Nice find on the physics.
How tire pressure affects the shape of a car tire is the reason you don’t want to play with more than a couple of pounds either side of recommend.
You get definite over inflated or under inflated types of tire wear. more than a couple of pounds and you’ll either wear the center of the tire out with lots of tread left toward the outside, or more commonly because tires leak down naturally, wear out the outside and have plenty of tread left in the middle.
compensation curve? what are you talking about?
those standard tire pressure gauges you find at the check out in parts stores are not rocket science. I have several. They check out against each other. 2 pounds is not a small error and I totally disagree that 2 psi is totally meaningless. Get the pressure off by 4 or 5 pounds and you’ll have wear problems not to mention totally changing the way the car handles. 4 or 5 pounds to high and you’re in trouble in a panic stop in the rain. How do you get from that to 2 pounds being meaningless. I know lots of gear heads to whom 2 pounds is not meaningless and lots of drag racers that would roll their eyes if they heard someone say it.
Even drag racing stock motorcycles you let pressure down on the back to get more grip and a wider squat on the hole shot, and stock motorcycle racing where you are matched by class it is ALL about the hole shot. He who wins the first one hundred feet wins the race. From there a chimp can shift gears, although I’ve seen guys dumber than chimps. lol . (and you pump the front to what the max is) Compensating wrong for air temp and track temp by one half a pound can be the difference between standing it on the rear tire and smoking the rear with excessive wheel spin. And that is with a stock tire. 2 pounds would be a ridiculous error.
My driveway runs north - south, basically, discounting a few mild curves. I always back-in when parking our many cars.
Occasionally the sun comes out and occasionally it actually creates warmth above the 45th parallel! The driver’s sides of my vehicles get the southern exposure to the sun, while the passenger sides remain in shade. Tires being flat black certainly helps create a temperature differential between the southern exposure tires and those exposed to the north. When humidity is low the temperature differential is exacerbated. Tires on one side feel quite hot and on the other side quite cold. I have never checked the difference with my infra-red non-contact thermometer, but I will when I go to my northern location.
Along with temperature change comes pressure change in the tires sometimes to the point that I wait for early morning or evening dusk to check and adjust tire inflation pressure.
I recently drove 1,500 miles with my car pointed south on I-75 and it was quite sunny during the majority of the trip. Can you see where I’m going with this discussion, yet?
In addition to rolling friction heating my tires fairly evenly I know that our nearest star was creating a temperature and pressure differential between the 2 sides of my car. I felt a mild pull and the car a little lower on the one side. Going the other direction causes the pull to the opposite side as well as lowering the opposite side.
So, it would be good to know when checking and adjusting tires while planning a trip heading north or south on a sunny day what tire pressures to use on each side of the car. What formula could I use that would take into account the day of year, latitude, ambient temperature, and relative humidity?
Oh, and on a long trip where the temperatures, humidity, and road directions are likely to change, is there a way to compensate for that that would be quick and easy?
Thank you in advance.
CSA 
I spent 20+ years as a development engineer riding and testing cars for ride and handling for GM. I spent 17 years as an amateur road racer. I am as OCD about tire pressures as anyone here and likely worse.
When we’d ride cars, we would set all 4 tires to the same pressure before every ride. The sunny side of the car was nearly always 1 to 2 psi hotter and we let out the pressure to equalize with the other side. This was to remove every variable we could so we could concentrate on the springs and dampers. This would be done in winter months in warm places like Mesa, AZ because cars ride poorly when cold and we needed reasonably even temperatures. 2 psi and 20 degrees mattered.
In racing you are using all of the tractive capabilities of the tires and balance front-to-rear is vitally important. 1 psi matters. It is the difference between 1st place and 3rd place. Notice I didn’t say 1st versus 10th place.
Those 2 examples require precision in the setting of tire pressures. Driving your car does not. Most people are not sensitive enough to notice 2 psi nor do they push their car hard enough in wet conditions to exceed the tire’s capabilities. And even rarer is the driver that exceeds the tire’s capabilities in the dry. Besides, modern cars have mandated stability control and ABS to save your bacon when you inadvertently do exceed the limits.
All that said… 2 psi does not matter in the real world. If it does, you should slow the heck down before you kill someone!
We don’t have much of it locally here, but then there’s that pesky thing while driving distances, that thing called elevation change.
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=167
CSA
All of my cars have had 60% of the weight on the front tires. Recommended pressure always 32-33 psi all around. For the last 20 years or so, I’ve been using 34 psi on the rears and 39 psi on the fronts and have gotten remarkably even tire wear (rotating tires every 10-15K miles). I can’t remember the last time I had an alignment done.
Good point. So gauge pressure goes up by about 1 PSI for every 2000 ft gain in altitude, with that delta decreasing a bit for high altitudes. At 10k ft, a 4.6 PSI increase.
And goes up 1 PSI for every 10ºF increase in temperature.
You just wrote some gray stuff when I appreciate it. I had already looked at your profile and read about your experience. I assumed you’d be working most reliable voices on here
And it’s true that with abs it’s harder out bake yourself in rain. But I can tell the difference in ride quality with 2 lb difference especially at the back of the car. That’s because it’s lighter back there making rebound more noticeable and I have refused to spend he money to replace the rear struts.
Some of the city streets around me are about Baghdad bad. I should probably make a separate post about how the back of my car bangs and bounces around over bumps. I’ve been under the back of the car and can’t find anything wrong so I figure it’s the bad shocks banging up through the shock Tower. I don’t know, maybe I should take it more seriously than I have cuz it is something I can notice in the rain. I love to drive and pretty much always push things a bit. Pretty much any car I own I put on a cloverleaf in the wet to get a feel for where the limits are both in under-steer and drop throttle oversteer. I my old prizm I have now I was doing that in a left hander, hit a little bump and it’s a good thing my reflexes and hands are still quick because it steeped out on me.
BTW, I ended up with oversized tires on it. In the dry and conditions where it’s not rear sensitive it really grips. The only time I ever had a problem was when I had the trunk loaded heavy and felt it trying to bottom a bit. It was a one time deal and i just was careful.
I just though of something though and you’d probably be the guy to ask. When it comes to factory cars and motorcycles sometimes it’s not so smart to try and outsmart the engineers. Do you think the oversized tires have anything to do with the way the rear feels? I don’t think so because I’m talking about banging over rough streets and I know it’s not hitting the wheel wells. I looked for marks on the back tires long ago for that. What do you think?
I’ve had an 82 Honda Magna V-45 for ten years not. I’ve had it fairly close to show quality before but it’s hard to keep that up now that I don’t have a garage (divorce). It was just as fast as I thought it would be but I never thought it turned very well. I thought it was because I was comparing it to the H series Kawasaki’s and the KZ550 that were lighter. It turned like a damned truck. Had to force it down into turns and felt it was unpredictable like it had a tendency to try and stand me back up all the time. So I was careful with it because I felt like if I was cornering and the rear slipped in the least, with that tendency to want to stand up it was a recipe for a high side. . It had raised white letter Dunlops on it that needed replaced a couple of years ago. When I looked I realized the front tire was ten over stock size. I didn’t think much of it because I’ve seen guys mess with tire sizes before. But I was pissed to find Dunlop didn’t make the raised white letter tire in that size any more. I goolged my way around the world looking for one for about a year before I gave up and bought a set of Bridgestones for it.
I wasn’t off the Iron Pony parking lot before I noticed the difference in feel. It totally re-balanced and reinvented the way it handles. It now turns on a lean the way my light bikes had and I can about drag ass on sweepers now with a cushion. I’ve had a damn lot of fun with it since it no longer felt like I was forcing it down into the turns. Now what I have to worry about is my messed up totally replaced left shoulder that is weakened by a post surgery tear the doctor is refusing to fix. I not only worry about not being about to ride out a wiggle but the stress under hard braking, especially if I had to panic stop.
Maybe in reality 2 lb too hard might not make enough difference in the rain but how about 5 lb? The point is it’s hard to unlearn what you already know. If I was about to take a trip through a whole tank of gas I’d put a couple of pounds all the way around. I don’t know if you would do anything like that but let’s say you did. I have a feeling if you were on that trip and drove into a thunderstorm that extra 2 lbs would come to your mind. I’ll bet your mind would go there just out of habit because you’ve been racer.
You can “…tell the difference in ride quality with 2 lb difference especially at the back of the car.” You state, “I should probably make a separate post about how the back of my car bangs and bounces around over bumps.” “…so I figure it’s the bad shocks banging up through the shock Tower.” But, “…have refused to spend he money to replace the rear struts.”
You changed tire size from the size specified and are now, after that, wondering if it is causing handling a problem? “Do you think the oversized tires have anything to do with the way the rear feels? I don’t think so because I’m talking about banging over rough streets and I know it’s not hitting the wheel wells. I looked for marks on the back tires long ago for that.”
" I don’t know, maybe I should take it more seriously than I have cuz it is something I can notice in the rain. I love to drive and pretty much always push things a bit."
Prizm? Car needs proper diagnosis and repairs. Banging suspension on rough roads? Ride quality issues? Experimental over-sized tires? 2 pound tire pressure differentials? Pushing things a bit while driving?
Wow… Seriously? Are you serious? Is this a put-on?
CSA
When your tire pressure is around 7 psi (top fuel dragster), 2 pounds over or under is huge. When your tire pressure is around 120 psi, (Tour de France bicycle racing) two pounds over or under is practically meaningless.
That’s why it’s important to use a gauge that only measures from 0 to 10 psi for low pressure tires like ATVs and drag slicks. A tire gauge that goes up to 150 psi barely registers on a drag tire.
Well, considering they haven’t made Prizms since '02, it probably needs rear struts. The struts themselves may not be leaking but the strut mounts may be shot and the jounce bumpers on the strut rods may be dust by now. Quick struts aren’t that expensive for this car ('cause it is a Corolla in Geo badging) and are easy to replace. If the car drives straight both before and after replacement, you may not even need an alignment after the swap. If the rear tires don’t rub, they should be OK but the extra weight isn’t making your ride any better!
As for the bike… naturally narrow rims with wider tires can make get you into problems a lot quicker than with a car - as you found out!
As for 2 or 5 psi in the rain… driving at the limit is more about balance than absolute grip. If all 4 tires are 5 psi high, at least they will all give up a bit early. And if you are finding that point, slow down, you are going to hurt someone.
A higher pressure may actually be less likely to hydroplane. Since the tire has a smaller footprint, it takes a higher speed to hydroplane.
Also, hydroplaning in the rain is not a all-of-a-sudden all or nothing event, you can be 50% hydroplaning, with only the rear part of the tire’s footprint in contact with the pavement and still be able to steer, you just don’t have as much traction.
When I noticed that to turn the bike I had to not only lo lean but had to put push the outside handle bar I knew that bigger tire was playing a role. I was just surprised at how radical the difference was. I had never really expected my Magna to handle as good as it does now.
As far as hard tires in the rain I’m always thinking about what’s going to happen in panic stop or other emergency.
Did you mean the extra weight of the tire? They are a bit wider and a bit taller then the originals. Taller profile tires have more rebound than low profile tires right?. (less rebound meaning less cushioning). I haven’t thought about it in a long time. now you’re going to have me writing he size down and comparing it to the owners manual. lol
Thanks for the advice on the struts. I don’t know with my back and shoulder problems whether I’m going to be up to trying to do them myself in the parking lot of my apartment building but I think I will move forward and at least get parts. Any thoughts on brand?
It’s an issue that I’ve let go too long and honestly it’s a safety issue because of the way it acts in rain, especially heavy rain. It’s twitchy. I haven’t been responsible about it. I don’t want to get into the habit of thinking problems on my car or bike are ok even though I know there are situations where the will not be ok. The nature of driving is such that sooner or later something random is going to happen. It’s one thing to be aware of it while driving and compensate but it’s another to find yourself in an unplanned maneuver, hit a bump and have it kick out faster than it can be caught. And even if caught it could leave you pointed where you don’t want to go.
I’m sitting here imagining how that could go wrong. Driving along in the left lane on the freeway and for whatever reason you have to Swerve to your right to avoid something. You have a hole to your right but just that. So you turn and while in the turn you hit a bump. The bump kicks the rear out to the right in an overseer. Now you have to steer right and keep the front in front of the rear but that moves you to the right and you’ve got cars on your right. Nothing good can happen. The odds are better that you catch the slide and end up bumping door-to-door with the car on your right then they are if you let it come around thinking it will stay in the lane and you won’t get run over be from behind. Discipline to not fight the spin with a big correction is something that even race car drivers have trouble with. But if it’s not door to door and ends up being the front or rear of the car to your right it could cascade across the lanes. If it kicks out nothing good can happen.
Although I seem to remember Danny Sullivan did just that spinning down the short shoot at Indianapolis after Mario drove him down on the apron in turn one. The Front end pushed right in an under-steer as it came back across the painted line on track. That forced him to lift and when he did the front end bit the rear end looped around. But if you look at the wheels you see he didn’t fight the spin. He knew if he steered to the right to catch it he’d be in the wall. His only chance was to let it loop around and spin straight down the backshute and that’s exactly what it did. A single 360 Degrees spend leaving the nose pointed the right direction. He was lucky BUT he also left the steering wheel centered as it spun. I don’t think I could fight that instinct. Were prone to thunderstorms in May. I’d better get it fixed.
I always make it a point to vacuum the glove-box and change the air,(Nitrogen), in my tires every spring and fall. I also rotate the floor mats if I can remember it. I usually 2psi extra in my higher profile tires in an effort to get a bit better mpg.