ZSA Motorsport

figured id ask the smart guys up here for perhaps a bit of a primer on limited slip differentials. figured it was a great place to clean up an area of confusion i have.

Been hearing about diffs and ramp angles, etc. and load under decel and accel, for a while. one of those areas that entrigues me and now id like to perhaps get a better understasnding.

i have a world challenge camaro. stock differential that came with it, as everyone said it would, blew apart the moment we got hustling on a track with 17 slow turns. wheelspin city. We looked at a few options and settled in on an aftermarket diff. installed it in the pumpkin and bang, thing works like a champ. I actually think the car is awsome under decl and accel.

In talking with another pro driver, they felt they'd like a bit "less locking" under decl , keep things a bit more free, help eliminate entry and midcorner understeer. i can visualize how more binding/locking, or whatever its called, on entry and midcorner will keep the car from rotating more tightly.

but im just curious whats really going on there technologically.

also, when speaking tot he supplier, he said the diff i have is setup 25/35.
25 under decel, 35 on accel.
i sort of asked what the 25 and 35 represented, a percentage? he said kilograms of force.
i mentioned that perhaps a bit less entry/mid understeer woudl be helpful, get that car pointed the right way and get back on the 6.2l motor.
he said they could take it up to a 35, 38 on decel, that would free up the car.

confusion sets in. i should have askd more questions at the time. i told him id get back to him.
i decided i needed to know more about this technical area.

anyone care to give a little primer, or explain the diff, the "25/35" and how a 25 to a 35 will help reduce entry udnersteer.

tx

chris walrod

On the racing side of things, ramp numbers typically express ramp angles. The lower the angle, the more internal forces are applied to the clutch packs with a given amount of input torque. I've noticed in the GT car area, ramp numbers are expressed as %/lock.

Increasing the decel ramp rates or forces on the clutch packs can indeed induce entry understeer as it is really acting as a yaw damper (clutch pack preload can also achieve yaw damping). This can, however, hurt entry quality as friction materials vary in break-away characteristics. There is the theory that increasing the decel lock amount can add entry over steer as keeping the rear locked longer and break grip on the inside tire. Playing with static preload can also be a **** to turn for fine tuning the diff.

In Champ Car years ago we played a lot with negative preload diff's. We had the luxury of an external [negative] preload adjustment that could be tuned quite easily and quickly. Another luxury we had was an oil pressurized clutch pack that kept diff temps constant, which kept the diff lock numbers consistent. The diff in these open wheel cars are a HUGE **** to turn with respect to tuning the overall chassis performance despite the rather large influence of aero factors.

Essentially the ramps apply lateral forces into the clutch packs as a function of torque input. You can also play around with number of friction surfaces based upon how you stack-up the clutch pack upon assy. I hope this helps some!!

KaiB

Oh my, that's gorgeous!!!

I need a shower....

jrgordonsenior

Chris remarkably I understood most of that.....

OP, In Porsche talk that diff would be known as 35/25 with the acel ratio first. Cups for instance come with 40/60 ratio diffs, 60 being ther decel number and Guard makes their GT diff either 40/60 or 50/80. Preload settings are just as significant as they set the point when the diff comes into play.

I haven't driven a WSC Camaro so I can't speak to it's handling characteristics, but a 25% lockup ratio under decel shouldn't be causing severe understeer. There are different theories regarding how to best drive a high lockup ratio diff like Guard's at 80%. It mostly involves going into turns a little hotter under braking and utilizing the diff lockup to induce rotation. Chris touched on this briefly above. With the motor up front in the Camaro I don't understand how a 25% lockup ratio can induce that much understeer. I've been waiting patiently for others with direct experience to post. Matt?

Bill Verburg

In the Porsche world the % and lockup factor and brake away torque and ramp angles are all related but not the same
lsd internals


ramps- Porsche Motorsports 40/65 LSD has a 50-degree ramp on the acceleration side and 37.5-degree ramp on the decel side pictured is a Guard nominal 50/80, I don't know what the ramp angles are


disks- inner, outer, inner, outer, belleville washer
inners are splined to the transmission, outers are splined to the axles, belleville + disk thickness contributes to initial preload




the ZF lsd as used in 915s could be set up as a nominal 40% or 80% lsd, the ramps would have the same symmetric 30* angles on both accel and decel side, the 40 or 80 came from the way the inner and outer disks were stacked, 40% has 2 inners then 2 outers on each side, 80% had alternating inner and outer so there were 2x's as many friction faces working. A further tuning technique was the stack preload, the disks come in varying thickness so that the initial preload can vary a lot, this afffects the actual breakaway torque.

modern lsds are asymmetric(acceleration/deceleration) , Porsche Motorsports 40/65 LSD has a 50-degree ramp on the acceleration side and 37.5-degree ramp on the decel side. the 40/65 is a nominal descriptive that has only a vague connection to actual brake away torque

starting ~'03 PMS 40/60's added more disks on each side, up to 4 inner and 4 outer alternately stacked on each side

different shops set their lsds up in different ways. 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.


set up variables
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).

lots of options

amso3

Great subject! Can't wait to hear more.

fatbillybob

Bill,

how do you know whether you are 40/60 50/80 etc. based on how and number of discs stacked? Is there a formula in a manual somewhere that tells you how things change based on what you do with discs?


Thanks!

Bill Verburg

in the early symmetric ZF's which all had 30 degree ramp angles on both sides the stacking order was the primary difference and thus used as a descriptor of the effectiveness of the lsd

on the late lsd's w/ asymmetric ramps, the ramp angles are the primary indicator of the lsd effectiveness and are thus nominally indicated by the 40/60, 50/80 etc.

you can stack the disks in these late lsd's as in the 40% ZFs, (OOII ) but that isn't normally done as it reduces the effectiveness of the lsd(the street lsd's are generally more like 25/40 or so but still use IOIO stack sequence). All the late lsd's I've seen stack the plates alternately(IOIO) and effectiveness is further tuned by stack height and belleville thickness

the final test of lsd effectiveness is a torque wrench which can measure the actual breakaway torque

fatbillybob

Bill,

Thanks for the schooling. So will with breakaway torque be different in the acceleration direction vs the deceleration direction of the typical asymetric ramp diff and if the? And lets say you get breakway in the acceleration direction of 40ftlbs with one diff and then with restacked discs 70ftlbs. What does that tell you about what that change to 70 would mean to the way the car might handle?

Bernie930

Hope Matt chimes in soon, some statements made here are incorrect, way off in fact. Diffs. are more voodoo than science, gear oils and friction material play a big part.

jrgordonsenior

+1... Matt wake up....

GTgears

Matt hasn't slept in the better part of a week. My baby girl was born midday Tuesday after my wife being in labor all night on Monday. My laptop is at the office and I'm posting from my phone which prohibits long replies and pictures.
There's a good "compendium" thread by Enthusiast over in the GT3 forum. The short answer is every chassis likes something different. 911 tuning has no relevance to Mustang and Camaro set up. Front engine rwd vehicles like way less lock up on braking than the 911 does. But the listed numbers for the Camaro still seem really low. On the BMWs and Audis (which are kind of rwd) we go with 50 or 60% lock up on braking. It's only the fwd cars that we take down to 25 or 35% on braking. But then I know nothing about Camaros and can't give specific advice, just generic experience.

cello

^ CONGRATS!! No sleep for you for a while.... ;-)

Link to the referenced "compendium" thread: https://rennlist.com/forums/997-gt2-...ntial-lsd.html

FFaust

Congratulations to you and the wife Matt, that's a lot of labor.

Thanks for the link.

Turn3 Autosport

Hey Matt, guess I didnt need to send the email to see if the new addition had arrived. Congratulations and welcome to the best part of being alive!!!!!

Although I gotta say I was hoping for a thursday birth, then I would never forget the date!

Cheers