42/40 Air pressue unsafe???

I have around 33,000 miles on my OEMs. I have always kept them at 42/40. The wear across the tires is even on all four tires.

If safety is our primary concern, we should not drive on these cheap quality tires. We definitely should replace them by 30,000 miles; the point at which the shallower rain groove on the side of the tread pattern disappears.

Tires are a lesson in compromise. To design a tire means to take a stab at an appropriate balance; knowing that you won't get anything right.

 

There are multiple factors at work when considering the effect of tire pressure on "handling". I put "handling" in quotes because it means a lot of different things, for instance:

* lateral grip
* feedback through steering wheel (BMWs are great at this)
* responsiveness of turn-in
* balance: oversteer vs understeer
* transition to/from oversteer and understeer in a corner (example: car goes into corner understeering and changes to oversteer during the exit)
* predictability/warning when one end of car begins to slide

I would think that most people with an engineering or physics background would recognize that lateral grip as a function of tire pressure can be graphed as upside-down parabola, with grip maximized in some pressure range, and it falling off with higher or lower tire pressures.


A few things I'm pretty sure of:

* Even pressure across the width of tire section maximizes grip
* Higher pressures mean stiffer sidewalls, therefore less tire deflection under load, which keeps tread in even contact with road.
* Lower tire pressure increases the length (the direction the tire rolls) of the contact patch, and a larger contact patch increases grip (that simple model of friction taught in first-year college physics is a simplification of the real-world). Think about a top-fuel dragster, they run so little tire pressure that the sidewalls wrinkle under acceleration.
* Tires change camber when the car leans in a corner, and when the wheels are turned. The tire has to be remain flexible enough to keep the tread face in even contact with the road while the camber is changing.
* Turn-in is crisper with higher tire pressure

So out of those five items (just the first five I could think of off the top of my head), one says higher pressures improve grip, one says higher pressures improve responsiveness, two say that lower pressures improve grip, and one says that it's important to keep the pressure within some optimum range, because grip falls off with extreme overinflation or underinflation. It's not simply "more is better" or "less is better".

Whether 44psi corresponds to the point in the graph of grip vs. tire pressure where the first derivative is zero, I have no idea. Subjectively, I don't care for the road feel through the wheel or road noise level at 44psi, so I run lower pressures (38/36).

 

ALL tires have built safety factor and the reason is because tire manufacturers know human nature. they know that people will see "max tire pressure" and read it as "recommended tire pressure" so the max tire pressure has a rather large safety factor built in. to account for pressure differences due to heating up tires, inaccurate tire pressure gauges, etc.

this is from my brother who has worked in the tire industry for nearly 20 years and can be considered an "expert" in the field. the primary reason for keeping tires with lower than max pressure is purely for comfort. a tire with a max pressure of 44 psi can under normal loads and conditions handle 50% more or 66 psi before tire failure becomes a real danger.

 

Sorry I should have qualified that statement to read "on snow/ice or soft sand."

I've gone offroading with some desert loving nuts near Moab, UT, and also on The Stage Coach Trail. Once off the highway, they'd always air their tires down to 25-30 psi, sometimes down to 20 psi, for desert driving.

Mind you, we're talking speeds under 40 MPH.

Once at the onramp to the Interstate, they'd use an air compressor or an air tank to quickly top the tires back to the recommended inflation.

My 1990 4Runner had the optional 31 x 10.5 R 15 tires and I think they recommended something like 26F / 28R. I never had any unusual wear, the tires lasted a long time, and the handling seemed ok.

Since I haven't seen unusual wear past 17,000km, I'll stick with the Toyota recommended inflation. FWD cars are usually harder on tires - at least the front tires - than traditional cars.

 

Isn't this largely to maintain full surface contact when centrifugal force pulls the center out as speed increases, and aerodynamic "lift" reduces the effective weight of the vehicle; and heat rises, causing expansion of enclosed gas, thereby raising pressure?

 

Karl, I appreciate your sharing your knowledge and experience with the rest of us. I don't want to kill the discussion, but I'm curious about your original post. The recommended tire pressures on my 2004 Prius' door frame says 35 front 33 rear. Has Toyota changed their mind about recommended pressures and lowered them by 3 psi?

My car was delivered to me with the tires inflated to 45 psi (their shipping pressure), which I reduced to 40/38 based on the discussions on this board. I have since reduced the pressure to 37/35 and find the ride and handling greatly improved, an no real effect on milage.

 

8AA: Thanks for your post. I boosted my pressure to 41/39 a couple of weeks ago but was considering lowering it. Your post sealed the deal for me. I had been reading a lot about the Integrety tires in this group and wondered if lowering the tire pressure wouldn't help compensate for some of the tire's shortcomings.

 

Low rear tire pressure (typically 4-5psi) in a top-fuel dragsgter allows the following:

1) Huge amount of sidewall flexing puts heat into the tire, increasing the stickiness of the tire compound
2) As the tire spins and grows in diameter, the contact patch changes from short and wide to long and narrow, which enhances traction. A contact patch that is longer in the direction that it needs to resist force is better. That's why a long (in the direction the car travels), narrow contact patch is good for acceleration and braking, and a short, wide contact patch is better for cornering.
3) When the tire grows, it momentarily increases the downward force on the rear tires (until the back of the car stops rising), also enchancing traction

As far as your comment about aerodynamic lift - that's exactly the opposite of what actually happens. The huge rear wing behind the rear tires is designed to put additional downward force on the rear tires to increase traction. You couldn't accelerate at 4Gs if you could only depend on mechanical grip between the tire and track surface - you need the aerodynamic aids to get much beyond 1G.

 

Re: 42/40 Air pressure unsafe???

With 2001 Prius I used the recommended 35/33 psi for the first 10k miles, and (finally!) noticed excessive tire edge wear. After a series of tread depth measurements at different pressures I concluded that 42/40 gave even wear across the tire. But that edge rubber was gone forever and the tires needed replacing at about 33k. The replacement tires now have about 42k miles and I've established that they also wear most evenly at about 42/40. When they need replacing at about 55k, I will again make a lot of tread depth measurements to find the optimum inflation pressure for the new set.

Incidentally I ignore the Toyota tire rotation pattern as well, and move the tire with the deepest tread to the fastest wearing position (left front, and then wear decreases RF, RR, LR). As you might expect from such tire obsession every 5k miles, the 4 tires are still quite similar in tread depth. This is another component of long tire life.

For those many who do not wish to make 96 tread depth measurements each time, I have read that putting masking tape across the tread will give a quick indication of wearing pattern.

While everyone gets to choose their own inflation pressure, I have not heard evidence that best traction (with passenger car tires on paved roads) will occur at anything other than the "minimum/even wear" inflation pressures.

Realize that the newer model Prius has a different tire setup, and it may be no more than coincidence that 42/40 is also popular there. I don't know of anyone with the new Prius who is making lots of tread depth measurements.
_____

DAS, with digital depth and pressure gauges

 

When I attended my Toyota dealer's open house shortly after buying my 2005 Prius, Elflida (long story), the subject of tire pressures came up. When doing their umpteen-point service, they fill up every tire they see to the same value. I think they said 32 psi, but I didn't pay attention. Moral: the dealer doesn't care about a couple of psi one way or another. Question: one wonders what else they don't care much about?

 

Good reference link, thanks. Now, if Michelin shows photos of higher inflation pressures, this topic may fade.

This discussion, however, has made me less offended that my tires were inflated, at delivery, to the 42/40 rate. It seemed that the dealer would stick to the mfg specs, so I reasoned that they merely wanted to demonstrate maximum mpg. My short ride in the back seat shortly after taking delivery, however, persuaded me to follow specs. The smoother, quieter rear seat ride convinced me to keep pressures there. Still seems the best choice.

 

Good link and very informative.

Obviously too low air pressure is bad. Going beyond the manufacturer recommended pressure isn't good either.

My 1990 Toyota 4Runner (Wish I had kept it) had the optional 31 x 10.5 R15 tire package. I think the correct inflation was 26F 28R. Those tires usually will accept a 50 psi maximum cold inflation pressure.

I did try 50 psi a few times and had some good scares. The back end got twitchy at Interstate speeds, and on the rare rainy days the back end got loose on me.

As far as MPG, I usually didn't notice any difference on the Trans Canada. A couple of times I *may* have gained around 2 MPG. It wasn't worth the bone jarring ride.

 

Well, by independent blind test, there is a noticable change in ride between 35/33 and 42/40.
Since I got my oil changed and tires rotated Friday, they changed the tire pressures. I admit, I didn't check them, but past experience tells me they did. I didn't have my compressor with me, so the are still at 35/33. My girl friend mentioned my car felt different this weekend. And it was a whole week since she rode in it with tires at 42/40.

 

First of all I should say I have had to use over the recommended pressures in every car I've owned, both for decent mileage, but also to prevent excess wear on the outside tire edges.

In my Prius I have tried std pressure and up to 42/40. I use the latter.

As to ride, I note the Toyota pressures are the best ride, but just a small extra and the ride gets about as bad as it gets. So going all the way to 42/40 has negligible further effects on
the ride.

The Prius seems to rely on soft tires more than most cars for ride quality! I noticed a huge
improvement in ride with my snow tires over the winter, I assume because of the soft
treads, since I used them also at the 42/40 pressures.

But every pound extra seems to help mileage as I go up in pressure. And the handling seems best at the higher pressures, though it is never good, on the stock tires.
I'm almost tempted to use the Toyota pressures just so my tires will wear out quicker, and I can get good tires for the car!

 

bumpy Boston

Tomdeimos;

I tried going up a little from 35/33, but the streets in Medford,
Arlington, Somerville, Lexington and just about everywhere else
here are in such horrible shape that I couldn't take the bumping.
I'm pretty sure a lengthy period would knock loose some rattles
in the car, although after 31,000 miles it hasn't happened yet.

This is driving the car through the suburbs about 85% of the time. If I was
commuting on 128, it might be a different story.

Richard