Tire Pressure, Altitude and Ambient Temperature?

When I lived in SLC at 4500' elevation and rode to Torrey 6800' I never noticed any issues. Last time we rode from sea level to Torrey our tires ran hard, and we measured high pressures in Torrey. I think the pressure in the tires doesn't change (assuming tire volume changes with pressure are insignificant), but the gauge reads as the difference between ambient pressure and tire pressure. Ambient pressure dropped, so the gauge reads higher. This begs the question, which inflation is right? What the gauge reads at altitude when I am at that altitude, or is there a reference altitude?

Tire Rack has a blurb that says "cold" means at ambient temperature in the morning before riding, and since at elevation these temps tend to be lower than mild coastal temps, it all works out. But they do suggest adjusting pressure to gauge reading at your destination and on return to home. Should one really be running with less air in the tire at elevation?

As well, they say for a car, a rough estimate of 1psi difference for every ten degrees F in temperature. Does this hold for the smaller volume of a moto tire? I think I see larger variations.

I always thought "cold" was defined as 60 F and pressure was referenced to sea level, but that is not what I am reading on tire sites.

Bottom line, riding in the Western US with road elevations from -282' to 14,130' and morning temps being very dependent on season, location and elevation, do you just ride and forget about it? We did that on our 4900 mile to the UnRally this summer and tire pressures measured at home on our return were spot on with departure. Never touched them. Elevation peaked on a chilly morning ride over the Beartooth at 10.947'.

For those not afraid of the asylum:

https://advrider.com/f/threads/tire-pressure-psa-something-i-just-learned.1337086/

you'll see a few posts from me if you wade far enough in, but they are of course useless from a practical standpoint

I just ride. Altitude is rarely more than a psi except for transient conditions. Temperature correction is small the last time I took the time to make the conversion and calculation. I'm more worried about making sure I get to Starbucks on my polished bike so that it can look pretty while parked.

For those not afraid of the asylum:

https://advrider.com/f/threads/tire-pressure-psa-something-i-just-learned.1337086/

 

you'll see a few posts from me if you wade far enough in, but they are of course useless from a practical standpoint

 

Thanks for posting this. I'm pretty anal about checking tire pressure and it was a good read. I do feel the difference on the front tire when its a few pounds down. I'm surprised I never knew the BMW spec was for setting the pressure at 68F and to compensate up or down depending on ambient air temp even when tires were cold. Lately, I've been trying to keep up with the 10% rule for upping the front tire over the 36 psi spec instead of just always using 38 psi or 40 psi to avoid cupping and bent a bent rim from potholes. (Yes its happened) My 10% results were varied with ambient air temps. I might try adjusting for ambient air temp on cold tires and see if that makes the 10% fluctuations a little closer.

Without having done exhaustive research on the topic, tire pressure is relative to temperature and atmospheric pressure (ideal gas law). To maintain the same tire flexibility characteristics at various altitudes, adjust the pressure so it reads "correctly" on your gauge. Yes, the number of molecules inside your tire will vary, but it's the pressure that you're worried about.

What the "correct" pressure really should be, of course depends on a variety of factors - road surface, riding style, personal preference and some other things that are beyond me.

And not to mention that just about every manufacturer recommends 36psi front and 42psi rear for every motorcycle in their range?

I think they have mountains in Europe.

I think the bikes go up and down.

I think you are better off worrying about how much tread is left.

We'd see tire pressures off.

Sometimes a little.

Once in a while much more. A couple of are you kidding me, you're riding this?

But, a change in tire air pressure based on elevation?

Yes.

Enough to matter?

Maybe.

Not likely for a trip with multiple elevation changes.

Notice, not saying it won't, just not enough to matter riding 4-5-6-7-8-900+ miles/day.

Now tread?

Yes it matters.

And, yes, I'll wager many of us push past tire wear bars.

Or worse.

That was something we saw ALL the time.

So I really don't think most riders even consider elevation change if they

don't monitor and respond to tread changes,

.02

Best wishes.

When I lived in SLC at 4500' elevation and rode to Torrey 6800' I never noticed any issues.

Elkroeger is right: gauge pressure is measured relative to ambient. The absolute pressure in your tire may remain constant, but if you change ambient pressure, then you are changing gauge pressure, and gauge pressure is what matters for tire behavior.

Here's a chart showing altitude, local air pressure at that altitude, and the incremental change from the previous line:

Altitude, ft.	Pressure, psi	tire gauge pressure increment
0	14.70	..
1,000	14.18	0.52
2,000	13.68	0.50
3,000	13.19	0.49
4,000	12.72	0.47
5,000	12.26	0.46
6,000	11.81	0.45
7,000	11.38	0.43
8,000	10.95	0.42
9,000	10.54	0.41
10,000	10.15	0.40
11,000	9.76	0.39
12,000	9.39	0.37
13,000	9.03	0.36
14,000	8.68	0.35

Too tedious to line all that text up, but if you eyeball it, you can see what's going on. Jan, if you're going from SLC to Torrey (4500' to 6800'), then you're looking at a ~2000' elevation gain, resulting in an increase in your tires' gauge pressures of about 0.9 psi. Depending on your tire temperature when you check your tires the next morning (and this can be affected by ambient temp, as well as whether or not the sun has been warming your tires or not), you may or may not notice that.

I used to notice it when I rode from Michigan (~1000 feet) out to Torrey (~7000'); if I didn't let some air out along the way, by the time I got to Torrey my tires would be reading about 3 psi high. It wouldn't be a huge problem to ride with tire pressures 3 psi above normal, but the problem that comes up for me is when I travel in the other direction, and my tire pressures decrease by that same amount after having let some air out while at Torrey. Then they wear at an accelerated rate. This came back to bite me in 2011, when my rear tire wore down to the cords in Iowa, and I had to get it changed at Gina before riding the last 500 miles home. If I had stuffed another few psi in my tire before leaving Denver, I might have made it home.

As it happens, riding to very high elevations (10K+) often coincides with a temperature decrease that tends to counteract the effect of local air pressure on your tires' gauge pressures. Also, as noted upthread, riding up to these altitudes is typically a transient thing. I check my tire pressures in the morning before heading out for the day, and that's it.

As well, they say for a car, a rough estimate of 1psi difference for every ten degrees F in temperature. Does this hold for the smaller volume of a moto tire? I think I see larger variations.

There's nothing special about tire volume here. Pressure change will be proportional to temperature change. The trick is that pressure and temperature must be measured on absolute scales.

So here's an example:

State 1:

temperature, 70F (absolute temp, 529R)

gauge pressure, 30 psi (absolute pressure, 44.7 psi)

State 2:

temperature, 80F (absolute temp, 539R)

What's the pressure at State 2? 44.7 * 539/529 = 45.54 psi absolute. That's 30.84 psi gauge.

The change is a bit more for motorcycle tires, which run around 40 psi: a 10F change means a 1.03 psi change. So "1 psi per 10F change" is a reasonable rule of thumb, at least at the kinds of pressures used on cars and motorcycles.

Elkroeger is right: gauge pressure is measured relative to ambient.

That is true, as long as we're talking about manual tire pressure gauges.

But it's not true in case of the common TPMS sensors that are internal to the tire, with no reference to outside ambient pressure.

The absolute pressure in your tire may remain constant, but if you change ambient pressure, then you are changing gauge pressure, and gauge pressure is what matters for tire behavior.

Right, the manual gauge pressure does change with elevation.

But I would argue that it doesn't matter.

My argument is that what matters is the absolute tire pressure.

Modern moto tires are not like balloons, the very strong carcass makes the tire behave more like a fixed volume container than a balloon.

=> The absolute pressure matter more to tire performance than the relative pressure.

I used to notice it when I rode from Michigan (~1000 feet) out to Torrey (~7000'); if I didn't let some air out along the way, by the time I got to Torrey my tires would be reading about 3 psi high. It wouldn't be a huge problem to ride with tire pressures 3 psi above normal, but the problem that comes up for me is when I travel in the other direction, and my tire pressures decrease by that same amount after having let some air out while at Torrey. Then they wear at an accelerated rate.

And I think you example here illustrates my point.

Had you left the tire pressure untouched on the way to Torrey, the gauge pressure would go up, and then down to original reading on the way back

But your tires would have been correctly inflated all the time.

By adjusting the tire pressure along the way, you kept the gauge pressure constant, but you ended up with incorrect tire pressure. Until you adjusted it back up again.

With a TPMS based system this problems doesn't exist.

Regardless of the elevation, the reading stays constant and tires remain correctly inflated.

I could be wrong.

With a quick Google search I didn't find any trustworthy information about which pressure (gauge or absolute) matters for moto tires.

Elkroeger is right: gauge pressure is measured relative to ambient.

That is true, as long as we're talking about manual tire pressure gauges.

But it's not true in case of the common TPMS sensors that are internal to the tire, with no reference to outside ambient pressure.

Any TPMS is going to have a barometric sensor somewhere on the vehicle chassis that it uses for calculating gauge pressure. Either that, or it uses the ECU's barometric sensor (every vehicle with EFI will have one of those anyway). If a TPMS reported absolute pressure to the driver/rider, it would confuse the hell out of anyone who measured their tire pressure with a handheld gauge and noticed that the number on the dashboard was about 14 psi higher.

My argument is that what matters is the absolute tire pressure.

Modern moto tires are not like balloons, the very strong carcass makes the tire behave more like a fixed volume container than a balloon.

=> The absolute pressure matter more to tire performance than the relative pressure.

Here's a thought experiment. Consider a motorcycle tire with no leaks, but it's filled to zero psi gauge (14.7 psi absolute) in your garage. (i.e. take out the valve core until pressure equalizes, then put it back in). Make a note of how compliant the tread surface is.

Now fill the tire to 40 psi gauge (54.7 psi absolute). And then take that tire and put it in a hyperbaric chamber, and bump the chamber pressure up to 54.7 psi absolute. In this environment, the tire's gauge pressure will be zero - and it will behave exactly as it did when it was at zero gauge pressure in your garage.

Now increase the hyperbaric chamber pressure to 100 psi. Your tire's absolute pressure is still 40 psi - but it'll be collapsed in on the rim. Get ready for a lumpy ride.

This should serve as a clear indication that gauge pressure is the thing that matters for tire performance.

And I think you example here illustrates my point.

Had you left the tire pressure untouched on the way to Torrey, the gauge pressure would go up, and then down to original reading on the way back

But your tires would have been correctly inflated all the time.

My tire pressure gauge disagrees with your assessment ("correctly inflated all the time"), and so does Bob Palin's.

I could be wrong.

With a quick Google search I didn't find any trustworthy information about which pressure (gauge or absolute) matters for moto tires.

How about Tire Rack?

TIRE TECH: THE INFLUENCE OF ALTITUDE CHANGES ON TIRE PRESSURE

Good link.

Like I said, tread not tire pressure is a bigger issue, IMO.

Best wishes.

Mitch, do KNOW this is the case, or do you expect it to be so?

"Any TPMS is going to have a barometric sensor somewhere on the vehicle chassis that it uses for calculating gauge pressure"

I totally agree that the TMPS readings are offset by say 14 PSI to show a familiar looking gauge pressure at some "normal" elevation.

But I'm not convinced the TMPS readings are adjusted dynamically to track the ambient pressure.

That Tire Rack link explains in long detail the reasons why the indicated gauge pressure changes with elevation.

There's no confusion about that.

But that link says nothing about which one we should pay more attention to, gauge pressure or absolute pressure.

Obviously, if the ambient pressure varies from 14.7 PSI to 54.7 PSI, the tire will behave very differently.

But that is not a realistic case. Tires are designed to work properly, at recommended tire pressure, at the normal conditions.

That would be same as saying the springs on the bike still follow the spring equation when the load is changed from 50lb to 1500lb.

The springs only work as springs within a reasonable load range.

I'll give this a try tomorrow.

I'll get a gauge reading tomorrow morning here at seal level, and also note the TPMS reading on my GS.

When I get to the higher passes on the Sierra's, I'll take the gauge reading again.

It will be higher due to two factors, the lower ambient pressure and higher tire temp.

If you are correct, also the TPMS reading will be higher (by about 3 PSI) because of the lower ambient pressure.

I'll admit to riding from sea level to the summit of Mauna Kea (13,803 ft.), and back, in one day. We had a wonderful time. I never stopped to check my tire pressures.

Mitch, do KNOW this is the case, or do you expect it to be so?

"Any TPMS is going to have a barometric sensor somewhere on the vehicle chassis that it uses for calculating gauge pressure"

You're right, I don't absolutely know this to be the case (except see below), but to not do so would mean a TPMS system could be off by 10-20%, depending on altitude.

That said, I did find this discussion this morning on a Corvette forum, which included the following:

After reading the explanation in the service manual ... you're correct the RCDLR (or RFA - the radio receiver for the TPM sensors and fobs) requests the current barometric pressure from the PCM to compensate for altitude and uses that to "correct" the tire pressures. BUT, here's the caveat, it only requests the pressure AT ENGINE START. Once it has the data it doesn't get a new reading of barometric pressure. So, if I start my car here in Redwood City (sea level) and drive non-stop to Lake Tahoe, my TPMS will be corrected for sea level, while the actual outside pressure is now at about 6,000 feet (or over 7,000 as I go over Donner Pass) ..... so the OP should have seen NO change in tire pressure until he Re-started his car at a high altitude .... at which point he would see a HIGHER pressure in the tires.

 

Conversely ... and this is where it could be a problem ... If I take my 30 PSI tires to Tahoe ... re-start the car, and let air out to bring them down to a compensated 30 PSI .... and THEN drive non-stop to sea level .... the TPMS will think I still have 30 PSI but I'm actually running under-inflated.

If he really did get that from the service manual, then there is at least one OEM TPMS that references local barometric pressure - or at least local barometric pressure as measured the last time the key was turned on.

On the other hand, I found this 2013 Infiniti service bulletin, which includes the following on the last page:

•At high altitude locations, a standard tire pressure gauge may show the tire pressure higher than the TPMS system. If the gauge reading is not accounted for, this could result in turning ON the Low Tire pressure Warning Light.

 

• Standard tire pressure gauge readings increase about 1.0 psi for every 2,200 ft of altitude increase above sea level (up to 10,000 ft). See Graph 3.

 

• For example, if the TIRE AND LOADING INFORMATION label reads 33 psi, then at an elevation of 5,280 ft, the cold inflation pressure using a gauge should be increased 2.5 psi to 35.5 psi."

OK, so there's at least one crappy TPMS that doesn't account for local barometric pressure - and the result is that Infiniti had to issue a post-production service bulletin that tells service techs how to avoid triggering a spurious Low Tire Pressure Warning Light for their customers (basically by inflating their customers' tires at high altitude to a gauge pressure higher than what's listed on the door jamb sticker). These customers may experience odd tire wear and a slight loss of traction, but at least they'll stop bitching to the service tech about the damn warning light.

On the other other hand, there's this aftermarket system, which includes the following info on page 33:

SmarTire® TPMS can be made to compensate for the pressure changes caused by elevation. To compensate for elevation, the system uses an ambient pressure sensor. The primary use of this feature is for off-highway equipment located in elevations other than sea level for extended periods of time. Unless an ambient pressure sensor has been installed on the vehicle, the Altitude Adjust setting should be set to “OFF”.

So it sounds like this system, if not equipped with a barometric sensor, simply subtracts 14.7 psi from the tire reading to give you a good approximation of gauge pressure at sea level. But if you want accurate tire pressure measurements somewhere other than sea level, then you're supposed to install the barometric sensor and turn on the Altitude Adjust setting.

Obviously, if the ambient pressure varies from 14.7 PSI to 54.7 PSI, the tire will behave very differently.

But that is not a realistic case.

I agree, it's totally unrealistic, but it illustrates the point that what matters for tire compliance is gauge pressure, not absolute pressure.

Tires are designed to work properly, at recommended tire pressure, at the normal conditions.

Yes. As measured by a pressure gauge, which measures...gauge pressure. I've never seen a tire or vehicle manual that tells the operator to adjust tire pressures based on altitude (the Infiniti service bulletin advice above is simply a band-aid to avoid having high-altitude customers complain about spurious warnings from their TPMS).

I'll give this a try tomorrow.

I'll get a gauge reading tomorrow morning here at seal level, and also note the TPMS reading on my GS.

When I get to the higher passes on the Sierra's, I'll take the gauge reading again.

It will be higher due to two factors, the lower ambient pressure and higher tire temp.

 

If you are correct, also the TPMS reading will be higher (by about 3 PSI) because of the lower ambient pressure.

Will be very interested to hear the results. If your system operates like the Corvette system I linked to above, you may need to restart the engine so the system grabs local barometric pressure at your current location. If your system is like the Infiniti system described in the service bulletin above, then it'll be inaccurate compared to your handheld gauge.

Interestingly, this Mitsubishi service bulletin from 2011 gets it dead wrong:

Increased altitude decreases tire pressure. A decrease of 0.5 PSI will occur for each 1,000 ft of altitude change. However, TPMS systems in 2007 Outlander and all 2008 and later models automatically compensate for altitude changes. Although It is not necessary to adjust tire pressure when driving to a higher altitude for a short period, if tire pressure must be adjusted, adjust it to the pressure listed in the Tire Pressure (PSI) for the altitude (see following chart). Adjusting pressure to the sea level pressure at higher altitude can lead to a too high inflation pressure (and a possible rough ride) when the vehicle is returned to lower altitude.

As for why the Corvette's TPMS corrects for altitude, but Infiniti's does not, it turns out that altitude correction is not required for TPMS systems. Here's the NHTSA Final Rule notice from 2005. Scroll down to section 16, quoted below ("ETV" = Eaton Vance Corporation, a maker of TPMS systems):

ETV also stated that the TPMS should be required to account for changes in atmospheric pressure that accompany changing altitudes. ETV commented that such atmospheric pressure changes could change tire pressure by as much as 10 psi.

OK, we already know that 10 psi is a bit of an overstatement; if you drove from sea level to 14,000 feet (or vice-versa), the biggest change in gauge pressure you'd expect to see is about 6 psi.

We have decided not to adopt requirements for temperature and altitude compensation because we believe that such requirements would introduce unnecessary complexity to the standard.

...

Regarding altitude correction, we do not believe that altitude will be a significant factor in tire pressure fluctuation. We expect that the effect of atmospheric pressure on tire pressure will not result in more than a 5-percent change in tire pressure over the atmospheric pressure extremes encountered during normal driving.

5% of 30 psi (for a car tire) is 1.5 psi, which is what you'd see with a ~3,000-foot elevation change. I guess you'd call that "normal" driving. Most people don't drive through massive elevation changes (and if they do, it usually takes multiple days, and they're supposed to check their tire pressures regularly), so this covers just about everyone, and allows manufacturers to save costs. This reinforces the idea that a TPMS, even if it displays numerical pressure values, shouldn't be trusted for fine-tuning performance adjustments, it should only be trusted for what the NHTSA mandated it to be: a crude warning system intended to prevent dangerous tire blowouts from major underinflation.

5% of 30 psi (for a car tire) is 1.5 psi, which is what you'd see with a ~3,000-foot elevation change. I guess you'd call that "normal" driving. Most people don't drive through massive elevation changes (and if they do, it usually takes multiple days, and they're supposed to check their tire pressures regularly), so this covers just about everyone, and allows manufacturers to save costs. This reinforces the idea that a TPMS, even if it displays numerical pressure values, shouldn't be trusted for fine-tuning performance adjustments, it should only be trusted for what the NHTSA mandated it to be: a crude warning system intended to prevent dangerous tire blowouts from major underinflation.

I'm not sure what "normal" driving is, but going through an elevation change of 6000 feet or more in a few hours is easy to do in a lot of places. I once rode from sea level to 8000+ feet (Whitney Portal) in a day, but it was a long day. By noon the next day I was at 14,000 feet but I walked up the extra 6000 feet so tire pressure wasn't an issue. That is kind of extreme but 3000 or 6000 feet is pretty common. The front range of Colorado is around 6000 feet and it is less than an hour to the top of the front range at 9000 feet and another hour to passes at 12,000 feet. Having said that I don't think many people worry, or even think, about changes of a few psi in tire pressures.

Good discussion! Thanks!

Remarkable amount of confusing info out there.

Well, the tires are now in Torrey, with inflation at sea level in Brookings. Morning temps are comparable. Yesterday riding they seemed a hair slick to me, consistent with an over-inflation condition. Since we'll be mostly above 5k' for the next week, I may let a little air out, and re-inflate when we start back down.

I agree with who ever said, a few psi makes a difference in moto tires. Both ride and wear.

After installing aftermarket TMPS on my bikes and cars a year ago and watching the real-time pressures and temperatures for the last year, I'm not quite so anal about the tire pressures on anything - as long as they are in a suitable range. The bike tire pressures will vary 5 PSI depending on riding conditions, speed, elevation changes, and even sunshine. PV=nRT, or more commonly P=T/V. As the elevation and tire temps go up/down, the tire pressure goes up/down, and sunshine can make a 2 PSI difference, either parked or rolling. Bottom line - tire pressures fluctuate several PSI CONTINUOUSLY, but not very quickly. So, what's the point of setting the tire pressure exactly when you know it will be changing shortly??

The TPMS lets you know if the pressure is "out of range", either due to temperature, elevation, or leakage, which is critical for the safety of bikes and riders.

A brief update on the TPMS from the Torrey trip.

At least on my 2014 GSA, the TPMS reading does not change with elevation.

Between 300ft and 10500ft elevation the front and rear pressure readings remained within one PSI.