hand brake turns with AWD

[Tmoose]

On most Subaru forums there are discussions that some model 'Roo have a center diff unlocking feature when hand brake is applied, inherited from the models designed to meet some kind of Rally homologation.

With an open center diff I picture locking or slowing the rear wheels significantly would cause the front wheels to try to rev way up, or the engine, or something if the clutch remains engaged. I picture that trying to suddenly snap the front tire(s) loose, and not what I'd want to happen in the middle of a stunt. Not that I'm likely to buy an AWD vehicle anyhow. But maybe I'll try to gently beat up a rental car someday.

1 - Am I misunderstanding something?
2 - Are most AWD vehickles incapable of hand brake turns?

thanks,

Dan T

 

[LateApexEE]

The STi models since about 2004 (in the USA anyways) have an active center differential. It is a magnetically controlled wet clutch and can sink over 5 amps at full lockup. The diff computer controls lockup based on throttle position, foot brake status, steered angle, and lat/long acceleration. There is a handbrake switch input which totally unlocks the center diff when the handbrake is active. The diff unlocks because it would basically stall the engine if the rear wheels lock and the diff is providing any sort of lockup. Unlocking the center diff also serves to get the revs up, so when you come off the hand brake and the diff locks back up, you get a little wheelspin rather than a bog and thus make it out of the corner quickly. Many AWD cars will not lock the rear wheels at all with the handbrake, and if they do it will usually bog the engine somewhat. I have seen drivers with mechanical center diff cars either use some sort of hydraulic declutching box on the rear driveshaft or just clutch in during handbrake applications.


Source: I engineer and build rally cars for a living.

 

[Tmoose]

So with 4Wd or all wheel drive I must declutch and thus can not power the fronts while locking the rears

 

[LateApexEE]

I suppose it depends on the type of center differential.

 

[yoshimitsuspeed]

AWD means the diff has some slip to it unlike a 4WD diff that locks the front and rear tires. In a 4WD if you used the hand brake it would lock up all four wheels. It is true that different diffs will respond differently but any AWD diff should have a little forgiveness. A quick blip on the brake will cause the diff to slip a little. This will tend to accelerate the wear of the diff.

I always feel like inexperienced drivers rely on a lot more hand brake and therefore this may be more of an issue. A good driver in a well balanced car only needs the e brake on the tightest of corners or the occasional rare moment. Even then when you are at the very limits it takes very little to get the rear end to step out and then for the most part you control it with gas and brakes.If you are riding on the gas and the ebrake for more than a split second you took the corner too slow and left unused units of traction on the table. More often than not proper weight transfer is plenty to get the slip angle you want. If that's not enough then for good drivers there is always the Scandinavian flick. If that's not enough then a light tap on the e brake may be needed.

 

[Tmoose]

This suggests the center "diff" on some Subarus is actually a clutch pack, and not a geared differential at all.

http://www.stanleysubaru.com/blog/2012/march/22/how-does-subaru-symmetrical-all-wheel-drive-work.htm

Sounds kind of wear prone and het generating if it is really providing some significant torque transfer at all.


Basic Geared differentials don't "slip." Their left/right or fore/aft output shafts can rotate at different rpm, but the average of those rpm will be the input rpm.

Seems like modern AWD is chock full of viscous couplings and other gizmos.
A far cry from Jeep's original full time AWD Quadratrack which I think basically had a center differential fitted with a clutch pack similar to a positraction except that could be manually locked by vacuum to become 4 wheel drive when "needed."

 

[murpia]

With a completely free geared centre diff, locking the rear axle will decelerate the engine to half speed. While this requires more torque from the handbrake than just locking the rear axle does, there's no reason this cannot be done.

So an active geared centre diff, which unlocks electronically when a handbrake turn is needed, sounds entirely logical to me.

Regards, Ian

 

[Tmoose]

"With a completely free geared centre diff, locking the rear axle will decelerate the engine to half speed."

If in gear, At the same time as the engine (the inertia multiplied overall gear ratio^2) is being slowed, the front wheels are trying to be accelerated. When traction is low one or both of the front tires will more likely spin. Probably Not the best thing when initiating a turn or mid corner.

if in neutral, most of the "engine" hardware disappears, so all that is lost is the ability for the front to provide tractive force to maintain speed while the rears are providing some drag.

It sounds like my OP suspicions regarding most AWD and recreational handbrake turns remain somewhat valid.

+++++++++++++++++
Supposedly Erik Carlsson's technique with SAABs was left foot braking and full throttle, which also produced somewhat of a net rear wheel brake effect to get the rear end to rotate/move outward.
3;18, 5:00, 11;00, 12;47 - 13:00, 18:39 here -

http://www.youtube.com/watch?v=gKxRiP2RLQs

I'm thinking that AWD and especially the various smart traction controls might make that ineffective too.

 

[GregLocock]

Aggressive use of the handbrake in motion was always a good way to break halfshafts on early range rovers.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[murpia]

'Aggressive use of the handbrake in motion was always a good way to break halfshafts on early range rovers.'

Front or rear?

Regards, Ian

 

[GregLocock]

I can't remember, since that was one of the few crimes I didn't commit.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?