carnut
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| posted on 2/5/03 at 01:17 PM |
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Weight of fibreglass body compaired to ali
Which is lighter 20swg ally or fibreglass?
I know the fibreglass is less dense but its also thicker. Was just wondering.
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stephen_gusterson
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| posted on 2/5/03 at 03:16 PM |
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i answered a similar question in the last month.
put simply, alu is lighter.
atb
steve
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carnut
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| posted on 2/5/03 at 04:02 PM |
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Thats good cos ive made all of mine from ali for the reason that i like the look of it and it should make the chassis a bit stiffer.
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Alan B
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| posted on 2/5/03 at 04:08 PM |
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Steve...with respect...
Is it bollocks...
GRP is lighter...
Typical GRP is 1.6 to 2.0 gms/cc
Typical Ali. is 2.7 gms/cc
However, if you compare strength/weight ratios or stiffness/weight ratios it may be another story..
I'd guess Ali had higher srength/weight ratio but, grp higher stiffness/weight..
Anyone confirm.
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carnut
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| posted on 2/5/03 at 04:10 PM |
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Ill work it out myself.
How thick are grp panels normally? Anyone know the density of grp?
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Alan B
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| posted on 2/5/03 at 04:29 PM |
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As I said 1.6 to 2.0 gms/cc
Just to clarify...I just answered the question.... "which was lighter in general terms"......not the question you actually
asked....oops
And sorry to Steve too if I mis-read...good job I put the smiley..
I'm almost certain the typical GRP bodywork will be heavier than 16g Ali...because it is double the thickness..
Hoope that clarifies things.
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carnut
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| posted on 2/5/03 at 06:58 PM |
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ok, sorry about asking for the density. Its just i work in kg/m^3 and was unsure of your units.
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stephen_gusterson
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| posted on 2/5/03 at 07:17 PM |
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Alan.
anyone can say to me summats bollocks, but be prepared for either an argument or my admitting your right.
I am glad you disagreed cos you seem to be in agreement way to much with me recently for your own good 
Lets use good old fashioned units.
specific gravity.
essentially, water is at 1.0.
alu is 2.7
grp is about 1.7
at this point you go HAH - im right he IS talking bollox.
not so.
if you panel with 1.2mm alu, with a SG of 2.7, then thats 2.7 x 1.2 'sg units thick' = 3.24 SG units
My panels I have made with 5 layers of glass are about 5mm thick.
thats 1.7 x 5.0 = 8.5 sg equivalent units.
So, for any given mm of area of the part, at the thicknesses given, it has a 'through density' of 8.5 for grp and 3.24 for alu.
My job is designing x-ray machines that inspect food for contamination. yes really. not many of us about. Basic density theory is how they work. But
im no expert.
atb
steve
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stephen_gusterson
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| posted on 2/5/03 at 07:23 PM |
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densities
densities
 
Rescued attachment grp1.jpg
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Alan B
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| posted on 2/5/03 at 07:59 PM |
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Steve, if you are saying 5mm GRP is heavier than 1.2mm then I'd agree...therefore GRP bodywork is heavier...
However, If you compare identical volumes then GRP is lighter.....
I'm pretty certain we are agreeing here...
Time to move on....
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stephen_gusterson
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| posted on 2/5/03 at 08:19 PM |
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ys we are both agreeing
but surely, there must be an argument in here somewhere.
You have been wearing that jumper for months now.......must be a bit smelly in that florida heat 
atb
steve
ps
according to our theory, reliants, having to be below 400 or so kg, should have been made from alu not grp.......
[Edited on 2/5/03 by stephen_gusterson]
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Alan B
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| posted on 2/5/03 at 08:44 PM |
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Yeah I could do with another avatar pic..
That page you showed with data on...is it out of a green book by Pilkington's?
It certainly looks familiar....
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stephen_gusterson
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| posted on 2/5/03 at 10:23 PM |
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nope its from
the glassfibre handbook
by rh waring
isbn 0 85242 820 0
ebay have it.
its 'ok' but not great.
other interesting stuff like cheimcal resistance and stress properties and how to cure star cracks etc. tells you how to build a garden pond too, but
I bet you have a swimming pool
atb steve
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Alan B
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| posted on 2/5/03 at 11:41 PM |
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Yes,..... and a pond.....
It's hard to live here without a pool...
Fortuneately they are pretty cheap to buy and run here.
Come and have a dip next time you are passing.....
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stephen_gusterson
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| posted on 3/5/03 at 07:22 PM |
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you never know.....I might one day!
atb
steve
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Simon
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| posted on 6/5/03 at 09:07 AM |
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Steve/Alan,
My f/glass side panels (seat/footwell area) are approx 2mm thick (ie 2 lam). Gone thin cos they're going to be silicon'd to the chassis
tubes.
I think they're lighter than ally (which I failed to make - see other posting. I think you replied to it Steve)
ATB
Simon
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svante
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| posted on 6/5/03 at 11:12 AM |
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Heard that Westfield are recomending to use ali-panels under the fibreglass...
the ali adds strength and glass the looks....
Svante
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carnut
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| posted on 6/5/03 at 12:13 PM |
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Ive just approximatly calculated the weight i will save by paneling my indy in ali rather than the usual fibreglass.
approx area of paneling = 81 square feet = 7.5 sq metres
volume of ali = 7.5 x 0.001 (thickness in metres) = 0.0075 m^3
volume of grp = 7.5 x 0.003 (thickness in metres) = 0.0225 m^3
mass = volume x density
ali mass = 0.0075 m^3 x 2700 kg/m^3 = 20.25kg
grp mass = 0.0225 m^3 x 1700 kg/m^3 = 38.25kg
This gives me a weight saving of 18 kg when you compair 1mm ali with 3mm grp. This may not be true for all fibreglass as some people make it thicker
and some thinner.
I also have the extra weight of rivets but this isnt very much even though ive used 500+ of them because the majority of the weight is in the stalk
which is thrown away.
Sorry to all you old farts who cant understand S.I. units but its what they teach you in a mechanical engineering degree so i have no choice.
[Edited on 6/5/03 by carnut]
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Simon
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| posted on 6/5/03 at 12:31 PM |
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Carnut,
It wouldn't be right for me to argue with someone doing/done a degree, however, I weighed one of my (bought) rear arches and it weighed about
2lb. Assuming all arches are the same size (which they ain't), that'd be 8lb on arches, 12lb for both side panels, say 10lb for the
nosecone 20lb for bonnet and 15lb for scuttle.Total 65lb (29.5 kg).
I'd say your est for f/g of 60.75kg and saving of 40.5 kg, might turn into a saving of 9kg.
I'm on a diet and have lost 4kg since Easter:-) Another 16 to go!!
Polished ally will look nice though!!
ATB
Simon
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carnut
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| posted on 6/5/03 at 12:46 PM |
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well id agree that the weight saved is over estimated but that was using the density and thickness given by others, if these values are inaccurate so
will the anwser.
I took in to account all panels including internal panels of which i havnt got panels in the inside of the outside panel (funny way of discribing it
but i allways was crap at english at school)
And im sure that you know as well as me that a degree is just a piece of paper and wether or not it means anything at all is debaitable. Practical
experiance counts more that double than formal education and im trying to increase my experiance by building a locost.
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Simon
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| posted on 6/5/03 at 01:06 PM |
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Carnut,
Interior panels - I've used MDF with radiused edged covered with foam and vinyl, till the SVA.
Very heavy, but very easy to make, and don't look too bad either.
If I get a chance tonight, I'll weigh the piece of ally I (badly) folded and compare it to the 'glass panel I made.
ATB
Simon
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stephen_gusterson
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| posted on 6/5/03 at 01:07 PM |
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the calcs are wrong - grp does not have the same density as alu - its more like 1.6 as shown in the table I posted.
This gives GRP weight of approx 36 kilos.
atb
steve
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carnut
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| posted on 6/5/03 at 01:39 PM |
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oh, i should have spotted that
Glad you pointed it out. I had it all correct on paper and cocked it up while typing it in. I thought it was a bit much.
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carnut
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| posted on 6/5/03 at 01:44 PM |
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I have now changed my calcs. The result is still quite good for the ali still being lighter.
It would be good to see how much the actual difference is in practice rather than theory.
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Alan B
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| posted on 6/5/03 at 03:13 PM |
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quote:
Originally posted by carnut
.....Sorry to all you old farts who cant understand S.I. units but its what they teach you in a mechanical engineering degree so i have no
choice..........
Oi young whippersnapper.....
(or did you mean Steve?, cos' he's old too?)
I did my degree in SI units.......just had to re-learn all the imperial nutsack when I moved to the US....
You are right though, you can't beat a practical project to make those boring classes now relevant...
(most of them still are a load of nutsack really...but there you go...)
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