Why you need a LSD if you like cornering!

[Sleepless]

If you lift a wheel, how much of the torque is transferred to the other wheel when using a clutch LSD? Is it configurable?

[wparsons]

If you lift a wheel with a clutch LSD, all the power goes to the wheel still on the ground. That's one of the few times it would be fully locked and driving both wheels at the same speed.

You can't really change that behaviour, but you can change how fast it reacts and how willing it is to lock with less speed differential between the wheels. Generally you really want to know what you're doing and have data to back up your changes to the diff setup though.

[ZDan]

Originally Posted by wparsons

If you lift a wheel with a clutch LSD, all the power goes to the wheel still on the ground.

No. It is limited by the amount of preload in the clutch packs. If you have 25 lb-ft breakaway preload, it won't take much torque from the engine to overcome the breakaway torque.

Still, IMO you want to run as light a preload as you can get away with for street/road course duty and let the RAMPS do the clamping for you. MUCH sweeter handling this way, AND the loss of preload over time will be minimal, well nigh negligible.

Some insist on running very tight preload, but IMO that's only suitable for drift. Gives a ton of understeer which you have to compensate for by biasing roll stiffness to the rear. Makes the car behave almost like the diff is welded. Will wear clutch plates rapidly, and now that you've set your car up to compensate for the push this will lead to oversteer AND inside wheelspin as the plates wear. Not a good solution, and totally unnecessary.

A lightly preloaded clutch-type will behave a lot like a Torsen, which IMHO is a good thing for most street/track usage and totally up to the task for most of us, even with reasonably well set up track cars. But of course the car already comes with a good Torsen limited slip...

[Ubersuber]

Originally Posted by ZDan

A lightly preloaded clutch-type will behave a lot like a Torsen, which IMHO is a good thing for most street/track usage and totally up to the task for most of us, even with reasonably well set up track cars.

Which is what Porsche apparently decided to do when they finally installed a clutch type lsd in their Cayman.

[Element Tuning]

In the quest for the fastest lap time I do believe in my experience that a clutch type differential will get you there where a torsen type can hold you back. Now you need to understand the limits of the torsen which is a lack of load on one wheel opens the diff up. Getting on the gas later just means you're driving around the problem.

So if you want to get on the gas hard and earlier you need to keep the rear tires on the ground and you can't get maximum droop with the rear sway bar in place. Taking off the rear sway bar will allow for maximum droop, keep the inside planted, your diff nice and tight, and you can drive out of the turn harder and faster!

Unfortunately as you get faster and faster you will get to the point with this suspension geometry that you'll be asking for so much spring rate and so much front roll stiffness to fight the rear roll that you'll be so stiff you can't curb and bumps can upset the car. It's at this point that adding a clutch type differential can take you to the next level and you might be able to soften up the rear spring rate, add a mild sway bar back in, and because your light inside wheel won't stop exit drive you might be faster overall. You'll also induce power on oversteer which will allow you to also soften up your front for more mechanical grip.

Get your car to work with the torsen first is my advice and that means more rear spring rate and no rear bar (for cars that have a track biased setup). When you've got that car so its starting to understeer too much then the clutch type diff can take you to the next level.

Thanks,
Phil Grabow

[Ubersuber]

The issue with this car, and what holds it back, is excess rear roll stiffness (or insufficient front roll stiffness if you prefer).

If the inside rear drive wheel is loaded at all then the Torsen should be sufficient. If your inside rear wheel is unloading too much then stiffen the front roll bar (or cut the rear bump stops shorter as that is really the problem with this chassis).

The cheapest mod you can do with this car to make it quicker is to shorten the rear bump stops. The rear bump stops are ludicrously long on these cars. As a result they are way too active and engage way to soon in the suspension travel. Now it is correct that rear bump stops work better than (and are way cheaper than) stiffer roll bars or stiffer springs for keeping the inside wheel down on the ground, because they only work in bump and have no effect on rebound in corners, but better yet would be to make the front work harder than the rear relative to stock.

This car is sufficiently rear biased in roll stiffness to slow it down. Rear tire wear is roughly twice the front tire wear, maybe more if you drive very aggressively. This is incorrect especially for such a low powered car.

Incidentally, this is why the BRZ should be and usually is quicker than the FRS or the GT86.

No doubt this is behind Toyota's decision to, apparently, fit BRZ front springs to the FRS and fit much better quality dampers to the rear axle.

[7thgear]

Originally Posted by Ubersuber

Rear tire wear is roughly twice the front tire wear, maybe more if you drive very aggressively.
.

I drive my car very aggressively and my number one concern at all events is tire management,


so far this season my fronts are wearing 2-3 times faster than the rear.


so I'm curious where you're getting your numbers from


unless you drift through every corner


if you even go to the track


which you've yet to post videos or data for

[wparsons]

Originally Posted by Ubersuber

If the inside rear drive wheel is loaded at all then the Torsen should be sufficient. If your inside rear wheel is unloading too much then stiffen the front roll bar (or cut the rear bump stops shorter as that is really the problem with this chassis).

The cheapest mod you can do with this car to make it quicker is to shorten the rear bump stops. The rear bump stops are ludicrously long on these cars. As a result they are way too active and engage way to soon in the suspension travel. Now it is correct that rear bump stops work better than (and are way cheaper than) stiffer roll bars or stiffer springs for keeping the inside wheel down on the ground, because they only work in bump and have no effect on rebound in corners, but better yet would be to make the front work harder than the rear relative to stock.

Based on what data? I love how you claim to know more than the chassis/suspension engineers at Toyota/Subaru.

If you cut the bump stops you're reducing the spring rate, which will give more body roll and will actually unload the inside tire more. Exactly the opposite of what you're claiming it will do.

Originally Posted by Ubersuber

This car is sufficiently rear biased in roll stiffness to slow it down. Rear tire wear is roughly twice the front tire wear, maybe more if you drive very aggressively. This is incorrect especially for such a low powered car.

Incidentally, this is why the BRZ should be and usually is quicker than the FRS or the GT86.

Except it isn't, fast drivers prefer the FRS over the BRZ because it understeers less at turn in.

Originally Posted by Ubersuber

No doubt this is behind Toyota's decision to, apparently, fit BRZ front springs to the FRS and fit much better quality dampers to the rear axle.

What do yo mean by "better quality"? Where's the data to back that up? How will better quality dampers significantly change the handling balance of the car on a smooth surface? Changing the dampening rate can affect things, but if the shocks are already consistent and work well then putting "better" ones on isn't going to make a huge difference in the balance.

Will MUCH better (talking $5k dampers) make it handle better on rough surfaces, absolutely. But with the same dampening rates, they alone won't change the balance on a smooth surface.

Originally Posted by 7thgear

I drive my car very aggressively and my number one concern at all events is tire management,


so far this season my fronts are wearing 2-3 times faster than the rear.


so I'm curious where you're getting your numbers from


unless you drift through every corner


if you even go to the track


which you've yet to post videos or data for

Mine aren't quite as bad as 3x, but they're definitely wearing at least twice as fast as the rears.

Even on wet pavement (not standing water, but wet) it's VERY hard to get my car to oversteer on a skidpad. On dry pavement, forget about it.

[Captain Snooze]

Originally Posted by Ubersuber

and fit much better quality dampers to the rear axle.

What does this mean? Not just "better" but "much better". Quality is also subjective.

[Element Tuning]

So I think there is some confusion about what causes the inside rear tire to lift in terms of roll. Too much roll doesn't lift the inside rear, too much rear roll stifness does.

As you inrease roll stifness with a sway bar it ties the two suspension arms together so as one side compresses it also tries to compress the other side. This keeps the car flat and on street tires grip gives up before the wheel lifts but on super sticky race tires there is enough traction for airtime.

Your FRS/BRZ's suspension is engineered for street use while on street tires. Change one part or the intended use and its up to you to reengineer to improve performance for your new mods or new use.

Thanks,
Phil Grabow

[ZDan]

Originally Posted by Element Tuning

In the quest for the fastest lap time I do believe in my experience that a clutch type differential will get you there where a torsen type can hold you back.

I don't think this is necessarily true. And anyway, it presupposes that all clutch types and all torsens are created equal. They aren't. T2R torsen in the Boss 302 Laguna Seca gets power to both wheels exiting corners better than the clutch-type unit standard in the "base" Boss 302. Also, in the GRM test referenced in this thread, one aftermarket clutch-type did better on a well-developed S2000 race car on Hoosiers, and another did not. So even at that level it's not the case that an aftermarket clutch-type will always benefit you over a factory Torsen.

Now you need to understand the limits of the torsen which is a lack of load on one wheel opens the diff up.

On a semi-decently balanced and set up FR car with halfway-reasonably lowish c.g., you shouldn't be lifting the inside wheel in corners past trail-braking at turn-in.

Getting on the gas later just means you're driving around the problem.

I've never had to adjust my driving style or "get on the gas later" at the track in the s2000 or RX-7. The Torsens in those cars have *always* gotten power to both rear wheels to the point of spinning both. Never gotten inside-only wheelspin in either.

So if you want to get on the gas hard and earlier you need to keep the rear tires on the ground and you can't get maximum droop with the rear sway bar in place. Taking off the rear sway bar will allow for maximum droop, keep the inside planted, your diff nice and tight, and you can drive out of the turn harder and faster!

You don't have to lose the rear sway bar to keep the inside rear planted. Stiffer front sway bar will do the same thing. Rear sways are connected on my torsen-equipped cars.
That said, in a car like the FR-S/BRZ, the FIRST thing I would do would be to get some decent front camber, which will allow a LOT more front roll stiffness without affecting handling balance (TONS more front grip with ~3 degrees front camber vs. the stock ~0). *IF* there is a problem with a stock FR-S/BRZ putting power to both rear wheels due to the Torsen (though post #1 of this thread to me indicates there is no problem there), addressing front camber and adjusting f/r roll stiffness to maintain handling balance should take care of most if not all of it.

Unfortunately as you get faster and faster you will get to the point with this suspension geometry that you'll be asking for so much spring rate and so much front roll stiffness to fight the rear roll that you'll be so stiff you can't curb and bumps can upset the car.

In my experience, I don't need metric tons of front roll stiffness to keep the Torsens working. And both my stock AP1 S2000 and modded FD have wheel stiffness rates biased to the REAR and have rear sway bars. Haven't tracked an FR-S/BRZ yet, but again, with decent front camber, you're naturally going to want to bias roll stiffness more to the front, which will put more load on the inside rear.

It's at this point that adding a clutch type differential can take you to the next level and you might be able to soften up the rear spring rate, add a mild sway bar back in, and because your light inside wheel won't stop exit drive you might be faster overall. You'll also induce power on oversteer which will allow you to also soften up your front for more mechanical grip.

I don't think going to a clutch-type means you can or should soften both ends of the car... For sure if you add a clutch-type that is stiff enough to perform substantially differently from a Torsen, that will induce understeer and you will need to bias roll stiffness to the rear accordingly. Which unloads the inside rear more. Which will want more diff clutch-pack tightness. Which warrants still more rear roll stiffness...

Get your car to work with the torsen first is my advice and that means more rear spring rate and no rear bar (for cars that have a track biased setup).

I think most track biased setups will have higher front and rear spring rates, and the setups I've seen tend to bias wheel rate heavily to the front anyway. Lots of "square" spring rate setups, which gives ~50% greater front wheel rates.

When you've got that car so its starting to understeer too much then the clutch type diff can take you to the next level.

If your desired/optimal suspension setup leads to inside wheelspin, by all means get a clutch type. But I would bet that the vast majority of people who track these cars should be getting power to both rear wheels while cornering with the stock Torsen...

FWIW, I *loved* the light-preload clutch-type in my street/track 240Z, and didn't buy into the hype over Torsens available for the R200 diff I saw on the Z boards. So I'm not biased against clutch-types at all. I just don't see them as being necessary for most people who will track these cars. Same as I didn't see a Torsen as "necessary" for my Z.

I just don't think that people should have the idea that the stock Torsen is inadequate, and that spending $$$$ on a clutch type is definitely going to benefit them. In most cases, it probably won't.

[CSG Mike]

I was having a discussion with a former S2k Challenge board member, and he brought up that Billy was trying to tell all the S2k owners that a clutch type diff was something that should be done very early in the stages of modding around 5 years ago

I'm starting to think he was on to something...

[Element Tuning]

Well given the FRS/BRZ does not have a solid rear axle and is not front wheel drive, we are just out of luck!

I'm not at all saying there aren't different ways to make the Torsen work well on the FRS and I do have track records and a national championship with the FRS to back that up. Making it work on a track day car and breaking unlimted track records however are two different things. We are running professional racing slicks that are 325" wide, have ridiculous grip, and with aero exceeding 700 lbs at 120 mph. The spring rates and front roll stifness required to keep a mac strut front end planted is huge. The rear multilink gains negative camber unlike the front so a softer rear with more independence can work really well. We focus on using the front to drive more weight to the rear to help "dig" it in with 400+ hp.

With how our car is set up, I'm very happy with the torsen but I know at some point I may be forced to increase the rear spring rate to deal with the aero weight that I'll give up too much mechanical grip or I'll be forced to avoid curbing which could hurt lap times.

Now when I was running stock power, slicks, and coilovers, ditching the rear bar netted me 2 seconds at VIR as I was able to drive out of the turns much, much earlier. While there are other ways to help this nothing costs less than disconnecting the end links and giving it a try. Lap times do not lie!

[Element Tuning]

Originally Posted by CSG Mike

I was having a discussion with a former S2k Challenge board member, and he brought up that Billy was trying to tell all the S2k owners that a clutch type diff was something that should be done very early in the stages of modding around 5 years ago

I'm starting to think he was on to something...

Same thing with the STi. A clutch type diff is a game changer on that car. If you don't do it at first, you can kiss all your suspension tuning goodbye.