TPM Accuracy

[narcosis]

First, have to say how much I like the tire pressure monitoring. It has alerted me twice now when punctures occurred and the tire wasn't flat but pressure was pretty low.

 

Just wondering if anyone has tested the TPM's accuracy? I have pencil style Slime digital gauge and the front tire TPM reading is off by about 1.5 psi and the rear by almost 3 psi. Not saying the Slime gauge is accurate but there certainly appears to be a difference between the front and rear sensors on the bike.

 

My sudden interest is the premature cupping on my front tire...at least I thought it premature at 6500 miles (it's really bad now at 8k. Thought it might be related to improper inflation.

[dirtrider]

Evening narcosis

 

 

You do realize that the BMW TPM is temperature compensated?

 

The only time that your tire pressure gauge should match your tire pressure gauge is at 68°f.

 

The front & rear tires have different air volume so are effected differently as the temperature changes.

 

Check them at about 68°f then see how close the front & rear matches your gauge.

 

I'm not saying the TPM sensors are precision pressures sensors but your finding might not show the true difference.

[Skywagon]

The TPM's on my car(s)have a front and back. They usually have to be recalibrated when I have a flat, rotate tires, or change tires. The tire dealers I buy from do it. I've watched them do it and pretty simple process. They calibrate the TPM to what their tire gauges say....non temperature calibrated. They are always pretty close to what my gauges say..1-2 pounds.

 

When I had the dealer put new tires on my 14RT..the TPM's were way off. I asked them to recalibrate. They tried to say that couldn't be done and margin of error was +- 5 lbs. I told them they were spot on before they changed my tires and that was too much margin. They took it in to the shop for about 30 minutes and now they read +- 1 lb of my gauge.

[NoelCP]

Evening narcosis

 

 

You do realize that the BMW TPM is temperature compensated?

 

The only time that your tire pressure gauge should match your tire pressure gauge is at 68°f.

 

The front & rear tires have different air volume so are effected differently as the temperature changes.

 

Check them at about 68°f

 

Speaking of temperature, pressure, I've read and have followed this approach to setting tire pressure from reading an article about it: for every 10F less than ambient cold tire temp of 68F, subtract 1F from the pressure setting using a quality manual gauge. So if it's 48F ambient on a cold tire and you want TP to equal 42psi at ambient of 68F, set pressure to 40F. Same same for >68F ambient. What I'm not sure about how ambient temp affects the Flaig gauge. It does have a ring calibration you turn to adjust to 0psi, so maybe this is what adjusts for temp.

 

Something is amiss w/ this approach however, even though I understand the logic behind it and it seems to make some sense. It may be my TP gauge is also ultimately temperature corrected (it's a metal non-digital Flaig heirloom grade German touted to be quite accurate). Anyway, what I find is TP set w/ this gauge at 68F ambient which is what is was in my garage yesterday, that TPMS is spot on the same in front and rear after the tire warms up from riding a couple of miles. If I use my old method as described above w/ adjusting for ambient, the exact same number of degrees I offset by is reflect by TPMS, so that logic isn't holding up. Like I say it could be the bi-metal strip of whatever is driving the Flaig gauge ultimately is temperature sensitive.

Edited September 19, 2016 by NoelCP

[dirtrider]

Morning NoelCP

 

Very/very seldom would a hand held tire gauge be temperature compensated (I've never seen one) as there is no reason for that. Most are just calibrated against local atmospheric pressure with no temperature compensation. (the hand held tire gauge has no idea of the temperature of the air inside your tire)

 

[hopz]

Glad to see a thread on this subject.

 

The indicated TPS on my '14 has been off since the day it was delivered. Sort of pisses me off but I can live with it. What is baffling me is that I have a very accurate and fairly expensive dial tire pressure gauge. I trust it.

 

I inflate to the pressure I want and then ride. The bike TPS is 1.5 or so off on each tire.

What is funnier is that as I leave the garage with cold tires I see the reading... after a spirited run and with hot tires... it remains the same.

 

Odd and counter to what I think of as the real world.

[dirtrider]

Glad to see a thread on this subject.

 

The indicated TPS on my '14 has been off since the day it was delivered. Sort of pisses me off but I can live with it. What is baffling me is that I have a very accurate and fairly expensive dial tire pressure gauge. I trust it.

 

I inflate to the pressure I want and then ride. The bike TPS is 1.5 or so off on each tire.

What is funnier is that as I leave the garage with cold tires I see the reading... after a spirited run and with hot tires... it remains the same.

 

Odd and counter to what I think of as the real world.

 

Morning hopz

 

That is the temperature compensation at work. (temperature compensation is sort of spelled out in your riders manual)_

[NoelCP]

What is funnier is that as I leave the garage with cold tires I see the reading... after a spirited run and with hot tires... it remains the same.

 

That is never the case for me. Always start out about 2psi lower with cold tires than after a few miles of riding.

[NoelCP]

Morning NoelCP

 

Very/very seldom would a hand held tire gauge be temperature compensated (I've never seen one) as there is no reason for that. Most are just calibrated against local atmospheric pressure with no temperature compensation. (the hand held tire gauge has no idea of the temperature of the air inside your tire)

 

FWIW, I wasn't suggesting temperature compensation was built in, but that perhaps w/ the parts involved it might vary w/ temp, hence the adjustable zero-point indicator.

[dirtrider]

Morning NoelCP

 

 

On uplevel tire pressure gauges some are adjustable so they can be calibrated to a known pressure-- (where I work we need to check our tire pressure gauge calibration once week against a master gauge). Same with our torque wrenches.

 

The other thing about adjustability in hand held tire pressure gauges are that the are not really zeroed against a true zero but zeroed against existing atmospheric pressure (tire pressure gauge measures pressure inside the tire against existing atmospheric pressure outside the tire)-- That is usually close to 14.7 pounds per square inch at sea level.

 

So if you zero at sea level then you need to re-zero if you move to Denver or move to death valley (or in between).

[Mike]

You guys have left me behind. I check my pressure with a gauge I trust. The TPM values are never identical, but are within one or two p.s.i.

 

The critical thing is that I keep an eye on those reported pressures to ensure that they are more or less constant. I don't rely on them as being spot on, only to watch for unexpected pressure drops.

 

Tests I've read suggest that many gauges are inaccurate, though the cheapo pencil gauges are often spot on. Still, I check my tires before every ride and I figure that puts me ahead of about 99% of folks on two wheels.

[NIM205]

 

 

You do realize that the BMW TPM is temperature compensated?

 

Compensated or not, my TPM reads 1-2 psi more after ten minutes riding.

[Dave_in_TX]

 

 

You do realize that the BMW TPM is temperature compensated?

 

Compensated or not, my TPM reads 1-2 psi more after ten minutes riding.

 

That frequently happens with my TPM too. I suspect the temperature compensation algorithm doesn't fully compensate.

[hopz]

Never thought about the sea level thing.

 

My house and garage is at exactly 7,000 feet. So- do I over inflate my target tire pressure, or under inflate?... or does it matter?

[dirtrider]

Never thought about the sea level thing.

 

My house and garage is at exactly 7,000 feet. So- do I over inflate my target tire pressure, or under inflate?... or does it matter?

 

Morning Hopz

 

Good question--answer depends on:

 

If your tire pressure gauge is calibrated (zeroed) for sea level or a lower altitude then you don't have what you think is in your tires.

 

But if your tire pressure gauge is calibrated (zeroed) for your altitude then you should be OK.

 

External (hand held) tire pressure gauges are just showing the pressure inside the tire vs the atmospheric pressure outside the tire. To get a true (non atmospheric pressure offset) pressure then you would have to add about 11.34 psi to your total)--At sea level you would have to add 14.7 psi to your total.

 

What you might do is visit a local (GOOD) tire store & see if they have a wall mounted tire pressure gauge calibration set-up, if so then see how your personal tire pressure gauge compares.

 

 

[realshelby]

At least this conversation explains why my TPM did seem to read less than I thought it should after a few hours on the road! I have a couple good quality gauges, while I won't swear to accuracy I think being very close is good enough. What I like most about the TPM is being able to check while going down the road if you feel a little wiggle in the bike. While I have not have a tire lose pressure on this RT, knowing it is easy to check without stopping is a nice thing.

[NoelCP]

At least this conversation explains why my TPM did seem to read less than I thought it should after a few hours on the road! I have a couple good quality gauges, while I won't swear to accuracy I think being very close is good enough. What I like most about the TPM is being able to check while going down the road if you feel a little wiggle in the bike. While I have not have a tire lose pressure on this RT, knowing it is easy to check without stopping is a nice thing.

 

It's saved me twice, on my F800GT. First time I had pulled into Casa De Fruita for lunch on 156 in California and upon entering the onramp to the highway TPM starting flashing, I pulled over and did a quick tire puncture repair and back on the road in about 20 minutes. Where it also shines is monitoring after a tire repair--nice to see how it's holding up. I rode about 200 miles home on that repair.

 

Next it was just a routine ride and some errands. Made it out the driveway and when TPM spooled up the rear tire was a 35psi, whereas I run it at 42 or 43. I turned around and sure enough found a heavy gauge staple was the culprit.

[hefy_jefy]

The TMP pressure *should* always read the cold (68F) pressure, i.e. 32f and 42r irrespective of the actual temperature. I just watch over a period of time and estimate if the readings are low or high and by how much. Then use a pressure gauge to adjust by that amount.

Front tires seem to show noticeable wear after about 5,000 miles (or less depending on the rider) changed my first set at 11,000 and the second at 21,000. The "shine" goes off a set of tires pretty quickly.

Geoff

[LIRider]

I'm confused ... shouldn't the tire pressure be set for the actual current ambient conditions? Showing me the cold tire pressure at 68 degrees is of little value if I'm riding in the Winter with temperatures around 30 or during the Summer with temperatures in the 90's. What am I missing?

[dirtrider]

I'm confused ... shouldn't the tire pressure be set for the actual current ambient conditions? Showing me the cold tire pressure at 68 degrees is of little value if I'm riding in the Winter with temperatures around 30 or during the Summer with temperatures in the 90's. What am I missing?

 

Morning LIRider

 

The tire air pressures are sort of a compromise but a good starting point is to set them based on 68°f at time of checking.

 

A good rule of thumb to use when comparing tire pressure to ambient air temperature is for every 10 degree of temperature change then the tire pressure will change by (appx) 1 psi. For example-- if the ambient air temperature increases by 10° then the tire pressure will increase by 1 psi. Or, if the air temperature falls by 10° then the tire pressure will decrease by 1 psi. (this assumes the tires are in the shade & allowed to equalize to the ambient temperature before checking)

 

The fudge factor here is--it isn't all ambient temperature as it is REALLY the air temperature inside the tire. So a tire is sitting out in 48°ambient it should show 2 psi lower than the same tire at 68°f ambient. BUT if the bike is sitting in the sun then even though it is 48°f outside the black tires could be much warmer due to sun load on the black tires.

 

Same with going down the road. It could be 48°f outside but road surface temps could be 75°f due to sun load on the surface & tire friction/engine heat from other traffic.

 

 

 

 

 

 

[Green RT]

I'm confused ... shouldn't the tire pressure be set for the actual current ambient conditions? Showing me the cold tire pressure at 68 degrees is of little value if I'm riding in the Winter with temperatures around 30 or during the Summer with temperatures in the 90's. What am I missing?

 

The tire temperatures start at the ambient temperature, but warm up from frictional heating within the tire as it flexes. That is why tires alway feel warm after they have run for awhile. So the specified tire pressures are designed so that if the pressure is set to that level at 68 F (20 C = standard temperature), when the tire warms up with use it will be at the desired pressure.

[LIRider]

Thanks guys but I do understand the variation of pressure with temperature (have a BS & MS in engineering fields). What confuses me is that I was always told you set the tire pressure to the recommended value(s) when they are cold or more specifically at the current ambient temperature (not heated by driving on them or sitting in the sun if you can help it). You don't correct for it not being 68 degrees out. Furthermore, the settings presented in the owners manual make no mention of the fact that they correspond to one temperature or altitude condition only. Yet, the display on the bike shows you the pressures corrected to 68 degrees. That's the part that I have a hard time with.

[dirtrider]

Thanks guys but I do understand the variation of pressure with temperature (have a BS & MS in engineering fields). What confuses me is that I was always told you set the tire pressure to the recommended value(s) when they are cold or more specifically at the current ambient temperature (not heated by driving on them or sitting in the sun if you can help it). You don't correct for it not being 68 degrees out. Furthermore, the settings presented in the owners manual make no mention of the fact that they correspond to one temperature or altitude condition only. Yet, the display on the bike shows you the pressures corrected to 68 degrees. That's the part that I have a hard time with.

 

Evening LIRider

 

It's probably not that critical as you usually don't see more than a 20°f temperature change in any one or two day frame (that is only about a 2 psi spread).

 

A pound or two off with the tires at or above the lower psi safe range really won't make much difference.

 

The thing is: We KNOW the tire pressure changes with tire temperature so if you set your tire pressure at 30°f cold then it sure won't be at that pressure at 68°f cold. (so right there it HAS to be off at one temp or the other don't ya think)

 

I work daily with very tightly controlled & precisely recorded tire pressures as we must test vehicles under very controlled & tightly monitored conditions. We calibrate out tire gauges daily for some testing & weekly for general development work. We only check tire pressures with the vehicle on the ground with full vehicle weight on it. We usually don't figure in cold tire temperatures as our building is climate controlled but it does vary slightly as well as the atmospheric pressure varies a bit from day to day.

 

I guess the bottom line here is nobody really cares what the cold tire pressures are at as we don't ride or drive at those pressures (we really only care about hot tire temps) but seeing as the working temps can vary a little & can vary by speed & road temps as well as sun load the only way to set them correctly is (before driven) cold soak.

 

It probably doesn't matter at what temps you check or set your cold tire temps but they really should be corrected to some common temperature (that can be 30°f or 100°f as long as you always use the same corrected-to temp, but most use 68°f as that is the commonly used standard.

 

I don't know how BMW sets their tire pressure specs but if like most auto companies they are set a bit to the high side as the motor company engineering community expects the drivers to allow the tire pressures to drop below the suggested set point as no one checks their tire pressures daily or even weekly. (the anal one that do probably wouldn't use the OEM suggested minimum specs anyhow)

 

 

 

 

[narcosis]

So what we need here is a little chart on what pressure to set according to temp. Maybe I'll get ambitious, apply the ideal gas law, create a spreadsheet. At least I don't have to account for altitude changes around here...it varies maybe 30 feet.

 

Heck, I won't see 68 degrees in my garage for a while yet. 78 this morning at 6:00 am

[dirtrider]

So what we need here is a little chart on what pressure to set according to temp. Maybe I'll get ambitious, apply the ideal gas law, create a spreadsheet. At least I don't have to account for altitude changes around here...it varies maybe 30 feet.

 

Heck, I won't see 68 degrees in my garage for a while yet. 78 this morning at 6:00 am

 

Afternoon Ken

 

You don't really need a chart, just figure 1 psi per 10°f change & you will be plenty close. If your garage is 78°f & the tires have cold soaked at that temp overnight then just set them 1 psi higher than you would set at 68°f.

[LIRider]

Okay, I think I get it now ... they show you the corrected pressure so you don't go thinking you've over-inflated your tires after you warm them up. However, I still believe the intent of the spec is the pressure you should set the tires to for your given conditions when the tires are cold (at ambient temperature) and not necessarily the spec for 68 degrees (although it certainly applies to 68 degrees if that is your current ambient temperature). Bottom line is that if I fill my tires to spec pressure when they are at 30 degrees and I check the TPM system, I should see a pressure close to 4 psi higher since the system is adjusting the pressure to 68 degrees. Or am I missing something else here?

[dirtrider]

However, I still believe the intent of the spec is the pressure you should set the tires to for your given conditions when the tires are cold (at ambient temperature) and not necessarily the spec for 68 degrees (although it certainly applies to 68 degrees if that is your current ambient temperature). Bottom line is that if I fill my tires to spec pressure when they are at 30 degrees and I check the TPM system, I should see a pressure close to 4 psi higher since the system is adjusting the pressure to 68 degrees. Or am I missing something else here?

 

Evening LIRider

 

Lets look at this as a worst case deal--If you fill your tires to max inflation pressure at 30°f then they will be over max spec at 68°f. I don't think BMW intends that riders put air in or let air out every time the temperature changes 20°f.

 

But I doubt that there is enough temperature change in any short time period to make it that critical.

 

On your TPM system-- if it is calibrated correctly & then it should show the air pressure adjusted to 68°f but I seriously doubt they are high enough quality to do that with precision.

[LIRider]

I agree that any temperature change on a given day will not matter all that much. I'm just trying to understand what the TPM might show after I properly inflate my tires. Although I've had my bike since January (when it was indeed cold out where I live), I never really played around to see what the readings were. I simply set the pressure in my garage, which was at worse 40 degrees (my gas heating system is in the garage and the two garage doors are fairly well insulated so it never drops to outside temperatures in the garage) and went about my business. I started monitoring the system much more closely after picking up a screw in my rear tire on the way home from a short trip in the Spring. I was fortunate in that the TPM alert didn't come on until I made it back into my garage (no doubt picked up the screw on my block). I can say the system did what I would want it to do and that is to let me know there might be a problem with a tire. What I do notice now (Summer) is that the TPM generally shows a slightly lower pressure (1 to 2 psi tops) than either of my gauges and the fact that it's showing me the pressure at 68 degrees probably explains that difference.

[RobCO80831]

DR,

 

I'm going to have to disagree on elevation changing what the tire gauges read and here is my reasoning.

 

I live at 6,800 ft of elevation, my family lives in the Midwest at around 800 feet of elevation. If the elevation made a difference with the tire pressures, then every time I would go to see my family (which I do twice a year), I would have to adjust the tire pressures during my drive to make up for the changes in elevation (losing ~6,000'). My TPMS would be constantly going off if my tires were either 15 pounds over/under-inflated.

 

I have also ridden motorcycles up to Pikes Peak (gaining 7,300') and my tire pressures have not changed in any of my vehicles. I have proven this with cars, motorcycles and bicycles that the tire pressures do not change, even with those large swings of elevation. And my cheapo dial tire-pressure gauge reads the same at sea level as it does at my current house.

 

I do agree that temperatures can make a difference but elevation cannot make a difference in tire pressures.

 

 

[lkraus]

Your gauge shows the difference between the tire pressure and the atmospheric pressure. The air pressure difference between sea level and 800 feet is only about 0.5 psia; from 800 feet to 7000 the difference is about 3.0 psia. ( Table here. ) A 15 psi change requires moving from below sea level to outside the atmosphere.

 

Without a precise pressure gauge, and careful temperature measurements, you are not likely to notice any significant differences in tire pressure with altitude. The quantity of air in the tire does not change with altitude, but the difference in pressure between the inside and the outside does.

Edited September 22, 2016 by lkraus

[dirtrider]

 

I do agree that temperatures can make a difference but elevation cannot make a difference in tire pressures.

 

 

 

Morning Rob

 

Good thing you don't design airplanes if you believe that altitude has no effect on pressure inside a closed container vs pressure outside that container.

 

Larry pretty well covered the PSI changes per altitude change.

 

You DO re-zero your tire pressure gauge at the altitude that you are at before using it at different altitudes correct? (the only way to see/measure the true pressure change differences)

 

As for your TPM system, that is not so defined as your hand held tire pressure gauge does in fact measure the difference between atmospheric pressure & pressure inside the tire so it definitely shows the tire pressure change vs altitude.

 

The difference with the TPM system is that the pressure sensors are INSIDE the tire (in the pressure chamber) so have no direct reference to the atmospheric pressure outside the tire (so about all they see is the difference in the pressure due to air volume changes from tire expansion/contraction/internal air temperature change, etc).

 

I suppose BMW could give a rider a better TPM system for altitude changes if they wanted to spend a little more money & write some more code. On the BMW the TPM system talks to the CAN (or Linbus ) & the fueling computer does have a built in altitude sensor so it could be done (probably not a priority item for them though)

_________________________

[NoelCP]

If you're not convinced of elevation changes on tire pressure just try playing tennis at 7500' with balls made for play at sea level! My wife and I played tennis a lot on the left coast, moved to Laramie WY. When we decided to go play tennis on the college courts we were shocked at how our timing was completely off! We learned later there are such things as high elevation tennis balls ;o)

 

This discussion re adjusting for ambient temp missed the point I made above vis a vis the TPM display. Here's what I'm saying: I have always followed the 1 degree per 10 degree F adjustment. But here's there rub: if it's 58F out, and I set my rear tire to 40psi w/ my allegedly very accurate Flaig gauge (i.e., so that at 68F ambient it *should* read 42psi, TPM will reflect a 2 degree difference--i.e., TPM AFTER the tire is road warmed, will display 40psi, not 42psi. Whereas when I set TP when ambient is 68F, and set it at 42psi w/ the Flaig gauge, TPM when the tire is road warmed will in fact display exactly 42psi. One potential explanation is that maybe my Flaig gauge actually adjusts for ambient temp. If that isn't it, what is the explanation?

Edited September 22, 2016 by NoelCP

[Mike]

If you're not convinced of elevation changes on tire pressure just try playing tennis at 7500' with balls made for play at sea level! My wife and I played tennis a lot on the left coast, moved to Laramie WY. When we decided to go play tennis on the college courts we were shocked at how our timing was completely off! We learned later there are such things as high elevation tennis balls ;o)

 

This discussion re adjusting for ambient temp missed the point I made above vis a vis the TPM display. Here's what I'm saying: I have always followed the 1 degree per 10 degree F adjustment. But here's there rub: if it's 58F out, and I set my rear tire to 40psi w/ my allegedly very accurate Flaig gauge (i.e., so that at 68F ambient it *should* read 42psi, TPM will reflect a 2 degree difference--i.e., TPM AFTER the tire is road warmed, will display 40psi, not 42psi. Whereas when I set TP when ambient is 68F, and set it at 42psi w/ the Flaig gauge, TPM when the tire is road warmed will in fact display exactly 42psi. One potential explanation is that maybe my Flaig gauge actually adjusts for ambient temp. If that isn't it, what is the explanation?

 

You hit the nail on the head. On page 42 of my 2014 RT manual, it says, "The tire pressures are shown adjusted for temperature on the multifunction display and are always relative to the following air temperature: 68°F (20 °C)."

[duegatti]

I think there have been some unintended consequences to the temperature compensation of our TPM systems. I would much prefer that it wasn't temperature compensated, and that it was as accurate and precise as the automobile systems that I've had.

 

BMW has given us a pressure spec, and told us that the TPMS corrects to 68F. That's it. They've never said that the TMPS should always read 36/42, and I don't think that it should.

 

 

if it's 58F out, and I set my rear tire to 40psi w/ my allegedly very accurate Flaig gauge (i.e., so that at 68F ambient it *should* read 42psi, TPM will reflect a 2 degree difference--i.e., TPM AFTER the tire is road warmed, will display 40psi, not 42psi. Whereas when I set TP when ambient is 68F, and set it at 42psi w/ the Flaig gauge, TPM when the tire is road warmed will in fact display exactly 42psi. One potential explanation is that maybe my Flaig gauge actually adjusts for ambient temp. If that isn't it, what is the explanation?

 

Noel, if it's a 1 lb/degF sensitivity that you're on board with (I use this myself), when you set your tires to 40 lb at 58F, the TMPS should correct to 68F and give you 41 lb, no? Not 42 lb. So, the TPMS is off by 1 lb, under those conditions. At 68F, you get the correct, expected reading of 42 lb, after you've set the pressure at 42 lb. The TPMS system is accurate in this case.

 

This deviation of TPMS readings of 1 lb, over a temperature delta of 10 degrees, is very much in keeping, I think, with what we all see with our systems. If the temperature correction was perfect, the readings should not change. But they do. I believe this is the explanation for what you have seen. To see how the system works, you would need to get clear of the system error by looking at ambient temperatures much further from 68F.

 

Moreover, I would say that at 58F, you should not set your rear at 40lb (or, 41 lb, if you more correctly adhere to the 1 lb/degF sensitivity). You should be setting it for the specified 42 lb, and then expect a properly functioning, temperature compensating TPMS system to read 43 lb throughout your ride.

 

[LIRider]

I think there have been some unintended consequences to the temperature compensation of our TPM systems. I would much prefer that it wasn't temperature compensated, and that it was as accurate and precise as the automobile systems that I've had.

 

I completely agree.

 

Moreover, I would say that at 58F, you should not set your rear at 40lb (or, 41 lb, if you more correctly adhere to the 1 lb/degF sensitivity). You should be setting it for the specified 42 lb, and then expect a properly functioning, temperature compensating TPMS system to read 43 lb throughout your ride.

 

Exactly the point I've been trying to get at ... you set the pressure to the spec numbers and don't try to compensate for anything. The TPM will, unfortunately, need to be compensated for since it's going to show you the equivalent pressure at 68 degrees, which may or may not be your current ambient temperature (or the temperature at which you set the spec pressures).

[dirtrider]

[John in VA]

TPM on my wethead seems no better than the TPM on my '13 camphead except perhaps as a warning of sudden pressure loss -- which I'm sure I'd detect through the seat of my pants before noticing a blip on the RID.

 

For example, this morning at 10am it was about 72F ambient temp at 500ft elevation. I checked my PR4 tires with a pretty good digital gauge and they read 40F and 42R. I checked it with a double-barrel foot tire pump with built-in dial gauge, same readings.

 

Fire up the bike and TPM says 33F and 36R, which is just not useful information.

[Mike]

TPM on my wethead seems no better than the TPM on my '13 camphead except perhaps as a warning of sudden pressure loss -- which I'm sure I'd detect through the seat of my pants before noticing a blip on the RID.

 

For example, this morning at 10am it was about 72F ambient temp at 500ft elevation. I checked my PR4 tires with a pretty good digital gauge and they read 40F and 42R. I checked it with a double-barrel foot tire pump with built-in dial gauge, same readings.

 

Fire up the bike and TPM says 33F and 36R, which is just not useful information.

 

I have a slightly different take on it. I still check my tires before every ride, but I do keep an eye on the TPM readings, to watch for a decrease that might signal a leak. I figure it might save me from a catastrophic failure if I can catch it before it completely deflates or drops to the point where the tire is destroyed.

 

[fastlarry]

I have a couple of dial-type tire pressure gauges. Neither agrees with the other and neither agree with the TPM. In fact the gauges are 4 psi off between themselves. They are not high dollar pieces, but they aren't garbage either. Is there a tire gauge that is accurate, but not exceedingly expensive? Is there a place one could send one for calibration? I never gave much thought to tire pressure gauge accuracy till this thread appeared.

 

Larry

[DaveCinNO]

Try this one - works great for me.

 

http://www.advantagebmwparts.com/oe-bmw/82121467187?origin=pla&gclid=CIbW9pS9pM8CFYtahgod8p8B9w

[NoelCP]

I think there have been some unintended consequences to the temperature compensation of our TPM systems. I would much prefer that it wasn't temperature compensated, and that it was as accurate and precise as the automobile systems that I've had.

 

BMW has given us a pressure spec, and told us that the TPMS corrects to 68F. That's it. They've never said that the TMPS should always read 36/42, and I don't think that it should.

 

 

if it's 58F out, and I set my rear tire to 40psi w/ my allegedly very accurate Flaig gauge (i.e., so that at 68F ambient it *should* read 42psi, TPM will reflect a 2 degree difference--i.e., TPM AFTER the tire is road warmed, will display 40psi, not 42psi. Whereas when I set TP when ambient is 68F, and set it at 42psi w/ the Flaig gauge, TPM when the tire is road warmed will in fact display exactly 42psi. One potential explanation is that maybe my Flaig gauge actually adjusts for ambient temp. If that isn't it, what is the explanation?

 

Noel, if it's a 1 lb/degF sensitivity that you're on board with (I use this myself), when you set your tires to 40 lb at 58F, the TMPS should correct to 68F and give you 41 lb, no? Not 42 lb. So, the TPMS is off by 1 lb, under those conditions. At 68F, you get the correct, expected reading of 42 lb, after you've set the pressure at 42 lb. The TPMS system is accurate in this case.

 

This deviation of TPMS readings of 1 lb, over a temperature delta of 10 degrees, is very much in keeping, I think, with what we all see with our systems. If the temperature correction was perfect, the readings should not change. But they do. I believe this is the explanation for what you have seen. To see how the system works, you would need to get clear of the system error by looking at ambient temperatures much further from 68F.

 

Moreover, I would say that at 58F, you should not set your rear at 40lb (or, 41 lb, if you more correctly adhere to the 1 lb/degF sensitivity). You should be setting it for the specified 42 lb, and then expect a properly functioning, temperature compensating TPMS system to read 43 lb throughout your ride.

 

I think I misstated what I meant. Let me try again. I happened to have ambient tempt very close to 68F the other day. I set tire pressure at 42psi. When TPM lights up after rolling a bit, TPM will say 40psi, then when the tire is road-warmed, it reads spot on 42psi. Sounds good so far.

 

Here's the part I don't follow: if ambient is 48F, and I set pressure at 40psi, when the tire gets fully road warmed TPM will read 38psi on the cold tire, and 40psi on the road-warmed tire. I guess I would have thought as the tire gets very road-warmed it might read 42psi, not 40psi, but it doesn't.

[NoelCP]

Moreover, I would say that at 58F, you should not set your rear at 40lb (or, 41 lb, if you more correctly adhere to the 1 lb/degF sensitivity). You should be setting it for the specified 42 lb, and then expect a properly functioning, temperature compensating TPMS system to read 43 lb throughout your ride.

 

Well, that is what I am wondering. Perhaps it doesn't make sense to follow the 1psi/10F rule. You are thinking no matter what ambient temp is, always set for the pressure you'd want it to be at at 68F? Here's why I'm still foggy:

 

I happened to have ambient tempt very close to 68F the other day. I set tire pressure at 42psi. When TPM lights up after rolling a bit, TPM will say 40psi, then when the tire is road-warmed, it reads spot on 42psi. Sounds good so far.

 

Here's the part I don't follow: if ambient is 48F, and I set pressure at 40psi, when the tire gets fully road warmed TPM will read 38psi on the cold tire, and 40psi on the road-warmed tire. I guess I would have thought as the tire gets very road-warmed it might read 42psi, not 40psi, but it doesn't.

[dirtrider]

I have a couple of dial-type tire pressure gauges. Neither agrees with the other and neither agree with the TPM. In fact the gauges are 4 psi off between themselves. They are not high dollar pieces, but they aren't garbage either. Is there a tire gauge that is accurate, but not exceedingly expensive? Is there a place one could send one for calibration? I never gave much thought to tire pressure gauge accuracy till this thread appeared.

 

 

Morning Larry

 

Motorcycle tire pressure gauges are a big compromise. At work I have a couple of very high dollar liquid filled tire pressure gauges that are usually calibrated daily (or at least weekly)-- Very accurate & very repeatable & easily calibrated. But neither are worth a darn for motorcycle usage as they are both large face dial gauges with 18" hoses.

 

Although they are super accurate they just allow way too much air to escape into the gauge at each testing for motorcycle use (between 1 & 2 pounds lower at each testing) -- So these great high dollar gauges are pretty worthless for small volume motorcycle tires unless I want to first overfill them then bleed the pressure down to proper PSI.

 

For motorcycle usage I have a couple of very small volume electronic gauges that allow almost no pressure drop when testing (just a very very small volume electronic sensor head). One came calibrated almost perfectly (& stayed that way for a long time now) & the other one I have our instramation department recalibrate for me (read very clos at 30-45 psi but the zero is off at lower PSI readings.

 

My suggestion is to find a small electronic gauge that allows almost no air escape at testing (very small internal volume & good sealing to valve stem) then find a tire store, or up-level garage, or race shop & check the calibration. If it is off 2 PSI (in the 30-45 psi range) then just figure that into your tire checking procedure.

 

All the accuracy in the world is pretty useless if you let 2 psi out at every check.

[NoelCP]

I have a couple of dial-type tire pressure gauges. Neither agrees with the other and neither agree with the TPM. In fact the gauges are 4 psi off between themselves. They are not high dollar pieces, but they aren't garbage either. Is there a tire gauge that is accurate, but not exceedingly expensive? Is there a place one could send one for calibration? I never gave much thought to tire pressure gauge accuracy till this thread appeared.

 

Larry

 

When I got back into riding 2.5h ago I researched gauges and settled on this which I have called an heirloom quality gauge for a motorcycle. Dunno if it deserves that but here's the deal: comes w/ both straight and 90 degree chuck, is claimed to be accurate to I think +/- 1%, is excellent at avoiding pressure loss when removing the chuck from valve stem because of its relief valve, and best of all results are highly reproducible. It's a mechanical dial type w/ an adjustable zero point. You might have to poke around for a straight chuck option which is where it's at for our 90degree valve stems. Made in Germany--google Flaig Tire Gauge.

 

 

 

 

[duegatti]

You are thinking no matter what ambient temp is, always set for the pressure you'd want it to be at at 68F?

 

No. The specification is not 42/36 on the TPMS readout. The spec is 42/36, the way we operated for a hundred years before TPMS, set on an equilibrated system prior to riding ("cold", if you will) for the ambient conditions of the ride. 42/36 at 38F; 42/36 at 68F; 42/36 at 98F.

 

Now, if we so choose, we assume the added burden of trying to figure out what the hell we're going to see on a TPMS that - for some reason I can't understand - attempts, but fails, to correct the pressures to a relevant but arbitrary 68F (a nice round 20C).

 

It's bad enough that the temperature compensation has added another calculation and one more source of error, but it has also derailed our understanding of how and why we inflate our tires.

[narcosis]

This is along the lines I've been thinking. There obviously is an ideal pressure, regardless of temperature, that produces the optimal tire profile. Why not put an external barometric pressure sensor on the bike and do the math compared with the internal tire pressures. Seems like that would be better.

 

Plus, is the same pressure ideal for Michelin PR4's as Metzeler Z8's, as Bridgestone T30's...?

 

[James L.]

The best way to get around all the calculations is to use nitrogen in the tire. It is expensive to get the setup, but it removes all the problems. It is used in racing tires to reduce the number of calculations to get the "wedge" in a round track car.

 

If you are going to spend that amount of time on tire pressure, you should already be using nitrogen.

 

Yes, if you need to add air on the road, that will create a problem, but back at home, you can rectify that situation.

 

If not using nitrogen, the exercise is quite moot. I think a gauge manufactured for your location and pressure checked before riding will suffice. But this is my opinion.

 

I know many of you are extremely anal like myself, but I find I dig way deeper than really needed.

 

James L

[dirtrider]

The best way to get around all the calculations is to use nitrogen in the tire. It is expensive to get the setup, but it removes all the problems. It is used in racing tires to reduce the number of calculations to get the "wedge" in a round track car.

 

If you are going to spend that amount of time on tire pressure, you should already be using nitrogen.

 

Yes, if you need to add air on the road, that will create a problem, but back at home, you can rectify that situation.

 

If not using nitrogen, the exercise is quite moot. I think a gauge manufactured for your location and pressure checked before riding will suffice. But this is my opinion.

 

I know many of you are extremely anal like myself, but I find I dig way deeper than really needed.

 

Morning James

 

What am I missing here? Are you saying that nitrogen doesn't adhere to the same ideal gas law as air does?

 

Exactly WHAT problems does nitrogen remove other then take some weight out of the rider wallet?

[Green RT]

The best way to get around all the calculations is to use nitrogen in the tire. It is expensive to get the setup, but it removes all the problems. It is used in racing tires to reduce the number of calculations to get the "wedge" in a round track car.

 

If you are going to spend that amount of time on tire pressure, you should already be using nitrogen.

 

Yes, if you need to add air on the road, that will create a problem, but back at home, you can rectify that situation.

 

If not using nitrogen, the exercise is quite moot. I think a gauge manufactured for your location and pressure checked before riding will suffice. But this is my opinion.

 

I know many of you are extremely anal like myself, but I find I dig way deeper than really needed.

 

Morning James

 

What am I missing here? Are you saying that nitrogen doesn't adhere to the same ideal gas law as air does?

 

Exactly WHAT problems does nitrogen remove other then take some weight out of the rider wallet?

 

My reaction exactly. Then I decided to look into it a bit before seconding your comment. It turns out that the water vapor included as a component of air, causes the tire temperature to vary more than pure nitrogen does. However, the pressure in nitrogen filled tires still varies with temperature. It is just muted somewhat because the temperature variation is more predictable than for tires filled with air containing water vapor.

[James L.]

The best way to get around all the calculations is to use nitrogen in the tire. It is expensive to get the setup, but it removes all the problems. It is used in racing tires to reduce the number of calculations to get the "wedge" in a round track car.

 

If you are going to spend that amount of time on tire pressure, you should already be using nitrogen.

 

Yes, if you need to add air on the road, that will create a problem, but back at home, you can rectify that situation.

 

If not using nitrogen, the exercise is quite moot. I think a gauge manufactured for your location and pressure checked before riding will suffice. But this is my opinion.

 

I know many of you are extremely anal like myself, but I find I dig way deeper than really needed.

 

Morning James

 

What am I missing here? Are you saying that nitrogen doesn't adhere to the same ideal gas law as air does?

 

Exactly WHAT problems does nitrogen remove other then take some weight out of the rider wallet?

 

First, nitrogen escapes a tire more slowly. then the gas is purely humidity free.

 

If you are going to check your tires with the uber small pressure head digital gauge that cost $90 bucks because it is a BMW item and uses/loses virtually no air, I think nitrogen would be the way to go. If a person is going to get that anal, nitrogen would help.

 

This is about precision, maybe it doesn't solve all problems(mis-statement on my part), but will get closer to the focal point of extremely anal people.

 

James L

Edited September 25, 2016 by James L.