Right Tire Pressure

[Texasdino]

Hi,

 

I have a 1958 Buick Special with radial tires and a question.

 

Which tire pressure is right for my dreamcar?

 

Thanks for a information.

 

Jens from Germany 

[Beemon]

32 to 35 is where I set mine. 

[JohnD1956]

The Diamondback company says that for wide white walled radials, inflate to the tire's maximum pressure as shown on the side wall.

[EmTee]

4 hours ago, Beemon said:

32 to 35 is where I set mine. 

 

Or, 220 to 240 kPa... 

[60FlatTop]

Or 2.5 Bar, PSI is so..... colonial.

[Bloo]

When in doubt, 35 pounds on any old American car with radial tires.

[Beemon]

4 hours ago, JohnD1956 said:

The Diamondback company says that for wide white walled radials, inflate to the tire's maximum pressure as shown on the side wall.

 

That's a little bit scary. Usually the psi value on the sidewall is the max tire pressure the tire can handle. The road heat will increase the internal pressure past the max pressure limit if inflated to the max pressure limit. I would check the OEM spec of a like sized SUV or truck (since cars these days run thin tires on big alloy rims) and see what they recommend. 

 

Unless it says max inflate pressure, I would cross check with a similar OE application. Max inflate pressure and max tire pressure are two different and very important numbers. 

Edited March 17, 2018 by Beemon (see edit history)

[Barney Eaton]

I recently read this ........it seems reasonable but ......maybe just one persons opinion/idea.

 

Lets say you have a new tire and the sidewall says  35 psi max and the load rating is 1753 lbs.  (this is the rating for a 225 70R15 BFG, other brands should be the same)

Lets say the Buick you are going to put them on weighs 4000 with 2 people. 

 

1753 x (4 tires) = 7,012 lbs    .......multiply 4000 lbs (the weight of the car) by the 35 psi and you get  140000,  divide that by 7,012 and you get  19.9 psi or 20 psi

The authors suggests that the 20 psi will support the weight of the car and should wear correctly based on the load rating  of the tires on that vehicle.

 

I am sure most of you think 20 psi is way too low,  but when you look at the numbers, it seems reasonable.

So just to be safe......run 25....or 28    think about it and maybe even run the numbers for your tires and vehicle then give it a try.  

 

[Bloo]

That is, I think how the factory numbers were arrived at on some cars. It results in bad gas mileage, mushy handling, short tread life, and overheated tires in hot weather.

 

The way I remember it, most drivers would complain about "flat tires" at 26 psi. Almost everyone ran 32 or 35 psi in radial tires. Since many of the cars originally came with bias, and the tire manufacturers (of the new radials) only specified a maximum, there was no "official" recommendation.

 

32, 35, and 44 were the maximum pressure ratings you would see on passenger car radials back then. I, and every other gas station jockey I knew used 35 pounds (or 32 pounds if that was the maximum rating) unless someone would ask for a different pressure. Maybe 1 in 1000 people asked for a different pressure.

 

Out here in Central Washington State, in the 110 degree summer heat, tires run down in the 20's would get hot enough to burn your hand when you checked the pressure, and the sidewall rubber would start to look rough. The speed limit was only 55mph then. It would be worse today.

 

I am not arguing the math. You are correct about the ratings, and your example gives a typical result. I still wouldn't do it.

Edited March 17, 2018 by Bloo (see edit history)

[Bloo]

FWIW "maximum pressure" on a tire sidewall is a "maximum cold pressure" rating on every tire I have ever seen. It is expected and normal that the pressure will run higher when the tire gets hot. It is part of the design.

 

[Grimy]

Big difference between bias and radial pressures, too.  Since DOT has mandated max tire pressure (TP) in sidewalls, those numbers are for COLD inflation.  And those are MAXIMUM pressures.

 

Back in the dark ages (late 50s/early 60s) when I worked in a gas station, contemporary cars had decals, usually inside the glove box door, with "recommended" (VERY LOW) tire pressures, like 24-26.  Tires were only good for 18-20,000 miles in those days, and those "recommended" pressures were intended to give a soft boulevard ride.  We gas jockeys always inflated those tires to 30-32 psi, EXCEPT...  My boss insisted that any car brought in for a tune-up always got a free wash, glass cleaned inside and out, and dashboard dusted and polished, AND the tp adjusted to the recommended amount.  Customers always said that was the best tune-up they'd ever had! 

[KongaMan]

6 hours ago, JohnD1956 said:

The Diamondback company says that for wide white walled radials, inflate to the tire's maximum pressure as shown on the side wall.

 

What does the wide white wall have to do with tire pressure?

 

2 hours ago, Bloo said:

When in doubt, 35 pounds on any old American car with radial tires.

 

Obama would be proud.  

[NTX5467]

When the P-metric radials came out, they were usually physically under-sized and needed the additional pressure to ensure they would carry the weight.  As things progressed, the "revs/mile" spec tended to apparently become more operative . . . also as many learned to get the metric measure conversion for their existing tire width.

 

When the '58 was built, it was deep into the "balloon tire" era, or toward the end of it.  Larger section width tires (still on narrow rims!) for more air volume at less pressure.  It was about "smooth ride", but many owners manuals might also contain info on how to modify the base recommendations if more passengers and luggage were in the car, plus if the car was driven at "highway speeds".

 

About this time, the base tire pressure tended to become 24psi.  If more passengers/load was in the car, more tire pressure.  If the car would see highway speeds and normal loads, then about 28psi cold was the recommendation.  These would be the MINIMUM COLD recommended inflation pressures!  NO bleeding of air pressure from a hot tire!

 

Although others have stated they generally run more air pressure, you might start with 30psi cold on the radials and see if that works for how the car is used.  Some in here go higher than that, but usually not more than what the sidewall pressure specs for "Max Load".

 

I used something akin to what Barney mentioned to first get the lighter-loaded rear tires to "min spec" for highway speeds, then increase the front tire pressure as that's where the greater axle-loading is, so that the front tires and rear tires have the same load-carrying relationship.  For a car with 55/45 f/r weight distribution, that ended up being 30 f/28 r pressures on bias ply and bias-belted tires.  I started to use 32psi front on radials and 30psi rr.  AND then you get to the relationship of the tire's tread width and wheel rim width, such that the tire's tread doesn't bow outward too much, which might be more of an issue with bias plies than with radials.  IF you think those repro 4.5" wide treads look skinny, consider they were usually on 5" wide rims, which worked pretty good.  The recommendation is that the tire's tread width be in a range of +/- 1 inch from the wheel's rim width (measured BETWEEN the "lips" on the rim's edge, where the tire's bead seats.  Using a little wider wheel width can necessitate the use of higher air pressure to get the tire's tread flat against the road surface AND make the tire's sidewalls more vertical than rounded, which can hurt ride softness and help steering response/handling.  Several things to consider!

 

NTX5467

[old-tank]

Start with 35 psi and experiment with what suits you.  I have the second set of radials on one of my 55's from Diamondback.  Built on the Toyo Extensa A/S, the ride and handling is what I like at 35 psi.  Previous set was built on the Firestone FR380 and that tire needed 44 psi to give it the same structure.  It depends on the tire and personal preference.

[KAD36]

Just happened to set them at 35 cold today before a short drive (Diamondback 225s). 

 

Used to keep the bias ply at 30-32 (L-78s).

[Airy Cat]

The side wall of my tires show a maximum inflation of 35 psi.  I have been inflating all of my tires to 32 psi when cold.

I have been doing this for at least 40 years and have had absolutely no problems with them.

[Barney Eaton]

This posting and the questions/answers stirred me to wonder how much the air pressure affect the height of the side wall.

My test vehicle is my Buick Reatta..... since the weight distribution front to rear is much greater than conventional rear drive cars,  it also gave a comparison between front and rear and the weight difference.

The tires are 215/60R16 Kumho V rated radials and the max pressure is 51 psi and the max load rating is 1521 lbs.

The car weight roughly 3450 according to different documents and the weight distribution is 60/40 according to the best info I could find.    That would translate to 2070 lbs front,  1380 rear.

I am attaching a picture of the crude set up I used to measure the height of the  rim from the floor.   Now that I know the total height difference I could do it again with a dial indicator. 
 I may plot it on a graph and post that later but here are my findings on this vehicle. ... I started by inflating the tires to the max psi of 51.    Measured a point on the rim from the floor.

Reduced the air pressure in 2 lbs increments, and took a measurement.     I started on the front and when I switched to the rear, I change the measurement point on the rim, thinking I could read it more accurately.

I am not going to list all the reading here, just some steps.

FRONT started at 3.60 inches and 51 psi.

at 40 psi the height was 3.46

at 30 psi the height was 3.29

On the REAR

51 psi was 4.27 (remember I change the measurement location)

40 psi was 4.18

30 psi was 4.08

Summary.....the Front dropped .31 when the air pressure was reduced 21 psi  (estimated 2070 lbs on front)

Rear dropped  .19 with a 21 psi air pressure drop (estimated 1380 lbs on rear)

I really didn't know what to expect....we usually do a visual on how much the sidewall flair out as the air is reduced,  .31 is roughly 5/16 (1/16 more than 1/4 inch)

.19 is roughly 3/16 (3/16 = .1875)

I don't know that this proves anything but it satisfied my curiosity on just how much the reduction of air pressure lowers/shortned the sidewall.   I also think I need a new hobby.

Edited March 21, 2018 by Barney Eaton (see edit history)

[NTX5467]

(NEW hobby?  How many round rocks are there in Round Rock?)

[Barney Eaton]

Been there, done that......lived in Round Rock for 20 years.    No round rock in Round Rock.....it is all limestone either in solid slabs or flat rocks.

Round Rock is named for the rock in the photo.....it was used as a trail marker for the Chisholm Trail cattle drives....the creek bed is solid limestone and has wheel grooves made by wagons crossing the creek.

In 1970 Round Rock was 2500 people, today it is over 100,000........population growth started in the late '70's when Austin residents moved to RR because of better schools and lack of gangs and drugs.

In the growth continued when Dell computers moved their campus from North Austin to just over the county line into Round Rock....... still growing,  I moved to Georgetown 20 years ago to get away from the traffic.

Edited March 21, 2018 by Barney Eaton (see edit history)

[KongaMan]

When I lived in Austin there was open space between Austin and Round Rock.  No more.  Glad I lived there then.  Wouldn't live there now.