ABS Normal?

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miniaustinmini
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ABS Normal?

Post by miniaustinmini »

I have an 05 Baja, NA, Manual Trans. We just had our first big snow and got to go play. I've never had this phenomenon on any other car. I'm trying to figure out if I have a problem or if this is normal:
When panic stopping on hard snow from it's as if the ABS doesn't have enough capacity to sustain ABS activation. The pedal falls away as the ABS first activates, then the "vibrating" noise stops and it the ABS makes a vroop noise as it pushes the pedal back towards me. It takes 3 or 4 cycles to come to a complete stop, with only about 30% of the time wasted to the ABS pushing the pedal back towards me. No ABS light, but it sure doesn't feel right.
I'm not sure in the inactive periods, if the wheels are full sliding or free rolling, but they're defiantly not stopping.
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kamesama980
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Re: ABS Normal?

Post by kamesama980 »

First off, keep in mind ABS isn't meant to improve straight-line stopping on loose surfaces. It's meant to retain steering ability on uneven or inconsistent surfaces (which would otherwise lead to a spin) or hard braking in corners (which would otherwise lead to understeer).

What may me happening is that there is some delay in action and response in the system... could also be that all 4 tires are locking up simultaneously making the ABS think the car's stopped and deactivate. could be the road surface isn't as uniform as you think too. It's hard to explain but it doesn't sound unusual.

Between an auto tech degree, autocross, liking to drive in the snow, and reading too much, I'm all too aware of the advantages and limitations.
-Russell
"You don't get to judge me for how I fix what you break"
03 sport 5mt
05 turbo 5mt vf39 and other STI bits, 3" turbo-back
miniaustinmini
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Re: ABS Normal?

Post by miniaustinmini »

Thanks Kamesama,
I've conceded that I am being too paranoid with my wife's car. I was trying it out on hard pack un-plowed, un-salted snow, so definitely the ABS's prime territory. Modern ABS also reduces straight line stopping distance since a sliding tire provides less stopping power than a rolling tire being resisted by the brakes. The Yaw and G sensor in the system will recognize that even though all 4 wheels are "stopped" there wasn't enough longitudinal load to have stopped the car.
As an auto engineer, I think I was over thinking the ABS recharge noise and thought that it was an issue. I realized I was letting my foot pressure off slightly once the ABS activated and the tires would have enough traction for the little braking I was asking them to do (an effect of the normal mushy Subaru pedal) When I stay hard on the pedal, the actuation would continue until I came to a stop.
All is well, I'll leave my notes here for anyone else.
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kamesama980
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Re: ABS Normal?

Post by kamesama980 »

To clarify my previous comments: In terms of four locked tires on firm pavement, yes, ABS can decrease straight line stopping vs locking the tires (simple comparison of static vs dynamic friction) but historically that wasn't the biggest safety push.

On snow or where you can pack material (loose snow, gravel, dirt) in front of the locked tire, ABS does not substantially reduce stopping distance in a straight line. This is because you go from using just the contact patch friction to the force involved in displacing the material. The biggest benefit in the snow is if one tire hits a patch with more or less friction, you don't initiate a spin (yaw).

While the newer systems you may be involved with have Yaw and G-sensors, older systems like the Bajas do not.

An engineer overthinking something? That never happens :mrgreen: lol I have several in the family, work in a building full of them, and frequently get asked if I am one. [smilie=coolup.gif]
-Russell
"You don't get to judge me for how I fix what you break"
03 sport 5mt
05 turbo 5mt vf39 and other STI bits, 3" turbo-back
w.rohr
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Re: ABS Normal?

Post by w.rohr »

Also Subaru abs sucks out loud, it is common where I drive for people to replace the abs fuse with a burnt out one. Why? because on dirt roads the abs will kick in and reduce the braking to about 1/3 and you will slide right through the intersection.
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anarchy1024
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Re: ABS Normal?

Post by anarchy1024 »

It doesn't need to be dirt, gravel or snow... Even bumps in the road can cause the ABS to kick in and substantially reduce the force applied to the brakes. At least on mine, the ABS partially releases the brakes to prevent (what it believes) the wheels from locking, but it feels like it doesn't reapply the brakes (or restore the pressure). Other people with Subarus from that time have said similar things, which suggests that it isn't a problem specific to my vehicle.
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kamesama980
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Re: ABS Normal?

Post by kamesama980 »

anarchy1024 wrote: Sun Dec 24, 2017 2:09 pm It doesn't need to be dirt, gravel or snow... Even bumps in the road can cause the ABS to kick in and substantially reduce the force applied to the brakes. At least on mine, the ABS partially releases the brakes to prevent (what it believes) the wheels from locking, but it feels like it doesn't reapply the brakes (or restore the pressure). Other people with Subarus from that time have said similar things, which suggests that it isn't a problem specific to my vehicle.
Yup, hitting a bump can cause the tire to leave the road briefly, reducing friction which allows it to lock (or even just decelerate more quickly than the others) causing the ABS to kick in. That's why good shocks/struts can make a big difference in braking (along with everything else): they reduce the bounce and keep the tires on the road.

The bajas are spring stiffly enough that they don't wallow obviously like many cars (stiffer springs also help the struts last longer...along with higher quality to start with) I recently changed mine along with king springs from Primative and the difference is pretty significant and I wouldn't have even called mine bad (And that's something since I was the guy in the shop that called them all bad). Makes such a difference in everything, even just steady acceleration (with stick shift) because less energy goes into articulating the car and thus less factors in getting a smooth shift (meaning you don't feel like a 16yo struggling to learn stick and shift halfway decently). Also my wife sometimes complained of carsickness driving around in town but doesn't anymore.
-Russell
"You don't get to judge me for how I fix what you break"
03 sport 5mt
05 turbo 5mt vf39 and other STI bits, 3" turbo-back
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anarchy1024
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Re: ABS Normal?

Post by anarchy1024 »

kamesama980 wrote: Mon Dec 25, 2017 8:36 am Yup, hitting a bump can cause the tire to leave the road briefly, reducing friction which allows it to lock (or even just decelerate more quickly than the others) causing the ABS to kick in.
I agree, but when friction is restored to normal (the weight has settled back on the tire that left the road), the ABS should also restore braking force. The problem is that it doesn't appear to (whether from a bump, ice, water, etc.), whereas other cars have no problem restoring braking force when the tire regains traction.
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