Wobbly brake at freeway speed

[ericsf]

I'm not sure how to best describe what I experienced this morning "Wobbly" is the first word that came up to me.

I was going downhill on US 101 just outside of San Francisco (southbound, just before Candlestick for those who live here). Gentle and regular slope. No bumps on the road. I was going around 50MPH and when I slightly pushed the brake I felt an irregular deceleration for the whole time (about 5-6 seconds). Obviously it stopped because I released the brake pedal.

It wasn't violently shaking the car and it didn't feel dangerous but I immediately noticed it. I would say the movement has a period of about 1 second. It didn't feel mechanical. Definitely not a vibration due to a pad or disk issue. If I had to make a guess it felt like the regen was somehow disengaging and re-engaging by itself. I didn't have the time to check the power meter to see if this was also going ON and OFF. Of course it didn't happen again during my commute but it's my last downhill.

I do get the grabby brake problem when I'm about to stop once in a while - I don't think I got the patch during my last visit at the dealer. But this felt completely different - also a very different speed.

Has anybody else experienced that ? I'm sensitive to this kind of stuff so I'm pretty sure it's the first time it happens to me. Could there be something wearing out in the regen? I'm getting close to 40K on this car.

 

[mwalsh]

Any emergency stops in your recent past? I'd say warped rotors, right off the top of my head.

 

[eclecticflower]

Why does that sound like ABS/traction control to me?? :?

 

[davewill]

Why does that sound like ABS/traction control to me?? :?

It does to me, too. Was there water on the road? Maybe a bit of hydroplaning tripped the ABS?

 

[ericsf]

The road was as clean and dry as it gets. Just a gentle downhill slope.

No emergency stop in the past 6 months (I think I triggered the ABS twice in the past 2 years).

What I did was to gently press the brake pedal and it instead of a smooth deceleration I got rocked back and forth a few times. Almost like if someone outside was shaking the car. It was very strange.

 

[Nubo]

Odd. A period of 1 second at 50mph doesn't correlate with wheel rpm so I don't think it's warped brake rotors.

Was the effect fore-aft, or was it side to side?

Are there grooves in the roadway at that point?

What was your battery state of charge? The car will "pull" regen when the state of charge is high; maybe you were right on the margin of that?

 

[ericsf]

It was a fore-aft movement. No groves in the road there. SOC was below 80%. I drove through the same road this morning and tried to replicate the same conditions but nothing bad happened this time. Not surprising: I've been driving there every weekday for the past 2+ years and it's the first time I notice it. But I'll keep on trying.