A-HA! So This Explains Why Shops "Overinflate" Your Tires!

[RidingOnRailz]

I just found this document, a Tech. Service Bulletin from a dealership service folder, that explains why your tire pressures might have been set so high when you left the garage or dealer service center:

https://static.nhtsa.gov/odi/tsbs/20...33726-9999.pdf

It's all about compensation!

[CapriRacer]

Originally Posted by RidingOnRailz

I just found this document, a Tech. Service Bulletin from a dealership service folder, that explains why your tire pressures might have been set so high when you left the garage or dealer service center:

https://static.nhtsa.gov/odi/tsbs/20...33726-9999.pdf

It's all about compensation!

When the testing guys used to set up a vehicle for testing the next day, they would mount all the tires needed to do the next day's tests, and overinflate them 2 to 3 psi to account for tire growth. I suspect that some of that is also in the picture.

[RidingOnRailz]

Originally Posted by CapriRacer

When the testing guys used to set up a vehicle for testing the next day, they would
mount all the tires needed to do the next day's tests, and overinflate them
2 to 3 psi to account for tire growth. I suspect that some of that is also in the picture.

Tire "growth"?

[CapriRacer]

Originally Posted by RidingOnRailz

Tire "growth"?

Yes, tires grow, especially in the first 24 hours.

As a matter of fact, we used to have a measurement test we called "24 hour measurements". Tires grow a bit after that - enough to measure, but not enough to worry about.

How much do they grow? I don't remember. but I do remember the 2 to 3 psi the test guys used, because when I mounted my own tires, I used that and it worked.

[RidingOnRailz]

Originally Posted by CapriRacer

Yes, tires grow, especially in the first 24 hours.

As a matter of fact, we used to have a measurement test we called "24 hour measurements".
Tires grow a bit after that - enough to measure, but not enough to worry about.

How much do they grow? I don't remember. but I do remember the 2 to 3 psi the test guys used,
because when I mounted my own tires, I used that and it worked.

When does this growth occur, when a tire is brand new, just installed on an application, or when?

And you mentioned a change in the pressure. Do they also 'grow' in dimension?

[CapriRacer]

Originally Posted by RidingOnRailz

When does this growth occur, when a tire is brand new, just installed on an application, or when?

And you mentioned a change in the pressure. Do they also 'grow' in dimension?

First, the change in pressure is due to the larger dimensions. Remember the Ideal Gas law: PV=nRT. V is for volume and that is where the dimensions come in. A 3 psi lose in pressure is about 7% - so the interior volume is changing up to 7% in the first 24 hours. (This is for passenger car tires. Other types are different.)

When does it occur? Right from the moment the tire is pressurized and doesn't stop. However, the growth after the first 24 hours is really small, but you can measure it. After a week, it's extremely difficult to measure any continuing change. For practical purposes, you could say that all the growth takes place in the first 24 hours and you would be mostly right.

It happens to all tires: new ones, and old, remounted ones. However, old tires don't grow nearly as much. The materials have been stretched and rebounded, but not rebound 100%.

[RidingOnRailz]

Originally Posted by CapriRacer

First, the change in pressure is due to the larger dimensions. Remember the Ideal Gas law: PV=nRT.
V is for volume and that is where the dimensions come in. A 3 psi lose in pressure is about 7% - so
the interior volume is changing up to 7% in the first 24 hours. (This
is for passenger car tires. Other types are different.)

When does it occur? Right from the moment the tire is pressurized and doesn't stop. However, the
growth after the first 24 hours is really small, but you can measure it. After a week, it's extremely
difficult to measure any continuing change. For practical purposes,
you could say that all the growth takes place in the first 24 hours and you would be mostly right.

It happens to all tires: new ones, and old, remounted ones. However, old tires don't grow nearly as
much. The materials have been stretched and rebounded, but not rebound 100%.

This is a fascinating aspect of tires!

Does this growth occur in every direction through a cross section of the mounted tire - latterally and vertically? And if so, this would increase tire volume, and cause indicated tire pressure to decrease over a certain period of time, correct?

So if I bought and installed a tire brand new and inflated it to spec pressure on Monday, then by Friday, assuming equal ambient temperature and atmospheric condition, a small amount of air would need to be added to maintain that same spec. pressure.

[CapriRacer]

Originally Posted by RidingOnRailz

This is a fascinating aspect of tires!

Does this growth occur in every direction through a cross section of the mounted tire - latterally and vertically? And if so, this would increase tire volume, and cause indicated tire pressure to decrease over a certain period of time, correct?

So if I bought and installed a tire brand new and inflated it to spec pressure on Monday, then by Friday, assuming equal ambient temperature and atmospheric condition, a small amount of air would need to be added to maintain that same spec. pressure.

The growth occurs in every direction, but not evenly. The belt really restricts the growth in the tread a lot more than where the belt isn't - the sidewalls. If there are cap plies, there's even less growth.

If you bought new tires on Monday and just after mounting set the pressures, 24 hours later (same time Tuesday), you would need to add air.

If you bought new tires on Monday and drove home, then set the pressures (after the tire cooled from the drive home), then you would still need more air on Tuesday, but not as much as above.

If you corrected the pressures on Tuesday by Friday, you'd need to add air in both cases, but the amount would be small - and probably not worth the effort - especially compared to the ambient temperature effect.

[RidingOnRailz]

Originally Posted by CapriRacer

The growth occurs in every direction, but not evenly. The belt really restricts the growth in the tread
a lot more than where the belt isn't - the sidewalls. If there are cap plies, there's even less growth.

If you bought new tires on Monday and just after mounting set the pressures, 24 hours later (same
time Tuesday), you would need to add air.

If you bought new tires on Monday and drove home, then set the pressures (after the tire cooled from
the drive home), then you would still need more air on Tuesday, but not as much as above.

If you corrected the pressures on Tuesday by Friday, you'd need to add air in both cases, but the
amount would be small - and probably not worth the effort - especially compared to the ambient
temperature effect.

Wow!

No WONDER so many ordinary folks are driving around on tires underinflated by as much as 5-10psi. “Oh.. I need new tires. I’ll have Town Fair slap some on while I visit Starbucks next door, come pay and pickup my ride, and I’m good for the next 3-6 months yipeeee!”

No, that’s not how this tire thing works. It’s constant maintenance, just like checking the oil, coolant, whatever. Tire growth is not all that different a concept from ‘shoe growth’. A brand new pair of shoes, even properly fitted by a shoe store rep who knows what they’re doing, will ‘break in’ over a period of days or weeks.

Thanks Barry, now I understand yet another, finer point, of tire maintenance.

[CapriRacer]

Remembering that this thread started out as an epiphany about why tires are overinflated when you leave the shop, I think there is more going on than technical reasons. Sometimes it's just to avoid the legal liability of underinflating tires.

[RidingOnRailz]

Originally Posted by CapriRacer

Remembering that this thread started out as an epiphany about why tires are overinflated
when you leave the shop, I think there is more going on than technical reasons. Sometimes
it's just to avoid the legal liability of underinflating tires.

Yes, I'm sure that is part of the pie. But no reason, as some customers have complained, to have their tires inflated to at or just below, the tire sidewall maximum value.

4-5psi over vehicle mfg. spec at the garage should be sufficient. Consumers must learn that tire pressure maintenance continues after leaving the shop, and cannot just wait until A, the next visit, or B, something goes wrong. I heard it in a movie some time ago, and it sticks in my mind years later: 'If something's important, you make the time'!

[Blue Bowtie]

Originally Posted by RidingOnRailz

...Consumers must learn that ... maintenance continues after leaving the shop, and cannot just wait until A, the next visit, or B, something goes wrong. ...

I took some liberties and altered your quote (just like the New York Times would) and believe it applies even more universally as a result.

I think you zeroed right in on the root of the problem. TMPS systems would not even exist if people could find 3-4 neurons to rub together and maybe even just read an owner's manual.

Incidentally, if you think passenger car or light/medium truck tires stretch a little, ask someone who replaces bias-ply ag or off-road equipment tires. Even if the temperature is higher days after mounting up a new tire and tube, the pressure can "slip a couple PSI. Liquid filling them makes that even more apparent. Take a unit out in the field and "work" the carcass a little to really get it loose, and it can "leak" even more.

[Blue Bowtie]

Originally Posted by RidingOnRailz

Yes, I'm sure that is part of the pie.

Incidentally, considering the season, what kind of pie? Pumpkin, apple, pecan, cherry, mince, or something else? Everything is on the table for this question.

[RidingOnRailz]

Originally Posted by Blue Bowtie

Incidentally, considering the season, what kind of pie? Pumpkin, apple, pecan, cherry, mince, or something else? Everything is on the table for this question.

Gotta be pumpkin for me!

“Group W”… back in my day that meant the Westinghouse Broadcasting division, back when they owned New York’s WINS, and 1010 played music!

[Blue Bowtie]

Group W - Since Thanksgiving is upon us, think "Alice's Restaurant" instead of Westinghouse.