Lets talk about Competition Diffs

[Storm]

Good comments so far.

The 20Kg center diff is great for loose surfaces because it's just about locked for full 4wd at all times. On gravel, it would be extremely useful to keep the car pointed where you want (with giant cajones for slow speed turn-in).

The 12Kg center diff seems like it'd be pretty forgiving on the street with definate improvements over the 4Kg unit when the car is pushed hard.

The Cusco Tarmac center diff has to be pretty darn close to ideal. I really-really wish the design could be duplicated in slightly different ratios to suit different tastes. Who am I kidding.....I'd love nothing more than for OBX to release an inexpensive center diff of similar design(to the Cusco), like the front helical LSD they have out now.

I had no problem keeping the rear wheels planted when I was on the 13's, even though my D2 coilovers don't exactly have a ton of droop travel. The increase in traction (and ride height) I have with the 275-35-15's has begun to get that inside rear off the ground again, but the old skool Fuji LSD still has enough ummph to keep the car driving forward on 3 wheels. Clearly a sign that I need more power!

Jay Storm

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[neilschelly]

Jay, what is the old-school Fuji LSD you've got in the rear? Is it a clutch-type from the pre-Impreza/Legacy Subarus?
-N

[KC]

Neil, where are you lifting a wheel? Do you know if you're even under power when the wheel is in the air? If the wheel lifts on steady state and decel, then there's no issue. When a wheel is in the air under power is when you need trick diffs. that can transfer the power to the outside wheel. Because, quite frankly, if it's in the air and there's no power going to it and it's down by the time you're starting to apply power, what does it matter if you have an open diff in the rear or not?

I doubt you can overcome the suspension limitations of lifting the inside rear wheel on cornering

and

I'm sure you can get the suspension to keep the inside rear loaded, but didn't ask that. Having the inside rear unload a bit though helps in getting rotated around the corners.

In the 1st, you state that you doubt overcoming the inside rear lift can be accomplished, and then you say you didn't ask about not keeping the inside rear loaded and that you're sure it can be done. Holy confusing!

Having the inside rear unload a bit though helps in getting rotated around the corners.

Having a rear tire lift is not a determination of how fast, or well a car can turn in/rotate.

You can still have the inside rear unloaded to a point that there's still contact where the diff won't send power to it. To accomplish that, you're going to need a high front spring rate should equate to less front dive (dive is a major "feature" of what sends the rear of the car up as the weight gets transferred). It can be touching, but still turning. It's not going to hurt the performance of the turn-in/rotation in the 'quick' stuff if the tire is touching, but totally unloaded isn't good.

Not only that, you want that tire touching because it's sooner you can get on the gas without having to wait for the rear tire to spin up.

--kC
(Or, did I miss your point entirely, *again*?)

[neilschelly]

Originally Posted by KC

Neil, where are you lifting a wheel? Do you know if you're even under power when the wheel is in the air?

Yes. I'm generally on the gas accelerating out of a turn before the apex, so there's still turning to do and the increasing g-forces will pull the inside rear off the ground enough to spin through the Phantom Grip. It's the wider turns that I will lift a rear tire on now (used to be the other way). Stiffer springs in the front this year and a much better F/R weight ratio (55/45) and running the struts at full stiff usually has kept the front from diving and the rear from lifting on the little stuff. I've also tried being lighter on the brakes as I've had a reputation for some left-foot-stomping in the past.

Originally Posted by KC

and In the 1st, you state that you doubt overcoming the inside rear lift can be accomplished, and then you say you didn't ask about not keeping the inside rear loaded and that you're sure it can be done. Holy confusing!

That was confusing yes. I tend to expect that you can get an Impreza to stay flat, but you will lose rotation capability. I don't seem to get adequate rotation except when the inside rear becomes unweighted at least. I don't consider it "overcoming the inside rear lift" unless you can do it without plowing every corner.

Originally Posted by KC

(Or, did I miss your point entirely, *again*?)

I don't think you missed my point before. Others were rewriting what I'd said about clutched and helical diffs and how autocross and road racing would have different requirements, and I was being corrected with the same arguments I'd given for a helical in front, a clutch in back, and some mystery diff in the center that may or may not be open. That's what I was trying to correct.
-N

EDIT: I think I see why you think I meant you missed my point. I said _I_ was missing something with regard to your comment, not you. I meant I'm still missing something about the right and wrong way to rotate. I'm still trying to learn to drive and I'm starting to think the Subaru's not a good car to learn to drive on because it's really, really forgiving. I can't count how many great turns I've made at events this year where I felt totally out of control, but suddenly found myself pointed the right way and kept going. I'm having fun being decently fast this year, but I really have so little confidence in it being me and not the car. When I get the rotation thing right, but it's luck.

[digitalseance]

Originally Posted by neilschelly

I imagine the WRX would be in the same boat as the NA Imprezas mostly. I doubt you can overcome the suspension limitations of lifting the inside rear wheel on cornering (unless maybe an '08 with the double-wishbone?). How much power can an ESP WRX make? Do you think you could transfer enough weight on corner exit to the rear axle in order to lift a front wheel?
-N

I'm only at 250whp and 260wtq on C16 with all the allowed power-adders, but I'm only a 2L. A fully built prepped 2.5L will see similar WHP levels but definitely will see 300+ WTQ, a local ESP 2.5L has at least that much but he can't seem to come to an event (don't get me started).

As for lifting the inside front wheel, you betcha. I do it quite often and it's very annoying b/c I know I'm loosing time; especially on the long sweepers and medium speed tighter courses. I've got video evidence where you can here the engine spinning the front inside like crazy and once it plants, you hear it bog back down some.

I will be getting the cusco 1.5 front lsd with the tarmac center...once I win the lottery and after I upgrade to 275s.

[Mechie3]

2.5L ESP here. Got protuned the week after prosolo and made 351 WTQ @ 3100 RPMS. If you look closely in this pic, i'm two wheeling this corner (hence my interest in diffs for next year). That little bit of "contact" you see is a shadow. This was actually a 180 degree off camper S curve that was taken around 55mph. Problem is, I'm a noob when it comes to diffs. I need to (re)read the diff FAQ. There's so many different possible setups and as many opinions. I want a setup that is competitive, doesn't require continuous maintenance/replacement of clutches, and doesn't wear out excessively due to DD.

[KC]

1) you need more front camber.... much more
1) you need more spring rate in the front
1) you need less bar in the front and rear

can you tell I'm a fan of more spring, less bar? It's all about having an independent suspension, a big bar takes away from the wheels operating independently.

--kC

[xcdhridr]

I'm with KC on this one... I just never got a chance to get the coivlover revalved for more spring. But I do have lots of front camber. I still need some caster though =(

[Mechie3]

Front camber is at -3. Any more (had it at -3.5 with camber plates maxed out) and it rubs on hard corners. I'm contemplating the stiffer springs and getting 275's. The only thing holding me back is that this is my only car and does DD duty.

[Scooby South]

well....to further this conversation along...The STi's have a slight advantage here...however inside wheel lift is still an issue and being able to put the power down, It is quite common....so to fix that...there is 2 ways of thinking.... get some long travel suspension system like zzyzx's to keep all 4 wheels planted and/or shim up the rear diff to a 'locking' rear end., essentially a 2.0 LSD sometimes a bit of overkill and pretty noisy and being able to drive the planted wheel and apply power to come off the corner...thats on a R180 Rear diff...
the R160 Diff is either open...or has a factory Viscious LSD which requires the diff to spin and get to a certain temperature before it will engage..

A definite improvement on the R160 Rear would be a mechanical 1.5 way...LSD unit...That would be the ultimate for our uses...IMO..

The Helical seems to be a decent fit for the front...My BSP STi had a factory Helical and I could feel the front "clawing" out of sweepers and Slaloms...the problem was the rear..and the infamous inside "lift"...when this particular situation happened...the car would inherently push..

My thought is that the center diff could be the key here...As Jose' pointed out...65/35 split would seem to be ideal...over the stock 50/50...maybe even a 60/40...to get rid of the push that develops when you add all the LSD's....

As KC pointed out...Rotation is important...We just need to figure out what a good solution is to get the car to do EXACTLY what we want...

Good stuff...Excellent discussion

Bill

[Splash]

The problem I have is that I feel I am trading front grip to keep the inside rear down so I can go forward. It's almost as if I must choose between being able to take sweepers and being able to exit corners.

I already have long stroke dampers and springs with helpers to allow the extension. Problem is, while rear bars do wonders for rotation, but they also hinder said extension once off the mainspring. I can eliminate the helper and get a longer mainspring, but then the rear droop travel suffers.

The other way to keep the inside rear down is to use a bigger front bar, which is what causes the loss of grip up front in sweepers... In the end, I'm trying to find out how small of a front bar I can get away with and how large a rear bar I can get away with.

To me, the main problem is that we have a car that wants to corner like a FWD car, and, in STI form, wants to apply power like a RWD car. I just can't seem to find that happy medium without sacrificing something important...

[Sp00L]

wouldnt the ultimate diff setup be 2.0 way(or even 1.5 way for a street car) clutch diffs in the front and back and a 12kg unit in the center, and to help out even more have some coilovers with good droop and adjustable rebound?

seems like that would stick pretty good, im not an expert on this matter though.

[KC]

Originally Posted by Mechie3

Front camber is at -3. Any more (had it at -3.5 with camber plates maxed out) and it rubs on hard corners. I'm contemplating the stiffer springs and getting 275's. The only thing holding me back is that this is my only car and does DD duty.

Not according to that picture. You barely have the wheel turned in and it's got positive camber. Sure, you lose camber as the suspension settles, but if that's -3*, I'm... I'm.... 21 again!

--kC

[solo-x]

Originally Posted by Sp00L

wouldnt the ultimate diff setup be 2.0 way(or even 1.5 way for a street car) clutch diffs in the front and back and a 12kg unit in the center, and to help out even more have some coilovers with good droop and adjustable rebound?

seems like that would stick pretty good, im not an expert on this matter though.

Aggressive clutch diffs carry a penalty on entry rotation. They are also much more speed dependent then a torsen. IE, in a tight corner, you might have the right amount of rotation, but in a more open, higher speed corner the car will push.

[Scooby South]

Originally Posted by solo-x

Aggressive clutch diffs carry a penalty on entry rotation. They are also much more speed dependent then a torsen. IE, in a tight corner, you might have the right amount of rotation, but in a more open, higher speed corner the car will push.

Exactly.....some of it could be dialed out with suspension and air pressure...but not all of it...then theres the whole "trying to get used to your car" again thing....

All these things add up...to me...it would seem that the car would be faster in and out of the corners..because obviously you have more Grip/traction...which in turn would allow MORE Go pedal in any situation..

Agreed???

Bill

[Mechie3]

Originally Posted by KC

Not according to that picture. You barely have the wheel turned in and it's got positive camber. Sure, you lose camber as the suspension settles, but if that's -3*, I'm... I'm.... 21 again!

--kC

Front right is severely compressed. The gap on the wheel is about 2 fingers or so (14" hub to fender). In that pic the wheel is probably a good inch up into the fender by that point. I only have street coils on.

[davisk6]

Originally Posted by solo-x

Aggressive clutch diffs carry a penalty on entry rotation. They are also much more speed dependent then a torsen. IE, in a tight corner, you might have the right amount of rotation, but in a more open, higher speed corner the car will push.

I have a KaaZ 2way Clutch Type in my 240sx and I can attest to this situation. The diff works great getting the car up and going, but since both wheels are providing traction, you need to be careful in how much "go" pedal you use. Not enough and you don't get enough traction. Too much and you over-rotate. Granted, this is from a RWD perspective, but I suspect the same situation occurs in an AWD.

[solo-x]

Bill, I'm not sure what you said in your last post. My reply was directed to Sp00L, explaining why you might not want aggressive clutch diffs at both ends. You should not be tuning the suspension around your diff. The suspension should work as good as possible with open diffs at all 3 points. THEN add whatever LSD is required at each point. Since the inside rear is prone to coming off the ground, I suggest a clutch type there. Since you want the car to rotate on entry in an autocross setting, I suggest a torsen up front. Since the front and rear axles will rotate at different speeds front vs. rear, I'd lean towards an open or at the very least a very "soft" center diff.

[Bryan @ JRZ]

Hi all,

Here goes a lengthy first post...

This is a great thread, I'm glad to see people talking about the differential. The diff is like - however clumsy of a parallel - a suspension for the engine. They allow us to change and re-distribute the forces and moments acting on the car as it moves about. Then we have the springs, shocks, and bars, which also distribute and react forces and moments. I'm sure most people are aware of this already; I reiterate to drive home a point.

When your're working on your setup, you have to treat the springs, shocks, bars, alignment, ride ht's and diffs (aero if applicable) in the same lump. All these things do is change the loading on the tire as you drive about. Changing one effects how the rest work. There is no magic differential setup, just as there is no magic suspension setup. Knowing both is going to get you a great car.

I think that with the varying power levels, uses of the car and suspension setups, there are going to be varying diff packages that are optimal. What I have seen in setting up race cars, is that making an educated diff choice is the first thing you do. They're harder (time+$) to change around and its easier to work a suspension around them. Some iteration is normally required. The biggest thing is to know what tires are doing what work when and how you can supplement or modify that behavior with the diff.

I would say that for cars with a lot of power and sticky tires, a lear LSD, a center 35/65 and a front open decel/lock-ish accel (which can be LSD or a torsen type) is going to work the best. Street cars with less power and more suspension compliance will need something different. I can't make a recommendation because I haven't worked with those much.

Lifting tires is generally bad, and using them to change balance should only be done as a band-aid when league rules don't allow the proper fix. After all, 4 tires is better than 3.


Food for thought.

Best,

Bryan

[Scooby South]

Originally Posted by solo-x

Bill, I'm not sure what you said in your last post. My reply was directed to Sp00L, explaining why you might not want aggressive clutch diffs at both ends. You should not be tuning the suspension around your diff. The suspension should work as good as possible with open diffs at all 3 points. THEN add whatever LSD is required at each point. Since the inside rear is prone to coming off the ground, I suggest a clutch type there. Since you want the car to rotate on entry in an autocross setting, I suggest a torsen up front. Since the front and rear axles will rotate at different speeds front vs. rear, I'd lean towards an open or at the very least a very "soft" center diff.

Sorry..I was following up your post and adding to it..

Bill

[Scooby South]

Originally Posted by Bryan @ JRZ

Hi all,

Here goes a lengthy first post...

This is a great thread, I'm glad to see people talking about the differential. The diff is like - however clumsy of a parallel - a suspension for the engine. They allow us to change and re-distribute the forces and moments acting on the car as it moves about. Then we have the springs, shocks, and bars, which also distribute and react forces and moments. I'm sure most people are aware of this already; I reiterate to drive home a point.

When your're working on your setup, you have to treat the springs, shocks, bars, alignment, ride ht's and diffs (aero if applicable) in the same lump. All these things do is change the loading on the tire as you drive about. Changing one effects how the rest work. There is no magic differential setup, just as there is no magic suspension setup. Knowing both is going to get you a great car.

I think that with the varying power levels, uses of the car and suspension setups, there are going to be varying diff packages that are optimal. What I have seen in setting up race cars, is that making an educated diff choice is the first thing you do. They're harder (time+$) to change around and its easier to work a suspension around them. Some iteration is normally required. The biggest thing is to know what tires are doing what work when and how you can supplement or modify that behavior with the diff.

I would say that for cars with a lot of power and sticky tires, a lear LSD, a center 35/65 and a front open decel/lock-ish accel (which can be LSD or a torsen type) is going to work the best. Street cars with less power and more suspension compliance will need something different. I can't make a recommendation because I haven't worked with those much.

Lifting tires is generally bad, and using them to change balance should only be done as a band-aid when league rules don't allow the proper fix. After all, 4 tires is better than 3.


Food for thought.

Best,

Bryan

However Lengthy it is...It's a good one.....Very good insight....especially about the different power levels...

-So whats optimum for one isn't necessarily good for another...
-How much does actual driving Style play into all this...It would seem the smoother the better obviously..to allow 'everything' to work as it should..

Bill

[Bryan @ JRZ]

Originally Posted by Scooby South

However Lengthy it is...It's a good one.....Very good insight....especially about the different power levels...

-So whats optimum for one isn't necessarily good for another...
-How much does actual driving Style play into all this...It would seem the smoother the better obviously..to allow 'everything' to work as it should..

Bill

Thanks Bill. Good point with respect to driving style. Smoothness is certainly one part of the equation. Another is response. Differentials can provide stability, which functionally, is the inverse of response. Your typical pitch-catch-hang-power driver is going to want less decel lockup than your typical drive it in-roll it-power early guy. Both "styles" can be fast. Decel lockup creates an understeering (read stable) moment about the yaw (cornering) axis. It plays a big role in entry and mid-corner behavior.

[KC]

Originally Posted by Bryan @ JRZ

Decel lockup creates an understeering (read stable) moment about the yaw (cornering) axis. It plays a big role in entry and mid-corner behavior.

Track or Auto-x? One doesn't translate to the other many times.

What could play a "big role" in corner entry at speeds above 60mph will not be the same as something that plays a "big role" at 35-55 mph. You're not going to see speeds about 65ish for the most part on an auto-x course, and I'm still thinking, sitting here, there's more time to be gained, or lost, in fine-tuning your suspension (and your driving) before one even remotely starts to consider trick differentials... almost akin from running street tires vs r-comps. R-comps mask many bad driving habits.

If your suspension isn't sorted out for cone dodging yet, differentials are only going to net you a marginal amount of time, if any at all. And in the meantime, frustrate you to all hell as the car stops driving as you like it to drive.

--KC
(PS: Drive better.)

[Bryan @ JRZ]

Originally Posted by KC

Track or Auto-x? One doesn't translate to the other many times.

--KC

Both. Track and autox are different driving situations with different levels of give and take. The physics is always the same. Whats different with autox is that the wheelspeeds difference is larger, and the cornering events happen in a shorter time frame. Id would say this situation favors responsive configuration. LSD differentials would have less preload as well.

[angryfist]

nate, maybe you can answer this one...

so for a clutch type rear diff, do you think a higher or lower initial torque on the clutches would be better for autox? specifically the design difference btwn the Cusco RS and MZ type diffs.