How does adding air to your tires affect safety and handling?

No it wont void the warranty, that is ridiculous. But yes when warm they will get even higher pressure. But when people on here give PSI numbers, those are cold PSI numbers so those are essentially the minimum PSI. 38/36 is still pretty low.

The handling is not really effected too much in normal city driving. If you take corners at speed, you will have more understeer. The ride quality is what really suffers. As the tire pressure increases, the "pillow" effect is gone, and little rocks and holes become boulders and canyons inside the car. I run my tires around 45PSI cold on the Prius and it is fine to me on the rough roads around here. If I want a cushy ride, I take the Lexus, that's what it is for.

As for safety, it is less safe. More chance of a blowout is up there but the biggest would be the point of increasing the tire pressure is to reduce the amount of contact between the tire and road. Instead of being a circle with a flat bottom, you now have a true circle. If you go even higher pressures, you start to get the tire bowing in the middle of the rim too so instead of the whole width of tire touching the ground, only the middle half or so is touching the ground and the rest is bowed upwards and not making contact.

The less tire in contact with the ground, the less energy the car has to expel in friction losses between the rubber and the road. It also means less traction. Dirt and rain become very slippery. Tire treads are also designed for full contact patches, which is the 32psi area that leaves that slightly flat buldge when filled. In the rain or snow, you have even less traction and will be more prone to hydroplaning. The other factor is braking. You can stop the wheels pretty easily, they are attached to the car. But stopping the wheels on a car that is moving fast is not as useful as stopping the car which makes the wheels follow suit. Hard braking will see longer and more frequent applications of ABS. The VSC and TC (Vehicle Stability Control and Traction Control) will also be more trigger happy. Because you have less contact with the ground, your stopping distance will also increase.

The list goes on and on.

Basically, 32PSI, 38PSI, whatever. Do what makes you feel comfortable to drive. Just don't exceed your tire's maximum pressure. This is stamped on the tire on the sidewall. Usually around 60PSI. And that is anytime. So if you fill to 60PSI cold, you will be over that limit. It will probably still work, but a smaller puncture source can turn a leak to an explosion or it just might shred off the wheel entirely.

 

A little over, makes the ride stiffer with better mileage.

A little under, makes a smoother ride while lowering mileage.

 

(This is just my opinion, everyone is entitled to my opinion, but do not mistake it for facts)


If your tires are wearing evenly the handling and tire life are maximized at the cost of mileage and comfort.
When I am adjusting my tires, I am shooting for even wear, good handling seem safest to me.

If your tires are wearing on the out side edges, tire pressure is too low, good comfort at the cost of poor mileage, bad handling, and poor life.

If you tires are wearing in the center, tire pressure is too high, good mileage at the cost of poor handling, comfort and tire life. Don't go here, you want tread on the pavement.

(this picture is a gross oversimplification and over emphasizes how the tread will bend)


I hesitate to put a number but I would start at 38f/36r and adjust as you see wear.

 

I disagree with the first point. Unless tire it pumped to some ridiculous high pressure (like near 100 PSI), there is no increase in blow-up. The opposite. The Ford/Firestone fiasco was due to too low pressure and not too high pressure. This is why we have the mandatory tire pressure sensors in USA. Furthermore, manuals of high end European cars talk about increasing cold tire pressure before high speed driving to decrease chanced of blow-ups. The higher pressure, the less heat buildup in tires (due to lower loses on flexing resistance).

Now, the second point (less traction and braking) is valid in theory, but I has not been scientifically proven to the best of my knowledge. There was a study on that and showed no clear results, except for overinflated tires getting more damage from nails (don't ask me why).

Link to the study results: Barry's Tire Tech

 

Sorry, but I feel obligated to call B.S. on your reply. You didn't identify yourself as a radial tire design engineer. If you are one, please say so and I'll tuck my tail between my legs and crawl away. If you are not, then why do you presume to describe changes in radial tire contact patches, handling, and safety based on changes in air pressure without tire industry training and experience? Am I a radial tire design engineer? No. But I do have the benefit of having read the educated opinions of one on this forum a few years ago, and the benefit of personal correspondence with that individual on this subject. To paraphrase his professional opinions, running a tire at the maximum cold inflation pressure molded into the sidewall is not in any sense "unsafe" under any conditions. It certainly does not increase understeer. Might it be intolderably rough-riding for most people's tastes? Probably. The MAX cold pressure is the pressure used by the tire maker to establish the tire's ratings. It is "MAX" only because a further increase does not increase a tire's load rating. Period. There is still ample safety margin for outside temperature, high-speed driving, etc. We do not have to second-guess how high actual pressure might be in normal use. Police officers who run their high-speed driving and maneuvering training courses have stated in writing that they routinely use 50 PSI cold pressures in their patrol cars on training courses because it greatly aids "handling" and keeping their cars on the road. Radial tires are designed to maintain flat contact patches under a variety of conditions/pressures. We non-engineers don't need to waste our time theorizing how a few pounds more or less might change them. We can trust the trained professional when he says that more cold tire pressure, up to the MAX stated on the tire, will make us safer and improve our gas mileage and tire life. Anything less than MAX is a compromise. In summary, run what you can or will tolderate, no less than the car's sticker, and no more than the MAX stated on the tire. I don't know or care how many angels can dance on a contact patch, and neither should you.

 

You are clearly not a tire engineer either. Passenger tire max load is rated at 35 PSI and not the max cold pressure. The max pressure figure is meaningless: Barry's Tire Tech

 

This is one of those personal philosophy and personal "history" issues that everyone approaches in various manners. To the OP's experience, I can fully understand why Costco, or any professional tire dealership would refuse to inflate the tire above the manufacturers recommendation, even if asked by the owner. For their own legal and/or liability reasons they are probably prohibited from knowingly or purposely exceeding the manufacturers recommendation.

As owners? We can do what we want.

I guess I'm still in the experimentation phase. But at least early I've been running 2 psi above Toyota's recommendation, so I'm 37 front, 35 rear.

I just kind of try to gauge EVERYTHING, ride comfort, handling, MPG efficiency, and tire wear. Very early at this PSI, I'm satisfied. My MPG has been great, and I find the ride to be comfortable. Unless I start to notice any abnormal wear patterns, I believe I will stick with this PSI.

I also think when dealing with Toyota service and/or any potential warranty issues, only running a couple PSI above the door jam recommendation gives me "plausible deniability" if I did have any problem. It's not so grossly above the door jam sticker that I couldn't argue that it "was" inflated at the recommended PSI.

Oddly....and I don't know if this was on purpose or not, but the first time I checked my PSI on my Prius, having the original inflation from the dealership, the tires were WAY over inflated. I actually had to let air out to get down to 37/35.

But I guess my ultimate standard operation procedure, is pretty conservative. I'll go a cold PSI or two above the Toyota doorjam recommendation BUT...I just don't feel I want to take the risk of going too much higher. But that's just me.

 

You forgot to include the differences between road surfaces, temperature and tire construction.

On very rough road surfaces with cold temps and hard tires, I would not suggest running high pressure for max safety. Braking effectiveness is likely to suffer. Surface type will also determine how pressure affects braking traction. Pliability and tread/sidewall deformation are important considerations we must acknowledge.

There is a big difference between smooth test tracks in ideal weather conditions vs. real world conditions.

 

Do you have any empiric evidence for your claim? I showed mine. How about facts?

 

Bottom line....for safety; do not exceed manufactures cold tire PSI on the tire side wall, at regular highway speeds. Hyper-milers pump their tires much higher, but they drive much slower speeds, which is more safe; but they do it at their own RISK.

 

Sure but it will have to wait until I get home. There are several papers that look similar so it takes me a bit to find the data so I know which one to link. Unfortunately none of them used pressures over 38psi in their tests but the trend was telling.

 

For what it is worth.....

I ran my stock Yokohama's at 44 PSI fronts and 42 PSI rears (Max PSI rated was 44 PSI). Even tire wear for the life of the tires which for me was ~2.3 years and 51,000 miles (2/32 on the tread). No hint of edge wear and no hint of center wear.

Your mileage may vary.

 

I was thinking Costco used N2 to fill tires? Not to re-open the N2 topic for the umpteenth time, but that may help explain why Costco is a little less willing to "waste" N2.

 

In my opinion, Toyota has chosen the tire pressure that provides maximum comfort.
Some owners will have other biases.

 

OK, couple of things.

1. the higher tire pressure, the lower contact path, the higher pressure at the contact path, the higher friction during braking. Simple physics. This is why studies show no difference in stopping with different inflation pressures. Not everything that "seems right" is right.

2. the passenger tires are rated for max load at 35 PSI. That is the industry standard. It Toyota engeeneers decided that prius can max out the tire load at say 90% of the max for the given tire size and load rating, that means that are going to recommend inflation at 35 PSI minus 10% (near 32 PSI).

Now, the real question is why the tire industry rates max load of tires at measly 35 PSI that is not optimal for traction and performance?

Here is the clue:

from: Tire - Wikipedia, the free encyclopedia

 

N2 is really cheap. I think there is another reason.

My experience with Costco tire service was that they are an extremely by-the-book company. I quit buying tires from them when they wouldn't sell me a non-stock size tire tire for my compact pickup when the non-stock tire size I wanted:
1. Was a more optimum size for the factory rim width.
2. Had more load capacity than the stock size.
3. Would make the speedometer more accurate.
4. Would improve handling.