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Old 09-07-2022, 03:30 PM   #1
Gerald81
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Default 07 STI Torsional Rigidity Measured (Results Inside)

Just thought I'd share this in case of interest.

I made a torsional rigidity test rig to test the effect of various chassis mods (present and future). This involved the following:

- Welding a set of coilovers to lock its length
- Fabricating brackets to mount to the wheel hubs
- Using aluminum extrusion profiles as axle beams to connect the left and right hubs (via the brackets)
- Rear axle beam lies flat on the ground with weights holding it down
- Front axle beam rests on a fulcrum made with pillow block bearings attached to a 30mm diameter shaft and 4 x levelling feet.
- Applying load to one end of the front axle beam while measuring the deflection in the chassis via the movement of the front axle beam using a dial indicator. I used a scissor jack on top of a corner weight scale pad to measure the amount of load. The angle of deflection for a given load can then be calculated.



It's not a perfectly accurate rig by any means, especially given the location of the fulcrum/pivot, and the fact that the hub brackets do not allow for any tilt as the chassis flexes. But I figured it would at least be useful for before-and-after comparisons on a relative basis.

For a start, I just measured the baseline outcomes at 30kg, 50kg, 75kg, 100kg and 125kg loads measured by the scale. I then took off the front tower strut bar and did the same. The outcomes are below, chart on the left shows the raw data points, while the chart on the right shows the line of best fit, with the gradient indicating the torsional rigidity in Nm/deg.

The data implies that the front strut tower bar improves rigidity by about 6.9% (8,249 Nm/deg vs 7,719 Nm/deg).



Also, the rigidity data is generally lower than I expected, as I've seen figures upwards of 20,000 Nm/deg quoted on the internet for the GD chassis. It could be the way the rig has been set-up (measuring hub-to-hub twist) or something else I've missed, but at least it can be used on a relative basis as mentioned above (kinda like a chassis dyno!).

I'm planning to replace my front and rear glass with polycarbonate shortly, and will measure again to see how much rigidity loss there is, and potentially consider other chassis bracing to make up for it.

For those interested in my chassis mods:

- STI titanium front strut bar
- Carlabs rear X brace
- Marche rear traction brace
- B-pillar harness bar
- Front MSI subframe
- Rear MSI subframe
- TSSFab T-bar
- TSSFab transmission mount
- Carbon roof panel bonded

For those interested in pictures of the rig:

Welded coilover:


Rear axle:


Front axle (a steel U-channel is used to reinforce the aluminum profile to minimize flexing):


Fulcrum assembly:



Overall setup (dial indicator was positioned below the beam, under the right front hub, measuring deflections upwards when load was applied from the scissor jack):
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Old 09-08-2022, 06:16 AM   #2
cueball89
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Very cool! Would it be possible to test the difference of the x brace? I think it was originally marketed as increasing rigidity by 10%.
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Old 09-08-2022, 08:36 AM   #3
Gerald81
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Quote:
Originally Posted by cueball89 View Post
Very cool! Would it be possible to test the difference of the x brace? I think it was originally marketed as increasing rigidity by 10%.
Actually I was very tempted to test the X brace as well - but then again my experience with playing around with those 4 bolts at the base of the X brace wasn't the best, so was a bit apprehensive with messing around with them.

But if a simple front strut bar gives a 7% increase, I certainly wouldn't be surprised if the X brace surpassed that figure.

I might try that the next time I measure the chassis flex, after the polycarbonate windshield and rear glass are in.
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Old 09-09-2022, 09:43 AM   #4
cboggess
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Quote:
Originally Posted by Gerald81 View Post
Actually I was very tempted to test the X brace as well - but then again my experience with playing around with those 4 bolts at the base of the X brace wasn't the best, so was a bit apprehensive with messing around with them.

But if a simple front strut bar gives a 7% increase, I certainly wouldn't be surprised if the X brace surpassed that figure.

I might try that the next time I measure the chassis flex, after the polycarbonate windshield and rear glass are in.
The wayback machine may have the info for the X-Brace from way back in the day when they introduced it. Would be interesting to compare.

BTW, that is very similar to the rig that they made to develop the X-Brace.
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Old 09-09-2022, 10:46 AM   #5
REX_WGN
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Weird how I can't see the pics on a desktop but can on my phone browser.

This is awesome! What compelled you to do this?

Have you considered an H-brace? I'd like to see how much it would improve the rigidity, or even the Oswald subframe but that's hard to acquire nowadays.

What about Fender braces? I think that would be a good one to test as well.
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Old 09-09-2022, 10:59 AM   #6
Gerald81
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Originally Posted by REX_WGN View Post
Weird how I can't see the pics on a desktop but can on my phone browser.

This is awesome! What compelled you to do this?

Have you considered an H-brace? I'd like to see how much it would improve the rigidity, or even the Oswald subframe but that's hard to acquire nowadays.

What about Fender braces? I think that would be a good one to test as well.
An unhealthy dose of curiosity, I think! Mostly to try and quantify the baseline, and how much rigidity might be lost when going from a glass to polycarbonate windshield.

I no longer have a H-brace as the MSI front subframe is not compatible with it. I do have a HKS Kansai front subframe brace, which originally has 8 points of connection when fitted with the stock front subframe, but again with the MSI front subframe, only 4 points are connected.

My 07 STI comes with factory installed fender braces, which seem pretty beefy on its own, so I saw no reason to upgrade them. I could test with and without, but given this is a professional shop which I'm leveraging on, there will be a labour cost involved. The X brace could be a simple test which I will consider trying when the car is next on the rig.
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Old 09-09-2022, 11:10 AM   #7
Gerald81
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Originally Posted by cboggess View Post
The wayback machine may have the info for the X-Brace from way back in the day when they introduced it. Would be interesting to compare.

BTW, that is very similar to the rig that they made to develop the X-Brace.
This is all I could find online currently:

https://forums.pelicanparts.com/porsche-911-technical-forum/404335-torsional-stiffness-1988-911-coupe.html
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Old 09-09-2022, 11:41 AM   #8
REX_WGN
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Quote:
Originally Posted by Gerald81 View Post
An unhealthy dose of curiosity, I think! Mostly to try and quantify the baseline, and how much rigidity might be lost when going from a glass to polycarbonate windshield.

I no longer have a H-brace as the MSI front subframe is not compatible with it. I do have a HKS Kansai front subframe brace, which originally has 8 points of connection when fitted with the stock front subframe, but again with the MSI front subframe, only 4 points are connected.

My 07 STI comes with factory installed fender braces, which seem pretty beefy on its own, so I saw no reason to upgrade them. I could test with and without, but given this is a professional shop which I'm leveraging on, there will be a labour cost involved. The X brace could be a simple test which I will consider trying when the car is next on the rig.
Very nice. And my bad on assuming it was your shop.

I don't think the STI fender braces were any different than the rest of the Impreza chassis but I could be wrong. I felt the difference when I upgraded to TIC fender braces over a decade ago. I was just curious if there was a difference between oem and any other aftermarket fender brace (although probably hard to find nowadays).
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Old 09-09-2022, 03:27 PM   #9
blurred
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Did you baseline your rig before testing?

How stiff is the rig/your suspension mounting points/your welded coilovers/deflection of bushings etc?
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Old 09-09-2022, 03:27 PM   #10
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&*^$#

Last edited by blurred; 09-09-2022 at 03:29 PM. Reason: dumbass forum
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Old 09-11-2022, 07:38 AM   #11
Gerald81
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Originally Posted by REX_WGN View Post
Very nice. And my bad on assuming it was your shop.

I don't think the STI fender braces were any different than the rest of the Impreza chassis but I could be wrong. I felt the difference when I upgraded to TIC fender braces over a decade ago. I was just curious if there was a difference between oem and any other aftermarket fender brace (although probably hard to find nowadays).
I think some of the older GD models had stamped steel braces in the fenders, which were changed to a steel tube design for the later GD series (like in my 07). The tube design seems stiffer. I'm not sure if this was only for STI and not the WRX, etc.
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Old 09-11-2022, 07:50 AM   #12
Gerald81
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Quote:
Originally Posted by blurred View Post
Did you baseline your rig before testing?

How stiff is the rig/your suspension mounting points/your welded coilovers/deflection of bushings etc?
No baseline done for the rig.

The aluminum profiles / beams are 100x100 with a mass of intertia of 447.2cm4. A U-channel made of steel is used to reinforce the front axle beam, due to the fact that it would be resting on a fulcrum in the centre with weight from the FL and FR resting on either end. There is definitely still some flex, but since the dial indicator is positioned on the same end as where the load is applied, I figured this is minimized. I did try taking dial indicator readings on the opposite end and got lower deflection numbers.

The welded coilovers should have minimal play. Top hat bushings are pillow ball all around.

Most of my suspension bushes are pillow ball, except for the front A-arm bushes which are rubber.

Overall, there would definitely be some movement from the rig as well as the suspension linkages, so I suppose the figures are best used in a relative sense, to measure contribution to stiffness from various braces, etc.
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Old 09-11-2022, 01:42 PM   #13
cboggess
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Quote:
Originally Posted by Gerald81 View Post
I think some of the older GD models had stamped steel braces in the fenders, which were changed to a steel tube design for the later GD series (like in my 07). The tube design seems stiffer. I'm not sure if this was only for STI and not the WRX, etc.
You are correct. 07 got the tubular which was an improvement, but still not as good as the aftermarket fender cowl braces. Someone did a really nice FEA study on iwsti back in the day.

Search for the username CarLabs here on Nasioc. The data may actually still be on this forum for chassis rigidity.
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