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PostPosted: Sat Jan 20, 2018 5:15 am 
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The ST5 rules impose a penalty on cars with upper a-arms of wishbones. As an example they list Miata's, S2000s and Boxsters however I don't believe Boxster's fit into this category. Boxster's don't have an upper a-arm, or wishbone on the front or back and instead have their wheel's position controlled by the strut housing with a MacPherson design. What do you guys think?

"2) If the vehicle has an OEM suspension design utilizing an upper “A-arm” or “Wishbone” type control arm on either the front or rear suspension, a Modification
Factor will be assessed. Conversion from an alternate design to an “A-arm” or “Wishbone” suspension design using non-OEM parts is not permitted

As an example of the difference here's a typical A-arm on a Miata and a Boxster setup

Miata
Image

Boxster
Image

Wikipedia definition of an A-arm suspension
"An automobiles, a double wishbone suspension is an independent suspension design using two (occasionally parallel) wishbone-shaped arms to locate the wheel. Each wishbone or arm has two mounting points to the chassis and one joint at the knuckle. The shock absorber and coil spring mount to the wishbones to control vertical movement. Double wishbone designs allow the engineer to carefully control the motion of the wheel throughout suspension travel, controlling such parameters as camber angle, caster angle, toe pattern, roll center height, scrub radius, scuff and more."


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PostPosted: Mon Jan 29, 2018 3:15 pm 
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The Boxster has an upper control arm in the rear. This allows variable camber with suspension travel just like an A arm suspension. A strut type suspension does not have this advantage. The rules don't specify what a multi link suspension falls under but it does have an upper control arm.

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PostPosted: Mon Jan 29, 2018 3:45 pm 
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The 911 has an upper control arm. The Boxster doesn’t.


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PostPosted: Mon Jan 29, 2018 5:12 pm 
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Interesting. When I searched for boxster suspension diagrams all of them directed me to pics with upper control arms. So if I was you I would not take the modification factor. If someone protested just remove a front and rear wheel and ask them to point to the upper arm. I would also send Greg an email with a recommendation to change the example in the rules.

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PostPosted: Mon Jul 09, 2018 3:39 am 
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Late response...

From the image in the first post, I do not see an upper A-arm or wishbone suspension member. But, it would be helpful if you can post actual photos of the suspension in question.

From my conversations with National Director Greg Greenbaum (and they must have been via telephone, since I can't find them in my emails), a car with a multi-link suspension would not take the upper A-arm/wishbone adjustment factor if it does not specifically have a single upper member that looks like an A-arm or wishbone. Just because a non-strut suspension has multiple links does not necessarily mean that it has an upper A-arm or wishbone. Multi-link suspensions in current Mustangs and Camaros (I specifically mention them because I owned/own one of each) do not contain upper A-arms/wishbones; as such, the adjustment factor does not apply to them. With this being said, it seems to me that these are some fairly advanced suspension designs that could out-perform others with upper A-arms/wishbones. I think that this term is a shared feature that was used to single-out specific cars without mentioning them by name.

If you remove all of the upper arms/links and scatter them across the garage floor, do any of them have the form of an A-arm or a wishbone?

Mark

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