Tire pressure formula

[Shannow]

RiceCakes,...

with qualifcations that extend to ubermod of the world's biggest forums, how do you know more than the engineers who designed your car ?

You blatantly say that you are experiencing a dramatic loss in traction, which is just plain stupid when you are interacting with other members of the public...particularly in a liability sense, when you've so much as explained same to the world.

Your own time on a race track, knock yourself out...public roads, irresponsible in the extreme.

 

[RiceCake]

Originally Posted By: Shannow
RiceCakes,...

with qualifcations that extend to ubermod of the world's biggest forums, how do you know more than the engineers who designed your car ?

You blatantly say that you are experiencing a dramatic loss in traction, which is just plain stupid when you are interacting with other members of the public...particularly in a liability sense, when you've so much as explained same to the world.

Your own time on a race track, knock yourself out...public roads, irresponsible in the extreme.


Thanks Shannow, I'll give your advice all the consideration it deserves.

 

[Boss302fan]

If you set them at the tire max sidewall pressures, you have a very good chance to ruin your suspension components... Hit a pot hole and say bye bye to your wheel. I would only go 2-3 psi higher then what the auto manufacture vehicle placard states.

 

[Hokiefyd]

Originally Posted By: RiceCake
Its a bad yardstick for inflation without judging a number of other factors into your decision, most notably the often dramatic levels in reduction of grip.


So how do you personally overcome the "dramatic levels in reduction of grip"? Physics that apply to "other drivers" apply to us all.

 

[johnsmellsalot]

I agree with this formula 100% especially if you check your tires in a garage. I usually check my tires every sunday morning before the sun shines on them. I use the recommended pressure. I put in exactly what the sticker says.
Originally Posted By: randomhero439
I like my tire pressures a bit higher than the stock pressure recommendations. The first of every month I set my tire pressures. Ive been using this formula on my cars, and thought I would share it with BITOG. This works best at night when there is no sun and the temperature has stabilized. Never go over the PSI posted on the side of your tire wall. The monthly record low temps can be found on the Weather Channel website.

(Current temperature - Record low temperature) / 10 + Specified tire pressure

Here is an example of my tire pressure for January:
(17 - (-20)) / 10 + 32 = 35.7psi rounded up to 36psi

For people who set their pressures very high to increase fuel economy and other things:
(Current temperature - Record high temperature) / 10 + Max tire pressure(the number on the side wall)

 

[RiceCake]

Originally Posted By: Hokiefyd
Originally Posted By: RiceCake
Its a bad yardstick for inflation without judging a number of other factors into your decision, most notably the often dramatic levels in reduction of grip.


So how do you personally overcome the "dramatic levels in reduction of grip"? Physics that apply to "other drivers" apply to us all.


Try not reading things into what I said; my car doesn't have dramatically reduced levels of grip by any means. Inflating the tire to the sidewall max PSI without judging a number of different things could result in a tire that is substantially harder then the car is designed for, which could cause dramatic shifts in the grip profile of the vehicle. A small, light car with essentially zero camber might have narrow tires with an acceptable pressure range upwards of 50 PSI. Running 50 on a vehicle like that would make for a "dramatic change", and likely a knife-edge point where loss of grip is rather sudden and difficult to control if not anticipated. Geo Trackers would be a fantastic example of this, as even stock tire pressures coupled with rear-wheel drive and a terrifically short wheelbase make it love to completely 180 when you lose traction on snow/ice. Overinflating those tires would be heinously crazy.

On the other hand, a heavier vehicle running some amount of negative camber might have fatter, heavier tires that only max out at 44 PSI. Okay; 44 PSI is still a lot, but its less then 50 (or over 50), the tires are wider, stanced differently, and the car is heavier. Couple that with being front-wheel drive and a long wheelbase, as is my car, and the only real effect on grip is easily manageable understeer, and even that only happens when you're caning it around a corner at a speed way beyond what you should be entering it at. The back end is nearly impossible to get loose in any fashion but on sheer ice; but ice is ice, its essentially gripless without studs anyhow.

 

[Shannow]

You said that your car was "looser", then claim understeer and an immovable rear end.

Which is it exactly ?

 

[SteveSRT8]

Unfotunately, I have yet to see a FWD vehicle without a pronounced front end weight bias.

Most of them will 'snap spin' in an instant under the right conditions. It can be quite surprising. Sure hope it never happens to anyone I know hre...

 

[RiceCake]

Originally Posted By: SteveSRT8
Unfotunately, I have yet to see a FWD vehicle without a pronounced front end weight bias.

Most of them will 'snap spin' in an instant under the right conditions. It can be quite surprising. Sure hope it never happens to anyone I know hre...


Hard to say, under normal driving conditions I've never experienced that. If you flick the back end though and find traction in the front (front on snow, back on ice maybe) the back end can quickly become the front in a smooth graceful curve if you let it. I'm sure most people would lock the brakes in that situation which would exasperate it heavily.

Situation depending though, you can use the throttle to pull the front end around before that ever happens. Again, thats my vehicle, ones with shorter wheelbases or different configurations might be way less forgiving.

It all depends on the vehicle. At the end of the day though the difference in handling and performance I've experienced is never going to cause any level of problem to be concerned with. Would I suggest everyone do this? No. Does it change how the car grips? Sure. Performance on rain-pooled asphalt and performance in snow is fantastic with really hard tires. Performance in other places is effected, be it different or possibly negatively. Loss of grip is more sudden with harder tires.

Does it effect handling more then bad suspension components, bad alignments, bad tires, worn tires, seriously underinflated tires? Absolutely Not and people drive around with that all the time. Of course though, when you're on the internet, bored in Australia and have no idea what I drive or the conditions I drive it in and have pride issues because their internet engineering degree doesn't get respected, I suppose I would try to blame every act of vehicular negligence on someone running 14 extra PSI.

 

[SteveSRT8]

No offense, but my daughter had "never experienced" this phenomena before either. Then one day she over cooked into a corner and hit her brakes late. Her car (FWD Pontiac) spun so fast she could do NOTHING and ended up in the weeds on the other side of the road!

Dry roads, nice tires, and a competent driver who simply does not damage cars.

On the road course my Neon R/T could be rotated perfectly by trail braking, there is usually MUCH LESS weight on the rear end of the typical Wrong Wheel Drive car. Then add that their polar moment of inertia is typically near the front end, this makes the spin particularly violent.

 

[CapriRacer]

RiceCakes,

I'm going to suggest that your picture of what a tire does or doesn't do isn't quite in sync with reality. You need to get recalibrated. Reading all your posts in this thread has revealed some mis-conceptions - enough that a single posting is not enough to correct them all.

I'll just do one so you can see where I'm coming from:

Originally Posted By: RiceCake
.....Mine say nothing about weight.......


Sorry, but the maximum load carrying capacity is required by law to be written on the sidewall of all tires. Somewhere on the tire it will say something about weight.

I see you are in Canada - and the Canadian regulations are very similar to the US regulations. They both require a maximum load and an inflation pressure.

The regulations in other parts of the world may differ, but it seems universal that both a max load and a pressure are required regardless of the country. The tire manufacturer I work for doesn't produce any tires without both. I'm sure that is true for other tire manufacturers as well.

***************************

And since this thought was followed up immediately by another mis-conception, I'll document that one as well.

Originally Posted By: RiceCake
........Besides, there's nothing wrong with running a tire overpressure (but not beyond the maximum specification).....


I am hoping you mean the tire won't explode on its own, but the way this is worded.........Well, let me put it like this:

If a vehicle manufacturer specifies 30 psi - and 45 psi is used, that's a 50% increase. That means the footprint is going to be much, much smaller. That shouid have a big affect on traction.

**************************

See what I mean?

 

[cp3]

Wouldn't it be easier to just add 1 pound of pressure for every expected 10 degree drop in temperature? Isn't that basically what this calculation is doing?

 

[RiceCake]

Originally Posted By: SteveSRT8
Then one day she over cooked into a corner and hit her brakes late. [...] a competent driver who simply does not damage cars.


Causing snap oversteer by entering a corner too violently, braking causing weight transfer to plant the front tires, and the rear end breaking free to pendulum around rapidly could happen regardless of tire pressure. You're trying to tell me that harder tires will guarantee this always happens, or you at least are inferring that the difference of 14 PSI will mean, on a car you don't know how its setup at all, will somehow be exponentially more likely to have this happen. No. I wouldn't want to drive a vehicle like that anyhow.

Quote:
On the road course my Neon R/T could be rotated perfectly by trail braking, there is usually MUCH LESS weight on the rear end of the typical Wrong Wheel Drive car. Then add that their polar moment of inertia is typically near the front end, this makes the spin particularly violent.


Well, again, of course. That's why you don't drive like that on regular roads. But you've admitted it can happen even with regular tire inflation so whats your point?

Originally Posted By: CapriRacer
If a vehicle manufacturer specifies 30 psi - and 45 psi is used, that's a 50% increase. That means the footprint is going to be much, much smaller. That shouid have a big affect on traction. See what I mean?


Yes, you obviously haven't read a thing I've posted then, because time and time and time and time again I've had to tell people; yes it will effect traction. Yes, it can effect it a lot. No, it depends on the vehicle and how the entire rest of the suspension is setup.

But feel free to try to blame all road accidents on me, I've never crashed once in over a decade, so I must be wrong according to internet experts who have plenty of opinions they've never tested out.