The Danger of Overloaded radials on old cars

Standard tire size for 1980 Ford LTD Crown Victoria 4-door 5.8L V-8 SelectShift automatic:
(in case of different tires in front and rear the dimensional data are valid for driving or rear wheels)

Standard tire size:

P 205/75 R 14

Tire width (mm):

205

Tire sidewall factor:

75

Rim size (in):

14

Total wheel diameter (mm / in):

663 / 26.1

 

Good stuff, thanks Bill.

 

While tire manufacturers may specify higher PSI for Max, they cannot load rate their SL tire at anything other than 35, soon to be standardized at 36.

A P-Metric SL tire with a Load Index of 98 has a Capacity of 1664 Pounds @ 35 PSI. The Hankook, or any P-Metric SL tire Rated under the Current DOT system is Max Load Rated @ 35 PSI. That is NOT the max inflation pressure as I read all the do***ents. See below

So that tire is indeed Rated at 1664 Pounds at a sustained 112 MPH, not sure what the old tire load rating was at, but unlikely 112 MPH

"Realistically most guys would run 30-32 psi in these, at 30psi the load capacity of that same tire is a hair under 1,200lbs. "

Actually @ 32 The Rating is 1587, @ 29 it is 1521, so the original thought while noble seems to be in error.
75 series 14 inch tires




Note the following cart is approximate depending on exact size, full chart is here as previously posted along with Load Rating at different pressures

 

Hey...Dan C. made a good point. Are the rims worthy?
A couple years ago I ordered six new Dunlop load range E 16" tires for my '67 C30 flatbed/dump dually shoptruck. The rims are factory originals. The recommended psi for the tires is 80 psi.
The tire guy asked me to come down to the shop for a sec. to show me something.
On the inside of each rim was a factory stamp that read 65 psi max. This was stamped on the rim, inside the tire so you would never see it unless the tire was dismounted.
We were not worried about it. It was just kind of interesting.
I run 65 psi rear and 50 psi front.
Truck can't do 65 mph anyway so no worries.

 

Thank you for posting this thread...I wish I had known about this a few years ago. We had a travel trailer, and I always kept the tire pressure at what the mfg. recommended, even though it was considerably less than the "maximum pressure" stamped on the sidewall of the tires. After two blowouts on the highway, and consulting with a local tire dealer, I upped the tire pressure to what max. tire pressure was stamped on the tires. And no more problems. Luckily, there was no damage, other than a shredded tire(s). It probably didn't help that that travel trailer was loaded to the gills with all sorts of camping "stuff". I never weighed it, but it was undoubtedly overloaded. (I was always a proponent of traveling light...but my wife, well not so much!)

 

A 9.50-14 would be roughly a 245/85-14 which of course doesn't exist.

A 215/75-14 is roughly a 7.75-14 (wider but lower-profile which reduces the load capacity)

You can see the problem here - the 215/75 is about five or six sizes smaller than the 9.50. Not remotely comparable.

To get an adequate modern tire for that car you have to go to a much larger rim diameter, I don't even think there's 15s or 16s that'll do it right.

Most of whats been said about the explorer is right tho I thought the reason for the 26psi pressure was to reduce tire grip so it'd slide rather than roll.

 

Tire Options = Original Estimated Load Rating 0f 950/14 = 1760 @ unknown MPH, 29.5 diameter.

There are options in this load range, appearance & diameters of course vary.



After reviewing this thread I have the following thoughts.

There are many reasons for an individual to choose an original type tire, appearance, authenticity, personal preference, and if they are writing the check for their car it is entirely at their discretion.

Saying that Radial Tires cannot be found with the proper Load Rating seems to be more of a justification for a personal "bias" and less of a safety concern.

Use your head, study the data, decide for yourself out of knowledge, jm2c!

 

I know my Dodge does not like Radials.. a wondering fool it is when it has those on.. I installed Bias Plys.. handled much better .. except for the ****py waved roads.. then it has a mind of its own..

 

Explanation of Maximum Inflation Pressure Vs Maximum Load PSI

Maximum Inflation Pressure

A tire's maximum inflation pressure is the highest "cold" inflation pressure that the tire is designed to contain. However the tire's maximum inflation pressure should only be used when called for on the vehicle's tire placard or in the vehicle's owners manual. It is also important to remember that the vehicle's recommended tire inflation pressure is always to be measured and set when the tire is "cold." Cold conditions are defined as early in the morning before the day's ambient temperature, sun's radiant heat or the heat generated while driving have caused the tire pressure to temporarily increase.

For the reasons indicated above, It is also normal to experience "hot" tire pressures that are up to 5 to 6 psi above the tire's recommended "cold" pressure during the day if the vehicle is parked in the sun or has been extensively driven. Therefore, if the vehicle's recommended "cold" inflation pressures correspond with the tire's maximum inflation pressure, it will often appear that too much tire pressure is present. However, this extra "hot" tire pressure is temporary and should NOT be bled off to return the tire pressure to within the maximum inflation pressure value branded on the tire. If the "cold" tire pressure was correctly set initially, the temporary "hot" tire pressure will have returned to the tire's maximum inflation pressure when next measured in "cold" conditions.

A tire's "maximum inflation pressure" may be different that the ***igned tire pressure used to rate the tire's "maximum load." For example, while a P-metric sized standard load tire's maximum load is rated at 35 psi, many P-metric sized standard load performance and touring tires are designed to contain up to 44 psi (and are branded on their sidewalls accordingly). This additional range of inflation pressure (in this case, between 36 and 44 psi) has been provided to accommodate any unique handling, high speed and/or rolling resistance requirements determined by the tire and vehicle manufacturers. These unique tire pressures will be identified on the vehicle placard in the vehicle's owner's manual.

The tire's maximum inflation pressure is indicated in relatively small-sized print branded near the tire's bead (adjacent to the wheel) indicating the appropriate value. Because tires are global products, their maximum inflation pressure is branded on the tire in kilopascals (kPa) and pounds per square inch (psi). These values can also be found in the industry's tire load & inflation charts.

 

I always go with the tire pressure on the tire not the door jamb sticker. I also run within 1-2 lbs away from max pressure as the tire wears better, for load and rolling resistance. I have 1 panel with P tires (it's empty inside) and the other panel has LT tires with a max pressure rating of 80lbs that I run at 70 psi.

 

Registered to this forum to react to this topic.
Dont have an old-timer, but got hold end 2007 of the formula that is used to determine advicepressures for cars with radial tires.
Went running with it , and things got a bit out of hand, now I constantly google for tire-pressure to see if I can spread my gathered information.
I live in Holland so my English can be a little odd once in a while, forgive me then.
Wont introduce myself in a seperate topic, because I only registered for tire-pressure information.

The 44psi up to 51 psi that is given on normal radial car tires( P-tires) is the maximum pressure of the tire, but is not the pressure that is used to carry the maximum load.
That pressure is called the reference-pressure( further used Pr) in the formulasheet, and the tire may bare the maximum load at this pressure up to maximum speed of tire, or if lower 160km/99m/h ( up to V speedrated tires) , without damaging the tires by driving alone, to the calculations of the tire-makers organisation. Extra between Pr and Pmax is used for higher speed and camber-angle above 2 ( alignment tires like this/-\ instead of this|-| on the axle). Pr has to be highened up with a system , depending on the speedrating of the tire (the S to Y printed on sidewall) .

This Pr is for radial P-tires ( normal car tires) 35 psi/2,4 bar for American tires always. For European marked made tires 2,5 bar/36 psi with exeptions to lower ( smaller tires , 80% Aspect ratio (2,4ba) fi).

For XL /Extraload/reinforced (RF) tires in America 41 psi/2,8bar, and Europe 2,9Bar/42 psi with exeptions to 2,8bar/41psi ( 80%AR) and 3.0bar/44psi.

Above that from C-load/6PR( Plyrating) to H-load/16PR the Pr is given on sidewall , and the maximum pressure is , so I concluded, 1.4 times as high.
If you read on sidewall something like this with AT in it "maximum load **** lbs AT yyy psi ( cold) " then the yyy is the reference-pressure (Pr).

This different giving of Pr and Pmax cources the confusion.
So now you know where it comes from.

The formula the tire-makers use for determining pressure for load, was introduced in 1928 for diagonal tires and has a power in it of 0,585 for diagonal tires.
The formula was meanth for tire-spe******ts ( if you had them in that time, so people who put new tires on your car is better ) to determine simple the needed pressure for the load, and is an aproaching formula.
But it missed its purpose , because almost nobody understands the power part in it.
Round 1970 the formula was adapted in it power for radial tires in Europe to 0,8 for all kind of radial tires, but in America to 0,5 ( root) for P-tires with Aspect ratio ( hight/width division) of above 50% , uo to 0,7 for C-load and up. And there it went wrong.
Only as late as 2006 American TRA swiched over to the 0,8 power of European ETRTO ( when the Ford/Firestone affaire was forgotten, wich was not courced by the wrong calculation that was repaired by 10% substaction, but because of the to high given maximum load on Off-road tires and low AR tires still happening).
But only for P-tires and XL//, for C-load and up still power of 0,7 is used for calculating loadcapacity for a sertain pressure.

I stumbled on an article of an American IR J.C.Daws with a new calculation and comparison with the old way, and concluded my own universal formula, of wich the old and the "Daws"formula can be made, by giving the right Power and construction load ( my own introduction= part of the maximum load that is carried by the construction of the tire at the given deflection) .
I now use that with , what I think are the right power and Lc in spreadsheets I made.

One is for making PDF's with pressure/loadcapacity-lists .
Made them for all the possible American tires and per tire , per axle single and dual, and placed them in a map on my public map of skydrive , that belongs to my hotmail-adress with same username as here, so you can look the savest loadcapacity ( sertainly saver then old American lists, and even saver then the European lists).
Will give the link to that map here, so you can use them.
https://skydrive.live.com/?cid=a526e0eee092e6dc#cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC!904
From this map , you can navigate my complete public map , lik in a forum, there is much more about tire-pressure also in English between the Dutch.

Enaugh for my first post , but if wanted, to be continued!!
Greatings from a Pigheaded Dutchmen
Peter

 

Great original post. I always run my tires at max rated psi. I put huge amounts of miles on my tires. It seems to me, my vehicles feel better on the road at the top psi number.

As far as the Explorer, Firestone issue. Never buy a tire or any product from a company with labor issue's. Goodyear had the same problem during their labor disputes. Call it what you want, but I happen to be in many Goodyear plants during their disputes. The Union Rubber workers where on strike, I say suit and ties makin tires with less care then makin cupcakes....

Anyone remember the Black Flag Bridgestone /Firestone movement at Indy? Many workers from around the country showed up, holding black flags, and walked 1 lap around Indy before the race.

 

I got a question to the tire guys. If you were to air the tire up on the ground with 50 psi then mount it to the equipment trailer. Lets say load 6000# on it and check the pressure will it be the same 50 psi or will it go up? I've never checked after it was loaded?

 

In theroy yes, in practice, No

What determies this is the Ideal Gas Law


PV = nRT

P = Pressure
V = Volume of the container
n = The amount of gas (air in this case)
R = 8.314472
T = Temperature of the gas (air) in the container (tire)

Solving for P you get:

P = (nRT)/V

so if you fill the tires to X PSI and then load them and see a higher PSI what has changed?

The amount of air, R, and the temperature remain constant. (The temperature actually increases a little but not enough to matter.) But the weight of the car slightly decreases the volume of the tire by deforming it. In terms of the equation, if V is smaller, P must be larger. So the pressure goes up.
I did a calculation of a 15" diameter, 6" wide tire on a 2600 pound car with 35psi. Dues to the amount of deflection, the pressure went up by .06 PSI - not registerable on most tire gauges. And that calculation doesn't take into account the bulging of the tire which would make the increase even smaller!

 

I knew somebody would know! Now I don't have to test it, Thanks!

 

Yep, but it is a downsized car. Call it what you will. If it's got 14's on it it's downsized.

 

Good stuff here, one of the problems not mentioned is front skinnys. Look at load rating on 135s and 145s X15 radials. last pair I had[145] were 770 lbs at 42 lbs.Real easy to be on the max rating with a light car. Then add in the highway speed we gotta run today, and its a disaster waiting. Good to know the corner weight of your car when looking at tires.

 

This is a good topic. A friend of mine has a '51 Buick Roadmaster and ran the specified Coker radials. He set the psi according to a fully loaded 4 guys and packed trunk. We were going up to Iola on the Freeway and had one blow out. We changed the tire with the spare and took side roads. Unfortunately, they all went on that one trip! He went with the bias-ply tires.

 

Good topic!
I always enter load capacity into the equation when buying tires for my F-350 or one of my trailers. Funny thing, it never ever even crossed my mind when buying tires for my 58 Ford !!
BTW I had one the Explorers with the infamous Firestones.The tires were ****...

 

Good topic

 

The 135R15 73 T is 805@118 MPH
The 145R15 77 T is 908@118 MPH

Either would be likely be below spec on a full size car with p***engers.

Two clarifications, all "standard load" or SL tires are Max Rated at 35PSI, the OP was incorrect on this point. If you are operating close to the max rated speed you may need to increase PSI up to the Maximum Cold Rated Inflation Pressure,this does NOT increase the load rating.

The Load Rating has a Speed rating with it. So it is Pounds @ Speed. It has to do with heat buildup at sustained operation. However no tire tech would "ADVISE" using a tire rated below it's weight,the slower you go the safer the tire. There is a Chart for de-rating or correcting the load capacity when you exceed the speed rating

So the Maximum PSI of 42 would not be for the load rating, that is always 35 PSI for SL, but you may need it to use all the speed rating.

Linkie

.

 

Actually the facts about the tire problems that Ford had on the Explorer's was caused by Ford/Bridgestone.. i.e. Ford wanted a cheaper tire, Bridgestone complied with Fords request, however, the tires Ford specified were not built to a DOT standard...
When the tire problem surfaced, Ford sent out a letter to all of it's Explorer customers, informing them to LOWER THE PRESSURE ten pounds per tire.. The letter should have told the customers to raise the pressure ten pounds.. The lower pressure coupled with an out of spec tire caused the tires to fail at a higher rate...
I use to have a copy of the letter Ford sent out, it was given to me by my GMC dealer because they were selling vehicles with Firestone/Bridgestone tires...
Many people are not aware just how important proper tire inflation is...

 

Very good thread, thank you for starting it and all whom have brought good information to the thread. I have learned a great deal and lots of info to ponder.

 

Full size LTD's in the late '70s and early '80s had 14" wheels. My parents used to have an '81 that they bought new. It had 14" wheels as original equipment.

 

Original Context

Originally Posted by rld14
"When Ford downsized their full sized cars in 79 I think it was they went back to 14s for a few years. If memory serves, from an old fitment guide, the 79s used Alphanumeric tires (H or J78-14s) but in 1980 they went to P Metric and I think it was the wagons but they did use a 225-75-14."


Originally Posted by sidevalve8ba

"When Ford went to the LTD II's (downsizing) they may have gone to a 14" wheel but the full size Fords were still on 15". "

You mentioned that the LTD II was 14 but the Full Size was 15. When Ford downsized the LTD ( 1979 Panther Platform) with 14"they still made the LTD II (77-79)which was a renamed Gran Torino Elite. The LTD II was not a downsized LTD. The LTD 3d Gen, 75-78 did indeed have 15 Inch wheels standard.

 

Interesting thread. As for the Ford 14/15" debate......Not sure of the exact years, probably 79 through mid 80's 14" was standard on the full size LTD/Crown Vic (not LTD 2) Fords and full size Mercs and 15" was optional. I had an 84 Merc Gran Marquis that had 14. My 87 Gran Marquis had 15. By then I think the 15" was standard equip.