I was discussing AWD drivetrains with a friend of mine recently and realized that I don't know jack.

Maybe someone could answer these examples:

1) If the US Evo 8 has an open front diff, how would a plate-type LSD or mechanical ATB diff affect performance... and why? What are the drawbacks of leaving the open-diff as is?

If you had more available traction in the front via an LSD, then would the power transfer or handling characteristics be adversely affected? To put it another way, with more traction (LSD) in the front, would the center diff transfer power to the rear in the same circumstances, or as often?

2) The Evo 8 has a viscous center and a plate-type LSD in the rear, which seem to work well, judging by the few magazine reviews that have been done. Besides durability, would there be any real advantage to a mechanical ATB diff over the rear plate-type LSD in the Evo 8? (Assuming they're even available...)

Thanks for any info... These ideas have been bugging me for weeks.

 

There are a lot of variables the answer is it depends...

What type of driving/ racing do you do?

What is your driving style/ experience level?

Lets start with that so we can focus on your theoretical best setup (which may differ from mine fro instance)

 

Put driving style and experience level aside. What I'm after is just a basic answer. There has to be a basic difference in the function and response of a both an open diff and LSD, and how they interact with the rest of the system as a whole, under identical conditions. Is it incorrect to believe that?

Let's say, for argument's sake, that I push the car hard on the street and do occasional autocross.

I'm not after the type of electronic and/or mechanical response of something you might find on a WRC car -- nothing that complicated or expensive. I'm just trying to understand basic differences.

Let's also say that I drive on slippery surfaces.... dirt or maybe winter conditons. We don't get ACD, so front/rear power bias settings are a moot point. Under ideal conditions, the center diffe splits power 1:1 front/rear. What happens if one of the front wheels slips when you have an front open diff AND when you have a front LSD. Is the same power % going to be transfered to the rear in both cases? I know that if you have an LSD or ATB diff, the outside wheel will get the most traction.... what happens to the rear wheels in both cases.

Or... is power transferred to the rear if the front is severely lacking in traction -- so then does having an open type or LSD in the front not matter?

Am I making sense or am I asking too many questions all at once? It's late. I'm tired. I'm just throwing all my thoughts down incoherently right now. Gotta start someplace, eh?

 

As chronohunter said earlier, there is no simple answer and it really depends on too many things.
Taking off the line, different phases of the corner, different driving styles,...to name just a few all make the same car to act differently.

There seem to be a common misunderstanding on what LSD does. To me it looks like that a lot of people assume that LSD sends power left and right (front and rear) wherever is more traction. This is at least what most of the dealers try to make us believe when their fancy vehicle that are trying to sell is equipped with that mighty LSD.Limiting slip diff is doing just that. It limits the slip between the two sides on the diff. Depending on how good those are, slip will be bigger or smaller. So, when one side starts to spin faster, there is a slip between the two sides and that is what diff is trying to correct. It tries to couple both sides to getter as much as its design will allow. So, in the best case, you will have 50:50 distribution on both sides. When it is fully open or slip doesn't work as well any more (or never did), you get more power on the side that has less traction and that one spins faster. Like in the open diff and FWD car, wheel that is on the ice will be spinning while the other one that is on the dry, would stay still.

It you want your diff to send power where ever you have more traction, you need to have device like AYC which is capable of doing such a thing.

As it was said earlier, it is not that simple. For example extra weight that is on the front wheels will affect this whole thing a lot. Now, when you get the drive the whole car and weight starts to move around your car from one corner to another, things get even worse.

So, from the static and ideal conditions on 50:50 weight distribution car, if you have an open front and center and ideal LSD rear diff, when you brake the traction level one of the front wheels will be spinning first. And it will stay like that and your car will probably stay in the spot without movement if that front wheel is on the ice or up in the air. And that is regardless of your AWD fancy car. You will not move!

Now by adding the ideal LSD in the center, things between the front and the rear will be loaded pretty much the same and this time car will move. Once the traction is exceeded, your front wheel will be spinning faster than anything else, but at least your rear wheels will be driven thanks to the center diff. So, (thanks to the ideal center LSD) you will be getting 50:50 between the front and rear drive shafts (assumption is that we have some kind of centrally located front/rear transfer unit, just to make things easier to explain). That would then be proportionally distributed between the rear wheels, while the front ones will go to the side with less traction and stay there. So, you will be getting something like 50% - 0% on the front wheels and 25%-25% on the rear wheels. This is still much better than the first case where you had all 100% on one of the front wheels.

If you now add ideal LSD in the front, then you would be getting 25% all around. The problem is that this is not preferred way, since you want to lower that amount on the front wheel that is turning. That is why you usually have some special type of the diff in the front which doesn't overpower the wheel that is doing most of the turning and still helps to put more power down!

Fedja

 

Mr. AWD did a good job of explaining the basic function of a Limited Slip. I've tried here to break it into small chunks (easier to digest!)

1. The center diff dosen't care what kind of diff are in the front of rear all it does is react to excessive wheel speed diferences front to rear. When wheel speeds are too different the viscous diff begins to "tighten" (because the fluid inside is designed not to sheer therefore it starts locking, much like a torque converter). This "sends" power to the other end.

2. If the front or rear diff can add traction (via some sort of LSD) then the center diff will have to respond less often because there is less slippage and therefore the wheel speeds (F/R) are ok.

The problem as Mr.AWD stated is that the diffs (all the different types) affect the way the car handles and handling balance is a very complex thing, especially when looking at all-wheel drive. That is why I initally asked about you driving experience and intent for the car. Ideal winter set-up is different from gravel which is different from autocross which is different fom roadrace etc. etc.

That's the advantage of an active set-up, it can be changed on command to suite the surface and the driving style at any given moment. With the eight WE will have to change our driving style to suit the CAR, that is untill we install the correct diffs for our intended purpose for the car. Who knows it may work brilliantly out of the box And all our worry will have been for nothing...We'll know in a few weeks....

 

I think I'm starting to understand why they left the front open. As chronohunter said, we'll have to wait and see how it works out of the box. But I'm begining to think also that this setup may not work too well for gravel and snow.

 

I believe that you are right here! As I mentioned above, the current US EVO setup in the best case could be doing something like 50% - 0% in the front and 25% - 25% in the rear. The front will kind of change regarding the state of the car (turning will unload inside front and it will get more power) and center will do as best as it can with some slip that has to be accounted.

What is really needed for the slippery stuff is more bias to the rear (like 60:40 or so), so you would get something like 40% - 0% in the front and 30%-30% for the rear. Here is one of the reasons why STi has 35:65 between the front and the rear!

Also, once you brake the grip levels on the dry, you get very similar effect as you have on the gravel where this happens all the time. This is especially true with the cars that have more power and capability to brake all four tires loose.


Fedja

 

Actually that is essentially what they do. You have to look at it from the power that is reaching the ground. An open diff will always put equal power to the ground on both sides. The wheel that has traction will still get power, equal to the amount of resistance on the wheel that is slipping(that resistance would be the power it is transfering to the ground). If there is little resistance, little power will go to the wheel that has traction which may not be enough power to move the car. So if you put an LSD into the mix it will act to lock out the differential. The one wheel still won't have traction, and won't get any more power to the ground. The power reaching the ground is however transfered to the other wheel, and it can be up to 100% on certain diffs if they completely lock out and the opposite side has 0 traction(like if it were off the ground).

 

Sorry Ben, but on open diff cars power goes wherever is easier and that is the fact. If you take for example any FWD or RWD with an open diff or AWD with all open diffs, and jack it up from one side so only two wheels (right or left side, you choose) are touching the ground, and then sit in the car start it, put in the gear and try to move, then you would be very surprised. Surprised because your car will not move!

You would see that outside wheel that is up in the air spinning twice as fast as your speedo is showing. If you would somehow grab that wheel and hold it in place, power would move to the other side and move the car (but don't try that ). That is basically what LSD does. When it "sees" one of the sides to spin faster than the other side, it locks and sends the power to the side that was slower or stopped.

I mentioned this one above as well, but you didn't take it serious enough! Once the two sides are locked (in the case when diff is in the lock mode), then you can send only 50% on each side. Doing anything else would brake laws of physics or your axles!

Only AYC type differentials with two pairs of the plates can send power up to 100% on either side.


Fedja

 

No, that is not how an open differential works.

Equal power ALWAYS reaches the ground on each side. It may not be much power if one side has little traction, and it may not be any power like in your example with a wheel off the ground.

Ok I'll try to make this simple. Lets only consider 2wd for simplicity. If both sides have enough traction to handle the power going through the diff, then each side will get 1/2 of the power. If one side only has enough traction to handle 10% of the power, then 10% will reach the ground on both sides for a total of 20% of your power reaching the ground. The other 80% of the power will just be lost to the spinning of the wheel with little traction. Still, equal power is reaching the ground.

Now what if it were an LSD? For simplicity, lets say that it fully locked out. The wheels are spinning at the same RPM. Well, the one wheel still only has traction for 10% of the power so that's all it will transfer to the ground and it may still be spinning. The other wheel has enough traction to handle the 90% so that's what it gets. Once the traction between the sides evens out, equal power will reach the ground on each side.

An open differential always sends equal torque to both sides.

 

Here, from howstuffworks.com, they can explain it simpler than I can. http://auto.howstuffworks.com/differential5.htm

"The open differential always applies the same amount of torque to each wheel. There are two factors that determine how much torque can be applied to the wheels: equipment and traction. In dry conditions, when there is plenty of traction, the amount of torque applied to the wheels is limited by the engine and gearing; in a low traction situation, such as when driving on ice, the amount of torque is limited to the greatest amount that will not cause a wheel to slip under those conditions. So, even though a car may be able to produce more torque, there needs to be enough traction to transmit that torque to the ground. If you give the car more gas after the wheels start to slip, the wheels will just spin faster. "

See equal torque will always be transfered through the differential to each wheel. That's the twisting force, and it's limited by the tire with the least traction.

With a limited slip differential, it transferes more twisting force(torque) to the wheel with more traction. Like I said, that can be up to 100% with an LSD that can fully lock, not 50/50 like you say.

 

Cool... I started a good thread.

I appreciate all this info. It's making me think quite a bit!

I think Ben tried to write what you re-wrote above, but your explanation was a bit more clear. I think I read the above posts 3-4 times before I started to get a better idea of how things work.

This may be stupid, but here's another question: What if you could put an ATB diff (i.e., Quaife) on the front and rear. Would that provide an overall advantage as far as traction (or getting power to the ground) is concerned? That more or less does the same thing as AYC, if I'm understanding these things correctly... Although, I don't think a Quaife can transfer 100% left or right. I'm going by memory, so I could be wrong here, too. Whether they make them for the Evo is another story. (No idea...)

I completely understand the idea behind an open diff. I hate my VW's tranny for this reason. More times than I can count, I've felt how my car doesn't really pull through and accelerate out of the corners because the inside front wheel is receiving the majority of the power once the weight of the car has been shifted to the outside. The car just "plows" through and you hope for the best. However, I can't quite afford to buy that Quaife just yet.

 

LOL Now I'm confused... or was... I read Ben's comments again and they make more sense now. I thnk MrAWD may have had the right idea, but was missing the details. I read through the Howstuffworks.com bit on differentials and things make a bit more sense.

So if I understand correctly, the open front diff in the Evo will behave the same as any other open diff..... just like on my VW. The outside tire may not put down enough torque if the inside wheel is slipping/spinning too much.

Then, if the center differential provides a full-time 50:50 split, then if the front tires are spinning, you'll get the car moving via the rear wheels, provided the rear tires have enough grip on the road surface. Are my thoughts becoming any more coherent?

Thanks again for the info and debate... I guess now's a bad time to ask about our S4's torsen differential, eh?

I also remembered this link I got from someone on AudiWorld:
http://home.attbi.com/~eliot_www/awd.html

It explains what Ben just wrote above.

 

Damn... double post.

 

Max Rebo just don't think too one dimensionally, as we origionally talked about all this locking makes for an understeering pig! I'm sure that's not what any of us want. Audis have a lot of understeer with the torsen diffs especially when used in the center (and compounded by their extreme front weight bias).

Start looking up 1, 1.5 and 2 way diffs and diffs that alter the torque split before slippage (35f/65r Cusco) for example. Combinations of these are the types of diffs that are used for most amateur types of racing (people who have to pay for the stuff themselves!).