Need advice for building carbon fibre strut tower/hatch brace

[brad76]

Hi,

I am planning on building a rear strut tower/hatch brace out of foam/carbon fibre.

As this is just a project I'm doing at home, I was going to use the method described in this article to build a bike frame (Polystyrene rod covered in fiberglass and then carbon fibre):

http://www.instructables.com/id/How-I-built-a-carbon-bike-frame-at-home-and-a-bam/

I plan on constructing the polystyrene frame in the vehicle, as to ensure correct dimensions.

I have attached an image of what I plan on constructing. I would just like peoples opinions on how I should modify the design or any tips on when I lay up the carbon. Alot of the carbon braces I've seen have aluminium strut tops which then have a hinge or a helm joint which joins to the carbon bar. If possible, id like to make the strut tops with polystyrene and carbon over them as well, making it a more holistic part.

http://i988.photobucket.com/albums/af7/b-rad_76/Composite/Render_1.jpg

http://i988.photobucket.com/albums/af7/b-rad_76/Composite/Render_2.jpg

Also how many layers of carbon fibre do you think I'd need for adequate strength? Do you think the 9 layers on the bars and 11 at the joints, used for the bike frame in the link above, will be okay?

Feel free to rip into my design as its not even correctly dimensioned & it's nowhere near the final form, it's merely done to give you an idea of what I'm aiming to achieve.

Thanks for looking,
Brad

 

[GregLocock]

You appear to have designed an assembly ideal for making from aluminium tube, and then covered it in carbon weave.

It might work.

Do you understand why people use a heim joint?



Cheers

Greg Locock


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[brad76]

Right, so you believe this will not be strong enough made from polystyrene wrapped in carbon fibre?

I was under the impression that most strut braces that include heim joints at the ends of the bar, use them for ease of assembly and adjustable length of the bar.

To my knowledge, the heim jointed or hinged bars are less favorable than a permanently fixed bar due to these type being only able to prevent horizontal separation and contraction of the strut towers, but do not prevent the independent vertical movement of the strut towers.

This is why i have not allowed for a heim joint at this stage.

Is there a reason i should be utilising these joints?

Thanks,
Brad

 

[GregLocock]

Because the strut towers flex. If you restrain them with a solid joint then you have to design for those forces. I must admit I had overlooked adjustability that is also a good idea.


Your thin- tube covered in weave design will work, it does not take advantage of the space available and all the details are wrong, so it will be heavy and inefficient, if it is strong enough, or it will break.

I very much doubt there is any real weight advantage in carbon fibre over aluminium if you design the CF part to look like an aluminium one.

Cheers

Greg Locock


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[patprimmer]

The carbon will be a lot more rigid than aluminium at similar dimensions, but has very low elongation at break, so the first time it is called on to restrain a big deflection it will break unless it is very strong.

Compared to carbon, aluminium will deflect a lot before it breaks or permanently deforms.

Greg is correct in the use of space.

To take good advantage of different materials we need to design for that material and it's unique properties.

Regards
Pat
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[Tmoose]

I never thought much about a tower brace resisting "vertical" motions. In building construction that is called a "moment connection." The resisting member and attachment need substantial "depth" in order to accomplish very much. AND, details that create a straight load path from one to the other

http://www.lusas.com/case/civil/images/hntb_conn_incr_46_front_iso.jpg

A pretty efficient use of material is a "truss", which your design generally resembles. Efficient truss design considers every joint a "pin" joint, and all members are straight, and must intersect. >>Every<< offset creates flexibility requires immense bending strength and stiffness.
Compare these 2.

http://www.streetsports.com/photo/v...excoupeb18c/images/Cusco_40mm_Strut_Brace.jpg

http://www.honsport.com/images/tanabe fstb.jpg

If I understand your design correctly, the deep X frame (resembling a truss) might be capable of offering useful resistance, but the pair of relatively thin (in the vertical direction) attachment lugs are going to have to be very stiff indeed to put the X frame to work. Gotta wonder if the lug/ears won't be forced to flex like a hinge, which they won't enjoy for long.

http://www.jennifersmithmayo.com/gallery/albums/FairsFestivals/ear_pinch.jpg

If the lower legs of your truss could pick up the structure it would let the stiff middle section get to work.

http://www.shelfmaster.com/img/steel_shelving.jpg

 

[evelrod]

I agree with what has been said, so far.

From a practical point of view, say for the rear strut towers of a 240/260Z (which have a flex problem when set up with race level suspension), what you propose will just NOT do the job. While there are definitely vertical forces present in the struts, the primary forces are horizontal. From what I have seen over the years is a standard, Rose/Heim jointed bar for the front McPherson struts. On the rear Chapman struts, most cars anchor the rear roll cage structure either to, or adjacent to, the top of the strut towers. Alloy steel tubular "X" bracing from the top of the strut towers to the rollover structure. All this in light weight tubing that is generally equivalent to equal strength structures in Al or carbon fibre. --- Not to say it cannot be used, I just doubt the weight savings (IF ANY) would justify the added complexity.

However, it does not need to be overly complex to get the job done. A couple of photos of very simple and effective installations, including one of a very neat street bar.

http://www.conceptcarz.com/view/photo/303367,2194/1970-Datsun-240Z_photo.aspx

http://www.datsunrestore.com/images/F.jpg

http://www.race-cars.com/carsold/other/1253306482/1253306482lm.htm

Rod