Bump tire pressure with cargo?

[Klutch9]

I haul about 400/500 pounds of equipment in my Sedona at all times, plus myself. The door placard states to run tires at 35 psi but, following my normal routine of filling the tires a tiny bit more, I have them all running at 38 psi. I actually have 3 different types of tires on the vehicle

The front axle currently has identical tires, with a 103 load rating. The rear tires, while non-matching, both carry a 101 load rating. All 4 tires have a maximum of 44 psi stated on the sidewall. I realize that 400/500 pounds isn't that much, and that a vehicle of that size is more than capable of hauling more people-weight that 400/500 pounds. Am I good staying at 38 psi?

 

[DoiInthanon]

Yes. The tire will increase contact area by 500 lbs / 38 psi ~ 13 inches squared. This 13 in^2 will be distributed around the normal-load contact area circumference in a band ~ 0.33 inches wide. Hard to notice.

 

[eljefino]

Sounds good particularly with winter coming... cold air will steal a couple PSI.

 

[Kira]

Hello, And isn't the "highway speed" adjustment to add 10%-15% ? Kira

 

[Chris Meutsch]

I am a big fan of slightly-overinflated tires. Going on a 650-mile round tripper soon and the Pilot's tires will be at 35PSI as always, calling for 32.

 

[dparm]

Some manufacturers specify a different tire pressure if you will be driving at high speed or carrying a lot of gear. Check the manual.

 

[JHZR2]

Originally Posted By: dparm
Some manufacturers specify a different tire pressure if you will be driving at high speed or carrying a lot of gear. Check the manual.


+1 MB and BMW cars seem to request an additional 4 psi for each of those per my recollection.

Maybe not as much anymore since tires seem to be getting aired up to higher pressures anyway - not as much 28 psi pressure requirement anymore...

 

[dparm]

Originally Posted By: JHZR2
Originally Posted By: dparm
Some manufacturers specify a different tire pressure if you will be driving at high speed or carrying a lot of gear. Check the manual.


+1 MB and BMW cars seem to request an additional 4 psi for each of those per my recollection.

Maybe not as much anymore since tires seem to be getting aired up to higher pressures anyway - not as much 28 psi pressure requirement anymore...



Mine has several:

Fully loaded, maximum speed of 100mph
Four passengers, speeds above 100mph
Fully loaded, speeds above 100mph

For each of the three scenarios, there are different pressures for snow tires, OEM 18" tires, and OEM 19" tires. Then they have it for the sedan, coupe, and convertible until MY 2011.5. And THEN they repeat for cars built after MY 2011.5.

 

[CapriRacer]

Originally Posted By: DoiInthanon
Yes. The tire will increase contact area by 500 lbs / 38 psi ~ 13 inches squared. This 13 in^2 will be distributed around the normal-load contact area circumference in a band ~ 0.33 inches wide. Hard to notice.


I hope you realize that the equation isn't true - that you can't predict the size of a footprint based only on inflation pressure and load.

If you are trying to figure out how much affect load has on the size of the footprint, then that equation is good ESTIMATE for passenger car tires, but not so good for other types of tires.

And, No, the additional area tends to be on the length of the footprint, and not the width.


But the answer to the OP's question is: If you aren't overloading the vehicle, then the tire pressure listed on the vehicle tire placard is appropriate. That is, it is calculated based on a fully loaded vehicle.

 

[meep]

CapriRacer may be able to shed some light on this, but I follow the guidelines I once read in a document published by bmw years ago.

check pressure cold. drive car for a while. check pressure hot. you want to see a ~2psi increase.
if it's under 2psi increase, lower the pressure.
if it's over 2psi increase, add pressure.

when I bothered to do it, this had my average sedans sitting around 32-34 psi cold.

these days I run everything between 32-36 and stay with it, unless the placard wants more.

 

[supton]

That's a very interesting procedure. I'd be curious if it's still valid. I'm also now wondering if my pressure gauge would be "good enough": it usually sits in my jacket pocket, or in the car, and thus it too would see the same temp swing. [I'm thinking of cold winter days here.] It shouldn't change, but it's not like I paid much for tire pressure gauges.

 

[CapriRacer]

Originally Posted By: meep
CapriRacer may be able to shed some light on this, but I follow the guidelines I once read in a document published by bmw years ago.

check pressure cold. drive car for a while. check pressure hot. you want to see a ~2psi increase.
if it's under 2psi increase, lower the pressure.
if it's over 2psi increase, add pressure.

when I bothered to do it, this had my average sedans sitting around 32-34 psi cold.

these days I run everything between 32-36 and stay with it, unless the placard wants more.


The theory is that since tires generate heat when rolling, the air chamber temperature also increases and that results in an increase in pressure. One can gauge the amount of temperature by monitoring the pressure. (Ideal Gas Law stuff!)

It has been a rule of thumb for as long as I remember that passenger car tires should not see any more than 3 psi (which is about 10% buildup)

I doubt that ANY vehicle manufacturer has published guidelines for pressure buildup. I suspect this is something along the lines of the game of "Telephone" - that is that as a tidbit of information has been passed from person to person, it gets changed slightly - and by the time it reaches your ears, it hardly resembles the original.

So while there is a bit of truth in what you've written, I just don't think it has all the caveats needed to be taken seriously.

 

[DoiInthanon]

Originally Posted By: CapriRacer
Originally Posted By: DoiInthanon
Yes. The tire will increase contact area by 500 lbs / 38 psi ~ 13 inches squared. This 13 in^2 will be distributed around the normal-load contact area circumference in a band ~ 0.33 inches wide. Hard to notice.


I hope you realize that the equation isn't true - that you can't predict the size of a footprint based only on inflation pressure and load.

If you are trying to figure out how much affect load has on the size of the footprint, then that equation is good ESTIMATE for passenger car tires, but not so good for other types of tires.

And, No, the additional area tends to be on the length of the footprint, and not the width.


But the answer to the OP's question is: If you aren't overloading the vehicle, then the tire pressure listed on the vehicle tire placard is appropriate. That is, it is calculated based on a fully loaded vehicle.


Of course, an estimate. I didn't think the AASHTO tire contact definition was appropriate for the discussion. Rather I was aiming for a word picture appropriate for the non-structural, non-geotech engineer.

 

[KenO]

Originally Posted By: dparm
Originally Posted By: JHZR2
Originally Posted By: dparm
Some manufacturers specify a different tire pressure if you will be driving at high speed or carrying a lot of gear. Check the manual.


+1 MB and BMW cars seem to request an additional 4 psi for each of those per my recollection.

Maybe not as much anymore since tires seem to be getting aired up to higher pressures anyway - not as much 28 psi pressure requirement anymore...



Mine has several:

Fully loaded, maximum speed of 100mph
Four passengers, speeds above 100mph
Fully loaded, speeds above 100mph

For each of the three scenarios, there are different pressures for snow tires, OEM 18" tires, and OEM 19" tires. Then they have it for the sedan, coupe, and convertible until MY 2011.5. And THEN they repeat for cars built after MY 2011.5.

Gotta love the crazy Germans. Covering all the bases for all levels of performance to a T

 

[zzyzzx]

Along the same lines, in front wheel drive cars I put about 2PSI more in the front tires than the back tires, even though the sticker on the car has the same spec for all 4 tires.

 

[gaijinnv]

Originally Posted By: zzyzzx
Along the same lines, in front wheel drive cars I put about 2PSI more in the front tires than the back tires, even though the sticker on the car has the same spec for all 4 tires.


Load is not the only factor in determining recommended TP from the manufacturer - handling is also a factor.

By increasing the TP of the front tires in a FWD vehicle, you are increasing understeer.

Just sayin'

 

[Ken2]

Quote:
check pressure cold. drive car for a while. check pressure hot. you want to see a ~2psi increase.
if it's under 2psi increase, lower the pressure.
if it's over 2psi increase, add pressure.

Knowing BMW, they may be looking for the best traction. Tires need to be normally warm for best traction. Less pressure causes them to flex more and heat more. More pressure causes them to flex less and heat less. Flex just right gives heat just right and traction just right.

 

[rslifkin]

Originally Posted By: gaijinnv
Originally Posted By: zzyzzx
Along the same lines, in front wheel drive cars I put about 2PSI more in the front tires than the back tires, even though the sticker on the car has the same spec for all 4 tires.


Load is not the only factor in determining recommended TP from the manufacturer - handling is also a factor.

By increasing the TP of the front tires in a FWD vehicle, you are increasing understeer.

Just sayin'

Not necessarily. If you increase their pressure too much, you'll increase understeer. However, increasing them some doesn't automatically mean you're going past the point of optimal grip.