996TT Discontinued Tire Sizes & AWD Implications - Page 4 - Rennlist Discussion Forums



996TT Discontinued Tire Sizes & AWD Implications

Reply
 
Thread Tools Search this Thread
Old 12-01-2016, 05:13 PM   #46
Third-Reef
Addict
Rennlist Member
 
Third-Reef's Avatar
 
Join Date: Oct 2014
Location: Penn Valley, Ca
Posts: 381
Default

Well Dang, going to have to call them back up and check this out. I was planning on getting them this month. It was about 2 months ago when i checked . wonder if they have some sort of back stock, I don't want old tires.
Third-Reef is offline   Reply With Quote
Old 12-01-2016, 06:04 PM   #47
Carlo_Carrera
2011 Rennlist F1 Champion
Rennlist Member
 
Carlo_Carrera's Avatar
 
Join Date: Feb 2010
Location: Nearby
Posts: 2,592
Default

Quote:
Originally Posted by strathconaman View Post
.... I think I am with Macster on this one. The 996 AWD system is next to useless.....
As someone who has driven my AWD Turbo quite a bit in the snow and on track I tell you first hand the AWD system works quite well if you are running the proper tire sizes. And the snow tire sizes I use are not what Porsche recommends. My winter tires closely match the ratios of the summer tire sizes.

There are many other folks here who use there TT in snow and enjoy every AWD minute of it.
Carlo_Carrera is offline   Reply With Quote
Old 12-01-2016, 06:33 PM   #48
Dock
Super User
 
Dock's Avatar
 
Join Date: Sep 2001
Location: Atlanta, Georgia
Posts: 5,621
Default

Quote:
Originally Posted by strathconaman View Post
I think I am with Macster on this one. The 996 AWD system is next to useless.
Agreed.
Dock is offline   Reply With Quote
Old 12-01-2016, 07:04 PM   #49
kmagnuss
User
 
kmagnuss's Avatar
 
Join Date: Nov 2013
Location: Englewood, FL
Posts: 895
Default

For fun I'm going to go out in the next rain and do a big smokey burnout and see if the awd kicks in... my guess is it will be a bunch of rear wheel hop and no front tire movement.
kmagnuss is offline   Reply With Quote
Old 12-01-2016, 07:14 PM   #50
T10Chris
Addict
Rennlist Member
 
T10Chris's Avatar
 
Join Date: Dec 2015
Location: Bellevue, WA
Posts: 527
Default

Quote:
Originally Posted by strathconaman View Post
I accept that the larger tires on the rear will cause braking on the front axle. It has caused me much googling over the past few days. Then I thought. How much braking? Well, the diff transfers between 5-40%...then, not a lot, at anything less than supersonic speeds.
I keep trying to wrap my head around this as well.

How is it causing braking? How is it anything more than the rolling resistance that would be present if the car is RWD?

Any explanation/math/diagrams would be appreciated.
T10Chris is offline   Reply With Quote
Old 12-01-2016, 08:04 PM   #51
Carlo_Carrera
2011 Rennlist F1 Champion
Rennlist Member
 
Carlo_Carrera's Avatar
 
Join Date: Feb 2010
Location: Nearby
Posts: 2,592
Default

Quote:
Originally Posted by Dock View Post
Agreed.
What is your experience driving a 996TT in snow?
Carlo_Carrera is offline   Reply With Quote
Old 12-01-2016, 08:09 PM   #52
Carlo_Carrera
2011 Rennlist F1 Champion
Rennlist Member
 
Carlo_Carrera's Avatar
 
Join Date: Feb 2010
Location: Nearby
Posts: 2,592
Default

Quote:
Originally Posted by T10Chris View Post
I keep trying to wrap my head around this as well.

How is it causing braking? How is it anything more than the rolling resistance that would be present if the car is RWD?

Any explanation/math/diagrams would be appreciated.
http://www.awdwiki.com/en/viscous+coupling/

Causing braking is not really accurate.

In static driving if you are using Porsche recommended winter tires sizes the front half of the coupling is spinning faster than the rear half and thus trying to put rotational power back into the rear drive line and engine.

If you are using summer tires sizes the power flow is always going from the engine and rear drive line to the front.
Carlo_Carrera is offline   Reply With Quote
Old 12-01-2016, 08:49 PM   #53
Dock
Super User
 
Dock's Avatar
 
Join Date: Sep 2001
Location: Atlanta, Georgia
Posts: 5,621
Default

Quote:
Originally Posted by Carlo_Carrera View Post
What is your experience driving a 996TT in snow?
I've driven my 996 Turbo in the snow/ice and wet many times.
Dock is offline   Reply With Quote
Old 12-01-2016, 09:10 PM   #54
Carlo_Carrera
2011 Rennlist F1 Champion
Rennlist Member
 
Carlo_Carrera's Avatar
 
Join Date: Feb 2010
Location: Nearby
Posts: 2,592
Default

Quote:
Originally Posted by Dock View Post
I've driven my 996 Turbo in the snow/ice and wet many times.
Prove it.
Carlo_Carrera is offline   Reply With Quote
Old 12-01-2016, 09:17 PM   #55
Carlo_Carrera
2011 Rennlist F1 Champion
Rennlist Member
 
Carlo_Carrera's Avatar
 
Join Date: Feb 2010
Location: Nearby
Posts: 2,592
Default

Carlo_Carrera is offline   Reply With Quote
Old 12-01-2016, 09:32 PM   #56
Dock
Super User
 
Dock's Avatar
 
Join Date: Sep 2001
Location: Atlanta, Georgia
Posts: 5,621
Default

Quote:
Originally Posted by Carlo_Carrera View Post
Prove it.
???
Dock is offline   Reply With Quote
Old 12-01-2016, 09:58 PM   #57
Carlo_Carrera
2011 Rennlist F1 Champion
Rennlist Member
 
Carlo_Carrera's Avatar
 
Join Date: Feb 2010
Location: Nearby
Posts: 2,592
Default

Quote:
Originally Posted by Dock View Post
???
???
Carlo_Carrera is offline   Reply With Quote
Old 12-01-2016, 10:19 PM   #58
996tnz
User
 
996tnz's Avatar
 
Join Date: Oct 2010
Posts: 1,762
Default AWD and front-rear tire diameters

The factory specified smaller diameter rears mean the rears are turning faster than the fronts already under full grip conditions, which effectively 'preloads' the front diff for when the rears might start going much faster than the fronts (rear wheels slipping/spinning up). So the car's response is just to increase the level of torque transfer in the direction it was already going, and so is both rapid and progressive. That makes it more predictable.

Running larger rears than fronts has that preload reversed. When the rears start to spin up, the front diff first has to pass through a neutral point where the front and rear axles are at the same speed before starting to engage again and transfer torque forward. Which would normally delay the response. If the rears are much bigger, the front diff is already preheated beyond normal levels so the momentary loss and then a - delayed but sudden - ramp up of torque transfer to the fronts jerks the car around much more than the factory intended. Not ideal in snow perhaps, but much more of a problem if tracking out on the limit on track when the rears start to spin up.

Perhaps PSM usually masks that problem for all I know, but I typically run with it fully disabled and have felt this jerky transition under two scenarios. The first when I put new roadies on out back, with fronts that were down to one third tread; and the second when running a set of 996 C2 wheels and tyres on my Turbo, during a week or two when my original wheels were getting custom painted. With 265s out back there was plenty of opportunity for the car to regularly shunt torque forwards but it felt rough and jerky rather than progressive. Still drivable but not confidence inspiring near the limit - not just loose and smooth but loose and rough.

I presume that Sumitomo HTRZIIIs in our OEM sizing and Nitto NT-01s in 245-40-18 + 315-30-18 (only slightly larger fronts, but used on track so the rears are slipping a bit a lot of the time anyway) will continue to be produced? No such issues with those...
996tnz is offline   Reply With Quote
Old 12-01-2016, 11:08 PM   #59
T10Chris
Addict
Rennlist Member
 
T10Chris's Avatar
 
Join Date: Dec 2015
Location: Bellevue, WA
Posts: 527
Default

Quote:
Originally Posted by Carlo_Carrera View Post
http://www.awdwiki.com/en/viscous+coupling/

Causing braking is not really accurate.

In static driving if you are using Porsche recommended winter tires sizes the front half of the coupling is spinning faster than the rear half and thus trying to put rotational power back into the rear drive line and engine.

If you are using summer tires sizes the power flow is always going from the engine and rear drive line to the front.
I know how VC units work as the link describes, but I have seen other people say that it causes braking, or makes the car fight itself, or other similar things in many posts and I don't see how that is possible...

All I can see it doing is removing the preload/being 100% RWD drive bias until there is tire slip from the rear for AWD to kick in?

Maybe it is just semantics/phrasing that is tripping me up.
T10Chris is offline   Reply With Quote
Old 12-02-2016, 12:17 AM   #60
kmagnuss
User
 
kmagnuss's Avatar
 
Join Date: Nov 2013
Location: Englewood, FL
Posts: 895
Default

Quote:
Originally Posted by Carlo_Carrera View Post
Prove it.
I lol'd.
kmagnuss is offline   Reply With Quote
Reply

Related Topics
Thread Thread Starter Forum Replies Last Post
C4S - Front / Rear Tire Size Question dunkd 996 Forum 16 12-01-2016 04:26 PM
Please help: 997.1 C4S wheel size ratio and its effect on the viscous coupler Joe Cool 997 Forum 6 04-26-2016 12:51 PM
996 TURBO AWD Question Ck986 996 Turbo Forum 30 12-09-2014 06:16 PM
315/25/19 Tires???? 928drvr86.5 996 Turbo Forum 8 08-22-2012 07:44 PM
Good discussion on wheels, tires and other performance mods - great for newbs Avid Fan Autocross 14 08-31-2007 11:12 AM


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Forum Jump


All times are GMT -3. The time now is 04:14 AM.


Copyright ę 1998 - 2015 Rennlist.com