Hi folks,
Many have already been following the buzz about the Prius brake problem so I wanted to post a summary. The good news are reports that Toyota has a fix and been putting it in new cars since January.
The symptom is momentary loss of braking force when lightly braking and running over a road surface imperfection. However, it doesn't always happen and about 50% of the PriusChat owners reported they've not experienced it. Only 20% call it a severe problem. In truth, I hadn't experienced the problem until Friday but even then, our Huntsville pavement becomes so super slippery when it rains that I would not have otherwise even noticed it.
Trying to understand what is going on, I collected photos of locations that triggered the braking problem:
. . .
The common elements were:
- slight, downhill grade
- light braking
- some discontinuity of the road surface
- intermittent - slamming the brakes worked RIGHT THEN!!!
- many NHW20 users report similar experiences but not as severe
- initial startle is soon replaced by irritation
So I bought a pair of accelerometers so I could loan one to folks having more luck replicating the problem. But the accelerometers arrived the same day we learned Toyota will be sending out a fix for the ZVW30. No word on the NHW20. Regardless, this is what the Gulf Coast Data Concepts accelerometer looks like in the car:
So this is my first capture of a speed bump:
The red line shows vertical acceleration of the speed bump. The blue line shows braking action with the brakes working right after passing the speed bump. This is normal braking behavior.
Then Friday it was drizzling and the first speed bump at the Redstone, Martin Road gate gave a "slip-n-slide" feeling. Now Huntsville streets are notorious for being slippery in the rain but I was recording the data:
You can see the two speed bump and the momentary pause when the guard checked my badge. It was the first bump that showed the problem including an 800 millisecond, delay in return of braking:
You can clearly see the braking action holds off for nearly 800 milliseconds. Integrating the acceleration gives velocity:
So here you can see how the velocity remains for 800 milliseconds at a speed of a little over 7.6 m/sec. This would add a little over 6 meters to the stopping distance. If a driver wasn't expecting this it could be bad news. But like I said, this is sort of how we expect wet Huntsville streets to behave.
We're expecting the ZVW30 braking fix to be a software download. No word on an NHW20 fix, yet.
Bob Wilson