tire presure
Burning Brakes
Joined: Feb 2014
Posts: 1,045
Likes: 226
From: Cape Neddick Maine
2020 C3 of the Year Finalist - Modified
OK here goes too broke may be on to something here hear me out
My 78 with a 496 BBC I used to run 26 psi in the front and 4.5 in the rears.
Front tires where 26" Good Years and rears 33 x 17 x 15 Hoosiers. Car did weigh 1850 with loaded with nothing in the weight bar LOL
any thing less than 4.5 it would dance on the top end and more it would just spin
My 78 with a 496 BBC I used to run 26 psi in the front and 4.5 in the rears.
Front tires where 26" Good Years and rears 33 x 17 x 15 Hoosiers. Car did weigh 1850 with loaded with nothing in the weight bar LOL
any thing less than 4.5 it would dance on the top end and more it would just spin
Last edited by jkippin; Dec 4, 2015 at 07:09 PM.
Drifting
Joined: Jan 2012
Posts: 1,908
Likes: 129
From: Fredericksburg Virginia
How was the wear pattern on the slicks?
I fully concede that there are certain instances and certain tires where low pressure is warranted. I run 8 to 10 lbs of air in my truck when I'm driving on the beach.
We're talking about everyday stuff over the course of many, many miles.
I fully concede that there are certain instances and certain tires where low pressure is warranted. I run 8 to 10 lbs of air in my truck when I'm driving on the beach.
We're talking about everyday stuff over the course of many, many miles.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
As we live in America you are free to run 100 psi if you want. Your tires may resemble black doughnuts but what does their appearance hurt?
When the centers wear out first they are over inflated.
When the centers wear out first they are over inflated.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
The original tire size was 275/40/18 and the door lists 32psi. I've talked to others who did run 32psi as listed on the door in their 305/30/19 tires and they had center wear. So, I ran with a slightly lower pressure and it seemed to fix the center wear. If you actually look at the numbers, 275/305 x 32lbs = 29lbs so that could either be a co-incidence or the change in width could be used to indicate how much to change the pressure.
Lets look at the tire numbers. Each of the tires is rated for 1900lbs at 50psi. The C5 has a curb weight of 3300lbs with a 50% weight split. The calculation suggested in this thread comes out to 22psi. This is not the correct tire pressure and illustrates that there is no truth to this calculation.
Last edited by lionelhutz; Dec 5, 2015 at 01:02 AM.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
I've recently replaced the rear tires on my C5 due to wearing them out driving the car. The take-offs were 305/30/19 Michelin PS2's with a 20k miles tread life warranty. I tried to keep between 28lbs and 30lbs cold in them. They went almost 40k miles and showed a nearly perfectly tread wear pattern.
The original tire size was 275/40/18 and the door lists 32psi. I've talked to others who did run 32psi as listed on the door in their 305/30/19 tires and they had center wear. So, I ran with a slightly lower pressure and it seemed to fix the center wear. If you actually look at the numbers, 275/305 x 32lbs = 29lbs so that could either be a co-incidence or the change in width could be used to indicate how much to change the pressure.
Lets look at the tire numbers. Each of the tires is rated for 1900lbs at 50psi. The C5 has a curb weight of 3300lbs with a 50% weight split. The calculation suggested in this thread comes out to 22psi. This is not the correct tire pressure and illustrates that there is no truth to this calculation.
The original tire size was 275/40/18 and the door lists 32psi. I've talked to others who did run 32psi as listed on the door in their 305/30/19 tires and they had center wear. So, I ran with a slightly lower pressure and it seemed to fix the center wear. If you actually look at the numbers, 275/305 x 32lbs = 29lbs so that could either be a co-incidence or the change in width could be used to indicate how much to change the pressure.
Lets look at the tire numbers. Each of the tires is rated for 1900lbs at 50psi. The C5 has a curb weight of 3300lbs with a 50% weight split. The calculation suggested in this thread comes out to 22psi. This is not the correct tire pressure and illustrates that there is no truth to this calculation.
First of all this is a C3 forum; not a C5 forum. The C3's used much larger tires than the C5's used so any uh "formula" for a C3 doesn't apply to a C5. I want you to imagine tires that are 1 mile in diameter and 1/4 mile wide (are you still with me?). If you put 4 of them on a modern aircraft carrier they would easily support the massive weight with about 2 psi in them. You see, the larger a tire gets it can support a given weight at a lower pressure. Like a 450# Yamaha or Honda quad that uses four 10" wide tires with only 2 psi in them. The larger the tire is the less pressure is required and that's why C3's with their HUGE 255X60R15's only require 20 psi.
Melting Slicks
Joined: Mar 2008
Posts: 2,989
Likes: 198
From: Meriden CT
First of all this is a C3 forum; not a C5 forum. The C3's used much larger tires than the C5's used so any uh "formula" for a C3 doesn't apply to a C5. I want you to imagine tires that are 1 mile in diameter and 1/4 mile wide (are you still with me?). If you put 4 of them on a modern aircraft carrier they would easily support the massive weight with about 2 psi in them. You see, the larger a tire gets it can support a given weight at a lower pressure. Like a 450# Yamaha or Honda quad that uses four 10" wide tires with only 2 psi in them. The larger the tire is the less pressure is required and that's why C3's with their HUGE 255X60R15's only require 20 psi.
I have 245/45R17 front, 275/40R18 back on my '79. Does that mean I have a C5 now?
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Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
Say what???? Both 275 and 305 are wider section widths than a 255 so how the hell can you say a 255 is larger? Also, you have never ONCE put any limits on your formula. Is it possible for anyone to back peddle any faster than this?
Burning Brakes
Joined: Jan 2012
Posts: 943
Likes: 73
I asked about putting 20 psi in my vette's 255/60/15 and right away the response was very strong to say the least. They said I would be absolutely crazy/negligent to under inflate a radial tire and the first sizable pot hole I hit would most likely pop the tire from the rim creating a very dangerous situation.
Right away they knew it wasn't me and I won't repeat what they said about the person I encountered that came up with the 20psi scenario.
Race Director
Joined: Dec 1999
Posts: 11,095
Likes: 990
From: Sierra Foothills CA
Stopped by and picked up a new spare for my 80 this morning at a place I've done business with for over 25 years.
I asked about putting 20 psi in my vette's 255/60/15 and right away the response was very strong to say the least. They said I would be absolutely crazy/negligent to under inflate a radial tire and the first sizable pot hole I hit would most likely pop the tire from the rim creating a very dangerous situation.
Right away they knew it wasn't me and I won't repeat what they said about the person I encountered that came up with the 20psi scenario.
I asked about putting 20 psi in my vette's 255/60/15 and right away the response was very strong to say the least. They said I would be absolutely crazy/negligent to under inflate a radial tire and the first sizable pot hole I hit would most likely pop the tire from the rim creating a very dangerous situation.
Right away they knew it wasn't me and I won't repeat what they said about the person I encountered that came up with the 20psi scenario.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Because I'm ignoring you. I will admit my term "cross section" was an error as it should have been volume so I am guilty of that mistake. For those of you who are running the larger 225 thru 275X60R15's PLEASE run 30+ pounds so you can witness your centers wearing out while the sides go untouched. There's no better way to learn than learning the hard way!
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I've been running BFG radial TA's since they first came out in sizes up to 295/50/15 and at 32lbs never encountered uneven wear of any sort. I don't know what your smoking but please pass it around so we can see these wear problems you are talking about.
Race Director
Joined: Dec 1999
Posts: 11,095
Likes: 990
From: Sierra Foothills CA
Because I'm ignoring you. I will admit my term "cross section" was an error as it should have been volume so I am guilty of that mistake. For those of you who are running the larger 225 thru 275X60R15's PLEASE run 30+ pounds so you can witness your centers wearing out while the sides go untouched. There's no better way to learn than learning the hard way!
Last edited by Les; Dec 5, 2015 at 04:43 PM.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
When the centers wear out first they are over inflated.
What happens to the diameter of an under inflated tire as it's rotational speed increases and centripetal forces act on the shape (width) and size of the tire.
I believe pressure not only controls the size of the tire patch on the road but also the shape and size of the tire as it's RPM's increase.
Isn't less pressure going to allow the center of the tire to deflect outward more at high RPM due to lack of side wall stiffness creating more wear on the center patch, not less?
Last edited by REELAV8R; Dec 5, 2015 at 05:23 PM.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
The same can be said about your formula for calculating the correct pressure. There is no evidence supporting it.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Go ahead and pump 30+ psi into your 255X60R15's and look at them 20,000 miles later. You'll find their centers are worn out when their sides still have 1/4" to 5/16" of tread left. That's what you call over inflation.
