Brakes "slip/skid" when hitting a bump?

This is pretty common. When the Prius detects wheelspin, it will shift braking from regen to friction. During that switch, there's a momentary loss of braking.

Do you notice that the traction control light (the amber skidding car) comes on when this happens? That shows you that wheelspin has been detected by the skid control ECU.

The original equipment Goodyear Integrity tires are not known for having outstanding traction. If you install a set of quality tires, then this issue may go away or at least happen less frequently.

 

Yeah, I get this too in my '06. It feels weird, but I've never once felt like the car was out of control... I've learned to just ignore it.

 

Yep, same here. As said, the integrity tires aren't the best for grip...combine this with a very sensitive traction control and you have the issue you described. As another poster, I just learned to ignore it. I have a similar thing happen when I accelerate sometimes from a stop going around a corner. If there's the slightest bit of slippage, the ecu completely chops power for what seems like an eternity when you're trying to merge into traffic during rush hour from a side street.

Mike

 

During braking? More likely wheels lock-up and cease spinning while braking. Wheelspin typically occurs during acceleration.

 

Hitting a bump can cause the tires to lose contact with the road surface for a moment; obviously there cannot be any braking action while a wheel is in the air. The loss-of-contact can also trigger the traction control/antiskid mechanism so that the wheel in the air won't start spinning too fast.

 

Really? I thought traction control was an acceleration thing. I didn't think traction control came into play during braking.

 

I am inclined to agree that it is more of a tire issue. As my Integrities get older and are on there last few thousand miles, it happens more and more for me.

 

I don't think its traction control, but rather the braking equivalent. Probably the ABS or stability controller? I guess they're all related. As Patrick said its probably a bit more pronounced feeling in the Prius as at the same time the ABS system would normally be reducing braking to prevent going into a skid, its also transitioning from electric to mechanical brakes in perpetration for an emergency braking situation. If you've ever tripped the ABS in a traditional vehicle it feels similar. I've had that happen under the same circumstances in other vehicles as well (hitting a bump while braking, freaking out the wheel speed sensors) though probably under somewhat harder braking due to better tire grip.

Rob

 

I think it's the traction control and not the ABS.

When I think of the ABS working, I think of noticeable pumping action/modulation of the brakes. At least that was the way it was on my prior car, a 1995 Volvo.

EDIT: Or maybe there can be instances when both the traction control and ABS are operating at the same time?

 

It is traction control. When you hit a bump and it unloads the front wheels, the traction control system detects that the wheels are slipping. As another poster suggested, this is not really wheel spin, but just the opposite. From an engineering or physics standpoint the two are identical, but with opposite signs. The loss of traction causes the Prius to switch from regenerative braking with the front wheels to friction braking with all four wheels. If the wheels continue to skid, the ABS system will modulate the friction brakes as needed. VSC does not come into play unless there is a loss of steering, in which case VSC will reduce power and actuate individual brakes as needed. All three systems interact with the brakes, but they are not the same.

Tom

 

Same here, and the traction-control light flickered 3-4 times.

 

Read near the end of this.
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This bothers me too. Most often when this happens I DO need to brake and I feel like valuable time is lost during this transition. Has made for some close calls over rough roads etc.

 

It's mostly just the sensation. If you really need to brake, you won't loose much if anything during the transition. Panic braking will automatically switch out of regen and invoke brake assist.

Tom

 

Yep me too. Thanks Patrick for the excellent explanation as thats exactly what's happening to me. See the VSC flashing real quick and the sensation of no braking. Usually happens when I'm going fast and hit a rut big enough to bounce the wheels.Just another peculiarity of the best car in the world.

 

Hmm. I've noticed this too in my 08, especially when I cruise into my driveway in a particular way (there's a little bump). The traction control light definitely comes on. Does this mean I'm going to have a problem in the winter when the roads get slippery?

I have the factory installed tires on my package 6...I live in Massachusetts, so winter driving is a concern. Just got my Prius in April, so have never driven it in snow or ice.

 

Try to avoid braking. You wont feel this sensation and you save some petrol. Actually you can save a lot of petrol.

 

It's not the VSC, it's the traction control. The two are often confused, but they are not the same. All Prius have traction control, but only some packages have VSC. VSC kicks in with loss of steering control, not simply loss of traction.

Tom