Speak EV - Electric Car Forums banner

81 - 89 of 89 Posts

·
Registered
Joined
·
450 Posts
Counter-intuitively, if the humidity of the atmosphere goes up, the density drops but the pressure increases (water vapour is lighter than air, but more mass overall).

... discuss ...
Forget about it:)
TPW ( total precipitable water ) is of the order of an inch or so in the uk. Compare the weight per unit area of that to 14.7 psi..
 

·
Registered
Joined
·
11,339 Posts
You said the atmospheric pressure doesn’t change. But it does with height.

Hence needing to adjust tyre pressure going up a hill.
 

·
I'm not crazy, the attack has begun.
Joined
·
23,354 Posts
You said the atmospheric pressure doesn’t change. But it does with height.

Hence needing to adjust tyre pressure going up a hill.
I didn't say it didn't change with height. I said it didn't change for a given area. Give me two areas you might get two answers.

No, do not adjust your tyre pressures on the move. Once you get to the top of the hill and you are going to stay there, maybe. But on the move you'll be surprised how high your tyre pressures get as an 'instantaneous' pressure due to them heating up. TPMS modify the tyre pressure reading according to tyre temperature, it is not a true gauge pressure.

Try it if you don't believe me. Come off the motorway after 50 miles and immediately measure your tyre pressures. Tell me what you find.
 

·
Registered
Joined
·
11,339 Posts
Fine if you never go anywhere in your car. The context of this thread is driving somewhere in different weathers and temperatures.

You need to think about the problem in context.
 

·
I'm not crazy, the attack has begun.
Joined
·
23,354 Posts
Fine if you never go anywhere in your car. The context of this thread is driving somewhere in different weathers and temperatures.

You need to think about the problem in context.
I am not sure I am discussing the thread topic any more ... what was it? ;)
 

·
Registered
Joined
·
11,339 Posts
I am not sure I am discussing the thread topic any more ... what was it? ;)
You were driving to the top of a mountain without adjusting your tyre pressures. They could be 20% overinflated.

I’ve probably saved your life.
 

·
Registered
Joined
·
3,441 Posts
As a semi-relevant comment. A tyre should expand as you go up a mountain. This 'square balloon' experiment proves that.

Expand ? Very slightly, but probably not visibly.

A tyre is pumped up to say 36psi, if you climbed a mountain and atmospheric pressured dropped from 14psi to 7psi it would be similar to over-inflating the tyre by 7 psi. Would you see an obvious visible difference in the shape looking at a tyre pumped up to 43psi instead of 36psi ? Only if you were looking closely...

A tyre is not a soft elastic vessel like a balloon with no strength of its own it's actually a very strong container with very high tensile strength. After all it has steel belts throughout the rubber to add strength as rubber alone would not but up to the task...

The sidewalls collapse without air because they have a low compression strength but when you inflate it sufficiently the load on the sidewalls from the weight of the car becomes a tensile load instead - further inflation beyond that point just increases the tensile load.

This is the same reason why you don't need to suddenly double the pressure in the rear tyres of an estate from 36psi to 72psi to maintain correct tyre shape when you double the load the car is carrying (including itself) - the needed pressure increase is only about 5psi or so. Most of the strength to support the weight of the car comes from the tyre carcass with the air pressure primarily converting what would be a compression load into a tensile load.
 

·
Registered
Joined
·
3,441 Posts
No, do not adjust your tyre pressures on the move. Once you get to the top of the hill and you are going to stay there, maybe. But on the move you'll be surprised how high your tyre pressures get as an 'instantaneous' pressure due to them heating up. TPMS modify the tyre pressure reading according to tyre temperature, it is not a true gauge pressure.
Interesting that TPM systems which display pressures on the dashboard display "temperature corrected" readings. I hadn't really thought about that but it makes sense from a user interface perspective that you don't want people to freak out when they see the reading climbing while driving as the tyres heat up.
Try it if you don't believe me. Come off the motorway after 50 miles and immediately measure your tyre pressures. Tell me what you find.
Yep, done that before many times. I suspect a few on this thread have not and therefore don't realise just how much the pressure varies with a driving session independent of seasonal changes.

Even if you're pootling around in the city you're going to see a rise of about 2psi between start and end of a journey - get on the motorway or drive spiritedly and you're going to get a rise of as much as 4-6psi maybe even more.
 

·
Registered
Joined
·
5,672 Posts
Expand? Very slightly, but probably not visibly.
Correct. Expand but imperceptibly.

Back in the late 70's I was made painfully aware of air pressure drop at altitude when I drove from Nice to Val D'Isere in one of the first BMW cars to be fitted with injection. Near to the top of the climb the car ran very raggedly. I pulled into a lay-by to check. I found three other cars there, all with the hood lifted - all injection system cars. We concluded that these cars had early injection systems that couldn't cope with the reduction in O2 and we were running on a rich mixture. We just had to limp along at low revs. Later controls solved this metering issue of course.
 
81 - 89 of 89 Posts
Top