Tire pressures Radial vs Bias ply does it make a difference

[Tire Whisperer]

Some background history of tire construction is necessary before arbitrarily picking a cold(cold as in, early morning, not yet driven) pressure to inflate the tires in older rides such as the OP's 1968 Polara.

The primary difference in the construction of bias-ply vs radial ply tires is the direction in which those plies - reinforcing belts from inner to outer bead. Bias ran diagonally, overlapping. Radial runs perpendicular between the beads(part of tire that mates with the rim or wheel).

This construction, plus different rubber compositions used back then, meant that bias tires were of generally stiffer construction, particularly the side walls. That is why you see such relatively low cold recommended pressures on the tire & load placards affixed to the door frame, trunk lid, etc of vintage cars. 24-26psi was a typical recommendation, but pressures as low as 15psi(think Corvair!) up to 28psi were not out of the realm back then.

My rule of thumb for converting an old MOPAR, or any other bias-ply era vintage, to modern radials of equivalent OE size, or Coker's look-like radials that mimic the bias shape, is to add between 5-10psi to the OE values on the original vehicle tire placard.
On that Corvair, that would equate 20-25psi cold in front, and 31-36 cold out back. As long as that front-rear offset is observed, the car should handle as well, if not even better, than it did when it left the showroom in 1961 or '66.

Now, some of you might be concerned that the cold pressures specified for bias tires back in 1960-something or the pressures specified for cars with radials more recently, 2010 or 2022 for instance, seem "low". Well, I did also, until a couple years ago...

I did something I never did before: I consulted both the Tire & Load, and Certification placards on on my 2010 Accord driver door pillar, and tire load inflation tables on tirepressure dot org. My uploaded image shows all three items consulted. The 'Certification' is the label with gross weights and build date on it.

I took the gross axle weight rating(GAWR) for each axle, and divided it in two - as in - per tire. (remember: 'Gross' = curb weight PLUS five passengers PLUS full tank of gas PLUS full trunk of luggage. ...

PLUS the family dog.)

Basically, the max load plus curb.

Then, I consulted the aforementioned table to see if Honda did their homework when selecting cold tire pressures for that year's Accord.

At 32psi cold, the OEM tire size for my car supports 110.9(111!) percent of front gross axle weight, and 129 percent of rear gross axle. In total, about 139 percent of Gross Veh weight.

A ten percent safety margin per axle is generally considered industry minimum, with 20 percent over, the average.

So in theory, at least, there is no acceptable or rational excuse to exceed the vehicle placard tire pressures at all, on passenger vehicles at least . Aside from higher shipping and storage pressures, to prevent flat spotting or cargo shift in transit. Some car makers, particularly certain German and Italian ones, provide a set of slightly higher cold pressures explicitly for sustained high speed (100-plus mph) operation.

In fact, I could run my rear tires 1-2psi cold lower than in the fronts, and still far exceed gross axle capacity back there.

A while later, I performed the same exercise, using the same sources, for my friend and neighbors 2019 Hyundai Tucson.

At 35psi cold per Hyundai, the OE tire size supports 130 and 135%, respectively, of Front and Rear gross axle weight. At 33psi, those figures dropped by about 10 percent, and the crossover drove more like a car than like a pair of ice skates...!

I think Hyundai was aiming more for fuel economy with that 35 figure, while Honda's pressures were a compromise between economy and handling.

Hopefully all of you now realize that the Max Cold Pressure stamped on tire sidewalls only provides a canvas, of sorts for that tire to be used on a wide range of vehicle sizes and classifications. It has ZERO relevance, whatsoever, for the load and handling requirements of any specific vehicle, from 1940 or so, to present. Regardless of how your car may 'feel' when running your pressures closer to or at Max. And 'personal preference'? I don't even know what that phrase means, lol!

And don't obsess over 'radial bulge' - where the rubber touches the pavement! Think of the caterpillars on an Army tank the next time you see one in the movies or at a parade.

So to summarize: The first order of business when a vehicle manufacturer selects wheels & tires, and cold tire pressures for a new model, or redesign of an existing, is load capacity. As explained several paragraphs back, the current industry norm for passenger cars is 10 to 20 percent over gross axle weight. A 10-20% safety margin. For a fully loaded car mind you.

Leaning toward 10% over gives good handling pressures, and going as high as 30% over is usually done for fuel economy.

The only challenge is to find gross axle/vehicle weight values for older cars, as such labels were not affixed to them as they are today.

I hope this example makes you and other drivers believers.

 

[MoPar~Man]

I think of tires this way: The more a tire deforms when it rolls, the more it's being stressed.

Higher air pressure naturally counteracts rolling deformation. But tires have max air pressure and load rates stamped on them. That's my primary guide when dealing with air pressure, not whats on the label of the door jamb. Higher air pressure gives you better milage, probably longer tire life and correct tread wear (moreso for wider tire sizes). But yes, you get a harsher ride.

I'd argue that half the consideration that car companies give to the air pressure numbers they put on the label was for ride comfort, using (sacrificing) the tires to counteract short comings in suspension design. I think this was more the case 25 - 60 years ago vs now. Remember the SUV's that were rolling over and Firestone tires blowing out because of the low air pressure on the decal?

So you want low rolling tire deformation. But the tire has to support a give weight. My own analysis of charts of tire sizes and load capacity shows that the more volume of air the tire has, the higher it's load rating. When you go to a wider tire, all else being equal, you increase the load rating, you can back off the tire pressure. Thats why the bigger (heavier) chryslers of the 60's usually increased the tire width first before going from 14 to 15" wheels.

I also think the biased tires from the 60's and 70's probably had 4-ply, probably like the trailer tires you can buy today. Stiffer side walls meant you could run lower air pressure.

But, I say, if you and your car's suspension can handle higher air pressure, a pressure that is higher than the door says but well below the max pressure stamped on the tire, then I say that's what you want.

 

[CapriRacer]

Hi! Tire Engineer here! I joined to help in this discussion.

First, bias tires came in different sizing conventions than modern radials. And the first modern sized radial tires came in the middle to late 1970's. So, sorry HWYCRZR (thread starter), the stock tires on your 1968 Dodge Polara were NOT P215/75R14.

According to Tire Guides, a publication that summarizes the vehicle tire placards (when they were introduced in the early 1970's) and whatever information can be found before that , a 1968 Dodge came with 8.24-14 inflated to 28/28 (except for Wagons which came with 8.55-14 inflated to 24/32.)

Both of those are "Super Low Section" tires - meaning 82 aspect ratio.
Also, these tires came with a "Rated Pressure" of 32 psi - meaning the load carrying capacity of the tire maxes out at 32 psi. We tire engineers call that max load the ""Rated Load".

Modern radial tires come in the form "P205/75R15" where P means Passenger, 205 is the width of the tire in mm, 75 is the aspect ratio, the R means radial, and the 15 is the rim diameter in inches. The rated pressure for these kinds of tires is 35 psi for Standard Load (SL) and for Extra Load (XL) tires, it's 41 psi. That is different than the max pressure which can be 35, 44 or 51 psi, for SL and 41 or 50 psi for XL tires.

Then I should tell you, not every tire manufacturer does this the right way and you will find variations that confuse matters.

Now onto history: Car manufacturers generally put on minimally sized tires until radial tires came along. I've noted 3 times that car manufacturers increased tire size and the first one occurred when they changed over the radials - and the last was the result of the Ford/Firestone controversy in August of 2000.

To help with things, I've created a webpage where I talk about a lot of this: Barry's Tire Tech: Vintage Tire Exchange

If you have any questions, ask in this thread for the next week or so. After that email me at Barry@BarrysTireTech.com

 

[CBODY67]

My knowledge of tire pressures dates back into the late 1960s, when ALL tires were on bias-ply or the newer bias-belted construction tires. So the OEM pressure specs applied to those cars and tires.

Back then, there was the "standard" tire pressure, which was usually 24psi. That would support the base car and its (usually 2-3 occupants and maybe 100lbs of "stuff", driven at up to 60mph) for an "easy" ride. It was also known that if you were going to drive at "Freeway Speeds", raising the base pressure to 28psi cold would help the tires last longer and the car handle more precisely (which could also mean "safer" to some). There were also "Maximum Loading" pressures for when the car was fully-loaded with (up to) 6 adults with 500lbs of luggage in the back. Those pressures maxed-out at 32psi, unless "Extra Load" tires were on the car that could then go up to 35psi and could be 6-ply rated tires.

In these years, it was also recommended that front tire pressures be about 2psi lower than the rears. I later determined this was a safety move to ensure the front tires slipped first in a corner, to augment the normal "understeer" orientation that most car companies desired. "Tire slip noises in corners" meant the driver was going too fast and to slow down. Better than having the rear end of the car break lose first!

In the later 1960s, as tire tech was evolving and improving, the old CAR LIFE magazine had a series of very informative and detailed tire articles! In one article, it even had a chart of tire pressures vs. weight that each pressure increment could support, up to 32psi for each tire size. That got me and my slide rule to investigating things!

I knew that higher tire pressure resulted in a decreased "slip angle" of the tire for each degree of initial steering in put, due to the tread not buckling under the car's weight and weight transfer. I also knew that having both ends of the car lose grip at the same time (at the limits) could make for a better-handling-feeling car, as if the car was the desired 50-50 weight distribution (which only Corvettes achieved, back then). All good, in theory.

My guinea pig was out '66 Newport Town Sedan. A few years prior to this point, POPULAR SCIENCE magazine had offered a free tire tread depth indicator as a perk of a year's subscription. We bit for that, which was interesting as I tracked the wear of the 8.55x14 Goodyear Super Power Cushion tires on the car from new.

Higher inflation pressures did decrease tire wear, so I advocated for 28/30 f/r, per Chrysler specs. The car felt firmer and more responsive to steering input on the highway. Like a Chrysler product should.

So I "balanced" the tire pressure vs load supported per axle in a different manner, but it worked well for me. With a typical 55/45 frt/rr weight distribution, starting with 28psi in the rear and 30 in the front.

 

[MoPar~Man]

Tire sizes in 1967 was spec'd as 8.25-14 (not sure what this spec is known as). By 1970 the convention was H78-15. The model year for this change-over in tire designation is obviously 1968, 69 or 70.

I have seen charts and tables that list equivalent sizes (ie 8.25-14 is equivalent to G78-14 and 215-75-14). This was a "best available" equivalence, not an exact match.

I may have seen loaded and rolling specs for, say, the G78-14 (or rotations per mile) - or maybe not, but I definately have never seen these numbers for 8.25-14 (but I would really like to).

Does the "78" in G78-14 refer to the profile? Were these 78 profile tires - the same way that for a 215-75-14 the "75" is the profile or ratio of height to width ? This would explain why a 214-75-14 is a close, but slightly smaller tire vs the G78-14.

And correct me if I'm wrong here:

Throw out the air pressure numbers in the service manuals. They are for biased tires, 4 or 6 or even 8-ply tires, and are completely and utterly non-applicable to the radial tires we all have on our C-body cars today.

---------------------

From their respective service manuals:


1967 Monaco / Polara:

Standard tire size: 8.25-14
For trailer-towing OR 440 engine, size went up to 8.55-14
For disk brakes, size went to 8.15-15 and up to 8.45-15 if A/C or trailer.
Station wagons got only 8.45-15 and possibly always came with disk brakes regardless of engine (?). Station wagon tires were 8-ply rated.
For all non-wagons, air pressure for max load was 28/28 otherwise it was 24/24 (front/rear). Wagons max load was 26/34 otherwise it was 24/28.

There is no '68 or '69 Monaco / Polara service manual on the net that I can find.

1970 Monaco / Polara:

Standard tire size: H78-15 on 5.5" wide rims, max air pressure 26/26, otherwise 24/24 (front / back)
Optional tire size: J78-15 or H78-15 on 6" rims
Station wagons tire size: J78-15 or L78-15 (depending on A/C and engine) on 6.5" rims, air pressure 22 / 32 (front/back) regardless of load

"Your vehicle is designed for bias belted or cross bias tires of the size indicated. The use of radial tires is not recommended, particulary on station wagons."

"Owners who prefer a softer ride may use the optional reduced inflation pressure if the load carried is 5 passengers or less (750 lbs max) and the vehicle speed does not exceed 75 mph."

 

[CBODY67]

Rather than totally "throw out" the FSM or Owner's Manual bias-ply tire pressures, use them as "a base number" instead. The "revs/mile" spec has been in the A.M.A Specs going back to at least the middle 1950s. Many of those can be found in the Automotive History Preservation Society's website under "Factory Information", or similar. I have downloaded many of them, including some Ford and GM models, plus Chrysler products. Otherwise, that information has been invisible to most car enthusiasts.

C-body wagons did come with 14" wheels in the middle 1960s, ONLY power disc brake cars came with 15" wheels, pre-1969, when all C-bodies transitioned into 15" wheels.

In the 1970 Polara/Monaco FSM (I have a paper copy), there is NO mention of changing inflation pressure for radial tires. Radials were available enough in that model year that they were mentioned in the Owner's Manual, too. In all cases, the max pressure recommended was that on the side of the tire sidewall, although 24psi and 28psi had their places, due to loading and/or speeds driven.

When I started to use different-than-OEM-recommended tire pressures in our '66 Newport Town Sedan, it was after careful analysis of axle loads compared to tire capability of supporting loads at particular pressures. There was a chart for this in one of CAR LIFE magazine's great tire tech articles in about 1968. My goal was that tire pressure be varied so that each tire was supporting the same amount of proportional weight, while still being at least 28psi in the rear tires. Only took +2psi in the front to make things right, according to my figures. A heavier-front fwd car could take about +4psi. With such an orientation, vehicle handling became more fun and tires lasted longer. When I put Pirelli P76 radials on the '70 Monaco and BFG Advantage TAs on the '67 Newport, I continued using these same orientations with excellent results. On my '80 Newport (which had radials as factory equipment), same orientations and results there, too.

When P-Metric tires happened, the conversion charts were geared toward "weight capacity", which generally resulted in tires one notch smaller in diameter being installed. When I first saw that, I knew it was the wrong approach. With these physically smaller tires, the higher inflation pressures were also needed. A better approach, which things tended to evolve into, was to match the sidewall width, which would result in the correct diameter tire (as in revs/mils) for the car to look correct. Simply multiplying the old section width by 25.4mm, then taking the next-largest size, upward rather than downward. Which then makes the H78x15 of 1968 or so into P225/75R-15.

ONE reason that Chrysler tended to stay with bias-belted-ply tires was the apparent poor build quality of tires from their main tire supplier. The term "radial pull" was first heard at Chrysler dealers' service dept. The fix? Alter the factory alignment settings to make the car "not pull". Didn't hear that from Ford (Michelin) or GM (UniRpyal) service departments, by comparison.

So, don't "throw out the baby with the bath water" just due to a different tire construction. It's the air volume that supports the tire, not specifically the tire's construction methods. The construction determines various handling/comfort parameters, but it's the air pressure which makes things happen as to weight carrying capacity.

Enjoy!
CBODY67

 

[CBODY67]

The transition in to the alpha-numeric tire sizing was related to federal-mandated consumer protection issues. Which also resulted in the tire pressure decals which were also started at the same time, noting minimum tire sizes and base inflation pressures . . . as "minimum values" rather than otherwise.

Seems that an average consumer had no knowledge of what minimum cold inflation pressures their tires should have in them OR when a flaky tire dealer might sell them a smaller tire than really needed to match the OEM tire. It was also a bit common for OEMs to undersize the base car's tire size to save a few pennies on the car's production price, if it came to that, by observation. Yet with "any" additional equipment, the next-larger tire size would be mandated. BTAIM

There were also minimum tire size requirements related to each tire size too, with up to a 7% margin above that minimum spec. This provided for uniformity of tire sizes among brands. Another consumer protection orientation. Tire sizes prior to this were usually of the 82 or greater "aspect ratio". At that time, the main orientation was to lower aspect ratios and wider section widths and wider tread widths, too.

I've been watching these things happen since about 1963, when I was in grade school. Lots of neat tech, over the years. Positioning what I was reading about in observations at the OEM service departments and service stations we patronized. Asking questions of people I trusted to tell me the truth. Lots of learning!

Enjoy!
CBODY67

 

[MoPar~Man]

What is the A.M.A ?
The Automotive History Preservation Society's website is not particularly easy to navigate. If there are tire specs there, please give me a hint as to how to find them.

 

[CBODY67]

In 1975, as radials were becoming much more available as standard or optional equipment on most new cars, GM had a novel idea, a "common tire performance criteria number" for each size of tire, no matter which tire company built it. The specs would be determined by GM's new Tire Performance Criteria unit rather than just working with a tire manufacturer of choice (or low bid). Resulting tires had a "TPC Number" cast into the sidewall conspicuously.

Each of these tires were built by the tire companies to GM Specs. Including the first all-season tread design and rubber compound. In GM's orientation, should a customer have a tire issue on a trip, they could go to any OEM tire brand and get a replacement tire that was the correct size and had the same performance characteristics as the other OEM tires, with no issues at all. I suspect that many main tire companies modified their molds and processes to put the GM tire onto their existing premium tire carcasses, but if it had the TPC Number, that meant that GM Tire had to sign-off on that.

GM having this "clout" over the tire companies tended to prevent anything like the "Ford and Firestone" issues, by observation. It continues to this day, but the tire companies have apparently increased the quality of their main tires so that no real performance differences happen.

Ford, using mostly BFG tires in the middle 1970s, had a similar (to the GM tread design) tread design, but without other inputs from Ford on tire design. Chrysler was mostly with Goodyear, which had a PolySteel tire and tread that was similar, but unique to them.

Some of the many things I learned and observed about tires over the past 60+ years.

Enjoy!
CBODY67

 

[CBODY67]

What is the A.M.A ?
The Automotive History Preservation Society's website is not particularly easy to navigate. If there are tire specs there, please give me a hint as to how to find them.

"A.M.A" is what was called the Automotive Manufacturers Association. An independent organization which oversaw the specs of vehicles being produced. The documents presented to them by the OEMs were pretty much like a "one-stop service manual" with all specs related to vehicle systems.

In about 1980 or so, it was re-named to something else, for whatever reason.

The AHPS website can be a bit challenging sometimes. Some parts of it require a yearly membership. I have one, which is a re0curring $20.00/yr. Road tests, service manuals, parts books, factory hot rod modification information, and other things. NOT uniform among all brands, though.

PM me your good email address and I can send a few of the A.M.A. files I have downloaded, regarding Chrysler products.

Enjoy!
CBODY67

 

[Turbo301]

Bias ply tires have an effectively stiffer sidewall and thus run lower pressures to maintain a given ride/overall height, relative to a similarly-sized radial. Suspension geometry is calibrated such that running radials on a car designed for bias-ply tires, or the other way around, may mess with your handling.

I typically run 37 psi in all my cars, though they've all been designed for radial tires; my '73 is the first car I've had designed for bias ply tires, but it's running radials now, and I set the pressure to the same 37 psi. I don't know how it would hold-up on the track, but I don't plan to find out LOL; on the lower-speed highways I tend to frequent, it feels fine.

 

[70bigblockdodge]

Bias ply tires have an effectively stiffer sidewall and thus run lower pressures to maintain a given ride/overall height, relative to a similarly-sized radial

Bias tires had to run more plies to maintain structure of the tire since cords in plies are running at a 45°angle to the bead. Radial runs perpendicular to the bead to give the tire it's integrity and 45° plies for more structure and puncture resistance (steel belts).
Radial construction makes them more flexible (compliant with road irregularities) this creating less internal friction and less heat as it rolls. Bias have the higher heat calculated into the cold tire pressure because at highway speeds it will get hotter, remember your high school physics volume remains the same , temperature go up, pressure will go up.
This is why radials will make better MPG , less energy goes into heating the tires, or less rolling resistance.
Your handling should not change, but your alignment can be made better now that tire wear is reduced at slight angles that would eat the tread on a Bias tire.

 

[CBODY67]

By observation, there were NO real changes in suspensions when radial tires came to be installed on USA-brand cars. They DID have better handling, due more to the fact the tread was always more-in-contact with the road surface, due to the radial construction of a stiff tread and more compliant sidewalls. The compliant sidewalls that allowed the chassis to lean as the tires stayed planted on the roadway, not matching the car's lean, too. It is THAT lean which the Chrysler geometry compensates for by their camber patterns (negative on the outside tire, positive in the inside tire), AS the Chrysler MasterTech course on front wheel alignment graphically illustrates.

Bias-ply tires have MORE total flex, radials have the bulk of their flex focused on the sidewall area only. After driving a radial tire on the highway, in pre-P-metric times, you could touch the sidewall and it would be warm, while the tread surface was even cooler. On a bias-ply tire in similar situations, definitely "no touch".

NOW, it must be understood that the current P-Metric radial tires are not all as much of a true radial as the 1970s Michelin radials were. If you look at the earlier cut-aways of Michelins, the steel belt (tread) cores were all almost exactly at a 0 degree angle to each other, while also being almost exactly at a 90 degree angle to the sidewall plies.

Now, look at the similar tire cut-aways of a P-Metric radial. Each of the belt tread layers have a slight criss-cross pattern of the tread-belt ply layers. Similarly, the angles between these tread plies and the sidewall plies are not the original 90 degrees, either. The complete tread ply stack will still be 90 degrees to the sidewall plies, but not the individual ply layers of that stack.

When P-Metrics were originally claimed to be needed for their higher rated inflation pressures and related increases in fuel economy, it was noted that their construction was "softened" a bit to not have a big impact on vehicle ride at the higher pressures. That "softening" was the angle of the tread ply stack layers between each other, not being nearly as parallel as they initially were.

In the more recent P-Metric times, some brands started to use a "tread belt cap" for alleged greater speed capabilities and tread stability at high speeds. Those were not needed previously, except possibly on some 160+mph radials. Not used universally, just by certain tire brands. Another indication of how much the prior radials were softened for the P-Metric designs, to me.

Even on radials, the basic tire "lost weight". A thinner carcass with less rubber thickness, a similarly thinner sidewall area, and less total tread-area thickness (with similar tread depth). End result, the P-Metric radials tended to feel more like a bias-belted tire as to ride and impact harshness, yet with better handling in cornering capabilities.

NO operational reason radials cannot run the same camber and caster angles as any other tire. Radials can abe a bit less picky as to camber, but certainly are more picky about toe-in settings. On the toe-in more for long life and a bit less rolling resistance, probably. YET, when I put new Michelins on one of my cars, it felt better (as to the Michelin feel), but still felt a bit flaky. No handling issues, though. Next time into the dealership, their complimentary alignment check noted that the toe-in was very "out" rather than "in". Which a full alignment confirmed. Somebody was not reading the digital displays very well that day when it was previously done. Getting things correct made for a nicer and "less-throttle" ride, by comparison.

The ONLY change to suspensions happened to coincide with the installation of rear sway bars on many cars, including Chryslers, which came factory with radial tires. Pontiac touted their Radial-Tuned Suspension as a selling point. At that time, in order to gauge later compliance in suspension systems, I would approach the rear of the car (when they still had metal bumpers), carefully place my right foot on the rear corner of the bumper, at a 45 degree angle to the wheelbase, and then push down at an angle to see how the suspension reacted. On 1974 Pontiac full-size cars, the car would quiver as it came back up against the shocks' resistance. On the 1975 RTS cars, the body just rebounded normally, with greatly-reduced lateral quiver. Meaning they took out a lot of the GM lateral compliance, to firm things up. Which they could have done previously, but didn't, apparently for "ride" purposes (which also caused the cars to waddle a bit as they stopped). Fords, by comparison, had a similar coil spring rear suspension, but No Waddle. The Fords were more like Chrysler products than GM products in that respect.

The then-new 1973 MonteCarlos were designed for "high caster" and radial tires from the outset. Even to the point of needing a steering stabilizer on the center "drag link" of the steering system. A few model years, later, those things were gone. After the advertising "buzz" of such was gone, by observation.

So, any car since about 1950 can use radial tires with the then-recommended alignment settings, other than minimizing the toe-in settings. It's been done more than suspected, over the decade when radials have been around, by observation. Only thing is that unless the car will be driven cross-country for pleasure or to car meets, many don't consider a pre-1955 car worth the extra expense of radials, not to forget some issues with "radial tire-rated" wheels on those old tubeless wheels. But those that did put radials on, raved about how much better the cars drive and ride. Main issues were "not following grooves/peaks in the pavement" for the newly-radialed car, plus being smoother on the highway.

CAR LIFE magazine had some very good articles on the subject of radials on prior-non-radial cars, in the later 1960s before Ford started offering Michelins as option equipment on their larger cars. In that time, offering the premium-orientation Michelin tires was a good luxury option to may people. By the earlier 1970s, there were probably more larger Ford/Mercury/Lincoln cars sold with Michelins than not, I suspect.

In the 1969 issue of CAR LIFE where they tested full-size cars with larger engines, termed "Power Cars", they had a test curve to the left where they had a camera set up to look at the front end/tires in that turn at 45mph. Each car in the test got their picture taken in that curve, on a public road in CA, somewhere. It was quite obvious that the Chrysler front end camber patterns worked well. In the one shot of the Ford LTD, that's when it became obvious just how much understeer (in the front suspension geometry) the Michelins on that car were counteracting and working against to keep the car aimed in the correct direction. Plus proved the merits of "safety-rim wheels".

Enjoy!
CBODY67

 

[CBODY67]

On Chrysler products specifically, the earlier articles (and in some "assistance" columns of car issues) in CAR LIFE, it was noted that the earlier radials transmitted more of the road into the car at about 45mph, due to their internal resonances. It seemed that those resonances, at that road speed, tended to be worse with uni-body construction cars, namely Chrysler Products. Body=on-frame cars, namely all Ford and GM cars, had no such issues due to the rubber isolation between the chassis and the body. Steel-belt radials were also noted to be worse in this than fabric or fiberglass-belt radials. Which led me to seek out some glass-belt Kelly-Springfields for my '67 Newport and the later BFG fabric-belt Advantage TAs, too.

In reality, I never did notice this resonance at 45mph issue on my '67 Newport, or the later '70 Monaco DH43N when I put radials on it. Just better cornering, ride, and a bit easier to come off-idle with the engines.

In the 1967 time frame, our Gulf oil company service station guy's preferred cars were '65-'66 Fury III 4-dr sedans. He had several of them over the years, one at a time. He was from the TX Panhandle area and ended up with some radial tires off of a cotton trailer, which were the same size as fit his Fury IIIs, in size and load capacity. They were branded "not for car use", or similar, but they worked fine on his cars. That's the only change . . . tires. He also kept them at 30psi, or a bit higher. No problems. With a nicer ride and handling. He did his homework before trying them and it seemed to be correct. I came to better-appreciate Chrysler products and their better engineering from him, plus from what I saw/heard at the local Chrysler dealership, back then.

I should also note that Chrysler was a bit slower to adopt full use of radial tires in the 1970s, as to what was standard and optional. Bias-belted tires were usually standard, although the base Valiants sometimes had bias-ply tires as base standard equipment, with ANY options resulting in getting the bias-belted as "needed".

Our local Chrysler dealer normally opted for bias-belted tires, unless radials were standard. The service manager did dot like radials, due to the issues when doing alignments Which tended to be confirmed with the TSB to align the car so it would not pull, although it started out to be "factory specs" initially. He, naturally, didn't like having to take longer to do an alignment that might need to be re-touched before the customer took the car. Apparently, there were some quality issues with the brand of tire Chrysler was using as that was the only place I heard those comments, back then. Radials and bias-belted all used the same OEM settings, as I recall.

Enjoy!
CBODY67

 

[Tire Whisperer]

Find out what the GAWR (Gross axle weight) is for the front and rear axles.

Once your tire dealer has installed the recommended size equivalent radial tires on your car, divide the front and rear gross axle weights by two, and find out what cold tire pressure will support 110-115 percent of that weight. Tirepressure . org has tables with this sort of info. I'm betting, even for massive C-bodies like those from 1969-'74 will require low to mid-30s PSI.