LocostUSA.com

I am trying to decide whether to use a steel, or aluminum floorpan, and what gauge to use.

If I go with steel I can just weld it on and be done with it. I know corrosion might be an issue using steel, but I am in Texas, so I don't have to worry about that. It will be heavier, but stronger, but if welded on, it is pretty permanent.
If I go Aluminum I have to drill and rivet a LOT of holes.... but it is then at least worst case removable.

Not sure which way to go, and I thought I would open it up for conversation.

What do you think are the pros/cons of each, what experience did you have, and what gauge did you use?

There have been a lot of threads about this...

Steel, stronger and more puncture resistant, can distort due to welding heat, and can be very heavy.

Aluminum, lighter, probably more expensive (depending upon thickness), easier to puncture, and more of a pain to attach.

Don't forget that unless the likely stitch-welded steel panel is fully sealed, driving through even one puddle is going to get water into the seam and start it rusting from the inside out. A compromise is to either fully seal the seam, or use stainless.

It really comes down to how much you care about total vehicle weight, and how concerned you are that some road debris might someday come up through the floor. I remember driving over one of those 6-ft long breaker bars on the freeway one time. I could only imagine the havoc that would cause if it got bounced up right before the next car drove over it. Too ugly to even think about... Much more likely would be a rock or piece of wood, but still. Remember, for most intents and purposes, what we're building aren't that much safer than motorcycles. You make your choices and take your chances.

_________________
Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

My car has a welded steel floor, I think 16 awg. If I build another, it will be an aluminum floor about .090" thick. That should be just as stiff as the thinner steel floor I have now, as well as lighter. I painted my steel floor, and the paint wears off pretty fast from shoes and grit, I also got a bit too much heat in it when welding it on, so that's a bit annoying. For the next one I would also make the floor pretty much cover the entire bottom of the car with small access holes for oil changes and diff/transmission access. A full under tray would make a small aerodynamic improvement, but a bigger improvement in chassis stiffness. Like KB58 mentioned on safety, I pretty much see it as a four wheeled motorcycle.
Kristian

_________________
V6 in a book frame build. Now registered.
viewtopic.php?f=35&t=7587
viewtopic.php?f=33&t=18172

I am going welded steel but still debating 16 vs 18 ga. I have sheets of both but still have alot of paneling left to do. I can either do the floor in 16 and the panel behind the seats in 18 or vice versa or buy another panel. I like the weight down low and the protection of the 16 ga floor but that will add more weight.

_________________
Build on

Chris
Build: NA Miata based +221 Se7en

I went with an 1/8" Aluminum diamond plate floor. It covers the entire bottom of the car. I think it weights about 55lb, which is heavier duty then most but I like my balls. I got an oil sucker and pull the oil out of the dip stick just like they use on boats.

_________________
2015 & 2016 EMod Florida State Autocross Champion
2013 & 2014 DSP Florida State Autocross Champion

Scrap Metal Build Log viewtopic.php?f=35&t=14558

I also went welded steel, 16 gauge, and I'm glad I did. Two years and 20K miles and rust hasn't been an issue. What I really like is how stiff and solid the chassis is. Aluminum probably would be as well, but I didn't much like the idea of rivets.

If (and that's a big IF) a steel floor is a serious rust issue, how does the frame still not rust if aluminum flooring is used? Steel floor for me and a continuous weld all along the outer edge. Stitch welded along all inside tubes with caulking between the welds to keep out "stuff". Nothing lasts forever. I think 30 years will be enough for me.

If choosing aluminum, consider the next step which is forming the aluminum along the bottom rail/floor and how std rivets may affect that forming and riveting. It is more work to use flush rivets. It is not a problem if a steel floor is chosen and welded along the outer edge of the square tube and ground flush. Either one will work. Success has been had with both materials. Your car, your choice.

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

I decided on the best of both worlds.

I put 12ga stainless on the vert. firewalls and forward floors of the driver & pass compartment. I did 14ga directly under the seat. I'll do the remainder of the car in .040 Alum, but also toying with using .018 stainless and welding it on.

As Wright said... I like the twins where they are and 12ga is bulletproof (.22cal) and I'll trade that weight off for the thin remaining under pan.

Rx7 hit the gorilla in the room though. Alum pan doesn't eliminate a rusting issue. It just moves it from the floor itself to the spaces between!

KS

_________________
Check out Firearm Finishes & Coating for options to ceramic coat your Locost parts. Hundreds of stock or custom colors including Chrome and Clear Coating options now available! High Temp options for hot bits!! Plastics too!!

Never become a pessimist. A pessimist is correct oftener than an optimist, but an optimist has more fun, and neither can stop the march of events.-Robert A. Heinlein

Because it's protected by paint/powder coat and an aluminium floor.

The big difference is that whilst an aluminium floor will not be affected by road gravel bouncing up underneath (and completely covers the steel, so protecting it), with steel such inevitable 'road rash' chips off whatever paint or rust-proofing you have applied and lets the rust take hold.

It will also usually be sealed against water ingress by Sikaflex or similar sealant/adhesive between the alloy and the steel (easy to apply a bead before riveting the floor on), whereas, as previously mentioned, a stitch-welded steel floor will trap moisture between the sheet and the tube, unless you take pains to seal it properly afterwards. This is why the sills (rocker panels, I think you guys call them?) are always amongst the first places to rust out on older cars.

Aluminium every time, for me.

absolutely no way you're protecting the internal portions of the frame rails from moisture the second you drill a hole for a rivet. powder coat the rails.. sure.. then drill through it, now you have an exposed hole that will not be 100% sealed by a rivet.

I'll be using steel, with seam sealer between the floor and rails, stitch welded every few finches.

_________________
Tucker

As I said above, it is normal practice to use a PU sealant/adhesive (eg. 'Sikaflex' or 'Tiger Seal') between the aluminium and the steel, when riveting.

This gives very effective protection where the tubes are drilled and riveted.

There are thousands of specialist sportscars running round the (salted in winter) roads of this depressingly damp island of mine, that are living testimony to the fact that this approach works.

I think it will be more resistant to damage from rocks under the car than steel. At least the reputation for small workboats is that the aluminum will deform a great deal before it actually tears and breaks.

I've always wanted to make a rig to hold a square piece of these materials and drop a pointed weight on it, like a window sash weight. How about a harbor freight anvil mounted on a 2x2 bolted to the basement ceiling joists? You could swing against a target sandbagged against the foundation. What do you folks think would be reasonable?

This is not really trying to represent something pointed but more like a round rock the size of a softball or more that you might go over at 100 KPH.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

Not to mention that even if you weld a steel floor on to avoid drilling the frame tubes for rivets, you're still going to attach the aluminum side panel with rivets to the same exact tube. At least the holes for attaching the floor are on the bottom of the tube so water will have as much chance of running out again as getting in.
Kristian

_________________
V6 in a book frame build. Now registered.
viewtopic.php?f=35&t=7587
viewtopic.php?f=33&t=18172

A sealer between the metals is good but does nothing to keep the water out. The individual rivets are dipped in waxoil (that you can make at home) or other sealant that doesn’t quickly setup.

The sides should wrap the edge of the floor. For an alloy floor, I’d seal, add rivets at the ends, then seal on the sides over the top and use rivets through the floor and the side skin overlap versus flush under the side skin overlap. For steel floor, stitched, painted, then seal the side skin overlap over the floors edge.

Horizon, here is a good article on the various hull materials: https://www.morganscloud.com/2013/08/06 ... o-impacts/

Quote: “Mild steel (until it corrodes) can be somewhat less fracture-prone in impacts than marine aluminum, and is also harder to abrade and easier to repair in the field.”

My reverse trike I’m building is using a .050” steel floor and firewall. My GO4s use aluminum sheet, about .080” thick which I have no intention of replacing with steel. I don’t know the alloy they used and it did corrode, allowing the powder coat to flake off in sheets. I used annealed 6061 to modify the floor on the P35 to get more air to the radiator.

Aluminum is a very broad term and all the alloys corrode in one way or another at different rates. Pure aluminum does not, but it isn’t a good structural material. Alclad sheet is an alloy with a thin layer of pure aluminum on top, but it can be scratched through and would be a poor choice for a floor.

In a fire, such as an accident, there is no comparison. Steel will stay solid long after the aluminum is a puddle. Also, in an accident, you do not know if you will be conscious or physically capable of escaping the car. I am not saying everybody should have steel; just sharing some info.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

My plan is to essentially weld a bunch of steel triangles into the chassis openings, with no overlapping the tubes. It's probably not worth the effort in the long run, and may actually wind up with a worse end result for all I know at this point, but apparently I'm a glutton for punishment when it comes to most of the ideas I find myself stuck wanting to pursue.

_________________
-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie