WRX ABS not working properly (snow/ice)

[1ofeach]

OK, so I've seen lots of posts about the ABS in the WRX not being very good, and a lot of people are either disabling it or adding a cutoff switch. I encountered a situation lately that was quite disturbing.

What happened is I had both passenger wheels on a patch of ice, and both driver side wheels on DRY pavement. When I tried to stop, the ABS kicked in on what seemed like ALL 4 wheels, even the ones on dry pavement (that weren't locked). What happened was that the car just slid like all 4 wheels were on the ice, instead of just the 2 on the passenger side.

Just to make sure I wasn't crazy, I told my friend with a WRX about it, and we did the same test in his car with the same results.

Needless to say, this disturbed me quite a bit. I checked on Subaru's web site, and it clearly says that the WRX has 4-channel ABS, which means that all 4 wheels should operate independently.

Has anybody else encountered this situation? If anybody knows of another thread that addresses this, I'd appreciate it if you could place a link in here.

* Registered users of the site do not see these ads.

[ANZAC_1915]

The two wheels on the pavement are now doing the work of all 4 wheels given the other two are one ice. I would expect them to lockup if you have stock rubber and really stomped on the brake pedal.

Glenn

[1ofeach]

Glenn,

I thought that myself. I thought that maybe I'm just not used to having only 2 wheels stopping and what felt like sliding was really in fact just 1/2 traction.

So I went back and really stomped on the brakes just to make sure and it seemed like no matter how much brakes I applied (once the wheels on ice locked up) the rate of decel was the same and the ABS would not kick in on the other 2 wheels.

I think I might try another experiment where I take out the ABS fuse and calculate my stopping distance (without locking any wheels). Then I can compare it with the stopping distance with the ABS enabled and see if they are too different.

I'll also go back with my S2000 and see how it feels in the same situation.

[SlideWRX]

be carefull with two wheels on ice, and two wheels off. without ABS, this will generally cause the car to spin several times.

[Bugsie1]

this is similar to the whacky ABS reaction when going over a bump... one wheel goes over a bump and the ABS kills all braking power on all four wheels.

-Richard

[1ofeach]

Richard,

Has anybody actually PROVEN that when one wheel's ABS kicks in it affects all 4 wheels? If so this sounds like a safety issue. Under hard braking in an emergency, if one wheel hits a spot of ice or snow or oil or whatever, the car can lose traction and hit something. Not good.

[Scottie]

As SlideWRX has already mentioned, what do you think would happen if the two wheels on dry pavement generated the same braking force as they would when all four are on dry pavement?

First off, with braking comes weight transfer to the front wheels. Since one of those front wheels is on ice and has little traction, your car is going to tend to pivot about the front wheel with traction. In otherwords, braking hard in such a situation will likely induce spin.

To prevent a spin in such situations, some braking systems have the means by which to reduce (delay) the build up of brake pressure to front wheel with traction.

From one of my posts elsewhere:
This is sometimes referred to as "yaw-monent buildup delay *1". Yaw-moment is the tendency of a vehicle to turn about a vertical axis. For instance, if both right tires are on a low friction surface like sand or ice, while both left tires are on a high friction surface like dry asphalt, then the car would tend to turn left under braking. This tendency will vary from vehicle to vehicle, but tends to be greater on smaller cars (And I believe the WRX sedan's wider track may also increase this tendency). To counteract this tendency, the ABS on smaller cars is sometimes supplemented with "yaw-moment builup delay". This feature delays the build up of brake pressure to the front wheel thats on the high friction surface (has the most traction).


*1. Bosch Automotive Handbook 4th Edition


Begin rant> It amazes me how so many like to quote "the laws of physics" in debates about quater mile times and such, but seem to totally forget those same laws when it comes to the ABS system. ABS is not a miracle pill. It can only do so much.

ABS can not get a wheel back up to speed if it has slowed or locked up during braking. All ABS can do is reduce the brake pressure so that the friction between the tire and surface it is sliding relative too, or torque via the driveshaft, can get it turning again. ABS by itself can not make or get a wheel to turn. It can only modulate the brake pressure. <End rant

[WRXSTi69]

Originally posted by Glenn Wallace
The two wheels on the pavement are now doing the work of all 4 wheels given the other two are one ice. I would expect them to lockup if you have stock rubber and really stomped on the brake pedal.

Glenn

The ABS would detect an impending lockup on those wheels as well, and prevent them from skidding by modulating the hydraulic pressure. Yes, stopping distance will be increased in a situation like that. Better than having the car make a hook turn to the left unexpectedly.

[Subdav]

I recently slid thru a "corner" the other nite. Wet road...expecting to slow down enough to make the right hand thing..nope! slid right thru and into a dirt field..lucky no fence! It felt like I had no brakes! Where do I go for my ABS training!!!!
dave

[twlai]

Subdav,

You don't need ABS training. We need SOA to fix the problem,
whatever problem it is. I was in the same situation as you
were, except I was taking an exit on a highway. It suddenly
lost all the braking power, (it felt like the ABS kick in, but I think
ABS should have SOME braking power, and slow me slow.)
and got off the ramp.

THere is definately some management problem with the
braking system.

[ANZAC_1915]

What tires do you have?

[twlai]

I have new snow tires on. (dunlop wintersport m2)
And the road was dry. I think the ABS should be a pulse
braking action that is like 80% brake and 20% with no brake.
This allow slow down quick without locking up the wheel and
allow steering. I think subaru works this way most of the time.
Except for a few times, it does 80% no brake and 20% brake.