Limited Slip and Torque Biasing Diffs - Explainer?
 

Hi -
I've done some searching and not found any good threads explaining the pros/cons of using either limited slip or torque biasing differentials. After reading through this: http://auto.howstuffworks.com/differential4.htm I have a very basic concept of how the differential works, but am still not clear on when each system would be an advantage or hinderance given either normal daily driving or track driving in a 911.

Thanks,
Tom

Article on LSDs from POC Velocity a while back.

http://members.rennlist.com/emcon5/velocity_lsd.pdf

Tom

The Velocity article that Tom referred you to is a great read, it mostly explains the advantages of the late 964 up clutch types over the earlier clutch types.

If I understand your question, it is more concerned w/ the relative merits/drawbacks of open, gear type and clutch type diffs.

The most transparent to use is, of course, the open type, here the only connection between the L/R output shafts is the internal friction in the diff, in normal operation these are smooth and not all intrusive in operation, drawback is you get little forward propulsion if one side loses grip as in a fast corner where one side is unweighted thus loses grip or when one wheel is on pavement and the other off.

the next most transparent is the gear type, it only works on acceleration but is just like the open diff on deceleration. Advantage is a larger forward push when one side loses grip, it's no perfect but does provide for more acceleration force than an open diff.

next is the clutch type. These work their magic on acceleration and deceleration. On acceleration the effect is like that of the gear type, providing forward more bite as long as one drive wheel has grip. These also work on deceleration stabilizing the car allowing you to brake deeper into a turn then either of the above 2 types. The cost here is that the car will have more "push", understeer to the sports car crowd.

the amount of linkage between the 2 sides of either of the 2 lsd types is variable and generally listed as a %, 2 % for the late asymmetric type(acceleration/deceleration), this is misleading because the breakaway torque between the 2 sides is the important feature but is only spec'd in a maintenance manual(if then).

higher % is generally better for track use and for deceleration, a 40% will be a better track lsd than a 20%, a 40/65, more plates can absorb more torque and tolerate greater breakaway forces then fewer plates.

Early ZF clutch types were 4 plate 40% units that could be made into 4 plate 80% diffs by rearranging the assembly order of the driven(d) and drive plates(D) they would be a 40% if the assembly order was dDD, 80% if it was DdD(more friction faces this way) brake away torque can also be adjusted by varying the preload on lsd assembly, the various plates come in different thicknesses to do this.

964t and 964RS were the first to use the asymmetric types they had 4 plate 20/100, this was found to be a bit much for most drivers , 993 use 2 plate 20/65 or 20/40, 993Rs use 4 plate 40/65 996GT3 use 6 plate 40/65. Thes asymmetrics are the ones praised in the Velocity article.

drawback of any lsd are apparent on totally slick roads, think an ice sheet that covers the whole road. In these conditions both drive wheels will spin frantically, the right hand torque rule shows that there will be a large force pushing the rear of the car to the left, there will be a major tendency to swap ends. This is a reason poems are somewhat reluctant to install lsd in civilian models and have favored the use of the electronic ABD versions that only work below ~20mph

Other drawbacks of high breakaway torque lsd are difficulty in parking lot type maneuvers the car just won' like to make tight turns.

the gear type is much preferred for A/X where the increased low speed understeer would generally be considered objectionable.

Asymmetric clutch type is considered best for track use, 40/65 or in that neighborhood for dual use, 50/80 or so for dedicated track use

Thanks for that link - I ran across a link to that article in my searching for info, but it was old and dead!

Bill - I will read that article (printing as I type) and then reread your post to hopefully better understand what the heck you wrote!

Thanks,
Tom

I have no experience on the track with any LSD. I do have a lot of experience on the street and AX courses with a TB style diff...in different cars.

The TB diff acts like a clutch type when accelerating in a straight line....get carried away & both rears spin. The same is true in wet conditions while turning.

In dry conditions it is a bit different. As stated before, the TB diff transfers more torque to the tire with the most grip.....right until there is no grip on one, then that one free wheels like an open diff.... this also means there is something wrong with your suspension set-up.

The good part is when both of your tires are loaded but the outside has more load and the torque is transferred there. It effect is it pulls (pushes?) the car around a corner....it tightens your line as you're accelerating thru a corner....good for the "fast out" crowd. The rear doesn't "step out", it just feels like it.....get carried away and it will, tho. Very useful if you like to throttle steer without sliding (loosing traction at the rear) the car. That is also available but is not the fast way around.....dirt track cars excepted.:D

This white paper tells about all there is to know about Torsen diffs.... enough to make ones head explode

http://www.torsen.com/files/Traction_Control_Article.pdf

It should be noted that all of these effects tend to diminish as speed increases (partially because you're not doing 90 degree turns under power when cooking along).

Take the LSD to the extreme, and you get the spool (no spider gears, both drive tires are 100% locked together). At slow speeds (parking lots, 90 degree right turns) one tire will bark and chirp all the way around the turn, and the entire drive suspension will feel severe stress. On black ice, you're all but guaranteed to spin the car, since both drive tires will spin together. Great for drag racing, good for dry track racing, terrible for any type of street driving.


High tire air pressure will lessen all of these effects.

Excellent coverage here from Bill as usual, so I'll just add a few things:

Torque biasing (gear type) differentials rely on gear friction to create the biasing force. This friction is reduced with age, so the amount of torque transmitted to the slipping wheel lessons over time. Eventually the unit wears out and it is basically not rebuild-able; you chuck it and buy another.

As Bill noted all clutch style units are not created equal- there is a performance and lifespan gap between "street" and "race" quality units. I'm not sure if the lowest end street units are all rebuild-able, but certainly most units and all race units are. Maintenance can also be performed to maintain the pre-load and breakaway torque as the unit ages. Even without this maintenance I've been told the clutch style is more consistent over time, meaning it "wears out" roughly half as fast. The clutch style is a few pounds heavier, and is sometimes audible and certainly more noticeable in general in operation.

I've used both gear type and clutch style, mostly at the autocross, in the past. I preferred the gear type in that roll, but as was mentioned you can set up a clutch type diff in many different ways for different applications, and mine was not ideally set up. For my new car, again largely for autocross use, we decided to go with a specially set-up clutch style unit. The experts have me largely convinced that they have configured it to perform similarly to a gear type at the autocross, but that it will out-last the gear type and perform better on the track. No experience yet.

Guard Transmission sells both types, and they are top quality- hard to go wrong.

Porsche autocross experts are few & far between, IMO......drivers, yes, there are plenty, but set-up/drivers are very hard to find. The only one I've met is Greg Fordhal. I went with his recommendation of the Guard TB diff for autocross.....expert opinions differ, it seems.

I'm looking forward to meeting up with you on some parking lot, Pete, so's we can"exchange notes". My low tech vs your hi tech should be interesting.:D

OK, I've done some more reading and have a slightly better understanding (I think). A few more questions too:

1. Does having an LSD create A LOT more wear on the internal transmission and drivetrain components as it is basically FORCING one wheel to turn when it would rather not?

2. I read a lot about heat generation - through friction especially w/the clutch types. Is a trans. cooler an automatic add on for longevity?

3. Seems like I read a lot about added understeer - my car already understeers!

4. The common theme in all discussions is that adding an LSD will drop your lap time - true?

I still don't completely understand it all - I can only take visualization so far. It'd be nice to have a few cars at a track set up w/the different options so I could do a few laps and try to "feel" the differences! (I'd like to have cake and eat it too!).

Tom

not really, all the stresss is at the lsd clutch plates(that's why they wear) or gear faces on the worm gear type, obviously the drive side sees more torque than it otherwise would w/ an open diff. None of these are issues to be concerned w/. The lsd will need very ocassional rebuid(new plates)


the gear type will geerate a small additional amout of heat, the clutch type more, most tracked 915s ought to have a cooler anyway, the g50s started needing coolers when they hit ~400hp or enduro events.

they both generate some additional power on understeer this is the cost of additional drive off a corner, the clutch type generates understeer on trailing throttle as well this is a good thing, the asymmetrics generate even more on trailing throttle this is an even better thing. As w/ every mod you need to consider how it works w/ the rest of the package, if you have too much understeer you tune it out w/ suspension, wheel and tire changes.

For most people, most of the time yes, but again 'courses for horses', gear type for A/X clutch type for track

I understand having a LSD can actually help transmission life. In a non LSD car the inside wheel spins up and then at some point grabs again- this can create shock loading stress that the LSD reduces. Thus I'm told if a gearbox is highly stressed a LSD can help.

Yea, me too. You'd just better hope it's a tight parking lot ;)