NA6 - lowering factory seat mounting rails to floor pan

[twr7cx]

Has anyone successfully lowered the factory seat mounting rails in their NA MX5 closer to the floor pan? e.g.

https://grassrootsmotorsports.com/forum ... 347/page1/

The author claims a 2" gain in headroom which would be ideal!

I'm looking at doing this in my JDM 89 NA6. I'm driving the car more regularly now, it's starting to get cold down here in Tasmania and I just find the height issue when the roof is up to be terrible. Foamectomy has been done already and I worry that if I remove more it would possibly effect the seats comfort.

I'm interested if there are any tips or better methods?

[93_Clubman]

Following is re racing with a hardtop vs roofless, however by end of page 1 gets into seats & seat mounting: viewtopic.php?f=19&t=76193

[bruce]

I do get concerned when seats and factory mounts get modified. It only takes a little bit of carelessness and you've got yourself a death trap.

[StuwieP]

yeah you'll have to go through an engineer. You'll need serious backing plates etc.

With that said, I've got a mate who went off backwards at PI in a stock HSV and the OEM seat mounts gave up. So assuming that the 'new' mounts to the floor are at least as tough as the original spot-welded brackets and cross-member, I don't see why it'd necessarily be more dangerous than original.

[twr7cx]

For the hyperlink provided in my OP what is involved is removing two rear raised parts which the rear of the seat rails bolt into. These are spot welded into place and a spot weld cutter should easily remove them.
Then heat up and bend the seat rails to suit the new angles.
Finally it's a matter of drilling through the floor pan so that the rear seat bolts go all the way though and using a large plate to distribute their load. Given that the original mounts were spot welded to the floor pan, I assume that the floor pan must already be strong enough as there would be little benefit welding to something that was not. I would suggest that the minimum side for the load spreading plate should be equal to the size of the removed hump so that the load is spread over at least the same sized area.

I'm not too fussed about the engineering process. I've been through it fairly recently for some significant work to my 4WD. Have also previously added rear seats to a cargo van that only had the front row. So I am happy to reengage the engineer for these seat modifications too.

EDIT: something like these CAMS eye bolt backing plates < http://www.autosport.com.au/harness-mounting-plate > might be suitable as both load spreaders and nuts for securing the rear of the seat in place. According to SCHEDULE I – SAFETY HARNESSES AND WINDOW NETS < http://docs.cams.com.au/Manual/GeneralR ... 2017-1.pdf > they are:

"where a safety harness is affixed to an un-reinforced section of the body shell, each attachment point shall be
reinforced by the use of a plate not less than 75mm x 50mm x 3mm thick"

[bruce]

A lot of assumptions.

[twr7cx]

A lot of assumptions.

Afternoon Bruce, can you expand any further on that statement to share the many other assumptions that you've identified? I ask as the only assumption I can identify is that of the strength of the floor bar being suitable when a load distribution plate is place behind it.

[bruce]

In a major accident everything goes under huge stresses. Cars are crash tested to ensure they meet a safe standard. Once you modify that it technically needs to be inspected by an Engineer. There are a lot of things to consider apart from 'does it look strong enough'. A few dodgy welds and you're in trouble.
Heating and bending changes metals strength, so I'd even question doing that to the seat frame.