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Learn me: Another sway bar thread (featuring math)

 
Old 07-25-2018, 11:56 AM
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Default Learn me: Another sway bar thread (featuring math)

Currently in the process of installing Xidas, 800/500 rates. The car currently has the spec miata adjustable rear bar with the stock front bar, which in combo with the prior 700/325 spec miata coilovers was pretty neutral. Doing some math, it looks like the front roll couple was about 59% with this setup. I found it to be pretty neutral on track, tending towards understeer, which I was okay with as I frequent Road Atlanta and it has a lot of walls.
So, what to do about the bars going forward? I plan to experiment with the options currently available to me, but it seems that the spec miata rear bar will make this thing an oversteer machine (FRC = 54%). However, with the stock front bar and no rear bar the car is back to FRC=59%. The roll stiffness of this setup is 2990lb-ft/deg, which is about 12% higher than with the spec miata combo I was running before. I didn't find that car to be too roly poly, and so (given that I'll be running a similar ride height) I'm not sure more is needed. I imagine the additional droop will help me put power down on the bumpy mountain roads that I frequent as well.
For reference, the recommended 1.125" hollow RB bar with a spec miata 14mm rear is also FRC=59% but the roll stiffness jumps to 3714 lb-ft/deg, a 30% bump over my prior spec setup.

I intend to try no bar, SM and stock rear bar before upgrading the front sway bar since that doesn't cost anything but I'm open to discussion.
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Old 07-25-2018, 02:23 PM
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Feel free to experiment for fun. But if you want to go fast the formula is on the same site where the shocks came from.
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Old 07-26-2018, 01:10 PM
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Pinch weld heights front and rear?
Camber front and rear?
Wheel width?
Offset?
Tire size/model?
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Old 07-26-2018, 02:55 PM
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You need a RSB to get the car to rotate on entry properly. You need the big FSB to compensate for this without requiring excessive front spring rate (you can do 1000/500 instead, but the car will be twitchy and unpleasant to drive). Buy the big FSB now and save yourself the headache, because the car isn't going to work right without it.
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Basically I've come over to the camp of "If something is a reliability problem on the track, just ask Andrew and do what he says".
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Old 07-28-2018, 09:17 AM
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Originally Posted by emilio700 View Post
Pinch weld heights front and rear?
Camber front and rear?
Wheel width?
Offset?
Tire size/model?
Haven't finished the install yet, but likely 4.75 front, a smidge higher in the rear (4 7/8 maybe). Camber TBD - was looking at street/track 949 setup but don't have elbj so might be limited on front camber. 15x9 +35 wheels, 225/45r15 Rival S tires.

Guess I'll head on over to the classifieds. How will that impact bumpy mountain road corner exit traction? On street boost I'm making over 200ft-lbs by 3500rpm
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Old 07-28-2018, 01:27 PM
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Originally Posted by Morello View Post
How will that impact bumpy mountain road corner exit traction?
You mean by going to a bigger front bar? I would think it would improve corner exit traction.
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Old 07-28-2018, 02:09 PM
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Originally Posted by hector View Post
You mean by going to a bigger front bar? I would think it would improve corner exit traction.
By running the requisite rear bar needed to compensate.

There's only one way to find out!
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