In reading some of the EVO 8 reviews, I noted that they mentioned that the lack of front LSD resulted in less
pull from the front wheels during hard turns. Does this result in a more FR like cornering behavior?

Cars like the Skyline and Turbo Porsche have computerized systems to make the AWD system behave like an FR, except when the extra traction is needed. The STI allows changing the
torque percentage to the front and rear whe via the DCCD. Is the lack of a front LSD, in what in every other respect is a track tuned car, a purposeful design element to give the car a more "fun" handling feel? Or am I off base here?

Just asking.

 

A front LSD will definitely be better than an open diff (if it is tuned correctly). When you understeer you are loosing tracking in the front wheels therefore, a good front LSD will prevent the lost of front wheel traction and prevent the uneven distribution of power between the front wheels.

 

Ah but the US evo was left with an open front diff for a reason.

Yes with a limited slip unit the front wheels will find more grip in corners, especially realy tight ones. In autocross the limited slip would turn the evo into an ungodly time killing machine.

BUT - one of the best characteristics of the US evo is its steering feel and turn in. When you add a big honk'n limited slip you have to deal with quite a few downsides. You basicly have introduced a little thing called torque steer and it very much takes away from steering feel. The US Evo is much more responsive than its UK and JDM counterparts thanks to that open diff. You can feel the road through your steering and you can feel the car's reactions as you turn. A limited slip puts a buffer in there, so while you are gaining great amounts of grip you are loosing feel and introducing rebounding torque into the steering system.

Now it can be done correctly so that there are virtually no downsides. The STi has a SureTrac(tm) front limited slip. That unit is virtually an open diff with electronic locking capability. So it dosn't introduce torque steer into the steering rack. But even the fancy electronicly controlled SureTrac(tm) reduced steering feel. You'll notice in Evo vs. STi compairisons how they talk alot about the steering feel in the Evo compaired to the STi. That would be one reason why they like the Evo better.

Now the UK and JDM evo's with their helical front LSD relies heavily on the ACD system not to put too much power up front basicly to control the bad characteristics of having a front LSD while mainting all the good characteristics...

Oops... long post. But yes, by leaving out the front LSD Mitsi has been able to increase road feel and steering response at the cost of front end grip.

 

The better steering is due to the shorter steering rack not the lack of LSD, the LSD is not mounted to the steering rack. The good turn in is due to the chassis, subframe, suspension, strut tower, front roll bar, etc. Also LSD reduces torque steer not the other way around.

Reason why STI don't have torque steer is because it has equal length front drive shaft since the engine is mounted in an inline position like a rear wheel drive car.

The reason they left the front LSD out on the USDM evo is because of cost.

 

An AWD car with the Evo's power level isn't going to torque steer.

My car doesn't even torque steer.

 

Actually since the 1995 Carrera 4, all awd porsches have had a purely mechanical awd system consisting of a viscous coupling and a limited slip diff in the rear with an open front differential. The first Carrera 4 did have an electronic ACD-type awd system but it was scrapped due to weight and complexity. The only electronics governing the new Porsche's awd system are the ASR and PSM systems which handle traction and stability but not power bias between the front and rear axles.

 

Is that true? In an fwd car, an LSD reduces wheelspin at launch, but doesn't enhance cornering ability. I thought that because the LSD wants to limit the speed difference between the axles, a front LSD will have a tendency to cause understeer in corners.

 

LSD transfers the power evenly between the two sides (wheels) not limiting it. You still get the same amount of power to the wheels but redistributed.

If you ever peel out in a car with no LSD you will only see one tire mark, that’s because in an open diff the power is transferred to the wheel that lost traction which create an even more slipping.

To answer your question LSD will improve an corning of a FF car to reduce understeer. (Understeer is cause by a lost of traction in the front wheels)

 

See the "Best Motoring International" tape, # 4 I believe. You can see "The Drift King" guy running the Turbo Porsche with rear wheels (and not front wheels) spinning. His voice over is where I got the information.

 

NYET!

Take the Mazdaspeed Protege or the Nissan Sentra SER Spev V for example. FWD cars that have horrible torque steer ENTIRELY because of their front LSDs. Their non LSD family car equivilents have very litte torque steer. The angle and length of drive shafts can cause the LSD to amplify torque steer by delivering more 'torque' onto the offending shaft section. Torque steer is caused by a couple different reasons, but one is suspension geometry changes due to shaft angle on a wheel that is receiving traction... it has nothing to do with the lack of traction... so how would adding an LSD reduce it? Shaft length and binding theory (small binding of metal drive shafts.. like twisting a rubber band) is another. Again, caused by traction, not the lack of.

Turn in... I'll not argue your point. The evo has the second best turn in of any car I've driven. But of course its not connected to the steering rack, its connected to your drive shafts which are connected to your wheels which is the thing your steering wheel is trying to move. I guess this is perceived turn in though, so I'll shut up. An LSD will require more effort and is not instantaneous so there's a pause while an LSD like a Helical starts to bind. So perception is that there is a delay in turn in, but in reality its really just the extra effort.

A front LSD does drasticly changes steering feel. It absorbs alot of road variations through the mechanism, it dulls the feedback into the wheel for the driver and in some cases it requires a manufacturer to up the level of power assist to deal with unpleasant nature of the feel.

AWD system aside the evo feels good when you add an LSD to the front, but a mechanical will have downsides in feel while giving major bonus points in grip.

Oh and the cost thing... the helical from the Evo VII costs about $400 retail. A quaife racing diff is less than $800 and the SureTrac in the STi costs less than $300... and those are retail prices so I'd imagine Mitsi could get a nice discount. The price was never an issue... but now we're talking politcal battles, not engineering excersizes.

 

LSD = Less torque steer
Torque steer is also a FWD thing only. The Honda Prelude SH has a pretty advanced version of a limited slip and ZERO torque steer. The regular Prelude has tons of torque steer. You can't compare a 180hp Sentra to a 130hp Sentra. The 130hp one doesn't have the guts to torque steer.

 

Torque steer is all about... well TORQUE! How can an LSD reduce torque steer when it will automaticly apply more torque to the offending "high grip" wheel?

No the cars that have reduced torque steer and also have LSDs were redesigned with other features that reduce torque steer, like strengthened suspension parts not prone to twisting or realigned or equaled drive shafts.

The Spev V and MSP didn't have anything done to prevent torque steer but then HP/TQ was raised and a limited slip was put in... so for your theory the any extra torque steer would have been eliminated... but instead an extra 30 HP resulted in 5 times the torque steer.

There is no mechanical method for a limited slip to reduce torque steer, its only mechanical function could be to increase it. Torque steer is NOT one wheel traveling faster than the other, its deformation of suspention that causes torque steer... you can peg leg burnout with unequal shafts and not have torque steer and you can make two solid black lines and pull hard to one side.

The LSD has no function to reduce it.

 

I wish I had your wagon.

 

Oops... didn't know I still had that listed. I sold the avant to buy an Evo. But then refused to pay over MSRP and lost my space. So for the racing season I bought a Mazdaspeed Protege.

I'm missing about 200+ horsepower compaired to my modified S4. But my MSP is way faster through the cones.

Not to get off topic.

 

So I'm researching as much as I can to figure out how an LSD can help reduce torque steer... and why my car and the Spec V don't benefit from such wonders...

I find that virtually every company that sells limited slips states that they help reduce torque steer... but each company that makes the LSDs those companies sell does not... except for Phantom.

So I'm asking the engineers I'm working with about it and everyone starts to argue.

Can an LSD limit the amount of suspension deformation neccessary to reduce torque steer? Two "yes" one "No way". The yes's argue that most clutch type LSDs will interveine before the twisting of the suspension or at very least reduce the effect. The no way says that only racing type LSDs are quick enough to prevent torque steer because both sides of the suspension deform and the LSD always sees a ballanced torque load.

I'm siding with the No Way engineer... but only on cars that have torque steer related to drive shaft angles AND length variances. If it was just length issues I think an LSD would reduce the effect.

Then again we're all sysadmins now not engineers... so I officially have decided not to give a flying monkey fart... but I'm still puzzled why the MSP and Spec V crank so hard to the right...

Sorry to take this post waaaaay off the orriginal topic.