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Old 03-20-2018, 03:27 PM   #1
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GT3 Control arms - help with shims

I'm going to start down the rabbit hole with my '01 986 for autoX and street use. I'm coming from the Miata world and am still figuring things out with this car. The Boxster is completely stock with 18" RE71R tires. The alignment was maxed in the front with -0.3* and a touch of toe out. -1.8* in the rear.

After doing my first autocross, it understeered quite a bit. GT3 LCA's will cure the issue while keeping the car properly classed. I'd like to get about -2* front and bring rear back to -1.6* or so. My question is what do I need for shims? 16mm? 10mm? Do I need to buy a assorted bunch? Obviously, I'd rather not throw money around on shims.

Thanks in advance
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Old 03-20-2018, 04:13 PM   #2
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What tire pressures were you running, and how about your fromt sway bar.

The easiest way to reduce understeer is to install a stiffer front sway bar and increase the tire pressure in front.

Another "understeer" problem is going too fast into turns, not trail braking (shifting the weight to the front) and not powering out of the turns. A common beginner mistake, often encouraged by instructors is not to brake when you turn and only accelerate when going straight. You are not managing weight transfer if you do that. Yes, hard braking needs to be done before you turn, but some braking when you enter the turn is required for fast times and makes the car much more responsive.
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Old 03-20-2018, 04:46 PM   #3
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10mm shim gets you about 1 degree. 1mm shim gets you the exact same length of a stock arm. Hope that helps.

Shims are outrageously expensive for what they are. Google Demon Speed for shims. $5 for 1mm. $13 for 10mm. That’s for *one* shim.

I run custom made camber plates and GT3 arms on my 986. 3.75 degrees in the front (4mm shim) running 255 71R tires. 2.75 degrees rear running 285 71R tires. Setups vary on driver, springs, sways, course, pavement, Yada... Yada...

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Old 03-20-2018, 05:50 PM   #4
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What he said, 10mm of shims gets you about -1 extra degree of camber. They can fit around 20mm, I'm running 18mm on mine, recently raised up from the 12mm that were on there before. I got my shims from Tarett. They are expensive...
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Old 03-21-2018, 06:08 AM   #5
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Thanks for the info. And it seems that Demon Speed is a better deal.
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Old 03-21-2018, 12:29 PM   #6
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Since you want 2 degrees in front, looks like you need a 10,6, and 3. This is per a side. You can fine tune with the strut slots on the top. Most likely range from 1.5 to 2.25 playing with the slots.

Not sure what rules you are playing by.... you can also mod the factory strut tops for more camber. Just redrill the 3 holes and remount the bolts. Shave the corner of the factory strut top that hits the inner fender well. Costs nothing. Won’t get you 2 degrees at factory height and no arms, but it’s a step in the right direction.

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Old 03-21-2018, 12:55 PM   #7
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Modifying the top mount isn't in the cards - that would put the car in a highly modified class. GT3 control arms are allowed, with rubber bushings not heim joints.

Getting more camber seems like the best compromise to meet class requirements for 2 different racing groups that each have their own classification system. Its a compromise. And it helps set limits for me to not go overboard modifying a car that my wife drives to work periodically.
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Old 03-21-2018, 03:15 PM   #8
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Quote:
Originally Posted by Anker View Post

The easiest way to reduce understeer is to install a stiffer front sway bar and increase the tire pressure in front.
A stiffer front sway bar will increase understeer not decrease it.
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Old 03-22-2018, 05:08 AM   #9
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Originally Posted by truegearhead View Post
A stiffer front sway bar will increase understeer not decrease it.
Not according to Vic Elford in Porsche High-Performance Driving Handbook. I tend to trust his advice and knowledge.
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Old 03-22-2018, 05:19 AM   #10
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Quote:
Originally Posted by Anker View Post
Not according to Vic Elford in Porsche High-Performance Driving Handbook. I tend to trust his advice and knowledge.
I'd love to see this - as it's counter to nearly everything I've ever read or done in terms of car setup.
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Old 03-22-2018, 09:41 AM   #11
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Not according to Vic Elford in Porsche High-Performance Driving Handbook. I tend to trust his advice and knowledge.
Yes according to Vic Elford as well. Page 75

https://books.google.com.mx/books?id=1X3P8L72Nb8C&pg=PA75&lpg=PA75&dq=Vic+Elford+in+Porsche+High-Performance+Driving+understeer+front+sway+bar&source=bl&ots=43rpp5FCPk&sig=g9yAPdp26E-APrp-KB1UWOltVoM&hl=en&sa=X&ved=2ahUKEwj76_icvIDaAhUPzWMKHchtBGEQ6AEwAHoECAYQA Q#v=onepage&q=Vic%20Elford%20in%20Porsche%20High-Performance%20Driving%20understeer%20front%20sway% 20bar&f=false
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Old 03-22-2018, 10:05 AM   #12
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I read it three times to be sure -- I'd cut and paste -- but it would not let me.
it states -- stiffer front end -- more understeer.

Please feel free to double check my understanding.


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Old 03-22-2018, 11:23 AM   #13
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agree with the consensus that a stiffer front bar will make understeer worse.
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Old 03-22-2018, 12:01 PM   #14
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As a general rule, a stiffer front swaybar makes the car understeer more due to a quicker load transfer.

But it is not always the case. It has to do with the camber curve of the macpherson strut suspension.
We gain negative camber as the control arm moves towards horizontial. after the control arm moves past horizontial it actually pulls the bottom of the front tires inwards (loses negative camber).

The stiffer front sway helps prevent the suspension from compressing past the point where the front control arms go past horizontial and you start losing negative camber.

The load transfer effect can lose out to the extra benefit of better camber curve up front.

I just wished we did not have a strut suspension in back and had a multi-link like the 911. Dynamic toe during compression and rebound in the rear sucks...

Last edited by SoloPierre; 03-22-2018 at 04:25 PM.
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