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TIRE PRESSURE

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My 1922 Dyke's Manual says to run them at 70-75 psi. The new 34 x 4.5 BF Goodrich cords being sold by Coker say load capacity 1250 lbs at 65 psi on the Coker website. Check to see if there is a max load rating on your tires and be sure not to exceed that. My tires are getting old and don't have any max ratings on them. I run mine at 65 psi with good results.

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hey! every one

I want share some information about tire pressure with you guy's. Check your driver’s manual for your car for the exact amount of pressure it suggests for your make and model of vehicle. The average pressure of most cars is 28 to 35 pounds of air pressure. It may vary from front to back tires. I hope you will got this information and keep it mind...thanks

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hey! every one

I want share some information about tire pressure with you guy's. Check your driver’s manual for your car for the exact amount of pressure it suggests for your make and model of vehicle. The average pressure of most cars is 28 to 35 pounds of air pressure. It may vary from front to back tires. I hope you will got this information and keep it mind...thanks

More or less true with tubeless tires supplied with Post WWII era vehicles to the present day. Not applicable to many if not most tire applications prior to the mid 1930's. Tire inflation pressures have often been used by American car makers to influence ride comfort, though usually at the sacrifice of handling and tire wear.

It is a fact though that no tire over five years old should be considered safe for driving more than around the block as general degradation will have begun. Those over 5 year old spare tires that have never been on the ground or that look good .....Not!

Jim

Edited by Jim_Edwards (see edit history)

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If you determine the total area of contact of each of the 4 tires with the ground in square inches and multiply that number by the psi of each tire and add up those four numbers the resulting number will be the total weight of the car. For example , if each tire has a footprint of 40 square inches and the psi is 30 the weight of the car would be 4800 lbs. 40 sq in x 30 psi x 4 = 4800 lbs. Increase the pressure to 60 psi and you will have 20 sq in in contact with the pavement.

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If you determine the total area of contact of each of the 4 tires with the ground in square inches and multiply that number by the psi of each tire and add up those four numbers the resulting number will be the total weight of the car. For example , if each tire has a footprint of 40 square inches and the psi is 30 the weight of the car would be 4800 lbs. 40 sq in x 30 psi x 4 = 4800 lbs. Increase the pressure to 60 psi and you will have 20 sq in in contact with the pavement.

Uh, huh! Try proving that theory with various wheel diameters and widths along with tread widths possible for being placed on a given vehicle.

Jim

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This also explains why if you have an empty truck with 50 psi in the tires and then load the truck the tire pressure will still be 50 psi.

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This also explains why if you have an empty truck with 50 psi in the tires and then load the truck the tire pressure will still be 50 psi.

So the load ratings for tires in your mind are virtually insignificant numbers so long as a given inflation pressure is maintained.

What has any of this to do with the original post of wanting to know the correct inflation for 34X4.5 tires?

Jim

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As a general rule of thumb~~~ the more skinny a tire is on the older cars~~~

The higher the operating pressure !

Weight of the car is also a big consideration.

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Check your driver’s manual for your car for the exact amount of pressure it suggests for your make and model of vehicle.

That is a very good suggestion for most vehicles, but surprisingly my 21 Stearns-Knight owner's manual does not mention anything about tire pressure. Not sure if that was common practice at the time or not. See Universal's tire catalog at http://www.universaltire.com/pdf/uvt-catalog.pdf, it has a section 'High Pressure Tire Inflation & Load Scale' which shows recommended pressure based on tire width and load for these high pressure tires. I still would not exceed the max on the side of the tires or if the tire has no max on it, I would not exceed the max rating of the new tires of the same size.

Edited by sleeve-valve (see edit history)

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Another consideration for the correct tire pressure is the type of rim and bead.

I would never consider less than 60psi in a clincher type rim. This pressure is necessary

to hold the bead seated.

Dennis

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Restorer32, you are absolutely correct about how tire pressure works. A tire inflated to 30 psi holds 30 pounds of the weight of the car per square inch of road contact, and 40 holds 40 and 50 holds 50. This is a fact that's hard to explain to some people, but it's simple physics.

It's always fascinating to me how little road contact a small brass car might have. For example, my 1910 Hupp weighs about 1200 pounds. At 60 psi in the tires, I only have 20 square inches, or 5 square inches per tire, on the road! Slightly more when I sit in it of course.......

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The first reply referred to a Dyke's Manuel I picked one up off ebay.

Twenty five dollars!! thirteenth printing. What a great source of info

Donny

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If you determine the total area of contact of each of the 4 tires with the ground in square inches and multiply that number by the psi of each tire and add up those four numbers the resulting number will be the total weight of the car. For example , if each tire has a footprint of 40 square inches and the psi is 30 the weight of the car would be 4800 lbs. 40 sq in x 30 psi x 4 = 4800 lbs. Increase the pressure to 60 psi and you will have 20 sq in in contact with the pavement.

Restorer, I'm curious where this formula is taken from?

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In this day and age, probably best to go with the tire mfr's recommendation... (referring to antique tires here, such as the 34x4.5 in the original post )

Tires are no longer made with cotton-canvas plies and natural rubber, like they were in 1910...

You want enough pressure in the casing to prevent rim / tire damage from bumps and chuck-holes, but not too much.

I'm sure the folks at Universal Vintage Tire or Coker Tire can offer some guidance...

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The fact that the pressure against the road per square inch has to equal the pressure against the tire (and thus, in the tire) is a simple concept. It's one of those things you don't think about, and I'll admit I went for years without realizing it.

The only exception I can think of is some of the run-flat tires now being produced. In that case, there's a structural element to the tire, and part of the pressure on the road comes from that structure itself.

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