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Is the toe setting affected by the negative camber gain that occurs when the rear suspension is loaded?
 

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Discussion Starter #24
Is the toe setting affected by the negative camber gain that occurs when the rear suspension is loaded?
i feel the two go hand in hand in that as you roll toward more negative camber you also get toe in.

Although, as the suspension extends you roll toward positive camber, but still toe in.

My conclusion - yes, there is some affect. How much, I'm not sure yet. That'll take some more measuring.
 

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here's a stupid question:

can you (cost effectively) move the inner mounting point of the toe arm upwards?
 

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Finally this is going somewhere and I would like to compliment your work but I think that you are getting to the conclusion that this is a snow ball effect and everyday more variable will need to be accounted for like the next subject would be suspension compliance. I know that it looks good to see all the numbers in front of you but are those truly the dimensions that we are dealing with in the real world. There is a lot more to this than just measuring values with hand tools and alignment machines. It is ok for marketing and demonstration purposes but there is too much room for error to use these values as the base of you research. Remember the famous school quote, garbage in garbage out. To accurately solve this problem you would need to scan the entire suspension to locate pivot points then take the car to a place like Morse Measurement (www.morsemeasurement.com) to compensate for all the compliance on the suspension components at operating conditions to generate a new corrected x,y,z model to import to a real simulation program like Adams Mitchell or even the Lotus version to plot out suspension travel according to ride height. Then after making the new projections based on your new parts, go back to the tester to validate your work. To finally realize that you might have to make new uprights, control arms and replace the rubber bushings for spherical bearing types which at this point is not cost effective. This is not a secret; in a nutshell this is what real race teams go through every year they just do not make parts for street cars.
 

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Discussion Starter #28
Finally this is going somewhere and I would like to compliment your work but I think that you are getting to the conclusion that this is a snow ball effect and everyday more variable will need to be accounted for like the next subject would be suspension compliance. I know that it looks good to see all the numbers in front of you but are those truly the dimensions that we are dealing with in the real world. There is a lot more to this than just measuring values with hand tools and alignment machines. It is ok for marketing and demonstration purposes but there is too much room for error to use these values as the base of you research. Remember the famous school quote, garbage in garbage out. To accurately solve this problem you would need to scan the entire suspension to locate pivot points then take the car to a place like Morse Measurement (www.morsemeasurement.com) to compensate for all the compliance on the suspension components at operating conditions to generate a new corrected x,y,z model to import to a real simulation program like Adams Mitchell or even the Lotus version to plot out suspension travel according to ride height. Then after making the new projections based on your new parts, go back to the tester to validate your work. To finally realize that you might have to make new uprights, control arms and replace the rubber bushings for spherical bearing types which at this point is not cost effective. This is not a secret; in a nutshell this is what real race teams go through every year they just do not make parts for street cars.
A very accurate model can be made measuring by hand, and compliance can also be figured as well. It'll just take more time, and patience.

Remember - folks were doing this long before computers and 3D scanners. Also remember - there hasn't really been all that much revolutionary in suspension setups in about 50 years. At least not for a production street car such as the STi.

I don't need a 3d scan or cloud of points to tell me the length of the toe arm. It's a simple part, and simple to measure.

Also, keep in mind the paper here is for general "what the heck does this thing do" information. As our data becomes more accurate and measurements just as accurate we'll be using that. For right now we're at the stage of how things work.
 

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From what I can see, the dynamic toe change wouldn't be linear because of the compliance of the bushing on the inside end of the link and the ball joint on the outside. This would in my eyes make the snap oversteer not very predictable. Is anyone actually experiencing it?
 

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Discussion Starter #30
From what I can see, the dynamic toe change wouldn't be linear because of the compliance of the bushing on the inside end of the link and the ball joint on the outside. This would in my eyes make the snap oversteer not very predictable. Is anyone actually experiencing it?
That and the change from arm motion isn't linear either. Take a look at the screenshot of the spreadsheet.

On the snap oversteer - lots of folks are seeing it, us included. Now, the question is - is the snap oversteer due more to the toe change or due more to bottoming out the suspension travel since it's so short. I have a feeling it's more a suspension travel issue, but we need to set up some tests to be sure.
 

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Re: 2008 rear suspension part II: Toe change/Bumpsteer

Additionally, it was noticed by our guy we use for alignment as his setup is one that can load the suspension and take measurements.
Were you able to take readings at various suspension heights? It would be useful to know the actual amount of toe change.

Frank
 

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That and the change from arm motion isn't linear either. Take a look at the screenshot of the spreadsheet.

On the snap oversteer - lots of folks are seeing it, us included. Now, the question is - is the snap oversteer due more to the toe change or due more to bottoming out the suspension travel since it's so short. I have a feeling it's more a suspension travel issue, but we need to set up some tests to be sure.
I suspect you are right on this. I have yet to experience the snap oversteer with the KW's, except for power oversteer at low speeds, which has always been there with subarus.

Frank
 

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Discussion Starter #33
Re: 2008 rear suspension part II: Toe change/Bumpsteer

Were you able to take readings at various suspension heights? It would be useful to know the actual amount of toe change.

Frank
I've got it roughly calculated, but I want to set up a toe board and dial gauges and just do a real world measurement first.
 

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OK, I know this is a dumb question and I have donned my flame suit so let em rip.

Would a 19" wheel on a lowerd STI exacerbate the problem described above?
 

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TiC, I'm not sure your analysis is completely accurate. You mention toe-in occuring above or below ride height because the toe link is horizontal to the ground. I know we talked about this, and I couldn't understand why Subaru would do this. I found the error.

The false assumption is that the wheel travels perpendicular to the ground, which would be a tangent to the toe link at ride height. This does not happen since the wheel travels in an arc determined by the upper and lower control arms.

At droop, it appears the lower control arm and toe link are- for the most part- parallel. (Confirm this, I have only looked at the pictures featured here.) This means that- despite neither arm being parallel to the ground- there will be no initial toe change (at least for the moment the links are parallel). Only camber and the track width will change.

However, since the two links are different lengths, the links become further from being parallel under compression, creating the toe-in. The toe curve will accelerate as the angle between the two links grows. Since the links appear to be parallel at full droop, you'll get toe-in starting very gradually (but accelerating) at anything beyond full-droop. This also means toe-out going from ride height back to full droop, albeit a minimal amount since that's towards the beginning of the toe curve.
 
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