How much more so they weigh over an ally one?

Lots

i didnt think it was that much as the alloy one is thicker than the steel but the alloy one is a bit lighter.

i could be wrong though

" The modulus of elasticity of aluminium (whatever grade) is only about 1/3 that of steel. So the deflection of an aluminium structure will be 3x that of a similar steel structure. If deflection is important you will need 3x more `second moment of area` (approx 3x more weight) of aluminium in order to achieve the same deflection as the steel structure. Aluminium is also 1/3 the weight of steel so both structures would now weigh the same."

Taken from Engineers Edge website.
If you are going to race the car, then you will have to fit a steel floor.

Not much for a road car probably of the order of 3kg.
Flak did the sums here http://www.locostbuilders.co.uk/viewthread.php?tid=21928

quote:

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1.2mm steel floor alone weighs 12.5Kg. So say around 13Kg all welded in.

2mm ali floor weighs 6.5Kg. Remember for every rivet you remove some of the loor too.... Probably 7Kg riveted. Probably about a tube of PU adhesive...about 0.5Kg?. Seat strengthening depends on whether you put 2 steel strips in, or plates. Say strips, probably about another 2.5Kg. So in total 10Kg maybe.

Saving a whopping 3Kg . Of course the weights for the adhesive and steel are guesses...

David

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Well thats one job for the future I won't bother with then

Cheers all.

Had something fall off of a lorry thursday on the A14 which went under my car , I'm glad I have a steel floor!

I worked out 3mm ali is the same weight as 1.2mm steel.

Also the weight is really low down, this will therefore help lower your centre of gravity and improve handling*


* i'm not completely serious with this comment.

a lot of small planes are still built with steel spaceframes so it can't be all bad