Aluminum vs. Carbon Fiber

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#1

I’m building a wheel prototype that is similar to a mountain bike wheel. It will have 6 (round) tubular spokes.

I’m concerned about the strength and lateral stiffness of the wheel…more so than it’s overall weight.

If I built 2 wheels out the same size tubing (OD and ID)…one with aluminum spokes, and one with CF spokes…which would be the better choice in terms of lateral stiffness and strength? (The hub and rim assembly would essentially be the same for both.)

If I go with CF I would likely be using (0.075" OD) pultruded CF tubing from Dragonplate http://dragonplate.com/ecart/product.asp?pID=744

(Other recommendations are welcome.)

TIA,
Steve

#2

0.075"… 1.9mm? That’s a very thin spoke… probably thinner than some of the heavier duty metal spokes? How will you be able to use only 6 of them? Or do you mean 0.075" wall thickness?

I looked into using CF rod/tube to make a sidestand for a motorcycle. From what I remember it seemed like pultruded tube wasn’t really suitable. Because the fibres all run down the length of the tube they won’t be equally as strong in all directions. You could put a CF sleeve over them, but that’s a lot of extra work - may as well just use a roll wrapped tube/rod? In addition to fibres running the length of the tube, they have fibres would around the diameter of the tube. These can withstand all forces equally.

As for the ally Vs. CF I don’t think they would compare equally over the entire size range. For example at smaller OD my gut says that ally would be stronger, but as the OD increases I think that the CF would start to come out on top. At least that’s how I think it would work out…

Hope that helps some.

#3

Thanks @Mugget!
That was a typo regarding the tube size. It should have read 0.75" (or six 3/4" OD tubes).
So, at that size tubing, would you say that (roll wrapped) CF would be stronger and stiffer than aluminum…length and wall thickness being equal?

#4

Ah, ok - that makes more sense!

As far as the question of CF vs. Ally strength at 0.75" OD, I don’t have the requisite experience to be able to offer an informed opinion… I can’t even offer an un-informed opinion. :o

Also what ID are we talking? The other thing to keep in mind is that even if the CF tube is equally as strong & stiff as the ally tube - the CF tube could be lighter. If you can’t get a strength advantage, you may as well take a weight advantage.

In cases like this I would recommend getting your hands on some ally tube and CF tube, and doing some tests yourself. You can’t always just look at data, sometimes doing your own testing will give you much more info.