Again...permissable tire sizes for AWD
04-30-2011, 05:34 PM
#16
Nordschleife Master
another alternative for 19" wheels is:
255x30x19
315x25x19
These are almost exactly the same overall diameter as the factory 18" tires. I am using Toyo T1R Proxies on my 993 Turbo R.
255x30x19
315x25x19
These are almost exactly the same overall diameter as the factory 18" tires. I am using Toyo T1R Proxies on my 993 Turbo R.
05-01-2011, 09:03 PM
#17
Nick, the answers are out there, but what is lacking is the tech to answer the question why. Perhaps this link will help you understand: http://www.awdwiki.com/en/viscous+coupling/
The 996 uses this type of viscous system and pre-loads the front wheels for 5% power. Which means that the viscous coupling is wearing out even with the factory tire setup.
Now, the question that I want more tech on is, why is 4% (or whatever magical number) the accepted variance between front and rear tire diameters?
EDIT:
I looked up the revs per mile for 996 Turbo tires on tirerack and calculated the percentage variation:
N3 Bridgestone S-02A 823/837 1.67%
N3 Michelin PS2 829/832 0.36%
Hankook Ventus V12 828/831 0.36%
Yoko ADVAN Neova AD08 831/831 0.00%
Sumitomo HTR Z III 826/834 0.96%
Michelin PSC 834/833 0.12%
Interesting to see that:
1- the N-rated Bridgestones has a whopping 1.67% variation in rev per mile between front and rear tires
2- the other tires have < 1% variation
3- the N-rated PS2 and Hankooks have the same percentage variation
Michael.
Viscous coupling is filled with silicone and is not computer controlled. A series of plates with holes and slots turn in the silicone fluid. Some plates are attached to the front axle driveshaft and some are attached to the rear axle driveshaft. Normally the plates turn at the same rate without relative motion. The silicone becomes very viscous as soon as it is heated by friction and shear caused by differences between the motion of the plates. This tends to lock the driveshafts. If the rear wheels and driveshaft are slipping and turning faster than the front, friction between the plates increases, slippage is reduced, the rear wheel spin is reduced and the torque from the input shaft is transferred to the front.
1. viscous coupling acting instead of a center differential
In this case, in normal conditions, all power is transferred to just one axle. One part of the viscous coupling is connected to the driving axle, another part is connected to the driven axle. When driving wheels slip, viscous coupling locks and torque is transferred to the other axle. This is an automatic all wheel drive system.
The disadvantage of a viscous coupling is that it engages too slowly and allows for excessive wheelspin before transferring torque to another wheels. This is especially critical in automatic all wheel drive systems - when cornering under acceleration, the rear end is engaged with a slight delay, causing sudden change in the car's behaviour fron understeer to oversteer. Also, when taking-off in sand, front wheels can become bogged down before all wheel drive is engaged.
In an attempt to reduce the coupling's activation time, VW Golf MkII Syncro always transfers 5% of torque to rear wheels (this is achieved by rear driveshaft rotating slower than front driveshaft in normal conditions, causing viscous fluid warm-up and slight solidification).
At the same time, pre-tensioning the coupling too much leads to undesireable transmission wind-up and makes the system too sensitive to uneven tread wear on front and rear tires. This is why Volvo first reduced the pre-tensioning in 2000 and then replaced the viscous coupling with Haldex clutch on their all wheel drive vehicles in model year 2003 (Volvo s60 has Haldex since 2002).
1. viscous coupling acting instead of a center differential
In this case, in normal conditions, all power is transferred to just one axle. One part of the viscous coupling is connected to the driving axle, another part is connected to the driven axle. When driving wheels slip, viscous coupling locks and torque is transferred to the other axle. This is an automatic all wheel drive system.
The disadvantage of a viscous coupling is that it engages too slowly and allows for excessive wheelspin before transferring torque to another wheels. This is especially critical in automatic all wheel drive systems - when cornering under acceleration, the rear end is engaged with a slight delay, causing sudden change in the car's behaviour fron understeer to oversteer. Also, when taking-off in sand, front wheels can become bogged down before all wheel drive is engaged.
In an attempt to reduce the coupling's activation time, VW Golf MkII Syncro always transfers 5% of torque to rear wheels (this is achieved by rear driveshaft rotating slower than front driveshaft in normal conditions, causing viscous fluid warm-up and slight solidification).
At the same time, pre-tensioning the coupling too much leads to undesireable transmission wind-up and makes the system too sensitive to uneven tread wear on front and rear tires. This is why Volvo first reduced the pre-tensioning in 2000 and then replaced the viscous coupling with Haldex clutch on their all wheel drive vehicles in model year 2003 (Volvo s60 has Haldex since 2002).
Now, the question that I want more tech on is, why is 4% (or whatever magical number) the accepted variance between front and rear tire diameters?
EDIT:
I looked up the revs per mile for 996 Turbo tires on tirerack and calculated the percentage variation:
N3 Bridgestone S-02A 823/837 1.67%
N3 Michelin PS2 829/832 0.36%
Hankook Ventus V12 828/831 0.36%
Yoko ADVAN Neova AD08 831/831 0.00%
Sumitomo HTR Z III 826/834 0.96%
Michelin PSC 834/833 0.12%
Interesting to see that:
1- the N-rated Bridgestones has a whopping 1.67% variation in rev per mile between front and rear tires
2- the other tires have < 1% variation
3- the N-rated PS2 and Hankooks have the same percentage variation
Michael.
Last edited by Michael-Dallas; 05-01-2011 at 09:29 PM.
05-02-2011, 01:46 AM
#18
Burning Brakes
I understand that perfectly, and considered it before I made my first post. My original question, put another way, was at what point does the size varience become too great for differential to operate within its designed parameters?
Does anyone know what mechanical components fail when a diff goes south? I can't imagine the ring and pinion, planetary gears or bearings. I can see the viscous coupling getting stressed as the viscosity of the fluid may be too great to deal with greater than designed speed differences from front to rear.
I'm interested to know more regarding this. Not trying to be a PITA, just my nature to question what not not perfectly clear to me. Thanks!
Does anyone know what mechanical components fail when a diff goes south? I can't imagine the ring and pinion, planetary gears or bearings. I can see the viscous coupling getting stressed as the viscosity of the fluid may be too great to deal with greater than designed speed differences from front to rear.
I'm interested to know more regarding this. Not trying to be a PITA, just my nature to question what not not perfectly clear to me. Thanks!
Perfectly fine question.
We do not know the design specification for the Porsche differential.
There are anecdotal answers, and "plenty of people run ABC with no problems" or "Porsche must have allowed Y%, so Z must be OK"
Personally my theory is that the increase in mismatched rotation cause the differential to heat up as it spins faster. Fluid breaks down, seals fail... I dunno, but the heat would be the first issue.
05-02-2011, 01:55 AM
#19
Nordschleife Master
The Viscous coupler is not part of either differential... it is actually bolted to the nose of the transmission. Too much heat will cause it to fail. When it fails, it stops transferring torque to the front diff... effectively, you have 2WD. I'm sure that many of us have failed viscous couplers and don't even know they are broken.
05-02-2011, 02:03 PM
#20
Drifting
Thread Starter
Viscous clutch is in the front diff housing of the 996. It seems like to keep the clutch happy, the input and output shaft speeds should be kept as close as possible, this considering the F&R final drive ratios are identical. This would be accomplished by having the F&R tire circumfrences the same.
I find it odd, if this was extremely important, that Porsche would not have warnings in the owners manual regarding usage of dissimular tire sizes F&R.
Maybe I don't understand the viscous clutch operation very well. I've read conflicting things regarding its operation. Somewhere I read it's filled with silicone fluid that when warmed it becomes more viscous, which I take to mean thinner and more fluid. I've also read when warmed it become very thick and near solid. So what is it? If it's near solid it would work like any fluid clutch and any constant differential between input and output speeds would want to sheer the gel or start tearing up the vanes.
http://www.autoatlanta.com/porsche-p...-05/360-02.php
I find it odd, if this was extremely important, that Porsche would not have warnings in the owners manual regarding usage of dissimular tire sizes F&R.
Maybe I don't understand the viscous clutch operation very well. I've read conflicting things regarding its operation. Somewhere I read it's filled with silicone fluid that when warmed it becomes more viscous, which I take to mean thinner and more fluid. I've also read when warmed it become very thick and near solid. So what is it? If it's near solid it would work like any fluid clutch and any constant differential between input and output speeds would want to sheer the gel or start tearing up the vanes.
http://www.autoatlanta.com/porsche-p...-05/360-02.php
05-02-2011, 02:44 PM
#22
Nordschleife Master
Is there any PCNA/PAG document that explains how our AWD system works?
Larry....does that mean that our cars start out at RWD and as the car gets warmed up become AWD?
Larry....does that mean that our cars start out at RWD and as the car gets warmed up become AWD?
05-05-2011, 11:40 AM
#23
Drifting
Thread Starter
I'd like to see a graph showing percentages of f to r drive ratios from 10-160 mph.
05-05-2011, 01:48 PM
#24
Rennlist Member
Adrien (996 Essential Companion) does a pretty good job explaining the AWD. On the 996, the car is always a minimum 5% FWD and as speed increases the viscous special silicone fluid thickens and the percentage of front drive increases. The maximum driving force of the front diff, 30% is reached at 155 MPH.
I'd like to see a graph showing percentages of f to r drive ratios from 10-160 mph.
I'd like to see a graph showing percentages of f to r drive ratios from 10-160 mph.
05-05-2011, 02:33 PM
#25
Nordschleife Master
Then I'm confused how you would benefit from tire slip if the AWD system kicks in at a later time when the fluid reaches a certain temp.
Meaning.....you are not at temp, you head around a corner, lose grip, and then?
Maybe I am confusing the actual function of AWD versus PSM. At this point I really don't see the benefit of AWD.
If we need to start a new thread that's fine as well.
Meaning.....you are not at temp, you head around a corner, lose grip, and then?
Maybe I am confusing the actual function of AWD versus PSM. At this point I really don't see the benefit of AWD.
If we need to start a new thread that's fine as well.
05-05-2011, 03:05 PM
#26
Rennlist Member
RWD conversion for the win here. I have found that my car acts as a rwd car in most cases when around turns but i feel that the engine in the rear acts as a weight and prevents a full blown rear end loss like in domestics. In a straight line the weight transfer seems to help a lot but i think this all becomes different bases on suspension components. I am still on stock suspension. At the end of the day i think the awd system helps but its not really AWD like people seem to assume.
05-05-2011, 03:19 PM
#27
Nordschleife Master
I'm beginning to think the AWD is almost a worthless system that we really can't exploit to the fullest extent. Maybe I'm just not understanding the intent of the AWD system for the daily driver crowd.
05-05-2011, 03:31 PM
#28
Rennlist Member
Well, my perspective is a little different. We got over 50 feet of snow this year and I use my TT as a daily driver. I can tell you for sure, that not warmed up, I am getting more than 5% power on my front axles. Otherwise I would have been stuck numerous times just trying to get out of my driveway. AWD has many benefits - quick acceleration due to limited wheel spin, ability to accelerate out of turns quicker. That also comes with the pitfall of added weight. For me and where I live, I couldn't get along with out it on any of my cars.
05-05-2011, 03:38 PM
#30
Rennlist Member
The viscous coupler will transfer power between the front and rear axles when one of the axles loses traction. The concept of it transferring 5% when cold and more as it heats up is a separate issue from when traction is lost. So regardless of temperature, if you lose traction in the rear, more power will be transfered to the front. If you are driving in a straight line without wheel slip, when cold, 5% power is transfered and apparently more as it heats up.