Adjusting tire pressures based on ambient temperatures

Recently read here the +1/-1 inflation guide based on 68 degree ambient temperature. In other words, if my garage is 98 degrees I would add 3#'s from the recommended pressure by bmw.

Example=42 rear cold would be inflated to 45 rear cold at 98 degrees in the garage this summer.
Example=42 rear cold would be inflated to 40 rear cold at 48 degrees in the garage this winter.

I recently posted this formula I just read about on another site and have caught all manner of flack from everyone except a few who agrees that recommended pressures are given based on 68 ambient and the adjustments make sense.

I'd like some definitive answers from the emeritas' here who can shed light on the above +1/-1 formula and why it's based on 68 degree ambient. How did 68 degree ambient recommendations become known? Did someone talk to engineers at bmw? Another maker?

Appreciate the assist on where this +1/-1 knowledge originated from and whether there's documentation of same from bmw, etc. that can shed definitive light on the subject.

Thanks.

It is those pesky engineers. Specifically European engineers. They expect accuracy. A pressure specification without a corresponding temperature specification is meaningless due to (I think it is ) Boyles Law of Gasses. If you dig into the brochures provided by Pirelli, Metzeler, Continental, Michelin, etc you will find that by convention (probably a ISO or EU standard) recommended pressures are at 20 degrees C. BMW of course follows the same standard.

PGlaves said:

It is those pesky engineers. Specifically European engineers. They expect accuracy. A pressure specification without a corresponding temperature specification is meaningless due to (I think it is ) Boyles Law of Gasses. If you dig into the brochures provided by Pirelli, Metzeler, Continental, Michelin, etc you will find that by convention (probably a ISO or EU standard) recommended pressures are at 20 degrees C. BMW of course follows the same standard.

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Thanks, I've not been able to find anything on line through any makers sites that mention recommended pressures 20 degrees C. If I could find that somewhere, the naysayers would be diminished exponentially.

On another note, is there paper/research that documents the +1/-1 formula anywhere by a manu of tires? Many are asking for more than just a writers opinion pieces.

Thanks

brownie0486 said:

Recently read here the +1/-1 inflation guide based on 68 degree ambient temperature. In other words, if my garage is 98 degrees I would add 3#'s from the recommended pressure by bmw.

Example=42 rear cold would be inflated to 45 rear cold at 98 degrees in the garage this summer.
Example=42 rear cold would be inflated to 40 rear cold at 48 degrees in the garage this winter.

I recently posted this formula I just read about on another site and have caught all manner of flack from everyone except a few who agrees that recommended pressures are given based on 68 ambient and the adjustments make sense.

I'd like some definitive answers from the emeritas' here who can shed light on the above +1/-1 formula and why it's based on 68 degree ambient. How did 68 degree ambient recommendations become known? Did someone talk to engineers at bmw? Another maker?

Appreciate the assist on where this +1/-1 knowledge originated from and whether there's documentation of same from bmw, etc. that can shed definitive light on the subject.

Thanks.

Click to expand...

As Paul says, most if not all of this stuff is dervived from tire manufacturers. BMW is just following what tire manufacturers have learned and passed on.

As for the 68F, I do believe that is simply a conversion from Celsius (20C). No idea where it comes from other than the thought that one must use some reasonable ambient temperature as a base and 20C is an easy one. My recollection is that the 1psi +/- per 10F is not exact but is a very reasonable rough guide that is easuy to remember and use and fine for day-to-day use.

I do beleive that your example is open to a little confusion (or perhaps it is just the wording/semantics) for two reasons:

brownie0486 said:

...
Example=42 rear cold would be inflated to 45 rear cold at 98 degrees in the garage this summer.
Example=42 rear cold would be inflated to 40 rear cold at 48 degrees in the garage this winter.
...

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What I mean is, I would word it slightly differently.

42psi cold inflated pressure (68F) would read 45psi at 98F out of the sun
42psi cold inflated pressure (68F) would read 40psi at 48F out of the sun

The use of "cold" in the second half,

40 rear cold at 48 degrees in the garage

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can be confused with "cold inflated pressure" which is meant to represent the pressure of a tire at 68F

The second thing is that other side of the practicle use of this rule-of-thumb is for identifying how much air to add or subtract to a tire based on it's recommended or desired inflated pressure. Therefore, if we use the example of a desired 42psi (@ 68F) and the tire is reading 42psi with an ambient tempeerature of 78F, you would rwant to add 1psi
Desired cold inflation = 42psi. Tire pressure reading at 78F is showing 42psi therefore the tire pressure should be increased (not reduced) by 1psi to = 43psi which would then be equal to 42psi at 68F.

Hope that makes sense.

That makes sense, but this confused me, seems to contradict each other

if we use the example of a desired 42psi (@ 68F) and the tire is reading 42psi with an ambient tempeerature of 78F, you would rwant to add 1psi
Desired cold inflation = 42psi. Tire pressure reading at 78F is showing 42psi therefore the tire should be reduced by 1psi to = 41psi which would then be equal to 42psi at 68F.

Help me out here, thanks

brownie0486 said:

That makes sense, but this confused me, seems to contradict each other

if we use the example of a desired 42psi (@ 68F) and the tire is reading 42psi with an ambient tempeerature of 78F, you would rwant to add 1psi
Desired cold inflation = 42psi. Tire pressure reading at 78F is showing 42psi therefore the tire should be reduced by 1psi to = 41psi which would then be equal to 42psi at 68F.

Help me out here, thanks

Click to expand...

Thanks for pointing out my mistake.

I've edited my post to read:

Desired cold inflation = 42psi. Tire pressure reading at 78F is showing 42psi therefore the tire pressure should be increased (not reduced) by 1psi to = 43psi which would then be equal to 42psi at 68F.

Click to expand...

I just looked through two large PDF files for Avon and Metzler tires...contains lots of technical data. Nowhere do I see it mention that tire pressure inflation are subject to outside temperature or based upon some standard temperature. One would think that if this were a critical factor, it would appear as a disclaimer or warning at some point. Just not seeing it.

AlanColes said:

Thanks for pointing out my mistake.

I've edited my post to read:

Click to expand...

20774,
So, you'd set the rear at 42 when the garage is 98 and the tire is deemed "cold" from sitting more than 30 minutes. If I then head up into the mountains and it turns 68, the tire is underinflated some amount [ presumed to be 1 degree for every 10 degrees F ]. If that's true, and we believe the colder temps reduce pressure in tires while warmer temps increase pressures, do we then stop and reset tires when there's a 30F disparity in a matter of an hour or two? How's that work when the tire is warm, have to wait the 30+ minutes to reset to 42 with a deemed cold tire again?

I've not been able to find anything like yourself in this matter, but there's a lot of people stating use the +1/-1 based on ambient temp over or under 68F/20C. I'm wondering how that thinking got started, how many people actually use the +1/-1 over/under 20C/68F when setting air pressures on unridden for at least 30 minutes [ described as cold ] tires.

I don't like running my tires underinflated, nor overinflated. Do the manu's talk at all about increase or decrease of pressures while riding into various environments. I ask because I can see out here several times a year or more the temps are 105, and a storm moves in and within an 30 minutes, the temps are down to 72-75.

It's my understanding and reading that tire makers see 30-40% of riders tires being ridden underinflated regularly. Underinflated creates more heat on the sidewall from increased flexing and I've been able to find articles that state the increased heat will deteriorate a tire more rapidly than when pressures are kept to recommended pressures.

When I talked to a Pirelli rep the other day, the scorpion rally 50/50 tire max pressure is 42 according to him, and he said run what bmw recommended for pressure. That's 42 on my GS. If I set that pressure to 42, then ride the bike and the tire heats up at all, I've gone past the limit of the max tire pressure. He didn't know what to tell me and just kept repeating run the tire at bmw's recommended setting for that bike. It's a little perplexing to say the least.

It's been a while since I really read my /7 users manual but I don't recall there being any instructions for modifying pressures due to changing temps. And in all the lists I participate in, I don't remember anyone discussing it either.

I do check tire pressure every time I get on the bike for either a short ride or on tour. I take what the pressure gage tells me. So, in that sense, my tires are inflated per the temperature at the start. But I think it borders on the side of WTF to get off at the top of a mountain when it's cold and adjust my tire pressure and then redo the adjustment when I got back to lower altitudes where it's warmer.

Hmmm...are there adjustment guidelines for altitude changes as well?

20774 said:

It's been a while since I really read my /7 users manual but I don't recall there being any instructions for modifying pressures due to changing temps. And in all the lists I participate in, I don't remember anyone discussing it either.

I do check tire pressure every time I get on the bike for either a short ride or on tour. I take what the pressure gage tells me. So, in that sense, my tires are inflated per the temperature at the start. But I think it borders on the side of WTF to get off at the top of a mountain when it's cold and adjust my tire pressure and then redo the adjustment when I got back to lower altitudes where it's warmer.

Hmmm...are there adjustment guidelines for altitude changes as well?

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I didn't suggest I'd be getting off the bike at the colder temps and adjust, just that it would make sense the tire was underinflated due to the much colder temps than when I started.

For most riders not on a racetrack a 2 psi difference from spec will seldom make any difference. So that pretty well covers temps from 50 to 90. I wouldn't say much in a little manual either. But physics is physics. You ought to see all the stuff on this topic on various car forums, especially BMW car topics.

FWIW, when I put the gauge to the tire, that is it until I put the gauge to it the next time. I’m pretty sure the tire manufacturers take all this into consideration.
I know the pressure will be less in the cold, more in the heat.
OM

brownie0486 said:

...
So, you'd set the rear at 42 when the garage is 98 and the tire is deemed "cold" from sitting more than 30 minutes ...

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No. What I am saying is that if you want to run a tire at 42psi "cold inflated pressure" (68F) and the ambient temperature is 98F than you need to adjust a cold tire (not been ridden more than a mile or within the last 30-60 minutes) to 45F.

brownie0486 said:

... do we then stop and reset tires when there's a 30F disparity in a matter of an hour or two? How's that work when the tire is warm, have to wait the 30+ minutes to reset to 42 with a deemed cold tire again? ...

Click to expand...

These are really rules-of-thumb and I don't think one shouldn't get too concerned about variations within the day. Start with properly inflated tires and they will to some degree self correct (more on that further along).

Tires are very basic in principal, but the issue of inflation is actually very complex and adjusting the pressure on a "warm" tire should only be done, in my opinion, if you have a probe-style pyrometer and already have identified target values, etc. We used to do this and keep detailed records when I was racing. However, those are far different circumstances and require far greater detail than is necessary for safe street/highway/off-road riding.

brownie0486 said:

...I've not been able to find anything like yourself in this matter, but there's a lot of people stating use the +1/-1 based on ambient temp over or under 68F/20C. I'm wondering how that thinking got started, how many people actually use the +1/-1 over/under 20C/68F when setting air pressures on unridden for at least 30 minutes [ described as cold ] tires...

Click to expand...

Really doesn't matter where it started, just that it is a common and accepted rule-of-thumb put forward by tire manufacturers. In my experience, most folks don't pay attention to this, even many motorcycle mechanics. Just like way too many motorcycle mechanics overfill a bike with oil. It happens way to frequently for them to truthfully be considered "Professional". Those mechanics/techs that do follow proper practices are a blessing to us all.

brownie0486 said:

...I don't like running my tires underinflated, nor overinflated. Do the manu's talk at all about increase or decrease of pressures while riding into various environments. I ask because I can see out here several times a year or more the temps are 105, and a storm moves in and within an 30 minutes, the temps are down to 72-75...

Click to expand...

I don't recall any doing so, but I wouldn't be surprised if a few did. Especially a if it were a BMW manual. Their tech department is well known for being overly anal and sometimes getting thing wrong even. ;-)

Just remember, it is best to never “bleed” or reduce air pressure when tires are hot. Therefore, check them before you ride and unless you suspect a leak, don't worry.

brownie0486 said:

...It's my understanding and reading that tire makers see 30-40% of riders tires being ridden underinflated regularly. Underinflated creates more heat on the sidewall from increased flexing and I've been able to find articles that state the increased heat will deteriorate a tire more rapidly than when pressures are kept to recommended pressures...

Click to expand...

That is correct. However, it is not just from sidewall flex, heat is also generated by tread squirm and contact patch. Under-inflated tires have a larger contact patch comared to properly inflated tires and over-inflated tires have a smaller contact patch.

brownie0486 said:

...When I talked to a Pirelli rep the other day, the scorpion rally 50/50 tire max pressure is 42 according to him, and he said run what bmw recommended for pressure. That's 42 on my GS. If I set that pressure to 42, then ride the bike and the tire heats up at all, I've gone past the limit of the max tire pressure. He didn't know what to tell me and just kept repeating run the tire at bmw's recommended setting for that bike. It's a little perplexing to say the least.

Click to expand...

The maximum pressure indication on a tire sidewall is the "Cold Inflation Pressure". Not the absolute maximum tire pressure while being ridden.

Very few tire reps, unless they are in the racing end of the field know much about the specifics. They are sales reps not engineers usually. The tire manufacturer will always state that you should follow the vehicle manufacturer's recommendations. That is their legal-liability avoidance manouver.

So, now a quick blurb on the "self-correcting" nature of tire inflation. As you rightly noted, an under-inflated tire will generate more heat when ridden compared to a properly inflated tire. Also, an over-inflated tire will generate less heat than a properly inflated tire. If the rate of increase/decrease were perfect, than the tires would actually self-correct for over/under inflation. Unfortunately, that is not the case, so unless one is using a computer or is an engineer, the only way to identify a tire's actual cold inflated pressure when it is hot is to have collected data.

We also have to recognize that tires have an optimal operating temperature (somewhat unique to each tire model) and that their optimal inflation is also dependent on vehicle load, etc.

Personally, I have found that 1-2 psi above the owner's manual and that setting/checking the pressure before riding and leaving it alone for the rest of the day works best for me.

Since there are variable pressures suggested for one-up, two-up/luggage in every owners manual and they do not state how heavy the first rider is...I have never worried about keeping on a number as the ride progresses.

It would drive me crazy going from low altitude desert, over a mountain pass and onto high plateau before dropping altitude into heat again. Have done that dance enough to not worry about any handling or wear issues
That's just way overthinking the ride for me.
I set mine at home altitude ambient and go ride. I do check any bike either of us has not ridden for weeks, but if I aired it Monday, I may not check it Tuesday. Only check and maybe adjust if I am staying at cold altitude more than a few days. I know this will agitate some, but, on most of our bikes, they seem to hold a steady pressure for days when I do check them.

Anyways YRMV and I'm gonna

I believe members have sufficiently answered the question of whether to use the +1/-1 formula. I'll ignore it and set the tires at recommended no matter what the temp is when starting out on cold tires [ cold being not ridden for at least 30 minutes, and in my case, overnight ].

I check the tires if I haven't ridden for several days, but when riding daily check them perhaps twice a week.

Thanks for the responses members, appreciate making it as easy as it used to be for me, until I saw the postings about +1/-1 per 10F formula here and the world got more complicated for a time. Why does anyone even bring that formula up to begin with if it's not something to follow?

Back to your regularly scheduled event, and thanks again.

Back in the day the rule of thumb was a 10% increase in tire pressure between cold and hot.
If you started out at 30psi cold, the psi should be 10% or 3 lbs more when hot.
If you weren't achieving that then start at a different psi when cold and reevaluate.
I used this with good success on both bikes and cars.

These days I'm too old and lazy.
I just start out with the proper cold psi.

The 10% rule refers to bias ply tires I believe.

My cold tire pressure, whenever I inflate the tire first thing in the day, is the recommended pressure in the users manual.

fmdualexhaust said:

Back in the day the rule of thumb was a 10% increase in tire pressure between cold and hot.
If you started out at 30psi cold, the psi should be 10% or 3 lbs more when hot.
If you weren't achieving that then start at a different psi when cold and reevaluate.
I used this with good success on both bikes and cars.

These days I'm too old and lazy.
I just start out with the proper cold psi.

Click to expand...

Practically, really, are you gonna take an IR gun and shoot your tires every 39 minutes and inflate/deflate? Are you gonna have an onboard compressor to adjust?

The true reality is that margin of error in your tire pressure gauge can be 2-3%, unless you spend $$$ every 6 months to calibrate.

These days I'm too old and lazy.
I just start out with the proper cold psi.

dieselyoda said:

Practically, really, are you gonna take an IR gun and shoot your tires every 39 minutes and inflate/deflate? Are you gonna have an onboard compressor to adjust?

The true reality is that margin of error in your tire pressure gauge can be 2-3%, unless you spend $$$ every 6 months to calibrate.

These days I'm too old and lazy.
I just start out with the proper cold psi.

Click to expand...

I have an on board compressor in a pannier, accurate pressure gauge [ as tested by a calibrated machine ] along with jump starter and patch kits at all times. Wouldn't ride anywhere without them along with other gear/items like rain suit, med kit etc.

When you're waking up at Laird Hot Springs in a tent, there's no compressor at the camp site to check tire pressures before heading out on the road. Better have that compressor and gauge if for some reason the tire runs near flat or flat over night in the middle of nowhere. Out on a fire road, same, there's you and what ya brought to get you back, unless one likes walking. Even between exits on a thruway, I'm not gonna get caught without a compressor, patch kit and have to walk or call for someone to get me and the bike.