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

NASA and other aviation sources, linked in several previous threads, disagree. Those tests showed hydroplaning resistance improving with higher pressure, without limit. I'm not recalling any substantiated claims to the contrary.

 

Agreed.

 

Here are a couple of papers on the subject of inflation pressures and their effects on vehicle braking distance.

This is the most commonly cited paper from NHTSA
http://nhthqnlas188.nhtsa.dot.gov/DOT/NHTSA/NRD/Multimedia/PDFs/VRTC/ca/capubs/tireinflationpressure.pdf

In it they tested pressures from 17psi to a high of 35psi. Despite some interesting data regarding wet braking, the trend for friction coefficient goes up with rising pressure up to 35psi. Unfortunately, they do not test higher pressures to see when this trend levels off or decreases.

These two papers do test higher pressures and they show an increase in stopping distance at pressures higher than nominal.

This paper tested a high pressure of 43psi.
http://www.transportproblems.polsl.pl/pl/Archiwum/2008/zeszyt1/2008t3z1_12.pdf

This one is odd but i'll post it anyway.
Tire Inflation Pressure Influence on a Vehicle Stopping Distances

 

Thanks for posting the links. I was familiar with the first study, but its irrelevant, as there is no overinflation in that experiment.

The other two are more interesting as they use more modern equipment with ABS (unlike the old study I linked that used a research trailer).

The Slovakian study has one glaring problem that the experiments were not well controlled for speed and the overinflated experiment had higher starting speed. I'm not sure how they didn't catch that (the editors of that journal are idiots). Furthermore, there is a huge range of results in the experiments and the authors didn't bother to calculate p value. This study is science fair grade and not quite fully scientific.

The Polish study seems to be more valid the best I can determine. I wish they did more than 2 measurements in each experiment, but at least the results are closely clustered (especially for dry experiments). I wold also change pressure in all 4 wheels rather than just front wheels, but this probably wasn't a big deal.

The difference is not huge, but measurable. I guess ABS makes a difference here.

Anyone wants to do similar experiments in Prius? Torque app should be able to measure deceleration too, right?

 

I agree. This is where Barry's (CapriRacer) point about there being a woefully inadequate amount of data out there rings true.

We do know that with colder temps, braking distance is increased due to a reduction in pliability of tire compounds. I postulate that increased pressure will exacerbate this issue due to a reduction in tread block movement and ability to mold to surface imperfections. This becomes particularly important as speeds increase and temps drop. Still, I think this is only a bigger issue in wet conditions as many tests have shown that tread depth doesn't significantly affect dry braking performance.

We've had similar conversations about high tire pressure on rough surfaces and the possible negative effect on FE due a tire's inability to grip surface imperfections and creates minute skipping which wastes energy. Again, there are no good papers on this but it makes sense in theory. lol

I know you understand this point but I am posting it again for lurkers. It helps illustrate my point about tread deformation.

 

LOL, think about disk brakes, the higher pressure on pads, higher friction.

Or better (for tire analogy), think about a brick pulled across a rough surface. You can lay the brick flat, the pressure will be lower (higher surface). They you can stand up the brick tall and the pressure will increase. The total friction should be about the same. This why the change in tire contact path surface (by pressure change) should not impact total friction (braking efficiency). In theory, at least.

This also explained here: Friction

 

It is related to friction but some of that friction is inside the tire as it deforms and uses energy without giving it back. Think bending a steel rod vs. a giant rubber band. The rubber band will give some of the energy used to deform it back. The steel doesn't.

Hysteresis - Hysteresis - Wikipedia, the free encyclopedia

Rolling Resistance - Rolling resistance - Wikipedia, the free encyclopedia

 

Correct. When your tire pressure is low, it gets hotter. The hotter the tire, the better chance you have of a blowout just driving around on a smooth road with no additional external influences. However the higher the pressure, the smaller the event needed to cause a blowout when you add it other influences. When you ram an object into another object, one will win and one will lose. If you put a tire over a pot hole, the pot hole generally does not deform much. The tire hits the hole, a pressure point is created. In a highly pressurized tire, that is an event for a blow out or at least a flat where with a properly inflated tire the risk is less. There is a "sweet spot" for inflation to get around road hazards.

More damage from nails or any other road debris for exactly above. That's why.

It has been proven, see below, first quote from NTHSA article.

No I'm not a tire engineer. But in all frankness personal conversations are similarly meaningless. I do race vehicles, like on a track, on occasion and know first hand what they do. Add to that the physics of the multiple engineering degrees I have, and I feel I can make an educated guess. Terminology or actual reasons may be wrong, but the practicality and reality of the opinions and anecdotes expressed are valid and real. Perhaps other contributing factors lead to such conclusions, but the overall conclusion is a real world event tied, even if somewhat loosely, to tire pressure inflation.

Well the NHTSA disagree's with that statement. Don't even have to get past page 1 of the above article.

From NTHSA: http://www-nrd.nhtsa.dot.gov/Pubs/811681.pdf

Underinflated tires experience a greater amount
of sidewall flexion than properly inflated tires, resulting in decreased fuel economy, sluggish handling,
longer stopping distances, increased stress to tire components, and heat buildup that can lead to
catastrophic failure of the tire, such as cracking, component separation, or blowout. These catastrophic
failures can cause loss of vehicle control and may result in a crash. Overinflated tires may be more easily
damaged by potholes or debris. Severe overinflation may increase stopping distance due to reduced
area of road contact and non-optimal traction, and may also contribute to vehicle instability.

Your statement is false. If your tires are overinflated then decreasing pressure will increase grip. Only to a point though. Once you go too low, you are also screwed. Again a "sweet spot". Increasing grip in the front of a car will increase the tendency to oversteer. Increasing the grip in the rear will increase the tendency to understeer. Decreasing grip on the front wheels, of a front wheel drive vehicle, increases understeer. You are now asking the vehicle to deliver power through the same tires that steer the vehicle (assuming you do not have a 4-wheel steer vehicle like some early 90's Hondas and other imports) with less grip. The Prius already understeers heavily in corners at speed. Why? Because it is easy to correct. Almost all cars today are built to understeer. When someone who is not trained goes around a corner too fast with understeer, the car just doesn't turn. They freak out, and let off the gas, and all is well. Same scenario with oversteer means the back end slips out and now you are facing the wrong direction as you crash into a tree. It takes medium skill to correct from an oversteering event and not overcorrect.

You can "correct" for the increase in understeer in general with higher pressure tires, but running different pressures in the rear vs. the front. Run higher in the rear and lower in the front will correct for the understeer but nobody should do this unless they know what they are doing. Same reason why the "good tires" on a FWD car should be on the front. Grip in the front is best for 99% of the drivers out there because it is brainless.

And to add another dimension, with decreased grip and traction any road hazard like oil, water, gravel, etc will be exaggerated with overinflated tires. So you will understeer even more.

I drive my Prius like I stole it sometime, taking hairpin turns at 65mph or purposefully body rolling the car the opposite direction to the upcoming turn to fish out the back end. On the Prius, and any other car I have driven as such, this is just the way it works.

There has to be a limit. Again, that sweet spot.

The NHTSA says that hydroplaning is very much related to how much tire touches the road. They also say that at low speeds (< 50mph) an underinflated tire has more contact with the road. However at higher speeds, the amount in contact with the road decreases. Why, I am not 100% sure. But that matches my anecdotal evidence from actually driving cars.

From NHTSA: NPRM on TIRE PRESSURE MONITORING SYSTEM FMVSS No. 138

Skidding and/or loss of control from hydroplaning
The conditions that influence hydroplaning include speed, tire design, tread depth, water depth on the road, load on the tires, and inflation pressure. At low speeds (less than about 50 mph), if your tires are under-inflated, you actually have more tire touching the road. However, hydroplaning does not occur very often at speeds below 50 mph, unless there is deep water (usually standing water) on the road. As you get to about 55 mph and the water pressure going under the tire increases, an under-inflated tire has less pressure in it pushing down on the road and you have less tire-to-road contact than a properly inflated tire as the center portion of the tread gets lifted out of contact with the road. As speed increases to 70 mph and above and water depth increases due to a severe local storm with poor drainage, the under-inflated tire could lose 40 percent of the tire-to-road contact area compared to a properly inflated tire. The higher the speed (above 50 mph) and the more under-inflated the tire is, then the lower the tire-to-road contact and the higher is the chance of hydroplaning.
Tread depth has a substantial impact on the probability of hydroplaning. If you make a simplifying assumption that the water depth exceeds the capability of the tread design to remove water (which most likely would occur with very worn tires), then an approximation of the speed at which hydroplaning can occur can be estimated by the following formula:
Hydroplaning speed� = 10.35 x inflation pressure [25]
Under this assumption of water depth exceeding the capability of the tread design to remove water:

At 30 psi, hydroplaning could occur at 56.7 mph ​

At 25 psi, hydroplaning could occur at 51.8 mph​

At 20 psi, hydroplaning could occur at 46.3 mph.​

That makes sense. But now if contact makes a difference, there is no denying that at high pressures (purposefully left at "high" and not a quantitative number) the contact is reduced. Whether this is 50PSI or 90PSI, I do not know. There is an underwhelmingly small number of studies on overinflated tires. But keeping with "there exists a point in higher pressures where tire contact is reduced" there should also be a point in a graph of pressure versus possibility of hydroplaning somewhere that is higher than the probability of normal pressures. If I had to guess, I expect a trough shape. A steeper downwards slope from a higher probability to a lower probability as tire pressure goes from extreme underinflation to a normalized 32PSI. I would then expect a flat region where it just doesn't really matter. This region would depend more on the tire than the inflation. Some tires as mentioned above keep a really nice contact patch profile over dynamic speeds and higher pressures. The point at which they can no longer maintain that flatter profile would be the second inflection point in the graph.

I would then hypothesize that when you run at a pressure high enough to cause visible uneven wear on the tires where the centers of the tire are worn more than the outside closer to the sidewalls, you have left the linear/flat region of the contact patch size vs. inflation graph and are in the increased hydroplaning area.

On Priuschat.com, we have seen members on here that develop these wear patterns. Different tires, different pressures that were being run, same sort of wear. That also means that a gas station air pump could inflate passenger tires to a pressure in this region. Although I can only recall of this happening to people running at 50+ PSI. Anyone remember a lower PSI number for such uneven wear?

The only top-of-my-head though on why perhaps this doesn't matter for hydroplaning is perhaps the extra pressure acts like a knife similar to icy driving. The 2 theories on ice driving are to:
(a) decrease your tire pressure (like to teens) so that you have maximum tire tread touching the ice, and more probability of something gripping
and
(b) increase your tire pressure so much that the pressure of your car is applied to a smaller area physically breaking through the ice and hopefully gripping into some extra snow or road below it.

Both work. But if we think of (b), then perhaps in hydroplaning the same sort of effect happens. No buildup of water underneath the tire because it is "cutting through" the water? If that is the case, then that second inflection point may never happen... I have no idea, but it seems like it might make sense.

 

I have no facts, but I have a theory. First look at another over done inflation Diagram.


Now lets think about spinning the tire at a zillion RPMs, The over inflated diagram can't change, it can't flex any more. The normal diagram will become a little more like over inflated, but not very much as it can't flex much. the under inflated tire hove ever can flex a lot and become even more like the over inflated pattern than the over inflated tire itself.

Lets watch drag slicks change shape

They become taller and thinner at high RPMs. (They run 7 PSI

Wide Tires
)

I suspect this is what is happening (much less dramatically) over 50 MPH.

Or maybe I am off base.

 

That was true with the old, cross-bias tires. Today's radials have a belt that in effect has no 'bow' function. On over inflation, the tire contact area is still normal and because of the higher psi in contact with the earth, grips. Better still, it 'pushes through' water to reduce hydroplaning.

Under inflation is still bad because of the higher heat and moving more of the weight to margins. This can lead to scalloping.

The tire rolling drag comes from hysteresis losses from flexing the tread and to a lessor extent, the sidewall. Higher pressure reduces the size of the contact patch and thus the amount of rubber subject to 'flex.' From the papers I've read, ~2/3 is lost in tread flex and the other 1/3 in sidewall and some interesting vibration modes (we'll save that for another day. <grins>)

Sad to say, the Sumitomo T4s I'm fond of are not longer rated at 51 psi, maximum sidewall. But they are like steel wheels in rolling resistance and running cool. At the house I've got some thermal graphs I generated several years ago testing the effect of tire-pressure on tread temperatures:

• higher pressure - moderated camber and toe errors . . . at pressures approaching the door jam, I could see in the heat pattern the numbers from the last wheel alignment. But at high temperatures, temperatures were even across all five treads.
• edge wear - high pressure tires take curves like nobody's business preserving momentum. But curves a 'Bobs Prius Speed' do lead to a little more edge wear (not excessive) compared to the center. The wear is most pronounced on the outside, not the inside tread.
• four-wheel alignment - doesn't do much for rolling resistance but it sure makes the tires wear even!

If you want to get the full miles out of a set of tires, keep the tire pressure up, highest tolerable being best. Top it off every two months . . . making one of those Harbor Freight tire inflation units a very cost-effective solution. Unless you have a reliable, free-air, the Harbor Freight pays for itself the first year and five years later when you get another year of tire service.

Bob Wilson

 

I guess your definition of what is scientifically proven is different from mine. I looked at that statement in the paper you provided and that claim was not referenced or supported by their study. It was clearly authors believes only. They also wrote (in the summary):

So much for science.

The studies F8L provided were more enlightening on the subject. Based on those, I decided one should not exceed 44 PSI (0.3 MPa) and probably stay closer to 40 PSI to be safe. This is what I have been doing so far in my cars.

 

Pump up your tires and go drive around a track. You will feel a difference.

 

When I bought my car I didn't realise that it had been sitting on the lot for almost 9 months. The tyres may have squared off a little as the ride was a little notchy. (Changing from a hydropneumatic Citroën C5 to a normal car doesn't help!) When I got a wheel alignment and mentioned the sensation the tyre guy pumped my tyres up from 38/36 to 50/50 and told me to drive on the nearby freeway at 100-110km/h (60-70mph) for 5-10 minutes and then deflate the tyres to "normal". That seemed to smooth things out.
I now run 47/45, which is probably a tad too high. And the woeful Ovation ecovision VI-682 tyres currently on the front apparently have a max pressure of 44psi.
I don't mean to sound reckless, but it shows me that tyres already have a high margin of safety for pressure.

The only instance when I would reduce tyre pressure is driving on sand or similar. For anything hard like potholes, I'd rather have too much pressure than too little. I can cop a mark on the outside of the tyre but I can't handle a bent rim and pinch-flat blowout.

Until the recent standardisation of national road rules in Australia, South Australia had a fine for over-inflation of tyres, but not under-inflation. I have no idea why either.

 

The side wall pressure indicated on the tire is 1/2 the bursting pressure, NYS (new york state) has outlawed electric bicycles on any road in the state, I find that ridiculous, but over inflating and getting a ticket, hold it, you guy's already drive on the wrong side of the road

 

Do you have any source for this? It contradicts claims, unfortunately also poorly- and un-documented, of significantly higher burst pressures.

 

I'm running with 37 psi on my Avid OEM tires. Seems to work out fine. The dealer put them at 32 PSI and my MPG dropped 5 immediately

 

There are several things that are kind of touched on in this thread but I want to point out.
1. Tire pressure is always intended to be measured when the tires are cold. The tire rating is based upon the cold air pressure.
2. Automobiles continue to specify tires pressures around the 35 PSI range, however many tires are rated at 44 PSI. I have found that when tires rated at 44 PSI and are only inflated to 35 PSI they tend to wear the outside edge significantly more than in the center due to being inflated to a lower pressure than they were designed for. Tires that are wearing the outside edge more than the center probably aren't providing the optimal traction across the tire surface either.
3. The flip side is if the suspension system was designed for tires inflated to 35 PSI and you inflate them higher then the tires are less compressible which could cause more wear and tear on the suspension system right down to the wheel bearings. Thankfully I am going to trust that Toyota makes better than average cars so it should be able to handle a little higher pressure.

So how does this apply to what I have experience with my Prius? When I purchased my 2010 it had about 76,000 miles. I noticed the steering seemed a bit flat, non-responsive. I measured the tire pressure and found the front to be 36 PSI and rear 34 PSI with the outer edge more worn than the center tread. I also noticed the tires are rated at 44 PSI max. To compensate for the uneven wear pattern I increased the pressure to 42 front and 40 PSI in the back. I immediately felt like the steering is more responsive with more road feel.

I do think its important to take into account both the car manufacture's and tire manufacture's intent and strike a compromise between them.

Now if your tires say their max cold pressure rating is 35 PSI then your problem is solved, use 35 PSI.