Aftermarket or Factory LSD?
09-22-2010, 12:57 AM
#46
Rennlist Member
It's certainly not that a 40% lock rate identifies how many friction surfaces there are. Your description of how an LSD works is, of course, accurate. But your definition of bias ratio is not. You can achieve any
bias ratio with any number and combination of friction plates depending on the differential.
bias ratio with any number and combination of friction plates depending on the differential.
"After the 356s and prior to about 1984, limited slips were pretty much cut and dry as supplied from the factory in a Porsche. These units were either called 40% or 80% limited slips, depending upon how the friction discs and outer plates were arranged. These limited slips had two sets of friction discs on each side of the thrust rings (for a total of four discs). The 40% limited slips had two of the friction discs against each other, without an outer plate between them. Because the two friction discs touch each other, without an outer plate to separate them, this effectively provides two friction surfaces. Since this occurs on each side of the differential, you end up with a total of four friction surfaces in the entire differential. The 80% limited slips separated the two friction discs with an outer plate. This provided four friction surfaces on each side of the differential, which makes the 80% limited slip have a total of eight friction surfaces. Although certainly the number friction discs that actually contact the outer plates affect the amount of friction that the differential can generate, the amount of pressure that the diaphragm springs have also influences the amount of friction that the differential can produce. The more friction discs and the higher the preload from the diaphragm springs, the more torque it takes for limited slip to actually slip. The amount of preload can be varied since Porsche offers the outer plates in different thicknesses, which can be used to change the releasing torque.
Early 40% limited slips (before 1984, in the 911 series vehicles) were designed to be set up so that it takes between 29 to 58 foot pounds of torque to make the differential slip. The 80% limited slips had higher settings. Beginning with the Carrera models (and on through the C-2 models), limited slips only had room inside for one friction disc and one outer plate on each side (like our illustration has). These limited slips are also called 40% limited slips, but have a setup that only allows 7 to 25 foot pounds of torque (and 25 foot pounds is almost impossible to get) before the unit begins to slip. How can one 40% limited slip have less pressure than another 40% limited slip and still be considered to have the same slip percentage? I have no idea, but this can't be good. We therefore call these single friction disc units "snow only" limited slips. They have limited benefits on the race course, because they release at very low torque numbers."
Early 40% limited slips (before 1984, in the 911 series vehicles) were designed to be set up so that it takes between 29 to 58 foot pounds of torque to make the differential slip. The 80% limited slips had higher settings. Beginning with the Carrera models (and on through the C-2 models), limited slips only had room inside for one friction disc and one outer plate on each side (like our illustration has). These limited slips are also called 40% limited slips, but have a setup that only allows 7 to 25 foot pounds of torque (and 25 foot pounds is almost impossible to get) before the unit begins to slip. How can one 40% limited slip have less pressure than another 40% limited slip and still be considered to have the same slip percentage? I have no idea, but this can't be good. We therefore call these single friction disc units "snow only" limited slips. They have limited benefits on the race course, because they release at very low torque numbers."
It's actually torque bias. The math is torque at the high friction wheel divided by torque at the low friction wheel. It's easier to just think of it as transfer rate, which while not technically accurate, is easier for people to understand and is close enough.
If the misunderstanding is that you thought I was confusing the way a TBD works compared to an LSD... I'm not.
If the misunderstanding is that you thought I was confusing the way a TBD works compared to an LSD... I'm not.
Can you send me a link or source where this is also used for a clutch type LSD? Since the internal function and performance characteristics of a torque biasing and clutch type LSD are quite different, its not immediately obvious that the same torque bias ratio would apply. But if so, how is the rating on the clutch type calculated or empirically measured?
I have not heard the types referred to as such, although I can guess what is meant. Its much more common to hear the terms symmetric and asymmetric in Porsche circles. I don't know of any Porsche LSDs that are 1-way.
09-22-2010, 01:09 PM
#47
Rennlist Member
Something must be getting lost in translation. I have no intention to be patronizing. What you wrote had some inaccuracies and I felt it was useful to share the correct information.
Take this with the right intent but even if I was wrong, that doesn't make you right by default. I could have said that LSD's are powered by puppies and rainbows, been unable to support that assertion, and that still wouldn't change the fact that your definition of lockup ratio was incorrect.
But back to our conversation:
1) The fact that some Porsche LSD's have a correlation between number of disks and lock rate is true... but as your own quote points out... only some Porsche LSD's. That was my point and an important clarification, don't you think? It's one thing to say there is correlation in some diffs, entirely another to say that's what the definition of lockup rate is.
If you're basing your stance on the beginning sentences from Greg Brown... those are written poorly. It should have been written as "These units were either 40% or 80% limited slips, depending upon how the friction discs and outer plates were arranged." The point being that they achieved different lockup ratios as a result of their disc arrangements, not because of their disc arrangements. The same disc arrangement in a different diff with different ramp angles and different springs would result in a different lockup ratio. Also the quote talks about test torque values but that actually doesn't equate to lock up ratio either. Which explains why he's confused. Again...different diffs, different friction disks, different ramps, different springs. You can have the same lockup ratio (the actual result of these variables) and have different breakaway torque values.
2) The fact that lockup ratio is actually an expression of torque bias does not mean that an LSD works the same as a TBD. Again, we shouldn't confuse that for something else. TBD's that are commonly sold don't have any torque bias ratio at all. They can send 100% of torque to a single wheel and hence the math says the torque bias ratio on a TBD is infinite. You do see a lot of rated TBD's when used as center diff's on AWD cars (e.g. max 80% send to the rear or max 30% send to the front, etc) but I don't think I've ever seen a rated TBD used on an axle. BTW, I don't believe I've ever seen the torque bias ratio of an TBD referred to as lockup ratio. Which make sense since they're different things.
This is a good explanation of lockup ratio although lengthy: http://forums.bimmerforums.com/forum.../t-322488.html
Take this with the right intent but even if I was wrong, that doesn't make you right by default. I could have said that LSD's are powered by puppies and rainbows, been unable to support that assertion, and that still wouldn't change the fact that your definition of lockup ratio was incorrect.
But back to our conversation:
1) The fact that some Porsche LSD's have a correlation between number of disks and lock rate is true... but as your own quote points out... only some Porsche LSD's. That was my point and an important clarification, don't you think? It's one thing to say there is correlation in some diffs, entirely another to say that's what the definition of lockup rate is.
If you're basing your stance on the beginning sentences from Greg Brown... those are written poorly. It should have been written as "These units were either 40% or 80% limited slips, depending upon how the friction discs and outer plates were arranged." The point being that they achieved different lockup ratios as a result of their disc arrangements, not because of their disc arrangements. The same disc arrangement in a different diff with different ramp angles and different springs would result in a different lockup ratio. Also the quote talks about test torque values but that actually doesn't equate to lock up ratio either. Which explains why he's confused. Again...different diffs, different friction disks, different ramps, different springs. You can have the same lockup ratio (the actual result of these variables) and have different breakaway torque values.
2) The fact that lockup ratio is actually an expression of torque bias does not mean that an LSD works the same as a TBD. Again, we shouldn't confuse that for something else. TBD's that are commonly sold don't have any torque bias ratio at all. They can send 100% of torque to a single wheel and hence the math says the torque bias ratio on a TBD is infinite. You do see a lot of rated TBD's when used as center diff's on AWD cars (e.g. max 80% send to the rear or max 30% send to the front, etc) but I don't think I've ever seen a rated TBD used on an axle. BTW, I don't believe I've ever seen the torque bias ratio of an TBD referred to as lockup ratio. Which make sense since they're different things.
This is a good explanation of lockup ratio although lengthy: http://forums.bimmerforums.com/forum.../t-322488.html
09-22-2010, 09:32 PM
#48
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I think we're getting into bickering for the sake of it here. Lets cut it short. The thread has confirmed my thinking on how it works and added some useful knowledge too so job done I feel.
Thanks guys.
Thanks guys.
09-23-2010, 11:56 PM
#50
Rennlist Member
I am editing to add this clarification and additional information for future reference:
My intent was not to say that actual 40% lock rate is created by the 4 friction surfaces. The point I was trying to make was that the 944 ZF LSD was somewhat arbitrarily called a 40/40 diff, in part, because of the 4 friction surfaces and the symmetric ramps.
And I based that partially on the article I posted above and also information and understanding similar to these comments from Guard and others relating to the ratings on clutch type LSDs. They tend to indicate the actual locking rates are somewhat of an unknown and the designated rates are a nominal rating. Start reading at post #37, and specifically post #41.
http://forums.pelicanparts.com/porsc...d-915-a-3.html
Anyone can argue that the accuracy of that should be questioned, as anything posted on the internet (or anywhere for that matter) should be viewed with a critical eye. But the credentials of Paul Guard should carry some weight. And the issue really becomes: to determine the actual lockup and slip characteristics of a clutch type LSD is seemingly difficult. As mentioned and known, various factors influence the function, from ramp angles, plate count, friction surface, and the various input variables (input torque, available traction, and wheel speed differential) etc. So to describe the lock, slip and/or release behavior by a single ratio or % is probably inaccurate at best.
Certainly something as simple as T1/T2 is really vague, because T1 and T2 are an unknown. There may be some calculations that take a known input torque and determine axial forces based on the ramp angles, and use that to determine a clamping force from known friction coefficients and disc area. But there may still be too many theoretical unknowns that would need to be determined or measured by actual testing (does the clamping force vary due to a range of input torque, does it vary due to a range of wheel speed differential).
Maybe ZF and maybe Porsche have equipment to measure differential output torque, but obviously these smaller aftermarket suppliers and shops do not. So those guys (like Guard) are somewhat arbitrarily designating the rates. And even if empirical torque outputs are used to identify a locking rate, that single locking rate may not describe the wheel torque differential throughout the operating range of the clutch type LSD.
This is a write up by Torsen, discussing the fuction of the Torque Biasing/gear type differential:
http://www.torsen.com/files/Traction...ol_Article.pdf
It does show both torque bias as T1/T2 and the ratio as (T1-T2)/(T1+T2) for a gear type. If T1 and T2 are known, then the characteristics of the diff can be shown either as a torque bias or a lock ratio.
I have not found an article of similar detail or of similar credibility that describes the locking ratio of a clutch type LSD. But will continue to look for something with equivalent detail in my spare time.
My intent was not to say that actual 40% lock rate is created by the 4 friction surfaces. The point I was trying to make was that the 944 ZF LSD was somewhat arbitrarily called a 40/40 diff, in part, because of the 4 friction surfaces and the symmetric ramps.
And I based that partially on the article I posted above and also information and understanding similar to these comments from Guard and others relating to the ratings on clutch type LSDs. They tend to indicate the actual locking rates are somewhat of an unknown and the designated rates are a nominal rating. Start reading at post #37, and specifically post #41.
http://forums.pelicanparts.com/porsc...d-915-a-3.html
Porsche's use of "percentage of anti-slip effectiveness" (what we just call percentage of "lock-up") makes a lot more sense to the layman than ramp angles.
All of Porsche's early LSDs came with 30-degree ramps (in both directions, as they were symmetrical). When 2 friction discs were utilized, the LSD was called a 40% LSD; when 4 friction discs were utilized (on both sides), the LSD was called an 80% LSD.
Porsche Motorsports 40/60 LSD has a 50-degree ramp on the acceleration side and 37.5-degree ramp on the decel side, utilizing a total of 4 friction discs. Most pro race teams opted for somewhat more aggressive 50/80 ramps. Some teams even used dual Belleville washers per side, for a tremendous amount of pre-load.
In '03 Porsche came out with a version of their 40/60 LSD that had thinner discs, allowing a total of 8 friction discs and 8 plain plates. Most teams used 40/60 rather than 50/80 ramps. These LSDs had no place on the street .... strictly track, due to their aggressive nature.
The "zero pre-load" idea is nothing new. A number of shops like to set up their LSDs with very little pre-load, utilizing much thinner Belleville washers and/or greater clearances. This sort of set-up is less aggressive in slow speed turns, as the clamping pressure provided by ramp-up is at a minimum during light throttle. More recently, set-ups with more friction discs and no Belleville washers have been experimented with. But this can be done with any LSD .... you don't need to spend 4K on an LSD that comes set up this way.
.....we have almost all the bits to now offer any of the following plate/Belleville combinations to any of our "traditional" GT LSDs (which covers all 911-based models from '64-on):
1.0, 1.5, 2.0, 2.2 Belleville washers... which can of course can be eliminated altogether
2, 4, 6, 8 friction discs
40/60, 50/80, 80/80 ramps (which can be reversed.... or figures halved by using only two friction discs or placing discs back-to-back).
So how do we rate LSD locking percentages today? With all these variables, it's pretty darn confusing. Is an LSD with 40/60 ramps and 6 or 8 friction discs still a 40/60 LSD? Of course not. So many factors now determine the true "degree of anti-slip effectiveness" (not least of which is the material and finish of the friction discs and plain plates, a story by itself). But likely 99% of GT LSDs sold will continue to have the standard 4 friction discs with 40/60//50/80 options.
All of Porsche's early LSDs came with 30-degree ramps (in both directions, as they were symmetrical). When 2 friction discs were utilized, the LSD was called a 40% LSD; when 4 friction discs were utilized (on both sides), the LSD was called an 80% LSD.
Porsche Motorsports 40/60 LSD has a 50-degree ramp on the acceleration side and 37.5-degree ramp on the decel side, utilizing a total of 4 friction discs. Most pro race teams opted for somewhat more aggressive 50/80 ramps. Some teams even used dual Belleville washers per side, for a tremendous amount of pre-load.
In '03 Porsche came out with a version of their 40/60 LSD that had thinner discs, allowing a total of 8 friction discs and 8 plain plates. Most teams used 40/60 rather than 50/80 ramps. These LSDs had no place on the street .... strictly track, due to their aggressive nature.
The "zero pre-load" idea is nothing new. A number of shops like to set up their LSDs with very little pre-load, utilizing much thinner Belleville washers and/or greater clearances. This sort of set-up is less aggressive in slow speed turns, as the clamping pressure provided by ramp-up is at a minimum during light throttle. More recently, set-ups with more friction discs and no Belleville washers have been experimented with. But this can be done with any LSD .... you don't need to spend 4K on an LSD that comes set up this way.
.....we have almost all the bits to now offer any of the following plate/Belleville combinations to any of our "traditional" GT LSDs (which covers all 911-based models from '64-on):
1.0, 1.5, 2.0, 2.2 Belleville washers... which can of course can be eliminated altogether
2, 4, 6, 8 friction discs
40/60, 50/80, 80/80 ramps (which can be reversed.... or figures halved by using only two friction discs or placing discs back-to-back).
So how do we rate LSD locking percentages today? With all these variables, it's pretty darn confusing. Is an LSD with 40/60 ramps and 6 or 8 friction discs still a 40/60 LSD? Of course not. So many factors now determine the true "degree of anti-slip effectiveness" (not least of which is the material and finish of the friction discs and plain plates, a story by itself). But likely 99% of GT LSDs sold will continue to have the standard 4 friction discs with 40/60//50/80 options.
Anyone can argue that the accuracy of that should be questioned, as anything posted on the internet (or anywhere for that matter) should be viewed with a critical eye. But the credentials of Paul Guard should carry some weight. And the issue really becomes: to determine the actual lockup and slip characteristics of a clutch type LSD is seemingly difficult. As mentioned and known, various factors influence the function, from ramp angles, plate count, friction surface, and the various input variables (input torque, available traction, and wheel speed differential) etc. So to describe the lock, slip and/or release behavior by a single ratio or % is probably inaccurate at best.
Certainly something as simple as T1/T2 is really vague, because T1 and T2 are an unknown. There may be some calculations that take a known input torque and determine axial forces based on the ramp angles, and use that to determine a clamping force from known friction coefficients and disc area. But there may still be too many theoretical unknowns that would need to be determined or measured by actual testing (does the clamping force vary due to a range of input torque, does it vary due to a range of wheel speed differential).
Remember that the degree of lockup is a function of input torque as well
as the characteristics of the differential (in most cases).
So a diff with 40/65 ramps actually locks up much harder under
acceleration than it does on deceleration. This also means that by using a
different gear in a given corner (especially on entry), you are changing
the input torque and hence the lockup.
Downshifting to the lowest possible gear generates the highest possible
degree of lockup, which may or may not be desireble.
Chris Cervelli
as the characteristics of the differential (in most cases).
So a diff with 40/65 ramps actually locks up much harder under
acceleration than it does on deceleration. This also means that by using a
different gear in a given corner (especially on entry), you are changing
the input torque and hence the lockup.
Downshifting to the lowest possible gear generates the highest possible
degree of lockup, which may or may not be desireble.
Chris Cervelli
Maybe ZF and maybe Porsche have equipment to measure differential output torque, but obviously these smaller aftermarket suppliers and shops do not. So those guys (like Guard) are somewhat arbitrarily designating the rates. And even if empirical torque outputs are used to identify a locking rate, that single locking rate may not describe the wheel torque differential throughout the operating range of the clutch type LSD.
This is a write up by Torsen, discussing the fuction of the Torque Biasing/gear type differential:
http://www.torsen.com/files/Traction...ol_Article.pdf
It does show both torque bias as T1/T2 and the ratio as (T1-T2)/(T1+T2) for a gear type. If T1 and T2 are known, then the characteristics of the diff can be shown either as a torque bias or a lock ratio.
I have not found an article of similar detail or of similar credibility that describes the locking ratio of a clutch type LSD. But will continue to look for something with equivalent detail in my spare time.
Last edited by Oddjob; 12-31-2010 at 02:45 PM.
