Aluminium or Stainless Steel for floor?

[Speedy5]

I have a spaceframe chassis on which I am replacing the floor. It is currently covered in a combination of .7mm aluminium and .5mm stainless steel. I would prefer to replace it with 1 complete piece. I know stainless steel is heavier but gives better protection against something coming up from underneath.
However, if I want to use stainless steel, how thin can I go? Will .3 or.4mm stainless give as much protection as .7mm ali ?
What do people generally use?

[Notso Swift]

The protection of 0.7mm Aluminium is, well, about none...

Seriously Ali will be better against larger blunter objects whail steel will resist piercing better

[phoenix]

For the same thickness of material the steel is about 2.78 x the weight of the ally and, if the ally is in T6 condition, 2.5 x the strength. The steel will be about 3 x the stiffness of the alloy too.

Out of interest, what is the area of the floor? We can then see just what the weight saving will be. 0.3 mm steel will be 17.6% heavier than 22 gauge alloy. For 1 square metre the weight difference between 22 gauge alloy and 0.3 mm stainless steel would be .35 Kg - 12.3 oz.

For this slight weight penalty, 0.3mm steel would 6% stronger and 23% stiffer than the existing 0.7 mm alloy. It would also have greater resistance to penetration and tearing.

[Speedy5]

Thanks for that info Phoenix. The floor area is 91cm x 230cm, i.e. about 2 sq metres. Hence .7kg heavier. I think that's worth it, don't you?

[Speedy5]

Further question about replacing floors:
I will, of course, drill out all the old rivets leaving holes in the chassis, which I will want to use for the new floor.
What's the trick to getting the holes drilled in the new floor to match the existing holes in the chassis?

[phoenix]

Originally Posted by Speedy5

Thanks for that info Phoenix. The floor area is 91cm x 230cm, i.e. about 2 sq metres. Hence .7kg heavier. I think that's worth it, don't you?

I would think so. And it's in the right place - as low as you can get it in the car.

[phoenix]

Originally Posted by Speedy5

Further question about replacing floors:
I will, of course, drill out all the old rivets leaving holes in the chassis, which I will want to use for the new floor.
What's the trick to getting the holes drilled in the new floor to match the existing holes in the chassis?

First warning: Drilling the old rivets out is likely to leave you with over-size holes so you may need to go up a rivet size.

The only way I can think of to line the holes up is to lay the old floor on the new floor and use it as a pattern for drilling. But I don't think it will work too well, to be honest.

IMHO, a better idea would be to use the old floor as a pattern to make new holes in the new floor and in the chassis between the old holes - if the spacing of the old rivet holes will allow this. Then you will have all news hole in both - and some of the 0.7 kg of the new floor will be offset by the extra holes in the chassis!

[terence]

Originally Posted by phoenix

First warning: Drilling the old rivets out is likely to leave you with over-size holes so you may need to go up a rivet size.

The only way I can think of to line the holes up is to lay the old floor on the new floor and use it as a pattern for drilling. But I don't think it will work too well, to be honest.

IMHO, a better idea would be to use the old floor as a pattern to make new holes in the new floor and in the chassis between the old holes - if the spacing of the old rivet holes will allow this. Then you will have all news hole in both - and some of the 0.7 kg of the new floor will be offset by the extra holes in the chassis!

And dont forget to de-burr all holes, this will stop the rivets from coming loose at a later date.

[ger80]

Originally Posted by phoenix

For the same thickness of material the steel is about 2.78 x the weight of the ally and, if the ally is in T6 condition, 2.5 x the strength. The steel will be about 3 x the stiffness of the alloy too.

T6 isnt very good in corrosion, isnt it?
Think I would use 2024T351 clad

[greenamex2]

Fibreglass sheet?

My kit car uses it and I've not have a problem in 28,000 miles.

[Speedy5]

Fibreglass sheet? Not considered that. What thickness do you use, and how heavy is it compared to Steel and Ali?

[greenamex2]

A couple of mm.

Not sure how the weight would compare or whether it would contravene any MSA regulation.

[rcarr]

I am building my Special Saloon Sunbeam Californian, personally I wouldn't touch steel for the floor! Aluminium or Glassfibre only.

The steel would be such a weight penalty, I am trying to remove as much weight as possible, if lyposuction was free I would use that! Anything to make my car faster up the straight.

[phoenix]

Nothing wrong with fibreglass as a floor, as long as it doesn't compromise the original design.

Panels, including floors, on space frame cars are often designed to carry part of the chassis loads, as they can increase the stiffness of the chassis considerably and so lighter (smaller diameter and/or gauge) tubes can be used. If replacing an alloy floor with any other material this should be considered. I had a Davrian with a fibreglass floor - but from memory it must have been 8mm thick.

Fibreglass has a Youngs modulus about 30% that of steel (I am using steel as a reference point only - I am not suggesting its use above any other material)

Therefore, to have a flat panel as stiff as steel made from fibreglass it would need to be 3.2 x as thick. And that is a minimum, as the layup glass fibre can be inconsistent so a tolerance thickness is normally added. Steel is only 3.12 x the density of GRP - so to get the same rigidity from fibreglass as steel you actually need a HEAVIER panel.

An 18 gauge steel floor in a saloon car, for example, would therefore need to be replaced with at least 5mm of fibreglass - if it was to offer the same rigidity as the steel. If the 18 gauge steel floor had pressings in it - which most if not all do - then it would be much stiffer - possibly 5-10 times as stiff. Ribs could be added to a fibreglass floor - but if they were not a flat fibre glass floor might need to be 25 mm+ thick. Glass fibres have incredible tensile strength - nearly 6 times that of steel - but once the glass is laminated with resin this falls to 1/3rd or less than the tensile stength of steel - even using the best ester resins.

Most designers who want to use composite floors would use a honey comb cored, twin skinned panel. This design gives the strength of a much thicker floor but without the weight penalty. This is how the earliest composite cars were built and how (civil) aircraft floors are made.

Going back to the Davrian, it had a contoured floor with some deep, box like structures designed into it. Curves and boxes add considerable strength to all materials, and composites are no exception. Another thing to avoid if using a GRP floor is heat. At 50 degrees C GRP has only 80% of the strength of GRP at 20 degrees. at 80 degrees it only hass 55% of the strength it has at 20 degrees. So running an exhaust under a fibreglass floor is not to be recommended!

[ger80]

GRP has a temperature problem, CFRP have a impact problem, steel in the needed thickness will be buckling if you are using a sheet only.
Sandwich plate sounds ok, but you have to take care than no water gets into it. Thats increasing the weight over the time.
Use Al, thats always ok. Maybe Al Li (AA2090), saves a few percent weight and gives a little higher stiffness, but of course more expensive
I wouls use 20mm Al (Li) plate and mill a sheet with rips on it out of that plate, but I think nobody else would use that solution here ...

[JohnD]

Having followed this thread with interest, learning all the time, I'm stumped by your post, Ger! "use 20mm Al (Li) plate and mill a sheet with rips on it out of that plate"? 20mm - that's nearly an inch thick! Are you building a tank?
Please explain!

I'm also interested in the composite question, Phoenix, having made non-loadbearing panels from composite GRP and paper honeycomb. Those achieved enormous weight loss and increase in stiffness (3 times lighter and ??100 times stiffer), but that I would not trust for a floor against penetration. Were you referring to alloy skins AND honeycomb?

John

[ger80]

Build a tank? No, I would use a 20mm plate and mill a sheet with integrated rips (for more stiffness) out of that plate. Well, I am working in aircraft industrie . 95% of the delivered material goes back as chips

[ger80]

For sandwich/honeycomb visit hexcel website.
There a some very interesting pdf files, for example one special guide for motorsports http://www.hexcel.com/NR/rdonlyres/C...ctorGuides.pdf

Honeycomb energy absorbing: http://www.hexcel.com/NR/rdonlyres/9...onBrochure.pdf

Design tips:
http://www.hexcel.com/NR/rdonlyres/8...and_Design.pdf

View all:
http://www.hexcel.com/Products/Downloads/Brochures/

[JohnD]

Thanks for those links, Ger, I'll have to look them up later - it's late!

And please translate 'rips'!
Would these be ribs, made by leaving thin strips un-milled when the rest of the plate is thinned by machining?
John

[ger80]

Originally Posted by JohnD

Thanks for those links, Ger, I'll have to look them up later - it's late!

And please translate 'rips'!
Would these be ribs, made by leaving thin strips un-milled when the rest of the plate is thinned by machining?
John

Yes, thats it. Sorry for my bad english ...

[phoenix]

Originally Posted by JohnD

I'm also interested in the composite question, Phoenix, having made non-loadbearing panels from composite GRP and paper honeycomb. Those achieved enormous weight loss and increase in stiffness (3 times lighter and ??100 times stiffer), but that I would not trust for a floor against penetration. Were you referring to alloy skins AND honeycomb?

John

In the civil aircraft industry resistance to penetration in a floor or a bulkhead is not a high priority. In the case of military aircraft it can become and important consideration. Some Helicopter floors are made from honeycomb cored laminates. The best cloths for resisting penetration use Kevlar or other aramid fibres - they are used in bullet proof vests for example. A layer of kevlar in the floor would almost certainly offer better penetration resistance than aluminium - probably on panels of the same thickness a thickness and certainly on panels of the same weight. So Kevlar skinned honeycomb panels would probably be a good - if expensive - solution. The honeycomb core could be aluminium, nomex or maybe even paper.

[ger80]

Penetration is not a problem in civil aircrafts in the upper deck of the fuselage but its a problem in the lower deck due to the condensation water

[phoenix]

Originally Posted by ger80

Penetration is not a problem in civil aircrafts in the upper deck of the fuselage but its a problem in the lower deck due to the condensation water

I was referring penetration by hard, pointy objects that could damage the driver's legs and other lower body parts

[trikesrule]

Hey what about a dimpled floor. I know someone else said they'd not recommend a f/glass floor but you could wrap your exhaust in Thermo-tec tape. The speedway team I crew chief for had a problem with melting the wires on the Starter. Wrapped wires and exhaust - problem solved and picked up some grunt at the same time (dyno proven). Back to dimples. I run a dimpled floor on the bottom of my gravity racer(s) (7 time consecutive champ) here in Tassie Australia. Faster with it than without. Was easy to make just laid some 450 woven rovings on a flat bit of steel sheet with a coat of PVA. Hey Presto one side had dimples the other side was nice and smooth. Trim with a 100mm diamond blade for a nice neat edge. Put some Black Pigment in the resin now everyone thinks I have a carbon floor - the twits. Also I use a Lazer Helmet with dimples and it is worth four kmh more than my Arai full face. Am fabricating a dimpled front spoiler at present for one of our cars. Always looking for more speed. Picture outside the box.........trikes

[trikesrule]

Worried about penetration using a frp floor. How about incorporating a sheet of alloy of your preferred thickness under the seat area? U could easily fit this in with lay-up........trikes