Bike Tire PSI Readings are Inaccurate

Tires were filled cold to 36 front and 42 rear PSI per the air gauge on the MotoPump.

PSI cold tires

Bike front and rear PSI, MotoPump front and rear PSI, Bike ambient temperature

And

PSI hot tires

Bike front and rear PSI, MotoPump front and rear PSI, Bike ambient temperature

So the climb in PSI is expected, fracturing in ambient temperature (+/-12 degree) change and tire heating up in the 30 mile ride home. Why the heck have the bike's readings never matched up to manually PIS readings. I've read through the Instruction manual manual looking to see if there's a reset for PSI. If there is a denotation in the manual, I've missed it. If it is in there please point me to it, or is it a shop reset when tire changes happen.

Tires were filled cold to 36 front and 42 rear PSI per the air gauge on the MotoPump.

 

PSI cold tires

Bike front and rear PSI, MotoPump front and rear PSI, Bike ambient temperature

 

And

 

PSI hot tires

Bike front and rear PSI, MotoPump front and rear PSI, Bike ambient temperature

 

So the climb in PSI is expected, fracturing in ambient temperature (+/-12 degree) change and tire heating up in the 30 mile ride home. Why the heck have the bike's readings never matched up to manually PIS readings. I've read through the Instruction manual manual looking to see if there's a reset for PSI. If there is a denotation in the manual, I've missed it. If it is in there please point me to it, or is it a shop reset when tire changes happen.

Evening Cohiba54

Your off-bike tire gauge reads TRUE tire air pressures-- The bike dash TPMs (RDC) system reads the tire air pressure then corrects it to 68°f.

So the ONLY time your MotoPump pressure gauge should match your dash TPMs (RDC) readout is when the tire & air in the tire is at exactly 68°f.

So figuring appx. 1 psi gain per 10°f of tire temperature-- if you put 32 psi in your tire cold (68°f) then ride the bike & raise your tire temperature to 108°f, your dash should still be reading 32 psi but your MotoPump pressure gauge should read 36 psi.

Look in your riders manual around page 117-120 or so under RDC-- (my manual shows it on page 119)

Then look under RDC, then look under TEMPERATURE COMPENSATION -- it should explain the temperature compensation.

Both of my BMWs have tpm and I never see more than a couple of PSI difference between my gage and the readout on the bikes. After the tires warm up, it's generally the same as the cold tire gauge reading.

I'd verify the accuracy of the Moto pump against a second gage.

The bike dash TPMs (RDC) system reads the tire air pressure then corrects it to 68°f.

This is my experience as well (on a 2016 RT).

Now *why* they would do this is a question I don't have an answer to. When its 100 degrees where I live, its not really very important to me what my cold tire pressure is at 68 degrees... I'm more interested in what it is at the current temperature, which means that I have to re-correct it in my head by adding 1 psi per 10 degrees.

The bike dash TPMs (RDC) system reads the tire air pressure then corrects it to 68°f.

 

This is my experience as well (on a 2016 RT).

 

Now *why* they would do this is a question I don't have an answer to. When its 100 degrees where I live, its not really very important to me what my cold tire pressure is at 68 degrees... I'm more interested in what it is at the current temperature, which means that I have to re-correct it in my head by adding 1 psi per 10 degrees.

Evening alegerlotz

No matter what the ambient or tire temperature you always have to add or subtract based on temperature when filling, or using handheld gauge, so the dash readout corrected to 68°f doesn't have any effect on this part.

As for the reason for correcting to 68°f, that is the industry standard that the recommended pressures are based on so it just keeps the rider from having to look at the dash numbers, guesstimate tire temperatures, then do the math in their head while riding.

The secondary importance of correcting to 68°f is to allow the low tire pressure warning to work correctly in all conditions. If it didn't correct to 68°f then the tire could actually be operating at too low of pressure in hot weather but the low pressure warning might not come on, OR in very cold weather the tire could be OK on pressure but the low air pressure warning could come on.

Most automobiles (unfortunately not all) also correct to about 68°f.

The bike dash TPMs (RDC) system reads the tire air pressure then corrects it to 68°f.

 

This is my experience as well (on a 2016 RT).

 

Now *why* they would do this is a question I don't have an answer to. When its 100 degrees where I live, its not really very important to me what my cold tire pressure is at 68 degrees... I'm more interested in what it is at the current temperature, which means that I have to re-correct it in my head by adding 1 psi per 10 degrees.

 

Evening alegerlotz

 

No matter what the ambient or tire temperature you always have to add or subtract based on temperature when filling, or using handheld gauge, so the dash readout corrected to 68°f doesn't have any effect on this part.

 

As for the reason for correcting to 68°f, that is the industry standard that the recommended pressures are based on so it just keeps the rider from having to look at the dash numbers, guesstimate tire temperatures, then do the math in their head while riding.

 

The secondary importance of correcting to 68°f is to allow the low tire pressure warning to work correctly in all conditions. If it didn't correct to 68°f then the tire could actually be operating at too low of pressure in hot weather but the low pressure warning might not come on, OR in very cold weather the tire could be OK on pressure but the low air pressure warning could come on.

 

Most automobiles (unfortunately not all) also correct to about 68°f.

I understand all of this D. R.

My ambient cold temperature never drops below 75F in the summer (at night).

If I adjust the air pressure to 37-38psi front and 43-44psi rear using a digital gauge. That would give me then 36 psi front and 42 psi rear on the BMW display?

Will my tires be now overinflated?

The bike dash TPMs (RDC) system reads the tire air pressure then corrects it to 68°f.

 

This is my experience as well (on a 2016 RT).

 

Now *why* they would do this is a question I don't have an answer to. When its 100 degrees where I live, its not really very important to me what my cold tire pressure is at 68 degrees... I'm more interested in what it is at the current temperature, which means that I have to re-correct it in my head by adding 1 psi per 10 degrees.

 

Evening alegerlotz

 

No matter what the ambient or tire temperature you always have to add or subtract based on temperature when filling, or using handheld gauge, so the dash readout corrected to 68°f doesn't have any effect on this part.

 

As for the reason for correcting to 68°f, that is the industry standard that the recommended pressures are based on so it just keeps the rider from having to look at the dash numbers, guesstimate tire temperatures, then do the math in their head while riding.

 

The secondary importance of correcting to 68°f is to allow the low tire pressure warning to work correctly in all conditions. If it didn't correct to 68°f then the tire could actually be operating at too low of pressure in hot weather but the low pressure warning might not come on, OR in very cold weather the tire could be OK on pressure but the low air pressure warning could come on.

 

Most automobiles (unfortunately not all) also correct to about 68°f.

 

 

I understand all of this D. R.

My ambient cold temperature never drops below 75F in the summer (at night).

If I adjust the air pressure to 37-38psi front and 43-44psi rear using a digital gauge. That would give me then 36 psi front and 42 psi rear on the BMW display?

Will my tires be now overinflated?

Evening Bernie

The manufactures give a bit of wiggle room on the tire inflation specs for this very reason as for most applications a pound or two makes no difference in tire performance or life.

What you stated above is no different than filling your tires at 68°f then having the ambient temperature go up to 75°f. (would they be overinflated then?)

A common rule that seems to work pretty good from 48°f to 88°f is to just adjust for the offset from 68°f so if the ambient temperature (ie tire temperature) is 88°f then add 2 pounds to the nominal pressure spec.

If starts getting foggy & requires a little common sense when the ambient temps start getting near the extremes as a rider wouldn't want to start out on a 32 psi nominal pressure tire at a corrected-to 25 psi (0°f morning) if the roads were cold & no riding heat gain as they could easily bend a rim if a pot hole was hit. Especially if the bike was fully loaded to GVW at the 0°f initial ride-off. (but few if any riders would actually check or fill a tire at 0°f due to the possibility of a little moisture freezing the valve stem partially open)

For practical everyday use, I set tire pressure cold with external gauge. When I ride I set the pressure in the info window. I leave it there 98% of the time. All I really use it for is to watch variation. If it starts losing air then hopefully I will notice. Here in Texas this time of year it's normally about 75 degrees at night and first thing in the morning when I check pressure. Riding on the freeways here in summer it is common for road temps to be well into triple digits. Last weekend the read of ambient temp was 95 but road temp reads 118... What did my bike say? Before it left the garage it said 37/42. After hours of hot road driving it said 38/44.

What my BMW says and what the external gauge says are usually very very close as MikeB60 said. Both of our cars have readouts too. They never very from a gauge more than a pound or two either. I use to obsess over the bike tires, but not so much anymore. I check it every 2 weeks or so and it almost never needs any air...and if it does...maybe 1-2 pounds. Should lead to a tire thread now...….PR 4GT's that are usually changed out at 12,000 mile check ups with plenty left. Sorry guys could not resist.

Most automobiles (unfortunately not all) also correct to about 68°f.

I have owned many BMWs (3, 5, and 7 series) and a Toyota Tacoma and none of them have done this in my experience. In hot weather the pressure is higher on the readout and in cold weather the pressure is lower. Earlier BMW cars got around this with simply having a color readout (green/yellow/red) rather than a number... but my 2014 535d showed numbers and they varied based on the outside temperature. My Tacoma does the same. The same is true of rental cars I've had in the past (I've rented a lot... for 6 years I had a travel job that had me on the road 30+ weeks/year).

The 2016 R1200RT is the only vehicle that has done this "correction" on the PSI display and I find it annoying. Fine, use a correction to avoid false warnings, but displaying a number based on it is not helpful IMO, its confusing and misleading.

I have a 2015 GSA with TPMS. I find the "correction to 20C/68F" easy to understand, and easy to use. I find it more useful to me than showing the actual pressure in a hot tire.

I set the pressure to 36F/42R whether it's hot or cold outside - whatever it is before I start off. The engineer in me thinks that it'd be nice if BMW would correct to the "real" temperature, but if I start off when it's 65 and it rises to 105 during the day, what should they correct it to? Tire manufacturers all say to set the pressure when it's "cold", and don't mess with it until it cools down again. So my solution is to declare that whatever my handheld gage says when the tires are cold is the real number. What shows on the display is therefore just a number that may or may not bear some relation to reality, and what's important is whether the number on the display changes, not what the number on the display is. And a chage of a pound or even two over the course of the day is trivial. Does it bug me? Sometimes, but I've mostly gotten over it

I've owned a '13 F800GT and my current '16 RT both w/ TPC as they call it. It's a magically reliable, trustworthy and effective instrument, in fact so much so, that BMW, despite the liability they are arguably exposed to with this, tell you to go by TPC and NOT external gauges, be they in your tool box or at a service station--this is how I interpret the rider manual on this topic.

Here are the caveats simply put:

1. Follow common practice for setting your INITIAL tire pressure w/ a tire gauge on a cold tire, using the temperature adjustment as indicated for ambient cold tire temp over or under 68F. Set the pressure maybe 2 psi higher than indicated, so for rear tire 44psi. Go 2 over on the front as well.

2. In both my BMWs the initial displayed temp on a cooled/cold/non-hot tire will be ~2psi lower than it will correct to once the tire is road warmed in a few miles etc. The pressure readings then you can consider reliable.

The reason to set them OVER by a few psi is so when you take her out for her first ride post inflation you can simply bleed off a little and get exactly where they will display your preferred pressures. Mine always indicated 36/42 until they need pumped up again, but that's not very often. Those of course show that way after the tire is road-warmed. You can expect they will stay w/in 1 psi no matter what the ambient temp is as well, at least that's been my experience and that is compatible w/ their design and intended use. In fact a good illustration of this was on a trip in 2016 where we took the amazing magic carpet ride out of Bishop, CA on the 168 on to St George, UT. When we left Bishop on June 5th very early in the day after road warming pressures displayed 36/42, and ambient temp was in the low 60's or high 50's I forget, but for hours of riding up hill into St George past LV despite reaching 112.9F for last few hours of the ride the display continued to read 36/42. As such it becomes easy to assess for air loss in the tire, versus needing to do the math first to do temp correction manually. TPC has saved me once from a potential bad outcome, and again as a convenience. I would not want a bike w/o after these two bike experiences.

Good Evening NoelCP.

What sort of tire life you get after using your method to adjust your air pressure in your tires.

I don't want to start a tire thread or hijack this thread, just wondering.

You are welcome to PM me if you want.

Thanks.

PS: My BMW TPS will read higher pressures, by 1 psi on the rear tire after extended high speed use at 98F.

Good Evening NoelCP.

What sort of tire life you get after using your method to adjust your air pressure in your tires.

I don't want to start a tire thread or hijack this thread, just wondering.

You are welcome to PM me if you want.

 

Thanks.

PS: My BMW TPS will read higher pressures, by 1 psi on the rear tire after extended high speed use at 98F.

Hi Bernie,

About 10-11K on either PR4GT or Angel GT which is what I've use on my '16 RT. I have Conti RoadAttack 3 on now and these are far and away the best tires in every aspect of the 3, and if I add Metz Z8 which were on hte F800GT best over them as well. I only have 5,300m on them so far but they retain their shape best, have the most inspiring grip, hold lines absolutely the best, don't make noise, and have the right turn-in which probably plays a role in why it holds line the best. I have a hunch they will easily keep up w/ the others for wear and likely exceed them. As a side point my TPC began not working the day they were installed. The shop thinks perhaps the combination the tire carcass and the age of the sensor batteries cause RF interference issues and indeed there were RF-i issue when I had my valves checked last week. For over 3000m TPC began acting erratically--not the values per se, they wouldn't display for up to 15 miles of use, then suddenly start working. Over time they returned to normal and they think as the tires wore this may explain why it came back as it were. Of note the rear was worse than the front always w/ regard to failing to display PSI on the dash, taking longer to 'wake up' as it were, and of course the rear has much more mass to it so this fits w/ the theory. I've not heard of anyone else with this issue but at least it finally totally normalized. I'll be sticking w/ these tires for the next set they are so much better, and I can run the front now back at 36-37 w/ zero scalloping, unlike PR4GT which were really bad at that issue as you know or have heard.

Thanks for the info on the tires, Noel.

Are those Conti RoadAttack 3 tires "GT" versions? I just wanting to make sure.

Thanks

Yes they are the GT version.