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Old 03-30-2020, 08:28 AM   #1
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Help with Control Arm Modification

I'm widening my car by 4"s by extending all of the suspension arms. I was hoping to get your opinion on my control arm solution, its not as clean as I was hoping it would be. I've posted pictures below to show you what I've done to add 2"s of length per arm. Do you think this is strong enough?


Here is the original Elephant Racing design


Here is the new design, with a 1' sleeve and threaded rod


This is the new design assembled





SPL makes a similar control arm, they extend the arm using this shiney piece, its the same dimensions as the sleave I'm using to extend the arm which is a good sign (my design also uses a threaded rod which should increase the strength


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Old 03-30-2020, 09:07 AM   #2
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Look at the size of the two tiny M8 studs used to hold the end on the OEM style GT3 inner bushing end, yours is way beefier than that.

I think this part gets most of its load in compression and tension vs shear.

Disclaimer: I am certainly no engineer
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Old 03-30-2020, 09:27 AM   #3
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Originally Posted by steved0x View Post
Look at the size of the two tiny M8 studs used to hold the end on the OEM style GT3 inner bushing end, yours is way beefier than that.

I think this part gets most of its load in compression and tension vs shear.

Disclaimer: I am certainly no engineer
Thatís a great comparison, itís shocking how small the studs are on the GT3 arms
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Old 03-30-2020, 09:29 AM   #4
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I'm no engineer, but that would make me nervous.
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Old 03-30-2020, 10:03 AM   #5
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I'm with Steved0x on this; compared to some of the other designs I've seen used, this is pretty stout.
I'm also not an engineer, so YMMV. But think about where the loads are, an dhow they're transferred; there's really very little opportunity for much shear there. You'll also get almost ZERO tension there (only as much as an unweighted inside wheel being drug towards the outside can create). Compression is really it. I think you're fine.
But don;t sue me if it breaks.
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Old 03-30-2020, 11:58 AM   #6
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How do you like Elephant Racing's stuff? Are they as good as they claim to be? They seem to be more popular with the aircooled crowd.
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Old 03-30-2020, 12:10 PM   #7
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Design wise probably not a big issue. Do you know the strength of the material? If it's equivalent to the bolts etc. probably won't be a problem.
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Old 03-30-2020, 12:11 PM   #8
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Originally Posted by bcrdukes View Post
How do you like Elephant Racing's stuff? Are they as good as they claim to be? They seem to be more popular with the aircooled crowd.
I run the control arms up front and the toe links in the back. Extremely well made, user rebuildable and any shop can adjust.
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Old 03-30-2020, 12:47 PM   #9
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Thanks for the feedback. Always liked their stuff, but hard to get up in Canada. I ended up getting a set of quick adjust camber plates from Vorshlag Motorsports instead of Elephant mainly due to cost and ability to ship to Canada
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Old 03-30-2020, 03:16 PM   #10
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Originally Posted by bcrdukes View Post
How do you like Elephant Racing's stuff? Are they as good as they claim to be? They seem to be more popular with the aircooled crowd.
They definitely make great stuff. If I were to do it again Iíd probably go with SPL though they have built in extenders and come with trust arms. I really wish someone sold control arms that could use extended ball joints. This would allow you to lower the car more without screwing up your geometry. Iím thinking of making custom arms next winter that will accept these. It would be great to drop the car another inch or so.

https://www.splparts.com/
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Old 03-30-2020, 09:30 PM   #11
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I am an engineer.

In theory, the forces on a LCA are considered to be almost entirely axial (tension/compression). However, in real life, there are bending forces transmitted to the LCA.

The magnitude of the LCA bending forces are hugely dependent on the suspension design and the exact geometry. A McPherson strut suspension is more concerned with small packaging and low manufacturing cost as opposed to optimal suspension performance. Thus, there may be more bending associated with that design than a racing suspension.

With that being said, I looked at some finite element analysis of simple double a-arm suspension designs - mostly because car manufacturers don't publicly publish their engineering reports and racing engineering students do. The analysis showed that the bending forces on the lower control arm to be very small compared to the tensile forces (on the order of 1/100th).

I am going to go out on a limb and guess that Porsche over engineered the LCA and took into account the exact geometry of the McPherson strut so even if there is some bending forces, they are likely to be minimal compared to the tension/compression forces.

What does all of this mean? If it doesn't buckle in tension/compression, it shouldn't be likely to bend.

After you drive it a bit, INSPECT IT CLOSELY. I can't emphasize this enough. Since there is no engineering analysis supporting your design modification and materials selection, you have to verify it manually. You have to inspect it. You need to check the sleeve for any sign of becoming out of round. Then drive it some more and INSPECT IT AGAIN. You will likely have to inspect it on a regular basis for the rest of its life because you have no idea of the longevity or durability of this design approach.

With all of that said, I wouldn't attempt your project. Too risky for me. Like Quad said, it would make me nervous.

And like any serious nerd, I am more than willing to cite a good reference:

Using FE Models to Predict Suspension Loads

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Last edited by thstone; 03-31-2020 at 03:24 PM.
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