct 13, 2020 6:32 PM I believe you can bend it the opposite way (upward) to provide a little more room. I think that's what I have done.
Did that affect the hook like things attached to each brace (that are kinda perpindicular). It seems that snap up against that sheet metal support to hold the bench in place (along with the center one that bolts up to the sheet metal).
cyclist
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Re: Cross brace removal under rear bench seat for subwoofer install

Post by cyclist »

Brad,
Is that the sub you bought from me? I would use the little black tabs that screw into the end of the sub and figure out a way to attach it to the seat bottom springs if no one sits there. You will also want to lay down some sort of dynamat etc if you plan on having it on the bottom like you have it pictured or it will vibrate like crazy. Are you installing this in a e24 or 8? If a e28 it should fit under the drivers side rear seat without any issues keeping the bar.
SlickDizzy
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Re: Cross brace removal under rear bench seat for subwoofer install

Post by SlickDizzy »

sixseries wrote: Oct 14, 2020 12:00 AM
I don't really get this, wouldn't they be held into place by the seat backs pushing down, and then that hook like thing on the front side that kinda 'snaps' down on the sheet metal support, along with the middle hook thing that you bolt to the sheet metal support?
The hook thing on the front is tied in with the brace, as shown in your picture. The seat frame itself is not rigid enough to maintain tension by itself or the braces you mention wouldn't be there. Also, the seat backs don't push down on the bench at all, they are held up by their own mounting hardware.

I accidentally bent the hook thing so that it didn't hold tension correctly when I refinished my rear bench, and the seat bench rattled annoyingly over bumps until I fixed it, so I speak from experience.
sixseries
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Joined: Jan 25, 2008 2:17 AM
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Re: Cross brace removal under rear bench seat for subwoofer install

Post by sixseries »

cyclist wrote: Oct 14, 2020 9:38 AM Brad,
Is that the sub you bought from me? I would use the little black tabs that screw into the end of the sub and figure out a way to attach it to the seat bottom springs if no one sits there. You will also want to lay down some sort of dynamat etc if you plan on having it on the bottom like you have it pictured or it will vibrate like crazy. Are you installing this in a e24 or 8? If a e28 it should fit under the drivers side rear seat without any issues keeping the bar.
It is. I couldn't really figure it out, so I just had it professionally installed. I had to bend the bar a decent amount so it would clear. Otherwise, the rear bench seat bottoms never really snapped in and stayed down as the bar was resting on the subwoofer
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