AWD Centre differential confusion

[randomthoughtguy]

Hi Everyone


Within the generally standard AWD or Fulltime 4WD drivetrain layout there is a Front, Centre and Rear differential. The Centre differential can either be open or locking through use of a (typically) Torsen type or Viscous type Differential unit.

Would it be viable to instead use a Clutch type LSD (KAAZ) instead to deliver a more controlled torque split front to rear, or would the design of the clutch type unit cause an issue within the system? Would there even be any benefit in using a Clutch type over Torsen type Centre differential for high traction road racing use?


Thanks in advance for any wisdom you can impart.

 

[BrianPetersen]

Haldex all-wheel-drive systems do exactly as you suggest. There is no "centre differential" - just a clutch, whose engagement is typically electronically controlled and modulated.

Haldex is a brand name ... there are others that work similarly. Probably 90+% of all-wheel-drive systems that are based on transverse engine front wheel drive "base" vehicles work similarly. Examples abound. Honda "Real Time AWD" (CRV, Ridgeline, Pilot, MDX, basically anything Honda makes that has all-wheel-drive), Toyota Rav4 and Highlander and various Lexus SUVs, Ford Escape/Kuga and Edge and a bunch of others, Mazda CX-whatever, etc.

 

[BrianPetersen]

Clarification, the Haldex and similar systems aren't even a "LSD" (limited slip differential) of the usual sense. It is JUST a clutch. No differential at all. Normally, these systems default to not transmitting any torque at all (clutch disengaged) - the vehicle is front wheel drive until the traction control system detects slippage, although some of them have more sophisticated control strategies that pro-actively start engaging under some other conditions.

There are some that combine an actual center diff with a controlled clutch, usually electronically controlled, Subaru and Audi do this, and I'm sure there are others.

 

[randomthoughtguy]

Thanks Brian

I'm aware of the Haldex system and it's not really what i'm looking for. I guess what i'm looking at is closer to a Transfer case differential in a longitudinal transmission. Usually these would come with either an open spider differential or a Torsen or Viscous differential system.

I'm contemplating replacing this centre transfer case differential, with a Clutch type Differential unit, commonly manufactured by KAAZ. However i've never seen one of these used or theorised in this placement, hence my appeal to minds greater than mine.

 

[BrianPetersen]

Back up and explain The Big Picture.

What do you want to do, that longitudinal-engine Audi, Subaru, BMW, and Mercedes center-diff-based systems don't already do? What do you want to accomplish, that BMW X-drive doesn't already do? It uses a center diff which by default sends 60% rear and 40% front, but there is an electronically-controlled clutch together with this which allows that diff to be partially or wholly locked up depending on how much clutch pressure is applied.

 

[BrianPetersen]

https://www.carmagazine.co.uk/car-news/bmw/bmw-vs-winter-all-you-need-to-know-about-bmw-xdrive/

 

[randomthoughtguy]

Please see attached.

I'm essentially thinking about a DIY Ford RS200 drivetrain system. Rear engine through transmission to a transfer case mounted at the front diff with the differential in the transfer case sending power to front and rear diffs.

The Differential in the Transfer case is the one i'd be looking at using the Clutch LSD unit.

 

[BrianPetersen]

Interesting project. The diff that you are proposing to use is a normal friction-clutch type. If it works the way I think it does, the two outputs are locked together until there is a prevailing torque difference that exceeds what the friction clutch can transmit.

I have a funny feeling - nothing more, because I have no experience with it! - that in a roadracing-type application, this probably isn't going to be a good thing. If you are rounding a bend at neutral throttle, whichever end wants to be going faster (due to different effective radius around the bend) will be trying to drag along the other end that wants to be going slower, front fighting the rear, unnecessarily using up "traction circle" lateral grip at both ends. If you had an ordinary open diff, or even a torsen type, it would simply allow the speed difference to happen, without using up grip by trying to fight it. If you had a viscous coupling, the speed difference wouldn't be enough for it to matter. If you had a Haldex, it would probably be uncoupled. If you had an electronically-controllable clutch around the diff (e.g. BMW X-drive), it would probably be uncoupled and acting like an open diff.

If it's a drag-racing situation, it will probably work fine, because using up lateral grip doesn't matter. If you are trying to extricate the vehicle from a snow-bank or ditch or beach sand, it will be fine. For that matter, if it is more of a rally-car situation, on loose surfaces, it will probably be fine.

 

[randomthoughtguy]

That's the thinking I was having around the LSD vs Torsen vs Viscous, and I guess this sort of explains why OEMs don't use clutch LSDs and use the other types.

The problem I can see with the X-Drive system for my use is the torque split is as standard 40:60 front to rear, and because id be using it backwards, this puts more torque to the front wheels, which isn't as sporty haha. I'm not sure whether the clutch pack in the X-Drive transfer could take the preload of a constant 50:50 or higher torque split to the front output(rear in my case).

Thanks for the Input Brian, i'll probably start looking towards Torsen transfer cases diffs as it's probably closer to what i'm looking for.

 

[RRiver]

I'm sorry I missed this.

The only thing RS200 race car did consistently well was act as a coffin. After killing 3 spectators and injuring a LOT in the Portugal WRC race and killing the driver and co-driver in the next race in Germany the future of the car was written. It lasted on year. The 200 road versions built lasted 2. The car was also a pig. Low rpm power and hard to control at high rpm's with unreliable power to weight (ratio) bias. Ford was prototyping an RS200 EVO to follow the RS200 but the growing reputation of the car and cooler heads prevailed. The EVO it's said did 0 to 60 in less than 2 seconds but still had many of the same handling issues. The street cars now are highly sought after because of their uniqueness. Not because of their driving pleasure. The car was 4WD, not AWD.

IIRC the car used a hybrid of the the Dana designed V Drive instead of a transfer case which at the time of the car, 1984, Dana had sold the design to VEMCO. Ford re-thought the use of the drive compared to Dana's concepts.

""A low vehicle profile is obtained in a front wheel drive system by providing a V-drive axle center section at the rear of a transmission with a pair of forwardly extending V-shaped output or drive shafts. The output shafts are adapted to drive a pair of angled forwardly extending propeller shafts disposed on opposite sides of the center section so that drive can be transmitted to separate gear units located at each front wheel for individually driving the same. Equalization of power between the front wheels is accomplished by means of a conventional bevel gear differential located in one of the output shafts with drive to the differential being through a pinion gear keyed to an adapter sleeve serving as the axle center section input shaft. The front wheel drive system is easily converted into a four-wheel drive system by merely removing the adapter sleeve and inserting in its place a second differential of the worm gear type. By this arrangement input drive is then transmitted to the second differential with the drive divided by the worm gearing in such a manner as to have a portion directed to the first differential located on the center section output shaft and portion directed to a rearwardly extending propeller shaft for driving the rear wheels of the vehicle while still maintaining the desired low vehicle profile.""

You don't identify what it is you're trying to accomplish but the V Drive may work for you.