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Yeah, well I run blah blah blah in my tires and so should you. This is why you go to a doctor when you're sick; not a forum. If your door jamb sticker tells you to run 20 psi in your 255X60R15's then run 20 psi. The pressure needed in tires is not a matter of majority rules but rather what the vehicle manufacturer calls for.
That information is what was used for the tire technology for that era, tire technology has changed considerably since then. What if your 20 psi on the sticker might be for use with bias ply tires are you going to go get a set?
Yeah, well I run blah blah blah in my tires and so should you. This is why you go to a doctor when you're sick; not a forum. If your door jamb sticker tells you to run 20 psi in your 255X60R15's then run 20 psi. The pressure needed in tires is not a matter of majority rules but rather what the vehicle manufacturer calls for.
The safe max inflation is embossed in the tire sidewall, by the tire manufacturer.
...but rather what the vehicle manufacturer calls for.
I'm sorry...let me fix that last statement for you so it suits the conversation better:
"...but rather what the vehicle manufacturer callED for [41 years ago]."
Fuel octanes have changed in 41 years, forcing us to modify how the engines are built (couldn't put an 11:1 370 horse LT-1 back in my car).
Fuel composition has changed in 41 years, forcing us to consider Methanol when purchasing rubber bits for our carbs.
Paint technology has changed in 41 years, giving us far greater opportunity for a long-lasting paint job than lacquer.
Tire technology has changed in 41 years, forcing us to THINK FOR OURSELVES when considering the little placard in the door jamb.
ADDITIONAL INTERESTING FACTS AND INFORMATION:
I just confirmed that the Corvette originally came with an F70-15 bias-ply tire (which is 225-70 tire). Not narrow, but not enormous.
It was VERY common for tire pressures to be modified to alter RIDE since a 30+ lb bias ply tire rode like a rock.
I'm not old enough to have a lot of bias-ply tire experience, but they kept their shape much better than radials, right? Wasn't there a whole thing where folks that were 'new' to radials were over-inflating them to get rid of the 'bulge' at the point where it contacts the road?
I can't find much conversation relative to bias-ply tires, but what I did find clearly indicated that folks were playing with pressures from 20 psi to 34 psi to improve ride.
Structurally, I can understand how a "bias" (crossing) ply-ing pattern would be more supportive of the contact patch. This is WHY they don't bulge like radials when loaded. The greatest tension would occur at the center of the tire, giving it the most support.
With that being the case, I can see how it would not be "as bad" to run lower pressures in a bias-ply tire.
With a radial-ply tire, the center of the contact patch is supported by little more than the internal air pressure. This is why they bulge. And this is why they are more sensitive to internal pressure to maintain their shape.
I totally get the "Tire Rack" article that Mr. MelWff directed us to. Makes perfect sense.
Last edited by keithinspace; Dec 2, 2015 at 09:43 PM.
For the record, the OP has gotten the direction he needed.
Yes, and now its a verbal battle between a mechanical engineer who knows what kind of pressures tires need versus weekend hobbyists who THINK they know what kind of pressures tires need. Its all about the tire's cross section area X the psi. With an air bag that is large enough in area you can lift a battleship with only 1 psi.
Yes, and now its a verbal battle between a mechanical engineer who knows what kind of pressures tires need.
So, you worked as a tire engineer? Post your resume which qualifies you as a tire expert and not just another weekend hobbyists when it comes to any expertise on tires. Otherwise, that "argument" is about as relevant as thumping your chest and saying "I'm smarter than you, na, na take that".
The simple fact is that tire technology has changed in 41 years so the number on the car door should not be blindly followed. Heck, even changing tire manufacturers on a new car could mean the pressure as specified on the door needs a small adjustment.
Last edited by lionelhutz; Dec 2, 2015 at 11:17 PM.
Are you suggesting the laws of physics have changed in the last 40+ years? The force required to support a given load will remain the same no matter how much time has passed. A tire is nothing more than a pressure vessel that contains the pressure whereas the pressure is the force required to support the load. In this case 20 psi is required in a 255X60R15 tire to support each corner of a C3.
Are you suggesting the laws of physics have changed in the last 40+ years? The force required to support a given load will remain the same no matter how much time has passed. A tire is nothing more than a pressure vessel that contains the pressure whereas the pressure is the force required to support the load. In this case 20 psi is required in a 255X60R15 tire to support each corner of a C3.
The OP is not utilizing a tire from the 70's. Support pressure for a load is totally dependent on the container design and construction, as verified by 2 Mechanical Engineer nephews.
You're correct but that 35 psi MAXIMUM pressure has nothing to do with the recommended pressure on a C3 which only requires 20 psi.
I think its funny how you ran to look at the sidewall of one of your tires, lol! So corvette tires only need 20 psi, and their vacuum secondary carbs shall not flow more than 600 cfm, lol! Got it, candy wrapper machine engineer!
Are you suggesting the laws of physics have changed in the last 40+ years? The force required to support a given load will remain the same no matter how much time has passed. A tire is nothing more than a pressure vessel that contains the pressure whereas the pressure is the force required to support the load. In this case 20 psi is required in a 255X60R15 tire to support each corner of a C3.
Are you sure you're a mechanical engineer? The design of the tire affects the pressure that should be used in the tire.
Yes, and now its a verbal battle between a mechanical engineer who knows what kind of pressures tires need versus weekend hobbyists who THINK they know what kind of pressures tires need. Its all about the tire's cross section area X the psi. With an air bag that is large enough in area you can lift a battleship with only 1 psi.
And here we have someone claiming to be a mechanical engineer for 30+ years that thinks he knows more about tires than tire manufacturers.
Are you suggesting the laws of physics have changed in the last 40+ years?
No, I am STATING (not suggesting) that the fundamental way a tire is constructed has changed in the last 40+ years.
For an F70 x 15 4-ply tire (BIAS), the recommended "cool" pressure was 24 PSI and the recommended "hot" pressure was 30 PSI.
When the radial tire was first introduced to the Corvette in 225 section, the recommended tire pressure was indeed 20/20 front/rear.
In 1976, chat was changed to 20/26 front/rear.
I found a 1982 Chevrolet Corvette tire pressure door label on this forum (Mr. BFI6603 on 07-22-2011, post #8 to a C3 General thread entitled "Tire Pressure") that clearly states the recommended pressures for the factory fitted P255/60/R15 tires to be 35 front and 35 rear.
My conclusion from all this is that YES, ABSOLUTELY something was happening. YES, ABSOLUTELY something changed.
I suspect one of two things.
1) They didn't quite understand/appreciate the long-term impact of recommending such low pressures in a radial tire.
or
2) The early radial tires were built like brick ****houses with enough steel in them to support a small village, negating the need to run a lot of pressure to keep the shape of the contact patch.
Whichever of those two things happened, the evidence is VERY clear.
The EARLY radials had low recommended pressures, but for the LATER radials...
---SAME MAKE of car
---SAME GENERATION of car
---SAME weight
---WIDER tire (225 early to 255 late)
...the manufacturer increased the recommended COLD tire pressure by 75% over the course of 8 years of fitting radials to that particular car (the Chevrolet Corvette).
(For those of you that aren't big math geeks, that's the better part of double.)
I mean this in the most sincere way: There isn't anything left to say. The tires manufactured today are more similar in construction to the tires fitted to the 1982 Corvette than the 1974 Corvette. Period. Every single scrap of evidence points to that fact.
I COULD say that I'm right because I'm a Mechanical Engineer. But that's not true. I am, indeed, a Mechanical Engineer, but that doesn't mean I know crap about tires. I'm basing the above FACTS on evidence derived from investigation and common sense.
ANYBODY CAN RUN WHATEVER PRESSURES THEY WANT IN THEIR TIRES!!! I'm not trying to convince someone to do something different. I'm just trying to debunk what is clearly erroneous information.
FWIW, personally, I'd start at 32 and tweak up and down from there based on tire wear.
Last edited by keithinspace; Dec 3, 2015 at 11:13 AM.
Good info Keith. You've hit on the major points exactly. I'd bet it's both, the tire construction changed and GM learned how to properly use radial tires.
I'll be honest...I was actually doubting I could resolutely prove my point until I found the sill placard from the 1982 car. That really tells the tale, doesn't it?
In case anyone doubts that is from a Corvette, look at the weight capacity and number of occupants. I don't know what other car Chevy put out that only carried 450#, 2 occupants, and used 255/60R15 tires.
As far as any "engineering" qualification, that's meaningless in my book.
---I could be a freaking ballet shoe designer for all anyone knows.
---I could have a business hand-crafting handles for #1 Phillips-head screwdrivers.
---I could be the Lead Toast Inspector for the Assistant Secretary of the American Bowling Association.
Doesn't matter. The maker of our vehicles saw it fit to recommend a thing. And that thing they recommended CHANGED. And those changes very accurately follows the flow of logic/learning from a bias-ply world to a radial-ply world, then to a modern radial-ply tire.
I really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really, really hope this issue is finally put to out to the pasture.
Then shot.
With a really big gun.
Multiple times.
[Reload]
And shot some more.
Because we need to be sure it's dead.
Last edited by keithinspace; Dec 3, 2015 at 01:37 PM.
No, I am STATING (not suggesting) that the fundamental way a tire is constructed has changed in the last 40+ years.
For an F70 x 15 4-ply tire (BIAS), the recommended "cool" pressure was 24 PSI and the recommended "hot" pressure was 30 PSI.
When the radial tire was first introduced to the Corvette in 225 section, the recommended tire pressure was indeed 20/20 front/rear.
In 1976, chat was changed to 20/26 front/rear.
I found a 1982 Chevrolet Corvette tire pressure door label on this forum (Mr. BFI6603 on 07-22-2011, post #8 to a C3 General thread entitled "Tire Pressure") that clearly states the recommended pressures for the factory fitted P255/60/R15 tires to be 35 front and 35 rear.
My conclusion from all this is that YES, ABSOLUTELY something was happening. YES, ABSOLUTELY something changed.
I suspect one of two things.
1) They didn't quite understand/appreciate the long-term impact of recommending such low pressures in a radial tire.
or
2) The early radial tires were built like brick ****houses with enough steel in them to support a small village, negating the need to run a lot of pressure to keep the shape of the contact patch.
Whichever of those two things happened, the evidence is VERY clear.
The EARLY radials had low recommended pressures, but for the LATER radials...
---SAME MAKE of car
---SAME GENERATION of car
---SAME weight
---WIDER tire (225 early to 255 late)
...the manufacturer increased the recommended COLD tire pressure by 75% over the course of 8 years of fitting radials to that particular car (the Chevrolet Corvette).
(For those of you that aren't big math geeks, that's the better part of double.)
I mean this in the most sincere way: There isn't anything left to say. The tires manufactured today are more similar in construction to the tires fitted to the 1982 Corvette than the 1974 Corvette. Period. Every single scrap of evidence points to that fact.
I COULD say that I'm right because I'm a Mechanical Engineer. But that's not true. I am, indeed, a Mechanical Engineer, but that doesn't mean I know crap about tires. I'm basing the above FACTS on evidence derived from investigation and common sense.
ANYBODY CAN RUN WHATEVER PRESSURES THEY WANT IN THEIR TIRES!!! I'm not trying to convince someone to do something different. I'm just trying to debunk what is clearly erroneous information.
FWIW, personally, I'd start at 32 and tweak up and down from there based on tire wear.
Uh, did you happen to notice the sticker says MAXIMUM LOAD?
