Correct Tire Pressure 3

[tomgranat]

I have OEM tires on my car with a 91 load index. The tire has a max inflation of 55psi. The car manufacturer only inflates to 44 psi. Can I assume that the load on the tire is (44/55)* rated load for a 91 load index?

I am getting new tires with same 91 load index however max inflation is 44 psi. Would I calculate the act load from the OEM tires and use this value to get the new inflation for the new tire (41 psi)?

Thanks for the help

 

[evelrod]

Sorry, my error---

165-70-13 Yokohama A032R(s)

That should read 165-70-10 Yoko's. My mini, "Rusty Booger" uses 10" Minilites.

Rod

 

[CapriRacer]

Norm said:

"....Can I infer from the above that a crude estimate of a tire spring rate curve could be made (given that you know something about the deflection at one load and pressure)? I should think that something simple that's still better than a flat "1500 lb/in" or "250 N/mm" exists...."

The problem is that getting to the part of the curve we generally work in (which is basically linear) requires going through the non-linear part at the low load part of the curve.

"....Wouldn't cantilevered construction tires demand slightly higher pressures than more conventional constructions, such that the conventional tire would want lower pressures relative to the cantilevereds?....."

Sorry, can't answer that. Cantilevered tires are a way around a rim width limitation rule. I just don't have experience with them.

"....I must be frequenting the "wrong" other automotive forums. Nearly every time that I've seen questions about tire:wheel width combinations outside the approved range, it's the other way around, with tires too wide for the wheel (example: 245/40 on a 7" wide wheel)...."

Yeah, I get that on other forums, too, and it's easy to counter with the "center of the tread wearing out". The ones I have trouble with are where they want to go wider and can't understand why the tire manufacturers won't allow it.

 

[damnaged]

CR,
From your response i get the understanding that tyre pressure do in fact increase when loaded, right? So when they say that tyre contact area equals to load on tyre divides by the tyre inflation pressure, how do i determine the tyre inflation pressure? is it manual check using gauge?? or can i just use the normal inflation pressure with assumption that the increase is very minimal and barely noticeable.

i hope my question is not spoiling the original purpose of this thread and thanks for the feedback.

 

[CapriRacer]

".....From your response i get the understanding that tyre pressure do in fact increase when loaded, right?...."

Compared to the unloaded state - yes, but the difference it is relatively small, and not worth trying to adjust for.

"....So when they say that tyre contact area equals the load on tyre divided by the tyre inflation pressure....."

No that is not correct. It is a common misconception because the units are the same - psi. What you get when you divide the load by the contact area is the average contact pressure.

"...how do I determine the tyre inflation pressure?...."

If you are asking how do you solve the puzzle when given 2 of the three bits of info, then the answer is you can't.

"....is it manual check using gauge??...."

Yes, you have to use some sort of measuring device

"....or can i just use the normal inflation pressure with assumption that the increase is very minimal and barely noticeable....."

As I said above, the difference in pressure between a loaded and unloaded tire is pretty small - ignore the difference.

 

[damnaged]

Thanks CR. I guess i need to continue my search to understand this further again. Thanks for your help.

 

[Knap]

These may be worth reading

http://www.sportcompactcarweb.com/tech/0208scc_tire_pressure_guide/index.html

http://www.toyo.com.au/Pit Lane.htm#Cold Inflation Pressures

I would assume with radials lower slip angle, less drag, and more added camber, which will effect spring rate.

 

[CapriRacer]

Great links, Knap.

But a word of caution: Setting tire pressures for a racetrack is a totally different exercise than tire pressures for the Mom's grocery getter.

 

[tireman9]

Good job CAPRIRACER of covering the complex topic of Regulatory and Association requirements of load and inflation.
Your long reply of Aug 15 should be required reading for anyone making a post on tire load or inflation.

Minor bit of info re "MAx inflation" on the sidewall.
In the past (about 2001 and earlier) the Max inflation on the sidewall of a P-Metric was usually 35 psi but as some vehicle mfg wanted to push the boundary of load capacity and others wanted increased max handling so wanted to run more than 35 not for load capability increase but for improved handling response for their specific vehicle. As a result T&RA changed the published guidelines. The load capacity of a tire did not change and the allowable load capacity did not increase as you inflated beyond 35 psi but the 44 psi max was placed as a safety item among others. Some "extra load" tires have 51 psi max and might have a higher inflation in the regulatory stamping "max xxx load at max XXX infl"

RE JADCOCK question on 35 vs 36 psi. Yes Chrysler was pushing the boundary by changing from a P-Metric size to a Metric size to gain the very minor load capacity increase because they did not want to spend the money or could not package a larger tire as the vehicle weight increased.

TO ALL Please go read the complete inofrmation on the sidewall of your tires, on the placard and in your owner's manual.

You should not run a lower inflation than the placard. You should not run a higher inflation than the max on the tire. You should adjust inflation if called for in the owner's manual or placard. If you change tires do not go with a lower speed rating ( S, T H etc) or lower load index ( 91, 85 101 etc). Always check your inflation with a good digital gauge at least once a month when the tires are cold (not driven on for at least 2 hours and not in sunlight).

37 yrs Tire Eng. Designed basic rain Firestone for CART. SCCA & IMSA Pro & Am. Set lap records at 6 different road courses in '89-91.

 

[Knap]

CapriRacer

Would be interested in your views on 30 year old handbook information, but would be off topic.

 

[CapriRacer]

Knap,

Why not start a new thread?