Braking on rough surfaces

Pump your tires up to 51PSI and drive 55 on a pothole road and tell me what happens.

 

If that happen to me, I think I would hear the tinkling fine white china... as my teeth fell out of my head.

 

CC, you may want to try a different denture adhesive,

 

Gorilla Glue???!

 

Interesting thought ... I wonder if it's possible Toyota calibrated the system for something closer to their spec pressure. I know a lot of PCers run somewhat harder than spec, but 51 is, what, nearly 50% over, in fact pushing the max sidewall rating....

It also means your contact area with the road is only 2/3 of normal ... like giving up one tire, and some change ....

-Chap

 

At best I'd say that depends on your point of reference. From my point of view at Toyota's tp the tire is underinflated and the center of the tread is not in full contact with the road. That underinflated tire also won't be able to carry a full load and will run hotter.

Some tires hold their shape better than others under low(er) pressure. On others the sidewall bows outward. When that driver has to execute an evasive maneuver there is a higher probability of tire failure and loss of control.

There's no one pressure that is right for everyone under every condition. To each their own. If you are happy with how the car handles and the tires wear that's all that matters. If not, try changing the pressure. It's inexpensive and reversible.

 

I've been hanging on PriusChat for a while, so sure, I've seen all those arguments. And to a point I agree with the 'to each their own' sentiment ... there are a lot of people on PriusChat who have learned a lot about tires and inflation, I had to myself as a Gen 1 owner, in order to be sure about whether the 84 load index and/or a specific pressure was needed to carry the load in front.

But while there's some sense in saying there's nothing special about my opinion or your opinion, it seems a stretch to say there's nothing special about Toyota's opinion ... they designed the car, and everything I've learned about Toyota design is that they are more than likely taking into account many more related engineering trade-offs than you necessarily see in the narrower discussions in PriusChat threads.

I do not know for sure whether their calibration of the ABS, VSC, or TRAC software has any dependency on tire pressure, but if it does, the pressure I would assume they calibrated for would be the same pressure they put on the placard.

Giving the benefit of doubt, I guess we could mean different things by full load. If the GVWR they print on the door placard came to more than 94% of the tires' load capacity at the printed placard pressure, they wouldn't have approval to sell the car in the U.S.

Edit: I had the thresholds wrong: the requirement is for GVWR to be less than or equal to the tires' load capacity at the placard pressure; it's the "normal load" (as defined by FMVSS) that has to be below 94% of that.

I guess that sums it up pretty well. In this thread, post 22 seemed to be from someone who wasn't happy with the braking-on-rough-surfaces behavior, after having chosen a roughly 50% deviation from a Toyota spec. I make no bones about anybody choosing what pressure they want, but I think if I ever chose to deviate that far from any of Toyota's specs in my car, and I didn't like the car's behavior under those conditions, I would be hesitant to post about that as if it were a fault of the car.

One note about reversibility ... when I bought my Gen 1, it had 125,000 miles and was as quiet inside as a new car. Inspired by PriusChat, I ran the tires at 42/40 for a few weeks, before compromising and settling on the 38/36 that I stuck with after that. Most of the effects were reversible, but the dash rattles and squeaks it picked up in those few weeks at 42/40 were with me the rest of its life.

-Chap

 

It is my humble opinion that everyone that drives should at least once in their driving life, take a hands on truck driving class. I had a truck driver come to our class and take us to a course back in the day (high school drivers ed). Hydro planing blew me away!!

 

I don't know which departments at Toyota have a vote in the selection of tires and pressures. Engineering? Legal? Accounting? Who's looking out for my interests and how would they even know what they are?

Toyota's recommended pressure is for Toyota's recommended tires. If these were the best choices we wouldn't have the replacement tire topic.

Montgomery, I agree. At the beginning of driving class everyone believes they know how to drive. Haven't come across anyone with a skid pad around here. I take the AAA classes for the insurance discount and on occasion run autocross with the SCCA.

 

I increased my 15" Toyo tire pressure to only 5 PSI over what the door placard recommends. It feels pretty good around town; the ride didn't seem much different.

But on the highway/freeway today, at speeds of 65 to 75, every little crack, bump, and hockey puck lane marker felt much more pronounced (and noisy.)

 

There's probably no need to hijack this thread into a rehash of the replacement tire topic. This thread's about the behavior of Prius ABS on rough surfaces, which for most of us is a minor effect that is easy to get used to, except as post 22 suggests it may behave more objectionably if the car is being used at tire pressures very significantly different from those it was designed for. Which may be true, but hey, if you want to second-guess Toyota's designs by as much as 50% or so, nobody stops you, but then you sort of own the outcomes, right?

-Chap

 

Why is this specific to the Prius? The feeling you've all described is what happen when you brake with ABS. I don't know how ESC gets involved but sure why not. The Prius doesn't seem any more sensitive than my Sonata, Civic, and a couple of rentals when I brake over railroad lines. The uneasy feeling I get in the Prius is the same uneasy feeling I get on all cars I've driven with ABS.

 

Thanks to everyone who related their experience with the Prius ABS. I have not had it activate yet. Thanks for the heads-up.

 

As a previous Prius driver, I have lots of experience feeling the "floating" feeling when braking on a rough surface. Not anything to worry about. You get used to it quickly. But, the first time or two it is weird.

 

In "normal" cars, all braking is done on all four wheels. With regular braking, when ABS is activated on one axle, braking still takes place on the other axle. A hybrid, such as the Prius, also uses regen braking, which only operates on a single (front) axle. If ABS is activated on the front axle while regen braking is active, you lose all braking until the system transfer the braking to the regular braking system. That is why it is more noticeable with a Prius.

 

^^ and also why the Prius does it only with light to moderate braking. Prius's hard and panic braking are not with regen (or at least regen-only), so it is already behaving as a 'normal' car.

 

I flash my lights and let truck drivers in trying to switch lanes. I admire and respect all drivers with a class D license. You guys know your stuff. And whenever I am on the highway and I see a truck driver suddenly put on his/hers hazard lights, I know something big is up ahead. I really like it whenever I am going up a long uphill climb behind a truck, and as soon as the truck driver see's a break ahead, they stick out there arm and wave that it is all clear to pass!! Keep on Truckin!! By the way, have you ever seen the movie "Duel" by Steven Spielberg? Not a good representation of truck drivers....LoL