The Danger of Overloaded radials on old cars

Ladies and Gentlemen,

I've mentioned this in a few threads before and thought it needed a topic of its' own.

Setting aside the usual "bias plus will kill you / radials aren't traditional and should be killed with fire" arguments that on rare occasion surface on here (PLEASE dont get this thread locked by arguing over bias vs radial!) there's something important that this board doesn't seem to be as aware of as maybe we should...

Radials generally have a lower load carrying capacity than bias plys. That's not the end of the world, but its important that we all realize that load carrying capacity of our tires is very important. This seems to be especially crucial on late 50s and early 60s larger cars that use big (8.50, 9.00, 9.50) 14s as original equipment, tires which simply cannot safely be replaced by 14" radials.

First, its important to realize that the maximum load carrying capacity of a tire is at a specified pressure, often the same as the tire's maximum pressure.

Most of us don't run our tires at maximum rated pressures.

Lower pressures mean a lowered carrying capacity. A 5 psi drop in pressure will lose you around 10% in load carrying capacity.

Lets say you have a 1960 Lincoln like me that weighs roughly 5,200lbs with air conditioning and your car originally came with 9.50-14s. These have a load capacity of 2,000lbs at 32psi. Running them at 26psi as the car calls for should give us roughly 88% of the 2,000 or 1,760 each or a total of 7,060lbs. That's a healthy safety margin.

But say you don't wanna spend $257 each at Coker... Say you grab a set of 215-75R14 (225-75R14s are now out of production in the mainstream market) at WalMart for $76 each. These have a rating of 1,664lbs, but at 44psi!

http://www.walmart.com/ip/Hankook-Optimo-H724-Tire-P215-75R14/17792573

Now, are you going to run them at 44psi? No, you're going to run them at MAYBE 35? Remember, you lose roughly 2% load capacity for every 1psi you run UNDER the pressure at which the max load rating is determined at. At 35psi you're down to 82% of 1,664 or 1,364lbs each for a total of 5,456lbs total. If you can find some old stock 225s you won't be but 5% if that over the 5,456.

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.

And I'm not even considering weight transfer under braking or cornering that can greatly increase the load on a tire.

Do you really want to put those under a 2 1/2 ton car once you and a full tank of gas and your tools and spare parts and friends or family climb on board?

Remember, the factory tire size has a load carrying capacity, as a rule, that's above the total permissible weight of the car (including maximum load) to take, amongst other things, weight transfer into account. That's why station wagons and cars with air often used the next size up, for a higher load carrying capacity. On 58-59 Lincolns air conditioned cars used the 9.50 while cars without it used 9.00s, and that's for a reason, the air conditioning added weight of about 200 pounds.

The theme of safety on this board comes up regularly and the general consensus on here is to "never take a chance". I think that this is very much worth bringing up.

Thoughts?

Bill

 

Good info!
Thanks.

 

215/14R is not the proper replacement size for a 9.50-14. A 235/14R would be more correct but I believe that size isn't readily available.
So you theory is flawed because of using undersized radials.

 

A 235-75R15 is shorter than a 9.50-14, 235 and 225 75R14s are no longer sold on the mainstream market, I believe only companies like Coker handle them anymore. 225-75-14s were OE on 79-80 or so Ford LTDs, not much call for them anymore I'm sure so they're obsolete.

215-75R14s are the tallest 14s still available, Aerostars used them if memory serves. That's why I pointed out that there's really no modern mainstream radial equivalent for the older huge 14" sizes.

 

Are the necessary larger sizes available for a 15" rim? I don't get the impression that 15" rims are all that hard to find, and I suspect that most of the folks who hang out here would be quite content to switch from 14" wheels to 15" wheels if there was a reason to do so.

(And frankly, 15" rims look better to my eyes than 14" rims on a really large car, anyway. Your mileage may vary, of course.)

What about changing rim diameter and perhaps being open to a tire with a different aspect ratio? Does that help? Are there somewhat appropriate tires available with an aspect ratio of, say, 70?

(Okay, don't crucify me for that last question, folks. I'm just asking. Keep in mind that this discussion is more about safety than maintaining a period correct look.)

Oh, and what about tire-fender well clearance issues? Would any of these "solutions" create possible problems with tires rubbing on the sheet metal?

 

I am NOT a tire expert. I noticed that radials are not used on all types or trailers. Probably for the reason you wrote this. The sidewalls on radials seem weak to me.

 

Full-sized Fords (LTD's, Galaxies, etc.) went to 15" wheels in 1965 and remained the standard rim size until 16" became standard.

 

This is a valid concern, but as a practical matter I think it has an extremely rare occurance.

The pre-1964 cars of this forum are generally lighter than later cars from the 70s (and later) that radial tires were engineered for. As cars "bulked-up" with crash protection and emission controls, they got much heavier for their size. Today, they're packed with so much stuff they're heavier than ever.

 

I would like to know more about these statements. Like where did you come up with this information? Is there a way to calculate tire pressure if car weight and tire size are known?

Knowing the weight of my car at the corners, I could adjust the tire pressure to suite. This would allow me to be sure that the tire is not over/under inflated.

Thanks for the post.

 

I am not posting this to stray from the original ,but take notice of a lot of the S.U.V.s & pickup trucks on the road that have had the tires replaced with P 235 XXX instead of the tires that were originally LT 235 XXX passenger versus light truck,bottom line the passenger tires were chaeper to buy and most folks do not look at load ratings ,traction ratings ETC. Sorry folks I had to throw this in.Bottom line it is all right there on the sticker and the sidewall.

 

The issue is not radial vs. bias ply. It is being sure that the tire and wheel combo has sufficient load carrying capacity at a given pressure for the weight of the vehicle. The Benett Garfield tire guide is the bible of the tire industry and should be referred to when unsure of the proper tire size for a given vehicle. Air pressure is what supports the load not the tire sidewall. Yes they do use radials on trailers but not passenger rated radials. One that is commonly used is the Goodyear Marathon. You will see them on alot of travel trailers. There is an old car tire guide thats been around for years but currently out of print. You can sometimes find them on EBAY. This would be your best reference source.

 

Eye opening information here, thank you.

 

Maybe that's why my 55 Olds squeeled like a banshee going around corners with 235 75 15s?

 

Didn't Ford find this out a few years ago on their exploders? They were using too light of a tire and when the public softened them up for a better ride they went poof and killed ford owners

 

Ford blaming Firestone tires was just scapegoating. At that time you could find rolled Explorers with any brand of tire you could imagine in the salvage yards (my Dad owned a yard at the time). Exploders had too high of a roll center and were easy to tip. Ford recommending the soft tire pressures didn't help but it wasn't just the tires.

 

As my friend, who was the engineer in charge of warranty claims for BGSTN/FST
when these problems came up explained, the contributing factors were:

· Ford had a lower recommended inflation on the door jamb sticker than FST did on the sidewall. (For ride quality)
· The tires were underinflated for the load. (See above or poor maintenance)
· The vehicles were overloaded. (Summer vacationers)
· The high road surface temps from summer heat and sustained high speed driving led to excessive heat build up in the tire, resulting in either tread separation or catastrophic casing failure. (All of the above)
· When the tire failed at speed, the high roll center of the Explorer coupled with an inexperienced driver slamming on the brakes, caused vehicle roll-over.

As the OP stated, tire load carrying capacity and correct inflation for the load and speed is the key.

 

To drift a moment.... I always blamed the Explorer roll over thing on people expecting a vehicle engineered for going slow over rough roads to handle like a sedan at highway speeds.

 

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.

 

Years agoi worked in a tire shop, maybe I'm a bit of a Savant but I have a memory that would put an elephant to shame. It's actually rather common knowledge in the tire industry. Google "tire load capacity pressure" and you should come up with some links..

I found this.. Pages 6-8. Note that not every manufacturer follows this 35psi rating that Firestone mentions, Hankook clearly doesn't.

http://www.tiresafety.com/images/Tire Replacement Manual.pdf

BobMcD is spot on, this has nothing to do with bias vs radial, its about load rating and safety.

 

It was a combination of factors. The original equipment Firestone ATX was actually a very good tire, they wore like iron. One of the places I worked while in college was a Firestone dealer, before that at a place called Tire America. TA didn't sell Firestones, except for the P235-75R15 ATX. If you read a fitment guide from back then the Explorer is notable for being the only car that mentions:

Only use P Metric tires on an explorer, never LT rated.
Never use all season or passenger car type tires, only all terrain type tires.
Never change tire size or use optional sizes
Inflation pressure is crucial.

Firestone wanted Ford to run 30psi, for ride comfort purposes Ford ran 26psi. Throw in people neglecting to check tire pressures and it was common to see these things running around with 20psi in them. Now, if you'll remember most of these accidents that made the news were where Mary J and John Q. Public had 5 people plus luggage on a Texas Interstate, likely at 80mph, in the middle of the summer and then blammo! Tire failure!

You also have to remember that when this happend, early to mid 90s, SUVs were a new thing and the public wasn't familiar with them. My first job selling cars was at a Ford store in 1995, we sold tons of Explorers and just about every trade in was a car.

 

M224speed brings up a good point here: passenger car radials have less plys than LT - Light Truck radials and Trailer tires have more plys than Light Truck. I am for sure no tire expert but isn't it 4 ply passenger, 6 ply LT. And 8 ply trailer as a general rule of thumb? The more plys the stiffer the sidewall and thus the harsher the ride....

 

Light truck (LT) and trailer tires are made and sold with Load Range Ratings.......Load Range C......Load Range D........Load Range E...for a few examples. It is not the size of the tire, but the Load Range (plies or ply rating) that differentiates them. They do, of course, come in various sizes and the weight capacity will vary between tires of the same size but different Load Range rating as well as between tires of the same Load Range but different size.

It simply requires knowledge of the weight to be carried, and the max weight capacity of the tire being selected, to get a safe combo, whether for cars, light trucks or trailers.


Ray

 

PSI Pct of rating
35 100.0%
32 95.9%
29 91.9%
26 86.9%

 

You are correct on this one.

 

Thanks for posting. I've been casually looking at Coker info,tire widths and heights,and weight ratings. For clearance on the front and back,I need a tall,skinny 15 for my Merc,but some look a little light on their ratings.
This is good stuff,thanks.

 

I like where you are going with this, BUT your statement of "A 5 psi drop in pressure will lose you around 10% in load carrying capacity." is pretty general and sweeping and according to a couple of tire manufacturers, not accurate. In reality the pressure to load rating is different for pretty much every tire. The number you used is probably on the safe side, but most tires manufacturers have the technical data available on their web sites. So the best way to figure out what you need, both load rating and inflation wise is to llok at the manufacturers charts. You should also really look at weight on the front wheels versus weight on the rear wheels as it will be different.
They have made some changes to the tire load rating, now using a numeric code called the Load Index instead of an actual number (although many tires have both). Here showing the load rating versus tire pressure for a bunch of Toyo Tires http://www.toyotires.ca/sites/default/files/loadinflationtable.pdf

 

Don't even get me started on the 'extra load' tires.....Good info here and a good crash course on what I had to learn when I worked in the tire biz.

 

i'd like to add a couple thoughts of my own: concerning the "rollover" problem on the ford explorer, i think a lot of that was due to drivers who panic when a tire blows and stomp on the brakes, rather than safely coasting to a stop. secondly, i'd hesitate to install radials on 55 or 60 year-old wheels due to age and the fact that those old wheels didn't use heavy enough (gauge size) metal as the newer wheels built for radials.

 

saw plenty of those rollovers on straight sections of interstate---load ratings are important for our safety,thank you for the info...

 

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".