Grabbing brakes when wet II - GreenHybrid - Hybrid Cars


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Old 01-12-2007, 07:24 PM
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Default Grabbing brakes when wet II

"...Wet brakes grab hard..."

NOT...!!

That statement is almost criminal...!!

First, a more likely explanation.

According to the US patent granted to Ford the instant that the anti-lock braking system, ABS, activates regenerative braking is totally disabled so as to prevent it from interfering with the ability to momentarily "free" the front wheels from any and all braking activity.

In the wet, especially in the WET, you may have an almost un-noticeable, brief, (single pulse) ABS activation which would mean a quick change-over from friction braking combined with regenerative braking to friction braking only.

Transparent to you, no change in foot pressure, the braking ECU will now have to increase the level of friction braking to make up for the loss of regenerative braking and conservative design would always require a little EXTRA frictional braking rather than a "seamless" changeover.

Only a guess but this might be an explanation for what you are feeling.

Additionally here in the great and beautiful pacific NW we often encounter wet brake rotors/pads and they do not GRAB nor do they instantly transistion from "oh my god I don't have any brakes..!! to sudden braking. More like a gradual, slow and scary gain in braking HP. Water between the brake pads and rotor quickly turns to a gas with the heat and pressure of brake application thereby forming a very slippery "gas bearing" which prevents the brake pad from "contacting" the rotor.

Last edited by wwest; 01-12-2007 at 07:37 PM.
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Old 01-12-2007, 10:55 PM
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Default Re: Grabbing brakes when wet II

I have experienced 'grabbing' probably only four or five times. All were in the first couple weeks of ownership with the exception of once at around 15k. The weather was either wet or very humid during each event. Every time it occured the same way, i.e., I was nearly stopped (<3mph), applying easy, consistent pressure on the pedal when they simply 'locked up' and put me to a complete stop. Just enough to get a dirty look from the misses
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Old 01-13-2007, 02:35 PM
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Default Re: Grabbing brakes when wet II

I posted something about this a few days ago (1/11) and was rather rudely closed down by the moderator because I "failed to read the FAQ". Whatever!

Here is what I've been experiencing in my first 1900 miles. Everytime it is raining there is definetly a big difference in slow speed braking. When dry, no grabbing whatsoever. When wet, there is a slight pulsing followed by a "grab" and quick abrupt stop. I have to feather the brake pressure I apply to give me a smooth stop. I am WELL aware of wet brake stopping and all that (this is not my first car!). I was just wondering if others experienced this and whether it was a problem or if it went away with time.
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Old 01-13-2007, 07:59 PM
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Default Re: Grabbing brakes when wet II

Quote:
Originally Posted by ohiocityprius View Post
I posted something about this a few days ago (1/11) and was rather rudely closed down by the moderator because I "failed to read the FAQ". Whatever!

Here is what I've been experiencing in my first 1900 miles. Everytime it is raining there is definetly a big difference in slow speed braking. When dry, no grabbing whatsoever. When wet, there is a slight pulsing followed by a "grab" and quick abrupt stop. I have to feather the brake pressure I apply to give me a smooth stop. I am WELL aware of wet brake stopping and all that (this is not my first car!). I was just wondering if others experienced this and whether it was a problem or if it went away with time.
Hmmm... wondered where that thread went. Its a common issue. I still seem to have it sometimes at 20,000 miles. The reason it seems so severe on this car is that in a normal car you apply the brakes softly at first and stronger later... the soft portion of braking tends to dry the rotors so the grab isn't as bad. On this car the soft portio of the braking is all done regeneratively so you end up with a nasty transition when the computer actually applies the brake shoes.

There is nothing wrong with the car, its pretty normal. You will get more used to it. It should happen only the first few times you brake when they are wet and then get better.

If you wish to "artifically" dry the brakes I suggest that coming to an intersection shift the car into neutral. This will disable the regenerative braking and only the real brakes will be in effect. Now when you gently apply the brakes the rotors will "dry" out and probably the next intersection you can drive compltely normally (leave it in D) and it won't happen again.

I haven't verified that much... because I kind of got used to the grabbiness and it doesn't bother me (I guess I kind of automatically let up a little when the grab starts). But the theory should be sound.
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Old 01-13-2007, 08:51 PM
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Default Re: Grabbing brakes when wet II

Quote:
Originally Posted by ohiocityprius View Post
I posted something about this a few days ago (1/11) and was rather rudely closed down by the moderator because I "failed to read the FAQ". Whatever!

Here is what I've been experiencing in my first 1900 miles. Everytime it is raining there is definetly a big difference in slow speed braking. When dry, no grabbing whatsoever. When wet, there is a slight pulsing followed by a "grab" and quick abrupt stop. I have to feather the brake pressure I apply to give me a smooth stop. I am WELL aware of wet brake stopping and all that (this is not my first car!). I was just wondering if others experienced this and whether it was a problem or if it went away with time.
"...When wet, there is a slight pulsing....."

That "pulsing" is most likely the Anti-lock system activating due to the "slippery when wet" surface. Once ABS activates the regenerative braking system is instantly disabled and the friction brakes must now take over the entire braking task. The transition from the combined braking to friction braking only is likely the cause of the "grab" you feel.

My guess is that what you are feeling is SOP, Standard Operating Procedure,
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2005, 2007, brake, brakes, car, damp, dry, drying, escape, grab, mariner, prius, slow, stopping, wet

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