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Tire pressure v load, and deflection

Tire pressure v load, and deflection

Tire pressure v load, and deflection

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,


RE: Tire pressure v load, and deflection

If you find a source of info, I'd like to know about it!


RE: Tire pressure v load, and deflection

most likely the graph is not findable - but if you want to start one:

1. the tire "foot print" is calculated by taking the tire presure and its load.  the footprint is equal to the load in pounds divided by the pressure in psi this gives you sq inches of foot print - except for some minor correctio for the edges where the actual "load" tapers off.

then it gets a little hairy. because you have to hit the geometry books to figure the secant [ thats the little line that flatens a circle ( like where the pavement is - i say line because your going to start in 2D before going to 3D in which case the line becomes a plane i.e. a surface. Then you figure the sq inches per inch of circumference of the true radius of your tire and the ratio of the two will let you compute how much the tire needs to be pushed in [ which is the deflection you want to know. In scool the instructor used to say intuitively you ......

RE: Tire pressure v load, and deflection

Sorry, it doesn't work that way. The tyre wall (and the belt) provides significant support which will strongly affect the vertical deflection.


Greg Locock

RE: Tire pressure v load, and deflection

Greg your right - to a degree. however that support does not come without deflection
so really the empirical solution [ for a given tire and load ] is to measure surface to rim with a caliper, or whatever, then reduce the pressure in increments and remeasure. BUT be sure you have a supply of pressure before you start deflation !
Then maybe you want to recheck on the way going up to see if there is a hysterisis effect ( like they talk about in motors - most likely not significant because this would be mushed out in motion - BUT not insignificant would be the fact that the deflection would decrease at speed - assuming no bumps or fooling around inside - because of the centrifugal force due to the spin of the tire.   Now let see - the coriolis effect in the western hem.....   Greg now that we've digressed this far - you seem savy enough to remind me of the name of that closed flask that only has one surface ( counting inside and outside).

And then mat, of course publish your table each time you do this with a new load.  And the car mfgr will say fit  the tire/pressure to the placard on the door jam - or what do they call that in tyre, bonnet, windscreen, country : - ).

RE: Tire pressure v load, and deflection

wouldn't the deflection reduce at speed?
and the pressure also will increase some as the tire warms up.

Hmm - the measurements still won't tell us the "proper" pressure at a given load, 'cause we don't know the ideal deflection?

- should we also look at the tire patch shape as pressure changes? maybe we can infer the desireable range of deflections by observing tire patch and tread squirm?


RE: Tire pressure v load, and deflection

him2 - Klein bottle

Original poster

I re-read your post and realised we've got off the subject a bit. The desirable pressure for a given load is specific not just to a tire size, but also the manufacturer, tire design, and , to some extent , the useage. Therefore your best bet is to get a chart from each manufacturer. The Tire and Rim Association book does NOT specify a table for this.

More confusingly, the recommendation will vary for a given tire depending on which vehicle it is fitted to. This is because the tire pressures are specified by a coven consisting of the fuel economy guys, brakes, ride, handling, NVH and durability, and a manufacturer's project engineer.

The manufacturer should be able to give you an estimate of the change in SLR (static laden radius) for a given change in load, or inflation pressure. He probably won't be as keen to give the change in rolling radius.


Greg Locock

RE: Tire pressure v load, and deflection

Another question along the same lines.  Is there a way to determine the tire's (linearized) spring rate at the normal loading and pressure?  Obviously the tire is nonlinear for significant deflections, but it seems that a linearized rate would be fairly accurate for "small" deflections.

RE: Tire pressure v load, and deflection

Again the manufacturer may be able to help. If you want to do it yourself then a very neat way is to measure the wheelhop frequency (add 80g of balance weight to one wheel and note the speed of maximum vibration). If you know the unsprung mass, referred to the tire contact patch, then you can work out the spring rate.

Typically 140-250 N/mm for automotive tires.


Greg Locock

RE: Tire pressure v load, and deflection

look here
the Load Range is molded into the sidewall of most tires today.
In the US the loads and inflation are set by the Tire & Rim Association and in almost all cases all manufacturers follow the same tables.
There is a formula but it is complex

RE: Tire pressure v load, and deflection

Michellin also has a chart that will give you a general guidline of loads for a given tire size at different pressures.  It is rather odd though as it only gives numbers based on size, load index (also range for light truck tires), or the width of the rim the tires are installed on.
Still it can be helpful in determining what air pressure changes are required when changing tire width and/or rim diameter.

RE: Tire pressure v load, and deflection

I was trying to follow the link to the Michelin site...it does not work anymore.
What I am after is an understanding on how to properly set tire pressure on non-OEM tire/wheel combinations. Is there any way to calculate the approximate pressure? What info would I need as a minimum? My current assumption of what I need are tires size, rim width, load index, wheel load and speed. I also understand that the geometry of the tire has an impact but that would make it too complex (I hope that this is not the main factor, though). In the past I had contacted a tire manufacturer for pressure recommendation, I am wondering if there is a list of addresses who to contact at the different manufacturers.
Would the tables of the Tire & Rim Association give me the answers I am looking for? Anyone have an example of what I would find there?

RE: Tire pressure v load, and deflection

Sorry the link is broken
This isn't as good but should work till I find a better one.

enter your size and then check the table

A dealer for the brand tire you have should have the load inflation table in their manuals. I know Firestone & Bridgestone do.
Formula is complex and ETRTO is slightly different than JATMA and T&RA.

An alternate method is to simply use the Service Description i.e.  87V  or 92H  which is a combination of Load Index and Speed rating. you should NEVER use a tire with a lower load Index than was on your OE tire. You also should not run a lower speed rating than was OE.

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.

RE: Tire pressure v load, and deflection

This page gives a quick listing of the Service description of one line of tires.
While the speed rating is a function of the individual tire design - in this case snow tire,
The Load index is a function of tire size so within one industry guide such as ETRTO for metric sizes all tires will have similar load ratings.

Hope this helps.

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.

RE: Tire pressure v load, and deflection

Thanks tireman9 for the info, I know a guy who owns 20 or so tire shops...will ask him for some tables.

RE: Tire pressure v load, and deflection

to determine specified pressure, ply rating, rolling circumference, deflection etc, tire engineers are using The tire and Rim Association book. Working as a tire enginner I do use a lot of calculation that are sligthly different for different type of tires.

RE: Tire pressure v load, and deflection

Hi tireman9 and z157,

  We all have computers.  My (don't I wish!) 2005 Corvette has 17 computers.  How about some of those complex formulas and calculations?  The tire datasheets on tirerack.com list "max. pressure" along with "max load".  What are we to do with those values other than do not exceed?

How about inflation pressure cold = 80% max. pressure posted x actual load/max. load posted, or maybe something needs to be to the three fifths power.

At this point in time, this old gas burner is looking for a combination of max. OD, min. rolling resistance, and max. safety.

RE: Tire pressure v load, and deflection

Ok, I guess a little more background is needed.
First you need to understand that there are number of "Associations" that most of the world's tire companies belong to. Second you need to know that the current formulas used by these associadions have been and are still in a state of continuous evolution.
The current formulas are an effort to provide guidance for the proper load for a given inflation and type of service. As you can well imagine there is no single formula that will properly address everything from a garden tractor tire to a Corvette tire to a 10 foot tall Off Road mining tire.
Another detail is that the formulas were developed to match existing load tables so there are sizes in the tables that do not match the calculations 100%.
The other fly in the ointment is the rounding issues. Do you calculate in metric and convert to SI then round or calculate in metric, round then convert or calc in SI...  etc  see the problem?
So much for the problems.

The guide for selecting the appropriate formula and appropriate load and inflation factors is  200 pages long so there is no way I can answer your question here in a single e-mail.

I will bring the books home next week and create a FAQ with a simple example, then return here and post the link to the FAQ.

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.

RE: Tire pressure v load, and deflection

Hi tireman9,

Thanks that would be very helpful, at least to me, and probably to several others.  I think matatdeneb specified common vehicle tires.  I would interpret that to mean passenger car / light truck.  If your example could address that realm, it might be more helpful, than garden tractors or turn-a-pulls.

RE: Tire pressure v load, and deflection

Hi tireman9,
   Thanks for taking the time to post the formulas for load vs. pressure on FAQ68-1148.  I did not realize how empirical the formula was until I read your faq.  Could you help me with



Sd =

I did not identify where H comes from.   Maybe some real world numbers would help.

RE: Tire pressure v load, and deflection

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.

RE: Tire pressure v load, and deflection

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?  


RE: Tire pressure v load, and deflection

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.

RE: Tire pressure v load, and deflection

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?


RE: Tire pressure v load, and deflection

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.

RE: Tire pressure v load, and deflection

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.

RE: Tire pressure v load, and deflection

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.

RE: Tire pressure v load, and deflection

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.


RE: Tire pressure v load, and deflection

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.

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