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-   -   Strut Bar Benefit Testing Before and After (http://forums.pelicanparts.com/porsche-911-technical-forum/1018553-strut-bar-benefit-testing-before-after.html)

tperazzo 01-19-2019 10:40 AM

Strut Bar Benefit Testing Before and After
 
I've always been curious and a bit skeptical about the installation of strut bars in a 911.

I was also curious about the effects of seam welding, so I devised a method to measure the effectiveness before and after. That thread is here:
http://forums.pelicanparts.com/porsche-911-technical-forum/1015116-seam-welding-works-before-after-testing-video.html
This week I repeated the torsional rigidity test with the addition of a strut bar (Both the two point and three point variety)

This weeks video is really long, sorry about that....so here are the results:

1) Increased torsional rigidity is about 10%
2) Max load in the horizontal bar was measured at 70 lbs
3) The loads alternate between tension and compression
4) Changes in camber are very small with or without the strut bar

I will repeat this test when the car is driveable, but I stressed the chassis a lot. Almost 1500 ft-lbs, I dropped the car on purpose and drug it across the floor.

Another great thread on this topic is here:
http://forums.pelicanparts.com/porsche-911-technical-forum/399470-lateral-stiffness-911-front-strut-tower.html

I tend to agree with burgermeister that the strut bar in a torsion bar 911 isn't that effective.

I would love to hear your opinions otherwise. Have you noticed a seat of the pants difference with either a two point or three point strut brace?

Here's the link to my video:
<iframe width="560" height="315" src="https://www.youtube.com/embed/QjGYaeYew8s" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

madcorgi 01-19-2019 11:30 AM

Like your videos--great stuff.

tperazzo 01-19-2019 11:54 AM

Quote:

Originally Posted by madcorgi (Post 10322735)
Like your videos--great stuff.

Thanks man! I'm open to constructive criticism by the way:)

Trackrash 01-19-2019 04:18 PM

Wow, actual strut bar data! This should end the controversy once and for all. :cool:

I believe the strut brace can have some benefits. Especially if you are running wide sticky tires on the track in a long hood.

In my case I put a lot of tension on my bar, pulling the strut towers closer together aiding in more neg camber.

Edit, it should also be mentioned that the rubber upper strut mounts will allow more movement of the struts than the actual towers. So with out mono balls a strut brace may not have much affect.

pkabush 01-19-2019 06:22 PM

Thatís a pretty informative video. I recently bought a aluminum X brace ala 935 for my 930
I have to admit it was mostly for the cool factor. Years ago a retired factory Porsche mechanic told me the benefits of a single strut bar were slim to none. I think it would be interesting to see your test done with the X brace. I gotta believe that the Porsche race team used them for a reason.
Although, as you pointed out in your video the difference is probably far greater on the track.
Whenís the sanding video? :D

icarp 01-19-2019 07:48 PM

Tom , once again , well done and thanks for your efforts . I believe that that making a chassis stiffer and more dimensionally stable happens with the collective from all the efforts. The same as with weight savings . Gold star results !!!!!
Icarp

tperazzo 01-19-2019 08:04 PM

Quote:

Originally Posted by Trackrash (Post 10323013)
Wow, actual strut bar data! This should end the controversy once and for all. :cool:

I believe the strut brace can have some benefits. Especially if you are running wide sticky tires on the track in a long hood.

In my case I put a lot of tension on my bar, pulling the strut towers closer together aiding in more neg camber.

Edit, it should also be mentioned that the rubber upper strut mounts will allow more movement of the struts than the actual towers. So with out mono balls a strut brace may not have much affect.

Thanks for your reply! What happens to your hood gap when you dial in tension? And how much extra camber do you get? Are you referring to the really early cars that don't have adjustable camber plates?

I agree that the rubber donuts give much more than the strut towers do. The torsional rigidity test I did doesn't care if the strut is connected to the tower with rubber or mono ball.

I also don't think mono balls change the actual force that the strut tower sees. You might transfer more vibration and load spikes, but the cornering loads and damper loads have to goto the same place.

The rubber connection just allows the strut geometry to change more, that's all.

Like Burgermeister said and I agree don't expect the strut bar to help much with camber control. Monoballs are much more effective at controlling camber.

tperazzo 01-19-2019 08:16 PM

Quote:

Originally Posted by pkabush (Post 10323147)
That’s a pretty informative video. I recently bought a aluminum X brace ala 935 for my 930
I have to admit it was mostly for the cool factor. Years ago a retired factory Porsche mechanic told me the benefits of a single strut bar were slim to none. I think it would be interesting to see your test done with the X brace. I gotta believe that the Porsche race team used them for a reason.
Although, as you pointed out in your video the difference is probably far greater on the track.
When’s the sanding video? :D

Clearly experiments and obsessing about small details is more fun for me than sanding. What will I come up with next to avoid sanding? I know, through the body sway bar strengthening plates!

Here are my thoughts on the 935 X brace:
1) I respect the factory and believe there's a benefit
2) I'm confident that the X brace over the spare tire would improve the torsional rigidity. (in my setup). Note that I measure the torsional rigidity at the suspension pan front pickup points which isn't really fair! Those points are not as strong as the front crossmember attachment points. Even though it would show an improvement in my test setup doesn't mean that it will change the way the car drives significantly. The forces in the front pick up points are very different than the rear ones because the location where the strut attaches to the suspension arm (biased toward the rear). The front points experience braking loads that try to pull the suspension mounts wider. There isn't much force in the front up and down wise.

The X brace may not help much with cornering loads??? I'm not sure. I'd love to know more from experienced racers and engineers! I was planning on adding an X brace too, but after this weeks results, I'm re thinking the effort versus value.

3) The 935 was a coilover car right? There is a lot more force in the strut towers with coil overs versus torsion bars.
4) The original 935 brace I found a picture of didn't have a diagonal bar between strut towers. The X brace becomes the triangulated support. Perhaps the diagonal bars in the same plane as the towers is redundant if using an X brace also.

Trackrash 01-19-2019 08:37 PM

Quote:

Originally Posted by tperazzo (Post 10323224)
Thanks for your reply! What happens to your hood gap when you dial in tension? And how much extra camber do you get? Are you referring to the really early cars that don't have adjustable camber plates?

I agree that the rubber donuts give much more than the strut towers do. The torsional rigidity test I did doesn't care if the strut is connected to the tower with rubber or mono ball.

I also don't think mono balls change the actual force that the strut tower sees. You might transfer more vibration and load spikes, but the cornering loads and damper loads have to goto the same place.

The rubber connection just allows the strut geometry to change more, that's all.

Like Burgermeister said and I agree don't expect the strut bar to help much with camber control. Monoballs are much more effective at controlling camber.

First off, IMO the function of a strut brace would be to help control camber change. You are measuring torsional rigidity of the chassis. Interesting that the brace makes an improvement there. Porsche used that ladder shaped brace later on, which ties in the front torsion bar mounts. I would imagine that would be intended to increase the chassis rigidity. I wonder how much change a roll bar, a 6 point roll bar, and a full cage would make.

As far as the hood gap, well that is a long story. Many years ago when I only drove on the street and my car was very low, I was concerned about too much negative camber. To correct this, I used a strut brace to jack the front strut towers apart. IIRC it amounted to almost one degree. At that time the hood gaps were noticeably larger.

Now, since I autocross, I have done the opposite. I have Heim joints on the ends of my brace and use it to jack the struts together. I never measured it, but I would guess we are talking about over 1/2" total. My hood gaps look fairly normal now, but it also increased the curvature of the cowl, which was a good thing.:D I can gain 1/2* or more negative camber using the strut brace. BTW, I believe the impact bumper cars are much more rigid to begin with, too bad there can't be a comparison.

I also used thick machined washers on top of my rubber strut bushings to limit the struts movement.

997991992 01-19-2019 09:45 PM

good work

ian c2 01-19-2019 09:52 PM

Quote:

Originally Posted by tperazzo (Post 10323231)
Clearly experiments and obsessing about small details is more fun for me than sanding. What will I come up with next to avoid sanding? I know, through the body sway bar strengthening plates!

Here are my thoughts on the 935 X brace:
1) I respect the factory and believe there's a benefit
2) I'm confident that the X brace over the spare tire would improve the torsional rigidity. (in my setup). Note that I measure the torsional rigidity at the suspension pan front pickup points which isn't really fair! Those points are not as strong as the front crossmember attachment points. Even though it would show an improvement in my test setup doesn't mean that it will change the way the car drives significantly. The forces in the front pick up points are very different than the rear ones because the location where the strut attaches to the suspension arm (biased toward the rear). The front points experience braking loads that try to pull the suspension mounts wider. There isn't much force in the front up and down wise.

The X brace may not help much with cornering loads??? I'm not sure. I'd love to know more from experienced racers and engineers! I was planning on adding an X brace too, but after this weeks results, I'm re thinking the effort versus value.

3) The 935 was a coilover car right? There is a lot more force in the strut towers with coil overs versus torsion bars.
4) The original 935 brace I found a picture of didn't have a diagonal bar between strut towers. The X brace becomes the triangulated support. Perhaps the diagonal bars in the same plane as the towers is redundant if using an X brace also.

Yes , the 935 was Coilover .
As was the 73 rsr .
If you are saying the only forces on the front points are from braking , and the force is from front to back only , you must only brake when the wheels are facing straight ?
I do the same in my garage ...

I might be a little drunk , but I kinda remember a porsche Motorsport engineer telling me the d-zug/x-brace was only added as an headlamp support for night racing (winning lemans etc) and as a belt-and-braces effort to help support bad welds .

I allways thought it was only fitted to stop fiberglass hoods having a huge dimple at speed .

:D

VFR750 01-20-2019 04:39 AM

Combined with mono balls, I use the strut brace to gain a little bit more negative camber by preloading it in tension.

burgermeister 01-20-2019 08:42 AM

I love seeing actual data!! Thanks!

Also love your equipment! Is there a source for reasonably priced load cells and associated equipment? I could see that coming in really handy!

I must admit to using a 2-point turnbuckle style struct brace on my car … I use it to get an extra -0.3 deg of camber, apparently like many others :)! That's as much as I personally am comfortable with - I don't want to bend anything permanently. It's visible in the hood gap if I look for it, but otherwise not.

I ran a blind test once - I set the brace to zero load (so no additional negative camber) and had a friend randomly connect & disconnect it over 3 or 4 sessions in the morning. Neither I nor the data I took could tell a difference. I was running RE11's at the time.

Also once upon a time I made a very flaky attempt at measuring the torsional stiffness of my 911 chassis. I got 5200 ft*lb / deg, likely on the low side of the ballpark - all the errors & unknowns, and there are a lot of them, would tend in that direction. But pretty silly by todays standards even if my error is 50%.
That said, it's significantly stiffer than the roll stiffness at either end of my car (22/29TB & 23mm SmartRacing ARBs), and thus sufficiently stiff to allow tuning the lateral load transfer distribution.
http://forums.pelicanparts.com/porsche-911-technical-forum/404335-torsional-stiffness-1988-911-coupe-3.html

tperazzo 01-20-2019 10:00 AM

Quote:

Originally Posted by ian c2 (Post 10323291)
Yes , the 935 was Coilover .
As was the 73 rsr .
If you are saying the only forces on the front points are from braking , and the force is from front to back only , you must only brake when the wheels are facing straight ?
I do the same in my garage ...

I might be a little drunk , but I kinda remember a porsche Motorsport engineer telling me the d-zug/x-brace was only added as an headlamp support for night racing (winning lemans etc) and as a belt-and-braces effort to help support bad welds .

I allways thought it was only fitted to stop fiberglass hoods having a huge dimple at speed .

:D

That is funny about the X brace keeping the hood and headlights in place, wow! Could very well be true, that's why bench racing is so much fun, ha ha.

About the up and down loading of the front suspension points:
Even if you are braking and turning the up and down load is small compared to the cross member attachment.
Sure you can load either front tire by braking and turning, but the location where the strut attaches puts most of that load at the rear suspension pickups. The front points do take a lot of braking loads but they act from side to side. The X brace would work better for up and down loads.

tperazzo 01-20-2019 10:06 AM

Quote:

Originally Posted by VFR750 (Post 10323367)
Combined with mono balls, I use the strut brace to gain a little bit more negative camber by preloading it in tension.

This is so interesting to me that many people use the strut bar as a bodymans tool to pull the stut towers in! Necessity is the mother of invention!


I have to ask, why not use offset ball joints or slot the camber holes/modify the camber plate for more camber?

burgermeister 01-20-2019 11:10 AM

Personally, I never trusted the offset ball joints - I have not seen sufficient data to trust my skin to them. The 911 loads the balljoint in tension, and creating an offset in the same space necessarily means a smaller - and thus more highly loaded - ball, custom aftermarket parts do not necessarily have the greatest quality control or reliability testing ...

Modifying the camber plate only goes so far, then the shock hat interferes with the body. Removing the shock hat helps a little more - but then the jounce bumper loses its efficiency. One could make a custom jounce cup to restore that, but then the shock rod is still exposed - bad for the seal if one lives on a dirt road in a state where rain is pretty common. And the shock body still has to clear everything.

The strut brace seemed the easy path - low cost, reversible, near zero cost if you have a welder and some time.

What I wonder is why there are no +10mm span lower control arms. Liability might make it financially unattractive to make such a thing of course, and it precludes running wider tires on the front as well. Also, see 1) above.

VFR750 01-20-2019 11:21 AM

I am also use tarret offset camber plates. And I took the strut sleeve off and replaced it with a rubber bellows.

Have not stepped up to raised spindles or offset ball joints.

Car had the Weltmeister bar and rubber bushings when I bought it. So I use what Iíve got.

tperazzo 01-20-2019 11:54 AM

Quote:

Originally Posted by burgermeister (Post 10323634)
I love seeing actual data!! Thanks!

Also love your equipment! Is there a source for reasonably priced load cells and associated equipment? I could see that coming in really handy!

I must admit to using a 2-point turnbuckle style struct brace on my car … I use it to get an extra -0.3 deg of camber, apparently like many others :)! That's as much as I personally am comfortable with - I don't want to bend anything permanently. It's visible in the hood gap if I look for it, but otherwise not.

I ran a blind test once - I set the brace to zero load (so no additional negative camber) and had a friend randomly connect & disconnect it over 3 or 4 sessions in the morning. Neither I nor the data I took could tell a difference. I was running RE11's at the time.

Also once upon a time I made a very flaky attempt at measuring the torsional stiffness of my 911 chassis. I got 5200 ft*lb / deg, likely on the low side of the ballpark - all the errors & unknowns, and there are a lot of them, would tend in that direction. But pretty silly by todays standards even if my error is 50%.
That said, it's significantly stiffer than the roll stiffness at either end of my car (22/29TB & 23mm SmartRacing ARBs), and thus sufficiently stiff to allow tuning the lateral load transfer distribution.
http://forums.pelicanparts.com/porsche-911-technical-forum/404335-torsional-stiffness-1988-911-coupe-3.html

That test equipment is pretty old and attainable on ebay. The data acquisition system in the HP/Agilent 34970a. It has removable test cards in the back, but the one I have is a 20 channel multiplexer. The software is free.

Connecting the instrument needs a GPIB to USB adapter which is a little harder to find.

The black thing with red wires is just a 5V power supply, but its very stable which is good for strain guages. If I had more time, I could configure the data acquisition system to cancel out power supply variation.
Load cells are also on ebay, but might be best to buy new or at least check the calibration. Mine is an old general sensors brand

Nice work on the torsional rigidity testing! These cars are a little flexible!

Thanks for your blind testing results too. That is what I was looking for to confirm my theory. Awesome....

tperazzo 01-20-2019 11:59 AM

Quote:

Originally Posted by burgermeister (Post 10323800)
Personally, I never trusted the offset ball joints - I have not seen sufficient data to trust my skin to them. The 911 loads the balljoint in tension, and creating an offset in the same space necessarily means a smaller - and thus more highly loaded - ball, custom aftermarket parts do not necessarily have the greatest quality control or reliability testing ...

Modifying the camber plate only goes so far, then the shock hat interferes with the body. Removing the shock hat helps a little more - but then the jounce bumper loses its efficiency. One could make a custom jounce cup to restore that, but then the shock rod is still exposed - bad for the seal if one lives on a dirt road in a state where rain is pretty common. And the shock body still has to clear everything.

The strut brace seemed the easy path - low cost, reversible, near zero cost if you have a welder and some time.

What I wonder is why there are no +10mm span lower control arms. Liability might make it financially unattractive to make such a thing of course, and it precludes running wider tires on the front as well. Also, see 1) above.

I too wondered about wider arms but agree that no one wants the liability.

I'd rather fabricate new dust covers than mess with my hood gaps. That's just me and my obsessive nature.

Thanks for chiming in, I love the discussion and learn something new everyday.

aoncurly 01-21-2019 12:03 PM

Not trying to be critical, but I see a couple of questionable elements to your setup and test. First, I question why you cut and instrumented the top brace in the middle of the brace? By so doing, you create a "pin" in the middle. In tension, this is not an issue, but in compression it would significantly weaken the strut/brace. For the loads you are seeing, I suspect you are nowhere near the buckling force (again, in compression) of the brace, so you would not measure much difference. At more extreme loads, the strut or brace would deflect under compression, creating a weak system, and in theory, a collapse mode. A better (but not ideal) solution would have been to move the load cell to the end of the brace, where there is already a "pin".
Secondly, the addition of a diagonal for the loads you showed earlier in your video will not see much if any load. If the top strut/brace is in tension or compression, the diagonal sees no load. Only if there is a vertical load at the strut tower applied to the system (acting perpendicular to the top strut/brace) will the diagonal see any load. Also, if the diagonal sees any vertical load, it all ends up at the lower diagonal attachment point. If you are designing for some vertical applied load at the strut tower, you should have a diagonal in each direction or cross.
So, in summary, for the loads you have described in your video, the addition of the diagonal has no or little effect on the torsional rigidity. If the top strut/brace is significantly loaded, the strut/brace would collapse under compression.
I offer this commentary for your consideration.


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