Reason's for Braking loss and possible way to resolve it.

I did not see any post explaining why this happens. I am sorry if I am repeating and for a longish post.

Having experienced this issue myself and recent news over last couple days here are my thoughts and analysis and also possible algorithm which may prevent this issue.

Why does this happen?
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Brakes of any type, regardless of whether regenerative or Frictional, can work only when wheels are in contact with ground and have enough friction to make them roll rather than slide.
Once the ground contact is lost brakes, again regardless of regenerative or frictional, can only stop wheels from rotating. But can not arrest momentum of the car which is what you feel.

Why only Prius and other hybrids based on similar design have this issue.
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I read today Ford fusion also has similar issue, although media does not seem to create as much hype about Ford.

The reason why Prius has this issue and not other cars is, Prius when using regenerative braking is using ONLY front wheels for braking. Thus if front wheels loose contact with ground, the loss in braking is complete loss.
For cars using conventional brakes as well this braking loss would indeed occur. But then it is a partial loss. Other pair of wheels is still braking, thus braking loss may not be felt as much.


Possible Algorithm to Resolve this.
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This alogorithm should prevent this issue without significant loss in fuel efficiency. I think all necessary sensors are already in place.
I assume when wheels loose ground contact, they would slow down much much faster since braking is arresting angular momemtum of wheels and transaxle alone rather than full momentum of the car.

This fact may be used in resolving the problem.
When all of following conditions are true,
a) user is braking
b) Regenerative brakes are being used
c) Front wheels are rotating slower than rear wheels, quickly engage mechanical brakes to rear wheels.

I said front wheels, because if rear wheels are airborn, there is no loss in braking when using regenerative braking. If already using frictional brakes there will be a loss in braking but then Prius is like every other car.

In a sense this should be something like VSC ( in my understanding, not necesarrily perfect). VSC in my opinion, would brake left wheels or right wheels, while here you would need to engage brakes on rear wheels.

I do not know how quickly this can be sensed and used to make user experience normal braking.

There may be another solution to always engage frictional brakes on rear wheels. I hope Toyota does not use this approach since loss in regenerative abilities will be in ALL braking situations while this problem is so rare.

If some similar solution can not be implemented, I would rather live with the current behaviour because normal human reaction to this issue, to step harder on brakes, is correct response.