Carbon Fiber?

[StevenMosley]

I'm looking at springs/sturts, sways, and tower bars for the BMW.

I am not sure what to think about Carbon strut tower bars. Are they more show then go or are they stronger. They are about the same price as aluminum ones.

http://i1.ebayimg.com/04/i/05/a6/3a/a2_2.JPG

[SUV-ETR]

I'm not an engineer, so take it for what it is worth, but I spent much of last summer working out a custom bar for the Scirocco with people who should know this stuff.

One of the only things CF doesn't handle well is compression stress. And as far as I'm aware, strut tower bars counteract a compression stress between the tops of the strut towers (i.e. the towers are trying to get closer together...not pulling away from each other). Thus, I've never been very keen on the idea of CF stress bars. IMHO, 4130 steel or properly selected aluminum would be the way to go.

For the same reason, avoid anything that utilizes spherical bearings, pivots, or anything other than a very rigid design. Unless it triangulates to something else (firewall, etc), it just turns an open box into a parallelogram with pivots at the corners, which is basically worthless. The downside is that any amount of chassis variation means the bar has to be basically custom-made or customized to fit your car. But, if your car is flexing itself out-of-whack, that just means that you REALLY need one, right?

Neal
(65hp econoboxes from the 70s don't seem to be much stiffer than the grocery bags they carried...)

[Rex Jr]

Neal
(65hp econoboxes from the 70s don't seem to be much stiffer than the grocery bags they carried...)

Someone once told me that the Mk1 VWs are the reason mass-produced stress-bars were invented. Having autox'd both Rabbits and Sciroccos, I tend to agree.

[AlexL]

I'm not an engineer, so take it for what it is worth, but I spent much of last summer working out a custom bar for the Scirocco with people who should know this stuff.

One of the only things CF doesn't handle well is compression stress. And as far as I'm aware, strut tower bars counteract a compression stress between the tops of the strut towers (i.e. the towers are trying to get closer together...not pulling away from each other). Thus, I've never been very keen on the idea of CF stress bars. IMHO, 4130 steel or properly selected aluminum would be the way to go.

For the same reason, avoid anything that utilizes spherical bearings, pivots, or anything other than a very rigid design. Unless it triangulates to something else (firewall, etc), it just turns an open box into a parallelogram with pivots at the corners, which is basically worthless. The downside is that any amount of chassis variation means the bar has to be basically custom-made or customized to fit your car. But, if your car is flexing itself out-of-whack, that just means that you REALLY need one, right?

Neal
(65hp econoboxes from the 70s don't seem to be much stiffer than the grocery bags they carried...)
Carbon fibers themselves have almost no compressive strength, so almost all the compressive strength comes from the resin. There are scores of different resins available, and they can provide a wide range of compressive strength characteristics. Properly made with the right amount of the right resin and cooked in an autoclave, a carbon fiber strut tower bar would work quite well. However, I seriously doubt you'll find a piece made with the proper process and materials to do as good a job as steel or aluminum for the same or less weight, and if such a device existed, I wouldn't want to pay for it.

Aluminum would be a slightly better option, but it's weak fatigue characteristics would turn me off. It seems that plain old steel may not be such a bad material after all. It may be heavy and low-tech, but it's strong, cheap, durable, and gets the job done.

[EWAustin]

For what it's worth, CUSCO's carbon strut bars are aluminum tubes with a carbon wrap on them. And in my experience CUSCO's suspension parts are very well designed and manufactured.

[washburn]

When racers use carbom fiber, the question is why? The answer is simple: Weight savings. Wrapping an aluminum bar with Carbom Fiber does not seem like the best plan to save weight. Building up a pure CF bar would also not be best since, as mentioned, to overcome the sucky compression resistance, you have to build up a structure so think that it now weighs more than aluminum or steel. (Reading about the new Lotus Elise, they now use steel for the pedal assembly instead of the cool billit aluminum. It actually lighter!) Don't use CF for the sake of using it. If it doesn't uctually save weight, it's a poser piece IMO.

[esper]

STi has equipped a few cars with various CF strut tower bars:

http://memimage.cardomain.net/member_images/12/web/540000-540999/540384_44_full.jpg

https://www.1stsubaruparts.com/catalog/images/i_strut_tower_brace_carbon_fiber.jpg

[SUV-ETR]

STi has equipped a few cars with various CF strut tower bars

...and as we know, Subaru has NEVER done ANYTHING just for the bling factor. ;)

<cough>

[StevenMosley]

If it doesn't uctually save weight, it's a poser piece IMO.

Agreed. Thanks all for your input.

I'm not sure how rigid the chasis is on E36. I'm not certain I need it on my daily driver, but what the hell. :)

[EWAustin]

I should have stated my view more clearly, my point in my original post is that they didn't go with a carbon fiber only bar, but instead they just wrapped some aluminum with carbon fiber to give it that appearance. Utilizing the strength and malleability of the aluminum and the fancy appearance of the carbon fiber.

As far as the "Poser" aspect, I think Carbon Fiber looks realy cool, and has it's weight and strength advantages, but there are definately places where carbon fiber doesn't belong. And CF stickers/applique is the single worst invention of the past 100 years.

[esper]

...and as we know, Subaru has NEVER done ANYTHING just for the bling factor. ;)

<cough>


At least its not as bad as this:

http://www.subaru-sti.co.jp/product/s201/c00.htm

[92MNstanger]

Formula 1 Cars have CF control arms, those see tremendous compressive and tensile loads (upper vs lower). Composites tend to offer great strength on one axis. Ever seen an F1 car clip the wall? I doubt however these "truly engineered cf components" are available on Ebay.