Strut Bar Benefit Testing Before and After

[tperazzo]

I totally agree that strengthening one section will move stress to another. Point well taken, thank you. I also look forward to driving this car and trying to bend stuff ��

[tperazzo]

I love before and after testing. Experimentalist, realist, or theorist??? I can't figure out which one fits me best.

Anyway, I did more torsional rigidity testing on my car for the heck of it. This time I tried to quantify the additional stiffness due to:

1) adding a weld in half cage (gusseted to the B pillar)
2) Removable door bars

Here are some pics of my car and the setup






It's been a while since I tested the seam welding (frunk only) and front strut bar but here are those results too. Note these results are all additive.

1) Seam welded trunk area 18% increase
2) Front strut bar with diagonal!!! 9% increase
3) Rear roll cage 9% increase
4) Removable door bars 4.6% increase

Granted my test isn't super scientific, I thought this group might like to see my results.

I would love to know how my car (not rusty) compares with a 964 or 993 model. Any volunteers?

[ChrisHoover]

alright enough of the fooling around, get to sanding! /s

[tperazzo]

Originally Posted by ChrisHoover

alright enough of the fooling around, get to sanding! /s

Ha ha, so true. Sanding is the bain of my existence.

Although I did sand some on Sunday which is rare for me.

[Flojo]

crazy level of "I-want-to-know"...

[tperazzo]

I repeated my strut bar loads test. This time it's a driving video measuring the strut bar forces with the same load cell as before.
Suspension is stock, tires in front are 7" with 205/60 tires.
The results are similar to my garage tests only 70lbs. Check it out. My feeling is the simple strut brace does nothing for our cars.
Video here :
https://youtu.be/pbX9luG637M

[Evan Fullerton]

Interesting data.

70lbs on that load cell is about .006in of deflection? Deflection would be more interesting data than load I think.

Street tires on a street car……. That’s probably in the noise of perceived feel…….. a track car with slicks hitting the rumble strips on a track at 1.5G of cornering is easily several times higher loading. On the 914 I autocross on Hoosier A7s, we see 1.6 to 1.7 mid corner G loading with shock loading pushing that higher. That’s been enough to fatigue crack the car in multiple places so it’s not always about just reducing alignment changes but also about prolonging the chassis life.

If you get the chance to drive a 911 with an extensive cage in it, the feel of the added chassis stiffness is immediately apparent just driving around the parking lot.

[PeteKz]

Tom, thanks for the follow up info.

I'm a mech engineer, so I tend to think about this kind of stuff a lot. I have never seen how the front strut braces could do much for a 911. The shock towers are already welded in at multiple points and braced from several directions. So I'm not suprised by your findings. I would be surprised if you found otherwise. Deflection of the shock tower by even .010" would change the camber hardly at all. The rubber bushings at the top of the shock probably deflect more than that.

As for Evan's comments about the increased rigidity of a chassis reinforced with a full roll cage, of course! In that case, the front and back of the car gets tied together diagonally with the roll cage, which is much stiffer than the middle of the car. But, that's not practical on a street car that you would actually want to drive.