How does ABS work on AWD cars?

[tentacles]

Can someone explain to me how the ABS works on AWD cars? I understand on some (all?) the center coupling is disengaged as soon as you hit the brakes, how does that work? Do they all work like that? What is the effect on handling?

[Mr_ET]

I would think it would work the same as on a rwd or fwd car man...

[SigmaRacing]

Can't tell you about how the centre diff behaves when I hit the brakes but I can tell you that the ABS on my wrx is by far the best ABS system I've ever used. I've driven RWD, FWD, and AWD cars and nothing is as stable as my current car under hard braking and low traction.

I can tell you that the centre diff on the non-STis are viscous coupling and as such do not have an electronic disengage...that's why its so hard on the car to do handbrake turns. Having said that, maybe the centre diff just tolerates the ABS action.

[tentacles]

My (obviously limited) understanding is that VC center differentials in particular need to be disengaged for the ABS to work properly, and most cars with VC center differentials do this. It sort of makes sense - the ABS works by wheel speed sensors sensing when one wheel is locked up or spinning much slower than the others, but the AWD drivetrain is constantly working to make all wheels spin at the same time.

[alloroc]

Originally posted by tentacles
Can someone explain to me how the ABS works on AWD cars? I understand on some (all?) the center coupling is disengaged as soon as you hit the brakes, how does that work? Do they all work like that? What is the effect on handling?

On some cars the center diff is disengaged (abs or not) but you are kinda thinking backwards.

ABS is actually easier on AWD differentials because it ensures all tires rotate at the same speed.

The question should how do non ABS equipped AWD vehicles handle one (or two) wheels locking up with the others wheels still turning?

dun dun dun.

[DonJuan]

Originally posted by alloroc

the question should how do non ABS equipped AWD vehicles handle one (or two) wheels locking up with the others wheels still turning?

dun dun dun.

TRU dat! How can you do a burnout/smokeshow without disengaging the ABS. As far as I know, the ABS is speed sensitive so if you are driving under 5kph you have no ABS. hence the burnout possibility etc.

The new question, how does the car know that you are travelling under 5kph and turnoff the ABS, if it calculates speed from the turning of the output shaft on the transmission. If doing a burnout above 5kph you still don't have ABS. Wierd no?

[hellraiser456]

in order to understand why abs works with SOME 4WD systems...you have to understand the various systems of 4WD itself. the main difference is how torque is distrubed when tires spin.

In the so called 4WD (actually called part time 4wd) when a tire starts to spin it will transfer torque to the other wheels or axle...and can send 100% or the torque to any one wheel if all three are spinning. when you hear about a manual diff lock or anything of that sort...you are actually changing your AWD system into this system...which is great for off road...but dangerous on road.

Full time 4WD is just the opposite of the above....when a tire loses traction it is simply allowed to spin.

AWD systems...that are commonly found in cars and SUV today are not like the part 4wheel drive systems. there are two main types...there is full time AWD or symmetric and automatic AWD. in full time AWD if a tire losses traction...it is simply allowed to stay that way...however the t case will try to ensure that you still get some torque to the other axle...not necessarily the other tire of the same axle.

The automatic system....is an advanced 2wd system. when the powered axle's drive shaft starts to spin faster than the unpowered axle's drive shaft (loss of traction) the t case will then send torque to the unpowered axle


As for ABS....anytime that you are in a part time 4wd system...ABS and TCS and the like will not work. my assumption is because of the fact that all four tires can be spinning at different speeds and the torque and power can be shifted to any one tire. In all the other systems...where it is the opposite....more so the automatic AWD...you can detect a wheel lock up....it could be as easy as simply measuring the speed of the front axle compared to the rear axle, since at optimum conditions...these should be spinning at the same speed.

[segfault]

Originally posted by SigmaRacing
Can't tell you about how the centre diff behaves when I hit the brakes but I can tell you that the ABS on my wrx is by far the best ABS system I've ever used. I've driven RWD, FWD, and AWD cars and nothing is as stable as my current car under hard braking and low traction.

I can tell you that the centre diff on the non-STis are viscous coupling and as such do not have an electronic disengage...that's why its so hard on the car to do handbrake turns. Having said that, maybe the centre diff just tolerates the ABS action.


Note: I would like to emphasize caution with the interpretation of the words "Issue", "Problem", and other such distinct quantifiers.


While I'm not one to argue with someones personal experience, I believe there is some justification in that there is a known 'issue' with the ABS in the Imprezas.

The problem, particularly with '02/'03 Impreza models, is when one wheel locks the ABS comes on for all four wheels. When you try to stop on an inconsistent surface one wheel can lock up fairly early, while the other still have lots of braking power left. This can happen on ice, snow, gravel or bumps (i.e. one wheel leave the ground for a split second causing it to lock).

As for an example; imagine you're driving down a street where there is slick ice along the curb and dry pavement in the middle. Let's say, if the brakes worked independently, you could use 30% of you braking power on the ice and 90% on the dry pavement before your wheels locked. With the system in my car as soon as I hit 30% and one wheel locks, the ABS comes on and it limits my braking power for ALL FOUR WHEELS to 30%. A proper ABS system should recognize that the other wheels still have a lot of braking ability left and don't need the ABS to be activated on them. Releasing and replying the brakes at this point can solve the problem but not if your still on the inconsistent surface.

I drive an '03 Impreza and while coming to an expedited stop at a light, there are many small bumps in the road which caused my ABS to kick in and limit my breaking across all wheels. I have since pulled my ABS fuse plug (2 years+ ago) and have been happily threshold breaking since. Further, I owned many older FWD/RWD vehicles without ABS and, quite frankly, just don't like it. I don't want my car doing something that I don't tell it to do.


Disclaimer 1: To the kiddies out there, please don't disengage your ABS without knowing the full ramifications of your actions. I have many friends that just jam on the breaks and swear by their ABS. Without ABS your wheels WILL LOCK and you WILL SLIDE (undesirably); just pray that it is not into someones rear end.

Also, If you turn off your ABS, there should be a yellow warning light displayed in your cluster. While it may be annoying, don't tape it over or disable it somehow. If you lend your car to someone (siblings, parents), it would be wisest for them to know that ABS is disabled and to react accordingly (ie, not pull back on the wheel and push the break with ones foot).

Disclaimer 2: I drive an 03 Impreza. If you drive a different vehicle your ABS most likely operates in a vastly different way (ie: electronically controlled in newer vehicles). Though that said, the 02-05 Impreza's share the same ABS system (by part number). That said, I do not know how the 04 STI's DCCD copes with the ABS and any surrounding electronics. I understand the 06+'s are vastly different.