Bent vs. straight tubing strength

[65Stang0_1]

Can anyone quantify how much weaker a 17"x 1.25" OD x 0.095" 4130 tubing with a 15 degree bend in the center is versus an identical straight piece?

Any help with the calculations would be most helpful.

Thanks,

Vic

[Z06killinSBF]

You must be building a Monte Carlo bar huh?

[65Stang0_1]

No, a lower control arm. Due to my car's lowered stance, a straight lower control arm doesn't have much travel before hitting the frame rail.

I am looking for some concept or theory about a bent tube strength in relation to a straight tube. Any takers?

Vic

[JohnUlaszek]

You need to give more information about how its loaded.

[65Stang0_1]

You need to give more information about how its loaded.

Here is a photo of the setup with the suspension hanging. I don't have one at ride height. The lower control arm is the one connecting the bottom of the spindle to the frame rail (just trying to be clear here for non-Mustangers). The lower control arm moves about 1.5" before it hits the frame rail due to it being lowered. I was considering options on how to correct this. One option is to build the arm with a slight bend to allow for more suspension travel. I was going to increase the diameter of the tube from 1" to 1.25" and make it out of chromemoly. Another option is to relocate the inner pivot point to allow for more travel, but this will negatively affect the suspension geometry. I like the idea of using a round tube for a clean look, but if the strength is really compromised I may move to a rectangular tube as a third option.

Once again, I am looking for some theory that explains exactly why (and how much) a bent tube is weaker.

Vic

[astroracer]

Don't worry about it. The true, direct, compressive force required to "bend" a control arm only happens during a direct lateral hit (from a wall or another car). The upper and lower control arms major reactive force is radial which is induced by the movement of the tire. Hard corning will induce some compressive force but this will be far below any force required to rebend a short tube. Most of this compressive force will be taken up in suspension articulation and the bushings.
Take a look at most of the aftermarket control arms out there. You will see many of them utilizing "bent" tubing in their manufacture. It is a necessary thing when long control arms, massive rubber and super deep offsets are used and it is not an issue because the normal loads will never be great enough to hurt them.
Mark

[52deluxe]

i agree with Mark....

[telite]

lets see a pic where it hits the arm? are you sure its not the sway bar?
do you see scatches/wear as evidence?

you can see the bumper for the upper arm is there to stop further compression. does the "morris" kit not meet the bumper?

go too far and the tire will hit the top inner fender.

[Z06killinSBF]

Notch the frame out

[65Stang0_1]

lets see a pic where it hits the arm? are you sure its not the sway bar?
do you see scatches/wear as evidence?

you can see the bumper for the upper arm is there to stop further compression. does the "morris" kit not meet the bumper?

go too far and the tire will hit the top inner fender.

I'm designing my own coilover front suspension at this point. My lower control arm is similar to the Ron Morris but will be 2" longer like the '67 Mustangs. With my current mockup, I get get about 1.5" upward travel of suspension before the lower control arm hits the frame. I was just looking for some feedback on the amount of strength reduction on a bent tube versus a straight. I heard some feedback that the TCP lower arms had broken due to their bends. Not sure if this is true or not.

I am leaning toward using a bent arm, but may slightly notch the frame and reinforce it internally??

Vic

[novanutcase]

Notch and box!

[telite]

not sure if this will help,

you could always raise it a bit
.
keep us updated with pics, good luck