Carbon fiber thickness

[perky416]

Hi everyone,

Im going to have a bash at creating my own carbon fiber motorcycle fairings. My original fairings are made from 2mm thick abs plastic. I was just wondering what people thoughts were as to how thick I should make the carbon fiber to give it the same strength as the original fairings?

Thanks

 

[MikeHalloran]

The word 'strength' probably means something different to you than it does to an engineer. Try making a couple of test pieces, say 1mm and 0.5mm thick, and see how you feel about them, then decide what you want.



Mike Halloran
Pembroke Pines, FL, USA

 

[PeterLester]

Motorcycle fairing.... Keep in mind that carbon fiber is brittle and will crack if too thin and/or not re-enforced. Start with a layer of kevlar fabric before adding several layers of carbon fiber. Or start with a couple of layers of fiberglass fabric. And always use epoxy.... not polyester resin.

I am guessing you will need a total thickness of 2 to 3 mm. Vacuum bag it to avoid excess work, sanding etc.

Graydon Tranquilla P.Eng Electrical
Industrial Oil and Gas Industry,
APEGBC, APEGGA, APEGS, APEGM

 

[RPstress]

2 mm of ABS was probably chosen for adequate stiffness and good surface finish. It also won't break if you fall off, as ABS has high strain to failure (it's likely to get a bit bent and scuff badly of course).

Modulus of ABS =~ 2.3 GPa (330 ksi).

If you use woven carbon (twill is quite pretty if it's bare with a surface ply of resin, but for a fairly curvy fairing you might find five harness satin easier to lay up) stiffness is going to be very roughly 50 GPa (7 Msi). Theoretically you could get the same bending stiffness as 2 mm of ABS out of 0.75 mm of carbon (bending stiffness goes with thickness cubed).

As PeterLester says, if you fall off then a carbon fairing is quite likely to break. You'll have to assess the likelihood of damage to you if so. If you're wearing leathers then the sharp broken edges might not do you too much harm.

It's also true that an aramid (Kevlar, Twaron, etc.) fairing would not be anywhere so likely to actually break in an impact (this is one reason that lightweight canoes are often Kevlar). For an aramid laminate with a carbon surface ply for esthetics the modulus would be likely to be about 3 Msi (20 MPa), giving a thickness of about 1 mm.

A hybrid of Kevlar and carbon might be good. There are some woven hybrids combining both fibres in the same ply. Oxeon’s TeXtreme wide tow fabrics look pretty cool (

http://www.oxeon.se/

).

 

[perky416]

Hi everyone,
Sorry i havent replied, my comp has been playin up, had to access the internet through my phone.

Thanks for all the usefull suggestions. Im going to be using the fairings for my track bike, when i came off it last the abs fairings broke into a million pieces anyway.

I was going to use a layer of 200gsm twill carbon fiber for the visible layer to make it look pretty and then 2 layers of the 300gsm kevlar carbon hybrid which hopefully will give some impact resistance. The end thickness should be around 1mm.

 

[RPstress]

Surprised the ABS broke up. It should have had about 30% elongation to failure. If it was glass fibre filled it would break at a low strain.

Your thickness sounds reasonable. Not sure how much service trouble Kevlar will give you. If you put carbon on just one side then it'll warp a bit, but shouldn't be too bad if it's a room temperature-ish cure.