Tire Pressure Question, Bias Ply Tires

[Centurion]

I had a new set of wide whitewall bias ply tires installed on my '59 Electra during the weekend, replacing the Coker bias ply tires I had installed in 2004.

The new tires are the B.F. Goodrich Silvertown - branded tires, sold by Universal Tires.

The tire store who installed the tires for me recommended running 35-psi in the tires, and aired them up accordingly.

I had always been very satisfied with the ride and handling of the old Cokers, but think I was packing pressures in the 30 to 32-psi range. My impression during my short drive home from the tire store during the weekend was that the car's ride is now much harsher.

Since I'll be leading a driving tour of about a thousand miles with the car in a couple of weeks, I'd rather not feel like my wife and I are riding in a "lumber wagon".

Any thoughts for me on the optimal tire pressure for bias ply tires? Will dropping the pressure from 35-psi to 32-psi make a significant difference in ride quality?

Thanks in advance for your input.

[buick5563]

The advantage of a little higher pressure is slow speed steering and parking. Disadvantage is a harsher ride and maybe a little more wander. I usually run mine at 35 for the easier parking with manual steering. I will run them lower for cross country cruising on the highway (32 range). I have personally not have any ill effects either way.

[Centurion]

Mike, very helpful! I think I'll dial the pressure back to 32-psi for the upcoming tour.

[Restorer32]

Growing up in Dad's tire shop in the late 1950's thru 1970 we almost universally aired tires to 32 psi. Only when radials started showing up did that rule begin to vary.

[Centurion]

Thanks for the confirmation, Restorer32!

[Yellowriv]

I find there is a tipping point at 28 - 30 psi. More than this and they are very harsh on my 53 Riv. I don't think cross plys were designed to run at more than 28 - 30.

[old-tank]

I had a new set of wide whitewall bias ply tires installed on my '59 Electra during the weekend, replacing the Coker bias ply tires I had installed in 2004.

The new tires are the B.F. Goodrich Silvertown - branded tires, sold by Universal Tires.

The tire store who installed the tires for me recommended running 35-psi in the tires, and aired them up accordingly.

I had always been very satisfied with the ride and handling of the old Cokers, but think I was packing pressures in the 30 to 32-psi range. My impression during my short drive home from the tire store during the weekend was that the car's ride is now much harsher.

Since I'll be leading a driving tour of about a thousand miles with the car in a couple of weeks, I'd rather not feel like my wife and I are riding in a "lumber wagon".

Any thoughts for me on the optimal tire pressure for bias ply tires? Will dropping the pressure from 35-psi to 32-psi make a significant difference in ride quality?

Thanks in advance for your input.

Can you post a picture of those tires that you got from Universal? The price is the same as Coker, but the picture is different (both Goodrich Silvertown). The Goodrich Silvertown from Coker rides and handles well at 32 psi, especially when new with tall tread. If these are in fact different than the Coker, and not just resellers, there may be differences in the construction that affect ride (I cannot believe that 2 vendors are selling completely different tires with the same name).

Willie

[Restorer32]

Doesn't Coker now own Universal?

[Centurion]

Willie, yes, I will post a picture within the next day. I'll be eager to hear whether you think they are the same tire or not.

[60FlatTop]

I replaced a set of Coker 8.20 X 15 Firestones with new Coker's of the same size last year. The only difference is more tread. I saved about $150 buying them through Summit Racing.

If the car was driving well and feels like a lumber wagon after a shop installed the new tires my first guess would be that they dropped it off the lift when you weren't looking. If they picked it up with a two post lift at the wrong points they could have screwed the car up.

I had to put my '60 on a two post lift one time. I hated to do it. I always use a drive on ramp lift.

In the 1990's I changed out a few Cadillac steering boxes. A nephew was working for me and I showed him how flexible the frames were. If you put the jack stands under the cowl and lower the jack so the rest on them, you can watch the front end bow downward another 3" or more as the jack is released. Imagine a shop leaving your car like that over night. Yeah, I'm too fussy.

I would have the camber checked right away. Incorrect camber will make the front wheels bang over every tar strip.

I wasn't born cynical and distrustful, professionals made me this way. Mechanics, doctors, and politicians contributed equally.

Bernie

Edited April 24, 2013 by 60FlatTop (see edit history)

[buick5563]

I replaced a set of Coker 8.20 X 15 Firestones with new Coker's of the same size last year. The only difference is more tread. I saved about $150 buying them through Summit Racing.

If the car was driving well and feels like a lumber wagon after a shop installed the new tires my first guess would be that they dropped it off the lift when you weren't looking. If they picked it up with a two post lift at the wrong points they could have screwed the car up.

I had to put my '60 on a two post lift one time. I hated to do it. I always use a drive on ramp lift.

In the 1990's I changed out a few Cadillac steering boxes. A nephew was working for me and I showed him how flexible the frames were. If you put the jack stands under the cowl and lower the jack so the rest on them, you can watch the front end bow downward another 3" or more as the jack is released. Imagine a shop leaving your car like that over night. Yeah, I'm too fussy.

I personally wouldn't worry about this^^^

I would have the camber checked right away. Incorrect camber will make the front wheels bang over every tar strip.

This is not a bad idea^^^

I wasn't born cynical and distrustful, professionals made me this way. Mechanics, doctors, and politicians contributed equally.

Bernie

I agree with this 100%

[Centurion]

Thanks guys. The car was on the floor of the shop the full time, with floor jacks used at each of the four corners. Nothing was dropped; I observed the entire operation.

The alignment was checked, and I saw everything. The alignment was remarkably close to specifications, despite having last been aligned in 2004. Old tires wore evenly.

[NTX5467]

The issue of tire pressure has NOT changed, by observation. Personally, I think I'd use 30frt/28rr. AS y'all might recall, the old "soft ride" (minimum) inflation pressure was 24psi, or even LOWER with the introduction of "balloon" tires in the mid-1950s. Soft ride w/related somewhat sloppy handling and steering response, plus greater tire wear from bias ply tires being bias ply tires. For "high speed"/highway speeds, up to about 75mph, "Add 4psi to the base inflation pressure" was usually what many owners' manuals noted in the 1970s. For "full load" times, that's when things got up toward the 32psi range (for the older tires, before P-metric sizing).

I soon learned that with the weight bias of most cars back then being about 55% front/45% rear (unloaded), if we use the "safe" +4psi orientation for all normal driving, then that puts things at 28psi "minimum". Adding the weight bias into the mix (after I found a tire weight capacity vs. inflation pressure chart in the later 1960s!), adding 2psi to the front tires only has some good results. The tires on our cars lost their "edge wear" from turning corners (even at normal speeds), and the tires wore "flat across". With the ocassional rear seat passengers and no real "stuff" in the trunk, the rear tires also wore "flat across" at their lower 28psi. End result was that each end of the vehicle's tires were carrying their proportional part of the vehicle's weight, by varying the tire inflation pressure to make that happen. The extra 2psi in front increased steering response, too, which added to the "Joy of Driving". For me, this is what works! I still use that 40+ years after I first came to the concept of having each set of tires support the same proportional part of the vehicle's weight, giving them the "just right" inflation pressure for what they were supporting.

Many front wheel drive, and smaller rear wheel drive V-8 cars (1982+ cast iron V-8 Camaros, for example) are more like 60% front, which would mean a "+4psi" orientation, but even the previous +2psi works for them too.

My experience with "tire store inflation pressures" is that they need a somewhat consistent inflation pressure recommendation for a few reasons. First, to ensure the tire's bead is fully seated on the rim. Second is to ensure that the tires left their shop with "enough air in them". Third, might be that it's less stuff for their employees to remember.

With bias ply tires, the "softness" of the ride allegedly results from the angle at which the carcass plies cross. This angle is within a particular range. I remember a Firestone 500 (pre-radial "500") tire add touting its better performance capabilities due to the 31 degree angle of its plies, meaning that 35 degrees would be more "softer ride" in nature, I suspect. Yet, as with any tire, the tread depth can also influence "impact harshness", as can inflation pressure.

In general, my orientation is 30 frt / 28 rear, possibly upping that to 32 frt and 30 rear for hot weather, "luggage and rear seat passengers" higher speed highway treks, etc. To me, it's proven to be possibly the best balance between steering response, fuel economy gains(?), tire longevity, and comfort.

In many cases, the vehicle manufacturer's recommended tire pressure, back then, were more oriented in "safety" (in having the pressures low enough to perform credibly but not high enough to encourage "spirited" driving, due to the earlier onset of "tire squeal"). Yet if you take a more engineering approach, as I did, you can soon realize that they WANTED the cars to understeer rather than otherwise. Be that as it may . . .

Just some observations,

NTX5467

Edited April 25, 2013 by NTX5467 (see edit history)

[old-tank]

Check to pressures with a known accurate gauge...there can be 5+ psi error in some cheap ones.

[buick man]

Wow. A lot of neat pointers here. I know when I tracked my porsche we corner balanced each tire. And yes as NTX5467 states, taking into consideration the weight bias from front to rear is vitally important in handling and ride quality as this tends to be a linear equation.

Always check and adjust tire psi "cold " and not warm or just off the road at the gas station cause PV=NrT always comes into play. ( The three sisters: Pressure, Volume, & Temperature)