Tire pressure v load, and deflection 1

[mattatdeneb]

Hi All,

I need some data which describes (a graph/table of) correct pressure vs load for common vehicle tires. Ideally, I'd also like data about how much the tire deflects under the given load/pressure combination.

Thanks in advance,

mattatdeneb

 

[tireman9]

A is the aspect ratio i.e. 75 or 60

H is the section height ( OD - rim dia)/2

Sd is calculated.

For S70 you can use the nominal section width i.e. 235 or 205 etc.

It may be easier to develop a formula where P is the variable for your current size tire.
With 26 psi as the minimum and 35 as the max for a P-Metric you can then calculate the theoretical load at each psi. Please remember the figure you get will probably not be an exact match due to the various rounding requirements.

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

 

[NormPeterson]

Thanks, tireman. I have a couple of additional questions.

How would K (and perhaps other variables) vary for aspect ratios below 50?

Is there a computation for estimating the radial/vertical tire spring rate that's possibly related to this one for load at inflation for tire size?

Norm

 

[tireman9]

As I pointed out in my earlier post, the book "Engineering Design Guide" is about 200 pages so I really can't go into all the details of the different formula for different size or application tires. K does change as well as other parts of the equasion for lower aspect ratio tires.

In general passenger loads are based on a relatively constant deflection. It might be possible, with sufficiently accurate measurements to set loads and inflations to hold to a constant but that would ignore the desirable reserve load which is normally in the 10 - 15% or greater range.

Don't forget that inflation changes with the weather and operating conditions and time so unlike most strength calculations you would need to include both a T and t function if you wanted that level of accuracy.

The reserve etc also addresses the fact that roads are not smooth and real life tire loading is dynamic ( cornering, accel & braking and aerodynamic ) so you should never be right at the max calculated load for a given infl.

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.

 

[zeusfaber]

Checking my own understanding here:

Working the formula in the FAQ backwards, or looking in the table in (eg) ISO 4000 gives what I think must be the *minimum permissible* inflation pressure for a given tyre loading in order to avoid eventual tyre damage.

Would I be right to assume that designers generally aim to keep recommended pressures down near (but not below) that limit, and that pushing pressures much higher than the minimum tends not to be good for handling?

A.

 

[ccw]

Hi zeusfaber,

Be careful!---minimum inflation pressures, ie. a soft tire for the load, is bad, from the standpoint of:

1. wear
2. hydroplane resistance (see sister thread on hydroplane)
3. heat buildup
4. handling
5. efficiency

You can go back to the web and research the Firestone/Bridgestone/SUV problems a few years back to see bad results of running minimum inflation.

I would say use the nominal for the load cold.

 

[tireman9]

To Zeusfaber
No, Vehicle engineers do not try and keep inflations at the minimum. Also you have the handling response backward.
Higher inflation is better for response and increases cornering force.
The vehicle recomended inflation is a compromise between soft ride i.e. low inflation vs good fuel economy and improved handling. There are also about 20 other trade-offs that are considered. Most cars come with about a 10 to 20% "reserve load" but some, like Ford on their Explorer decided to specify 0% reserve load.



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.

 

[tireman9]

To CCW
Not sure what you mean by "Nominal".
In years past ( 70's) with bias tires "Nominal" meant 26 psi.

The proper inflation should never be lower than that specified by the vehicel manufacturer and never higher than the max stated on the sidewall of the tire.

I personally run 2 to 3 psi higher than MFG on my 2003 Hyundai Tiburon.

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.

 

[zeusfaber]

Thanks to both for a timely nod of caution.

All the resources I've been able to find to date run long on the dangers of underinflation to the point where the tyre is overloaded, but say little or nothing about the qualitative effects on handling once you vary pressure outside the overloaded zone.

Can anyone recommend a good place to look to understand how higher pressures affect handling (and how this depends (if it does) on tyre location - front, rear, driven, dead, steering, nonsteering, trailer...).

The thought that there are about 20 factors is a bit daunting - if it weren't for that, I'd be tempted to ask if anyone feels up to writing another FAQ.

Job in hand is actually to identify optimum tyre pressures for a single-axle boat trailer which comes without OEM documentation (all I have is the tyre designation and a weighbridge ticket - plus access to a well-stocked standards library). Because the tyres are comfortably within their Load Index, I've got some space to play with. What I need to learn is a strategy to make best use of that space.

Having said that, I also have a long-standing but non-work passenger car tyre pressure confusion.

Grateful for any help - any worries that this is diverting the thread unduly, please say so and I'll start a new one.

A

 

[ccw]

Note to Tireman9,

"Nominal" was a place card that I was hoping you would define. I just wanted to redirect thinking something good about "minimum" pressure. 0-10 % margin below max. load for "reserve" sounds good to me.

Note to Zeusfaber,

One of the caveats early on from Tireman9 was that his current examples should not be extrapolated to other sizes and situations eg. not lawn tractors, not earth movers, (and I would suppose) not trailers.

Tireman9 has really taken me to school on this pressure vs. load thing and I appreciate his efforts. The burgeoning aftermarket for custom and high performance wheels and tires for car, van, and light truck lovers has got to be giving tire/rim engineers gray hairs.