Sidewall pressure is the pressure required for the tire to carry its maximum rated load, not the recommended pressure which is specific to each model. That said, most of the pressures given in the manuals for these vehicles would be for bias ply tires. Radials are likely different. I start with 32 PSI in the 225/75R15's on my car and adjust pressure according to how they wear. The only time I have ever used sidewall pressure is on trailers and the rears of my truck when towing a 5th wheel. 80 PSI in the rears of an empty 1T is not very comfortable.
Tire pressure?
- Thread starter Rusty Muffler
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Rusty Muffler
Active Member
Shocks are brand new KYB's.
Check this thread, they are no afraid to raise pressure to suit.
Tire pressure for a 1962 Valiant with 14" tires
Tire pressure for a 1962 Valiant with 14" tires
Darter6
Active Member
Agreed. Rides like a wooden wheel buckboard wagon.
Top Cat
Member
What you're feeling on rough pavement may be "lack of structural rigidity". Auto testers including Consumer Reports noted this characteristic on all of the new Forward Look '57 Mopars.
If you lower your tire pressure, the car will ride less harshly. I'm in the mid-20s on radials on my '60 Dodge Dart and I do just fine. If your Plymouth has manual steering it will steer easier with more pressure in the front tires. (I have power steering.)
Here's what Consumer Reports had to say, April 1957:
Rusty Muffler
Active Member
Very interesting, thanks!
ynst
Member
I’m surprised that nobody has raised the issue of drastic front suspension changes when manufacturers switched over to radial tires.
The old bias ply tires did not sit flat on the pavement when cornering like radial tires do.
There was, also, very little flex in the sidewall on bias ply tires.
Radial tires are supposed to lay flat on the pavement and one of the ways this is accomplished is to soften the sidewall. The other way is to drastically modify the front suspension.
Bias ply tires tend to break into a skid quite gradually whereas radial tires suddenly break loose when the over-stressed sidewall snaps the tread back under the center of the rim, thus suddenly breaking traction.
I’ve been running radial tires on a bunch of bias ply vehicles since radials first came out.
My hemi powered 34 Ford has them and I DRIVE that car from the Boston area to Louisville, KY every year.
I put radials on Tex Smith’s old ‘48 Chrysler in Rawlings, Wyoming in ‘86 when I bought it and drove it home to eastern Mass from Tex’s place in Driggs,ID.
My ‘72 Dart Swinger has had them since I got it in the early eighties. It's a drum brake car and was set up for bias ply tires. It was my daily driver until I hurt my back at work and couldn’t comfortably sit in the bench seat and my 47 year old daughter got it as her first car when she got her license in high school.
My bought- it-new ‘68 GTX got switched over to radials nearly twenty years ago when I restored it
I run all of these vehicles between 32 and 35 psi.tire pressure (just like I do with my three Dodge Magnums)
Also, ANY gas filled shock is going to give you a harder ride. That’s to keep the radial tire firmly planted on the pavement. If you’re unsatisfied with the ride, put the original type shocks back on it.
On the oil topic, I run Valvoline VR-1 20-50 in the ‘34 and GTX and the rest of the fleet gets Shell Rotella with a bottle of ZDDP. Rotella used to have lots of zinc but not anymore.
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Tire Whisperer
New Member
Inflated to 33psi cold that 255/45R22 107H rated tire can support a gross/max weight of 1,786lbs.
Divide the heavier GAWR(gross axle weight)/2 = 1,488lbs per wheel.
Divide that tires load capacity at 33psi 1,786 / 1,488 = 1.2002 or 120% of the maximum rear axle load of that Nautilus vehicle. 20 percent safety margin.
Do the same calc. for the front GAWR, and you should arrive at a 125-130% of front axle load. 25-30percent margin.
Ford/Lincoln did their due diligence at arriving at that front/read cold tire pressure value, and there is ZERO REASON why anyone would want to run even 1psi higher pressure, except to keep ahead of falling autumn temperatures.
Tire Whisperer
New Member
The vehicle determines the maximum load, not the tire.
Write on that on the chalk board 100 times while the rest of the class is at recess.
That is why a wheel, and corresponding tire size with suitable margin, is selected for a new or revamped existing vehicle.
The 44psi or 51psi max pressure on the tire itself qualifies that size tire for use on a wide range of categories or sizes of vehicles,
As long as a cold inflation pressure is selected, after testing, that supports 110-120 percent of the heavier gross axle weight (curb plus 4-8 passengers plus fuel and cargo/luggage), the vehicle mfg did their job. A cold tire pressure of 130 percent or higher of heaviest gross axle weight, and you're entering fuel economy priority territory.
Lower pressure (105-110 percent of gross axle) is for ride comfort, or in the case of the Explorer, a cheap way to combat tip over tendency. 110-120 percent = blend of ride and handling.
"Drastic suspension changes" did not happen with the earlier radial tire options. Same suspension architectures as with bias-ply or bias-belted tires. Radials needed a little bit of alignment setting differences, being much more critical of toe-in settings due to their less-tread-scrubbing rolling down the road. So toe-in needed to be toward the minimum OEM setting, even down to less than 1 degree of toe-in with the mid-1980s then-new all-season radials (which also needed frequent tire rotations due to their tread design. Radials are more tolerant of camber settings, due to their more-flexible sidewalls. Many also desire to have higher caster, when possible.
How the tire is planted on the roadway is a function of the vehicle's designed-in camber patterns when the wheels turn. Chrysler products have put the outside tire into negative camber and the inside wheel into positive camber so better ensure the wheels are more perpendicular to the road surface. GM and Ford geometry did not do this, so when the car bodies leaned, so did the tires. There is one picture of a 1969 LTD in a sharper turn at 45mph, where the optional Michelin radial's tread was hanging onto the road surface as the outside of the rim it was on was several inches outboard of the tire's contact patch! Good thing for "safety rim wheels"!
All tires are designed to be "flat" on the road surface. Bias-ply tires do that just as other tires do, when they contact the road. The "contact patch" is always flat, unless the rim is too narrow for the tread width, and/or the tire has too much inflation pressure for the load. "Center tread wear" can result. With not enough air, the bias-ply tread can "buckle-up" in the middle and "edge wear" can happen, plus a marked decrease in handling precision and capabilities. The old rule was that tread width should be + or - 1 inch of the rim width, for best results. In the 1960s, few rims were 6" wide, so tire treads MIGHT be that wide in the larger sizes. When the high-perf 70-series tires came to be, rim widths of 6"+ started to become normal on cars where those tires were options. or a part of some HP package. By the later 1960s, Corvettes were using 15x7 and then 15x8 wheels for these new HP tires, markedly up from their prior 15x6" wheels of the middle 1960s.
A tire's loss of traction "at the limit" is more a function of several factors than just "bias-ply" or not. It might also be that the same tire on a '72 Chevelle SS 396, or a '70 GTX, or a '70 Torino GT might all act differently, due to their differences in suspension geometries. The '70 GTX would probably do well due to the Chrysler geometry, keeping the tires more perpendicular to the road surface. The Chevelle SS 396 might be "in that hunt" due to its 14x7 wheels and a "new-fangled" F-41 suspension and rear sway bar. The Torino GT would be a "typical understeering Ford", by comparison.
The same cars with radials would be similar, no doubt, but at higher cornering speeds. Some earlier radials (usually more brand-specific than not) DID have grip and then 1mph faster, did not, when pressed to their limits. Later evolutions gave a little bit more notice, though. In any event, "tire squeal" should be an indication that something MIGHT happen as to loosing grip on solid pavement. YET people trained in high-speed driving can know how far they can safely take their cars in such situations. As in trained law enforcement operatives. Tire tests by CAR AND DRIVER and others have noted the break-away characteristics in their testing. Some are more gradual than others, they noted, in wet or dry. Later TireRack tire comparisons also note the same things. Nothing "universal" in this respect, then or now.
There are THREE levels of gas shocks. The normal "gas shocks" from Monroe or similar have just enough internal gas pressure to need a cotton string tie to keep them compressed prior to installation. Like about 8psi or so. Just enough to help prevent "air bubbles" on the oil on rough roads. The NEXT step up would be like KYBs, where their internal pressure keeps the tire on the road more for better ride control . . . about 20% stiffer than OEM, from what a KYB installer told me. The FINAL step up is the Bilstiens, with the highest internal pressure. These can even raise the ride height of the car a bit.
How "hard" a car rides is more a function of the valving and its type than just on internal gas pressure. The valving is tuned to the vehicle the damper is installed in.
ZDDP in motor oil? Recent oil analysis posted to the www.bobistheoilguy.com indicate that Shell Rotella T6 5W-40 (diesel-rated oil, NOT the "Gas Truck" variant) has less zddp than in prior times, BUT it is till more than 1100ppm, which is above the "SL" level of 1000ppm zddp. Adding more zddp is not really needed, as the additional zddp can counteract the calcium detergency part of the additive package. Less calcium (as the current "SP" oils have) lets the zddp be more effective, even at a slightly-reduced zddp level. According to Lake Speed, Jr., architect of the "Driven" brand of motor oils. BTAIM He has several YouTube videos on the subject of motor oil.
When I transitioned into radial tires on my cars, I ran the normal air pressures that I normally did, about 30psi frt and 28psi rr. I prototyped that pressure bias on our '66 Newport Town Sedan with bias-ply, then BFG bias-belted Silvertowns, and continued on my '70 Monaco Brougham 383 "N" car with Pirelli P76 radials on the factory W23 15x6 road wheels. On that car, with its factory dual exhausts, I chose a JR78-15 over the stock equivalent H78x15 size to use the "radial sidewall flex" to maintain the stock ride height and not "drag the rear pipes" on sharp entry approaches and such.
Just my experiences in the diverse driving and temperature environment of N TX. Your experiences might vary in other parts of the world.
Enjoy!
CBODY67
Just Carbs
Well-Known Member
King pin inclination or included king pin angle produces camber changes in a turn. The more included angle the king pin spindle has, the more the camber will change when the steering wheel is turned. Both the outside and inside tires gain more positive camber. The inside can and generally does benefit from the increased camber, the outside it is assumed does not. The more caster the car has, the more the inside gains positive camber and the less the outside gains positive camber, so positive caster can be good from that standpoint. But, more positive caster also produces noticeable outside body lean do to more inside spindle drop, not a desirable feature.
These camber changes are miniscule in comparison to the unavoidable body lean induced camber changes from control arm geometry that can never be ideal for any passenger road car. That's one of the reasons sway control devices produce such big results, they greatly limit body lean induce camber changes and help keep the front and back tires singing the same song.
As for old car radial tire pressure? I set the back tire pressure as a compromise between rear end waddle and ride comfort, "then" set the front for understeer/oversteer scrub in relation to the rear adhesion. 30 to 34, depending on, well, everything.
These camber changes are miniscule in comparison to the unavoidable body lean induced camber changes from control arm geometry that can never be ideal for any passenger road car. That's one of the reasons sway control devices produce such big results, they greatly limit body lean induce camber changes and help keep the front and back tires singing the same song.
As for old car radial tire pressure? I set the back tire pressure as a compromise between rear end waddle and ride comfort, "then" set the front for understeer/oversteer scrub in relation to the rear adhesion. 30 to 34, depending on, well, everything.
Tire Whisperer
New Member
I was looking through some old service manuals going back to Kennedy and Eisenhower, and I noticed a negative caster spec on some, but not all, vehicles from that period. EG: "-0.5°, +-0.5°", allowing for 0 up to -1.0° of caster to be dialed in.
That is, the top of the steering axis, viewed from car side, was ahead of the bottom, through the tire contact patch.
Did some cars actually benefit from this counterintuitive set up, or did it have something to do with the dynamics of then prevalent bias-ply tires?
That is, the top of the steering axis, viewed from car side, was ahead of the bottom, through the tire contact patch.
Did some cars actually benefit from this counterintuitive set up, or did it have something to do with the dynamics of then prevalent bias-ply tires?
In a Chrysler MasterTech segment (in the printed material) I found in about 1967, although authored earlier than that, has illustrations to this affect. It is obvious that body lean is an influencer, too. It also illustrates the Chrysler geometry to counter-act the outside wheel "leaning over" in turns and how much better the Chrysler orientation of negative camber (for the outside wheel) better braces the sidewall against cornering forces by keeping it more perpendicular to the road surface. To me, the fact that this was obviously designed in the earlier 1950s, well before ultimate cornering capabilities was "a thing", was operable for anybody except law enforcement or emergency-responders.
I remember when, in about 1967, when Mustangs and Camaros were being raced in SCCA and also the Fairlane 500 GTA cars, Ford changed the lower control arm pivot to change the front suspension "roll center", keeping the outside wheel more perpendicular to the road surface, had benefits in total cornering power on the race courses. At the time, this was presented as "hard core suspension" improvements, but they also could not specifically follow the Chrysler geometry as it was probably copyrighted as a part of Chrysler Corp. vehicles, I suspect.
Suspension geometry is one of those things which costs the same if you "do good" or "just mediocre". There were many engineers working on the Chry. Corp. Neon vehicle design that were avid SCCA autocross competitors. They basically built an autocross car from scratch, as to geometry. Then varied the spring rages and damper calibrations to cover the base models and the HD suspension orientations. Plus a few tweaks for the HP 4cyl engine. In their first year of national competition, Neons swept the first 10 places in their class competition. Leaving the prior year's winners (Mazda Miatas owners) scratching their heads as to how Miatas got de-throned suddenly by a new vehicle they didn't see coming. Same thing happened for a few more years! Eventually, the Miata owners had had enough, so they petitioned the SCCA to allow "trunk kits" (of non-production line parts to be installed on their Miatas so they could become competitive again. The ultimate NEON in this case became the "factory racer" Neon ACR version, as the Neons aged-out of that class. Geometry costs nothing. Damper valving costs "the same", with springs being very minorly-more expensive. Performance tires cost a little more. Of the few Neons I rented, it took a while to realize that I didn't have to slow down to turn corners in town (as with other cars). Which increased the "fun factor" many notches higher. A performance car hiding in econobox clothing.
Check out that Chrysler MasterTech program.
CBODY67
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