Lets' Talk Tires

[Deathdeelr]

Now that DOES bring up a hypothetical question. When a tire manufaturer states ** psi to be put in the tire, is this actually the weight that should be in a tire with it off the ground, or the psi with the weight of the bike on the tire? I'm sure most do it with the weight of the bike on it except for when getting a new tire put on.Tod

This being my very first post, hello to all.

It doesn't matter if the tire is off the ground or has weight on it. The pressure in the tire doesn't change unless you change the volume of the space the air must fill which is impossible when it comes to tires. A tire off the ground will just take the air faster when you fill it than one with weight on it but 38psi is 38psi. I just recently proved this point to an "engineer" at the job. I filled a set of truck tires to 130psi while off the ground then dropped the pressure and refilled the same tires while loaded. They still touched. So much for engineering degrees.

[Yard Dogg]

I bought Dunlop 404's for my bike and am happy with them so far. I don't know how well they wear, but I can drag my kickstand and stick to the pavement at 70 ......

You see, the first eek is for when you first drag your kick stand, and the second eek is for when your brain kicks in and you realize what you just did. Know what I mean?

[cywelchjr]

This being my very first post, hello to all.

It doesn't matter if the tire is off the ground or has weight on it. The pressure in the tire doesn't change unless you change the volume of the space the air must fill which is impossible when it comes to tires. A tire off the ground will just take the air faster when you fill it than one with weight on it but 38psi is 38psi. I just recently proved this point to an "engineer" at the job. I filled a set of truck tires to 130psi while off the ground then dropped the pressure and refilled the same tires while loaded. They still touched. So much for engineering degrees.

Ok, the engineer was an idiot then. Of course if you set the pressure loaded and unloaded it would be the same. However, if you put 130 PSI in them, and then put a load on them the pressure will go up. Just as if you put the pressure in them loaded and then remove the load the pressure will do down. Tires are not a rigid container, they are elastic and as such, putting pressure on them will vary the pressure.

[crazy steve]

Elite II??? You better check the dates on those tires as they don't make them anymore.

I wondered when somebody would bring up 'expiration dates'.....

'78E original owner

[fredintoon]

This being my very first post, hello to all.
It doesn't matter if the tire is off the ground or has weight on it. The pressure in the tire doesn't change unless you change the volume of the space the air must fill which is impossible when it comes to tires. A tire off the ground will just take the air faster when you fill it than one with weight on it but 38psi is 38psi. I just recently proved this point to an "engineer" at the job. I filled a set of truck tires to 130psi while off the ground then dropped the pressure and refilled the same tires while loaded. They still touched. So much for engineering degrees.

Hi deathdeelr and welcome,
alas that your practical demonstration lacks rigor and your theory is in error.
Yes, modern tire materials don't stretch worth a darn so the tire's surface area will remain the same whether it's resting on the ground or up in the air.
Not so for the volume.
Squashed out of it's perfect shape by being pressed onto the ground the tire's volume will indeed be less.
So as p1v1 = p2v2 the tire pressure goes up.
Even if not by much.

[Yard Dogg]

That's a rough way to treat a first poster you f$*(@#% A@$^&&*# M#$%^ F$!#%&! Just kidding. Maybe you should welcome the FNG now.

[Deathdeelr]

Hey Yard thanks for the boost. I haven't been called a FNG in a long time. Makes me feel like a kid again. Anyway, Fred, I think you just contradicted yourself. If the tire doesn't stretch and the surface area doesn't change, I still don't see how the volume decreases. When I inflated the tires on the jack and dropped them on the ground the pressure didn't change. If anything the sidewalls deformed to accommodate the volume displaced when the weight of the truck was dropped on the tire. If you want to get technical your equation p1v1=p2v2 doesn't apply here (the surface area of the container being constant causes the volume to be constant). I'm more likely to buy cywelchjr's explanation. Either way I think we're talking semantics here and I think I got us on a tangent talking about truck tires that take the same amount of air to fill as can fill maybe 20 of our bike tires. Even if I'm wrong (as the many bruises I have can attest to) the amount of pressure difference that may or may not apply when you air up a rear tire then drop the bike on the ground would be minuscule. Therefore I'll stick by my previous statement that it doesn't make a difference; air it up on the ground or off; same thing.

[jmnjrpa]

'expiration dates'

When dealing with tires and plastics the amount of UV radiation is by far the biggest age factor (90+%). If the item was in storage then there is usually very little age deterioration. The rare exception to this rule is VERY high oxygen chemical exposure. I have seen a set of the first original NON-SKID tires mounted on a 1913 Metz that had been in continuous storage for 70+ years. The tires were in incredible shape with minimal cracking.

[jmnjrpa]

Btw

Fred is right on the volume.

[fredintoon]

- - - Anyway, Fred, I think you just contradicted yourself. If the tire doesn't stretch and the surface area doesn't change, I still don't see how the volume decreases. - - -

Hi Dearhdeelr,
let's look at a simple analog to the tire thing.
Open a can of beer.
Pour the beer into a glass.
Partially crush the can.
Try and pour the beer back in.
It won't all go back in.
Although the can's surface area has not changed it's volume has been reduced.
Same deal with a tire.
flatten it at the bottom and it's volume gets smaller although it's surface area remains the same.
As the tire is a sealed system containing pressurized gas, Boyle's Law does so apply.
Admittedly, the typical volume change due to the tire resting on the road under vehicular load vs being suspended is so small that you wouldn't see the difference on the average tire gauge.

[DenyP]

Interesting Discussion!

Interesting Discussion!

Deny

[Deathdeelr]

Once you mentioned the beer can I got it.

[fredintoon]

Once you mentioned the beer can I got it.

Hi Deathdeelr,
yeah, beer, the universal language! I'm on Kieth's IPA right now. I'm usually on Rickard's red but my son brought a 12 of IPA and there were several cans left over after we fixed his bike.

[justme1100]

I have a tire that is only flat on one side, the bottom. If I turn it upside down will it be ok?

[psycoreefer]

Sure, turn it upside down, then put some armorall on it to help soften up the rubber. You could also try taking it off the bike and soaking it in brake fluid for a while, that should make it nice and soft again too.

Really though, don't do any of those things...