savagecommander

how thick (or how many layers of what weight cloth) would i need to duplicate the strength of 3mm aluminum? i also was considering some carbon/glass layering as well, but im trying to save on weight.

c-cat

Alot depends on which characteristics of 3mm aluminum you wish to have. Aluminum is fairly ductile, while fiberglass generally isn't. What are you building or replacing with fiberglass?

savagecommander

its an attempt at a monoque(?) chassis. the original is aluminum, and i want to shave weight, so instead of cutting out the alum, was just going to lay up a glass one. its a simple box shape, so i was just going to pink foam the structure and lay it up. i didnt think glass would be strong enough alone so maybe carbon is my next best approach?

planebuilder66

My Feedback: (8)

You could do an inner skin and outter skin construstion, it would give it a composite constuction like a boston whaler has, ties the whole structure together and gives it rigidity like a unibody, but the space between is nothing more than foam.

MTK

Aluminum, aircraft grades have tensile strengths in the 50-60Kpsi range. Stiffness is also fairly high. Ductility is good also.

Glass layups with a good quality epoxy such as Pro Set or West Systems will produce tensiles in the 100Kpsi regime. Carbon lay-ups with the same epoxy systems will produce tensiles in the 200-300 Kpsi regime but with much greater stiffness than either glass or aluminum for the same thickness. Something in the 1000Kpsi is about right for stiffness. These will not be brittle either.

Carbon-glass composites will be somewhere in between the two sets of properties depending on the ratios you use. Foam or balsa inner with a carbon shell outr will surpise you at its stiffness and strength. BUT, and this is very important, you must capture the inner slab completely, especially around the sides. If not, it will separate quickly at this weak spot.

Weight savings of the balsa-carbon composite compared to the aluminum part will be substantial....my guess is around 40% the original weight unless you use really heavy wood. Carbon will be lighter than glass by around 40%; the glass-balsa composite will also save weight (not as much), around 30% over the original ally part.

All this assumes equivalent thicknesses are used and depends greatly on the chassis size and load carrying requirements

If the car is around 10-15 lbs, I would use 4-5 layers of 6oz glass over wood and extra around the perimeter. You would only need about 2-3 layers of 6K carbon cloth. Both sides must be covered with either with extra around the perimeter. Vacuum bagging is the best technique for building this composite....it draws out the excess epoxy leaving a greater percent of carbon, meaning lighter and stronger. But two layers of window glass work (in a sandwich) as long as you apply enough weight. Go for around 2 pounds per square inch, evenly distributed.

BTW, I've built a firewall for a 40 cc gas engine using the composite route....carbon/balsa. It's approximately 35 square inches in area and weighs 35 grams. The equivalent thickness plywood for example weighs 100 grams. The aluminum piece would weight in at around 200 grams. Your layup will need more carbon than mine requires, to carry the shock loads of road racing or off road racing.

This should give you a pretty good idea whether you want to tackle the project. It's wortwhile but it requires some special techniques and knowledge to achieve the results you are after

MattK