7 replies to this topic

[mrman_3k]

With the launch of the Ferrari F430, they advertise as one of the features derived from Formula 1 the electronic differential. My question is what is this, and what are its advantages. The more info the better.

Thanks in advance!

[ivanalesi]

it is limiting the torque at given speed or throttle input, like the WRC differentials, this way you have less chance to spin your wheels... but it will has only rear e-diff...

[panzani]

And, as I understand it, regarding F1 and not the F430, whilst ED is doing its job at the rear wheels it does exactly what ABS and Torque Transfer would do on the front ones - if they were not banned. I could be wrong, though.

[desmo]

The term "electronic" diff is a misnomer. Electrohydraulic would be more accurate. Generally the electronics merely control the hydraulics which actually do the actuating of the diff clutches.

[mrman_3k]

I'm not fully versed on the techical working of an LSD. Could you possibly explain in greater detail how a standard LSD in say my road car works and then how an electrohydraulic diff works?

Thanks!

[indigoid]

This page has a bit about Salisbury (clutch+spring) LSDs:

http://eaglewoman.ra...rehelp/diff.htm

And these a bit about Torsen LSDs:

http://auto.howstuff...ferential10.htm
http://www.torsen.co...ts/products.htm

Google is your friend. Don't ignore him

[BRIAN GLOVER]

Indigoid has posted some URLs that explain quite a bit about LS diffs. Some things are over simplified and his conclusions only apply to the one particular vehicle which he fails to describe. IE: For the same size engine in terms of HP and Torque, the diff set up will be completely diffent for a mid front engine, mid rear engine, rear engine, the weight of the car, CG location, transfer, etc etc etc. The Lemans GTS Corvette would probably have the same settings as the GTS 550 Ferrari, but different to the Saleen GTS. The same coast settings may cause understeer in the Vette and not in the Saleen. If you reduced HP by 2 or 3 hundred on each car, the diff would have completely diffent set ups again. Put a Chevy LS1 into a lotus 7, there would have to a few other changes apart from the diff.

Here is some more over simplification.
If you jack a car up at one of its drive wheels that has a normal or an open diff and crank up the engine and engage a gear, the driver shaft will rotate and the wheel up in the air will just spin harmlessly and the car will not move.
With a limited slip differential, this wheel will still spin in the air provided that there is no torque. Press your foot on the gas pedal and the 'spider gears' will try to come out of mesh. The gears splined to the side shafts will be pushed toward the wheels. Now suppose you place a bunch of clutches between this gear and the carrier, they will begin to get the squeeze. The wheel in the air, after spinning twice the axle ratio will begin to receive torque. Since the spider gears are attached to the carrier that is being driven by the ring gear, the torque will be transferred to the wheel on the ground and the car will fall off the jack.

If you are an off roader, a lifted wheel is a common occurrence and in such a tricky situation, the last thing you want is a limited slip differential without a locking device. You may be in a precarious situation and wish to proceed with idle power and/or judicial use of progressive throttle linkage. Idle power wont engage the clutches so in the case of GM locking differentials found on the Z51 off road package, a little cam lever falls into place which locks the gears. After the wheel comes into contact with the ground and torque is applied the cam lever disengages and the clutches do their job. The ramps on the gears will determine the clutch pack pressure and therefore the amount of torque split for coast or power. The higher the ramp, the less the pressure.

The text in Indigoid's post describes an electrically operated locking diff. The GM axles do the same thing, but mechanically and automatically.
An electro-mechanical diff is a different animal. The pressure on the clutches is determined by hydraulic pressure like the drums inside an automatic transmission and the information controlling the pressure is fed into the ECM which includes, steering input, mass acceleration, (both longitudinal and lateral), shaft speed, gear selection and throttle position.(Torque) The clutch packs are mounted outside of the carrier. If you look at the new Volvo AWD systems, these packs are huge on the rear axle. I think Ferrari tried something that looked like this to eliminate the engine clutch, but it was banned.

There are some very fancy all wheel drive systems these days with 3 electronically controlled diffs. Off road vehicles with auto-locking front wheel hubs and electronically controlled two speed transfer cases that can apply 100% of the torque to the back or the front which is one step up from shifting to 2 and 4 wheel drive and high and low range while moving at any speed.

Originally posted by mrman_3k
I'm not fully versed on the techical working of an LSD. Could you possibly explain in greater detail how a standard LSD in say my road car works and then how an electrohydraulic diff works?

Thanks!

I am the NRA.

[indigoid]

Firstly, to clarify Brian's post... the text referred to as "his text" in this case is not mine :-) I just did some quick googling.

Secondly, and somewhat offtopic, while Brian was probably citing an LS1 in a 7 chassis as a pie-in-the-sky extreme case , there is actually an Australian company, (Elfin), that has done exactly that, and somehow managed to make it road-legal. They had to extend the wheelbase a small amount, and the car ended up weighing 875kg (350-400kg more than your average 7). Still... 2.66kg/hp isn't bad at all, and being a 7 it is reasonably capable around the twisties.

http://www.elfin.com.au/ms8.htm

edit: updated with data from Elfin's site