Quick question on tire pressure and braking..

Hi Jaqueh2, welcome to PriusChat! Both of your questions, and many more, have been asked and answered before. Many forums have sticky topics listed at the top that serve as FAQs. Also, the Search link, found at the top of every page, is your friend.

Tire pressure above the manufacturer's recommended 35 psi front / 33 psi rear is a matter of personal preference. Many on PriusChat run 42/40. It's good to have a couple of more psi in the front to support the weight of the HSD.

One of the main ways the Prius gets such good fuel efficiency is by recapturing forward motion and turning it into electrical energy for re-use when you step on the brake pedal. This is what is called regenerative braking; you're right in a sense, the motor becomes a generator. In a conventional car, stepping on the brake pedal causes the brake pad to rub against the disk or drum, turning forward motion into wasted heat. In a Prius, the brake pads don't make contact, under normal circumstances, until your speed is 7 mph or under. All the energy that is wasted as brake heat in a conventional car is used to charge the high-voltage battery. Prius rarely need new brake pads.

 

For *maximum* efficiency, psi at the rated limit of the tire... standard rolling-resistance tests actually use max + 3psi! (don't try this at home, folks... or especially on the road!) This maximizes "efficiency" in part by deforming the tread to make a too-narrow contact patch.

42/40 is essentially a decent compromise we have found to provide a good contact patch and safe pressure level (reports of blowouts at this psi must be rare... I'm not sure we've seen any posts).

 

<div class='quotetop'>QUOTE(bobdavisnpf @ Jul 9 2007, 11:43 AM) [snapback]475525[/snapback]</div>

At 45psi in the Integrities the yellow idiot light will flash on probably once in a 35 mile commute when braking on dry pavement. I don't see any evidence of deformation.

 

<div class='quotetop'>QUOTE(bobdavisnpf @ Jul 9 2007, 11:43 AM) [snapback]475525[/snapback]</div>

I read the standard for the LRR procedure, and +3 lbs is not as extreme as it looks. They run the tire for a certain period of time so that it is warm, as it would be in driving. Then they set the pressure at max +3. So this standardizes and compensates for the difference in pressure which is usually measured while the tire is cold. I would suspect a 3 lb increase in pressure for a tire when warmed up to be normal. Hence, the test is running the tire at or close to its maximum recommended pressure.

Unless I am missing something.

 

<div class='quotetop'>QUOTE(Doc Willie @ Jul 12 2007, 07:27 PM) [snapback]477882[/snapback]</div>

Well, ok, you've got me there. If I had to drive on the stock tires at full pressure and full load I'd probably be more conservative; I just don't trust that big sidewall on a 17 lb tire to hold up well, when braking & swerving hard.

Then again, maybe they test for that, just to be sure they won't get sued if a blowout causes a fatality. No, wait -- NHTSA and Goodyear say the correct pressure is what the car's tire spec plate lists (35 front, 33 rear)... if we're off of that, we're on our own.

Come to think of it, a blowout in evasive maneuvers is probably *less* likely with an overinflated tire than an underinflated one: the sidewall will deform less, since the high-pressure tire is more rigid; so it's less likey to peel off the rim, inside edge or outside.

JimN, are you sure you're getting a full contact patch at 45psi plus the 3-4psi from driving warm-up? I'd think you need the full 1168 lbs load over each tire to get a good contact patch at that pressure. With the rears only carrying maybe 500 lb each, I doubted we'd get a full contact patch even at 40 psi.

Maybe I'll try testing for contact patch at different tire pressures... what the heck, that's a good excuse to go buy that electric air pump...!

 

When you increase tire pressure above factory specs you reduce the tire's contact patch. That reduces friction (hence higher MPG), and it also reduces the tire's GRIP. Less grip means a lower threshold before the tires slide in braking.

 

<div class='quotetop'>QUOTE(bobdavisnpf @ Jul 14 2007, 12:26 PM) [snapback]478665[/snapback]</div>

I have no idea what the size of the contact patch is at any given psi.

 

Most people on the forum recommend tire pressures at 42/40 as it helps on the MPG; Toyota, with all its research, recommends tire pressures at 35/33. Does anyone know why Toyota recommends such low pressures, especially when it could use the higher pressures recommended on the forum and advertise their car as having an even higher MPG! Does it have to do with comfort or handling or safety or all three, or is it something else?

 

I believe ride comfort is the reason.