Is L track on top of the rigid foam a bad idea?

If you have to ask, you probably shouldn't be installing DIY seating.
Mounting the L-track to the floor of the van securely requires steel plates underneath that you bolt through. Plenty of threads in the archive that talk about this. New Zealand is the only place I found with legal standards for camper van conversions and they require metal-to-metal all the way through the floor (no wood) and many other conditions.
Given the difficulty of doing this right, the one-piece floor that travoisusa just posted looks like a bargain.

 

I routered out the plywood and insulation to allow flanged l track to mount to the floor of the van...blocked the gap between them with aluminum bar 1 1/4" x width ot the gap.

 

Forgot to mention that the NZ standard forbids flanged L-track because in a crash the flange may end up carrying loads other than the seat. Presumably it's ok as long as your plywood is itself bolted through the van floor, but it makes sense not to use the L-track flange to hold down the floor if the track might also be holding a passenger.

Here's the NZ standard: https://lvvta.org.nz/documents/standards/LVVTA_STD_Seats_&_Seat_Anchorages.pdf
According to the doc, "The Low Volume Vehicle Technical Association Incorporated (LVVTA) represents ten specialist automotive groups who are dedicated to ensuring that vehicles, when scratch-built or modified, meet the highest practicable safety standards"

 

I’m comfortable on how to attach attach the seats to the van. Just curious about bolting from on top of XPS or not. I’ve done plenty of concrete slabs over foam board. But that is spread across a wide area whereas L track is so narrow

 

Plywood as a base

If you have ever seen objects, properly engineered and secured for safety, in an automobile, torn from their anchors during an accident flying forward and injuring others, (including seats ) one would have to wonder why anyone would use plywood as any form of anchor. It isn't even a solid piece of wood. Which isn't much better.

Just mho.

Semper Fi

 

Of course. It will be reinforced with 1/4” steel under the floor. Just curious about staring on top of foam board or putting in flush against the van floor.

 

OK I'm taking a bit of time for a very short and very simplified engineering lesson (it's more a common sense lesson though...) because no matter if you want to do a legal install (following legal rules) or not, safety of the passenger should be the main concern here.

If you fix your L-Tracks directly on the floor, with metal screws for example but even with bolts and washers : you pull hard towards the top but also towards the front of the van (hard like in a accident..) on the L-Track -» Screws or bolts will mainly work in traction -» the floor sheet metal will shear very easily (it's the weak "link") -» flying seat -» not good.

Now, to compensate for that and improve the weak link, what's to be done is to reinforce the sheetmetal anchor points. You can't make the floor metal thicker (the original one), even if you spread big-macs all over it for a few weeks... So one easy thing to do it is to spread the load -» Thicker sheetmetal plate under the original thin floor (1/4 in thick is mostly used but it doesn't only depend of the thickness, it depends of the surface of that plate) -» So you take the factory floor in sandwich between your L-Track and this thicker plate. My plate for example is 3in wide, 6in longer that my L-Track (which is 24in long), 1/4in thick steel) -» Tighten the bolts (Grade 10 blots are a good extra precaution).. and install a seat...
Now you have an accident -» the seat is thrown forward, sideways and/or upwards -» the seat pulls the L-Track -» the L-Track pulls the thick plate -» large surface, it might bend the original floor a bit but won't shear -» no flying passenger.
How does the L-Track pull directly on the backing plate ? trough the bolts, which are used as they are designed for -» They hold together 2 SOLID parts, with the slim van floor material in between. This material, while being weak because it's slim, is solid material, it won't compress -» Bolts are good to transfer load in traction (when you pull them straight), which is what we have here.

Now, we take this last mounting pattern, but we add a thick (1/2in ?) piece of compressible material between the L-Track and the factory floor... "compressible material" can be plywood or even worse foam... anything that will leave a mark even with a small hit with a hammer.
An accident happens, the seat pulls the L-Track (not only upwards, remember, it will mainly pull to the front or to the side), the L-Track tries to pull the thick reinforcement plate BUT while doing that some of the "compressible material" actually compresses... the bolt don't work on pure traction anymore, you're adding bending, which is some kind of shear for a bolt (in the thread) -» The bolt is not working as intended anymore, and actually bolts and screws are not that good with bending... the bolts will break -» Flying passenger -» NO GOOD !

No compressible material in an bolted assembly for such a use, none, never !


This is of course very simplified but I hope you get the point... even with my far from perfect english (not my first language, obviously).
Have a great day.

 

Another Method of attachment

You could do what mobility up fitters do, attach a steel plate across the flooring , secured through the original floor to a reinforced attachment under the vehicle. Then secure your brackets to that steel plating. You could then add whatever flooring you want around your attachment brackets if you didn't want to just have a steel plate floor

Good Luck

Semper Fi

 

Bolts need to be properly Torqued to meet their load rating. You will not be able to properly Torque a bolt through foam or wood. As Ben was alluding to - the idea is to keep the bolt under constant tension, which prevents shear loads and reduces fatigue from cyclic loading.

20 years ago I engineered secondary structure mounting on passenger trains, including bolt joint analysis. Hope this helps.

 

Yep, steel/aluminum plate or tubing all the way to floor to match foam/ply height as needed.

MAYBE metal spacers like washers, but that gives a fulcrum point for movement.

 

Obligatory "I am not an engineer and am not liable" warning.

I fabricated steel spacers by welding tubing to flat bar (see photos). The L track flange is barely touching the floor. All the real torque is going through the spacer. 100% agree on @surly Bill about the fulcrum point. To combat the fulcrum issue, the spacer is wider than the L-track, the spacers are directly over the boron steel stiffeners. Additionally, the fact that the seat has two legs, spaced across the axis of the fulcrum point, combats any fulcrum movement.

FWIW, I had a friend who is an engineer at tesla (formerly boeing) help out and he gave his unofficial thumbs up.

My floor is a mix of wood and foam to fill in floor ribing, then 1" cork insulation, then 3/8" ACX plywood, then vinyl garage flooring.