Hi Matt.

Thanks for the answer, and yes it makes sense.
So I guess it is correct to say that it is the torque difference that really makes the diff lock and unlock, and the ramp angles are a way of "resisting" this force until the desired (or rather undesired) difference in wheelspeed are achieved before locking up.

So with one wheel on the ground and one wheel airborn, the diff locks up so that wheelspeed is the same on both wheels: the diff will then stay locked until both wheels are back on ground beacuse of the torque difference between the two wheels even when the wheelspeed is now equal between the wheels, correct?

 

That's actually a very good example to show that it is the difference in torque and not the difference in wheelspeed that makes the LSD lock.

 

Thanks Matt.
I know these details are not important for understanding what the diff do and why it is important on these cars, but I really like to learn about the forces involved and I feel like I finally understand how this works on a detailed technical level now.

Anybody mentioned that this thread is great?

 

no, in my opinion it is just the other way round!
the diff is locked (via preload) and/or as soon as torque is applied from the engine.
the more engine torque, the more lockup! no matter if you are cornering or driving in a straight line.

as soon as the difference in left and right wheel torque exceeds a certain value (due to heavy cornering or difference in road conditions or one wheel loosing contact to the road) the diff opens up!!!

 

You are talking about preload alone and nothing else.
The discussion in the end here between me and Matt is related to the locking through the climbing of the ramps and disregard preload completely.

I suggest you read this thread in full, as everything you are missing regarding how the diffs in these cars work are in here.

 

nevermind the preload!
i am talking about the way a clutch type lsd works in general. and as soon as you apply force to the lsd via the engine it gets locked.
the tires can only unlock it!

but i do not want to convince anybody of my point of view.
just think about everything in detail and you will get the point...

matt, do you agree with rbt that a clutch type lsd locks when one wheel lifts off the ground? or does it unlock in that situation, what i believe it does?

 

I will suggest once more that you actually read the thread you are commenting on, I think that is regarded common courtesy anyway.

The type of LSD you are describing sounds like one with a spring pack: http://auto.howstuffworks.com/differential4.htm

The one in a Porsche GT3 do not have a spring pack, but spider gears and ramps in the center. Both types is clutch type LSDs but they work in opposite ways.
The link here is from within the thread as well.

The question that you are still asking is answered (by Matt and others) in a very detailed way in this thread.

 

a porsche lsd does not have spring packs? matt thinks different i guess!
and the one oe-lsd of a 997 gt2 (with broken cover... so i would need a new one... matt, got my emails?) i just have in front of me does actually have two springs.
but they are not important to understand the basic behaviours of a clutch type lsd in general.

 

You think an LSD unlocks when there is one wheel off the ground? Then you think wrong. For decades one of the reasons racers prefer a true clutch type LSD over a gear driven torque sensing/biasing differential is that while the TBD acts open with a wheel in the air, the LSD locks because of the difference in torque across the wheels. It still sends power to the wheel that is on the ground and continues to drive the vehicle forward, even if it's only able to transmit a percentage of the engine's power.

rbt initially asked when do the pins come back down the ramps in the pressure rings, releasing the clamping force on them. Whether it's an LSD with preload or one of the new zero preload designs like Porsche is using in the PDK and the Cayman 6MT gearboxes isn't really relevant to understanding the answer to his question. I deliberately removed it from the conversation to simplify things with respect to directly addressing what he wanted to know.

Coes, I responded to your email quite some time ago. I use a yahoo mail account and oftentimes my emails end up in the junk folder when sent to foreign clients. Try emailing me again and be sure to add my email to your safe sender's list so that my reply comes through. But the short answer is we don't sell just LSD bodies or end covers.

I'm glad I checked in today since I hadn't seen this back and forth. The reason I checked in was to give you all a small bit of bad news. Well, it's not so much bad news as a heads up. Even though we increased production of ramps by 20% this year for expected rebuilds that people would want to do while there was snow on the ground, you guys cleaned us out again. We sold more ramps in Jan and Feb than we did in the previous 6 months combined. We're making more as fast as we can, but any of you 997 GT3 guys who want to change locking factor will have to sit tight for a few weeks. We're hoping to be back in stock before April 1st. And if you just want the clutches done or have a 996 that doesn't need improvement, let us know. We've got rebuild parts in stock, just not pressure ring ramp kits.

 

Actually it's closer to 30 including practice time. Regardless, one of the common questions we get is how frequently am I going to need to rebuild my LSD if I use your parts. I think it's a valid question, though in the last couple of years the nature of the question has been skewed by certain other manufacturings talking about how more plates is better and that according to them their infinity plate LSD never needs rebuilding as a result. Well, here's some empirical show and tell for you.
These are the actual friction discs that were used in Bullet Racing's car that competed in the Rolex 24 Hours of Daytona at the end of January.

You can go back to page one of Mike's install if you want to see what they look like when brand new. Obviously they are no longer new. However, when measured, the friction discs show .01mm of wear on them. And while they are showing swirl marks, the hardened steel plates show no measurable wear. I pulled these out, and if it weren't for the fact that I've completely rebuilt the gearbox and sold it to a new client, I would put them right back in and keep running them. On one of BGB's LSDs in one of their Conti cars, we saw more than 80 hours of dedicated motorsports usage on their unit before we serviced it. I'll be sending these off to Bullet to keep in their kit of spares and next time they have a worn out LSD in one of the cars in their fleet, these parts can go right in with the expectation of another 50 or 60 hours of use before they need to be retired.

Just for grins, I measured the clutches from the 2010 Cup Car LSD that was initially part of the donor box for this build. I was told by the previous owner that it had 18 hours on it. Like mine, the steel plates showed no measureable wear. The clutches were .05mm off of their new spec. Quite a bit more wear.

 

Matt,

Thanks for sharing info on this! This thread is a huge contribution IMHO. Am I to assume that your friction plates are hardened steel with a rough friction finish? Or do they also have another friction additive added to the steel surface or the actual material. Not asking for proprietary info here. Just curious as pertains to wear. Also, does this material make the transmission oil service intervals shorten? Just curious if these materials can be more damaging to bearings if they get in the bearings?

Obviously when aggressive grip is needed, a compromise is always at hand. And the abrassives that are sometimes required in things like Diff friction plates or synchros are more destructive the more affective they are. And these being the best discs on the market makes me curious as to why Porsche doesn't use them in factory street cars verses the less effective brass discs? Brass is actually pretty expensive compared to steel. Just a thought.

 

VAG,

It's rare that I'd presume to butt in on a question directed toward Matt. However, as Matt took over at Guard Transmission AFTER these friction discs & plates were perfected, I'm probably in at least as good a position to answer your queries, in part by relaying a bit of GT's hair-pulling history.

Guard Transmission's early friction discs were too aggressive, AND they released some grit into the gear oil. Considering the expensive Plasma bombarding process that we were paying for, this sloughing wasn't supposed to occur. (We had been assured that all friction particles would embed themselves into the base material, virtually becoming a component of the surface composition. After all, this was NOT merely a coating or inexpensive surface etching process.)

During this same time period, we became aware of another issue .. some ZF & Porsche plain plates were softer than others, and these would gall badly if used in a rebuild alongside our friction discs (.. and sometimes even with Porsche's own replacement friction discs). In our renewed quest for the best possible plain plate, we also arrived at the solution to our overly aggressive friction disc.

The steel plate material used for our friction discs & plain plates is harder than the ZF/Porsche plates. After cutting out the individual discs & plates, rather than merely tumbling them to remove sharp edges, we began experimenting with REM'ing them to polish the surface .. and then it came to us: Why not try REM'ing our friction discs a little bit, too? (REM = Isotropic Normalizing, in our case without the liquid solution added)

So that was the answer. REM the friction discs just enough to remove any particles that had failed to fully embed themselves, without compromising the function or longevity of the discs.

The friction discs used by Guard Transmission for the past few years really don't release much of anything into the oil, so long as they are used with GT's REM'd plain plates. Both Matt and I are extremely pleased with the long-term feedback regarding both function and longevity of these parts (even though from my standpoint, it's now merely a pride thing).

Paul Guard

 

The day I complain that Paul Guard came out of retirement to contribute to a discussion is the day I need to be taken out back and have some sense knocked in to me.

VAGscum,
Just to add to what Paul has said, always consider your source. There's one shop in particular that has made it their mission to soil the GT name and talks a lot of trash about our friction material. Most of what they have to say on the subject is outright lies. There's also a lot of ignorance out there and people assume that all LSDs are created equal. Because Carbonetics tells you to change your gear oil every 3000 miles, people sometimes mistakenly assume that any aftermarket clutch type LSD should see the same sort of treatment.

To further complicate things there are some reputible shops who never did much volume in GT LSDs and are actually unaware of the evolution of the friction discs. Some of these shops may even currently buy 1 or 2 LSDs a year and don't realize that our products are constantly updated and improved and are working under the impression that the discs are still really gritty but buy the LSD because it's the best one out there. And then they just advise the client to change the gear oil often because they don't realize that things have changed.

My gearbox oil service interval recommendations have nothing to do with LSDs at all. Porsche's factory interval is a joke. You should NEVER go over 15k mi on your gear oil. If you track the car, that interval gets shortened exponentially. I tell people as a rough guideline use one weekend at the track as 2500 mi. If you only drive your car at the track, that means you are changing the gear oil after every 4-6 events. That's for a DE vehicle. For a racecar? Personally I'd change it after every race weekend, or at most, 2 weekends.

As a matter of policy, we do tell people to change the gear oil after the first 5 heat cycles or 1000 mi once they install new GT clutches in a diff. While they are REM polished, there is still a little sloughing that occurs and I consider it one of those better safe than sorry sorts of things. However, I will go on to say that those pictures above from Daytona are from a gearbox that did NOT see the oil changed after "break-in". We ran that gearbox at the Roar testing sessions, and practice, qualifying and the race all on the same gear oil. If you look elsewhere on the board, you can find pictures of the gears and ring and pinion that came out of that gearbox after the race. If our friction material damages gears and bearings and whatnot there's no evidence of it in the gearbox we raced at the Rolex 24 Hours of Daytona this past year.

 

Isotropic normalization of course . Way above my head, but great info. Thank you soo much for sharing your knowledge and honesty. Great info.

 

What function of this part ? I had a chance to got a used Cup LSD. Upon opened for inspection, all parts looked good except this one cracked. Anyone know part number and where to order?