Question Regarding Breaking on Ice

While I don't claim to be an expert on prius braking, and may be totally incorrect

1) Regenerative braking is disabled when the traction control light goes off, ie when the prius notices the wheels start to slip, like on ice.

2) the prius has disk brakes in the front and drum brakes on the rear.

3) all braking is dependent on the availability of traction... if the tires can't get any, the prius will follow newton's first law of motion... and go into a straight line.

4) one of the reasons why the rear tends to fishtail is because the weight of the engine/transmission is more focused on the front, instead of being evenly distributed... thus the front has more traction.

 

I believe that when you take your feet off of both the accelerator and the breaks, that there is some regeneration happening that slows the car down a bit more than just friction and aerodynamics. So this is a slight negative for the Prius on ice, IMO.

BUT, the Prius engine does not slow the car as much as a normal engine in a normal car (unless you are in B mode on tranny), so this is a slight positive for the Prius on ICE.

My suspicion is that the slight positive outweighs the slight negative and/or they are so close as to be a wash.

Yes, I believe that the brakes are not used at all in the Prius under normal (moderate) braking conditions above the threshold speed.

No, I think regeneration on the front is not normally combined with braking in the rear, unless braking is hard enough the front brakes are activated also.

These are my thoughts and understandings. If someone thinks I'm wrong, I'm sure they'll pipe up.

 

First off, can we please get the word right? BRAKING. Not
breaking. You want your prius to BRAKE, not break. It breaks
when it slews around sideways and hits something, probably due
to poor braking technique.
.
Front wheel braking is probably not the main cause of rear end
spin, and in fact allowing those wheels to keep turning probably
helps keep things on an even keel rather than destabilizing.
Ask any trucker about trailer brakes in snow. If you've somehow
put enough sideways force on the rear end to slide it, it will
probably do so with or without the wheels turning because you've
overcome whatever side friction the rear tires have left.
.
In fact it's pretty hard to spin this car *unless* you stab the
e-brake to deliberately lock up the rear end, and you have to
help it quite a bit by going into a pretty tight turn beforehand.
Playing around in empty, snowy parking lots is really good practice.
.
_H*

 

On top of what's been said before, your Prius has Vehicle Stability Control, with sensors that look at your steering angle and the car's yaw. If the back end starts to slide one way or the other, VSC applies force in the other direction to maintain steering direction. Applying force could mean a little braking on one wheel, or reducing braking on another.

If you are driving on Goodyear Integrity slicks by next winter and happen to drive on a sheet of ice, your tires will have no traction anyway, so no amount of VSC can help you. Winter driving means having appropriate tires and driving appropriately for speed and conditions.

 

As Hobbit said, engine braking on the front wheels will not usually cause the car to spin because the rear wheels are turning with no force slowing them (unless you apply some).

Example - I had a Subaru Wagon 4WD (older model - manually shiftable). In FWD it would become "unstable" on ice, but wouldn't spin, just scare me by "wandering". I would shift it to 4WD and that would stabilize it, because it locked the front and rear wheels together. But it would handle "funny" in corners that way due to the front/rear lockup. The current Subarus don't do that as they have clutchs/diffs. in the front to rear drive.

Example 2 - I was following a small pickup on the highway in winter (in the Subaru). Unknowingly we entered a 5 mile stretch of highway that had just been hit with freezing rain. We were doing 80 km/hr (50 MPH). I didn't notice anything happening to me, but he started to loose the rear one way, then the other, etc. He almost recovered, but then it snapped in one direction and he went head on into the ditch (at low speed thankfully). It took me 3 miles to come to a stop from about 60 km/hr (40 MPH or so). Now THAT was slippery!

Example 3 - I only have one winter with Pearl so far. I drove in freezing rain twice. No problems whatsoever (city only). I do have good tires on the car (Nokian WR which are rated as good as the Mich. "Ice tire"). I leave them on all summer as well. I did see the anti-lock light a few times while braking, but mostly tripped the cars' systems when accelerating (traction control) and twice in a corner (VSC). Under normal accel or braking I had no problems all winter. I even tripped the traction control last week on some still not cleaned up winter sand/gravel when taking off around a corner from a light.

 

I run very aggressive studded snow tires in winter. Available at Canadian Tire as the Goodyear Nordic, it's the same as the Goodyear Ultra Grip 500 sold in Europe

Goodyear Eur-Winter Tire-UltraGrip 500

With these tires on, I never worry about icy roads myself. I just make sure when I hit the brakes, that nobody is behind me. The tires really do work that well

I've never had any issues with stability or traction on ice, running the aggressive studded winter tires. Though last winter, I did witness a brand new Ford F-450 do a very interesting spin on ice at Deacon's Corner.