Cross-Post- Max allowable tyre diameter differential for AWD Turbos / 4Ss??
07-05-2011, 03:11 PM
#1
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Thread Starter
Cross-Post- Max allowable tyre diameter differential for AWD Turbos / 4Ss??
https://rennlist.com/forums/993-foru...4s-turbos.html
Anyone here know the definitive maximum tire diameter differential that will still keep the viscous coupling happy and "in neutral" so that power is not sent to the front wheels during dry road driving?
original tire sizes =
225 - 40 X 18 = 25.1" = 825 rev's per mile
285 - 30 X 18 = 24.7" = 836 rev's per mile
my poor math skills tell me that works out to ~ 1.1% difference with the rears spinning slightly faster than the fronts.
I'm a bit surprised that P designed the system to have the rear tyres "slipping" as compared to the fronts due to supplied tyre sizes.
Now that i've reinstalled the AWD, i'm wondering if proposed
235 - 40 X 18 = 25.5" = 817 rpm
295 - 35 X 18 = 26.1" = 798 rpm
2.3% differential
will work?
note, the rear tyres would be spinning more slowly with these tyres.
Thanks,
Craig
a few threads with pertinent info, but still no direct answer
https://rennlist.com/forums/993-turb...s-coupler.html
https://rennlist.com/forums/993-turb...s-coupler.html
Anyone here know the definitive maximum tire diameter differential that will still keep the viscous coupling happy and "in neutral" so that power is not sent to the front wheels during dry road driving?
original tire sizes =
225 - 40 X 18 = 25.1" = 825 rev's per mile
285 - 30 X 18 = 24.7" = 836 rev's per mile
my poor math skills tell me that works out to ~ 1.1% difference with the rears spinning slightly faster than the fronts.
I'm a bit surprised that P designed the system to have the rear tyres "slipping" as compared to the fronts due to supplied tyre sizes.
Now that i've reinstalled the AWD, i'm wondering if proposed
235 - 40 X 18 = 25.5" = 817 rpm
295 - 35 X 18 = 26.1" = 798 rpm
2.3% differential
will work?
note, the rear tyres would be spinning more slowly with these tyres.
Thanks,
Craig
a few threads with pertinent info, but still no direct answer
https://rennlist.com/forums/993-turb...s-coupler.html
https://rennlist.com/forums/993-turb...s-coupler.html
Last edited by cgfen; 07-05-2011 at 04:23 PM.
07-05-2011, 04:45 PM
#2
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Actually that is exactly what you want. The rear tires spinning a little faster "pre-loads" or slightly activates the VC so you have a teeny bit of torque (5% according to Mike Schatz) sent up front.
This will make your handling pretty consistant.
Remember the rear tyres wear much faster than the front. With the factory setup, you will always have your VC enaged a bit.
If you start off with the rear tires bigger than the front, you will have some VC preload. As the difference wears down to zero, you will have no preload and then it will start to kick in again as the rear tire becomes smaller than the front.
I just read your post on the 993 board:
Since you want "facts" instead of "opinions", I'll refer you to Michael Schatz and Jimmy the Felon.
This will make your handling pretty consistant.
Remember the rear tyres wear much faster than the front. With the factory setup, you will always have your VC enaged a bit.
If you start off with the rear tires bigger than the front, you will have some VC preload. As the difference wears down to zero, you will have no preload and then it will start to kick in again as the rear tire becomes smaller than the front.
I just read your post on the 993 board:
Since you want "facts" instead of "opinions", I'll refer you to Michael Schatz and Jimmy the Felon.
Last edited by ScottMellor; 07-05-2011 at 05:28 PM.
07-05-2011, 05:52 PM
#3
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Thread Starter
07-09-2011, 09:02 AM
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I think both combos will work. I've had both on my previous 993TT without problem. The rear was 295/30/18 though, not 295/35/18. You shouldn't worry too much if you stay within 2% + or -. Remember that this is no exact science, since the ratio will change all the time with tirewear as the rear tire wears down. So if you start with the rear tire being slightly larger (1%), after a while it will actually be smaller than the front due to tirewear. You'll be fine with either combo IMHO. I have rolled fenders and run 245/40/18 front and 315/30/18 rear (Michelin PS2).
07-15-2011, 05:40 PM
#6
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Thread Starter
I think both combos will work. I've had both on my previous 993TT without problem. The rear was 295/30/18 though, not 295/35/18. You shouldn't worry too much if you stay within 2% + or -. Remember that this is no exact science, since the ratio will change all the time with tirewear as the rear tire wears down. So if you start with the rear tire being slightly larger (1%), after a while it will actually be smaller than the front due to tirewear. You'll be fine with either combo IMHO. I have rolled fenders and run 245/40/18 front and 315/30/18 rear (Michelin PS2).
