A Arms

[shapelessheart]

I was thinking of using Mild Steel A Arms coated with Kevlar Fibres....to reduce weight without compromising on strength.
MS 1020-
OD 3/8 "
WALL .035"
COATED WITH KEVLAR TO MAKE FINAL OD 14 MM

PLEASE SUGGEST / COMMENT

[shapelessheart]

I was thinking of using Mild Steel A Arms coated with Kevlar Fibres....to reduce weight without compromising on strength.
MS 1020-
OD 3/8 "
WALL .035"
COATED WITH KEVLAR TO MAKE FINAL OD 14 MM

PLEASE SUGGEST / COMMENT

[TGrau]

my concern would be that the steel and the kevlar/matrix structure might have different strain rates under loading.

[Jersey Tom]

What school are you from?

I suggest you (a) use the search feature on the forum to look up this discussion which has been gone over before, (b) don't bother with coating it in kevlar, (c) do the engineering and run the numbers and you tell us what you think quantitatively.

[fade]

3/8 od .035 wall coated in more steel might work better

[LU-Bolton]

Shapelessheart,

It's good that you are thinking outside of the box, but I think you should put a little more thought into it.

First, people do not use kevlar fibers in their a-arms because it is really strong. The reason they do it(UWA can comment here), is because kevlar is renowned for its energy absorption characteristics. So teams will coat their leading edge arms with kevlar in case they hit cones, to avoid brittle failure when an arm is put into bending. Look up the numbers yourself and you should see that carbon fiber is a far better material as far as strength is concerned. A steel a-arm is not especially brittle as compared to carbon, so why coat steel with something that can absorb energy(kevlar) if you're looking for something that is strong and stiff.

Second, the problem with this is that you will not lose much weight, which is your main goal here. Without doing the calculations myself, I would guess that you could lose more weight by moving to a 3/4" steel-only tube with an 0.028" wall as compared with the 3/8" tube covered in composite.

Third, the bond between your composite and steel is suspect to me, unless you are very meticulous and have the correct materials to do it properly.

So here's my advice: You have two options.

1) Your best option. Go with a steel tube design that has a larger OD than 3/8" (remember that moment of inertia is very important in the design of a-arms) Remember.... I = 0.25*pi*r^4

2) Make all carbon arms. Buy the carbon tube from a supplier seeing as you probably don't have good enough facilities to make them properly. They don't cost that much, look it up. But if you choose carbon.... test, test, test. I guarantee the modulus you use or guess to use with carbon will be incorrect. When suppliers quote strengths, they are often over-inflated values. I found this last year when I tested carbon tie-rods and pushrods. My factor of safety was anywhere from 6-10 depending on the rod I was testing, and not all of them passed design load.

Also be careful with carbon a-arms when hitting things like cones, I remember that Queen's had multiple failures like this when they ran them a few years back. They lost whole corners more than once if I remember correctly.

Hope this helps.

Aaron Cassebeer

[John Valerio]

we did lose a rear wheel in 04 enduro but that was due to a cv joint failure. we have had our fair share of cone impact failures though we've never had any tubes or bonded joints fail in compression or tension.

[Tommo]

You are correct in the use of the kevlar on the arms that are taking the cone hits. The Kevlar was there purely for its energy absorbing abilities. Carbon is known for its notch sensitivity and brittle failures, the kevlar is there to stop these. That said, this years car has done away with the kevlar altogether (the kevlar doesn't save weight, it adds it), no dramas with failures yet (our steering arm failure at oz was something else).

I would agree there is probably little to gain by wrapping your steel tubes in kevlar, unless you're planning on making them bullet proof. I would go with one of Aaron's suggestions. or fade's.
Testing is the key to the carbon arms, particularly buckling failures, will be significantly less (50% or so) of theoretical. If scotty DK still hangs around here he'll be able to chime in

cheers

[carboneater]

I saw a laminating handbook of a-arms from F1 car. There are many UD layers in it.
I also think that covering a steel tube with cevlar isn't the right way. You'll save no mass.
As Tommo said – the buckling failures are the big problems.
We tested bought-in carbon tubes (20mm dia and 1,5mm wall thickness) – they were absolutely unsuitable for using as a-arms. They had plastic buckling at 3000 N.
It is advisable to laminate the rod ends or inserts in, than you'll have no problems with pulled out rod ends in the enduro.

[Jack Reany]

Thought this might be a good place to insert a couple of pictures of our '06 car suspension. The a-arms are made from unidirectional carbon fiber, vacuum infused with vinylester resin onto a Divinycell foam core. We loaded each a-arm to 3 times the load we 'expect' them to see on the car (1.5 g lateral, braking, and accel, 3 g bump, and all possible combinations of these load cases.) All composite parts of the suspension went through similar Instron tests, (with some surpise failures and re-designs) and the inboard flexing portion on the a-arm was tested through full suspension travel for 10^6 cycles.

http://i110.photobucket.com/albums/n114/lehigh_jack/DSCN3987.jpg

http://i110.photobucket.com/albums/n114/lehigh_jack/DSCN5992.jpg

http://i110.photobucket.com/albums/n114/lehigh_jack/LowerFrontA-armTest.jpg

http://i110.photobucket.com/albums/n114/lehigh_jack/FSAE06005.jpg

[kwancho]

Wow. That's fricking awesome. Weight?

[Bill Kunst]

ooooooo, they look like wings, can it fly?

[LU-Bolton]

Alex,
Difficult for me to remember the weight off the top of my head... but the uppers were around 0.6-0.7 lbs and the lowers which were more severely loaded were around 0.9-1 lb. Again, don't take those numbers to be absolute, I haven't weighed them in over 8 months. Hopefully you'll get to see them this year when we come to West.

Bill,
As you can see, the upper "airfoil" creates lift while the lowers create downforce. Theoretically I guess the two would cancel each other out, but further wind tunnel testing will be necessary to explore this phenomenon. And yes, when we hit exactly 88 mph, the car does actually fly. lol. http://fsae.com/groupee_common/emoticons/icon_wink.gif Jokes aside, making the a-arms look like airfoils was the easiest way to manufacture them.

Aaron Cassebeer

[Bill Kunst]

just senseless humor. They look great. I thought that the first time I saw them and once again in this post. Nice job.
Bill