pnebbs

Ernie

A LSD can help prevent wheelspin, if one wheel is on ice it will prevent that wheel spinning needlessly whilst the other stays still, it will make no difference if both wheels are spinning.

On an open diff the torque split can be what ever is set up by the gearing, however as soon as a wheels spins all the torque goes to that wheel, becoming a 100:0 torque split.

As far as I can work out, with a fuly locked diff you cannot have anything other than 50:50 as it has become one system.

With an adjustable diff, torque split can be set by internal gearing to 70:30, 60:40 etc and it will stay that way until one end starts to slip. With a rear biased system the maximum torque is under maximum acceleration when there is a weight shift to the rear, thus although the rear wheels have more torque they also have more traction, hence the reason under full power on a car such as a UK Impreza the front wheels can start to spin under hard acceleration before the VC reacts

Paul
Getting on more shaky ground as he goes on!!!

MorayMackenzie

If memory serves correctly, All AWD imprezas have LSDs at the rear and in the centre. Some Type RA/R cars have been specified with front LSD as well, but I think this is chosen on that specific car's option sheet, I don't think there was a model that came with a front LSD as standard.

Moray

ernie

How does a differential work? Is it true that the Scooby has three? The only one I can see is the rear LSD (UK MY00). I thought that you only have a diff to distribute power from the propshaft on rear wheel drive vehicles. On STI's how does the adjustable diff work? If you "wind the diff up", what does it mean?

Any info/comments most appreciated

pnebbs

A differential has one input shaft and two output shafts, it splits the torque between the two output shafts but allows them to rotate at different speeds

Yes the scooby has 3, one at the rear to supply torque to the rear wheels, one at the front to split torque between the front wheels and one in the centre to split torque between front and rear, I believe the front and centre are built into the gearbox casing on the Impreza.

If you didn't have the front and rear diffs the left and right wheels would not be able to rotate at differnt speeds, such as happens during cornering.

Some part time4 4wd vehicles do not have a centre diff, but these vehicles are designed to only use 4wd in slippery conditions when the tyres slip against theground and so can travel at the same speed.

Wind up is when a diff is locked, imagine no diff, but the wheels are connected by a solid axle, as they dry to travel at different speeds they will end up twisting the axle, or winding it up.

Some models have limited slip diffs, this allows the wheels to travel at different speeds, but restricts the speed difference to a predermined ratio, the locking ratio.

An adjustable centre diff is a diff that be adjusted from fully open (normal diff) through limited slip (at variable ratios) to fully locked.

Most Imprezas have a limited slip centre coupling (similar to a diff, but not identical) and some have a limited slip rear diff. Some STi's and 22Bs have the adjustable centre diff

I stand to be corrected on some of the above

Paul

ernie

Hello Paul

Thanks for the excellent information. Does a LSD prevent wheelspin? If you have an adjustable centre diff, does locking it mean the power ratio of front:rear is 50:50 and that adjusting it gives 40:60, 30:70 etc? If you were to have a front:rear ratio of, say, 30:70, would'nt it mean the rear wheels would wheelspin? If so, whats the point of anything other than 50:50? Do you know of any sites that show the actual mechanical workings of the inside of a diff and how they physically work?

Thanks again

pnebbs

Moray

Are you sure about that, I didn't think the earlier models or non-turbos came with rear LSDs, although I'm probably wrong

Paul