Another Tire Pressure Question! Front-Rear Considerations

[CapriRacer]

It is my impression that lower rear tire pressures will cause the rear to swing out further and that's a bad thing. - BUT - I suspect this phenomenon depends on the particular maneuver. I don't think contact patch size enters into the equation when we are talking about transient responses.

Frankly, I don't think cars as old as yours matter so much about precise inflation pressures. The bushings are probably loose and that likely has more effect that small changes in inflation pressure. The only thing would be the direction. Going towards more oversteer is likely to lead to more issues.

[RidingOnRailz]

Originally Posted by CapriRacer

It is my impression that lower rear tire
pressures will cause the rear to swing out further and that's a bad thing.
- BUT - I suspect this phenomenon depends on the particular maneuver.
I don't think contact patch size enters into the equation when we are
talking about transient responses.

Frankly, I don't think cars as old as yours matter so much about precise
inflation pressures. The bushings are probably loose and that likely has
more effect that small changes in inflation pressure. The only thing
would be the direction. Going towards more oversteer is likely to lead to more issues.

1. "Transient response" - Such as swerving around a parked car door suddenly swung open into a narrow right-of-way?

2. Worn Bushings - I know what I'll hear even from taking an 80,000mile car to even three of my most trusted local mechanics: "Come back when you're over 150,000mi!"

What are signs of advanced wear in bushings - from a behind the wheel perspective?

[CapriRacer]

Originally Posted by RidingOnRailz

1. "Transient response" - Such as swerving around a parked car door suddenly swung open into a narrow right-of-way?

2. Worn Bushings - I know what I'll hear even from taking an 80,000mile car to even three of my most trusted local mechanics: "Come back when you're over 150,000mi!"

What are signs of advanced wear in bushings - from a behind the wheel perspective?

I'm an amateur mechanic, but worn bushings can have quite a few symptoms - noise, banging, sloppiness, and the like.

[RidingOnRailz]

Originally Posted by CapriRacer

I'm an amateur mechanic, but worn bushings
can have quite a few symptoms - noise, banging,
sloppiness, and the like.

I haven't noticed much of those, and my steering on this car is still quite direct, especially with higher pressures than specified by Honda.


I just found this, Capri:

https://www.rimex.com/images/TyreSen...n-pressure.pdf

What does it mean?

If I set my tire pressures to 32psi on a given morning, is their actual pressure lower than what's indicated on the gauge?

By the reasoning and examples in this article, I should, depending on morning ambient temperature, be setting my tires to between 34-36psi(via gauge) to get true 32psi.

What?!

[CapriRacer]

Originally Posted by RidingOnRailz

I haven't noticed much of those, and my steering on this car is still quite direct, especially with higher pressures than specified by Honda.


I just found this, Capri:

https://www.rimex.com/images/TyreSen...n-pressure.pdf

What does it mean?

If I set my tire pressures to 32psi on a given morning, is their actual pressure lower than what's indicated on the gauge?

By the reasoning and examples in this article, I should, depending on morning ambient temperature, be setting my tires to between 34-36psi(via gauge) to get true 32psi.

What?!

First, Rimex is selling a device, but they have taking some liberties with the truth to convince people to buy that device.

The first concerns "Cold Pressure". They define it differently than all the tire manufacturers define it. "Cold pressure" is the pressure in a tire when the tire hasn't been operated for some time - that is "cold".

The fact that if you set the cold pressures when the ambient temperature is - say - 50° F, then later measure the pressure when the ambient temperature is 90° F, you get a different pressure reading - is just an unfortunate fact of Physics. Rimex is trying claim that their device works off a different law of Physics - that the proper pressure in a tire is defined as occurring at a particular set of temperature and atmospherically pressure conditions - which oddly enough are different that STP (Standard Temperature and Pressure) as defined in Chemistry.

That is incorrect - and I suspect they are deliberately doing this incorrectly because their device would require more complication to do correctly.

So while there is a lot of correct physics in the article, their starting premise is incorrect.

[RidingOnRailz]

Originally Posted by CapriRacer

First, Rimex is selling a device, but they have taking some liberties with the truth to convince people to buy that device.

The first concerns "Cold Pressure". They define it differently than all the tire manufacturers define it. "Cold pressure" is the pressure in a tire when the tire hasn't been operated for some time - that is "cold".

The fact that if you set the cold pressures when the ambient temperature is - say - 50° F, then later measure the pressure when the ambient temperature is 90° F, you get a different pressure reading - is just an unfortunate fact of Physics. Rimex is trying claim that their device works off a different law of Physics - that the proper pressure in a tire is defined as occurring at a particular set of temperature and atmospherically pressure conditions - which oddly enough are different that STP (Standard Temperature and Pressure) as defined in Chemistry.

That is incorrect - and I suspect they are deliberately doing this incorrectly because their device would require more complication to do correctly.

So while there is a lot of correct physics in the article, their starting premise is incorrect.

Thanks for clarifying that. Longacre Racing also sells a "temperature-compensating gauge" similar to what Rimex is hawking.

I think the premise is that if you want, IE: 32psi when you are driving, the gauge will indicate a compensating pressure based on current ambient F/C.

IE: With an outside ambient of F°40, the gauge will recommend 28-30psi. About 5-10 minutes of driving, and your tires will peak out around your goal - 32psi. Something which I suspect is more important for competition, not every day street driving.

Let me know if I'm on the right path with that, or totally out to lunch, lol!

I'm simpler than that: I use a gauge that indicates actual PSI, and adjust my tires to desired pressure cold - just before sunup, car not driven since yesterday. That's my definition of 'cold' pressures.

[CapriRacer]

One of the things that has always bothered me about "Hot Pressures" is that if you start at 40°F, you get a different operating temperature than if you start at 90°F (with the same "cold" inflation pressure". So I can see why some would want to use a standard temperature and pressure reference when setting pressures.

But the problem is everyone focuses in on the air chamber pressure/temperature, when it's the temperature at the edge of the belt that is the important temperature. This is something that could be determined, but I don't think anyone has sat down and done the homework to actually quantify how this actually works.

[RidingOnRailz]

Originally Posted by CapriRacer

One of the things that has always bothered me about
"Hot Pressures" is that if you start at 40°F, you get a different
operating temperature than if you start at 90°F (with the same
"cold" inflation pressure". So I can see why some would want to
use a standard temperature and pressure reference when setting
pressures.

But the problem is everyone focuses in on the air chamber pressure/
temperature, when it's the temperature at the edge of the belt that
is the important temperature. This is something that could be
determined, but I don't think anyone has sat down and done the
homework to actually quantify how this actually works.

Re: Operating temperatures

I'm sure it take longer for a tire to heat up when driving during lower ambient temperatures. I guess that's a factor these temperature-compensating gauges takes into consideration when it reads you a pressure. Confusing as heck to me!

I just make sure my tires read, in my specific case, 32psi first thing in the morning, be it 20°F or 80F outside.