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Tire diameter % difference effect on ABS

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Old Nov 12, 2011 | 01:02 PM
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Default Tire diameter % difference effect on ABS
I am planning on running some non-OE tire diameters on my 996 coupe for track and AX duty.

Is there a % difference front to rear that effects the function of the ABS...?
What is the largest % difference I can run without confusing the ABS?

I'm looking at 245x40x17 front with overall diameter of 24.7"
265x40x17 rear with overall diameter of 25.4"

1999 996 C2

Thanks
Last edited by DanielT; Nov 12, 2011 at 01:39 PM.
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Old Nov 12, 2011 | 01:22 PM
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c2 or c4?
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Old Nov 12, 2011 | 01:56 PM
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1999 996 C2.
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Old Nov 13, 2011 | 12:00 PM
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C2,....Probably doesn't much matter for ABS,.... PSM/TRAC(virtual LSD), certainly.

ABS reacts to the rate the tire is slowing during braking, circumference has little or no effect.
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Old Nov 13, 2011 | 12:14 PM
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Originally Posted by DanielT
I am planning on running some non-OE tire diameters on my 996 coupe for track and AX duty.

Is there a % difference front to rear that effects the function of the ABS...?
What is the largest % difference I can run without confusing the ABS?

I'm looking at 245x40x17 front with overall diameter of 24.7"
265x40x17 rear with overall diameter of 25.4"

1999 996 C2

Thanks
have you lowered your car? those diameters are within acceptable range.
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Old Nov 13, 2011 | 12:25 PM
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I would say no problem,...but what is the goal...softer ride?
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Old Nov 13, 2011 | 12:37 PM
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as wwest said ABS doesn't care about diameter. Those fronts are fat compared to the rears 245/265. I doubt you'll have to worry about understeer. I moved from 205/255 to 225/255 and it helped reduce understeer quite a bit.
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Old Nov 13, 2011 | 06:15 PM
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PCA website usually states the 4% is the rule of thumb for front/rear error.
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Old Nov 13, 2011 | 08:20 PM
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Some 996's I know of run up to 6% difference without issues.
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Old Nov 13, 2011 | 10:34 PM
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Originally Posted by ivangene
I would say no problem,...but what is the goal...softer ride?
The goal is to reduce understeer and improve turn in for AX and track duty. Also on corner exit this should also help free up the rear end a bit.
Later next year I will be installing ROW M030 to help decrease body roll and help with overall car balance.
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