Rear Shock Tower Gussets Fabricated For Coilovers

[tperazzo]

There are some aftermarket steel pieces to strengthen the rear shock mounts for coilovers, but I decided to make my own because 1) I also want to attach the rear roll cage to this area and 2) I hope this is a bit stronger. Here are a few pics, but more info in the video link below:






YouTube Video Link:
https://youtu.be/OvwP5Iy0864

Pelican threads about this car:
https://forums.pelicanparts.com/porsche-911-technical-forum/995074-hotrod-build-project-video-series-longhood-backdate-250-a.html
https://forums.pelicanparts.com/porsche-911-technical-forum/1010328-suspension-pan-rust-repair-video-series.html

[Walt Fricke]

I see you also reinforced the vertical seam where the cross member meets the longitudinals. I've had to weld up that area (and on the other front side also) a couple of times on my SC track car, due to cracks, and that's on a torsion bar car. Wish I could weld as well as you, though.

Do the raised edge punched holes contribute significant strength, like maybe reducing the slenderness ratio or something?

[brighton911]

Fine work there Tom. Video was very well done too. Thanks for sharing.

[175K911]

Like Walt said, important to gusset more than just the shock tower if you're converting to coilovers.

[tperazzo]

Originally Posted by Walt Fricke

I see you also reinforced the vertical seam where the cross member meets the longitudinals. I've had to weld up that area (and on the other front side also) a couple of times on my SC track car, due to cracks, and that's on a torsion bar car. Wish I could weld as well as you, though.

Do the raised edge punched holes contribute significant strength, like maybe reducing the slenderness ratio or something?

Wow, congrats on driving the car hard enough to crack the seam even without coilovers!

I doubt the dimpled holes provide any strength in this case. They are mostly there for style points. Notice that I didn't drill the plate behind it because I thought the holes weakened the gusset. One benefit of the holes is I can shoot epoxy primer in that hollow cavity to prevent rust!

Thanks for the welding compliment.

[tperazzo]

Originally Posted by 175K911

Like Walt said, important to gusset more than just the shock tower if you're converting to coilovers.

Thanks Ed,
I found those pictures somewhere before and decided to plagiarize some. Your's looks even stronger than mine.
I want to attach my rear roll cage here too. Makes more sense than attaching to the thin firewall parcel shelf area.
Thanks,
Tom

[175K911]

Just going to the firewall sheetmetal doesn't properly triangulate the chassis for coilovers. Have to go from the main hoop through the firewall to the rear gussets. In my case we went through the firewall with 1.5" x .120 DOM from the top corners of the main hoop to the rear reinforcing plates as seen in the pic in my earlier post, then welded .125" plates around the tubing onto the firewall to properly seal the firewall back up. And yes my cage is probably overkill. Oh, and it's white now too. At one point I wanted the entire car Ferrari Fly Yellow until my wife discovered the paint was $800/gallon and decided Glacier White was much better.

[KTL]

Nice fabrication work! You did a great job capturing all of the shock tower tube and also seam welding the "cap" on top of the tube. That cap has been known to fail with coilovers.

I agree that it's a good idea to take a look underneath the crossmember and plate weld the corners down below. It does indeed crack down there too.

[Inkblot]

Love the speed holes! Style points, for sure- and everyone knows that enough style points make your car go faster

In all seriousness, it must have been a real pain to weld all of that in such tight quarters. Great job!

Jake

[tperazzo]

Originally Posted by KTL

Nice fabrication work! You did a great job capturing all of the shock tower tube and also seam welding the "cap" on top of the tube. That cap has been known to fail with coilovers.

I agree that it's a good idea to take a look underneath the crossmember and plate weld the corners down below. It does indeed crack down there too.

Thanks, I'm going to have a look down there and see what I can do to improve strength. Here's a pic of the factory weld BTW. Not only is it porous but they missed the top seam completely 😀



Do you happen to have any pics of where the cracks appear underneath the car?

[tperazzo]

For those of you who run coilovers successfully, what do you think about this horizontal bar between the shock towers? I'm inclined to leave it off, because I don't think it will add much strength, but what do you think? It's about 2lbs.

[Ken911]

it's only two pounds and will stiffen things up more.

[Walt Fricke]

I'm monkey see, monkey do with some of this stuff. Peter Dawe welds in a cross bar like that, which has had me thinking about doing it, but without some cracking to prompt doing it it is mainly in the back of my mind if ever I cut out the center of the torsion tube (to lose the weight of the T bar sockets there, and make dropping the transmission easier), which requires a lot of other stuff..

[tperazzo]

Thanks guys for the feedback. I'm going to wait until the rear cage is finished and then think about it again.... The rear cage is going to tie into these gusset plates, so maybe that will be enough???

[Walt Fricke]

I think you'll never know. Do you have a way of measuring torsional rigidity? Got a fancy program which can model stresses and so on? Finite element analysis? For sure you won't crash test to see what happens. Fortunately, the rest of us don't crash really hard often enough to see which mounts work best.

[tperazzo]

Originally Posted by Walt Fricke

I think you'll never know. Do you have a way of measuring torsional rigidity? Got a fancy program which can model stresses and so on? Finite element analysis? For sure you won't crash test to see what happens. Fortunately, the rest of us don't crash really hard often enough to see which mounts work best.

I agree with both your sentiments (monkey see monkey do and lack of scientific method).

I for one find this stuff really interesting and I'm willing to do some real world testing with strain guages. Trouble is it will be a while before this thing drives, so for now it's all bench racing.

I'm currently making the front strut tower braces and it would be fairly easy to instrument the strut bar to determine forces in the bar. I saw the thread where the guy put the bathroom scale in there and did some static tests. Anyone else on here played with strain guages?

[Walt Fricke]

As I recall, he found that the stock front strut towers were pretty rigid as they are. The guy who did it was a genuine automotive engineer, but doubtless it would be interesting to use strain gauges or other instrumentation to get a better quantified idea of what forces do what. His conclusion was that the shock towers are pretty stiff as it is.

But like everyone else, I have a brace on one car, and a welded in structure on the other up there.

Movement of the rear shock towers ought not to affect the suspension much, since nothing articulates up there. I'd think there would have to be way more movement than there possibly can be there for coilover rate or shock characteristics to change enough that you could see an effect in your data from the track. Movement in the front can change camber, and cars are quite sensitive to that, but not here in the rear.

But of course things can fatigue and crack. And lots of us are always curious to read the results of testing.

[icarp]

Originally Posted by tperazzo

For those of you who run coilovers successfully, what do you think about this horizontal bar between the shock towers? I'm inclined to leave it off, because I don't think it will add much strength, but what do you think? It's about 2lbs.

Tom, the factory shape has a pinch weld across the top of the main structure , that is for further managing the stress of the hollow shape of the shock housing member . I think it would be very helpful to add the cross bar to the tower tops . I would also add a small vertical link from the pinch , where the small box is already in place , to the cross bar .
As far as deflection goes imagine just the entire shock mounting system and invert it,
and add pressure to the outer edges . Maybe that can help to see how the stress is transmitted . The extra height and the center tie will help stabilize the rear of the chassis, then add the roll bar and wow! just some thoughts . I like your project !
Ian

[Flojo]

Originally Posted by 175K911

looks like you got carried away welding, haha

[175K911]

Originally Posted by Flojo

looks like you got carried away welding, haha

Just a bit. Wanted to make sure the required additional bars that go through the firewall to the rear by the shock towers is proper reinforced and triangulated. I guess the old mechanical engineer in me took over, amazing how much of that old schooling comes back to you after 30 years of not using the knowledge.